Frontiers in Quantum Materials and Devices Workshop 2016
Poster Abstracts
Imaging Cyclotron Orbits of Electrons in Graphene
Sagar Bhandari, Gil-Ho Lee, Anna Klales, Kenji Watanabe, Takashi Taniguchi, Eric Heller, Philip Kim, Robert M. Westervelt
Electrons in graphene can travel for several microns without scattering at low temperatures, and their motion becomes ballistic, following classical trajectories. When a magnetic field B is applied perpendicular to the plane, electrons follow cyclotron orbits. Magnetic focusing occurs when electrons injected from one narrow contact focus onto a second contact located an integer number of cyclotron diameters away. By tuning the magnetic field B and electron density n in the graphene layer, we observe magnetic focusing peaks. We use a cooled scanning gate microscope to image cyclotron trajectories in graphene at 4.2 K. The tip creates a local change in density that casts a shadow by deflecting electrons flowing nearby; an image of flow can be obtained by measuring the transmission between contacts as the tip is raster scanned across the sample [1,2]. On the first magnetic focusing peak, we image a cyclotron orbit that extends from one contact to the other. In addition, we study the geometry of orbits deflected into the second point contact by the tip [3].
Supported by Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319 and U.S. DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under grant DE-FG02-07ER46422.
References:
1. M.A. Topinka et al., Nature 410, 183(2001).
2. K. E. Aidala et al., Nature Physics 3, 464 (2007).
3. S .Bhandari et al., Nano Lett. DOI: 10.1021/acs.nanolett.5b04609 (2016).
Ground State Electroluminescence
M. Cirio, S. De Liberato, N. Lambert, F. Nori
Electroluminescence, the emission of light in the presence of an electric current, provides information on the allowed electronic transitions of a given system. Our work shows that, together with usual electroluminescence, systems in the ultrastrong light-matter coupling regime emit a uniquely quantum radiation when a flow of current is driven through them. While standard electroluminescence (SE) relies on spontaneous emission from excited states of the system, the process we describe (Ground State Electroluminescence, GSE) extracts bound photons from the dressed ground state and it has peculiar features that unequivocally distinguish it from usual electroluminescence.
Signatures of Majorana Modes in the Dynamics of HgTe Josephson Junctions
R.S. Deacon, E. Bocquillon, J. Wiedenmann, T.M. Klapwijk, H. Buhmann, S. Tarucha, L.W. Molenkamp, and K. Ishibashi
We study Josephson junctions with weak links of the HgTe Quantum spin hall insulator. The presence of a 4pi-periodic current phase relation due to the topologically protected gapless Majorana modes of the junction are revealed in measurements of the ac-Josephson effect. We present two methods of detecting this mode. First the Shapiro steps are measured in the presence of an rf-drive. We observe a doubling of the Shapiro step voltage indicating a fractional ac Josephson effect. In the second method we detect the Josephson emission from a voltage biased junction and detect a peak in the emission power at half the Josephson frequency of the junction again indicating the gapless mode.
Fractional Quantum Hall Effect in the N=2 Landau Level in Bilayer Graphene
Georgi Diankov, Chi-Te Liang, Francois Amet, Patrick Gallagher, Menyoung Lee, Andrew J. Bestwick, Kevin Tharratt, William Coniglio, Jan Jaroszynski, K. Watanabe, T. Taniguchi, and David Goldhaber-Gordon
We report transport measurements of fractional quantum Hall (FQH) states in the N=2 Landau level (LL) (filling factors 4<ν<8) in bilayer graphene. In contrast with recent observations of particle-hole asymmetry in the N=0/N=1 LLs of bilayer graphene, the FQH states we observe in the N=2 LL are consistent with the CF model: within a LL, they form a complete sequence of particle-hole symmetric states whose relative strength is dependent on their denominators. The FQH states in the N=2 LL display energy gaps of a few Kelvin, comparable to and in some cases larger than those of fractional states in the N=0/N=1 LLs.
Terahertz Spectroscopy of Single Molecules Using Sub-Nm Scale Gap Electrodes
Shaoqing Du, Kenji Yoshida, Ya Zhang, Kazuhiko Hirakawa
Terahertz (THz) spectroscopy is a powerful tool for clarifying electronic structures and vibrational dynamics of various kinds of molecules. However, it is a formidable challenge to greatly exceed the diffraction limit and perform single molecule spectroscopy, because there is a huge size difference (a factor of ~10^5) between the THz wavelength (~100 um) and the size of single molecules (< 1 nm).
In this work, we have performed terahertz (THz) spectroscopy of single molecules by using nanogap electrodes as the THz antennas. From transport measurements, we confirmed that we successfully captured a single molecule (C60 and Ce@C82) in the nanogap. When the sample was illuminated with a THz radiation from a blackbody light source, we observed a very small (less than 100 fA), but finite THz-induced photocurrent in the single molecule transistors. The Fourier spectra of the THz photocurrent exhibited equally-spaced low-energy multiple peaks and peak splitting, suggesting that the electron transport is strongly affected by molecular vibrations. This is the first demonstration of THz spectroscopy of single molecules.
Investigation of Superconductive Heavily Doped Boron Diamond for Device Fabrication
Delroy Green 1; Gary Harris1; R.D. Vispute 2;
Howard University 1) Bluewave Semiconductor Inc.2)
Diamond has a wide bandgap of 5.47 eV at room temperature and is the hardest known naturally occurring material with a Knoop hardness of 10,400 kg/mm2 or 10 on the Mohs scale. Due to the structure of the covalent bonding of its carbon atoms, diamond is extremely strong having each carbon bonded to four neighboring carbon atoms.
Although diamond is hard, its toughness, when compared to most engineering materials, is poor. However, because of its hardness, it is an efficient cutting and drilling tool. With the exception of naturally occurring blue diamonds, which are semiconductors, diamond is a good electrical insulator. However, unlike most insulators, diamond has the highest thermal conductivity of 22 W/cm-K among naturally occurring materials. Although diamond is a good electrical insulator, it also shows semiconducting properties when doped with impurities. When diamond is heavily doped with boron the resulting material possess excess electron holes and as such it is classified as a p-type material. If excess boron doping is achieved, then the resulting material is found to behave like a superconductor at very low temperatures. In this superconducting state, the doped diamond conducts electricity.
A series of boron-doped diamond films were grown by hot filament chemical vapor deposition (HFCVD) and tested to determine the optimum technique for doping diamond with boron for superconductivity. The first technique involved the insertion of boron powder (B2O3) around the sample holder to dope seeded poly and nano diamond during growth. The second technique involves doping with diborane gas (B2O6).
Various processing parameters were optimized for diamond quality, structure, morphology, and doping. A combined analysis of scanning electron microscope, Raman mapping and Hall measurements at various temperatures were conducted to ascertain the superconductive nature of the material. Preliminary results of the boron solid source doping on diamond show a superconductive transition temperature of 2.3 oKelvin at a doping concentration of 2.3 x 1020 cm-3. NSF –DMR 1231319
Green's Function of Magnetic Topological Insulator in Gradient Expansion Approach
Yusuke Hama and Naoto Nagaosa
In this research, we investigate the Keldysh Green's function of Weyl-fermion surface state of three- dimensional magnetic topological insulator in gradient expansion approach. By using it, we study various transport properties induced by the spatially and temporally slowly-varying magnetization such as electric charge, electric current and spin densities, as well as energy density and current in terms of emergent electromagnetic fields.
Edge States of Silicene, Germanene and Stanene Nanoribbons with Edge Hydrogen Terminations
Ayami Hattori, Masaaki Araidai, Yasuhiro Hatsugai, Keiji Yada, Kenji Shiraishi, Masatoshi Sato, Yukio Tanaka
We study electronic properties of monolayer zigzag silicene, germanene, and stanene nanoribbons with and without hydrogen terminations based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, not only \pi but also \sigma orbitals influence seriously on the energy spectra of edge states, in contrast to graphene nanoribbons. This orbital degree of freedom is also important for describing the hydrogen termination effects. Similar to zigzag graphene nanoribbons, various intriguing in-gap edge states appear depending on the types of edge hydrogen terminations. We propose zigzag stanene nanoribbons as the candidate of topological quantum field effect transistor from our calculations.
Crystalline Spin-Orbit Interaction and The Zeeman Splitting in Pb1−Xsnxte
Hiroshi Hayasaka and Yuki Fuseya
A ratio of the Zeeman splitting to the cyclotron energy (M =) characterizes the relative strength of the spin-orbit interaction in crystals. We calculated M for the narrow gap IV-VI semiconductors, PbTe, SnTe and their alloy Pb1-xSnxTe on the basis of the k・p theory, the group theoretical analysis and the relativistic empirical tight-binding band calculation. We clarified that M<1 for PbTe, M>1 for SnTe and found M=1 at the band inversion point. This shows that the band inversion is clearly characterized by the ratio M. By using this property, we can detect the transition to the topological crystalline insulator only from the bulk measurements of quantum oscillation. Also we show that, by using the deviation from M=1, we can evaluate how the electrons in crystal is close to the Dirac electrons.
Optical Response of Carbon Nanotube Quantum Dot
Akira Hida and Koji Ishibashi
The optical response of the quantum dot made of single-walled carbon nanotubes and collagen model peptides were investigated. In the photoluminescence excitation spectroscopy, a series of peaks corresponding to the discrete exciton emission levels were observed. When we increased the intensity of excitation laser, a single peak split into clear doublet. This behavior could be explained in terms of the Rabi splitting because the observed splitting was directly proportional to the square root of the excitation laser intensity. The anti-crossing in the dispersion relation was also observed, which supported above interpretation. A remarkable phenomenon occurred when the quantum dot was excited by picosecond laser pulse pairs. The pulse pairs strengthened or weakened the exciton emission depending on their phase difference. These results might lead us to the coherent control of excitons.
Angular Momenta of Two Valleys And Valley Coupling in Finite-Length Single-Wall Carbon Nanotubes
Wataru Izumida, Rin Okuyama, Ai Yamakage, Riichiro Saito
Recent studies of quantum transport have revealed that single-wall carbon nanotubes (SWNTs) contain richer phenomena, especially in terms of spin and valley. Fourfold degeneracy of discrete energy levels has been considered as an intrinsic property in finite-length SWNTs reflecting the two spin states and the two valley states. Measurements for ultraclean SWNTs show lift of fourfold degeneracy caused by the spin-orbit interaction. Here we focus on angular momenta of two valleys in SWNTs, to discuss the valley coupling as another mechanism of lift of degeneracy. It is shown that the two valleys have the same angular momentum in the majority (82 %) of SWNTs. The present study reveals that the coupling of two valleys occurs in the majority of finite-length SWNTs, even for ultraclean tubes with clean edges, which conserve the angular momentum of ideal bulk states of electrons.
Ballistic Electron Transport in Suspended InAs Nanowires
H. Kamata, R. S. Deacon, S. Matsuo, S. Baba, K. Li, H. Q. Xu, A. Oiwa, and S. Tarucha
InAs nanowires have a surface charge accumulation layer subject to a strong spin-orbit interaction (SOI), and therefore are expected to generate helical states under a magnetic field and furthermore Majorana Fermion bound states when contacted to a s-wave superconductor. Although ballistic transport in nanowires plays an important role in creating such exotic states, conductive electrons in the accumulation layer experience so strong surface roughness scattering and ionized impurity scattering that the transport is entirely diffusive. Here, to suppress such scatterings we employ suspended nanowire structures across a gap of approximately 100 nm, and observe a feature of ballistic electron transport.
Critical Current for the Breakdown of Quantum Anomalous Hall Effect Measured in Hall Bars with Various Sizes
Minoru Kawamura, Ryutaro Yoshimi, Kei. S. Takahashi, Atsushi Tsukazaki, Masashi Kawasaki, and Yoshinori Tokura
Quantum anomalous Hall effect (QAHE) is a novel quantum transport phenomena which has been demonstrated recently in ferromagnetic topological insulators [1, 2]. In the QAH state, the Hall resistance is quantized to h/e^2 and the longitudinal resistance becomes zero, as similar to the quantum Hall effect (QHE). Although there are many similarities between the two effects, the magnetization origin of the QAHE makes it different from the QHE qualitatively. One of the big differences is that the QAHE can be realized even at zero external magnetic field while the QHE requires high magnetic fields. Therefore the QAHE has a potential to be utilized as a new type of a quantum resistance standard without using any external magnetic field [3, 4]. For such an application, robustness of the QAHE against the measurement current is an important issue of discussion.
In this poster, we report robustness of the QAHE against the bias current. We observed the QAHE in MBE-grown Cr-doped topological insulator thin films and measured the current-voltage characteristics using Hall bars with various sizes. The Hall resistance deviated from the quantized value when the applied current exceeded a certain critical current. The critical current for the breakdown was proportional to the Hall bar width, suggesting that the Hall electric field which is responsible for the QAHE breakdown is homogeneous across the Hall bar.
[1] Cui-Zu Chang et al., Science 340, 167 (2013).
[2] J.G. Checkelsky et al., Nature Phys. 10, 731 (2014).
[3] A. J. Bestwick et al., Phys. Rev. Lett. 114, 187201 (2015).
[4] Cui-Zu Chang et al., Nature Mat. 14, 473 (2015).
Single-shot Readout of Three Two-electron Spin States in a Quantum Dot Coupled to Quantum Hall Edge States
H. Kiyama, T. Nakajima, S. Teraoka, A. Oiwa, and S. Tarucha
We demonstrate the single-shot readout of three two-electron spin states in a gate-defined GaAs single quantum dot. The three spin states are a singlet and two triplets having z-components of spin angular momentum of 0 and +1, and distinguished by detecting spin-dependent tunnel rates that arise from the spin filtering by spin-resolved edge states and the spin-orbital correlation with the orbital-dependent tunneling. We confirm the ternary spin readout by observing the spin relaxation dynamics among the three spin states.
Towards Single Layer Ferromagnetism
Dahlia R. Klein, Efrén Navarro-Moratalla, Pablo Jarillo-Herrero
Van der Waals heterostructures of layered 2D materials have been widely explored for conductors, insulators, and a whole family of semicondutors. However, there has been little development of magnetic 2D crystalline layers, which could lead to interesting new physical states when coupled with other atomically thin materials. Thus, we are attempting to expand this field through the study of an intrinsic magnetic layered material for the isolation of single layer ferromagnetism. We have optimized the growth of layered magnetic materials including the insulating ferromagnet chromium triiodide (CrI3). From the bulk crystals, we have successfully exfoliated them onto solid substrates. In light of their sensitivity to ambient moisture, we have also developed the necessary techniques to assure the integrity of the flakes during their manipulation. We are now evaluating the effect of dimensionality on the bulk ferromagnetic order using graphene-based Hall sensor devices.
Quantum Optics with Giant Artificial Atoms
Anton Frisk Kockum, Lingzhen Guo, Mikhail Pletyukhov, Arne L. Grimsmo, Sankar R. Sathyamoorthy, Alexandre Blais, Per Delsing, Franco Nori, and Goran Johansson
In quantum optics experiments with both natural and artificial atoms, the atoms are usually small enough that they can be approximated as point-like compared to the wavelength of the electromagnetic radiation they interact with. However, a recent experiment coupling a superconducting qubit to surface acoustic waves shows that a single artificial atom can be coupled to a bosonic field at several points which are wavelengths apart [1]. This situation could also be engineered with an xmon qubit coupled to a microwave transmission line [2].
Here, we present results of theoretical studies of such "giant artificial atoms" [2, 3, 4]. In the Markovian regime, where the travel time between coupling points is negligible, we find that interference effects due to the positions of the coupling points give rise to a frequency dependence for the strength of the coupling between the giant artificial atom and its surroundings [2]. The Lamb shift of the atom is also affected by the positions of the coupling points. We discuss possible applications for these frequency dependencies (which can be designed). In the non-Markovian regime, where the distance between coupling points is large, an excited giant atom exhibits revivals and non-exponential decay [3]. In this regime, we have also studied novel features that occur in the correlation function g^2(t). Finally, we also explore setups with several giant atoms coupled to a transmission line in various configurations [4].
[1] M. V. Gustafsson et al., Science 346, 207 (2014).
[2] A. F. Kockum et al., Phys. Rev. A 90, 013837 (2014).
[3] L. Guo et al., in preparation (2016).
[4] A. F. Kockum et al., in preparation (2016).
Bistable Photon Emission in Hybrid Circuit-QED
Neill Lambert, Christian Flindt, Franco Nori
We study the photon emission from a voltage-biased double quantum dot coupled to a microwave cavity [1,2], an example of so-called hybrid circuit-QED. We find that the resulting photonic statistics can exhibit a dynamic bistability, which we validated by showing that the distribution describing these statistics has the shape of a tilted ellipse. The switching rates which describe the bistability can be extracted from the electrical current and the shot noise in the quantum dots, and used to predict this elliptic form of the photonic distribution. We also explore how the state of the cavity can be manipulated and made "more coherent" with techniques from feedback control.
[1] N. Lambert, F. Nori, and C. Flindt, Physical Review Letters 115, 216803 (2015).
[2] N. Lambert, C. Flindt, and F. Nori, Euro. Phys. Lett. 103, 17005 (2013)
Fabrication of Quantum-Point Contacts in Graphene-on-Boron Nitride Heterostructures
Andrew Lin, Sagar Bhandari, Robert Westervelt
Graphene is an atomically thin hexagonal lattice of sp2-hybridized carbon atoms with unique electronic and physical properties. These properties, such as ballistic motion of electrons and Landau quantization which results in observation of the quantum Hall effect, are contingent on its mobility [1]. The mobility of electrons is limited when graphene sits on a silicon substrate due to uneven ripples and surface distributions caused by the substrate-level roughness of silicon wafers. Therefore, stacking graphene in between layers of atomically flat hexagonal boron nitride (hBN) creates a smooth substrate greatly increasing the mobility of the graphene samples [2]. We report in this poster the successful fabrication of a 100-nanometer quantum point contact (QPC) in a layered graphene-on-BN heterostructure, along with its characterization via atomic force microscopy, scanning-electron microscopy, and future characterization via electronic transport measurements and scanning-probe microscopy. From our scanning probe measurements, we hope to image the flow of electrons in graphene through a quantum point contact. In addition, we propose the imaging of coherent nature of electrons in graphene by verifying the existence of electron wave interference fringes.
[1] Nakaharai, S., J. R. Williams, and C. M. Marcus. "Gate-defined graphene quantum point contact in the quantum Hall regime." Physical review letters 107.3 (2011): 036602.
[2] Dean, C. R., Young, A.F., Meric I., Lee C., Wang L., Sorgenfrei S., Watanabe, K., Taniguchi, T., Kim, P., Shepard, K. L., & Hone, J. (2010)."Boron Nitride Substrates for High-quality Graphene Electronics." Nature Nanotechnology, 5 (10), 722-726.
Optically Detected Magnetic Resonance Of High-Density Ensemble Of NV Centers In Diamond
Yuichiro Matsuzaki, Hiroki Morishita, Takaaki Shimooka, Toshiyuki Tashima, Kosuke Kakuyanagi, Kouichi Semba, W. J. Munro, Hiroshi Yamaguchi, Norikazu Mizuochi, Shiro Saito
Optically detected magnetic resonance (ODMR) is a way to characterize the NV centers. By using our model, we have reproduced the ODMR with and without applied magnetic fields. Also, we theoretically investigate how the ODMR is affected by the typical parameters of the ensemble NV centers such as strain distributions, inhomogeneous magnetic fields, and homogeneous broadening width. Our model could provide a way to estimate these parameters from the ODMR, which would be crucial to realize diamond-based quantum information processing.
Conductance Fluctuations in High-mobility BN/graphene/BN System
M. Mineharu, C. R. da Cunha, M. Matsunaga, N. Matsumoto, Y. Ochiai, G.-H. Kim, K. Watanabe, T. Taniguchi, D. K. Ferry, and N. Aoki
We have fabricated a high-mobility graphene transistor using h-BN as the gate insulator. The mobility is close to 10 m^2/Vs and the transport regime is in the quasi ballistic regime. Nevertheless, we observed conductance fluctuations at low temperature. We will discuss its characteristics focusing on the differences between this and a conventional low mobility sample fabricated on a SiO2 layer.
Cancellation of Half-Quantized Anomalous Hall Conductivities in Top And Bottom Surface States of a Topological Insulator Heterostructure
M. Mogi, M. Kawamura, R. Yoshimi, A. Tsukazaki, Y. Kozuka, M. Kawasaki, and Y. Tokura
Three-dimensional topological insulators possess spin-momentum locked massless Dirac surface states due to topologically non-trivial insulating bulk states. The magneto-electric responses of the exotic surface states can be described as axion electrodynamics. The electrodynamics lead to presences of half-quantized Hall conductivity (e^2/2h) and novel ’topological magnetoelectric effect’ by breaking time reversal symmetry. The recently observed quantum anomalous Hall effect in magnetically doped topological insulators can be considered as a summation of half-quantized top and bottom surface states.
In this work, we grew a magnetic (Cr) modulation doped topological insulator ((Bi,Sb)2Te3) thin film with vertical asymmetry along growth direction. We observe stabilized zero Hall conductivity plateau and zero longitudinal conductivity without an external magnetic field. We have identified that the effect originates from a state where the magnetization of Cr modulation doped layers are anti-parallel, and hence the surface state has a gap in the whole of the film without edge state. In such magnetic configuration, we can assume a subtraction of Hall conductivities of top and bottom surface states, and at the same time, topological magnetoelectric effect may be directly detected.
Control of Charge States of NV Center by Nin Diamond
T. Murai, T. Makino, H. Kato, Y. Doi, Y. Suzuki, M. Hatano, S. Yamasaki, M. Shimizu, H. Morishita, M. Fujiwara, N. Mizuochi
Nitrogen-vacancy (NV) centers in diamond are the most promising candidate for various applications such as quantum information science, magnetometry, and biosensing. For these applications, controlling the charge state of the NV centers is crucial, because optical initialization and readout of the spin state of the NV centers are only possible in negatively charged one (NV−). However, upon illumination, the NV centers undergo stochastic charge-state transitions between NV− and neutral charge state of the NV center (NV0). In case of 532 nm excitation, the steady-state-population of NV− is about 70%. This charge-state interconversion occurs upon illumination at any wavelength, so the steady-state NV− population is always less than 75%–80%.
Recently, we showed Fermi level control by phosphorus doping generates 99.4 ± 0.1% NV− under 1 W and 593 nm excitation which is close to maximum absorption of NV−. Here, to realize such stabilization of the charge state of NV center in intrinsic undoped diamond film, we investigate nin diamond junction.
This work was supported by CREST and KAKENHI (No. 15H05868, 16H02088).
Artificial Two-dimensional Lattice Structures Assembled by Atom Manipulation Technique
M. Nantoh, K. Takashima, T. Yamamoto and K. Ishibashi
We have fabricated artificial two-dimensional lattice structures of Fe atoms and CO molecules on a Cu(111) surface using atom manipulation technique with a LT-STM. A triangular lattice and a square lattice of Fe show different standing waves patterns, reflecting different electronic states. On the other hand, the tunneling spectra measured within a CO triangular lattice show difference near the Fermi level depending on the measurement locations which correspond to sublattice A or sublattice B of the honeycomb of molecular graphene. This result indicates that the symmetry of the sublattice degree of freedom of this lattice structure is originally broken.
Simultaneous Feedback Control in Coupled Mechanical Resonators
Ryuichi Ohta, Hajime Okamoto, Daiki Hatanaka, and Hiroshi Yamaguchi
We demonstrate simultaneous feedback amplification and damping of several mechanical modes in hexagonally coupled mechanical resonators. Six doubly clamped beam resonators connecting each other form coupled mechanical modes which spatially distributes in several resonators. Opt-mechanical feedback loop using photothermal stress with appropriate delay times enhances or suppresses the multiple mechanical vibrations at the same time. This simultaneous feedback control will enable the manipulation of phonons in hybrid mechanical systems.
Observation Of Topological Faraday And Kerr Rotations In Quantum Anomalous Hall State By Terahertz Magneto-Optics
Ken N. Okada, Youtarou Takahashi, Masataka Mogi, Ryutaro Yoshimi, Atsushi Tsukazaki, Kei S. Takahashi, Naoki Ogawa, Masashi Kawasaki and Yoshinori Tokura
Electrodynamic responses from three-dimensional (3D) topological insulators (TIs) are characterized by the universal magnetoelectric term, i.e., the axion term, constituent of the Lagrangian formalism. The quantized magnetoelectric coupling, which is generally referred to as topological magnetoelectric (TME) effect, has been predicted to induce exotic phenomena including the universal low-energy magneto-optical effects. Here we report the experimental demonstration of the long-sought TME effect, which is exemplified by magneto-optical Faraday and Kerr rotations in the quantum anomalous Hall (QAH) states of magnetic TI surfaces by terahertz magneto-optics. The universal relation composed of the observed Faraday and Kerr rotation angles but not of any material parameters (e.g. dielectric constant and magnetic susceptibility) well exhibits the trajectory toward the fine structure constant (= 1/137) in the quantized limit. Our result will pave a way for versatile TME effects with emergent topological functions.
Prediction of Enhanced Edelstein Effect using interband Rashba Effect
K. Okamoto, A. Sawada, H. Chen, T. Yamashige and T. Koga
We will show our recent prediction of enhanced Edelstein effect based on the interband Rashba Effect in a double quantum well made of narrow gap semiconductors. We will show our preliminary experimental results as well.
Excitation Spectroscopy of Carbon Nanotube Quantum Dot by Cotunneling Transport
Rin Okuyama, Atsushi Iwasaki, and Mikio Eto
Transport through carbon nanotube quantum dot (CNT QD) in Coulomb blockade regime can be a new tool for the excitation spectroscopy because the inelastic cotunneling takes place when the bias voltage matches the excitation energy [1]. We theoretically examine the two-electron state in CNT QD and cotunneling transport. First, we elucidate the electronic state in this "multi-valley artificial atom," considering two valleys of K and K' points, spin-orbit interaction, and electron-electron interaction. Second, we calculate the cotunneling current and current fluctuation, which are in good agreement with recent experimental results [2].
[1] T. S. Jespersen et al., Nat. Phys. 7, 348 (2011).
[2] R. Fujiwara, T. Arakawa, and K. Kobayashi, private communications.
Electrical Transport in Metal-Deposited CVD-Graphene
Youiti Ootuka, Jin Aoki, Ayato Horie
We will present experimental results of the electrical transport of CVD graphene deposited with nickel and indium clusters at low temperatures. The magnetoresistance of Ni-deposited graphene is measured in order to investigate the interaction between the metal and the electrons in graphene, especially the effect onto the scattering of carriers. The magnetoresistance is well described by the weak-localization theory for graphene below about 1T, and the phase-breaking time, the inter-valley scattering time, and the transport scattering time are obtained by fitting to the theory. In the case of In-deposited graphene, the system shows behaviour similar to the granular superconducting films, i.e., the BTK-like global superconducting transition and the superconductor-to-insulator transition. They are tuned by the field-effect and the degradation of graphene by plasma processing.
Theoretical Study of Magnetoresistance in Bismuth Under Strong Magnetic Fields
Mitsuaki Owada, Yuki Fuseya
The magnetoresistance of bismuth has been studied for a long time, but its properties have not been clarified theoretically yet. We calculate the magnetoresistance of bismuth on the basis of the semi-classical theory and the quantum theory. In our calculation, the specific properties of bismuth are considered: 1) the magnetic field dependence of electron and hole carriers, 2) the effective Hamiltonian: the extended Dirac and the Smith-Baraff-Rowell models, 3) the effective mass and the g-factor tensors estimated from experiments. In the case of semi-classical theory, we found that the magnetoresistance increases linearly at strong magnetic fields (B>9T), which can explain the experimental mystery qualitatively. However, the semi-classical result doesn’t agree with experiments “quantitatively” It is much larger than the experimental magnetoresistance. On the other hand, the quantum result shows the different field dependence from the semi-classical one at strong magnetic fields (B>9T). We discuss the difference between the semi-classical and the quantum theories at strong magnetic fields.
Tight-Binding Theory of Surface Spin States on Bismuth Thin Films
Kazuo Saito, Hirokatsu Sawahata, Takashi Komine, and Tomosuke Aono
The surface spin states for bismuth thin films are investigated using an sp3 tight-binding model. The model explains most experimental observations using angle-resolved photoemission spectroscopy, including the Fermi surface, the band structure with Rashba spin splitting, and the quantum confinement in the energy band gap of the surface states.
Phonon-Modulated Graphene Nanomesh for Thermal Transport Control
Takuya Sekiguchi, Yuma Yasui, Yuki Anno, Kuniharu Takei, Seiji Akita, Takayuki Arie
Since the main heat carrier in graphene is phonon, the structural change in graphene modifies the phonon propagation, thereby changing the thermal transport properties. Here we study the graphene nanomesh structures with periodically aligned nanoscale holes for phonon modulation. The electrical and thermal properties of graphene nanomesh show that the structural modification does not alter the electrical conductivity but reduces the thermal conductivity by approximately 80% when the neck width between holes is accurately controlled. This is due to the mean free path of phonon in graphene is much longer than that of electron. From the viewpoint of thermoelectric application, where the device performance is positively correlates with electrical properties but negatively correlates with thermal properties, the way of reducing thermal conductivity we propose is useful for enhancing the performance of graphene thermoelectric devices.
Inter-Edge-Channel Scattering and Nuclear Polarization in InSb Two-Dimensional System
K.Sekine, K.Kakuta, K.Nagase, Y.Hirayama
In the quantum Hall system, the ratio of Zeeman energy to cyclotron energy can be changed by tilting the device in a magnetic field, which causes switching of the spin-resolved Landau levels for a large g-factor system such as InSb two-dimensional (2D) system. We realize an unequal population of edge channels using a gate controlled through a metal-insulator-semiconductor structure, and evaluate a scattering rate between two unequally-populated edge channels. The relaxation length is clearly modified when the tilting angle is changed, reflecting the switching of the spin in the neighboring two edge channels (i.e. spin parallel or antiparallel). The temperature dependence will be discussed in the presentation.
Local Heating In Superconducting Atomic Point-Contacts
Yukihiro Shibata and Youiti Ootuka
In atomic point-contacts, the current crowning and the electric-field concentration take place in the atomic vicinity of the junction, and easily drive the system far from equilibrium. In such situations, it is not trivial that the both systems, the electron and the lattice, have the same temperature.
We made an aluminum mechanical-break junction (MBJ) device that had small thermometers very close to the junction, and successfully measured the lattice temperature change as a function of biasing voltage and current of MBJ at low temperatures. We also found an evidence of super-to normal-conducting transition in the I-V curve of Al-MBJ, that indicates the electron temperature exceeds Tc. We will present the experimental data and discuss the energy dissipation and the heat flow in the device.
Efficient Control over Wavefunctions of Electrons in Self-Assembled InAs Quantum Dots using Side-Gating
R. Shikishima, H. Kiyama, S. Baba, T. Hirayama, N. Nagai, K. Hirakawa, S. Tarucha, and A. Oiwa
InAs self-assembled islands work as quantum dots (QDs) without complicated gate tunings and show a lot of fascinating properties. One of the expected applications of InAs QDs is spin-based information processing. In InAs QDs, all-electrical fast manipulation of single electron spin with Rabi frequency as high as 60 MHz has been reported due to the strong spin-orbit interaction (SOI). Moreover, SOI in self-assembled InAs QDs can be tuned by applying an electric field to alter the QD confinement. Much stronger SOI and thus much faster manipulation may be realized by the SOI tuning with side-gating. However, the efficiency of the side-gate tuning is typically about 10 times smaller than that of the back-gate tuning mainly due to the screening effect by source and drain electrodes directly contacted to the QD. This prevents efficient electrical control of SOI in self-assembled QDs.
In this work, we show the improvement of the side-gating efficiency by fabricating narrow source and drain electrodes, which considerably suppress the screening effect. In the devices with a source-drain electrode width of 40 nm, leverarms of side-gates are improved to be more than 2 times large compared with those of conventional width of 200 nm. Using the improved efficiency of side-gate control, we observe a peak-like feature in the side-gate dependence of tunnel couplings evaluated from Coulomb peaks. This can be interpreted in terms of the spatial shift of electron wavefunction in the InAs QD across the narrow source-drain electrode. This result indicates that efficient control using side-gates enables us to largely modulate the QD confinement, which is needed for a large control of SOI.
Fast Phase-Control of the Single Nuclear Spin by Electric Field Effects
T. Shimo-Oka, Y. Tokura, Y. Suzuki, N. Mizuochi
We propose a fast phase-gating of a single nuclear spin, which interacts with the single electronic spin of the nitrogen-vacancy center in diamond. Our gate operation is to be called the geometric quantum gate; a geometric phase shift of the electron spin induced by a rotating electric field and magnetic field, is utilized to control the nuclear-spin phase. We estimate that the phase-gate time is orders shorter than hitherto reported. This work was supported by CREST and KAKENHI (No. 15H05868, 25220601).
Theory of Quantum Information Hardware
Peter Stano, Chen Hsuan-Hsu, Guang Yang, and Daniel Loss
I will present our recent achievements in the quantum theory of condensed matter with a focus on spin and phase coherent phenomena in semiconducting and magnetic nanostructures. In particular, the team investigates the fundamental principles of quantum information processing in the solid state with a focus on spin qubits in quantum dots, superconducting qubits, and topological quantum states such as Majorana fermions and parafermions. This involves the study of decoherence in many-body systems and scalable quantum computing technologies based on surface codes and long-distance entanglement schemes. We also study nuclear spin phases, many-body effects in low-dimensional systems, quantum Hall effect, topological matter, spin orbit interaction, and quantum transport of magnetization.
Theory for Diabatic Mechanisms of Higher-Order Harmonic Generation in Semiconductors
T. Tamaya, A. Ishikawa, T. Ogawa, and K. Tanaka
We theoretically investigated higher-order harmonic generation (HHG) in semiconductors under a high-intensity ac electric field. Consequently, we discovered that the diabatic processes, namely, ac Zener tunneling and semimetallization of semiconductors, are key factors for nonperturbative mechanisms of HHG. These mechanisms are classified by the field intensity and could be understood by an extended simple man model based on an analogy between tunnel ionization in gaseous media and Zener tunneling in semiconductors.
Micrometer-Scale Electron Paramagnetic Resonance Spectroscopy Using a Superconducting Flux Qubit
Hiraku Toida, Yuichiro Matsuzaki, Kosuke Kakuyanagi, Xiaobo Zhu, William J. Munro, Kae Nemoto, Hiroshi Yamaguchi, and Shiro Saito
Electron paramagnetic resonance (EPR) spectroscopy using superconducting devices is extensively investigated to achieve a sensitivity of a single spin. Flux qubits, which consist of micrometer-scale superconducting loop structure, can interact with spin ensembles strongly due to their large circulating current. We use this feature to realize a highly sensitive magnetization detector and EPR spectroscopy. We prepare a flux qubit directly coupled to an electron spin ensemble (Er:YSO) to perform experiments. First, we measure magnetization of the spin ensemble as a function of an in-plane magnetic field and temperature. Magnetization of the spin ensemble increases lineally as a function of Zeeman and thermal energy ratio, which is consistent with Curie-Weiss's law. We also perform EPR spectroscopy by applying a radio frequency excitation signal with an on-chip microstrip. Resonance peaks appear and positon of the peaks increases linearly as increasing the in-plane magnetic field. We derive g-factors of the spin ensemble to be ~6.0 and ~15.9, which is consistent with literatures. Sensitivity and the sensing volume of this method are estimated to be less than 1000 spins per unit time and ~0.05 pL, respectively.
Formation of Tunnel Barriers in a Multi-Walled Carbon Nanotube by Focused Ion Beam Irradiation
H. Tomizawa, K. Suzuki, T. Yamaguchi, S. Akita, and K. Ishibashi
We report on formation of tunnel barriers in a multi-walled carbon nanotube. The nanotube was irradiated by Ga focused ion beam with a diameter of 10 nm and the local damaged region acts as a tunnel barrier at liquid helium temperature. Ion dose dependence of the resistance increase due to the irradiation was investigated and correlation between the room temperature resistance and barrier height was evaluated. Single electron transport was demonstrated in two-barrier samples and the characteristics were compared between side-gate and top-gate samples.
Microwave Resonance through the Superconducting Circuit Cavity Coupled with InSb Double Quantum Dots
Rui Wang, Russell S. Deacon, Diana Car, Erik P. A. M. Bakkers, Koji Ishibashi
InSb nanowires exhibit large g-factor and strong spin-orbit interaction, making them a promising candidate to construct the hybrid QD-cavity architecture to realize the potential strong spin-coupling regime. Here we demonstrate a mechanical transfer technique to align single nanowires with micron accuracy onto prefabricated surface gates followed with one step lithography for the contact electrodes and the microwave cavity. This method ensures a high fabrication yield of hybrid device in which the nanowire interacts with the strongest microwave field. A double QD is formed in the InSb nanowire with local electric gates. The charge state is read out from the amplitude and phase response of resonator as well as the dc transport measurement. The interaction strength between charge dipole and photons are investigated by the dispersive readout measurement.
Large Unidirectional Magnetoresistance in Magnetic Topological Insulator
K. Yasuda, A. Tsukazaki, R. Yoshimi, K. S. Takahashi, M. Kawasaki, and Y. Tokura
Unidirectional magnetoresistance (UMR), odd magnetoresistance with respect to the current and magnetization directions, is a hallmark of conductor with both broken space-inversion and time-reversal symmetries, that will provide a new pathway to investigate magnetoelectric response in magnetic / nonmagnetic heterostructures. In this study, we measured the UMR in magnetic topological insulator (TI) heterostructures. The size of UMR terned out to be quite large because of the notable feature of perfect spin-momentum locking in two-dimensional surface electrons. In the presentation, we discuss the microscopic origin of UMR in TI heterostructures from the angular, magnetic field and temperature dependence.
Photon-Assisted Tunneling in Single-Molecule Transistors Induced by Terahertz Radiation Enhanced in the Sub-nm Gap Electrodes
Kenji Yoshida, Kenji Shibata, and Kazuhiko Hirakawa
We have investigated the electron transport in single C60 molecule transistors under the illumination of intense monochromatic terahertz (THz) radiation. By employing an antenna structure with a sub-nm wide gap, we concentrate the THz radiation beyond the diffraction limit and focus it onto a single molecule. The photon-assisted tunneling (PAT) in the single molecule transistors has been observed both in the weak-coupling and Kondo regimes. The THz power dependence of the PAT conductance indicates that, when the incident THz intensity is a few tens mW, the THz field induced at the molecule exceeds 100 kV/cm, which is enhanced by a factor of ~100000 from the field in the free space.
Towards the Electrical Detection of Magnetic Domain Wall Conduction in Ferromagnetic Topological Insulators
R. Yoshimi, M. Mogi, A. Tsukazaki, K. S. Takahashi, M. Kawasaki, Y. Tokura
The three-dimensional topological insulator (TI) is a novel state of matter as characterized by two-dimensional Dirac states on its surface. Quantum transport in Dirac electron systems such as half integer quantum Hall effect (QHE) and quantum anomalous Hall effect (QAHE) have recently been attracting much attention by breaking time reversal symmetry. In the QAHE, the dissipation-less edge channel appears due to the effective magnetic field in the gapped Dirac states, resulting in the quantized Hall conductivity and zero longitudinal conductivity. Because the sign of the effective magnetic field depends on the magnetization direction, there should be the one-way propagating edge states between two magnetic domains. In this poster, we report the attempt towards the electrical detection of magnetic domain wall conduction in a ferromagnetic topological insulator Crx(Bi1−ySby)2-xTe3. By etching the sample we have succeeded in changing the coercive field and intentionally creating magnetic domains. In electrical measurement, we observe the difference in longitudinal resistance across the magnetic domain wall on one side of the device and the other. This suggests that the potential difference between two edges and existence the chiral conduction along the magnetic domain wall.
Spin-Orbit Qubit on a Multiferroic Insulator in a Superconducting Resonator
P. Zhang, Z. L. Xiang and F. Nori
We propose a spin-orbit qubit in a nanowire quantum dot on the surface of a multiferroic insulator with a cycloidal spiral magnetic order. The spiral exchange field from the multiferroic insulator causes an inhomogeneous Zeeman-like interaction on the electron spin in the quantum dot, producing a spin-orbit qubit. The absence of an external magnetic field benefits the integration of such a spin-orbit qubit into high-quality superconducting resonators. By exploiting the Rashba spin-orbit coupling in the quantum dot via a gate voltage, one can obtain an effective spin-photon coupling with an efficient on-off switching. This makes the proposed device controllable and promising for hybrid quantum circuits.
Templated Growth of Diamond Optical Resonators via Plasma-Enhanced Chemical Vapor Deposition
Xingyu Zhang, Evelyn L. Hu
Color centers in diamond, including the nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers, have diverse applications for nanoscale sensing and quantum information science. Diamond photonic structures assist the manipulation and optical readout of color centers’ electron spin state by increasing collection efficiency and emission rate using the Purcell effect. Such micro- and nanostructures have typically been created using traditional “top-down” fabrication techniques including reactive ion etching. However, it is useful also to explore “bottom-up” approaches in which three-dimensional photonic nanostructures are formed directly through templated growth of diamond. This allows incorporation of color centers directly during growth, and minimizes the impact of energetic ions or electron beams that can incur damage to embedded color centers. We utilize plasma-enhanced chemical vapor deposition through a patterned silica mask for templated diamond growth to create optical resonators. Pyramid-shaped resonators have quality factors Q up to 600, measured using confocal photoluminescence (PL) spectroscopy; and mode volumes V as small as 2.5(λ/n)^3 for resonances at wavelengths λ between 550 and 650 nm and refractive index n, obtained using finite-difference time-domain simulations. PL and Raman spectroscopy indicate NV and SiV formation in the high-quality grown diamond. The resonator design and fabrication technique obviates any etching of diamond, and preserves emitter properties in a pristine host lattice.
Excited-State Charging Energies In Quantum Dots Investigated By Terahertz Photocurrent Spectroscopy
Y. Zhang, K. Shibata, N. Nagai, C. Ndebeka-Bandou, G. Bastard, and K. Hirakawa
In zero-dimensional nanostructures such as quantum dots (QDs), electrons are confined in a nanometer-scale volume and electron-electron interactions play important roles in determining their electronic and optical properties. The charging energies, which arise from the Coulomb repulsion among electrons, can be determined from the Coulomb stability diagrams when an electron is added to the manybody ground states (GSs). However, since device operation always requires excitation of electrons, the Coulomb repulsion energies for excited states (ESs) are as important as those for the GSs.
In this work, we have investigated the ES charging energies in QDs by measuring a terahertz (THz)-induced photocurrent in a single electron transistor (SET) geometry that contains a single InAs QD between metal nanogap electrodes. A photocurrent is produced in the QD-SETs through THz intersublevel transitions and the subsequent resonant tunneling. We have found that the photocurrent exhibits stepwise change even within one Coulomb blockaded region as the electrochemical potential in the QD is swept by the gate voltage. From the threshold for the photocurrent generation, we have determined the charging energies for adding an electron in the photoexcited state in the QD. Furthermore, the charging energies for the ESs with different electron configurations are clearly resolved. The present THz photocurrent measurements are essentially dynamical experiments and allow us to analyze electronic properties in off-equilibrium states in the QD.
Sagar Bhandari, Gil-Ho Lee, Anna Klales, Kenji Watanabe, Takashi Taniguchi, Eric Heller, Philip Kim, Robert M. Westervelt
Electrons in graphene can travel for several microns without scattering at low temperatures, and their motion becomes ballistic, following classical trajectories. When a magnetic field B is applied perpendicular to the plane, electrons follow cyclotron orbits. Magnetic focusing occurs when electrons injected from one narrow contact focus onto a second contact located an integer number of cyclotron diameters away. By tuning the magnetic field B and electron density n in the graphene layer, we observe magnetic focusing peaks. We use a cooled scanning gate microscope to image cyclotron trajectories in graphene at 4.2 K. The tip creates a local change in density that casts a shadow by deflecting electrons flowing nearby; an image of flow can be obtained by measuring the transmission between contacts as the tip is raster scanned across the sample [1,2]. On the first magnetic focusing peak, we image a cyclotron orbit that extends from one contact to the other. In addition, we study the geometry of orbits deflected into the second point contact by the tip [3].
Supported by Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319 and U.S. DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under grant DE-FG02-07ER46422.
References:
1. M.A. Topinka et al., Nature 410, 183(2001).
2. K. E. Aidala et al., Nature Physics 3, 464 (2007).
3. S .Bhandari et al., Nano Lett. DOI: 10.1021/acs.nanolett.5b04609 (2016).
Ground State Electroluminescence
M. Cirio, S. De Liberato, N. Lambert, F. Nori
Electroluminescence, the emission of light in the presence of an electric current, provides information on the allowed electronic transitions of a given system. Our work shows that, together with usual electroluminescence, systems in the ultrastrong light-matter coupling regime emit a uniquely quantum radiation when a flow of current is driven through them. While standard electroluminescence (SE) relies on spontaneous emission from excited states of the system, the process we describe (Ground State Electroluminescence, GSE) extracts bound photons from the dressed ground state and it has peculiar features that unequivocally distinguish it from usual electroluminescence.
Signatures of Majorana Modes in the Dynamics of HgTe Josephson Junctions
R.S. Deacon, E. Bocquillon, J. Wiedenmann, T.M. Klapwijk, H. Buhmann, S. Tarucha, L.W. Molenkamp, and K. Ishibashi
We study Josephson junctions with weak links of the HgTe Quantum spin hall insulator. The presence of a 4pi-periodic current phase relation due to the topologically protected gapless Majorana modes of the junction are revealed in measurements of the ac-Josephson effect. We present two methods of detecting this mode. First the Shapiro steps are measured in the presence of an rf-drive. We observe a doubling of the Shapiro step voltage indicating a fractional ac Josephson effect. In the second method we detect the Josephson emission from a voltage biased junction and detect a peak in the emission power at half the Josephson frequency of the junction again indicating the gapless mode.
Fractional Quantum Hall Effect in the N=2 Landau Level in Bilayer Graphene
Georgi Diankov, Chi-Te Liang, Francois Amet, Patrick Gallagher, Menyoung Lee, Andrew J. Bestwick, Kevin Tharratt, William Coniglio, Jan Jaroszynski, K. Watanabe, T. Taniguchi, and David Goldhaber-Gordon
We report transport measurements of fractional quantum Hall (FQH) states in the N=2 Landau level (LL) (filling factors 4<ν<8) in bilayer graphene. In contrast with recent observations of particle-hole asymmetry in the N=0/N=1 LLs of bilayer graphene, the FQH states we observe in the N=2 LL are consistent with the CF model: within a LL, they form a complete sequence of particle-hole symmetric states whose relative strength is dependent on their denominators. The FQH states in the N=2 LL display energy gaps of a few Kelvin, comparable to and in some cases larger than those of fractional states in the N=0/N=1 LLs.
Terahertz Spectroscopy of Single Molecules Using Sub-Nm Scale Gap Electrodes
Shaoqing Du, Kenji Yoshida, Ya Zhang, Kazuhiko Hirakawa
Terahertz (THz) spectroscopy is a powerful tool for clarifying electronic structures and vibrational dynamics of various kinds of molecules. However, it is a formidable challenge to greatly exceed the diffraction limit and perform single molecule spectroscopy, because there is a huge size difference (a factor of ~10^5) between the THz wavelength (~100 um) and the size of single molecules (< 1 nm).
In this work, we have performed terahertz (THz) spectroscopy of single molecules by using nanogap electrodes as the THz antennas. From transport measurements, we confirmed that we successfully captured a single molecule (C60 and Ce@C82) in the nanogap. When the sample was illuminated with a THz radiation from a blackbody light source, we observed a very small (less than 100 fA), but finite THz-induced photocurrent in the single molecule transistors. The Fourier spectra of the THz photocurrent exhibited equally-spaced low-energy multiple peaks and peak splitting, suggesting that the electron transport is strongly affected by molecular vibrations. This is the first demonstration of THz spectroscopy of single molecules.
Investigation of Superconductive Heavily Doped Boron Diamond for Device Fabrication
Delroy Green 1; Gary Harris1; R.D. Vispute 2;
Howard University 1) Bluewave Semiconductor Inc.2)
Diamond has a wide bandgap of 5.47 eV at room temperature and is the hardest known naturally occurring material with a Knoop hardness of 10,400 kg/mm2 or 10 on the Mohs scale. Due to the structure of the covalent bonding of its carbon atoms, diamond is extremely strong having each carbon bonded to four neighboring carbon atoms.
Although diamond is hard, its toughness, when compared to most engineering materials, is poor. However, because of its hardness, it is an efficient cutting and drilling tool. With the exception of naturally occurring blue diamonds, which are semiconductors, diamond is a good electrical insulator. However, unlike most insulators, diamond has the highest thermal conductivity of 22 W/cm-K among naturally occurring materials. Although diamond is a good electrical insulator, it also shows semiconducting properties when doped with impurities. When diamond is heavily doped with boron the resulting material possess excess electron holes and as such it is classified as a p-type material. If excess boron doping is achieved, then the resulting material is found to behave like a superconductor at very low temperatures. In this superconducting state, the doped diamond conducts electricity.
A series of boron-doped diamond films were grown by hot filament chemical vapor deposition (HFCVD) and tested to determine the optimum technique for doping diamond with boron for superconductivity. The first technique involved the insertion of boron powder (B2O3) around the sample holder to dope seeded poly and nano diamond during growth. The second technique involves doping with diborane gas (B2O6).
Various processing parameters were optimized for diamond quality, structure, morphology, and doping. A combined analysis of scanning electron microscope, Raman mapping and Hall measurements at various temperatures were conducted to ascertain the superconductive nature of the material. Preliminary results of the boron solid source doping on diamond show a superconductive transition temperature of 2.3 oKelvin at a doping concentration of 2.3 x 1020 cm-3. NSF –DMR 1231319
Green's Function of Magnetic Topological Insulator in Gradient Expansion Approach
Yusuke Hama and Naoto Nagaosa
In this research, we investigate the Keldysh Green's function of Weyl-fermion surface state of three- dimensional magnetic topological insulator in gradient expansion approach. By using it, we study various transport properties induced by the spatially and temporally slowly-varying magnetization such as electric charge, electric current and spin densities, as well as energy density and current in terms of emergent electromagnetic fields.
Edge States of Silicene, Germanene and Stanene Nanoribbons with Edge Hydrogen Terminations
Ayami Hattori, Masaaki Araidai, Yasuhiro Hatsugai, Keiji Yada, Kenji Shiraishi, Masatoshi Sato, Yukio Tanaka
We study electronic properties of monolayer zigzag silicene, germanene, and stanene nanoribbons with and without hydrogen terminations based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, not only \pi but also \sigma orbitals influence seriously on the energy spectra of edge states, in contrast to graphene nanoribbons. This orbital degree of freedom is also important for describing the hydrogen termination effects. Similar to zigzag graphene nanoribbons, various intriguing in-gap edge states appear depending on the types of edge hydrogen terminations. We propose zigzag stanene nanoribbons as the candidate of topological quantum field effect transistor from our calculations.
Crystalline Spin-Orbit Interaction and The Zeeman Splitting in Pb1−Xsnxte
Hiroshi Hayasaka and Yuki Fuseya
A ratio of the Zeeman splitting to the cyclotron energy (M =) characterizes the relative strength of the spin-orbit interaction in crystals. We calculated M for the narrow gap IV-VI semiconductors, PbTe, SnTe and their alloy Pb1-xSnxTe on the basis of the k・p theory, the group theoretical analysis and the relativistic empirical tight-binding band calculation. We clarified that M<1 for PbTe, M>1 for SnTe and found M=1 at the band inversion point. This shows that the band inversion is clearly characterized by the ratio M. By using this property, we can detect the transition to the topological crystalline insulator only from the bulk measurements of quantum oscillation. Also we show that, by using the deviation from M=1, we can evaluate how the electrons in crystal is close to the Dirac electrons.
Optical Response of Carbon Nanotube Quantum Dot
Akira Hida and Koji Ishibashi
The optical response of the quantum dot made of single-walled carbon nanotubes and collagen model peptides were investigated. In the photoluminescence excitation spectroscopy, a series of peaks corresponding to the discrete exciton emission levels were observed. When we increased the intensity of excitation laser, a single peak split into clear doublet. This behavior could be explained in terms of the Rabi splitting because the observed splitting was directly proportional to the square root of the excitation laser intensity. The anti-crossing in the dispersion relation was also observed, which supported above interpretation. A remarkable phenomenon occurred when the quantum dot was excited by picosecond laser pulse pairs. The pulse pairs strengthened or weakened the exciton emission depending on their phase difference. These results might lead us to the coherent control of excitons.
Angular Momenta of Two Valleys And Valley Coupling in Finite-Length Single-Wall Carbon Nanotubes
Wataru Izumida, Rin Okuyama, Ai Yamakage, Riichiro Saito
Recent studies of quantum transport have revealed that single-wall carbon nanotubes (SWNTs) contain richer phenomena, especially in terms of spin and valley. Fourfold degeneracy of discrete energy levels has been considered as an intrinsic property in finite-length SWNTs reflecting the two spin states and the two valley states. Measurements for ultraclean SWNTs show lift of fourfold degeneracy caused by the spin-orbit interaction. Here we focus on angular momenta of two valleys in SWNTs, to discuss the valley coupling as another mechanism of lift of degeneracy. It is shown that the two valleys have the same angular momentum in the majority (82 %) of SWNTs. The present study reveals that the coupling of two valleys occurs in the majority of finite-length SWNTs, even for ultraclean tubes with clean edges, which conserve the angular momentum of ideal bulk states of electrons.
Ballistic Electron Transport in Suspended InAs Nanowires
H. Kamata, R. S. Deacon, S. Matsuo, S. Baba, K. Li, H. Q. Xu, A. Oiwa, and S. Tarucha
InAs nanowires have a surface charge accumulation layer subject to a strong spin-orbit interaction (SOI), and therefore are expected to generate helical states under a magnetic field and furthermore Majorana Fermion bound states when contacted to a s-wave superconductor. Although ballistic transport in nanowires plays an important role in creating such exotic states, conductive electrons in the accumulation layer experience so strong surface roughness scattering and ionized impurity scattering that the transport is entirely diffusive. Here, to suppress such scatterings we employ suspended nanowire structures across a gap of approximately 100 nm, and observe a feature of ballistic electron transport.
Critical Current for the Breakdown of Quantum Anomalous Hall Effect Measured in Hall Bars with Various Sizes
Minoru Kawamura, Ryutaro Yoshimi, Kei. S. Takahashi, Atsushi Tsukazaki, Masashi Kawasaki, and Yoshinori Tokura
Quantum anomalous Hall effect (QAHE) is a novel quantum transport phenomena which has been demonstrated recently in ferromagnetic topological insulators [1, 2]. In the QAH state, the Hall resistance is quantized to h/e^2 and the longitudinal resistance becomes zero, as similar to the quantum Hall effect (QHE). Although there are many similarities between the two effects, the magnetization origin of the QAHE makes it different from the QHE qualitatively. One of the big differences is that the QAHE can be realized even at zero external magnetic field while the QHE requires high magnetic fields. Therefore the QAHE has a potential to be utilized as a new type of a quantum resistance standard without using any external magnetic field [3, 4]. For such an application, robustness of the QAHE against the measurement current is an important issue of discussion.
In this poster, we report robustness of the QAHE against the bias current. We observed the QAHE in MBE-grown Cr-doped topological insulator thin films and measured the current-voltage characteristics using Hall bars with various sizes. The Hall resistance deviated from the quantized value when the applied current exceeded a certain critical current. The critical current for the breakdown was proportional to the Hall bar width, suggesting that the Hall electric field which is responsible for the QAHE breakdown is homogeneous across the Hall bar.
[1] Cui-Zu Chang et al., Science 340, 167 (2013).
[2] J.G. Checkelsky et al., Nature Phys. 10, 731 (2014).
[3] A. J. Bestwick et al., Phys. Rev. Lett. 114, 187201 (2015).
[4] Cui-Zu Chang et al., Nature Mat. 14, 473 (2015).
Single-shot Readout of Three Two-electron Spin States in a Quantum Dot Coupled to Quantum Hall Edge States
H. Kiyama, T. Nakajima, S. Teraoka, A. Oiwa, and S. Tarucha
We demonstrate the single-shot readout of three two-electron spin states in a gate-defined GaAs single quantum dot. The three spin states are a singlet and two triplets having z-components of spin angular momentum of 0 and +1, and distinguished by detecting spin-dependent tunnel rates that arise from the spin filtering by spin-resolved edge states and the spin-orbital correlation with the orbital-dependent tunneling. We confirm the ternary spin readout by observing the spin relaxation dynamics among the three spin states.
Towards Single Layer Ferromagnetism
Dahlia R. Klein, Efrén Navarro-Moratalla, Pablo Jarillo-Herrero
Van der Waals heterostructures of layered 2D materials have been widely explored for conductors, insulators, and a whole family of semicondutors. However, there has been little development of magnetic 2D crystalline layers, which could lead to interesting new physical states when coupled with other atomically thin materials. Thus, we are attempting to expand this field through the study of an intrinsic magnetic layered material for the isolation of single layer ferromagnetism. We have optimized the growth of layered magnetic materials including the insulating ferromagnet chromium triiodide (CrI3). From the bulk crystals, we have successfully exfoliated them onto solid substrates. In light of their sensitivity to ambient moisture, we have also developed the necessary techniques to assure the integrity of the flakes during their manipulation. We are now evaluating the effect of dimensionality on the bulk ferromagnetic order using graphene-based Hall sensor devices.
Quantum Optics with Giant Artificial Atoms
Anton Frisk Kockum, Lingzhen Guo, Mikhail Pletyukhov, Arne L. Grimsmo, Sankar R. Sathyamoorthy, Alexandre Blais, Per Delsing, Franco Nori, and Goran Johansson
In quantum optics experiments with both natural and artificial atoms, the atoms are usually small enough that they can be approximated as point-like compared to the wavelength of the electromagnetic radiation they interact with. However, a recent experiment coupling a superconducting qubit to surface acoustic waves shows that a single artificial atom can be coupled to a bosonic field at several points which are wavelengths apart [1]. This situation could also be engineered with an xmon qubit coupled to a microwave transmission line [2].
Here, we present results of theoretical studies of such "giant artificial atoms" [2, 3, 4]. In the Markovian regime, where the travel time between coupling points is negligible, we find that interference effects due to the positions of the coupling points give rise to a frequency dependence for the strength of the coupling between the giant artificial atom and its surroundings [2]. The Lamb shift of the atom is also affected by the positions of the coupling points. We discuss possible applications for these frequency dependencies (which can be designed). In the non-Markovian regime, where the distance between coupling points is large, an excited giant atom exhibits revivals and non-exponential decay [3]. In this regime, we have also studied novel features that occur in the correlation function g^2(t). Finally, we also explore setups with several giant atoms coupled to a transmission line in various configurations [4].
[1] M. V. Gustafsson et al., Science 346, 207 (2014).
[2] A. F. Kockum et al., Phys. Rev. A 90, 013837 (2014).
[3] L. Guo et al., in preparation (2016).
[4] A. F. Kockum et al., in preparation (2016).
Bistable Photon Emission in Hybrid Circuit-QED
Neill Lambert, Christian Flindt, Franco Nori
We study the photon emission from a voltage-biased double quantum dot coupled to a microwave cavity [1,2], an example of so-called hybrid circuit-QED. We find that the resulting photonic statistics can exhibit a dynamic bistability, which we validated by showing that the distribution describing these statistics has the shape of a tilted ellipse. The switching rates which describe the bistability can be extracted from the electrical current and the shot noise in the quantum dots, and used to predict this elliptic form of the photonic distribution. We also explore how the state of the cavity can be manipulated and made "more coherent" with techniques from feedback control.
[1] N. Lambert, F. Nori, and C. Flindt, Physical Review Letters 115, 216803 (2015).
[2] N. Lambert, C. Flindt, and F. Nori, Euro. Phys. Lett. 103, 17005 (2013)
Fabrication of Quantum-Point Contacts in Graphene-on-Boron Nitride Heterostructures
Andrew Lin, Sagar Bhandari, Robert Westervelt
Graphene is an atomically thin hexagonal lattice of sp2-hybridized carbon atoms with unique electronic and physical properties. These properties, such as ballistic motion of electrons and Landau quantization which results in observation of the quantum Hall effect, are contingent on its mobility [1]. The mobility of electrons is limited when graphene sits on a silicon substrate due to uneven ripples and surface distributions caused by the substrate-level roughness of silicon wafers. Therefore, stacking graphene in between layers of atomically flat hexagonal boron nitride (hBN) creates a smooth substrate greatly increasing the mobility of the graphene samples [2]. We report in this poster the successful fabrication of a 100-nanometer quantum point contact (QPC) in a layered graphene-on-BN heterostructure, along with its characterization via atomic force microscopy, scanning-electron microscopy, and future characterization via electronic transport measurements and scanning-probe microscopy. From our scanning probe measurements, we hope to image the flow of electrons in graphene through a quantum point contact. In addition, we propose the imaging of coherent nature of electrons in graphene by verifying the existence of electron wave interference fringes.
[1] Nakaharai, S., J. R. Williams, and C. M. Marcus. "Gate-defined graphene quantum point contact in the quantum Hall regime." Physical review letters 107.3 (2011): 036602.
[2] Dean, C. R., Young, A.F., Meric I., Lee C., Wang L., Sorgenfrei S., Watanabe, K., Taniguchi, T., Kim, P., Shepard, K. L., & Hone, J. (2010)."Boron Nitride Substrates for High-quality Graphene Electronics." Nature Nanotechnology, 5 (10), 722-726.
Optically Detected Magnetic Resonance Of High-Density Ensemble Of NV Centers In Diamond
Yuichiro Matsuzaki, Hiroki Morishita, Takaaki Shimooka, Toshiyuki Tashima, Kosuke Kakuyanagi, Kouichi Semba, W. J. Munro, Hiroshi Yamaguchi, Norikazu Mizuochi, Shiro Saito
Optically detected magnetic resonance (ODMR) is a way to characterize the NV centers. By using our model, we have reproduced the ODMR with and without applied magnetic fields. Also, we theoretically investigate how the ODMR is affected by the typical parameters of the ensemble NV centers such as strain distributions, inhomogeneous magnetic fields, and homogeneous broadening width. Our model could provide a way to estimate these parameters from the ODMR, which would be crucial to realize diamond-based quantum information processing.
Conductance Fluctuations in High-mobility BN/graphene/BN System
M. Mineharu, C. R. da Cunha, M. Matsunaga, N. Matsumoto, Y. Ochiai, G.-H. Kim, K. Watanabe, T. Taniguchi, D. K. Ferry, and N. Aoki
We have fabricated a high-mobility graphene transistor using h-BN as the gate insulator. The mobility is close to 10 m^2/Vs and the transport regime is in the quasi ballistic regime. Nevertheless, we observed conductance fluctuations at low temperature. We will discuss its characteristics focusing on the differences between this and a conventional low mobility sample fabricated on a SiO2 layer.
Cancellation of Half-Quantized Anomalous Hall Conductivities in Top And Bottom Surface States of a Topological Insulator Heterostructure
M. Mogi, M. Kawamura, R. Yoshimi, A. Tsukazaki, Y. Kozuka, M. Kawasaki, and Y. Tokura
Three-dimensional topological insulators possess spin-momentum locked massless Dirac surface states due to topologically non-trivial insulating bulk states. The magneto-electric responses of the exotic surface states can be described as axion electrodynamics. The electrodynamics lead to presences of half-quantized Hall conductivity (e^2/2h) and novel ’topological magnetoelectric effect’ by breaking time reversal symmetry. The recently observed quantum anomalous Hall effect in magnetically doped topological insulators can be considered as a summation of half-quantized top and bottom surface states.
In this work, we grew a magnetic (Cr) modulation doped topological insulator ((Bi,Sb)2Te3) thin film with vertical asymmetry along growth direction. We observe stabilized zero Hall conductivity plateau and zero longitudinal conductivity without an external magnetic field. We have identified that the effect originates from a state where the magnetization of Cr modulation doped layers are anti-parallel, and hence the surface state has a gap in the whole of the film without edge state. In such magnetic configuration, we can assume a subtraction of Hall conductivities of top and bottom surface states, and at the same time, topological magnetoelectric effect may be directly detected.
Control of Charge States of NV Center by Nin Diamond
T. Murai, T. Makino, H. Kato, Y. Doi, Y. Suzuki, M. Hatano, S. Yamasaki, M. Shimizu, H. Morishita, M. Fujiwara, N. Mizuochi
Nitrogen-vacancy (NV) centers in diamond are the most promising candidate for various applications such as quantum information science, magnetometry, and biosensing. For these applications, controlling the charge state of the NV centers is crucial, because optical initialization and readout of the spin state of the NV centers are only possible in negatively charged one (NV−). However, upon illumination, the NV centers undergo stochastic charge-state transitions between NV− and neutral charge state of the NV center (NV0). In case of 532 nm excitation, the steady-state-population of NV− is about 70%. This charge-state interconversion occurs upon illumination at any wavelength, so the steady-state NV− population is always less than 75%–80%.
Recently, we showed Fermi level control by phosphorus doping generates 99.4 ± 0.1% NV− under 1 W and 593 nm excitation which is close to maximum absorption of NV−. Here, to realize such stabilization of the charge state of NV center in intrinsic undoped diamond film, we investigate nin diamond junction.
This work was supported by CREST and KAKENHI (No. 15H05868, 16H02088).
Artificial Two-dimensional Lattice Structures Assembled by Atom Manipulation Technique
M. Nantoh, K. Takashima, T. Yamamoto and K. Ishibashi
We have fabricated artificial two-dimensional lattice structures of Fe atoms and CO molecules on a Cu(111) surface using atom manipulation technique with a LT-STM. A triangular lattice and a square lattice of Fe show different standing waves patterns, reflecting different electronic states. On the other hand, the tunneling spectra measured within a CO triangular lattice show difference near the Fermi level depending on the measurement locations which correspond to sublattice A or sublattice B of the honeycomb of molecular graphene. This result indicates that the symmetry of the sublattice degree of freedom of this lattice structure is originally broken.
Simultaneous Feedback Control in Coupled Mechanical Resonators
Ryuichi Ohta, Hajime Okamoto, Daiki Hatanaka, and Hiroshi Yamaguchi
We demonstrate simultaneous feedback amplification and damping of several mechanical modes in hexagonally coupled mechanical resonators. Six doubly clamped beam resonators connecting each other form coupled mechanical modes which spatially distributes in several resonators. Opt-mechanical feedback loop using photothermal stress with appropriate delay times enhances or suppresses the multiple mechanical vibrations at the same time. This simultaneous feedback control will enable the manipulation of phonons in hybrid mechanical systems.
Observation Of Topological Faraday And Kerr Rotations In Quantum Anomalous Hall State By Terahertz Magneto-Optics
Ken N. Okada, Youtarou Takahashi, Masataka Mogi, Ryutaro Yoshimi, Atsushi Tsukazaki, Kei S. Takahashi, Naoki Ogawa, Masashi Kawasaki and Yoshinori Tokura
Electrodynamic responses from three-dimensional (3D) topological insulators (TIs) are characterized by the universal magnetoelectric term, i.e., the axion term, constituent of the Lagrangian formalism. The quantized magnetoelectric coupling, which is generally referred to as topological magnetoelectric (TME) effect, has been predicted to induce exotic phenomena including the universal low-energy magneto-optical effects. Here we report the experimental demonstration of the long-sought TME effect, which is exemplified by magneto-optical Faraday and Kerr rotations in the quantum anomalous Hall (QAH) states of magnetic TI surfaces by terahertz magneto-optics. The universal relation composed of the observed Faraday and Kerr rotation angles but not of any material parameters (e.g. dielectric constant and magnetic susceptibility) well exhibits the trajectory toward the fine structure constant (= 1/137) in the quantized limit. Our result will pave a way for versatile TME effects with emergent topological functions.
Prediction of Enhanced Edelstein Effect using interband Rashba Effect
K. Okamoto, A. Sawada, H. Chen, T. Yamashige and T. Koga
We will show our recent prediction of enhanced Edelstein effect based on the interband Rashba Effect in a double quantum well made of narrow gap semiconductors. We will show our preliminary experimental results as well.
Excitation Spectroscopy of Carbon Nanotube Quantum Dot by Cotunneling Transport
Rin Okuyama, Atsushi Iwasaki, and Mikio Eto
Transport through carbon nanotube quantum dot (CNT QD) in Coulomb blockade regime can be a new tool for the excitation spectroscopy because the inelastic cotunneling takes place when the bias voltage matches the excitation energy [1]. We theoretically examine the two-electron state in CNT QD and cotunneling transport. First, we elucidate the electronic state in this "multi-valley artificial atom," considering two valleys of K and K' points, spin-orbit interaction, and electron-electron interaction. Second, we calculate the cotunneling current and current fluctuation, which are in good agreement with recent experimental results [2].
[1] T. S. Jespersen et al., Nat. Phys. 7, 348 (2011).
[2] R. Fujiwara, T. Arakawa, and K. Kobayashi, private communications.
Electrical Transport in Metal-Deposited CVD-Graphene
Youiti Ootuka, Jin Aoki, Ayato Horie
We will present experimental results of the electrical transport of CVD graphene deposited with nickel and indium clusters at low temperatures. The magnetoresistance of Ni-deposited graphene is measured in order to investigate the interaction between the metal and the electrons in graphene, especially the effect onto the scattering of carriers. The magnetoresistance is well described by the weak-localization theory for graphene below about 1T, and the phase-breaking time, the inter-valley scattering time, and the transport scattering time are obtained by fitting to the theory. In the case of In-deposited graphene, the system shows behaviour similar to the granular superconducting films, i.e., the BTK-like global superconducting transition and the superconductor-to-insulator transition. They are tuned by the field-effect and the degradation of graphene by plasma processing.
Theoretical Study of Magnetoresistance in Bismuth Under Strong Magnetic Fields
Mitsuaki Owada, Yuki Fuseya
The magnetoresistance of bismuth has been studied for a long time, but its properties have not been clarified theoretically yet. We calculate the magnetoresistance of bismuth on the basis of the semi-classical theory and the quantum theory. In our calculation, the specific properties of bismuth are considered: 1) the magnetic field dependence of electron and hole carriers, 2) the effective Hamiltonian: the extended Dirac and the Smith-Baraff-Rowell models, 3) the effective mass and the g-factor tensors estimated from experiments. In the case of semi-classical theory, we found that the magnetoresistance increases linearly at strong magnetic fields (B>9T), which can explain the experimental mystery qualitatively. However, the semi-classical result doesn’t agree with experiments “quantitatively” It is much larger than the experimental magnetoresistance. On the other hand, the quantum result shows the different field dependence from the semi-classical one at strong magnetic fields (B>9T). We discuss the difference between the semi-classical and the quantum theories at strong magnetic fields.
Tight-Binding Theory of Surface Spin States on Bismuth Thin Films
Kazuo Saito, Hirokatsu Sawahata, Takashi Komine, and Tomosuke Aono
The surface spin states for bismuth thin films are investigated using an sp3 tight-binding model. The model explains most experimental observations using angle-resolved photoemission spectroscopy, including the Fermi surface, the band structure with Rashba spin splitting, and the quantum confinement in the energy band gap of the surface states.
Phonon-Modulated Graphene Nanomesh for Thermal Transport Control
Takuya Sekiguchi, Yuma Yasui, Yuki Anno, Kuniharu Takei, Seiji Akita, Takayuki Arie
Since the main heat carrier in graphene is phonon, the structural change in graphene modifies the phonon propagation, thereby changing the thermal transport properties. Here we study the graphene nanomesh structures with periodically aligned nanoscale holes for phonon modulation. The electrical and thermal properties of graphene nanomesh show that the structural modification does not alter the electrical conductivity but reduces the thermal conductivity by approximately 80% when the neck width between holes is accurately controlled. This is due to the mean free path of phonon in graphene is much longer than that of electron. From the viewpoint of thermoelectric application, where the device performance is positively correlates with electrical properties but negatively correlates with thermal properties, the way of reducing thermal conductivity we propose is useful for enhancing the performance of graphene thermoelectric devices.
Inter-Edge-Channel Scattering and Nuclear Polarization in InSb Two-Dimensional System
K.Sekine, K.Kakuta, K.Nagase, Y.Hirayama
In the quantum Hall system, the ratio of Zeeman energy to cyclotron energy can be changed by tilting the device in a magnetic field, which causes switching of the spin-resolved Landau levels for a large g-factor system such as InSb two-dimensional (2D) system. We realize an unequal population of edge channels using a gate controlled through a metal-insulator-semiconductor structure, and evaluate a scattering rate between two unequally-populated edge channels. The relaxation length is clearly modified when the tilting angle is changed, reflecting the switching of the spin in the neighboring two edge channels (i.e. spin parallel or antiparallel). The temperature dependence will be discussed in the presentation.
Local Heating In Superconducting Atomic Point-Contacts
Yukihiro Shibata and Youiti Ootuka
In atomic point-contacts, the current crowning and the electric-field concentration take place in the atomic vicinity of the junction, and easily drive the system far from equilibrium. In such situations, it is not trivial that the both systems, the electron and the lattice, have the same temperature.
We made an aluminum mechanical-break junction (MBJ) device that had small thermometers very close to the junction, and successfully measured the lattice temperature change as a function of biasing voltage and current of MBJ at low temperatures. We also found an evidence of super-to normal-conducting transition in the I-V curve of Al-MBJ, that indicates the electron temperature exceeds Tc. We will present the experimental data and discuss the energy dissipation and the heat flow in the device.
Efficient Control over Wavefunctions of Electrons in Self-Assembled InAs Quantum Dots using Side-Gating
R. Shikishima, H. Kiyama, S. Baba, T. Hirayama, N. Nagai, K. Hirakawa, S. Tarucha, and A. Oiwa
InAs self-assembled islands work as quantum dots (QDs) without complicated gate tunings and show a lot of fascinating properties. One of the expected applications of InAs QDs is spin-based information processing. In InAs QDs, all-electrical fast manipulation of single electron spin with Rabi frequency as high as 60 MHz has been reported due to the strong spin-orbit interaction (SOI). Moreover, SOI in self-assembled InAs QDs can be tuned by applying an electric field to alter the QD confinement. Much stronger SOI and thus much faster manipulation may be realized by the SOI tuning with side-gating. However, the efficiency of the side-gate tuning is typically about 10 times smaller than that of the back-gate tuning mainly due to the screening effect by source and drain electrodes directly contacted to the QD. This prevents efficient electrical control of SOI in self-assembled QDs.
In this work, we show the improvement of the side-gating efficiency by fabricating narrow source and drain electrodes, which considerably suppress the screening effect. In the devices with a source-drain electrode width of 40 nm, leverarms of side-gates are improved to be more than 2 times large compared with those of conventional width of 200 nm. Using the improved efficiency of side-gate control, we observe a peak-like feature in the side-gate dependence of tunnel couplings evaluated from Coulomb peaks. This can be interpreted in terms of the spatial shift of electron wavefunction in the InAs QD across the narrow source-drain electrode. This result indicates that efficient control using side-gates enables us to largely modulate the QD confinement, which is needed for a large control of SOI.
Fast Phase-Control of the Single Nuclear Spin by Electric Field Effects
T. Shimo-Oka, Y. Tokura, Y. Suzuki, N. Mizuochi
We propose a fast phase-gating of a single nuclear spin, which interacts with the single electronic spin of the nitrogen-vacancy center in diamond. Our gate operation is to be called the geometric quantum gate; a geometric phase shift of the electron spin induced by a rotating electric field and magnetic field, is utilized to control the nuclear-spin phase. We estimate that the phase-gate time is orders shorter than hitherto reported. This work was supported by CREST and KAKENHI (No. 15H05868, 25220601).
Theory of Quantum Information Hardware
Peter Stano, Chen Hsuan-Hsu, Guang Yang, and Daniel Loss
I will present our recent achievements in the quantum theory of condensed matter with a focus on spin and phase coherent phenomena in semiconducting and magnetic nanostructures. In particular, the team investigates the fundamental principles of quantum information processing in the solid state with a focus on spin qubits in quantum dots, superconducting qubits, and topological quantum states such as Majorana fermions and parafermions. This involves the study of decoherence in many-body systems and scalable quantum computing technologies based on surface codes and long-distance entanglement schemes. We also study nuclear spin phases, many-body effects in low-dimensional systems, quantum Hall effect, topological matter, spin orbit interaction, and quantum transport of magnetization.
Theory for Diabatic Mechanisms of Higher-Order Harmonic Generation in Semiconductors
T. Tamaya, A. Ishikawa, T. Ogawa, and K. Tanaka
We theoretically investigated higher-order harmonic generation (HHG) in semiconductors under a high-intensity ac electric field. Consequently, we discovered that the diabatic processes, namely, ac Zener tunneling and semimetallization of semiconductors, are key factors for nonperturbative mechanisms of HHG. These mechanisms are classified by the field intensity and could be understood by an extended simple man model based on an analogy between tunnel ionization in gaseous media and Zener tunneling in semiconductors.
Micrometer-Scale Electron Paramagnetic Resonance Spectroscopy Using a Superconducting Flux Qubit
Hiraku Toida, Yuichiro Matsuzaki, Kosuke Kakuyanagi, Xiaobo Zhu, William J. Munro, Kae Nemoto, Hiroshi Yamaguchi, and Shiro Saito
Electron paramagnetic resonance (EPR) spectroscopy using superconducting devices is extensively investigated to achieve a sensitivity of a single spin. Flux qubits, which consist of micrometer-scale superconducting loop structure, can interact with spin ensembles strongly due to their large circulating current. We use this feature to realize a highly sensitive magnetization detector and EPR spectroscopy. We prepare a flux qubit directly coupled to an electron spin ensemble (Er:YSO) to perform experiments. First, we measure magnetization of the spin ensemble as a function of an in-plane magnetic field and temperature. Magnetization of the spin ensemble increases lineally as a function of Zeeman and thermal energy ratio, which is consistent with Curie-Weiss's law. We also perform EPR spectroscopy by applying a radio frequency excitation signal with an on-chip microstrip. Resonance peaks appear and positon of the peaks increases linearly as increasing the in-plane magnetic field. We derive g-factors of the spin ensemble to be ~6.0 and ~15.9, which is consistent with literatures. Sensitivity and the sensing volume of this method are estimated to be less than 1000 spins per unit time and ~0.05 pL, respectively.
Formation of Tunnel Barriers in a Multi-Walled Carbon Nanotube by Focused Ion Beam Irradiation
H. Tomizawa, K. Suzuki, T. Yamaguchi, S. Akita, and K. Ishibashi
We report on formation of tunnel barriers in a multi-walled carbon nanotube. The nanotube was irradiated by Ga focused ion beam with a diameter of 10 nm and the local damaged region acts as a tunnel barrier at liquid helium temperature. Ion dose dependence of the resistance increase due to the irradiation was investigated and correlation between the room temperature resistance and barrier height was evaluated. Single electron transport was demonstrated in two-barrier samples and the characteristics were compared between side-gate and top-gate samples.
Microwave Resonance through the Superconducting Circuit Cavity Coupled with InSb Double Quantum Dots
Rui Wang, Russell S. Deacon, Diana Car, Erik P. A. M. Bakkers, Koji Ishibashi
InSb nanowires exhibit large g-factor and strong spin-orbit interaction, making them a promising candidate to construct the hybrid QD-cavity architecture to realize the potential strong spin-coupling regime. Here we demonstrate a mechanical transfer technique to align single nanowires with micron accuracy onto prefabricated surface gates followed with one step lithography for the contact electrodes and the microwave cavity. This method ensures a high fabrication yield of hybrid device in which the nanowire interacts with the strongest microwave field. A double QD is formed in the InSb nanowire with local electric gates. The charge state is read out from the amplitude and phase response of resonator as well as the dc transport measurement. The interaction strength between charge dipole and photons are investigated by the dispersive readout measurement.
Large Unidirectional Magnetoresistance in Magnetic Topological Insulator
K. Yasuda, A. Tsukazaki, R. Yoshimi, K. S. Takahashi, M. Kawasaki, and Y. Tokura
Unidirectional magnetoresistance (UMR), odd magnetoresistance with respect to the current and magnetization directions, is a hallmark of conductor with both broken space-inversion and time-reversal symmetries, that will provide a new pathway to investigate magnetoelectric response in magnetic / nonmagnetic heterostructures. In this study, we measured the UMR in magnetic topological insulator (TI) heterostructures. The size of UMR terned out to be quite large because of the notable feature of perfect spin-momentum locking in two-dimensional surface electrons. In the presentation, we discuss the microscopic origin of UMR in TI heterostructures from the angular, magnetic field and temperature dependence.
Photon-Assisted Tunneling in Single-Molecule Transistors Induced by Terahertz Radiation Enhanced in the Sub-nm Gap Electrodes
Kenji Yoshida, Kenji Shibata, and Kazuhiko Hirakawa
We have investigated the electron transport in single C60 molecule transistors under the illumination of intense monochromatic terahertz (THz) radiation. By employing an antenna structure with a sub-nm wide gap, we concentrate the THz radiation beyond the diffraction limit and focus it onto a single molecule. The photon-assisted tunneling (PAT) in the single molecule transistors has been observed both in the weak-coupling and Kondo regimes. The THz power dependence of the PAT conductance indicates that, when the incident THz intensity is a few tens mW, the THz field induced at the molecule exceeds 100 kV/cm, which is enhanced by a factor of ~100000 from the field in the free space.
Towards the Electrical Detection of Magnetic Domain Wall Conduction in Ferromagnetic Topological Insulators
R. Yoshimi, M. Mogi, A. Tsukazaki, K. S. Takahashi, M. Kawasaki, Y. Tokura
The three-dimensional topological insulator (TI) is a novel state of matter as characterized by two-dimensional Dirac states on its surface. Quantum transport in Dirac electron systems such as half integer quantum Hall effect (QHE) and quantum anomalous Hall effect (QAHE) have recently been attracting much attention by breaking time reversal symmetry. In the QAHE, the dissipation-less edge channel appears due to the effective magnetic field in the gapped Dirac states, resulting in the quantized Hall conductivity and zero longitudinal conductivity. Because the sign of the effective magnetic field depends on the magnetization direction, there should be the one-way propagating edge states between two magnetic domains. In this poster, we report the attempt towards the electrical detection of magnetic domain wall conduction in a ferromagnetic topological insulator Crx(Bi1−ySby)2-xTe3. By etching the sample we have succeeded in changing the coercive field and intentionally creating magnetic domains. In electrical measurement, we observe the difference in longitudinal resistance across the magnetic domain wall on one side of the device and the other. This suggests that the potential difference between two edges and existence the chiral conduction along the magnetic domain wall.
Spin-Orbit Qubit on a Multiferroic Insulator in a Superconducting Resonator
P. Zhang, Z. L. Xiang and F. Nori
We propose a spin-orbit qubit in a nanowire quantum dot on the surface of a multiferroic insulator with a cycloidal spiral magnetic order. The spiral exchange field from the multiferroic insulator causes an inhomogeneous Zeeman-like interaction on the electron spin in the quantum dot, producing a spin-orbit qubit. The absence of an external magnetic field benefits the integration of such a spin-orbit qubit into high-quality superconducting resonators. By exploiting the Rashba spin-orbit coupling in the quantum dot via a gate voltage, one can obtain an effective spin-photon coupling with an efficient on-off switching. This makes the proposed device controllable and promising for hybrid quantum circuits.
Templated Growth of Diamond Optical Resonators via Plasma-Enhanced Chemical Vapor Deposition
Xingyu Zhang, Evelyn L. Hu
Color centers in diamond, including the nitrogen-vacancy (NV) and silicon-vacancy (SiV) centers, have diverse applications for nanoscale sensing and quantum information science. Diamond photonic structures assist the manipulation and optical readout of color centers’ electron spin state by increasing collection efficiency and emission rate using the Purcell effect. Such micro- and nanostructures have typically been created using traditional “top-down” fabrication techniques including reactive ion etching. However, it is useful also to explore “bottom-up” approaches in which three-dimensional photonic nanostructures are formed directly through templated growth of diamond. This allows incorporation of color centers directly during growth, and minimizes the impact of energetic ions or electron beams that can incur damage to embedded color centers. We utilize plasma-enhanced chemical vapor deposition through a patterned silica mask for templated diamond growth to create optical resonators. Pyramid-shaped resonators have quality factors Q up to 600, measured using confocal photoluminescence (PL) spectroscopy; and mode volumes V as small as 2.5(λ/n)^3 for resonances at wavelengths λ between 550 and 650 nm and refractive index n, obtained using finite-difference time-domain simulations. PL and Raman spectroscopy indicate NV and SiV formation in the high-quality grown diamond. The resonator design and fabrication technique obviates any etching of diamond, and preserves emitter properties in a pristine host lattice.
Excited-State Charging Energies In Quantum Dots Investigated By Terahertz Photocurrent Spectroscopy
Y. Zhang, K. Shibata, N. Nagai, C. Ndebeka-Bandou, G. Bastard, and K. Hirakawa
In zero-dimensional nanostructures such as quantum dots (QDs), electrons are confined in a nanometer-scale volume and electron-electron interactions play important roles in determining their electronic and optical properties. The charging energies, which arise from the Coulomb repulsion among electrons, can be determined from the Coulomb stability diagrams when an electron is added to the manybody ground states (GSs). However, since device operation always requires excitation of electrons, the Coulomb repulsion energies for excited states (ESs) are as important as those for the GSs.
In this work, we have investigated the ES charging energies in QDs by measuring a terahertz (THz)-induced photocurrent in a single electron transistor (SET) geometry that contains a single InAs QD between metal nanogap electrodes. A photocurrent is produced in the QD-SETs through THz intersublevel transitions and the subsequent resonant tunneling. We have found that the photocurrent exhibits stepwise change even within one Coulomb blockaded region as the electrochemical potential in the QD is swept by the gate voltage. From the threshold for the photocurrent generation, we have determined the charging energies for adding an electron in the photoexcited state in the QD. Furthermore, the charging energies for the ESs with different electron configurations are clearly resolved. The present THz photocurrent measurements are essentially dynamical experiments and allow us to analyze electronic properties in off-equilibrium states in the QD.
