FQMD Poster Abstracts
Sensitive Radio-frequency Reflectometry
Natalia Ares, Felix Schupp, Tian Pei, Aquila Mavalankar, Matthias Mergenthaler, Gregory Rogers, Jonathan Griffiths, Geb Jones, Ian Farrer, David Ritchie, Charles Smith, Jamie Warner, Audrey Cottet, Andrew Briggs, and Edward Laird
Gate-defined quantum dots are promising for spin qubits, but this requires fast and sensitive measurements, which are hindered by poor impedance matching to the device. We experimentally show how to achieve controllable perfect matching with a high device impedance; a gate-defined GaAs quantum dot. Voltage-controlled capacitors allow in situ tuning of the matching condition, even accounting for parasitics, and an absolute calibration of the capacitance sensitivity. We benchmark our results against the requirements for single-shot qubit readout using quantum capacitance. We measure the complex impedance of a Coulomb-blockaded quantum dot, finding that the capacitance changes in proportion to the conductance, in agreement with a quasistatic model of electron tunneling.
This reflectometry circuit also allow us to probe the motion of a suspended carbon nanotube. By using a gate voltage to tune the carbon nanotube into resonance with the radio-frequency resonator at 300 MHz, the mechanical signal is transduced efficiently to an electrical signal. We evaluate the suitability of this readout scheme for monitoring mechanical motion near the ground state.
Coupling Light to Circuit QED via Mechanical Transducers
G. Arnold, M. Wulf, A. Rueda, M. Peruzzo, S. Barzanjeh, O. Painter, and J. M. Fink
Superconducting circuits are promising candidates for future quantum processors. However, when it comes to long distance communication, small energy microwave qubits show significant drawbacks compared to optical photons, since optical fibers offer ultra-low-loss transmission and unmatched resilience to thermal noise and environmental interference. High efficiency conversion between microwaves and optical wavelengths would therefore represent a key feature to establish a long distance quantum network of superconducting processors. First experiments have already shown the basic possibility of microwave-optical conversion using mirror-membrane-systems [1,2] and photonic crystals  reaching conversion efficiencies as high as 10% at temperatures as low as 4 K. Our work aims at the development of an integrated on-chip transducer that couples microwaves to optical photons with efficiencies exceeding 50%. We use a mechanical transducer between both photonic regimes by combining photonic crystals and a capacitive electromechanical modulator on one SOI platform fully compatible with superconducting qubits. Exploiting the mechanism of parametric up-conversion one can couple both microwave and optical photons to the same mechanical mode. This would allow the full transfer of quantum information from microwave to optical frequencies and vice versa without a collapse of the quantum state under ideal conditions. In order to accomplish conversion efficiencies close to unity and lossless operation, the challenge is to (1) achieve sufficient electromechanical coupling at comparably high mechanical frequencies exceeding the optical linewidth and (2) maximize the optomechanical coupling to minimize any heating due to optical absorption. We have designed and optimized various transducer geometries using detailed finite element simulations. The resulting geometries are currently fabricated in our clean room as well as characterized regarding their cavity characteristics. Consequently, we will present our progress in the design, fabrication, and characterization of first prototypes, which may find application not only in quantum communication but also for ultra-sensitive RF detection.
 Bagci, T. et al. Optical detection of radio waves through a nanomechanical transducer. Nature 507, 815 (2014).
 Andrews, R. W. et al. Bidirectional and efficient conversion between microwave and optical light. Nat. Phys. 10, 321-326 (2014).
 Bochmann, J., Vainsencher, A., Awschalom, D. D. & Cleland, A. N. Nanomechanical coupling between microwave and optical photons. Nat. Phys. 9, 712-716 (2013).
Lithium ion intercalation at Singular Van der Waals Heterostructures
D. Kwabena Bediako, Mehdi Rezaee, Frank Zhao, Takashi Taniguchi, Kenji Watanabe, Tina L. Brower-Thomas and Philip Kim
The relatively weak coupling between the layers of Van der Waals(VdW) materials makes it possible to use these materials as hosts for the inclusion of guest species like atoms, ions, or molecules. This intercalation process can lead to profound alterations in the physical and chemical properties of the host, in effect resulting in the creation of a new material. While such reactions have been known for decades and successfully leveraged in technologically crucial systems like rechargeable batteries, the recent discovery of methods to isolate and reassemble individual atomic layers of VdW materials has opened an exciting new avenue for the creation and study of complex but modular materials for energy storage and electronic applications. We demonstrate the deterministic assembly of atomically thin layers of transition metal dichalcogenides, hexagonal boron nitride, and graphene, thereby creating well-defined and atomically precise interfaces that can act as host sites for intercalated Li ions. We have developed an approach to interrogate electro-intercalation reactions at the level of these singular artificial VdW heterostructures, excluding any contributions from inhomogeneity in particle size, inter-particle charge transfer, or accompanying side reactions. In situ spectroelectrochemical studies (Raman and photoluminescence spectroscopies) as well as low temperature charge transport and magnetoresistance measurements are leveraged to reveal the mechanism of intercalation at these atomic layers and the precise degree of charge transfer to individual heterolayers in a complex architecture. We achieve ultrahigh carrier densities in graphene by intercalating tailored vdW interfaces, and also demonstrate strong spatial control over the electron density in graphene through intercalation of longitudinally varying heterostructures.
Anomalous Nonlocal Resistance and Spin-Charge Conversion Mechanisms in 2D
Huang Chunli, Yidong Chong, Miguel Cazalilla
We uncover two anomalous features in the nonlocal transport behavior of two-dimensional metallic materials with spin-orbit coupling. Firstly, the nonlocal resistance can have negative values and oscillate with distance, even in the absence of a magnetic field. Secondly, the oscillations of the nonlocal resistance under an applied in-plane magnetic field (Hanle effect) can be asymmetric under field reversal. Both features are produced by direct magnetoelectric coupling, which is possible in materials with broken inversion symmetry but was not included in previous spin diffusion theories of nonlocal transport. These effects can be used to identify the relative contributions of different spin-charge conversion mechanisms. They should be observable in adatom-functionalized graphene, and may provide the reason for discrepancies in recent nonlocal transport experiments on graphene.
Capacitance Measurements of van der Waals Heterostructures
Ahmet Demir, Spencer Tomarken, Yuan Cao, Jason Yuanhong Luo, Ray Ashoori, Pablo Jarillo-Herrero
We report capacitance measurements on a dual-gated twisted bilayer graphene (TBLG) heterostructure as well as tunneling-capacitance measurements on a bilayer graphene based tunneling heterostructure. The high mobility, small twist-angle TBLG devices shows clear evidence of insulating states at the superlattice band edges. Additionally, insulating states appear whenever the electron density is tuned to an integer number of electrons per superlattice site -possibly as a result of interaction induced gap opening. We also report initial tunneling capacitance measurements of a vertical bilayer graphene and thin hexagonal boron nitride structure. We see clear evidence of symmetry breaking in the zeroth Landau level despite a strong screening source roughly one nanometer away. The tunneling-capacitance measurements support the feasibility of performing time domain tunneling measurements on vertical van der Waals heterostructures to extract excitation spectra (currently underway).
Modeling the Twisted Geometry and the Strain Effects with Layer Materials
Shiang Fang, Stephen Carr, Bertrand I. Halperin, Efthimios Kaxiras
The two-dimensional layered materials are very interesting classes of materials, which can host a variety of physical properties such as topological phases, charge density waves, superconductivity, and the magnetism. To overcome the theoretical challenges in simulating the applications with these materials, we employ the Wannier transformation technique to bridge the ab initio density functional theory calculations and the efficient tight-binding models. Based on these models, we study the electronic coupling between layers in the twisted geometry and the effects of strain in each layer.
Engineering Topological Electronic States in Graphene Nanostructures
Oliver Gröning, Shiyong Wang, Pascal Ruffieux and Roman Fasel
On-surface synthesis has proven to be a versatile tool to produce a large variety of graphene based nanostructures with atomic precision using a fundamentally bottom-up approach. This has enabled the engineering of electronic properties by the synthesis of graphene nanoribbons with a well-defined atomic structure. A key design parameter in this respect is the tuning of the band gap by precisely controlling the width of arm-chair graphene nanoribbons (AGNR). We will discuss how specific structural edge modifications can be used as a further tool to engineer the electronic properties. In this respect we are using the localized states, present at short zig-zag segments on the edges of AGNRs, to produce realizations of the Su-Schieffer-Heeger model. By modifying the spacing of these localized states along and across the AGNR it is possible to change the coupling parameters between these states in such ways to produce topological distinct electronic phases.
Layer-Dependent Ferromagnetism in a van der Waals Crystal down to the Monolayer Limit
Dahlia R. Klein, Efren Navarro-Moratalla, Bevin Huang, Genevieve Clark, Xiaodong Xu, Pablo Jarillo-Herrero
Since the discovery of graphene just over a decade ago, the field of 2D materials has expanded to include a broad range of materials including conductors, insulators, semiconductors, and superconductors. Surprisingly, no intrinsic magnets made it into the 2D materials family until just very recently. Our group along with collaborators at the University of Washington have, for the first time, experimentally demonstrated long-range ferromagnetic order in an atomically-thin crystal, chromium triiodide (CrI3), down to the monolayer limit. This is the first ferromagnet in the family of 2D materials. In the same way in which graphene permitted studying the transport of electrons in a true 2D lattice, monolayer CrI3 opens the door for the study of magnetism in the true 2D limit. Our results pave the way for further studies of interfacing 2D CrI3 with other atomically-thin materials in van der Waals heterostructures, which could revolutionize technologies in magnetoelectronics, information, and spin-based data storage.
PDOS of Metal Clusters: Sodium Versus Silver
Peter Koval, Mathias Per Ljungberg, Marc Barbry, Daniel Sanchez-Portal
Simple metal clusters respond to an external perturbation rather differently to noble metal clusters. For instance, there is an anomalous red-shifting of the plasmonic frequency for smaller simple metal clusters, which is opposite for clusters of all the other materials including noble metals and semi-conductors. Models based on the classical electrodynamics and on the time-dependent density functional theory (TDDFT) with simple semilocal functionals are capable of reproducing this behaviour. However, the microscopic picture provided by these two approaches is dramatically different. While the former leads to an appealing picture of the induced-charge clumping outside (simple metals) or inside (noble metals) the surface, the latter involves a competition between the quantum-size effect and the interaction screening to explain the experimentally observed trends, which results in the induction of charge in the whole volume of the noble-metal clusters rather than only in the vicinity of the surface. However, the absence of Fock exchange in semilocal functionals renders the electronic structure rather "metallic" even for very small clusters containing just a dozen of atoms. This situation can potentially lead to an inaccurate microscopic picture generated now by TDDFT. In contrast, Hedin's GW approximation is built upon the Fock exchange operators and provides a far more accurate quasiparticle spectrum -- a prerequisite for an accurate optical spectrum when solving a Bethe-Salpeter equation. In this contribution, we investigate the density of states of small sodium and silver clusters to illustrate this concern.
Ultrafast Time- and Angle-resolved Photoemission Spectroscopy
with 11 eV Photons
Changmin Lee, Timm Rohwer, Edbert Sie, Hiroshi Eisaki, and Nuh Gedik
Time- and angle-resolved photoemission (tr-ARPES) is a powerful technique that measures the band structure of various condensed matter systems. In a tr-ARPES setup, one laser pulse is used to pump the system to an excited state, and another ultraviolet pulse is used to probe the photoemitted electrons at different time delays after the arrival of the pump pulse. In the past, most of the tr-ARPES experiments have been carried out with 6 eV photons, limited to a small measurement range of the momentum space. In other approaches, 20 -30 eV photons through a high-harmonic generation process were used with a worse energy resolution (> 70 meV). Recently we have succeeded in achieving record time and energy resolutions of 250 fs and 16 meV, respectively, using 11 eV laser pulses. Extreme ultraviolet pulses were obtained through third harmonic generation of 346 nm laser pulses in Xe gas. Our accomplishments will allow unprecedented study of transient dynamics of electronic structure in various condensed matter systems that have interesting phenomena far way from the center of the Brillouin zone. Examples include cuprate and iron-based superconductors and topological crystalline insulators.
Probing the Spin Dynamics of Ferromagnetic 2D Atomic Crystals
T.D. Rhone, H. Idzuchi, S. Harvey, T. Zhou, C. Du, J. Chee, R. Walsworth, D. Ham, P. Kim, A. Yacoby
Layered 2D atomic crystals of transition metal chalcogenides (TMC) are on the forefront of research on magnetism in reduced dimensions. Monolayer magnetic 2D atomic crystals offer new venues for applications in spintronics as well as an unprecedented means to probe the microscopic origins of magnetism. Theoretical studies of monolayer TMCs have recently shown that CrGeTe3 exhibits ferromagnetic order while CrSiTe3 exhibits antiferromagnetic order. Whereas bulk (multilayered) CrGeTe3 and CrSiTe3 are both known to exhibit ferromagnetic behavior, distinct magnetic order arises in reduced dimensions. Ferromagnetic order persists for monolayer CrGeTe3, while a transition to antiferromagnetic order is predicted for monolayer CrSiTe3. Here we report studies of the spin properties of thin films (multiple layers) of CrGeTe3 and CrSiTe3 using conventional ferromagnetic resonance (FMR) spectroscopy. FMR is a powerful tool for probing magnetic properties such as saturation magnetization and spin-precession damping. Since conventional FMR lacks sufficient sensitivity to probe a single atomic layer, future studies of monolayer magnetic TMCs require more sensitive probes of magnetic properties. Magnetic resonance studies based on Nitrogen Vacancy centers in diamond may provide a sufficiently sensitive probe of the magnetic properties of monolayer 2D atomic crystals with magnetic order paving the way for studies of spin dynamics in reduced dimensions.
Fabrication of Bipolar Dopant Devices with Hydrogen Resist Lithography
Tomas Skeren, Sigrun Koester, Nikola Pascher, Andreas Fuhrer
Hydrogen resist lithography is a technique capable of preparing atomic scale dopant devices. It is enabled by large difference in chemical reactivity of a bare and hydrogen passivated Si(001):2x1 surface. Starting with a hydrogen passivated Si surface, the hydrogen layer can be locally desorbed by Scanning tunneling microscope (STM) with nanometer precision leaving behind exposed areas of reactive, bare Si. When exposed to suitable gas, the hydrogen layer acts as a resist and the gas sticks only to the desorbed areas, opening the possibility for localized doping. Compared to conventional fabrication methods, hydrogen resist lithography enables doping with nanometer resolution and extremely abrupt doping profiles.
It is relatively established to create N-type dopant devices by using phosphine gas. The interaction of the phosphine with silicon surface is well studied and understood. In order to create P-type devices one needs to be able to incorporate acceptor atoms into the silicon matrix. One choice for the source of P dopants is diborane gas, however, its reaction with Si surface is substantially slower and electrical activation of boron atoms requires higher temperatures. Nevertheless, we have recently developed a method to dope Si with diborane gas and demonstrated the possibility to create P-type devices.
In this work we present for the first time the fabrication of a bipolar dopant device (a PN junction) created with hydrogen resist lithography by sequentially doping with both phosphine and diborane. Our preliminary electrical measurements confirm that the PN junction is electrically active and we observed resonant inter-band tunneling similar to the behavior of Esaki diode.
Prediction of Intrinsic Spin Hall Effect in the TaAs-family of Weyl Semimetals
Yan Sun, Yang Zhang, Claudia Felser, and Binghai Yan
Since their birth topological insulators have been expected to be ideal spintronic materials due to the spin currents carried by the surface states with spin-momentum locking . However, the bulk doping problem still remains to be an obstacle that hinders such application. In this work, we predict that a newly discovered family of topological materials, the Weyl semimetals, exhibits large intrinsic spin Hall effects that can be utilized to generate and detect spin currents. Our ab-initio calculations reveal large spin Hall conductivity that is comparable to that of 4d and 5d transition metals. The spin Hall effect originates intrinsically from the bulk band structure of Weyl semimetals that exhibits large Berry curvature and spin-orbit coupling, naturally avoiding the bulk carrier problem in the topological insulators. Our work not only paves a way to employ Weyl semimetals in spintronics, but also proposes a new guideline to search for spin Hall effect materials in various topological materials.
Stability of Chaos in a Generalized Sachdev-Ye-Kitaev Model
The Sachdev-Ye-Kitaev (SYK) model, N Majorana fermions in 0+1 dimensions with infinite-range interactions, is attracting a lot of interest as a toy model for quantum holography. Here we show that a generalized SYK model with an additional one-body infinite-range random interaction, which is a relevant perturbation in the infrared, is still quantum chaotic and retains most of its holographic features in the limit of weak perturbations. However, results from level statistics and out-of-time correlation functions strongly suggests the existence of a chaotic-integrable transition for sufficiently strong perturbations.
This work has been done in collaboration with Antonio M. Garcia-Garcia and Aurelio Romero Bermadez.
Imaging of Incompressible Strips in Quantum Hall System
by Scanning Gate Microscopy
Toru Tomimatsu, Katsushi Hashimoto, Shunsuke Taninaka, Ken Sato, and Yoshiro Hirayama
Edge channel transport plays an essential role in the electronic properties of quantum Hall systems (QHS). Scanning gate microscopy (SGM) on QHS enables local investigation of the edge transport by inducing selective backscattering with a local potential perturbation by using the scanning tip in the electron gas. Recent SGM studies for resolving edge channels highly rely on the narrow constriction system, such as a quantum point contact or a narrow wire, to enhance the sensitivity of resistance signals.
Here, by incorporating non-equilibrium experiments to SGM, we achieved the high sensitive visualization of incompressible strips, where local inelastic scattering between edge and bulk predominantly occurs, on QHS without any aid of the narrow constriction. We performed SGM on a standard Hall bar and imposed relatively higher current to induce the non-equilibrium potential gradient in the vicinity of edge, which allows sensitive detection of scattering events. The SGM images of longitudinal voltage showed the voltage changes (enhanced peak or suppressed valley), which was distributed along current direction. The position of this SGM signal was insensitive to current in the lower current region, whereas it varied with filling factor nu. We found good agreement with nu dependence of positions of the SGM signal and that of incompressible strip calculated by the analytical model. The systematic evaluation of SGM images at wide region of filling factor were performed with a discussion including the effect of screening on the SGM images.