CIQM Micro-presentation Abstracts
Annual Meeting
October 13-14, 2016
Graduate Student and Postdoctoral Fellow
Micro-Presentation and Poster Abstracts
Group 1: Engineering New Topological Crystals
1. Cui-Zu Chang
Postdoctoral Fellow, Massachusetts Institute of Technology
Observation of the Quantum-Anomalous-Hall Insulator to Anderson Insulator: Quantum Phase Transition in Magnetic Topological Insulators
Cui-Zu Chang, Jagadeesh. S. Moodera
The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without external magnetic field, which can be realized by time reversal symmetry breaking in a topologically non-trivial system [1,2], and in thin films of magnetically-doped TI [3]. A QAH system carries spin-polarized dissipationless chiral edge transport channels without the need for external energy input, hence may have huge impact on future electronic and spintronic device applications for ultralow-power consumption. The observation of QAH effect has opened up exciting new physics and thus understanding the physical nature of this novel topological quantum state, can lead to a rapid development of this field. In this talk, we will report our recent progress about the experimental observation of a quantum phase transition from a quantum-anomalous-Hall (QAH) insulator to an Anderson insulator by tuning the chemical potential, and finally discuss the existence of scaling behavior for this quantum phase transition [4].
References
[1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988).
[2] R. Yu et al, Science 329, 61-64 (2010).
[3] C. Z. Chang et al, Science 340, 167(2013); Nature Materials 14, 473(2015); Physics Review Letters 115, 057206 (2015).
[4] C. Z. Chang et al, Phys. Rev. Lett. 117, 126802 (2016).
Work done in collaboration with W. Zhao, J. K. Jain, C. Liu, and M. H. W. Chan from Penn State U. and J. Li from Princeton U.
2. Aravind Devarakonda
Graduate Student, Massachusetts Institute of Technology
Synthesis of Topological Semimetals
Aravind Devarakonda and Prof. Joseph Checkelsky
Building on the experimental confirmation of 2D and 3D topological insulators, the condensed matter physics community has begun searching for topological phases that are protected by symmetries other than time reversal [1-3]. Recently it has been predicted that a topological semimetal phase with a line-like bulk Fermi surface and topologically protected surface states can be achieved in BaNbS3 and related materials [3]. The topological character of this phase is protected by a non-symmorphic symmetry element, a screw rotation along the c-axis of the crystal. Through single-crystal synthesis and electronic transport measurements, we aim to uncover the properties of the line-like bulk Fermi surface and the possible role of the topological surface modes.
[1] Fu, L., Phys. Rev. Lett., 106, 106802 (2011)
[2] Wan, X. et al., Phys. Rev. B, 83, 205101 (2011)
[3] Liang, Q. et al., Phys. Rev. B, 93, 085427 (2016)
3. Shiang Fang
Graduate Student, Harvard University
Topological Antimony Surface Relaxation and the Surface States
Shiang Fang,Yau Chuen (Oliver) Yam, Jennifer Hoffman, Bertrand Halperin, Efthimios Kaxiras
The antimony (111) surface with normal bilayer termination can host topological nontrivial surface states near Gamma point, which have both Rashba and Dirac band features in the dispersion. In the scanning tunneling microscope (STM) measurement carried out in Hoffman’s group, the differential conductance spectra (dI/dV) at bias V reflect the density of states (DOS) from these surface states. Though DOS only provides the momentum-space integrated information, complementary probes of Landau level spectroscopy (LL) in the magnetic field and the quasi-particle interference (QPI) for the scattering near the surface impurity can yield momentum-space resolved information of the surface bands. These local surface probe approaches complement the angle-resolved photoemission spectroscopy (ARPES) and plays a crucial role in characterizing the quantum materials and their nanostructure.
During the cleavage process of the sample, a different kind of the surface may emerge compared to the conventional bilayer termination. Surprisingly enough, the main features of the surface states remain robust under LL and QPI measurements. We investigate the crystal relaxation and the associated structural soliton, their impacts on the surface states, including the magnetic breakdown in the presence of additional bands near the original set of surface bands.
4. Junwei Liu
Postdoctoral Fellow, Massachusetts Institute of Technology
Discovery of Robust In-plane Ferroelectricity in Atomic-thick SnTe
Kai Chang, Junwei Liu, Haicheng Lin, Na Wang, Kun Zhao, Anmin Zhang, Feng Jin, Yong Zhong, Xiaopeng Hu, Wenhui Duan, Qingming Zhang, Liang Fu, Qi-Kun Xue, Xi Chen and Shuai-Hua Ji
Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1–unit cell (UC) limit. The ferroelectric transition temperature Tc of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.
5. Yun Liu
Postdoctoral Fellow, Harvard University
High-throughput Computational Discovery of Topological Insulators
Yun Liu and Alán Aspuru-Guzik
Computational materials design and prediction has been guiding the discovery of topological insulators (TI) ever since the first observation of time-reversal symmetry protected edge states. With the development of the theoretical interpretation of topological order and modern supercomputers, it is now possible for us to computationally predict the topological order of thousands of materials at the same time. Here, we propose a high-throughput materials design infrastructure for the discovery of TI using ab initio simulations. We employ electronic structure calculations and pattern recognition techniques to identify TI from millions of potential candidates. Our infrastructure will integrate computational predictions with experimental exploration and validation through a web-based materials data portal that will allow the data be shared across the center.
6. Pheona Williams
Graduate Student, Howard University
Terahertz Spectroscopy and Topological Insulators
Pheona Williams, Thomas Searles
Terahertz (THz) time domain spectroscopy (TTDS) is a contactless experimental method used for interrogating complex conductivity (permittivity) that measures not just changes in amplitude of the complex E-Field but also differences in phase. Our TTDS system at Howard University will have large bandwidth (>6 THz), high spectral resolution (1 GHz) and flexibility to add on other instrumentation to explore future materials, systems and other experimental methodologies such as optical pump-THz probe phenomena. The asynchronous optical sampling technique that will be employed will avoid common error associated with mechanical delay systems like spot size variation and pointing instabilities. In this presentation, we will highlight the advantages of our THz spectrometer and detail the usefulness of this instrument to study new topological insulator crystals grown by our CIQM collaborators.
7. Linda Ye
Graduate Student, Massachusetts Institute of Technology
Anomalous Hall Effect of a Half-Metallic Kagome Ferromagnet Co3Sn2S2
Linda Ye, Joe Checkelsky
Large single crystals of the half-metallic compound Co3Sn2S2 [1] with a layered kagome structure of Co have been synthesized and investigated in detail in terms of anomalous Hall effect. This compound exhibit a non-distorted kagome lattice structure of Co and out-of-plane Using anisotropy which are favorable ingredients in a theoretical model hosting topological flat bands [2]. We further separate the extrinsic and intrinsic contributions to the anomalous Hall effect and found that the intrinsic part corresponds to a sum of Berry curvature up to 0.62 e^2/h per individual kagome layers.
[1] M. Holder et al, Phys. Rev. B 79 205116 (2009)
[2] E. Tang et al, Phys. Rev. Lett. 106 236802 (2011)
8. Changmin Lee
Graduate Student, Massachusetts Institute of Technology
Toward Nonlinear Optical Domain Imaging of Magnetic Topological Insulator Surfaces
Changmin Lee, Carnia Belvin, Ferhat Katmis, Pablo Jarillo-Herrero, Jagadeesh S. Moodera, and Nuh Gedik
A magnetic topological insulator is predicted to host a variety of exotic physical phenomena such as the quantum anomalous Hall effect, Majorana fermions, and dissipationless chiral edge currents. Based on our previous work on second harmonic generation (SHG) spectroscopy, we are currently developing a tool to visualize magnetic domains at the interface between a topological insulator (Bi2Se3) and a ferromagnetic insulator (EuS). Our novel SHG imaging technique is expected to determine the direction of the magnetic domains and characterize the spin texture at the domain boundaries.
Group 2: van der Waals Heterostructures: Stacking Atomic Layers
1. Kwabena Bediako
Postdoctoral Fellow, Harvard University
Ion Intercalation in Van der Waals Systems
D. Kwabena Bediako, Frank Zhao, Hiroshi Idzuchi, Philip Kim
Layered materials comprising 2-dimensional sheets of covalently bonded atoms that are stacked via relatively weak Van der Waals (VdW) bonds may be exfoliated to yield individual atomic sheets and, if desired, re-assembled in arbitrary configurations. Besides the exciting opportunities in the study of the pristine as well as newly assembled layered crystals, these systems are also exciting materials to explore the (electro)chemistry of intercalation at the 2D limit. We discuss the efforts in our group to imbue new fundamental properties and functionality to VdW heterostructure systems through intercalation. We believe that systems such as these will form the basis for the next generation of electronic devices and energy conversion/storage schemes.
2. Shiang Fang
Graduate Student, Harvard University
Efficient Electronic Structure Modeling to Weakly Interacting Layered Heterostructure: Twisted Bilayer Graphene
Shiang Fang, Stephen Carr, Bertrand Halperin, Efthimios Kaxiras
The 2D layered materials can host a variety of physics properties, vertical and lateral heterostructure can be formed. The geometry also provides access to gating and surface probe and manipulations. When devising the applications with vertical heterostructure, it is challenging to navigate the massive parameters space in assembling the heterostructure including the stacking order, relative translation and twist for the layers in the stacks. Therefore, it is imperative to have a reliable and efficient numerical scheme for simulating the heterostructure. In our work, we provide the building block for this by employing Wannier function method on density functional theory calculations to model both the layered materials themselves and the interlayer coupling in between suitable for generic layer stacking.
As an example, we model the graphene stacks and compare with the magneto-transport measurements in Jarillo-Herrero’s group on a 2 degree twisted bilayer graphene sample. We found good agreement on the Hall conductance and the effective mass from Lifshitz-Kosevich analysis on the magnetoresistance oscillations. The comparison is carried out with a commensurate supercell simulation. However, we further generalize the theoretical method to incorporate incommensurate twist structure. Our preliminary results show good agreement compared to the scanning tunneling microscope local probe spectra and we also show the quantized Hall conductance in an incommensurate cell using Streda formula.
3. Dahlia Klein
Graduate student, Massachusetts Institute of Technology
Synthesis and Characterization of Ferromagnetic CrI3 2D Crystals
Dahlia R. Klein, Efrén Navarro-Moratalla, Pablo Jarillo-Herrero (MIT)
The van der Waals heterostructures of 2D conductors, insulators, and a whole family of semicondutors have already been widely explored. 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 and characterized the crystal structure of the insulating ferromagnet chromium triiodide (CrI3). From the bulk crystals, we have successfully exfoliated few-layer flakes on SiO2/Si substrates and obtained Raman and optical contrast data. 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. These development paves the way for realization of single layer ferromagnetism.
4. Tewa Kpulun
Graduate Student, Howard University
Transfer and Characterization of Graphene and Hexagonal Boron Nitride
Tewa Kpulun, Gary Harris, Jing Kong
The growth and development of 2D materials consists of several different techniques. In this experiment we are looking at the techniques of metal etched transfer and electrochemical delamination (bubble) transfer. These two techniques aid in the transfer of thin film materials from one substrate to the next without the use of toxic gasses. To ensure that the transfer is successful, Raman spectroscopy and scanning electron microscope (SEM) images are taken and analyzed. When transfer is complete, the samples will be analyzed further with an atomic force microscope (AFM Cypher). Their sheet resistance, transmission, and absorbance will be determined. Hall measurements will also be conducted of the graphene samples using the Vander Pauw method. (*This experiment was started over the summer and I am still working on the transfer phase of my project. Because of this my presentation and poster will mainly focus on the two transfer techniques.)
5. Jason Luo
Graduate Student, Massachusetts Institute of Technology
Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene
Yuan Cao, Jason Luo, Valla Fatemi, Shiang Fang, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Pablo Jarillo-Herrero
Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moiré physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov–de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moiré wave vector.
6. Mehdi Rezaee
Graduate Student, Howard University
Decoration of Graphene with Transition Metal and Alkali Ion
Mehdi Rezaee, Kwabena Bediako, Tina L. Brower-Thomas, Philip Kim
We have been investigating the interaction of transition metal (TM) and alkali ions (AI) with the surface of graphene using chemical and electrochemical reactions. Graphene open the door to research in the area of 2D materials, but despite all of graphene’s wonderful properties such high mobility, high thermal conductivity, graphene lacks some special properties which restrict its usage in a variety of industrial applications such as electronics (zero bandgap), spintronics(nonmagnetic) and etc. During the past decade after successful exfoliation of graphene from bulk graphite, several researches have worked to improve the properties of this 2D material. Properties of transition metal and alkali ions lend them appropriate candidates for functionalizing graphene with the aim of achieving improved graphene function without fundamentally effecting graphene’s desired properties. Theoretical investigations about the interaction of TM an AI and graphene have been reported lately. Experimental approaches are needed to challenge and substantiate theoretical work. Lack of comprehensive experimental analysis in this area has further motivated us to perform research in this field. This investigation has been divided into two efforts first; exploring of TM interaction with monolayer graphene through chemical process and characterize the results using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscope(STM), second; modification of mono and bilayer graphene properties via electrochemical intercalation. For the intercalation process we will fabricate hall bar with encapsulated graphene then we use ion liquid salt to intercalate ions (Ca) between layer through two step process. After successful surface functionalization, we will analyze the transport, plasmonic and other properties of decorated graphene.
7. Trevor Rhone
Graduate Student, Harvard University
Probing Ferromagnetic 2D Atomic Crystals
T.D. Rhone, H. Idzuchi, S. Harvey, T. Zhou, C. Du, R. Walsworth, P. Kim, A. Yacoby
Layered 2D atomic crystals of transition metal chalcogenides (TMC), such as CrGeTe3 (CrSiTe3) could realize the first 2D atomic ferromagnet (antiferromagnet). Whereas bulk (multilayered) CrGeTe3 and CrSiTe3 are known to exhibit ferromagnetic behavior, distinct magnetic order could arise in reduced dimensions. Ferromagnetic order is predicted to persist for monolayer CrGeTe3, while a transition to antiferromagnetic order is predicted for monolayer CrSiTe3. Experimental studies of magnetic properties of monolayers of these TMCs have not yet been performed. Here we report the first studies of the spin properties of bulk crystals (multiple layers) of ferromagnetic CrGeTe3 and CrSiTe3 using conventional ferromagnetic resonance (FMR). FMR is a powerful tool for the characterization of magnetic materials, capable of extracting magnetic properties such as the saturation magnetization and the damping constant. Since conventional FMR lacks sufficient sensitivity to probe a single atomic layer, future FMR studies of monolayer CrGeTe3 and CrSiTe3 require other 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 magnetism in reduced dimensions.
8. Erick Ruiz
Graduate Student, Harvard University
The Motivation Behind Imaging Techniques: Quantum Dots
Erick Ruiz, Robert M. Westervelt
An innate trait of human beings is the need to see things with our own eyes in order to learn and process information. (Hence the idiom, "Seeing is believing.") At the macroscopic-level, this is not a problem. However, things begin to break down at the microscopic level and fall apart completely once past the mesoscopic level. As a result, imaging techniques have been developed in order to capture images of physical phenomena which would be impossible to see even with an optical microscope. Scanning capacitance microscopy (SCM) and scanning probe microscopy (SPM) have enabled us to image quantum dots in two-dimensional electron gases and, more recently, in two-dimensional materials, further promoting our understanding of the nature of such physical phenomena and how to manipulate them.
9. Spencer Tomarken
Graduate Student, Massachusetts Institute of Technology
Capacitance Based Spectroscopy of van der Waals Heterostructures
S.L. Tomarken, A. Demir, N.E. Staley, K. Watanabe, T. Taniguchi, R.C. Ashoori
Tunneling spectroscopy measurements offer direct access to the density of states of a strongly interacting electron system, allowing exploration of the full excitation spectrum as the chemical potential is changed. Such measurements in the quantum Hall regime remain technically challenging due to the substantial in-plane resistivity that develops between Landau levels. Attempts to tunnel into many van der Waals materials are further hampered by poor contact resistances. Our lab has previously developed a time-domain capacitance spectroscopy (TDCS) technique that exploits a contactless, completely vertical tunneling scheme, eliminating the major pitfalls of conventional tunneling spectroscopy measurements. TDCS has successfully probed large area GaAs quantum wells in the quantum Hall regime, revealing a host of correlated electron effects. In order to extend TDCS to smaller exfoliation-based van der Waals heterostructures, improvements in the low temperature electronics are needed to recover the concomitantly weaker signals. Towards this goal, we report improvements to our measurement scheme as well as preliminary tunneling data on GaAs quantum wells.
10. Jundong Wu
Graduate Student, Harvard University
Hydrodynamic Oscillation With 2D Electron Gas
Jingyee Chee, Jundong Wu, Shannon Harvey, Amir Yacoby, Donhee Ham
The rapid oscillations of the electron density in conductors form plasmonic waves. A typical example of plasmonic wave would be the surface plasmons on bulk metals. While the surface plasmons usually appears in optics regime, plasmonic waves in 2D conductors can achieve a frequency that ranges from GHz to THz. For electrons in such a system, the equation of motion contains a hydrodynamic term under a DC current, which would give a reflection coefficient larger than unity on an AC open boundary. Our group's work focus on the direct observation of such mechanism, and we're devoting ourselves on making useful devices such as an oscillator using this phenomenon.
11. Yuan Yang
Graduate Student, Harvard University
Light Absorption and Scattering in Multilayer Graphene
Yuan Yang, Eric J. Heller
Multi-layer graphene is becoming a benchmark material in layered material studies. Translations and twists of multi-layer graphene change the relative geometry between layers, and lead to different inter-layer interactions. and thus different electronic structures. We study the effects of geometry distortions on optical absorption and light-matter scattering in the multi-layer graphene in the project.
12. Lili Yu
Graduate Student, Massachusetts Institute of Technology
High Yield MoS2 Circuit
L. Yu, D. El-Damak, U.Radhakrishna, Y.-H. Lee, D. Antoniadis, J. Kong, A. Chandrakasan and T. Palacios
2D electronics based on single-layer (SL) MoS2 offers significant advantages for realizing large-scale flexible systems. However, the reported devices and circuits based on this material have low yield because of various variation sources inherent to the growth and fabrication technology. In this work, we develop a variation-aware design flow and yield model to evaluate the MoS2 technology and provide a guideline for the co-optimization of the material, devices and circuits. Test chips with various inverters and basic logic gates (such as NAND and XOR) are fabricated as demonstration of the close-to-unit yield of the proposed technology platform.
13. Xu Zhang
Graduate Student, Massachusetts Institute of Technology
Controllable and Nonintrusive Doping in Graphene
Xu Zhang, Allen Hsu, Theanne Schiros, Yong Cheol Shin, Jing Kong, Mildred Dresselhaus, Tomas Palacios
Plasma-based chlorination is a promising technique to achieve controllable doping in graphene, while maintaining its high mobility. Synchrotron-based X-ray spectroscopy offers an effective route to investigate the surface states of functionalizing dopants in graphene. In this work, we systematically studied the electronic states of chlorinated graphene on different substrates, including surface binding energy, dopant concentration and work function shift by use of XPS, Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy and photoemission threshold measurements. It is remarkable that the sp2 carbon core-hole exciton (291.85eV) retained its sharpness even after treatment, indicating the long-range periodicity in graphene is largely preserved. A high post-doping mobility of 1535 cm2/Vs in CVD-grown large scale graphene was achieved by optimizing the Cl plasma parameters.
14. Lin Zhou
Postdoctoral Fellow, Massachusetts Institute of Technology
Synthesis of High-Quality Large-Area Homogenous 1T′ MoTe2 from Chemical Vapor Deposition
Lin Zhou and Jing Kong
1T′ MoTe2 has recently sparked great interest due to its novel properties and promising electronic applications. The scalable production of high-quality, large-area two-dimensional 1T′ MoTe2 is challenging but crucial both for fundamental research and applications. Here we synthesize high-quality large-area few-layer 1T′ MoTe2 films with high homogeneity by the controlled tellurization of a MoO3 film. The resulting 1T′ MoTe2 film has resistivity value even lower than that reported for bulk 1T′ MoTe2. We find that the Mo precursor plays a key role in determining the quality and morphology of the as-grown 1T′ MoTe2. Furthermore, the amount of Te strongly influences the phase of the MoTe2 grown from MoO3. The investigation of the role of the Mo precursor and the amount of Te for the growth of MoTe2 provides insights into the controllable synthesis and phase engineering of MoTe2. Our growth method paves the way towards studies of the exotic properties of 1T′ MoTe2 and the scalable production of high-quality 1T′ MoTe2-based applications.
Group 3: Engineering Topological Superconductors
1. Valla Fatemi
Graduate Student, Massachusetts Institute of Technology
Magnetoresistance and Quantum Oscillations of a Gate-Tuned Semimetal-to-Metal Transition in Ultra-Thin WTe2
Valla Fatemi, Kenji Watanabe, Takashi Taniguchi, Robert J. Cava, Pablo Jarillo-Herrero
Semimetals are of resurgent interest due to their unique transport phenomena and new topological phases. Huge, non-saturating magnetoresistance has been reported on several compounds, particularly including WTe2. Here, we report on electronic transport measurements of electrostatically gated nano-devices of the semimetal WTe2. High mobility metallic behavior is achieved in the 2D limit by handling the thin flakes in an inert atmosphere. At low temperatures, we find gate-tuneable magnetoresistance and quantum oscillations indicating an electrostatically induced semimetal-to-metal transition. In particular, we see a complete turn-off of the magnetoresistance in the simple-metal case. We also find that the magnetoresistance does not behave according to the classically expected quadratic behavior seen in the bulk, but rather with a non-integer sub-quadratic power law, pointing to new physics in the 2D limit of WTe2.
2. Jun Yong Khoo
Graduate Student, Massachusetts Institute of Technology
Gate-Tunable Spin-Orbit Interaction in Graphene
Jun Yong Khoo, Leonid Levitov
We propose a novel strategy to achieve gate-tuning of the spin-orbit (SO) interaction strength in two dimensional electronic gas systems. Our method exploits the substantial di
erence in proximity- induced SO interaction strengths between the two layers of a bilayer system. Distinct from previous methods, tuning is achieved by gating the bilayer, which introduces a polarizing electric field that localizes the carriers to either of the layers and consequently, the strength of the SO interaction they experience. We illustrate this idea by placing bilayer graphene on a transition metal dichalcogenide substrate, from which it acquires a proximity-induced SO interaction as observed in recent experi- ments. We show how this gate-tuning strategy can be extended to include other proximity-induced properties, such that when the substrate also has a superconducting pairing potential, the resulting system allows for gate-control of Majorana Fermions.
3. Cyprian Lewandowski
Graduate Student, Massachusetts Institute of Technology
Photoexcitation and Particle-hole "Jets" in Dirac Materials
Cyprian Lewandowski, Leonid Levitov
We are investigating the problem of photon absorption in an undoped disorder-free graphene, where a single photon can initiate creation of multiple particle-hole excitations. We argue that with the photoexcitation pathway occurring via an off-shell mechanism, it is possible to relieve an energy and momentum bottleneck arising in a conventional on-shell process. Created particle-hole excitation undergoes then a subsequent relaxation producing a cascade of near-collinear particle-holes. In the approximation of negligible electron-phonon scattering, we study behavior of so formed "jets” in the framework of relativistic hydrodynamics. Motivated by recent work on chiral transport in quark-gluon plasmas, we will search for dependence of particle-hole beam on the topological properties of the process’ host material.
4. Efrén Navarro-Moratalla
Postdoctoral Fellow, Massachusetts Institute of Technology
Single-layer Ferromagnetism in CrI3
Efrén Navarro-Moratalla, Dahlia R. Klein, Bevin Huang, Genevieve Clark, Xiaodong Xu and Pablo Jarillo-Herrero
Since the discovery of graphene, a wide variety of 2D materials with different optoelectronic properties have already been explored. Though local magnetic moments may be introduced in the latter via doping or defect engineering, the vast majority of these materials are non-magnetic. The isolation of crystalline single-layers with long-range magnetic order open the door to low-dimensional spintronics in van der Waals heterostructures. We focus our study in the isolation of few-layer crystals of CrI3, which exhibits a transition to a ferromagnetic phase below 60 K in bulk. The magneto-optic Kerr effect enables to probe the magnetic properties of few-layer specimens down to the single-layer. We observe that ferromagnetism prevails down to the monolayer. In addition, there is also a strong effect of the thickness on the ferromagnetic order, which manifests as a different response of the crystal magnetization to an external applied field depending on the number of layers.
*Sharing Poster with Dahlia Klein
5. Yunbo Ou
Postdoctoral Fellow, Massachusetts Institute of Technology
Find Solid Evidence for Topological Magnetoelectric Effect and Majorana State
Yunbo Ou, Jagadeesh Moodera
Since the concept of “topological state” is proposed in 2005, lots of such topological phenomena have been observed experimentally. One decade later, only some tough but meaningful proposals, such as topological magnetoelectric effect, magnetic monopole and Majorana fermion, are waiting to be observed by experimental physicists. Here we propose some experiments and try to observe topological magnetoelectric effect and Majorana fermion.
6. Michal Papaj
Graduate Student, Massachusetts Institute of Technology
Teleportation-based Majorana Transistor
Michal Papaj, Zheng Zhu, Liang Fu
We study a three-terminal Majorana island by the means of numerical renormalization group. We show that the DC conductance in this setup exhibits fractional quantized value of 2/3 e^2/h at low temperatures. The crossover temperature is much larger than in the previously studied cases and the result is not sensitive to channel coupling anisotropy or moving away from the charge degeneracy point, which makes our proposal experimentally feasible. We predict as an experimental signature a non-trivial crossover between two and three lead cases with a DC conductance plateau at 2/3 e^2/h emerging while tuning the lead tunnel coupling at sufficiently low temperature. Based on those results we propose a Majorana transistor, in which the DC conductance between two leads is controlled by the tunnel coupling of the third one.
7. Falko Pientka
Postdoctoral Fellow, Harvard University
Topological Superconductivity in 2d Josephson Junctions
Falko Pientka, Anna Keselman, Erez Berg, Ady Stern, Amir Yacoby, Bertrand Halperin
We consider a planar Josephson junction in a two-dimensional electron gas with Rashba spin-orbit coupling and an in-plane magnetic field. We show that this quasi-one-dimensional system can support a topological superconducting phase hosting Majorana bound states at its ends. Compared to other proposals for realization of topological superconductivity, this setup requires much less fine tuning of experimental parameters in order to drive the system into the topological phase. Once the phase difference between the superconductors is close to π the topological phase is obtained for almost any value of chemical potential and Zeeman field. The system can also self-tune into a topological phase in a range of in-plane Zeeman fields, for which the ground state is a π junction. In this case, the critical current carries a signature of the 0-π transition, and is therefore a natural diagnostic of the topological transition.
8. Andrew Pierce
Graduate Student, Harvard University
Proximity-coupled Quantum Hall Edge Modes in HgTe Quantum Wells
A. T. Pierce, H. Ren, S. Hart, M. Kosowsky, C. Brüne, L. W. Molenkamp, A. Yacoby
A promising approach to the problem of engineering a topological superconductor involves coupling a superconductor to a normally non-superconducting material. The so-called "proximity effect," whereby electrons near but outside of a superconducting electrode are induced to form Cooper pairs and hence carry supercurrent, forms the basis of this approach. In particular, it is believed that proximity-coupling a two-dimensional electron gas in a quantum Hall phase to a trivial superconductor gives rise to a two-dimensional chiral topological superconductor. Motivated by the goal of realizing and controlling such a topological superconductor, we study transport phenomena mediated by Andreev processes at the interface of a quantum Hall phase and a superconductor. We carry out these experiments using HgTe quantum wells, in which the electron g-factor is large, and which can be doped with Mn to reduce the magnetic field at which Hall quantization is observed. We discuss preliminary results on HgTe devices fabricated with Nb and NbN contacts of various geometries, and describe plans for future work.
9. Hechen Ren
Graduate Student, Harvard University
Tunneling Spectroscopy of Topological Superconductivity
Hechen Ren, Sean Hart, Andrew Pierce, Philipp Leubner, Christoph Brüne, Hartmut Buhmann, Amir Yacoby, Bertrand Halperin, Laurens Molenkamp
We engineer superconductivity into the first discovered two-dimensional topological insulator – HgTe/HgCdTe quantum wells - through proximity effect. The resulting topological superconductor would be capable of supporting localized Majorana fermions, particles whose braiding properties have been proposed as the basis of a fault-tolerant quantum computer. One key feature of such localized Majorana states is that they always appear at zero energy, right in the middle of the superconducting gap. Therefore tunneling spectroscopy is the ideal method to investigate the conditions under which they are created and manipulated. By measuring the differential conductance with a tunnel probe on a phase-controlled and proximitized edge, we hope to provide evidence for and achieve a microscopic understanding of these topological superconducting states.
10. Oles Shtanko
Graduate Student, Massachusetts Institute of Technology
Theory of Relativistic Edge States in Dirac Materials
Oles Shtanko, Leonid Levitov
We predict that Dirac materials can host electronic states confined to generic sample edges by atomic-scale potentials. In contrast to the tightly localized Tamm states in semiconductors, relativistic edge states have confinement scale compared to Fermi wavelength. Their existence at a generic boundary is explained by nature of Dirac carriers. Due to the large confinement scale, energy modes diffract over edge impurities and roughness and propagate quasi-ballistically for mesoscopic distances. This makes relativistic edge states comparable to weakly guided light in fiber optics. The high propagation rate determines the application of edge states in nanotechnology as transport channels and carriers of quantum information.
Group 4: Diamond Color Center Engineering
1. Rodrick Kuate Defo
Graduate Student, Harvard University
Diffusion of Vacancies in SiC
Rodrick Kuate Defo, Xingyu Zhang, David Bracher, Evelyn Hu, Efthimios Kaxiras
Defect centers in silicon carbide (SiC) have emerged as strong contenders in the quest to realize quantum devices due to the material’s lower cost as compared to its counterpart diamond, and due to the microfabrication techniques now available and favorable optical emission wavelengths and spin properties. We investigate the stability and diffusion of the negatively charged silicon vacancy in 4H-SiC, using density-functional-theory, which should serve as an invaluable guide in controlling defect positions within devices.
2. Jin-yong Hong
Postdoctoral Fellow, Massachusetts Institute of Technology
A Rational Strategy for Graphene Transfer on Substrates with Rough Features
Jin-Yong Hong, Jing Kong
Degradation in intrinsic properties of chemical vapor deposition (CVD)-grown graphene, as a result of the imperfect transfer process, is a crucial issue that must be solved for successful applications of graphene. In this work, we develop a very simple, yet effective, approach, based on distinctive features (i.e. physical, mechanical, and chemical properties) of ethylene vinyl acetate (EVA) as a support/carrier material, for transferring CVD-grown graphene from a growth substrate onto substrates with rough features. This novel and facile method can not only result in satisfactory transfer on substrates with terraces or grooves, but also gives rise to a successful result for uneven growth substrates (textured and also crumpled). The outstanding mechanical properties of EVA as a support/carrier material provide a conformal graphene transfer onto such irregular substrates and avoiding tears in the graphene. Moreover, the well-matched solubility parameters between the polymer and the solvent give rise to a highly cleaned graphene surface with minimal polymer residue. Consequently, the graphene transferred with EVA support/carrier material exhibits superior electrical performance compared with most presently used transfer methods.
3. Amirhassan Shams Ansari
Graduate Student, Howard University
Toward Quantum Wavelength Conversion with Diamond Nonlinearities
Amirhassan Shams Ansari, Gary Harris, Pawel Latawiec, and Marko Loncar
Diamond is a promising platform for on-chip photonics. Its relatively high nonlinear refractive index, and absence of two-photon absorption have enabled diamond as an attractive candidate for nonlinear optics. Solid-state single photon emitters in diamond are important for scalable quantum information technology. Photons from single-photon emitters such as negatively charged NV and SiV do not propagate over long distance due to their visible emission (532 nm and 738 nm, repsectively). In this case there is a need for designing micro-cavities for single photon quantum frequency conversion to convert the color-center photons to low-loss telecom frequency channels for long distance communication. Third order nonlinearity in diamond micro-photonics is the dominant parametric nonlinearity. Four-Wave Mixing (FWM) is the process that enables this third-order nonlinear process. In this research, we have been investigating the Quantum wavelength conversion using third-order nonlinearity in diamond using FWM-bragg scattering. To ensure this, high-Q resonators and efficient coupling of light to the chip are required which are studied in a diamond Raman Laser platform.
4. Linbo Shao
Graduate Student, Harvard University
Wide-field Microwave Sensing using Nitrogen-vacancy Centers in Diamonds
Linbo Shao, Mian Zhang, Ruishan Liu, Marko Loncar
Negatively-charged nitrogen-vacancy (NV) center is a lattice defect in diamond and the photoluminescence of a NV center depends on its electron spin state, which is sensitive to the microwave signal. We demonstrate wide-field microwave field microscopy and spectrometry using NV centers; ensembles of NV centers serve as transducers from microwave field to fluorescence intensity. Several applications of this wide-field microwave sensing are proposed from 2D materials devices to IC chip securities.
5. Young-Ik Sohn
Graduate Student, Harvard University
Strain Engineering of Diamond Silicon Vacancy Centers in MEMS Cantilevers
Srujan Meesala, Young-Ik Sohn, Haig Atikian, Jeffrey Holzgrafe, Mian Zhang, Michael Burek, Marko Loncar
Co-author and poster presenter:
Srujan Meesala
Graduate Student, Harvard University
The silicon vacancy (SiV) center in diamond has recently attracted attention as a solid state quantum emitter due to its attractive optical properties. We fabri- cate diamond MEMS cantilevers, and use electrostatic actuation to apply controlled strain fields to single SiV centers implanted in these devices. The strain response of the four electronic transitions of the SiV at 737 nm is measured via cryogenic (4 K) photoluminescence excitation. We demonstrate over 300 GHz of tuning for the mean transition frequency between the ground and excited states, and over 100 GHz of tuning for the orbital splittings within the ground and excited states. The interaction Hamiltonian for strain fields is inferred, and large strain susceptibilities of the order 1 PHz/strain are measured. We discuss prospects to utilize our de- vice to reduce phonon-induced decoherence in SiV spin qubits, and to exploit the large strain susceptibilities for hybrid quantum systems based on nanomechanical resonators.
6. Jordan Stroman
Graduate Student, Howard University
Electrical Excitation of Silicon Vacancies in 4H SiC
Jordan Stroman, Evelyn Hu, Gary Harris
The Silicon Vacancy defect in 4H Silicon Carbide (SiC) could serve as a quantum bit or emitter. In the last year spin coherence, manipulation, and optical readout have been demonstrated using this defect. In this presentation I will describe my work to electrically excite Silicon Vacancies. I first irradiated a 4H SiC pn junction with high energy electrons and then fabricated a mesa structure to probe this junction. I will also present a preliminary investigation on the effects of electron irradiation on the electroluminescence spectrum of pn junctions in 4H SiC under forward and reverse bias conditions.
7. Michael Walsh
Graduate Student, Massachusetts Institute of Technology
Fabrication of High Quality Quantum Emitters in Diamond Nanostructures
Michael Walsh, Matthew Trusheim, Tim Schroder and Dirk Englund
The controlled creation of defect center-nanocavity systems is one of the outstanding challenges for efficiently interfacing spin quantum memories with photons for photon-based entanglement operations in a quantum network. We demonstrate two methods. The first being direct: maskless creation of atom-like single silicon-vacancy (SiV) centers in diamond nanostructures via focused ion beam implantation with <50 nm positioning accuracy relative to the nanocavity. The second method relies on autonomously imaging emitters and registering them relative to an on-chip coordinate system. This technique can be performed on a larger variety of emitters, as it does not require a focussed ion beam. The repeatability of this method suggests an accuracy down to ~20 nm.
8. Amber Wingfield
Graduate Student, Howard University
Investigating Incorporation of Nitrogen in HF-CVD Diamond
Amber Wingfield, Gary Harris, James Griffin
Doping diamond for the production of nitrogen-vacancy (NV) centers has recently come to the forefront as a potential candidate for quantum computing. Growth and doping of diamond can be achieved by way of employing hot filament chemical vapor deposition (HF-CVD). Thus, understanding how dopant concentration is effected during diamond growth is of interest. Therefore, the focus of our current research is to determine nitrogen incorporation in relation to diamond growth conditions. This will allow leverage into determining the relationship between nitrogen concentration and NV-center incorporation Prepared silicon and silicon carbide substrates are placed into a HF-CVD environment with conditions set for diamond production. Within this environment, the substrates are doped with nitrogen and subjected to varying growth conditions, such as pressure and flow rate changes. Secondary ion mass spectroscopy (SIMS analysis) is then conducted to provide insight on how nitrogen concentration changes occurred during growth.
9. Jundong Wu
Graduate Student, Harvard University
Hydrodynamic Oscillation With 2D Electron Gas
Jingyee Chee, Jundong Wu, Shannon Harvey, Amir Yacoby, Donhee Ham
The rapid oscillations of the electron density in conductors form plasmonic waves. A typical example of plasmonic wave would be the surface plasmons on bulk metals. While the surface plasmons usually appears in optics regime, plasmonic waves in 2D conductors can achieve a frequency that ranges from GHz to THz. For electrons in such a system, the equation of motion contains a hydrodynamic term under a DC current, which would give a reflection coefficient larger than unity on an AC open boundary. Our group's work focus on the direct observation of such mechanism, and we're devoting ourselves on making useful devices such as an oscillator using this phenomenon.
10. Xingyu Zhang
Graduate Student, Harvard University
Optical Cavities for Emission Enhancement and Characterization of Defects in 4H-SiC
Xingyu Zhang, David Bracher, Rodrick Kuate Defo, Efthimios Kaxiras, Evelyn Hu
Similar to color centers in diamond (e.g. the nitrogen-vacancy center), defects in silicon carbide (SiC) have optical and spin properties that make them promising qubit candidates. In addition, the diverse polytypes and more complex lattice structure of SiC lead to a greater variety of defects with long spin coherences and optical addressability. Using nanobeam photonic crystal cavities, we have demonstrated resonant emission enhancement of the zero-phonon transitions of one such defect, the negatively charged silicon vacancy, in 4H-SiC. By measuring the photoluminescence and its enhancement from these and related defects, the optical cavities can also be used to study defect formation and migration under different annealing conditions. This may ultimately allow control of defect position within photonic nanocavities, providing greater enhancement of defect emission.
