Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A15: Graphene: Quantum Hall Effect |
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Sponsoring Units: DCMP DMP Chair: Paul Campbell, Naval Research Laboratory Room: 314 |
Monday, March 14, 2016 8:00AM - 8:12AM |
A15.00001: Valley Hall Effect in the presence of strain fluctuation in graphene systems Wenyu Shan, Di Xiao We develop a theory of valley Hall transport in graphene on hexagonal boron nitride substrate, where the strain-induced random gauge potential becomes the dominant source of disorder. We find a large value (if not quantized) of valley Hall conductivity in the band transport regime, for a wide class of strain-fluctuation modes. Such exotic property is a consequence of a generic enhanced coordinate shift under vector disorder scattering for Dirac systems. When applied to monolayer or bilayer graphene, our theory reproduces the large valley Hall angle and finite-temperature behaviors of valley Hall conductivity observed in experiments. Our theory provides an alternative interpretation of experimental results for nonlocal transport in graphene, instead of previous understanding based on the equilibrium current in ground state. [Preview Abstract] |
Monday, March 14, 2016 8:12AM - 8:24AM |
A15.00002: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 8:24AM - 8:36AM |
A15.00003: Spin and valley resolved Landau level crossing in tri-layer ABA stacked graphene. Biswajit Datta, Vishakha Gupta, Abhinandan Borah, Kenji Watanabe, Takashi Taniguchi, Mandar Deshmukh We present quantum Hall measurements on a high quality encapsulated tri-layer graphene device. Low temperature field effect mobility of this device is around 500,000 cm$^{2}$/Vs and we see SdH oscillations at a magnetic field as low as 0.3 T. Quantum Hall measurements confirm that the chosen tri layer graphene is Bernal (ABA) stacked. Due to the presence of both mass-less monolayer like Dirac fermions and massive bi-layer like Dirac fermions in Bernal stacked tri-layer graphene, there are Landau level crossings between monolayer and bi-layer bands in quantum Hall regime. Although most of the Landau Level crossings are predominantly present on the electron sides, we also observe signatures of the crossings on the hole side. This behaviour is consistent with the asymmetry of electron and hole in ABA tri-layer graphene. We observe a series of crossings of the spin and valley resolved Landau Levels. [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A15.00004: Landau Level Crossings in Dual-Gated Bilayer Graphene at Large Displacement Fields Cheng Pan, Yong Wu, Bin Cheng, Shi Che, Chun Ning Lau, Marc Bockrath Previous work shows that Landau levels in bilayer graphene with the same orbital index $N$ but different spin and valley degrees of freedom can form superpositions at a finite electric field[1-2]. Using the technique of one dimensional edge contacts[3] we fabricate dual-gated boron-nitride-encapsulated bilayer graphene device with a graphite local gate, which enables us to apply large electric fields and observe Landau level crossings between levels with neighboring orbital indices. We will discuss our latest results. [1] R. T. Weitz et al., Science 330, 812-816 (2010). [2] K. Lee et al., Science 345, 58-61 (2014). [3] L. Wang et al., Science 342, 614-617 (2013) [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A15.00005: Quantum Hall Effect in Bernal-stacked tetralayer graphene Yanmeng Shi, Shi Che, Timothy Espiritu, Ziqi Pi, Takashi Taniguchi, Kenji Watanabe, Chun Ning Lau Bernal-stacked few layer graphene is of particular interest due to its unique tunable band~structure. Here we study the electric transport of Bernal-stack tetralayer graphene that are encapsulated by boron nitride sheets. The device shows a clear Landau fan with multiple Landau level crossing features. We will present the dependence of its quantum Hall properties on electric and magnetic fields, and compare with theoretical calculations.~ [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A15.00006: Quantum Hall resistance standard in graphene devices under relaxed experimental conditions F. Schopfer, R. Ribeiro-Palau, F. Lafont, J. Brun-Picard, D. Kazazis, A. Michon, F. Cheynis, O. Couturaud, C. Consejo, B. Jouault, W. Poirier Large-area and high-quality graphene devices synthesized by CVD on SiC are used to develop reliable electrical resistance standards, based on the quantum Hall effect (QHE), with state-of-the-art accuracy of 1x10$^{\mathrm{-9}}$ and under an extended range of experimental conditions of magnetic field (down to 3.5 T), temperature (up to 10 K) or current (up to 0.5 mA). These conditions are much relaxed as compared to what is required by GaAs/AlGaAs standards and will enable to broaden the use of the primary quantum electrical standards to the benefit of Science and Industry for electrical measurements. Furthermore, by comparison of these graphene devices with GaAs/AlGaAs standards, we demonstrate the universality of the QHE within an ultimate uncertainty of 8.2x10$^{\mathrm{-11}}$. This suggests the exact relation of the quantized Hall resistance with the Planck constant and the electron charge, which is crucial for the new SI to be based on fixing such fundamental constants. These results show that graphene realizes its promises and \underline {demonstrates its superiority} over other materials for a demanding application. \underline {Nature Nanotech. 10, 965-971, 2015}, \underline {Nature Commun. 6, 6806, 2015} [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A15.00007: Scaling behavior of the quantum Hall plateau-plateau transition in graphene p-n-p junctions Wei-Hua Wang, Cheng-Hua Liu, Po-Hsiang Wang, Tak-Pong Woo, Chun-Wei Chen, Chi-Te Liang We present the observation of scaling behavior in graphene p-n-p junctions achieved by controlled metallic diffusion. Generally, metal deposition on graphene surface introduces substantial carrier scattering, which undermines the high mobility of intrinsic graphene. However, we discover a weakly functionalized regime of the deposited contact with small carrier scattering, while p-type doping of graphene is realized due to the metal oxide formation. Consequently, the resulted graphene channel are composed of p-type doped and an intrinsic regions. The high-quality graphene p-n-p junctions is evidenced by a pronounced quantum Hall effect and Shubnikov-de Haas oscillations. Remarkably, we observed a well-defined QH plateau-plateau transition of zeroth Landau level, yielding a scaling exponent of {\$}$\backslash $kappa$=$0.21$\backslash $pm0.01{\$}. Moreover, the graphene p-n-p junctions exhibit weak localization behavior, and the coherence length was found to be correlated to carrier scattering in the graphene devices. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A15.00008: Landau Level Mixing in the $\nu=0$ Quantum Hall State of Graphene Braden Feshami, Herbert Fertig The $\nu=0$ quantum Hall state in graphene has been the focus of many studies over the last several years. Recent experimental developments have allowed for the possibility of tuning the strength of the Zeeman interaction by tilting a graphene sample in the presence of an external magnetic field. Many of the theoretical frameworks for these systems involve projecting into the zeroth Landau level (LL) and specifying effective interaction parameters to simplify the calculation. We explore the effects of keeping a larger number of LLs, allowing for the possibility of Landau level mixing, within a self-consistent Hartree-Fock theory of the system. We include a SU(4) symmectric, Coulombic-like, interaction, and introduce microscopic on-site and nearest-neighbor interactions. Phase diagrams are constructed over a range of these two microscopic interaction strengths for different magnetic field strengths and Zeeman couplings. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A15.00009: Unconventional Quantum Hall Edge-Bulk Correlation in Gated Graphene Devices Yong-Tao Cui, Bo Wen, Eric Ma, Georgi Diankov, Zheng Han, Francois Amet, Takashi Taniguchi, Kenji Watanabe, David Goldhaber-Gordon, Cory Dean, Zhi-Xun Shen The quantum Hall effect in a two dimensional electron system (2DES) has been understood as chiral edge states circulating a highly insulating bulk when the bulk Landau levels are completely filled. We combine edge-sensitive charge transport with scanning Microwave Impedance Microscopy that probes the bulk transition through Landau levels, to directly examine such edge-state picture in gated graphene devices. Surprisingly, comparison of these measurements reveals that quantized transport typically occurs below complete filling of bulk Landau levels, different from the conventional picture in semiconductor-based 2DESs. We suggest that gate-induced charge accumulation near edges leads to a complex edge state configuration in which counter-propagating edge states could exist at the same edge. The backscattering between these states, together with an incompressible strip separating the bulk and the edge region, can explain the observed deviations. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A15.00010: Collective Bulk and Edge Modes through the Quantum Phase Transition in Graphene at $\nu=0$ Ganpathy Murthy, Efrat Shimshoni, Herbert Fertig Undoped graphene in a strong, tilted magnetic field exhibits a radical change in conduction upon changing the tilt-angle, which can be attributed to a quantum phase transition[1] from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at filling factor $\nu=0$. This behavior signifies a change in the nature of the collective ground state and excitations across the transition. Using the time-dependent Hartree-Fock approximation, we study the collective neutral (particle-hole) excitations in the two phases, both in the bulk and on the edge of the system[2]. The CAF has gapless neutral modes in the bulk, whereas the FM state supports only gapped modes in its bulk. At the edge, however, only the FM state supports gapless charge-carrying states. Linear response functions are computed to elucidate their sensitivity to the various modes. The response functions demonstrate that the two phases can be distinguished by the evolution of a local charge pulse at the edge[3]. 1. M. Kharitonov, Phys. Rev. B {\bf 85}, 155439 (2012). 2. G. Murthy, E. Shimshoni, and H. A. Fertig, Phys. Rev. B {\bf 90}, 241410(R) (2014). 3. G. Murthy, E. Shimshoni, and H. A. Fertig, arxiv:1510.04255 (2015). [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A15.00011: Observation of helical edge states and fractional quantum Hall effect in a graphene electron-hole bilayer Jason Yuanhong Luo, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero 1D electronic systems are common theoretical building blocks for constructing quantum circuits, motivating a search for new experimental systems where 1D edge states of different quantum numbers can be coupled to each other by design. Twisted bilayer graphene allows for the stacking of two separate 1D quantum hall edge states, thus providing a natural sandbox for studying different types of edge state interactions. Via doping to form an electron-hole bilayer at moderate magnetic fields, we can induce edge modes of opposite chiralities and spin polarizations on different layers, thereby creating helical edge states reminiscent of a two-dimensional quantum spin Hall system. We report magnetotransport measurements of high-quality twisted bilayer graphene, showing how non-local measurements allow us to elucidate the nature and robustness of the helical edge states, as well as hints of fractional edge state interactions that are observable at higher magnetic fields. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A15.00012: Emergence of Helical Edge Conduction in Graphene in the $\nu=0$ Quantum Hall State Herbert Fertig, Pavel Tikhonov, Efrat Shimshoni, Ganpathy Murthy The conductance of graphene subject to a strong, tilted magnetic field exhibits a dramatic change with tilt-angle, interpreted as an evidence for the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) $\nu=0$ quantum Hall state. We develop a theory for the electric transport in this system based on the spin-charge connection, whereby the evolution in the nature of collective spin excitations throughout this quantum phase transition is reflected in the charge-carrying modes. To this end we study quantum fluctuations of the spin-valley configuration in a system with an edge, and derive an effective theory describing collective charge edge excitations coupled to neutral bulk excitations. Focusing particularly on the FM phase, naively expected to exhibit perfect conductance due to the emergence helical edge modes, we analyze the mechanism whereby the coupling to bulk excitations assists in generating back-scattering. Finally, we calculate the conductance as a function of temperature and the Zeeman energy – the parameter that tunes the transition between the two phases. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A15.00013: Edge State Structure of the $\nu=0$ quantum Hall State in monolayer Graphene Angelika Knothe, Thierry Jolicoeur Single-layer graphene at neutrality under a magnetic field is a many-body insulator whose phase structure is under intense scrutiny. When tilting the applied magnetic field, there is a phase transition towards a conducting state. A plausible description is to start from a SU(4) spin-valley symmetric quantum Hall ferromagnet and add some lattice-scale anisotropies in valley space. In the manifold of ground states captured by this approach, it has been proposed that graphene undergoes a transition between a canted antiferromagnetic state and a ferromagnetic state. While this picture is clear in the bulk of the system, it remains to understand the effect of this phase change on the current-carrying edge states that are formed a the physical boundaries of a real sample. We use an extended Hartree-Fock approach to describe a finite-size system with a simple model for the edge and extract the one-body spectrum. We then describe the current-carrying edge textures. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A15.00014: Andreev reflection in edge states of time reversal invariant Landau levels K. G. S. H. Gunawardana, Bruno Uchoa We describe the conductance of a normal-superconducting junction in systems with Landau levels that preserve time-reversal symmetry. Those Landau levels have been observed in strained honeycomb lattices. The current is carried along the edges in both the normal and superconducting regions. When the Landau levels in the normal region are half filled, the Andreev reflection is maximal and the conductance plateaus have a peak as a function of the filling factor. The height of those peaks is quantized at 4e$^2$/h. The interface of the junction has Andreev edge states, which form a coherent superposition of electrons and holes that can carry a net valley current. We identify unique experimental signatures for superconductivity in time-reversal-invariant Landau levels. [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A15.00015: Andreev conversion of quantum Hall edge state in graphene Gil-Ho Lee, Sean Hart, Di Wei, Katie Huang, Dmitri Efetov, Takashi Taniguchi, Kenji Watanabe, Amir Yacoby, Philip Kim Understanding the interplay between superconductivity (SC) and quantum Hall effect (QHE) has been a long-sought theoretical and experimental problem. SC contacts to QHE systems enable us to study interesting physics, such as Cooper pair injection into ballistic 2D channels, Andreev edge states, and emergent excitations of non-Abelian anyons. We developed an in-situ etching technique for highly transparent superconducting contact (NbN) to hBN encapsulated graphene channels. The high critical field of NbN electrodes ($H_{c2}$ $>$ 30 T) and the high quality of our graphene devices allows us to experimentally access a wide range of magnetic field where SC and QHE coexist. In order to probe the Andreev conversion of QH edge states, we measure the chemical potential of normal electrodes located on the upstream and the downstream QH edge states relative to a narrow grounded superconducting electrode. We observed that the chemical potential in downstream has sign opposite to the one measured in upstream suggesting Andreev conversion of incident electrons to outgoing holes across the narrow superconducting contact. We systematically investigated this phenomena as a function of temperature, magnetic field, bias voltage and the width and length of the superconducting electrode. [Preview Abstract] |
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