Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B40: Graphene Quantum Hall Effect |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Jairo Velasco Jr., Univ of California-Santa Cruz Room: LACC 501C |
Monday, March 5, 2018 11:15AM - 11:27AM |
B40.00001: Quantum Spin Hall and Quantum Valley Hall Effect in Trilayer Graphene and their topological structure. Majeed Ur Rehman The present study pertains to the tri-layer graphene in the presence of spin-orbit coupling (SOC) to probe as quantum spin /valley Hall effect. The spin Chern-number C_{s} for energy-bands of tri-layer graphene having the essence of intrinsic SOC is analytically calculated. We find that for each valley and spin, C_{s} is three times larger in tri-layer graphene as compared to single layer graphene. We also study the tri-layer graphene in the presence of both electric-field and intrinsic SOC and investigate that tri-layer graphene goes through a phase transition from quantum spin Hall to a quantum valley Hall state when the strength of the electric field exceeds the intrinsic SOC strength. The robustness of associated topological bulk-state of tri-layer graphene are evaluated by adding various perturbations such as Rashba SO interaction α_{R}, and exchange-magnetization M. In addition, we consider a theoretical model, where only one of the outer layers in tri-layer graphene has the essence of intrinsic SOC, while the other two layers have zero intrinsic SOC. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B40.00002: Quantum Hall Interferometry in Graphene Matthew Yankowitz, Yihang Zeng, Shaowen Chen, Jia Li, Kenji Watanabe, Takashi Taniguchi, Cory Dean Electrons in two dimensions can exhibit unique correlated phenomena such as the fractional quantum Hall effect under strong magnetic fields, in which excitations of composite fermions are fractionally charged. These excitations are predicted to be neither fermionic nor bosonic, but rather should exhibit anyonic exchange statistics in which the wave function acquires a complex phase upon exchange of two identical quasiparticles. However, so far there has been no direct confirmation of these anyonic exchange statistics due to numerous challenges performing the necessary braiding operations in high-mobility 2DEGs. Here, we develop Mach-Zehnder and Fabry-Pérot interferometers in ultra-high quality graphene devices operated in the fractional quantum Hall regime in order to test these predictions. Our efforts to achieve this utilize a variety of device geometries, primarily focused on the use of pn junctions or electrostatically-defined quantum point contacts in graphene. |
Monday, March 5, 2018 11:39AM - 11:51AM |
B40.00003: Progress towards probing quantum Hall edge states in Graphene Heterostructures Haoxin Zhou, Alexander Zibrov, Eric Spanton, Takashi Taniguchi, Kenji Watanabe, Andrea Young Capacitance measurements reveal graphite gated, hexagonal boron nitride encapsulated graphene to be an extremely clean two dimensional electron system. In these devices, well-developed incompressible fractional quantum Hall peaks are evident down to magnetic fields as small as 1 Tesla, with a variety of even denominator fractional quantum hall states appearing at high magnetic fields. However, these states appear less accessible in multiterminal transport measurements due to fringe field effects, etch-induced edge disorder, contact resistances and other unknown issues. This makes mesoscopic physics experiments that rely on edge properties--such as edge state interferometry--an experimental challenge. I will give a progress report on our efforts to probe edge states with transport measurements, including efforts to engineer all-electrostatically defined devices. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B40.00004: Direct Imaging of Current and Dissipation of Quantum Hall Edge States in Graphene with Scanning Nanoscale SQUID-on-Tip Microscopy Marec Serlin, Charles Tschirhart, Avi Shragai, Jiacheng Zhu, Martin Huber, Andrea Young When a strong out of plane magnetic field is applied to a two-dimensional electron system, the electrons in the bulk are localized, while the electrons near the edge remain conductive as the energy bands bend and cross the Fermi level. These conductive edge states are theoretically immune to back scattering and give rise to quantized Hall conductivity, the hallmark of the quantum Hall effect. Here we explore the utility of scanning nanoscale SQUID on tip (nSOT) microscopy for investigating edge state physics, focusing on the current- or doping-induced breakdown of quantization in the integer quantum Hall effect. nSOT microscopy provides direct, non-invasive, nanoscale resolution spatial imaging of current and temperature in ambient magnetic fields up to 2T, matching the regime of integer quantum Hall effects in high mobility graphite-gated graphene heterostructures. I will discuss edge state signal estimates and preliminary measurements made with our newly built 4.2 Kelvin scanning nSOT microscope. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B40.00005: Interplay of Andreev physics and quantum Hall effect in graphene Tibor Sekera, Rakesh Tiwari, Christoph Bruder The interplay of Andreev physics and the quantum Hall effect has recently attracted a lot of interest and gained experimental relevance. A quantizing out-of-plane magnetic field in the normal region (N) leads to propagating states confined along the interface with the superconducting region (S). If N is a normal metal, there are magnetoconductance oscillations that can be understood as a result of an interference between the two interface states with wave vectors ±k, where k≈π/2l_{c} and l_{c}=2\hbar k/eB can be semiclassically interpreted as a cyclotron diameter. When N is graphene which is characterized by a Dirac dispersion, the conductance was found to be independent of the magnetic field [1]. This is confirmed in our numerical simulations in a certain range of parameters. Outside this range we find magnetoconductance oscillations if the pair potential Δ is larger than the Fermi energy E_{F} (measured from the Dirac point) and (or) the penetration depth of the magnetic field λ_{B} is larger than the graphene lattice constant a. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B40.00006: Directly Probing Quantum Hall Physics in Graphene with Tunneling Transistors John Davenport, Eberth Quezada, Junyan Liu, Takashi Taniguchi, Kenji Watanabe, Arthur Ramirez, Jairo Velasco Jr. Scanning tunneling spectroscopy (STS) is a conventional method used to directly probe the electronic structure and phonon spectrum of graphene and its multilayers. However, STS measurements require extreme mechanical stability, and thus are difficult to incorporate into experimental setups with ultra-high magnetic fields and extremely low temperatures. We report an alternative method that employs tunneling field effect transistors to directly probe the electronic and phononic spectrum of graphene and its multilayers. These transistors are composed of 2D materials that are stacked into ultra-clean heterostructures and can function in magnetic fields and temperatures that are difficult or inaccessible to STS. We will discuss the latest progress towards using these nanodevices to measure and manipulate quantum hall states and phonons in graphene and its multilayers. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B40.00007: Observation of even denominator fractional quantum Hall states in monolayer graphene Alexander Zibrov, Eric Spanton, Haoxin Zhou, Carlos Kometter, Takashi Taniguchi, Kenji Watanabe, Andrea Young We report magneto-capacitance measurements of high quality monolayer graphene encapsulated in boron-nitride (hBN) and few-layer graphite. In addition to the usual odd-denominator hierarchy states at partial fillings ν=p/(2p±1), we observe even-denominator ν=±½ and ±¼ states, which appear only for a narrow range of magnetic fields. Similar behavior is observed in 3 different samples, but at dramatically different B: in two of the samples, the even-denominator states appear at B≈28 T, while in the third sample they appear at B≈5.5 T. We trace the difference to the measured zero-magnetic field sublattice splitting, which arises due to the interaction with the hBN subtrate. The ν=±½,¼ FQH states are accompanied by a series of phase transitions and crossovers in the neighboring odd-denominator states, indicative of an underlying isospin phase transition. Mean field analysis suggests this transition is between a low-field valley ordered Kekule phase and a high field canted antiferromagnetic phase. The observed behavior cannot be explained within the composite fermion picture, suggesting a possible intermediate phase between the Kekule distorted phase and the antiferromagnet. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B40.00008: Trigonal warping and even denominator fractional quantum Hall effect in trilayer graphene Carlos Kometter, Alexander Zibrov, Eric Spanton, Jia Li, Takashi Taniguchi, Kenji Watanabe, Maksym Serbyn, Andrea Young We report magneto-capacitance measurements of high quality trilayer graphene devices fabricated by encapsulating the trilayer in both boron nitride and few-layer-thick graphite. The resulting devices are exceptionally low-disorder. We resolve Landau levels at magnetic fields well below 1 T and odd-denominator fractional quantum Hall (FQH) states at B = 6 T. At high displacement fields and low magnetic fields, we observe the emergence of three-fold quasi-degenerate Landau levels, signifying the emergence of new massless Dirac points. At high magnetic fields, we observe numerous fractional quantum Hall states between filling factors ν = -6 and +6. In addition to the expected hierarchy of odd-denominator states, we also observe even denominator states at ν =9/2 (at B = 30 T) and 9/2 and 11/2 (at B = 40 T). |
Monday, March 5, 2018 12:51PM - 1:03PM |
B40.00009: ν=0 Quantum Hall state in a finite graphene sheet and at finite temperature Malcolm Kennett, Hank Chen, Sujit Narayanan, Matthew Fitzpatrick, Bitan Roy The quantum Hall state at ν=0 in graphene arises due to electronic interactions, giving rise to ordered states via the mechanism of magnetic catalysis. We consider the situation in which the ordered state is a canted antiferromagnet (AFM), supporting easy-plane AFM accompanied by an easy-axis ferromagnetic moment. Accounting for both strong Landau level mixing and finite size effects, we demonstrate that the canted-AFM undergoes a continuous quantum phase transition to a ferromagnetic state in the presence of a strong tilted magnetic field. We use parameters (such as interaction strengths) consistent with measurements of the bulk gap in a perpendicular magnetic field to study the edge states and establish a semi-quantitative agreement with recent experimental observations of such transition. In addition, we also include the effects of thermal fluctuations for the order parameters in the canted-AFM phase and proprose scalings for the transition temperature of the quantum Hall ordered phases at fillings ν=0 and ν=1, which can directly be verified in experiments. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B40.00010: Exciton Condensate Stability in Double Bilayer Graphene in the Quantum Hall Regime Ming Xie, Allan MacDonald Semiconductor double-layers in the quantum Hall regime favor exciton condensate ground state when the two layers are brought into close proximity and the total Landau level filling factor is close to an integer. The exciton condensate state has spontaneous interlayer phase coherence, and is easily detected by drastic change in the drag resistance. Colossal drag has been observed in conventional semiconductor quantum wells at total filling factor 1, and in bilayer graphene with partially occupied N=0 Landau levels. Here we consider the dependence of stability of the exciton condensate state on the orbital degree of freedom that exists within bilayer graphene’s N=0 Landau level, which contains separate Landau levels with wavefunctions like those of parabolic bands with n=0 and 1. The phase diagram in this case is enriched by the competition between orbital order within each layer and interlayer phase coherence. By studying the stability of the exciton condensate relative to fluctuations that break translational invariance in different ways, we explain the absence of exciton condensate except when both bilayers have n=0 and address behavior near n=0/1 boundaries of individual layers. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B40.00011: Formation of the n = 0 Landau level in hybrid graphene Paul Cadden-Zimansky, Min Kyung Shinn, Gavin Myers, Matt Dalrymple, Henry Travaglini The minimum of 4-terminal conductance occurring at its charge neutral point has proven to be a robust empirical feature of graphene, persisting with changes to temperature, applied magnetic field, substrate, and layer thickness, though the theoretical mechanisms involved in transport about this point – vanishing density of states, conventional band gap opening, and broken symmetry quantum Hall mobility gaps, and hydrodynamic flow – vary widely depending on the regime. Here we report on observations of a regime where the 4-terminal conductance minimum ceases to exist: transport in monolayer graphene connected to bilayer graphene during the onset of the quantum Hall effect. As monolayer and bilayer graphene have distinct zero-energy Landau levels that form about the charge neutral point, our observations suggest that competitions between the differing many-body orderings of these states as they emerge may underlie this anomalous conductance. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B40.00012: Long-Distance Spin Transport Through a Graphene Quantum Hall Antiferromagnet Petr Stepanov, Shi Che, Dmitry Shcherbakov, Jiawei Yang, Kevin Thilahar, Greyson Voigt, Marc Bockrath, Dmitry Smirnov, Kenji Watanabe, Takashi Taniguchi, Roger Lake, Yafis Barlas, Allan MacDonald, Chun Ning Lau An important goal in spintronics is to establish mechanisms that minimize dissipation in the devices that are to exhibit the action of spin currents. In magnetic insulators the easy plane ordered spin currents could be carried dissipationless in the form of spin-supercurrents. Spin superfluidity transport has been theoretically predicted in a graphene quantum Hall insulator [1]. Here we report on the first experimental demonstration of the robust spin-current transport through graphene anti-ferromagnet insulator (AFMI) in the quantum Hall regime. The charge neutrality point (CNP) forms canted anti-ferromagnet (CAF) in the ground state that effectively serves as AFMI for spin currents propagation. By utilizing quantum Hall (QH) edge modes as injector, filters and detector we find large non-local signal across 5-μm long graphene CAF region. Our work demonstrates a long-distance spin transport through AFMI in graphene and shows that QH states can serve as a powerful tool for fundamental studies of ferromagnet and anti-ferromagnet spintronics. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B40.00013: Electrostatically Gate-defined Structures in Graphene Shaowen Chen, Rebeca Ribeiro-Palau, Jia Li, Matthew Yankowitz, Takashi Taniguchi, Kenji Watanabe, James Hone, Cory Dean Graphene, a tunable 2D material, can support novel fractional quantum hall (FQH) states with large energy gaps, which makes it an emerging platform to study fractional or even non-Abelian statistics with quantum Hall interferometer. However, electrostatic confinement, which is required to fine-tune interference area, is elusive in graphene since it is a gapless semiconductor. Moreover, experience from GaAs/GaAlAs quantum well system indicates electrostatic gating may degrade device quality. Here we present hBN-encapsulated monolayer and bilayer graphene gate defined devices with graphite gates on both sides, without sacrificing FQH quality. With electrostatic gating, we define the active device region by putting the off-regions at nu=0, which has energy gap both in bulk and edge. We observed in transport, FQH states at magnetic field as low as 6 T with enhanced energy gaps, the presence of four-flux states and reentrant integer quantum Hall states which arise from electron solid phases. Our result paves the way for electrostatic constrictions under the FQH regime in graphene. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B40.00014: Corbino geometry as a bulk probe in ultra-high mobility graphene. Yihang Zeng, Jia Li, Olivia Ghosh, Takashi Taniguchi, Kenji Watanabe, James Hone, Cory Dean In the quantum Hall regime, transport measurement of the edge states in a Hall bar geometry give an indirect signature of the bulk incompressible states, making the edge-bulk interplay a subject of continued importance. The Corbino geometry has the advantage of having no edges therefore providing a direct probe of bulk property. We report recent progress on designing and fabricating high quality graphene device with dual graphite gate, in a Corbino geometry. In addition to the usual fractional quantum Hall states associated with the 2-flux composite fermion sequence, we also resolve for the first time in graphene both 4-flux and 6-flux states , demonstrating the excellent quality of the Corbino devices. The dual gated geometry provides high charge carrier density, allowing us to access N=3 Landau level at magnetic field B~ 30 T. Here we observed features in conductance resembling reentrant integer quantum Hall effect for the first time in the N=3 Landau level, at filling fraction of around 10+1/4 and 10+3/4. Additionally, sdH measurements are performed to measure quantum life time in the low field limit, as a characterization of bulk disorder, and the result of such measurement is compared to Hall bar geometry. |
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