Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H14: Graphene: MagnetotransportFocus
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Sponsoring Units: DCMP Chair: Erik Henriksen, Washington Univ Room: BCEC 153C |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H14.00001: Shape of the zeroth Landau level in graphene with non-diagonal disorder Rajesh Malla, Mikhail Raikh Non-diagonal (bond) disorder in graphene broadens Landau levels (LLs) in the same way as random potential. The exception is the zeroth LL, n=0, which is robust to the bond disorder, since it does not mix different n = 0 states within a given valley. The mechanism of broadening of the n = 0 LL is the inter-valley scattering. Several numerical simulations of graphene with bond disorder had established that n = 0 LL is not only anomalously narrow but also that its shape is very peculiar with three maxima, one at zero energy, E = 0, and two others at finite energies ± E. We study theoretically the structure of the states in n = 0 LL in the presence of bond disorder. Adopting the assumption that the bond disorder is strongly anisotropic, namely, that one type of bonds is perturbed much stronger than other two, allowed us to get an analytic expression for the density of states which agrees with numerical simulations remarkably well. On the qualitative level, our key finding is that the delocalization of E = 0 state has a dramatic back effect on the density of states near E = 0. The origin of this unusual behavior is the strong correlation of eigenstates in different valleys. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H14.00002: ABSTRACT WITHDRAWN
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Tuesday, March 5, 2019 2:54PM - 3:06PM |
H14.00003: The role of edge effects in magnetotransport measurements of graphene hall bars Olivia Ghosh, Yihang Zeng, J.I.A. Li, Kenji Watanabe, Takashi Taniguchi, Cory Dean The development of the Van der Waals (VdW) assembly technique has led to greatly improved bulk quality of graphene devices. However, little is known about the quality of the sample edge. Here we investigate how the nature of the sample edge affects transport measurements, particularly for the case of the quantum Hall effect regime where the chiral edge modes are presumed to be topologically protected and therefore insensitive to details of the edge. We compare in a single device the transport response along electrostatically-defined versus naturally occurring boundaries, and discuss possible implications for the measurement of boundary edge modes more generally. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H14.00004: Magnetotransport in graphene 30-degree quasicrystal Rebecca Engelke, Hyobin Yoo, William Weiter, Philip Kim Introducing a twist angle between two layers of graphene realizes a variety of new lattice configurations with different electronic structures. A 30-degree twist angle creates a quasicrystal (QC) structure with 12-fold symmetry [1]. We fabricate a graphene QC by tearing apart and stacking a single crystal of monolayer graphene. The sample is encapsulated in hBN and studied under transmission electron microscopy (TEM) to determine the precise twisting angle. We find the angle of the twisted heterostructure to be within 0.2 degrees of 30. We fabricate multi-terminal contacted devices with top and bottom gates. We will discuss magnetotransport data on the graphene QC in the extreme quantum limit as function of density and displacement fields. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H14.00005: Novel two-component ground state at 1/3 + 1/3 filling in strongly coupled graphene double-layer J.I.A. Li, Yihang Zeng, Qianhui Shi, Olivia Ghosh, Kenji Watanabe, Takashi Taniguchi, James Hone, Cory Dean In the composite fermion (CF) picture, fractional quantum Hall effect (FQHE) states can be reinterpreted as effective integer quantum Hall effect (IQHE) for CFs. This provides a natural framework for interpreting the relative hierarchy of observable FQHE states, and leads to the prediction and observation of a Fermi surface of CFs at half filled LL. It is recently demonstrated that quantum Hall ground states in a strongly coupled graphene double-layer could be described by an expanded CF picture with two-component correlation that accounts for both interlayer and intralyer interaction. Apart from a novel sequence of two-component FQHE states that emerge as effective IQHE of CFs, this new construction implies a Fermi sea of CFs at 1/3 filling in each graphene layer, which could host incompressible ground state resulting from pairing instability. Here we report experimental observation of incompressible state 1/3 + 1/3 filling. Multiple phase transitions are observed as a function of magnetic field and density imbalance, and possible ground state order will be discussed. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H14.00006: Extraordinary magnetoresistance in encapsulated monolayer graphene Bowen Zhou, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen We report a study on the phenomenon of extraordinary magnetoresistance (EMR) in boron nitride encapsulated monolayer graphene devices. Each device is circular, with an internal circular metal shunt made by edge contact to the graphene. Extremely large EMR values--calculated as (R(B) - R0) / R0, can be found in these devices due to the vanishingly small resistance values at zero field. In many devices the zero-field resistance can become negative, likely due to ballistic carrier transport. This enables R0 to be chosen arbitrarily close to zero depending only on measurement precision, resulting in very large EMR; conversely, small values of R0 can have large uncertainty in short term measurements, leading to very large error bars on the EMR. We critically discuss the dependence of EMR on measurement precision and device asymmetry. Finally, the gate-voltage-dependent resistance at zero field also shows a strong electron-hole asymmetry, which we trace to the nature of the metal-graphene edge contact: as in the well-studied case of metals deposited on graphene, the graphene at one-dimensional edge contacts also appears to be heavily electron-doped ~100s of nm away from the contact. We also report the effects of the sizes of the devices and the ratios of metal shunt to graphene on EMR. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H14.00007: A different Landau level energy diagram in bilayer graphene proximitized by WSe2 Ya-Wen Chuang, Jing Li, Hailong Fu, Kenji Watanabe, Takashi Taniguchi, Jun Zhu The eight Landau levels of bilayer graphene in a magnetic field near the charge neutrality point is a rich playground to explore the effect of competing many-body interactions. Studies have uncovered numerous level crossings that correspond to the change of the ground state ordering. Previous work from our group has established an empirical and quantitative energy diagram[1]. Here we report measurements on the Landau level energy gaps of bilayer graphene adjacent to 1-2 layer WSe2 through van der Waals stacking. In comparison to pristine samples, a significant enhancement of the orbital splitting between the N=0 and N=1 levels was observed. The coincidence point of the ν=0 gap splits at field as low as 4T. Meanwhile, the magnitude of the large ν =2 gap was reduced to roughly half of its value in pristine bilayer. Using thermally activated transport and a pair of top and bottom gates, we performed a set of gap measurements at ν =1, 2, 3 as a function of the carrier density and the applied perpendicular electric field D. We analyze the dependences, compare to the results in pristine bilayer and discuss the impact of WSe2. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H14.00008: Observation of Cyclotron Resonance in Dual-Gated Bilayer Graphene Jordan Russell, Jordan Pack, Yashika Kapoor, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen We present measurements of the cyclotron resonance in boron nitride encapsulated, dual-gated bilayer graphene by way of infrared transmission magnetospectroscopy. We have observed both intra- and inter-band resonances, and study these as a function of the Landau level filling factor, ν ∝ n/B, for zero electric displacement field, D, and constant magnetic field, B. The magnetic field dependence and effects of nonzero displacement field on the Landau level resonances will also be discussed. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H14.00009: Graphene Quantum Point Contact for Quantum Hall Interferometry Thomas Werkmeister, Yuval Ronen, Si Young Lee, Young Jae Shin, Danial Haei Najafabadi, Kenji Watanabe, Takashi Taniguchi, Philip Kim Anyons are particles which possess exchange properties beyond the typical bosonic and fermionic quantum statistics. Several varieties of anyons are expected to emerge as quasi-particles in the regime of the fractional quantum Hall effect (FQHE) where strong magnetic fields are applied on a 2-dimensional electron gas with strong electron-electron interactions. Traditionally, experiments in GaAs heterostructures have attempted to demonstrate anyonic exchange properties via quantum Hall interferometry, but results remain inconclusive. Recently, graphene based heterostructures have emerged as an alternative platform for studying these exotic anyonic quasi-particles within several robust FQHE phases that show larger gaps, particularly in even-denominator states. Here we present preliminary results utilizing hBN-encapsulated monolayer graphene with top and bottom graphite gates to electrostatically define interferometers in the FQHE regime. We will present a quantum point contact (QPC) that functions in the quantum Hall regime, a necessary ingredient for interferometers, and several devices that we characterize through transport measurements at cryogenic temperatures and strong magnetic field. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H14.00010: Aharonov-Bohm Phase and Valley Splitting in Strained Graphene P-N Junction Rabindra Nepal, Sanjay Prabhakar, Roderick Melnik, Alexey Kovalev Veselago lens focusing on a graphene P-N junction is promising for the realizations of electron-optical devices such as electron lenses and electron beam splitters. We study two effects in a strained graphene layer with P-N junction: the Lorentz force due to fictitious magnetic fields and Aharonov-Bohm (AB) phase. In a strained P-N junction graphene layer, the strain induced magnetic field modifies the electron trajectories. Furthermore, the Lorentz force on electrons engenders the valley-splitting of electron beams as well as the shifting of caustics formed by refracted electron waves. We also calculate a non-zero AB phase associated with the electron beams due to a localized in-plane strain produced by a ripple. This non-zero phase is accumulated from the electrons that avoid traversing through the locally strained region and experience a non-vanishing vector potential associated with the fictitious magnetic field induced by the localized strain. We believe that these effects can be realized in a simple ballistic experiment and might be useful for mapping strain profiles by analyzing the interference pattern observed in electron-optical devices. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H14.00011: Influence of a Gaussian nanobubble on quantum Hall conductance across a p-n junction in graphene Nojoon Myoung, Hee Chul Park Structural deformations in graphene can emerge during the fabrication processes. Although the nanobubbles in graphene has a capability to be applied in valleytronics and nano electromechanics, they also act as disorders causing inelastic scatterings in Dirac fermion transport. It is important to locate where the nanobubbles are formed in graphene surface not only for exploiting them to applications but also for avoiding them to acheive ballistic transport. Here, we report a theoretical investigation of influence of a Gaussian nanobubble on quantum Hall conductance with a p-n junction in graphene, showing strain-induced conductance oscillations as the position and strain strength of the Gaussian nanobubble vary. We reveal that the reported conductance oscillations stem from the rotation of valley isospin along the p-n junction interface. Due to the pseudo-magnetic field produced by the Gaussian nanobubble, there are localized states for given strain strengths, and Fano resonances can appear in the quantum Hall conductance as a consequence of coupline between the localized and extended states. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H14.00012: Splitting of conductance resonance through a magnetic quantum dot in graphene Nojoon Myoung, Jung-Wan Ryu, Hee Chul Park, Seung Joo Lee, Sungjong Woo We report a dual resonance feature in ballistic conductance through a quantum Hall graphene nanoribbon with a magnetic quantum dot. Such a magnetic quantum dot localizes Dirac fermions exhibiting anisotropic eigenenergy spectra with broken time-reversal symmetry. Interplay between the localized states and quantum Hall edge states is found to be two-fold, showing Breit-Wigner and Fano resonances, which is reminiscent of a double quantum dot system. By fitting the numerical results with the Fano-Breit-Wigner lineshape from the double quantum dot model, we demonstrate that the two-fold resonance is due to the valley mixing that comes from the coupling of the magnetic quantum dot with quantum Hall edge channels; an effective double quantum dot system emerges from a single magnetic quantum dot in virtue of the valley degree of freedom. It is further confirmed that the coupling is weaker for the Fano resonance and stronger for the Breit-Wigner resonace. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H14.00013: Giant pseudo-magnetic fields, valley polarization and topological channels by nanoscale strain engineering of monolayer graphene Chen-Chih Hsu, Jiaqing Wang, Marcus Teague, Nai-Chang Yeh We report the use of nearly strain-free PECVD-grown graphene [1] to induce controllable strain and pseudo-magnetic fields (Bs) by placing graphene on periodic nanostructure arrays [2]. We fabricated these arrays using focused ion beam and electron-beam lithography. These nanostructures were covered by a monolayer h-BN followed by a monolayer graphene, we found that graphene appeared to wrinkle up along the nanostructures. Each wrinkle results in four parallel channels of alternating positive and negative pseudo-magnetic fields, which are natural topological channels for valley-polarized propagation. Properly designed arrays of nanostructures could induce the desirable BS values and spatial distributions, which can function as a valley splitter to separate valley-unpolarized currents, or a valley propagator to guide valley-polarized currents. To enable valleytronic applications, we pattern strained graphene with these topological channels into Hall bar geometry to study the valley Hall effect. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H14.00014: Scaling and quantum phase transitions of disordered graphene in the quantum Hall regime. Tatiane Dos Santos, Leandro Lima, Caio Lewenkopf We investigate the magneto-transport properties of disordered graphene samples in the quantum Hall (QH) regime. Despite the huge attention that the QH effect in graphene has received in the past decade, there are still open questions about the effects of the disorder on the transition states between Hall plateaus, especially regarding scaling properties. Such transition states have been intensively studied in the context of two-dimensional electron gases and found to exhibit universal scaling behavior. The situation is less clear in graphene, which displays an anomalous quantum Hall effect as a consequence of the valley degeneracy. For a sufficiently strong disorder that causes valley mixing, theoretical works have predicted Landau level splittings and even a transition to the conventional quantum Hall regime. The latter behavior has not been experimentally reported. To elucidate this issue, we address the problem by calculating the longitudinal and Hall resistances of graphene systems up to 10^5 atoms in a Hall bar geometry via the Landauer-Büttiker formalism. We consider both scalar and chiral disorder with different disorder correlation lengths. Finally, we discuss how intervalley/intravalley scatterings affect the QH transition states in graphene. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H14.00015: Spin-relaxation in superconducting graphene Denis Kochan, Jaroslav Fabian 2D materials in the proximity to a superconductor are expected to host a |
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