Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M65: Fractional Quantum Hall Effect: ExperimentRecordings Available
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Sponsoring Units: DCMP Chair: James Nakamura, Purdue University Room: Hyatt Regency Hotel -Grant Park C |
Wednesday, March 16, 2022 8:00AM - 8:12AM |
M65.00001: Unconventional fractional quantum Hall states on the N=1 Landau level in bilayer graphene Ke Huang, Hailong Fu, Jun Zhu, Kenji Watanabe, Takashi Taniguchi In a magnetic field, strong electron-electron interaction in 2D leads to the celebrated fractional quantum Hall (FQH) effect. FQH states riding on the n=1 orbital Landau levels (LLs) are particularly fascinating. Even-denominator states occurring at half fillings have long been suspected to be non-Abelian in nature and carry excitations that are potentially useful in topological quantum computing. States with odd denominators such as 2/5 and 3/7 are also expected to be non-Abelian, with even more complex wave functions [1][2]. In prior work, we showed the appearance of a new even-denominator FQH state at nu = 5/2 in ultra-clean Bernal-stacked bilayer graphene and the spontaneously broken particle-hole symmetry at a number of even-denominator FQH states [3]. In this talk, we discuss findings at nu = 7/5, which exhibits three distinct ground states induced by a varying perpendicular electric displacement field D that moves the state from the N=0 to the N=1 LL of bilayer graphene. We measure the gap energy of all three ground states and construct a phase diagram of = 7/5 as a function of the D- and B-fields. Connections to theoretical predictions are made. |
Wednesday, March 16, 2022 8:12AM - 8:24AM |
M65.00002: Interferometry measurements on candidate non-Abelian states and their nearby Abelian states Robert L Willett, Kirill Shtengel, Loren N Pfeiffer, Edwin Y Chung, Kirk Baldwin, Milton Peabody An extensive series of interferometry measurements using illuminated, high density, ultra-high mobility heterostructures have shown results consistent with proposed non-Abelian properties at filling factors 5/2 and 7/2. These results expose features that correspond to non-Abelian e/4 and Abelian e/2 charges, specifically in B-field sweeps even-odd effect for e/4 braiding e/4 and expected accumulation of both Aharonov-Bohm and statistical phases for braiding of e/2 charges. Here we will summarize measurement results at both Abelian and proposed non-Abelian states using 3 methods to alter the interferometers enclosed quasiparticle (qp) number: 1) B-field sweeps which systematically change both enclosed qp and flux number, 2) side-gate sweeps which differently change both enclosed qp and flux number, 3) small interior central interferometer gate sweeps which alter the enclosed qp number alone. We show here results at Abelian states 7/3 and 16/5, expressing their Aharonov-Bohm and statistical phase contributions to interference. 7/2 and 5/2 results in the three measurements are presented with their correspondence to the non-Abelian even-odd model. Overview of interference from filling factors 2 to 3 and 3 to 4 will be shown. |
Wednesday, March 16, 2022 8:24AM - 8:36AM |
M65.00003: Experimental study of bulk-edge coupling in quantum Hall interferometers and the impact on detection of anyonic statistics James R Nakamura, Shuang Liang, Geoff Gardner, Michael J Manfra Interferometers have been used in the quantum Hall regime to probe fractional charge and anyonic statistics of quasiparticles. Theoretical analysis indicates that coupling parameters between charge in the bulk and at the edge of the interferometer are crucial in determining interference behavior in integer and fractional quantum Hall states. We discuss measurements of an interferometer with significant bulk-edge coupling in which the relevant parameters can be determined experimentally, and we compare results to theory. In the integer quantum Hall regime, the device exhibits Aharonov-Bohm regime interference at ν = 1 but Coulomb dominated interference at ν = 3, and the difference in behavior can be accounted for by the experimentally extracted values of the electrostatic coupling parameters KI and KIL. At the ν = 1/3 state, the device exhibits Aharonov-Bohm interference near the center of the conductance plateau with a few discrete jumps in phase, while at high and low fields oscillations with period Ф0 occur, consistent with signatures of anyonic statistics when the bulk is incompressible and when it is compressible. The values of KI and KIL can account for the deviation of the discrete phase jumps from the ideal theoretical value of 2π/3. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M65.00004: Fractional Interference in a Mach-Zehnder Interferometer HEMANTA K KUNDU, Sourav Biswas, Nissim Ofek, Vladimir Umansky, Moty Heiblum Interference of electrons and anyons in the quantum Hall regime have always been of fundamental interest. Such measurements allow studying the quantum statistics of quasiparticles. With recent observations of fractional interference in a Fabry-Perot interferometer, fractional interference was never observed in a Mach-Zehnder interferometer (MZI). The MZI is a true ‘two-path’ interferometer and is free of Coulomb interactions, responding solely to Aharonov-Bohm (AB) phase. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M65.00005: Assessment of quantum point contact stability in GaAs/AlGaAs heterostructures with trench gates Shuang Liang, James R Nakamura, Geoffrey C Gardner, Michael J Manfra GaAs/AlGaAs heterostructures designed to probe fragile quantum Hall states in the 2nd Landau level typically suffer from poor gate stability. We study the stability and edge mode transmission of quantum point contacts utilizing etched trenches and low temperature illumination. We observe that devices etched past the doping layer above the quantum well show low hysteresis and conductance drift after an initial burn-in sweep of the gate voltage. Nevertheless, the electron density within the constriction is typically reduced from the bulk value after the burn-in process. We report on fabrication and gate operation techniques to reduce this density difference. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M65.00006: Identification of an Isolated Non-Abelian Channel Bivas Dutta, Vladimir Umansky, Mitali Banerjee, Moty Heiblum Possiblities of long range entaglement in non-abelian anyons make them one of the most sought after in the field of condensed matter physics. This particular quality also brought them in the front line as prospective candidates for fault-tolerant topological quantum computation. Interestingly, what makes these quasiparticles elusive to most conventional measurement techniques is their charge-neutral characteristics. A proposed host of such quasiparticles is the ν=5/2 quantum Hall effect state. Their gapless edge modes are ideal candidates for braiding experiments, which can reveal state’s topological order and its robustness to decoherence. Since the ν=5/2 state hosts a variety of edge modes (integer, fractional, neutral), a robust technique is needed to isolate the non-abelian modes while assuring that their original character remain intact. Recent thermal conductance measurements of the ν=5/2 state already showed a fractional character [1], compared to an integer of abelian states. However, the nature of the isolated ν=1/2 mode must be demonstrated. In this work we exploit a novel technique to gap-out the integer modes of the ν=5/2 state by interfacing it with integer states ν=2 & ν=3 [2]. We observed a thermal conductance of 0.5κ0T(with κ0=π2kB2/3h a universal constant), assuring the non-abelian nature of the ν=1/2 channel and its Particle-Hole Pffafian topological order [3]. Our result opens new avenue to manipulate and test other QHE states, as well as braid, via interference of the isolated exotic modes. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M65.00007: Observation of ballistic upstream modes at fractional quantum Hall edges of graphene Ravi Kumar, Saurabh K Srivastav, Christian Spanslatt, Kenji Watanabe, Takashi Taniguchi, Yuval Gefen, Alexander D Mirlin, Anindya Das The structure of edge modes at the boundary of quantum Hall (QH) phases forms the basis for understanding low energy transport properties. In particular, the presence of upstream modes, moving against the direction of charge current flow, is critical for the emergence of renormalized modes with exotic quantum statistics. Here we discuss noise measurements performed at the edges of fractional QH (FQH) phases realized in dual graphite-gated bilayer graphene devices. A noiseless dc current is injected at one of the edge contacts, and the noise generated at contacts at L=4µm or 10µm away along the upstream direction is studied. For integer and particle-like FQH states, no detectable noise is measured. By contrast, for hole-conjugate FQH states, we detect a strong noise proportional to the injected current, unambiguously proving the existence of upstream modes. The noise magnitude remaining independent of length together with a remarkable agreement with our theoretical analysis demonstrates the ballistic nature of upstream energy transport, quite distinct from the diffusive propagation reported earlier in GaAs-based systems. Our investigation opens the door to the study of upstream transport in more complex geometries and in edges of non-Abelian phases in graphene. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M65.00008: Thermally non-equilibrated to topologically equilibrated thermal conductance for hole-conjugate fractional quantum Hall state in graphene Saurabh K Srivastav, Ravi Kumar, Kenji Watanabe, Takashi Taniguchi, Anindya Das Transport in integer and particle-like fractional quantum Hall (FQH) state, occurs via downstream edge modes (Nd). On the contrary, for hole-conjugate FQH states, the presence of upstream modes (Nu) leads to complex transport behavior. Here, we report the thermal conductance (GQ) measurement of integer and FQH states in graphene. We found the measured values of GQ for integer and particle-like states (1/3 and 2/5) are consistent with theoretical predictions of Nd κ0T, where κ0T is the quanta of GQ. Surprisingly, for the hole-conjugate states (2/3 and 3/5), the measured GQ shows (Nd + Nu) κ0T below 35 mK, consistent with vanishing thermal equilibration. However, with increasing temperature GQ decreases and eventually saturate to theoretically predicted topological value of (Nd - Nu) κ0T for 3/5 above 50mK and for 2/3, GQ remains finite (~0.4 κ0T) at 60mK. Our results for the first time benchmark the temperature induced transition of thermal conductance from no-equilibration to full-equilibration for FQH states in graphene. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M65.00009: Domain textures in the fractional quantum Hall effect Ziyu Liu, Ursula Wurstbauer, Lingjie Du, Kenneth W West, Loren N Pfeiffer, Michael J Manfra, Aron Pinczuk Domain textures in the disordered bulk of fractional quantum Hall effect (FQHE) systems are probed by resonant inelastic light scattering. Domains with different characteristic sizes are found to have distinct impacts of breakdown of wave vector conservation on low-lying neutral excitations. We demonstrate that large domains of quantum fluids in the bulk support long-wavelength neutral collective excitations with well-defined wave vector (momentum) dispersion that could be interpreted by theories for uniform phases [1]. At filling factor v = 1/3 we consider interpretations based on graviton excitations predicted by geometrical theories [2,3] and earlier interpretations based on two-roton bound states [4,5]. The results offer significant experimental access to strong electron correlation physics in the FQHE. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M65.00010: Anomalous quantized plateaus in two-dimensional electron gas with gate confinement Jiaojie Yan, Yijia Wu, Shuai Yuan, Loren N Pfeiffer, Kenneth W West, Hailong Fu, X.C. Xie, Xi Lin The even denominator 3/2 fractional quantum Hall plateau has been reported in a regular two-dimensional electron gas with gate confinement at a bulk filling factor of 5/3 [1]. Besides the 3/2 plateau, a series of unexpected plateaus with similar gate confinements are observed in our experiments at different bulk filling factors. All of these plateaus are quantized to a fraction of quantum resistance (h/e2) within 0.02% to 0.05%. The appearance of these anomalous plateaus and their relationships to bulk filling factors will be discussed in this talk. |
Wednesday, March 16, 2022 10:00AM - 10:12AM |
M65.00011: Phase competition of the ν=9/5 fractional quantum Hall state and the reentrant integer quantum Hall Wigner solid Sean A Myers, Vidhi Shingla, Loren N Pfeiffer, Kirk Baldwin, Gabor A Csathy In the two-dimensional electron gas fractional quantum Hall states and electron solids often compete. We recently observed such a competition near the Landau level filling factor ν=9/5. In this area we observed a Wigner-solid-like phase at a new filling factor ν=1.79. Because of the reentrant nature of the transport, we call this phase the reentrant integer quantum Hall Wigner solid. This Wigner solid develops in a range of Landau level filling factors that is related by particle-hole symmetry to the so-called reentrant Wigner solid that forms near ν=1/5. Thus, our finding of the integer quantum Hall Wigner solid underscores the pervasiveness of particle-hole symmetry in quantum Hall systems and further illustrates the delicate interplay of topological physics and strong Coulomb interactions. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M65.00012: Correlated-electron phases in the very low Landau level limit near ν = 1/7 Edwin Y Chung, David E Graf, Lloyd W Engel, Kevin A Villegas Rosales, Kirk Baldwin, Pranav Thekke Madathil, Kenneth W West, Loren N Pfeiffer, Mansour Shayegan A subject of long discussion in the condensed matter community is the ground state of clean two-dimensional electron systems (2DESs) in the very low Landau level limit (ν < 1/6.5). While theory suggests a transition from a series of fractional quantum Hall states (FQHSs) to a Wigner solid at some point, the exact filling at which this occurs is still controversial. Experimental studies on this regime are still rather limited because they require 2DESs with exceptional quality. Following the recent breakthrough in ultra-high-quality GaAs 2DESs [1], we revisit this problem by systematically probing correlated-electron phases near ν = 1/7 using magnetotransport measurements. In our magnetoresistance (Rxx) data, we find deep minima developing at ν = 1/7 and 2/13 in the midst of an insulating background, strongly suggesting the existence of FQHSs flanked by Wigner solid phases at these fillings. We also evaluate the magnetic field (B) dependent activation energy (EA) values we obtain from the relation Rxx ~ exp(EA/2kT), where k is the Boltzmann constant and T is the temperature. Similar to the Rxx data, minima occur at fillings ν = 1/7 and 2/13 in the B vs. EA plots, further corroborating our view. |
Wednesday, March 16, 2022 10:24AM - 10:36AM |
M65.00013: Nonlinear I-V characteristics and noise in transport in the Wigner solid regime Pranav Thekke Madathil, Kevin A Villegas Rosales, Chengyu Wang, Edwin Y Chung, Loren N Pfeiffer, Kirk Baldwin, Kenneth W West, Mansour Shayegan Two dimensional electrons in a perpendicular magnetic field at very low temperatures and Landau level fillings (v) crystallize into a Wigner solid which is typically pinned by the disorder potential. A sufficiently strong external electric field/current de-pins the solid and causes it to slide. This is reflected in the non-linear behavior of the differential resistance (dV/dI) as a function of current (Idc),1 along with an onset of noise and an oscillatory current whose frequency depends on Idc.2 We report non-linear I-V and noise measurements at small fillings (v < 0.2) in an ultra-low-disorder 2D electron system confined to a GaAs quantum well with a density of 2.83x1010 cm-2 and an extremely high mobility of 15x106 cm2/Vs.3 We observe complex, non-monotonic, oscillatory features in dV/dI at the onset of the sliding regime, which disappears at higher temperatures. Narrow-band noise measurements in the sliding regime reveals the presence of a characteristic frequency that varies linearly with Idc but is ~1000 times smaller than the expected washboard frequency, suggesting filamentary conduction through the sample. |
Wednesday, March 16, 2022 10:36AM - 10:48AM |
M65.00014: Large many-body enhanced gap near the ν = 1 Landau level crossing in a dilute GaAs 2D hole system Chengyu Wang, Meng K Ma, Edwin Y Chung, Loren N Pfeiffer, Kenneth W West, Kirk Baldwin, Roland Winkler, Mansour Shayegan At Landau level (LL) filling factor ν = 1, the ground state of a low-disorder, 2D electron system (2DES) can be a quantum Hall ferromagnet (QHF), a fully-spin-polarized state with a large energy gap for charge excitations, even in the absence of single-particle Zeeman splitting. Many-body enhancement of the ν = 1 gap and signatures of Skyrmions have indeed been observed in GaAs 2DESs [1]. In GaAs 2D hole systems (2DHSs), there are LL crossings at finite magnetic fields due to the strong spin-orbit coupling, offering a viable platform to study QHF and Skyrmions at certain filling factors. Here we report a study of the ν = 1 quantum Hall state in a high-mobility, dilute 2DHS. We measure the ν = 1 energy gap near the crossing of the lowest two LLs through magneto-transport measurement. We observe large ν = 1 gaps, significantly enhanced over the values calculated in a single-particle picture over a wide range of density in the vicinity of the LL crossing. The gap remains large even at the LL crossing, indicating a QHF ground state, and scales as the square-root of magnetic field, further supporting its many-body origin. However, no signature of Skyrmions is observed. |
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