Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B46: Fractional Quantum Hall Effect II: Bubbles, Stripes, and MoreLive
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Sponsoring Units: DCMP Chair: Gabor Csathy, Purdue Univ |
Monday, March 15, 2021 11:30AM - 11:42AM Live |
B46.00001: Hidden quantum Hall stripes in AlxGa1-xAs/Al0.24Ga0.76As quantum wells Xiaojun Fu, Yi Huang, Boris Iona Shklovskii, Michael A Zudov, Geoff C Gardner, Michael Manfra We report on transport signatures of hidden quantum Hall stripe (hQHS) phases in high (N > 2) half-filled Landau levels of AlxGa1−xAs/Al0.24Ga0.76As quantum wells with varying Al mole fraction x < 10−3. Residing between the conventional stripe phases (lower N) and the isotropic liquid phases (higher N), where resistivity decreases as 1/N, these hQHS phases exhibit isotropic and N-independent resistivity. Using the experimental phase diagram we establish that the stripe phases are more robust than theoretically predicted, calling for improved theoretical treatment. We also show that, unlike conventional stripe phases, the hQHS phases do not occur in ultrahigh mobility GaAs quantum wells, but are likely to be found in other systems. |
Monday, March 15, 2021 11:42AM - 11:54AM Live |
B46.00002: Noise spectrum measurements in reentrant integer quantum Hall states Jian Sun, Jiasen Niu, Yifan Li, Yang Liu, Loren Pfeiffer, Ken W. West, Pengjie Wang, Xi Lin Two-dimensional electron gas (2DEG) system in strong magnetic field can host electron solid phases such as reentrant integer quantum Hall (RIQH) states in the second and higher Landau levels. Electron bubbles forming in RIQH states can be driven by an external electric field, providing a platform to study the dynamic collective behaviors. By performing noise spectrum measurements in a GaAs/AlGaAs 2DEG sample, we observed field-driven noise spectra in RIQH states including broad-band and narrow-band noise. Remarkably, unexpected dynamic ordering alternations of moving electron bubbles are revealed according to the noise patterns. |
Monday, March 15, 2021 11:54AM - 12:06PM Live |
B46.00003: Thermal and Quantum Melting Phase Diagrams for a Magnetic-Field-Induced Wigner Solid Meng Ma, K. A. Villegas-Rosales, Hao Deng, Edwin Yoonjang Chung, Loren Pfeiffer, Ken W. West, Kirk Baldwin, Roland Winkler, Mansour Shayegan In a strongly interacting two-dimensional (2D) electron system, when the Coulomb energy dominates over the kinetic energy, the electrons tend to arrange periodically and form a so-called Wigner solid (WS) ground state. In a GaAs 2D electron system, a magnetic-field-induced quantum WS forms at very low temperature and high magnetic field near filling factor v = 1/5 when the kinetic (Fermi) energy is quenched and the Coulomb energy dominates. In a dilute GaAs 2D hole system, on the other hand, the WS phase forms near v = 1/3 because of the significant Landau level mixing caused by the large hole effective mass. Here we report our measurements [1], using a newly developed technique which probes the melting of the WS via its screening efficiency, of the fundamental temperature vs. filling phase diagram for the 2D holes' WS-liquid thermal melting. Moreover, via changing the 2D holes' density, we also probe their Landau level mixing vs. filling WS-liquid quantum melting phase diagram. We find our data to be in good agreement with the results of very recent calculations [2]. |
Monday, March 15, 2021 12:06PM - 12:18PM Live |
B46.00004: Screening properties of bubble phases in excited Landau levels Kevin Villegas Rosales, Siddharth Kumar Singh, Hao Deng, Kirk Baldwin, Ken W. West, Loren Pfeiffer, Mansour Shayegan Bubble phases, electron solids that possibly contain more than one electron per site, form in very high quality (low-disorder) two-dimensional (2D) electron systems in the excited Landau levels. At the lowest temperatures, magneto-transport traces exhibit a vanishing longitudinal resistance at certain filling factors, implying insulating states associated with the bubble phases. As the temperature is raised, a peak in resistance is observed, which is interpreted to signal the melting of the bubble phase. Other techniques such as nonlinear transport, microwave spectroscopy, surface acoustic wave propagation, and thermo-power measurements support the picture of an electron solid. By employing a capacitance technique and using very high-quality GaAs 2D electron systems, we report an unexplored bulk property of the bubble phases, namely, their screening efficiency, measured as their response to a perpendicular electric field. We find that the bulk of the 2D electron system does not screen the electric field and behaves as an insulator at the lowest temperatures. As the temperature is raised, the screening reaches a local maximum close to the temperature at which the resistance shows a peak. These observations are consistent with the electron solid interpretation of the bubble phases. |
Monday, March 15, 2021 12:18PM - 12:30PM Live |
B46.00005: Spontaneous ferromagnetism in a dilute 2D electron system Md. Shafayat Hossain, Meng Ma, Kevin Villegas Rosales, Edwin Yoonjang Chung, Loren Pfeiffer, Ken W. West, Kirk Baldwin, Mansour Shayegan The ground states of an interacting, low-density electron system are of fundamental interest in the physics of strongly correlated electrons. Here we report experiments in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well, revealing a cascade of transitions as the density is lowered [1]. The large electron effective mass in this system allows us to reach very large values of the interaction parameter rs, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases: a paramagnetic metallic phase at large densities, a transition to an insulating state at rs = 27 that is stabilized by interaction and disorder, a spontaneous transition to a ferromagnetic state when rs surpasses 35, and then a phase with strongly non-linear current-voltage characteristics, suggestive of a pinned Wigner solid, when rs exceeds 38. Remarkably, with the application of a perpendicular magnetic field, we observe clear signatures of a fractional quantum Hall state at filling factor v = 1/3 at rs as large as 38. |
Monday, March 15, 2021 12:30PM - 12:42PM Live |
B46.00006: Record-quality two-dimensional electron systems Edwin Yoonjang Chung, K. A. Villegas-Rosales, Kirk Baldwin, P. T. Madathil, Ken W. West, Mansour Shayegan, Loren Pfeiffer We present our recent results on the growth of record-quality GaAs quantum wells via molecular beam epitaxy (MBE). These results were achieved by a careful purification and assessment of source material and vacuum quality, as well as innovation in MBE chamber design. Two-dimensional electron systems (2DESs) hosted in our new samples have a factor of ~2 higher mobility than the best previous 2DESs, with a peak mobility of electrons to μ = 44×106 cm2/Vs at an electron density of only n = 2×1011 /cm2. This is the highest mobility observed in any material. In representative samples with n ≈ 1×1011 /cm2, low temperature magnetotransport data display many-body states that have never been seen before. Furthermore, we find that the robustness of exotic interaction-driven states in these samples is unprecedented. For example, the ν = 5/2 fractional quantum Hall state shows an extraordinarily large activation gap of Δ = 820 mK, significantly larger than the previous record value of ~620 mK in samples with much larger electron densities of n ≈ 3×1011 /cm2. |
Monday, March 15, 2021 12:42PM - 12:54PM Live |
B46.00007: Dependence of the fractional quantum Hall effect energy gap on electron layer thickness Pranav Thekke Madathil, K. A. Villegas-Rosales, Edwin Yoonjang Chung, Loren Pfeiffer, Ken W. West, Kirk Baldwin, Mansour Shayegan Low-disorder, two-dimensional electron systems (2DESs) at large perpendicular magnetic fields provide an ideal platform for studies of exotic, correlated phenomena such as the fractional quantum Hall effect (FQHE). While the simplest theoretical models assume zero electron layer thickness, 2DESs confined to semiconductor heterostructures have finite layer thickness which results in weakened Coulomb interactions and reduced FQHE energy gaps (Δ) [1]. We systematically address the changes in Δ, for the FQHE at filling factor 1/3, as a function of electron layer thickness by studying 2DESs confined to GaAs quantum wells of varying width, ranging from 20 to 80 nm. All the 2DESs have a density of 1x1011cm-2. Experimentally, we obtain Δ from fitting the temperature dependence of the longitudinal resistance minimum in an Arrhenius plot. We present a Δ vs. layer thickness plot and contrast it to different theoretical calculations [2,3]. Even allowing for the reduction of the energy gaps by disorder, we find that the experimentally deduced gaps are smaller than the calculated values by over 10%, especially in the limit of very small layer thickness. |
Monday, March 15, 2021 12:54PM - 1:06PM Live |
B46.00008: Stability of Multielectron Bubble Phases in High Landau Levels Dohyung Ro, Sean Myers, Nianpei Deng, John Watson, Michael Manfra, Loren Pfeiffer, Ken W. West, Gabor Attila Csathy Reentrance to integer quantum Hall effects in two-dimensional electron gases is attributed to the formation of electronic bubbles, intriguing solid phases of electrons predicted by Hartree-Fock theories. Although most properties of the reentrant insulators are consistent with predictions for the bubble phases, none of the efforts could find the proliferation of bubble phases in high Landau levels, a missing puzzle of bubble interpretation. The recent observations of two different types of reentrance in the N=3 Landau level changed this and confirmed the possibility of such proliferation. Here, we investigate temperature evolution of magnetotransport of the reentrant states in the N=3 Landau level and discuss the stability of multielectron bubble phases in the N=2 and N=3 Landau levels. Our results considerably enhance the validity of bubble interpretation on reentrant integer quantum Hall states and enlighten the energetics of multielectron bubbles offering further understanding on the role of electron-electron interactions in bubble formation. |
Monday, March 15, 2021 1:06PM - 1:18PM Live |
B46.00009: Competition between fractional quantum Hall liquid and Wigner solid at small fillings: Role of layer thickness and Landau level mixing Siddharth Kumar Singh, K. A. Villegas-Rosales, Meng Ma, Md. Shafayat Hossain, Edwin Yoonjang Chung, Loren Pfeiffer, Ken W. West, Kirk Baldwin, Mansour Shayegan The Wigner solid, an ordered-array of charges, and the fractional quantum Hall liquid, an incompressible state, compete in very clean (low disorder) two-dimensional (2D) electron systems at small Landau level fillings. Here we report magneto-transport data for a new 2D |
Monday, March 15, 2021 1:18PM - 1:30PM Live |
B46.00010: Magnetotransport patterns of the nu=1 integer quantum Hall state in a high mobility two-dimensional electron gas Sean Myers, Haoyun Huang, Loren Pfeiffer, Ken W. West, Gabor Attila Csathy Through continual advancements in sample growth procedures, GaAs quantum wells host two-dimensional electron systems with extremely low disorder and remarkably high mobilities. In a sample belonging to the newest generation having record high mobilities, we observe a pattern of structures in the magnetotransport near the integer quantum Hall state at the filling factor nu = 1. This magnetoresistance pattern reveals electron localization effects beyond Anderson localization. The filling factor range displaying complex structure of magnetotransport overlaps with that of prior microwave absorption features of the Wigner solid in the flanks of integer quantum Hall plateaus. Moreover, the transport signatures are also consistent with the formation of the Wigner solid. Our measurements show that, contrary to the widely held belief, the Wigner solid in the flanks of integer quantum Hall plateaus does have signatures in dc transport in the cleanest electron gases and indicate the possibility of enriched physics in the latest generation of high mobility samples. |
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