Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session A19: Integer Quantum Hall Effect |
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Sponsoring Units: DCMP Room: Room 211 |
Monday, March 6, 2023 8:00AM - 8:12AM Author not Attending |
A19.00001: Electron phase jumps in a graphene quantum Hall interferometer Thomas R Werkmeister, Yuval Ronen, James Ehrets, Danial Haie Najafabadi, Kenji Watanabe, Takashi Taniguchi, Bertrand I Halperin, Amir Yacoby, Philip Kim We have developed a gate-defined graphene heterostructure to observe Fabry-Pérot interference of electrons propagating in quantum Hall edge channels [1]. Similar to recent experiments in GaAs quantum wells [2], the graphene heterostructure suppresses charging effects, revealing Aharonov-Bohm interference [1,3]. Moreover, the electron density in our graphene channel can be tuned electrostatically using local graphite gates. Here, we will discuss modulations of the Aharonov-Bohm phase appearing when multiple edge modes are present in the cavity. We observe periodic phase jumps in the Aharonov-Bohm oscillations of the interfering edge due to charge added to an adjacent mode. This observation is consistent with a new relation between the interference phase and the total electron charge contained in the interferometer [4]. The observation and proposed mechanism, arising from interaction between the spin-split Landau levels, may yield insight about anomalous reports of pairing and correlation between edge channels [5,6]. |
Monday, March 6, 2023 8:12AM - 8:24AM |
A19.00002: Shining magnon on quantum Hall ferromagnetic junctions Chun-Chia Chen, Lingfei Zhao, Ethan G Arnault, Trevyn Larson, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein The quantum Hall (QH) ferromagnetic state in graphene is a unique platform for investigating the properties of magnon excitations, which can be generated by separately biasing two QH edge states of opposite spin. In our work, we explore the magnon interaction with a QH ferromagnet junction. The state of the junction can be controlled by non-local magnon excitations created elsewhere in the sample. Our results may assist in developing the evolving understanding of the ground states and excitations in this broken SU(4) symmetry system, as well as enable the future development of spin-based devices in 2D materials. |
Monday, March 6, 2023 8:24AM - 8:36AM |
A19.00003: Confinement-induced chiral edge channel interaction in quantum anomalous Hall insulators Ruobing Mei, Lingjie Zhou, Yi-Fan Zhao, Ruoxi Zhang, Zijie Yan, Deyi Zhou, Wei Yuan, Morteza Kayyalha, Moses H Chan, Cui-Zu Chang, Chaoxing Liu Quantum anomalous Hall (QAH) insulators are well-known for their insulating bulk and non-dissipative chiral edge states, but the penetration depth of the chiral edge states and how they interact with the bulk state have not been fully understood. Some previous studies suggested that the edge states decay exponentially into the bulk with a penetration depth of a few nm, while the others argued that this length is much longer (~ 102 nm). A recent experiment found that at the charge neutral point, the QAH effect persists in narrow devices with widths down to ~72 nm, and the Hall resistance slowly deviates from the quantized value as the width is reduced. To understand these observations, here we theoretically investigate the chiral edge states in a QAH system. We show that the chiral edge states exhibit a short, intrinsic penetration depth of a few nm, featuring the exponential decay. We also demonstrate that disorders can extend the chiral edge states into the bulk, which further facilitates the interaction between two edge states, and thus leads to slight deviations from the perfect QAH. |
Monday, March 6, 2023 8:36AM - 8:48AM |
A19.00004: Edge-Induced Pairing States of the Josephson Current in a Spin-Polarized Quantum Anomalous Hall Insulator Ryota Nakai, Kentaro Nomura, Yukio Tanaka The Josephson junction of a spin-polarized quantum anomalous Hall insulator connecting conventional superconductors is studied from the perspective of pairing symmetry. Induced pairing states in the chiral edge mode are equal-spin triplet, a combination of the even- and odd-frequency components, nonlocally extended, and have a finite momentum. The signature of the equal-spin triplet pairings is confirmed via the dependence on the interface-magnetization direction, and that of the finite-momentum pairing states via the width dependence. In the presence of disorder, the robustness of the chiral edge mode leads to high sensitivity to disorder configurations, which is resulting from the interference of current-carrying channels. |
Monday, March 6, 2023 8:48AM - 9:00AM |
A19.00005: Two-particle interference in quantum Hall edge states: the role of channel mixing Matteo Acciai, Preden Roulleau, Imen Taktak, Christian D Glattli, Janine SPLETTSTOESSER We consider a two-particle interferometer in the quantum Hall regime, where voltage sources applied to ohmic contacts inject electronic excitations into a pair of copropagating edge channels. In this context, Hong-Ou-Mandel measurements (based on the current fluctuations at the output of the interferometer) have been shown to be a very useful tool to probe the quantum statistics and indistinguishability of the injected excitations. For an ideal scenario, the noise is completely suppressed when the incoming excitations are synchronized. However, recent experiments [1] performed in the integer quantum Hall regime at filling factor 2 have observed an incomplete reduction of the noise. Interestingly, given the injection scheme implemented in the experiment, this effect cannot be explained by Coulomb interactions. Here, we analyze the impact of channel mixing due to inter-edge tunneling on the current noise measured at the output of the interferometer. We show that this mixing can be responsible for an incomplete suppression of the noise, thereby reducing the visibility of the interference [2]. Our model provides a good quantitative agreement with the experimental data. Finally, we also investigate to which extent the impact of mixing on the interference visibility depends on different shapes of the voltage drives used to inject electronic excitations in the interferometer. |
Monday, March 6, 2023 9:00AM - 9:12AM |
A19.00006: Phase Diagram of the ν=2 quantum Hall phase in bilayer graphene Efrat Shimshoni, Udit Khanna, Ke Huang, Ganpathy N Murthy, Herb Fertig, Jun Zhu Bilayer graphene exhibits a rich phase diagram in the quantum Hall (QH) regime, arising from the interplay of the spin, valley, and orbital degrees of freedom. In particular, at very high magnetic fields, a perpendicular electric field drives a transition between valley-unpolarized and valley-polarized states in several QH phases. Here, we explore the behavior of this transition as the magnetic field $B$ is reduced, focusing on the phases around ν=2. We find that the variation of the critical electric field (D*) with filling factor is qualitatively different at lower B, and that for ν=2, D* may even vanish if B is sufficiently small. We present a theoretical model for the lattice-scale interactions which correctly accounts for these surprising observations; contrary to earlier studies, it involves finite-ranged terms comprising both repulsive and attractive components. Furthermore, we (theoretically) analyze the nature of the ν=2 state as a function of the magnetic and electric fields, and find that a valley-coherent phase may emerge in the D*~0 regime. This suggests the existence of a Kekule bond-ordered phase at low magnetic fields. Such phases have been recently observed in the ν=0 phase of monolayer graphene through STM measurements. |
Monday, March 6, 2023 9:12AM - 9:24AM |
A19.00007: Enhanced thermal activation in the flanks the ν = 1 integer quantum Hall plateau Haoyun Huang, Sean A Myers, Waseem Hussain, Loren N Pfeiffer, Ken West, Gabor A Csathy In the region of the nu=1 quantized Hall plateau of two-dimensional electron gases of modest quality transport is activated, and the activation energy monotonically decreases with an increasing quasiparticle density. We found that in some of the highest mobility electron gases confined to GaAs/AlGaAs quantum wells transport along the nu=1 plateau is activated as well. However, in contrast low mobility samples, the activation energy in high quality electron gases is non-monotonic with the quasiparticle density. We find that in the flanks of the nu=1 plateau the activation energy is enhanced. Furthermore, the non-monotonic dependence of the activation energy on the quasiparticle density correlates with the regions of single particle localization and collective pinning. Indeed, the activation energy reaches local maxima in the middle of the single particle localization and the collective pinning regimes. In addition, we find that at the boundary of single and collective localization regimes the activation anergy has a conspicuous local minimum. Our results determine the thermodynamic properties of the two-dimensional electron gas and constrain the theory describing the transition between single particle localization and collective pinning. |
Monday, March 6, 2023 9:24AM - 9:36AM |
A19.00008: Localization renormalization and quantum Hall systems Bartholomew Andrews, Dominic Reiss, Fenner Harper, Rahul Roy The obstruction to constructing localized degrees of freedom is a signature of several interesting condensed matter phases. We introduce a localization renormalization procedure that harnesses this property. To demonstrate our method, we distinguish between topological and trivial phases in quantum Hall and Chern insulators. By iteratively removing a fraction of orthogonal basis states, we find that the maximally-localized length scale supported in the residual Hilbert space exhibits a power-law divergence as the fraction of remaining states approaches zero, with an exponent of ν=0.5. In sharp contrast, the localization length converges to a system-size independent constant in the trivial phase. |
Monday, March 6, 2023 9:36AM - 9:48AM |
A19.00009: Numerical study of Integer quantum Hall transition in Harper-Hofstadter model with local random disorder Chakradhar Rangi, Ka-Ming Tam, Juana Moreno Anderson localization plays a crucial role in the integer quantum hall(IQH) systems. The degenerated Landau level is split by random disorder and the increase of disorder strength leads to the localization of the states in the tails of the Landau level. The recently developed Cluster-Typical Medium Theory (Cluster-TMT) can be modified to study lattice models for the integer quantum effect, in particular the Harper-Hofstadter model with local random disorder. The cluster-TMT method is successful in reproducing the full phase diagram of the three-dimensional Anderson model. Its success is hinted by the observation that the local density of states of the localized phase follows a log-normal distribution. We investigate the statistics of the local density of states of the Harper-Hofstadter model with local random potential. We estimate the exponent corresponding to localization length method by performing the finite size scaling analysis for the smallest Lyapunov exponent obtained from Transfer Matrix method. In light of the recent studies in critical scaling at the IQH transitions, we carefully investigate the role of irrelevant exponent in our finite size scaling analysis. We use these results to perform the finite size scaling of the typical local density states. We find that the proposed order parameter, typical local density of states, fits well to the scaling hypothesis from which we obtain the corresponding critical exponent. We also find that the local density of states qualitatively changes very closely from normal to log-normal distribution as the disorder strength increases. This opens a possibility to apply cluster-TMT to the integer quantum hall systems. |
Monday, March 6, 2023 9:48AM - 10:00AM |
A19.00010: Modifying the integer quantum Hall effect with cavity vacuum fields Josefine U Enkner, Felice Appugliese, Gian Lorenzo Paravicini-Bagliani, Mattias Beck, Christian Reichl, Werner Wegscheider, Giacomo Scalari, Jerome Faist At 50mK sample temperature we measure electron transport in the dark. We immerse a Hallbar into the cavity vacuum fields of a 2-dimensional metamaterial, complementary a split ring resonator resonant in the THz. With this platform, we recently showed that the interaction with vacuum fields not only modifies the finite resistances of the Shubnikov de Haas oscillations in the diffusive transport regime, but it also breaks the topological protection in the integer quantum regime. As a topological insulator, the quantum Hall system is paradigmatic for being robust under the influence of short-range perturbations. We observe the loss of the zero-resistance states in the longitudinal traces and of quantization of the Hall plateaus due to the cavity induced long-range perturbations. The accompanying picture to this long-range perturbation is called "cavity mediated electron hopping". It posits that we can lift an electron occupying the disordered eigenstate Φλn from the nth Landau level into the (n+1)th via the absorption of a virtual photon. The inverse process then drops the electron back into the nth Level but into a disordered eigenstate Φλ' n, in a different location. Effectively, with this process electrons can now scatter between the topologically protected edge states via the insulating bulk and break topological protection. We quantitatively characterize this loss of quantization as vacuum field induced resistivity ρvacxx. |
Monday, March 6, 2023 10:00AM - 10:12AM |
A19.00011: Theory of Electron-phonon Couplings in 3D Quantum Hall Effects Kaiyuan Gu, Kai Torrens, Biao Lian The 3D quantum Hall effect modulated by electron-phonon interactions has been observed recently and has aroused many theoretical discussions. In 3D, because the Landau levels in x-y plane have dispersions in z direction, the bulk state is metallic unless the charge-density-wave (CDW) opens the gaps at Fermi energy. In this research, we developed a numerical way to approach the ground states of the system by considering all possible phonon modes and minimizing the ground state energy. In the process of modulating the effective magnetic field and the effective electron-phonon coupling strength, we demonstrated the ground state phase transitions from (n+1)-levels to n-levels. More importantly, we discovered a novel metallic phase within the transitions, which may account for the plateau in 3D quantum Hall resistance. |
Monday, March 6, 2023 10:12AM - 10:24AM |
A19.00012: Quantum Percolation Effect in Quantum Anomalous Hall Insulators Lingjie Zhou, Yi-Fan Zhao, Ruoxi Zhang, Deyi Zhuo, Zijie Yan, Wei Yuan, Moses H Chan, Cui-Zu Chang The quantum anomalous Hall (QAH) effect is the zero magnetic field manifestation of the integer quantum Hall effect. In our recent experiments, we find that the magnetic TI sandwich devices show the perfect QAH state as long as the magnetic domain switching ratio is greater than the quantum percolation threshold of magnetic domains. To quantitively examine the quantum percolation effect on the chiral edge channels (CECs), we fabricate the QAH devices with a narrow constriction down to 50 nm. By performing magnetotransport measurements, we find that the magnetic transition peak near the coercive field is broadened when the width of the constriction is less than 300 nm. This observation suggests that the narrow constriction is more sensitive to magnetic domain switching. Moreover, our minor loop measurements show that the reversion of a single magnetic domain destroys the appearance of the dissipation-free CECs when the width of the constriction is less than 100nm. Our work paves the way for the potential applications of dissipationless CECs in energy-efficient electronics and spintronics, as well as topological quantum computations. |
Monday, March 6, 2023 10:24AM - 10:36AM |
A19.00013: Three-dimensional quantum Anomalous Hall effect in hundred-nanometer-thick magnetic topological insulator trilayers Ruoxi Zhang, Yi-Fan Zhao, Lingjie Zhou, Deyi Zhuo, Zijie Yan, Ke Wang, Moses H Chan, Cui-Zu Chang Magnetic topological states refer to a class of exotic phases in magnetic materials with their non-trivial topological property determined by magnetic spin configurations. The quantum anomalous Hall (QAH) state, a zero magnetic field manifestation of the integer quantum Hall state, is one example of magnetic topological states. To date, most of the experimental efforts on the QAH insulators still focus on the magnetic TI films/heterostructures with thicknesses less than 12nm, which are still in the 2D regime or near the 2D-to-3D boundary. In this work, we optimize the molecular beam epitaxy (MBE) growth conditions and parameter space of magnetic TI sandwiches and succeed in realizing the QAH state in magnetic TI sandwiches with the middle undoped TI layer up to 100 quintuple layers (QL). We find that the magnetic TI sandwiches with the middle undoped TI layers from 4 to 100 QLs show the perfect quantization and similar transport behaviors under different gate voltages. These observations suggest the negligible role of the side surface states in our magnetic TI sandwich samples. For the thick magnetic TI sandwiches with Cr- and V-co-doping in both the top and bottom surface layers, we find that the peak value of the longitudinal resistance near the coercive field is greatly boosted by tuning the bottom gate, implying the feasibility of the realization of the gate-induced QAH to axion insulator state phase transition in a single magnetic TI sandwich sample. The successful synthesis of thick QAH insulators and the possible realization of the gate-induced QAH to axion insulator phase transition pave the way for energy-efficient electronic and spintronic devices and topological quantum computations. |
Monday, March 6, 2023 10:36AM - 10:48AM |
A19.00014: Spin-Orbit Torque Switching of the Edge State Chirality in Quantum Anomalous Hall Insulators Wei Yuan, Ling-Jie Zhou, Kai-Jie Yang, Yi-Fan Zhao, Ruoxi Zhang, Zijie Yan, Deyi Zhuo, Ruobing Mei, Yang Wang, Hemian Yi, Moses H Chan, Morteza Kayyalha, Chaoxing Liu, Cui-Zu Chang A quantum anomalous Hall (QAH) insulator is a topological state of matter, which processes the quantized Hall resistance and dissipationless chiral edge current (CEC) without the need for an external magnetic field. CEC flows along the edges of the sample in either a clockwise or counter-clockwise direction dictated by the spontaneous magnetization direction. To date, the chirality of CEC in QAH insulators can be only controlled by sweeping back and forth the external magnetic field. However, this is clumsy and cumbersome as a means to operate in real CEC-based electronic devices. In this work, we fabricate mesoscopic QAH sandwich Hall bar devices with a width of 1~10 μm and succeed in switching the CEC chirality in QAH insulators through spin-orbit torque (SOT) by applying a current pulse under a suitably controlled gate voltage. Since the SOT magnetization switching ratio is easily greater than the quantum percolation threshold of magnetic domains, the well-quantized QAH state with the opposite CEC chirality can appear after the magnetization switching. The realization of SOT switching of the edge state chirality in QAH insulators will not only advance our knowledge of the interplay between magnetism and topological states but also expedite easy and instantaneous manipulation of the QAH state in energy-efficient electronic and spintronic devices as well as quantum information applications. |
Monday, March 6, 2023 10:48AM - 11:00AM |
A19.00015: Laughlin's argument and thermal Hall effect Ze-Min Huang We study the effective action of hydrostatic response to torsion in the absence of spin connections in gapped $(2+1)$-dimensional topological phases at low temperatures. In previous studies, a torsional Chern-Simons term with a temperature-squared ($T^2$) coefficient was proposed as an alternative action to describe the thermal Hall effect with the idea of balancing the diffusion of heat by a torsional field. However, the question remains whether this action leads to a local bulk thermal response that is not suppressed by the gap. In our hydrostatic effective action, we show that the $T^2$ bulk term is invariant under variations up to boundary terms considering the geometry dependence of local temperature, which precisely describes the edge thermal current. Furthermore, there are no local boundary diffeomorphism anomalies or bulk inflow thermal currents at equilibrium, and also there is no edge-to-edge adiabatic thermal current pumping upon changing the gravitational background. These results are consistent with exponentially suppressed thermal current for gapped phases. |
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