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 V42: Hall Effects in 2D SystemsLive
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Sponsoring Units: DCMP Chair: Enrique Cobas, United States Naval Research Laboratory |
Thursday, March 18, 2021 3:00PM - 3:12PM Live |
V42.00001: Quantum spin Hall edge states in twisted bilayer 1T’-WTe2 Dacen Waters, Felix Lüpke, Anh Pham, Jiaqiang Yan, David George Mandrus, Panchapakesan Ganesh, Benjamin Matthew Hunt The quantum spin Hall (QSH) effect, characterized by counter-propagating spin-polarized edge states, was recently demonstrated in monolayers of the transition metal dichalcogenide 1T'-WTe2. The presence of the QSH edge state is expected to be protected by time-reversal symmetry, however, the robustness of this topological protection and any potential scattering mechanisms remain largely unexplored in van der Waals heterostructures containing one or more layers of a QSH insulator. In this work, we use scanning tunneling microscopy and spectroscopy (STM/STS), to study the QSH edge state in WTe2 bilayers for three different twist angles. We observe the characteristic spectroscopic signature of the QSH edge state in the twisted bilayers, including along a coinciding edge. In addition, we report the presence of the QSH edge state at a junction of a monolayer and an as-grown bilayer. By comparing our experimental observations to first principles calculations, we conclude that the constituent layers of the twisted bilayers are weakly coupled, in contrast to the topologically trivial as-grown bilayer, due to their larger interlayer spacing. |
Thursday, March 18, 2021 3:12PM - 3:24PM Live |
V42.00002: Giant nonlinear Hall effect in strained twisted bilayer graphene Cheng-Ping Zhang, Jiewen Xiao, ZHOU Tong Benjamin, Jinxin Hu, Yingming Xie, Binghai Yan, Kam Tuen Law Recent studies have shown that moire flat bands in twisted bilayer graphene(TBG) can acquire nontrivial Berry curvatures when aligned with hexagonal boron nitride substrate, which can be manifested as a correlated Chern insulator near the 3/4 filling. In this work, we show that the large Berry curvatures in the moire bands lead to strong nonlinear Hall(NLH) effect in a strained TBG with general filling factors. Under a weak uniaxial strain ~0.1%, the Berry curvature dipole which characterizes the nonlinear Hall response can be as large as ~ 200Å , exceeding the values of all previously known nonlinear Hall materials by two orders of magnitude. The dependence of the giant NLH effect as a function of electric gating, strain and twist angle is further investigated systematically. Importantly, we point out that the giant NLH effect appears generically for twist angle near the magic angle due to the strong susceptibility of nearly flat moire bands to symmetry breaking induced by strains. Our results establish TBG as a practical platform for tunable NLH effect and novel transport phenomena driven by nontrivial Berry phases. |
Thursday, March 18, 2021 3:24PM - 3:36PM Live |
V42.00003: Quantum anomalous Hall effect in two-dimensional Cu-dicyanobenzene coloring-triangle lattice Yixuan Gao, Yu-Yang Zhang, Jiatao Sun, Lizhi Zhang, Shengbai Zhang, Shixuan Du Magnetic two-dimensional (2D) topological insulators with spontaneous magnetization have been predicted to host quantum anomalous Hall effects (QAHEs). So far, most materials predicted for realizing the QAHE are inorganic. However, several theoretical works suggested the possibility of 2D organometallic frameworks as organic topological insulators (OTIs). For OTIs, the QAHE only exists in honeycomb or Kagome organometallic lattices. Recently, coloring-triangle (CT) lattice has been found to be mathematically equivalent to a Kagome lattice, suggesting a potential 2D lattice to realize QAHE. Here, based on first-principles calculations, we predict an organometallic CT lattice, Cu-dicyanobenzene (DCB), to be a stable QAH insulator. It exhibits ferromagnetic (FM) properties as a result of the charge transfer from metal atoms to DCB molecules. Moreover, based on the Ising model, the Curie temperature of the FM ordering is calculated to be around 100 K. Both the Chern numbers and the chiral edge states of the semi-infinite Cu-DCB edge structure, which occur inside the spin-orbit coupling band gap, confirm its nontrivial topological properties. These make the Cu-DCB CT lattice an ideal candidate to enrich the family of QAH insulators. [1] |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V42.00004: Anomalous Hall Effect in Ultrathin Crystalline Strontium Ruthenate Membranes Patrick Blah, Edouard Lesne, Martin Lee, Ana Monteiro, Dmytro Afanasiev, Thierry van Thiel, Mattias Matthiesen, Jorrit Hortensius, Ulderico Filippozzi, Yingkai Huang, Peter Steeneken, Andrea Caviglia SrRuO3 (SRO) is a complex oxide that hosts a plethora of exotic magneto-transport properties due to its strong spin-orbit coupling and itinerant ferromagnetism. In particular it is an excellent candidate to investigate the intrinsic Berry-phase driven Anomalous Hall Effect. A recent breakthrough1 has allowed complex oxides, epitaxially grown via pulsed laser deposition, to be exfoliated and released via a sacrificial layer. These freestanding complex oxide membranes are an exciting new platform for investigating and tuning the interplay between structural and electronic properties2. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V42.00005: Quantum Hall crystals, liquids, and bubbles in graphene from direct measurement of the chemical potential Fangyuan Yang, Alexander A Zibrov, Ruiheng Bai, Yu Saito, Xiaoxue Liu, Takashi Taniguchi, Kenji Watanabe, Jia Li, Mark Oliver Goerbig, Michael Zaletel, Andrea Young We describe an experimental technique to measure the chemical potential in atomically thin layered materials with high sensitivity and in the static limit. We apply this technique to a high quality graphene monolayer to map out the evolution of chemical potential with carrier density up to the N=4 Landau level. In the N=0 level, the measured ground state energy per particle shows exceptional agreement with numerical calculations when the screening of the Coulomb interaction by filled Landau levels is accounted for. In the N=1 Landau level, the comparison reveals the importance of valley anisotropic interaction and the presence of valley-textured electron solids near odd integer fillings. Finally, in the N=2,3 and 4 Landau levels, we find a series of oscillations in the chemical potential that match Hartree-Fock calculations based on single- and mixed phases of multi-electron bubble states. We also report our recent results on measuring chemical potential in the flat band of twisted bilayer graphene. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V42.00006: Evidence for non-abelian spin-singlet state in graphene quantum Hall bilayer Yihang Zeng, Cory Dean The even-denominator fractional quantum Hall state (FQHE) at 5/2 filling is believed to host Ising-type non-abelian anyons - exotic excitations that are thought to enable a subset of operations in topological quantum computation (TQC). It has been proposed that in a spinful system, a non-abelian spin singlet (NASS) state could host Fibonacci anyons, a more robust kind of non-abelian whose braiding properties are theoretically capable of realizing the entire set of operation in TQC. The 2/3 FQHE state has been identified as a candidate for these more exotic non-abelians but, experimental evidence is lacking. Here we discuss measurements of the 4/3 state In double-layer graphene, consisting of two graphene monolayers separated by an interlay dielectric, where the layer degree of freedom plays the role of pseudo spin. Using a combination of Corbino parallel flow and counterflow, together with Hall bar drag measurements, we identify evidence for existence of the k=2 NASS state. The implication of our results on the search for Fibonnacci anyons will be discussed. |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V42.00007: Edge Channels of Broken Symmetry Quantum Hall States in Graphene probed by Atomic Force Microscopy Sungmin Kim, Johannes Schwenk, Daniel T Walkup, Yihang Zeng, Fereshte Ghahari Kermani, Son Le, Marlou Slot, Julian Berwanger, Steven Blankenship, Kenji Watanabe, Takashi Taniguchi, Franz J Giessibl, Nikolai Zhitenev, Cory Dean, Joseph Stroscio The quantum Hall effect, a topologically non-trivial quantum phase has brought into focus the concept of topological order in physics. The topologically protected quantum Hall edge states are the essential features of the QH effect, however microscopic local probe studies of edge states have been conducted with limited success. The QH edge states in graphene are special since they emerge from four-fold nearly-degenerate Landau levels. In this talk, we present a microscopic study a of the QH edge states originating from the broken symmetry states of zeroth LL in graphene using AFM. The KPFM detects the chemical potential transitions when Landau levels are being filled or empty as a function of back gate potential and show the same fidelity for Landau level spectroscopy as STS measurements. In particular broken symmetry states can be resolved at integer filling factors ν=0, ±1 inside the N=0 Landau level manifold, showing the lifting of the graphene four-fold degeneracy due to spin and valley. The edge channels emerging from integer filling factors are spatially mapped across the quantum Hall edge using AFM. The microscopic properties by scanning probe measurement of quantum Hall edge channels can be correlated with macroscopic properties measured with in-situ magnetotransport. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V42.00008: Strange metal behavior of the Hall angle in twisted bilayer graphene Zachary Tuchfeld, Rui Lyu, Nishchhal Verma, Haidong Tian, Kenji Watanabe, Takashi Taniguchi, Jeanie Lau, Mohit Randeria, Marc Bockrath Twisted bilayer graphene (TBG) has emerged as a tunable system exhibiting a number of correlated phases including Mott-like insulators, superconductivity, and magnetism. A linear in temperature normal state resistivity has been attributed to an exotic Planckian dissipation[1] mechanism but can be equally well explained in terms of conventional electron-phonon scattering [2, 3]. Here we discuss combined temperature-dependent transport measurements of both the longitudinal and Hall resistivities in close to magic-angle TBG. While the longitudinal resistivity is consistent with previous reports [1, 2], the Hall resistance shows an anomalous T dependence, providing new insight and a cotangent of the Hall angle cot(ΘH)∝T2. Boltzmann theory for quasiparticle transport predicts that both the resistivity and cot(ΘH) should have the same T dependence, contradicting the observed behavior. This provides strong evidence that TBG is incompatible with ordinary quasiparticle transport, reminiscent of other correlated strange metals such as cuprates. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V42.00009: Trigonal Warping and anomalous valley-Hall conductivity in twisted double bilayer graphene Priyanka Mohan, Unmesh Ghorai, Rajdeep Sensarma We calculate the topological phase diagrams of both ABAB and ABBA stacked twisted double bilayer graphene as a function of both twist angles and perpendicular potential. The two transitions with Chern number changes of −3 and +1 can be explained by the splitting of the band crossing point into three satellite cones around a central Dirac cone in the moiré Brillouin zone due to the presence of trigonal warping. Considering the overlap of the bands in energy, which leads to metallic states, we construct the experimentally observable phase diagram of the system in terms of the indirect bandgap and the anomalous valley-Hall conductivity. We show that while most of the intermediate phase becomes metallic, there is a narrow parameter regime where the transition through three insulating phases with different quantized valley Hall conductivity can be seen. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V42.00010: Anomalous Quantum Hall States in Bilayer Graphene Jung-Jung Su, Chun Ning Lau, Allan MacDonald Bernal stacked bilayer graphene has a collection of competing broken symmetry states characterized by flavor-dependent spontaneous valley polarization.[1] The possible states include polarized and unpolarized orbital magnets and various quantum anomalous Hall states. By carefully evaluating the orbital magnetizations of the competing states, we find that the phase diagram is profoundly altered by a B field by favoring states of which its natural filling factor νN ≡ σH /(e2/h) is close to the actual filling factor ν, increasingly so as the B field strengthens. We have explored how coupling of orbital magnetization to external B fields is manifested in the dependence of ground state properties on weak gate Vg and magnetic fields B that favor particular states. We construct the ground-state phase diagram of this system, vs. Vg and B at fixed ν, and vs. Vg and ν at fixed B, and obtain excellent agreement with two-point conductivity measurements. This work explains why the νN = ±4 quantum Hall Effect in bilayer graphene is stable to anomalously weak B fields, and suggests that νN = ±2 anomalous quantum Hall effects could occur in the absence of a B field in samples of sufficiently high quality. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V42.00011: Anomalous Hydrodynamic Flow in Interacting Noncentrosymmetric Metals Riki Toshio, Kazuaki Takasan, Norio Kawakami In highly conductive metals with sufficiently strong momentum conserving scattering, the |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V42.00012: Scanning tunneling spectroscopy of quantum Hall ferromagnetic states in graphene Xiaomeng Liu, Cheng Li Chiu, Gelareh Farahi, Kenji Watanabe, Takashi Taniguchi, Michael Zaletel, Ali Yazdani Under strong magnetic fields, cyclotron motion of two-dimensional electrons causes Landau quantization. In graphene, the quantized Landau levels are four-fold degenerate, due to spin and valley degrees of freedom. Electron-electron interactions, however, breaks the Landau level degeneracy, and establish quantum Hall ferromagnetic (QHFM) states. The unique SU(4) symmetry breaking of QHFM states in graphene have attracted extensive studies, mostly using transport and capacitance techniques. However, these studies only probe the system near the Fermi energy. Exotic excitations of QHFM in graphene, such as skyrmions, still wait to be explored. In this work, employing scanning tunneling microscope, we measure tunneling spectrum as a function of electron filling. We demonstrate well-developed symmetry breaking QHFM states at all integer fillings and directly measure the exchange energy. Besides Laudau levels, we discover rich spectral features stemming from the symmetry broken gaps. These excitation features near /nu=-1 and /nu=1 are very similar and are distinctly different from excitation features near \nu=0, suggesting they are related to the nature of the symmetry breaking. |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V42.00013: Spectroscopic imaging of graphene in the quantum hall regime with the scanning tunneling microscope Gelareh Farahi, Xiaomeng Liu, Cheng Li Chiu, Michael Zaletel, Ali Yazdani In strong magnetic fields, the kinetic energy becomes suppressed and Dirac fermions in graphene form a highly interactive system that resides in quantized Landau levels. In this limit, a myriad of interaction-driven phenomena can emerge in the zeroth Landau level, including quantum Hall ferromagnetism, Wigner crystallization and skyrmionic excitations, some of which that have been studied in transport and capacitance measurements. With its capability to determine the electronic density of states in real space, the scanning tunneling microscope (STM) is a robust tool to shed light on the interactions in both occupied and unoccupied levels. Landau orbits have been previously imaged near point defects on the surface of Bismuth at high magnetic fields with the STM. We aim at extending such spectroscopic imaging techniques to graphene in the quantum Hall limit, by measuring its local density of states as we tune the carrier density with electrostatic gating. Our energy-resolved measurements detect signatures of quantum Hall ferromagnetism that arise from broken valley and spin symmetries. We seek to complement these measurements with spatially resolved studies that will unveil real-space features of these and other interacting electronic states that form in graphene in high fields. |
Thursday, March 18, 2021 5:36PM - 5:48PM On Demand |
V42.00014: Electric field induced anomalous Hall effects and nematic phases in carrier doped rhombohedral trilayer graphene Youngju Park, Allan MacDonald, Jeil Jung The enhanced density of states near charge neutrality due to the flattening of the bands in ABC trilayers makes this type of multilayer graphene systems prone to form correlated ordered phases in sufficiently clean devices. We show by means of mean field Hartree-Fock calculations with long-range Coulomb interactions that the energetically favored ground state for ABC trilayer graphene turns from the layer antiferromagetic (LAF) phase to the valley polarized anomalous Hall (AH) phase upon a small carrier doping and application of a perpendicular electric field. The uneven filling of the three electron or hole pockets near the band edges upon carrier doping makes the system prone to form nematic phases with broken rotational symmetry due to momentum space condensation, an exchange driven mechanism that tends to lower the energies of electronic states that are closer together in momentum space. Our result suggests the possibility of switching electrically on and off the trigonal 120o rotational symmetry of the carrier doped ground states. |
Thursday, March 18, 2021 5:48PM - 6:00PM Not Participating |
V42.00015: Intrinsic anomalous Hall conductivity in non-uniform electric field Vladyslav Kozii, Aleksandr Avdoshkin, Shudan Zhong, Joel Ellis Moore We study how the intrinsic anomalous Hall conductivity is modified in two-dimensional crystals with broken time-reversal symmetry due to weak inhomogeneity of the applied electric field. Focusing on a clean non-interacting two-band system without band crossings, we derive the general expression for the Hall conductivity at small finite wavevector q to order q^2 , which governs the Hall response to the second gradient of the electric field. Using the Kubo formula, we show that the answer can be expressed through the Berry curvature, Fubini-Study quantum metric, and the rank-3 symmetric tensor which is related to the quantum geometric connection and physically corresponds to the gauge-invariant part of the third cumulant of the position operator. We further compare our results with the predictions made within the semiclassical approach. By deriving the semiclassical equations of motion, we reproduce the result obtained from the Kubo formula in some limits. We also find, however, that the naive semiclassical description in terms of the definite position and momentum of the electron is not fully consistent because of singular terms originating from the Heisenberg uncertainty principle. |
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