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 L42: Topology in GrapheneLive
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Sponsoring Units: DCMP Chair: Jianhao Chen, Peking Univ |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L42.00001: Moiré band topology in twisted bilayer graphene Chao Ma, Qiyue Wang, Scott Mills, Xiaolong Chen, Bingchen Deng, Shaofan Yuan, Cheng Li, Kenji Watanabe, Takashi Taniguchi, Xu Du, Fan Zhang, Fengnian Xia Recently twisted bilayer graphene (t-BLG) emerges as a strongly correlated physical platform near a magic twist angle, hosting the Mott-like insulating phases and unconventional superconducting behavior. Besides, band topology may be another critical element in strongly correlated twistronics. In this work, we performed a systematic nonlocal transport study and revealed the nontrivial moiré band topology in t-BLG. Pronounced nonlocal responses are observed both in the electron and hole superlattice gaps of t-BLG, which are robust to the interlayer electric field, twist angle, and edge termination. We elucidate that two nontrivial Z2 invariants characterize the topology of the moiré bands. Our findings provide a new perspective for understanding the emerging strongly correlated phenomena in twisted van der Waals heterostructures. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L42.00002: Nodal Structure of Chiral Twisted Bilayer Graphene Model and Implications for Higher Magic Angles Jie Wang, Yunqin Zheng, Andrew Millis, Jennifer Cano This talk presents a mathematical and numerical analysis of the flatband wavefunctions occurring in the chiral model of twisted bilayer graphene at the "magic" twist angles. We show that the chiral model possesses an exact intra-valley inversion symmetry. Writing the flatband wavefunction as a product of a lowest Landau level quantum Hall state and a spinor, we show that the components of the spinor are anti-quantum Hall wavefunctions related by the inversion symmetry operation introduced here. We then show numerically that as one moves from the lowest to higher magic angles, the spinor components of the wavefunction exhibit an increasing number of zeros, resembling the changes in the quantum Hall wavefunction as the Landau level index is increased. The wavefunction zeros are characterized by a chirality, with zeros of the same chirality clustering near the center of the moire unit cell, while opposite chirality zeros are pushed to the boundaries of the unit cell. The enhanced phase winding at higher magic angles suggests an increased circulating current. Physical implications for scanning tunneling spectroscopy, orbital magnetization and interaction effects are discussed. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L42.00003: Correlated and topological states in twisted graphene multilayers Minhao He, Yuhao Li, Yahui Zhang, Jiaqi Cai, Yang Liu, Zaiyao Fei, Kenji Watanabe, Takashi Taniguchi, David Cobden, Xiaodong Xu, Matthew Yankowitz Twisted van der Waals heterostructures have recently emerged as versatile platforms for investigating strongly correlated and/or topological states of matter. The key advantage of these systems lies in the ability to dynamically tune the width and spectral isolation of their flat bands, the Coulomb interaction strength, and the band topology using a combination of tuning knobs including twist angle, doping, displacement field, magnetic field, and pressure. Here, we report electrical transport measurements of twisted monolayer-bilayer graphene (tMBG) and twisted double bilayer graphene (tDBG) over a small range of twist angles around 1°. We identify a variety of tunable correlated states, including those with various types of magnetic ordering driven by either electron spin or orbital motion. In tMBG, the intrinsic band topology additionally leads to the emergence of tunable Chern insulator states. Our results highlight the wide tunability of the correlated and topological states in twisted graphene multilayer heterostructures. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L42.00004: Landau Levels as a Probe for Band Topology in Graphene Moiré Superlattices QuanSheng Wu, Jianpeng Liu, Yifei Guan, Oleg Yazyev Flat bands in twisted multilayer systems are often characterized by non-trivial topology due to the pseudo-gauge field caused by the moiré potential. In this work, we will show that Landau levels are a good probe for the topological character of electronic bands in two-dimensional moiré superlattices. Twisted double bilayer graphene (TDBG), a system constructed by twisting two AB-stacked bilayer graphene (BLG) counterparts placed on top of each other, is considered as an example. TDBG has AB-AB and AB-BA stacking configurations, that have different valley Chern numbers of the flat bands, yet have very similar band structures. Different valley Chern numbers in these two configurations of TDBG manifest as different Landau level sequences in the Hofstadter butterfly. The results are explained from the point of view of the distribution of orbital magnetization in momentum space that is governed by the rotational C2 and time-reversal T symmetries. Our results can be readily extended to other twisted graphene multilayers and h-BN/graphene heterostructures thus establishing the Hofstadter butterfly spectra as a powerful tool for detecting the non-trivial valley band topology. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L42.00005: Moiré Superlattices at Fractional Band Fillings: Emergent Fermi Liquids and Fractional Chern Insulators Ahmed Abouelkomsan, Zhao Liu, Emil Bergholtz We consider the core problem of Coulomb interactions within fractionally filled Moiré flat bands and demonstrate that the dual description in terms of holes, which acquire a non-trivial hole-dispersion, provides key physical intuition and enables the use of standard perturbative techniques for this strongly correlated problem. We find that the single-hole dispersion has a profound impact on the phase diagram: in experimentally relevant examples such as ABC stacked trilayer and twisted bilayer graphene aligned with boron nitride, it leads to emergent Fermi liquid states at band filling fractions down to 1/3 and 2/3 respectively. Moreover, we predict both twisted bilayer graphene aligned with boron nitride and gate-tunable twisted double bilayer graphene to be versatile platforms for the realization of fractional Chern insulator states like spin-singlet Halperin states and spin polarized states in bands with Chern number C =1 and C = 2 at zero external magnetic fields. |
Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L42.00006: Spectroscopy of Correlation-enabled Chern Insulators in Magic-Angle Twisted Bilayer Graphene Youngjoon Choi, Hyunjin Kim, Yang Peng, Alex R Thomson, Cyprian Lewandowski, Kenji Watanabe, Takashi Taniguchi, Jason F. Alicea, Stevan Nadj-Perge Electronic interactions play a central role for the formation of various correlated phases in magic-angle twisted bilayer graphene (MATBG). Here we report results on local properties of newly discovered correlated Chern insulating phases that develop in the presence of finite magnetic fields. We identify Chern numbers of these topological phases using a novel spectroscopic technique hinging on scanning tunneling microscopy (STM) that enables us to reconstruct a local, spatially dependent, Landau fan diagram. The corresponding spectroscopy in finite fields directly reveals that these Chern phases arise from the strong exchange-like interaction that acts on the Hofstadter sub-bands. Moreover, the spatial mapping in an area where twist angle slowly changes further reveals that these phases form in fields B<8T only in close vicinity (0.1deg) of the magic-angle value. Our results highlight the importance of strong interactions in the formation of topological phases in MATBG. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L42.00007: Dirac Magic Angles and Topological Floquet Flat Bands in Twisted Multilayer graphene Yantao Li, Adam Eaton, Babak Seradjeh, Herb Fertig We characterize the energy and quasienergy (Floquet) bands in various twisted multilayer graphene structures. We find new magic angles in the static AA-stacked structures that we name “Dirac magic angles” in which multiple Dirac cones from different bands coexist at magic twist angles. We also find Floquet flat bands carrying non-zero Chern numbers in the irradiated AB-stacked and twist-and-twist back structures. The Dirac magic angles may offer a new platform to study the itinerant electron magnetism or robust superconductivity controlled by the twist angles. The dynamically produced topological flat bands under irradiation can potentially realize the fractional Chern insulator. |
Wednesday, March 17, 2021 9:24AM - 9:36AM Live |
L42.00008: Electrically tunable correlated and topological states in twisted monolayer–bilayer graphene Shaowen Chen, Minhao He, Yahui Zhang, Valerie Hsieh, Zaiyao Fei, Kenji Watanabe, Takashi Taniguchi, David Cobden, Xiaodong Xu, Cory Dean, Matthew Yankowitz Twisted van der Waals heterostructures have emerged as a tunable platform for correlated and topological states. In graphene-based moiré heterostructures, the correlated phase diagram and band topology depend on the number of graphene layers and the details of the external environment from the encapsulating crystals. I will report that the system of twisted monolayer–bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as well as topological magnetic states. Because of its low symmetry, the phase diagram of tMBG approximates that of twisted bilayer graphene when an applied perpendicular electric field points from the bilayer towards the monolayer graphene, or twisted double bilayer graphene when the field is reversed. In the former case, we observe correlated states that undergo an orbitally driven insulating transition above a critical perpendicular magnetic field. In the latter case, we observe the emergence of electrically tunable ferromagnetism at one-quarter filling of the conduction band, and an associated anomalous Hall effect. The direction of the magnetization can be switched by electrostatic doping at zero magnetic field. Our results establish tMBG as a tunable platform for investigating correlated and topological states. |
Wednesday, March 17, 2021 9:36AM - 9:48AM Live |
L42.00009: A local compressibility study of magic angle twisted bilayer graphene, Part 1 Andrew Pierce, Yonglong Xie, Jeong Min Park, Seung Hwan Lee, Yuan Cao, Patrick R Forrester, Eslam Khalaf, Takashi Taniguchi, Kenji Watanabe, Ashvin Vishwanath, Pablo Jarillo-Herrero, Amir Yacoby Magic-angle twisted bilayer graphene (MATBG) exhibits fascinating correlated phases due to the dominance of electronic interactions within its extremely flat low-lying bands. Recent tunneling spectroscopy and magnetotransport experiments have revealed topologically insulating states with Chern number C=±1, 2, 3 at the moiré band filling factors ν=±3, 2, 1, respectively, which are predicted within a minimal model consisting of eight topological bands with C=±1. Here, we report high-resolution local compressibility measurements of MATBG using a scanning single-electron transistor. We identify incompressible states with finite Chern number, and discuss their connection to recent tunneling spectroscopy and magnetotransport results. |
Wednesday, March 17, 2021 9:48AM - 10:00AM Live |
L42.00010: A local compressibility study of magic-angle twisted bilayer graphene, Part 2 Yonglong Xie, Andrew Pierce, Jeong Min Park, Seung Hwan Lee, Yuan Cao, Patrick R Forrester, Eslam Khalaf, Takashi Taniguchi, Kenji Watanabe, Ashvin Vishwanath, Pablo Jarillo-Herrero, Amir Yacoby Magic-angle twisted bilayer graphene (MATBG) hosts a rich variety of electronic states including correlated insulators, superconductors, and topological phases. The interplay between the isospin degeneracy and the electronic interactions in the system leads to a cascade of transitions between different ground states, as observed in recent tunneling spectroscopy and local compressibility measurements. In this talk, we will present local compressibility measurements which provide evidence for such transitions. We will also discuss their response to an applied magnetic field and other fine features in our experiments. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L42.00011: Spectroscopy of a Tunable Moiré System with a Correlated and Topological Flat Band Cheng Li Chiu, Xiaomeng Liu, Jong Yeon Lee, Gelareh Farahi, Kenji Watanabe, Takashi Taniguchi, Ashvin Vishwanath, Ali Yazdani Moiré superlattices created by twisted stacking of 2D materials can host highly tunable electronic structure. In twisted bilayer graphene, the twist angle has been proven a powerful knob to tune the electron bandwidth. When the twisted angle is close to the magic angle (~1.1degree), the non-trivial correlated insulating phase and the superconductivity emerge. Besides twisted angle another tuning knob – electric field can also be effective. In twisted double bilayer (TDBG) which is made by twisting two Bernal bilayer graphene, the electric field can gap the Bernal bilayer graphene and change the band structure. In previous studies, multiple correlated insulators have been found in the multi-dimension phase diagram. Here we report the STM study of TDBG[1]. With the gate-tuned scanning tunneling spectroscopy, we reveal the single particle band structure under different twist angle which is hard to measure otherwise. Also, we will show, under certain conditions, the evidence of correlated insulating phase and giant orbital g-factor that arises from the valley Chern bands. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L42.00012: Probing orbital Chern ferromagnet phase in twisted bilayer graphene Charles Tschirhart, Marec Serlin, Hryhoriy Polshyn, Avi G Shragai, Zhengchao Xia, Jiacheng Zhu, Yuxuan Zhang, Kenji Watanabe, Takashi Taniguchi, Martin E Huber, Andrea Young Electrons in the moiré flat bands of magic angle twisted bilayer graphene aligned to hexagonal boron nitride can break time reversal symmetry and open an interaction-driven, topological gap. The resulting magnetic order and associated quantized anomalous Hall effect have properties that diverge substantially from quantized anomalous Hall effects observed in other systems. I will present transport data and scanning probe magnetometry data acquired using a nanoSQUID-on-tip microscope. A quantitative analysis of the magnitude of the magnetization of the Chern magnet shows that the magnetic moment per moiré unit cell substantially exceeds 1 μB and grows rapidly in the topological gap, consistent with an orbital origin for the magnetic order. We find that the Barkhausen jumps observed in transport measurements can be mapped directly to microscopic motion of ferromagnetic domain walls. These domain walls are strongly pinned to disorder in the device and are reproducible across thermal cycles, suggesting coupling between the magnetic degrees of freedom and structural inhomogeneity. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L42.00013: Topological charge pumping in twisted bilayer graphene Yinhan Zhang, Yang Gao, Di Xiao We show that a sliding motion between the two layers of a \moire superlattice induces an electric current and realizes a two-dimensional version of the topological Thouless pump when the Fermi energy lies in one of the minigaps. Interestingly, a chiral charge pump, namely, a transverse current induced by the sliding motion, is possible in twisted homobilayers. This result is confirmed by a concrete calculation of the adiabatic current in twisted bilayer graphene. Our work reveals an interesting link between mechanical motion and electricity unique to \moire superlattices, and may find applications in nanogenerators and nanomotors. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L42.00014: Strongly Correlated Chern Insulators in Magic-Angle Twisted Bilayer Graphene Kevin Nuckolls, Myungchul Oh, Dillon Wong, Biao Lian, Kenji Watanabe, Takashi Taniguchi, Andrei B Bernevig, Ali Yazdani Most topological electronic phases appear in systems with weak electron-electron interactions, making instances where topological phases emerge only as a result of strong interactions rare. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new strongly correlated topological phases. I will describe the use of a novel local spectroscopic technique using a scanning tunneling microscope (STM) to detect a sequence of topological insulators in MATBG with Chern numbers C = ±1, ±2, ±3 near v = ±3, ±2, ±1 electrons per moiré unit cell respectively, which are stabilized by modest magnetic fields [1]. One of these phases (C = +1) was previously observed when the sublattice symmetry of MATBG was intentionally broken by a hexagonal boron nitride (hBN) substrate, with interactions playing a secondary role [2]. We demonstrate that strong electron-electron interactions alone can produce not only the previously observed phase, but also new Chern insulating phases in MATBG by breaking time-reversal symmetry [1, 3]. |
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