Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T56: Twisted Multilayer Graphene Away from Magic AngleRecordings Available

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Chair: Yutao Li, Columbia University Room: Hyatt Regency Hotel Burnham 
Thursday, March 17, 2022 11:30AM  11:42AM 
T56.00001: Phasons in twisted trilayer graphene Rhine Samajdar, Yanting Teng, Yunchao Zhang, Subir Sachdev, Mathias S Scheurer Twisted trilayer graphene (TTG) is the latest addition to the growing family of novel moiré systems. Motivated by recent experimental advances in this system, we study its longwavelength lattice fluctuations on the moiré scale and, in particular, its phason modes. Unlike twisted bilayer graphene, TTG hosts two sets of phasons, which can be classified as either odd or even under the mirror symmetry of the system. Taking into account the important effects of lattice relaxation, we calculate the phason spectra by solving the continuum equations of motion for infinitesimal vibrations and illustrate the differences between the mirroreven and odd sectors. An outofplane displacement field provides an experimental knob to tune the physics of TTG, and we analyze the effects of such a field, which breaks the mirror symmetry and couples the even and odd modes. Finally, we discuss the interactions between the phasons and the electronic degrees of freedom and consider the implications thereof for superconductivity. 
Thursday, March 17, 2022 11:42AM  11:54AM 
T56.00002: Orbital magnetism in twisted multilayer graphene with two moiré interfaces Priyamvada Jadaun, Bart Soree Recent years have seen an intense interest in the field of moiré materials formed by stacking layers of 2D crystals that have a small difference in their lattice constant or orientation. 2D moiré systems often show both strong electronic correlations and nontrivial band topology leading to novel properties such as quantum anomalous Hall effect, superconductivity and emergent orbital ferromagnetism. Here, we design heterostructures of graphene layers composed of two moiré interfaces. We study the system by varying the relative stacking chirality of the moiré interfaces along with parameters such as external field and strain. Specifically, we focus on the variation of topological states and orbital magnetism in these systems as a function of these parameters. Our results show that multimoiré systems provide an attractive platform for a variety of applications owing to their novel and tunable electronic properties. 
Thursday, March 17, 2022 11:54AM  12:06PM 
T56.00003: Theoretical study ofChern transitions in Hofstadter bands of twisted double bilayer graphene Dibya K Mukherjee, Pratap C Adak, Subhajit Sinha, DEBASMITA GIRI, L. D. Varma Sangani, Kenji Watanabe, Takashi Taniguchi, Herb Fertig, Arijit Kundu, Mandar M Deshmukh We study Hofstadter physics of twisted double bilayer graphene (TDBG) in presence of an outofplane electric field. In this system Hofstadter bands have unequal distribution of charges in the layers. Moreover, such layer polarization depends on the valley index. These together lead to nontrivial changes of Chern number as a function of the applied electric field. In addition, if we include an assumed spinexchange, the resulting cascade of even and odd change of Chern numbers matches with the experimental findings. This talk will focus on the theoretical analysis of the work, where the experimental results will be presented in a separate talk. 
Thursday, March 17, 2022 12:06PM  12:18PM 
T56.00004: Interplay of moiré and Landau level physics in twisted double bilayer graphene Yulia Maximenko, Marlou R Slot, Sungmin Kim, Son T Le, EnMin Shih, Steven Blankenship, Kenji Watanabe, Takashi Taniguchi, Fei Xue, Yafis Barlas, Paul Haney, Nikolai Zhitenev, Fereshte Ghahari Kermani, Joseph A Stroscio After superconductivity was first observed in magicangle twisted bilayer graphene, a new field of moiré materials quickly formed. Moiré materials are weakly coupled 2D crystals where a new periodicity is created due to a small lattice parameter or angle mismatch. Moiré systems can host a variety of quantum phases due to the flatness of the electronic bands and enhanced electronelectron interaction. Smallangle twisted double bilayer graphene (TDBG) made up of two Bernalstacked bilayers is tunable by electric field which can drive the system into a correlated phase for a continuous range of twist angles. Here, we use a multimode instrument combining force microscopy, tunneling microscopy, and electrostatic gating to study TDBG at 10 mK under magnetic field up to 15 T. We demonstrate the interplay between the band physics, electronelectron correlations, and fieldinduced Landau levels in TDBG with atomic resolution. We use the experimental data and theoretical calculations to further unravel the complexity of quantum phases in this tunable moiré system. 
Thursday, March 17, 2022 12:18PM  12:30PM 
T56.00005: Electric field drives Chern transition in Hofstadter bands of twisted double bilayer graphene Pratap C Adak, Subhajit Sinha, DEBASMITA GIRI, Dibya K Mukherjee, L. D. Varma Sangani, Kenji Watanabe, Takashi Taniguchi, Herb Fertig, Arijit Kundu, Mandar M Deshmukh Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, a major focus has been to gain new insights into correlatedelectron physics hosted by the flat bands in smallangle moiré systems. However, the topological properties of these moiré bands such as Chern numbers are little explored. We study the effect of band topology on Hofstadter physics using twisted double bilayer graphene (TDBG), where the perpendicular electric field tunes the band structure and its topological properties. In addition to the signatures of nontrivial topology in the Hofstadter spectra, we observe a cascade of Chern gaps that switch their Chern numbers oneatatime as we tune the electric field. Our experiments together with theoretical calculations suggest a crucial role of charge polarization in TDBG that changes concomitantly with topological transitions. The layer polarization is likely to play an important role in the topological states in fewlayer twisted systems. Overall, our work establishes TDBG as a novel Hofstadter platform to switch magnetic moment tunable with an external knob of the electric field. 
Thursday, March 17, 2022 12:30PM  12:42PM 
T56.00006: Berry curvature dipole senses topological transition in a moiré superlattice Subhajit Sinha, Pratap C Adak, Atasi Chakraborty, Kamal Das, Koyendrila Debnath, L. D. V Sangani, Kenji Watanabe, Takashi Taniguchi, Umesh V Waghmare, Amit Agarwal, Mandar M Deshmukh Berry curvature and Chern number define the topological structure of electronic bands. While Berry curvature and its effects have been studied, detecting changes in the topological invariant, Chern number, is challenging. In this regard, twisted double bilayer graphene (TDBG) has emerged as a platform to gain electrical control over the Berry curvature hotspots and the valley Chern numbers of its flat bands. In addition, strain induced breaking of the threefold rotation (C_{3}) symmetry in TDBG, leads to a nonzero first moment of Berry curvature called the Berry curvature dipole (BCD), which can be sensed using nonlinear Hall (NLH) effect. We reveal, using TDBG, that the BCD detects topological transitions in the bands and changes its sign. In TDBG, the perpendicular electric field tunes the valley Chern number and the BCD simultaneously allowing us to probe the physics of topological transitions. Furthermore, we find hysteresis of longitudinal and NLH responses with electric field that can be attributed to switching of electric polarization in moiré systems—this holds promise for nextgeneration Berry curvaturebased memory devices. Probing topological transitions, as we show, can be emulated in other 3D topological systems. 
Thursday, March 17, 2022 12:42PM  12:54PM 
T56.00007: Dichotomy of Thermodynamic and Transport Properties in Twisted DoubleBilayer Graphene Yimeng Wang, Jonah HerzogArbeitman, G W Burg, Jihang Zhu, Kenji Watanabe, Takashi Taniguchi, Allan H MacDonald, Andrei B Bernevig, Emanuel Tutuc The emergence of controlled, twodimensional moiré materials has uncovered a new platform for investigating topological physics. Twisted double bilayer graphene has been predicted to host a topologically nontrivial gapped phase with Chern number equal to two at charge neutrality, when half the flat bands are filled. However, it can be difficult to diagnose topological states using a single measurement. The challenge lies in probing the bulk and edge properties at the same time. A combination of chemical potential measurements, transport measurements, and theoretical calculations provides insight to the band topology of twisted double bilayer graphene. The thermodynamic gap extracted from chemical potential measurements shows that twisted double bilayer graphene is a bulk insulator under an applied transverse electric field, while transport measurements show metallic edge transport. A LandauerBüttiker analysis of measurements on multiterminal samples allows us to quantitatively assess edge state scattering. We interpret these results as signatures of the predicted topological phase at charge neutrality. 
Thursday, March 17, 2022 12:54PM  1:06PM 
T56.00008: Spontaneous time reversal symmetry breaking in twisted double bilayer graphene Manabendra Kuiri, Aswin Vishnuradhan, Kenji Watanabe, Takashi Taniguchi, Joshua Folk Twisting two individual graphene layers at small angles can dramatically alter the band structure, leading in some cases to nearly flat bands dominated by electronelectron interactions, where correlated electronic states have been observed. A particular case of such structures is twisted double bilayer graphene (tDBG), composed of two Bernal stacked bilayer graphene sheets with a twist between them, where the emergence of correlated states can be controlled by twist angle (θ), carrier density, and displacement field. Signatures of broken spin and/or valley symmetries have been reported in both insulating and metallic correlated states in tDBG, although the precise description of such states remains elusive. Here, we study the magnetotransport properties of a dual gated tDBG sample with twist angle (θ∼1.3°), similar to samples measured in previous reports where inplane magnetic field dependence of the resistivity gave evidence for spin polarization of the correlated insulators at integer filling, = 2 and 3. The inplane magnetoresistance of correlated insulator states in our sample is consistent with previous reports, but in addition we observe hysteresis in longitudinal and transverse resistivity as a function of outofplane field for the correlated metallic state at fractional filling 3.5 4. These hysteretic anomalous Hall signatures suggest spontaneous timereversal symmetry breaking associated with valley ferromagnetism. The outofplane hysteresis is suppressed but not eliminated by additional inplane fields up to 1T, perhaps reflecting an interplay between spin and valley degrees of freedom in the system. 
Thursday, March 17, 2022 1:06PM  1:18PM 
T56.00009: Local spectroscopy of a gatereversible Chern insulator in twisted monolayerbilayer graphene Canxun Zhang, Tiancong Zhu, Tomohiro Soejima, Salman A Kahn, Kenji Watanabe, Takashi Taniguchi, Stefano Cabrini, Alex K Zettl, Michael P Zaletel, Michael F Crommie Stacking twodimensional van der Waals materials is a novel approach to realize Chern insulators featuring topologically protected edge states that arise from nonzero total Chern number (C_{tot}) for occupied bands. Signatures of correlated insulating states and quantized Hall conductance were recently detected in twisted monolayerbilayer graphene (tMBLG) – a graphene monolayer rotationally misaligned with respect to a Bernalstacked bilayer. We have fabricated gatetunable tMBLG devices and characterized their local electronic structure using scanning tunneling microscopy and spectroscopy. We observe charge gaps in the local density of states at ½ and ¾filling of the tMBLG conduction flat band, consistent with the emergence of symmetrybreaking insulating phases. In an outofplane magnetic field the ¾filling correlation gap evolves into two separate gaps at different electron filling levels that follow the Středa formula with C_{tot} = 2 and C_{tot} = 2, thus signifying gatecontrolled switching of the spontaneous valley polarization. Our observations are consistent with reported transport studies and can be understood theoretically as resulting from a competition between bulk and edge contributions to the overall magnetization. 
Thursday, March 17, 2022 1:18PM  1:30PM 
T56.00010: Correlated Insulating States and Transport Signature of Superconductivity in Twisted Trilayer Graphene Superlattices Xi Zhang, KanTing Tsai, Ziyan Zhu, Wei Ren, Stephen T Carr, Mitchell Luskin, Efthimios Kaxiras, Ke Wang Layers of twodimensional materials stacked with a small twist angle give rise to beating periodic patterns on a scale much larger than the original lattice, referred to as a “moiré superlattice” [1] [2]. Here, we demonstrate a higherorder “moiré of moiré” superlattice in twisted trilayer graphene with two consecutive small twist angles. We report correlated insulating states near the half filling of the moiré of moiré superlattice at an extremely low carrier density (∼10^{10} cm^{−2}), near which we also report a zeroresistance transport behavior typically expected in a 2D superconductor. The fulloccupancy (ν = −4 and ν = 4) states are semimetallic and gapless, distinct from the twisted bilayer systems. 
Thursday, March 17, 2022 1:30PM  1:42PM 
T56.00011: Strongly correlated states in twisted graphene multilayers Dacen Waters, Anna Okounkova, Yahui Zhang, Kenji Watanabe, Takashi Taniguchi, Matthew A Yankowitz Moiréinduced flat bands in twisted graphene systems have recently emerged as a highly tunable platform for the study of strongly correlated and topologic states. Extremely flat bands are formed in twisted bilayer and twisted trilayer graphene when they are tuned to their respective magic angles. In contrast, twisted monobilayer graphene (t1+2) and twisted double bilayer graphene (t2+2) have a broader range of angles over which flat bands are formed, in part due to the ability to tune the moiré bandwidth with a displacement field. However, as the number of constituent graphene layers is increased, the number of bands that must be hybridized also increases. So far, it is unclear whether a tunable moiré flat band hosting strongly correlated states can emerge in systems approaching the graphitic limit. In this work, we present transport results for a series of twisted multilayer graphene systems (tM+N) in which signatures of strong correlations are observed. We further study the ground state ordering and topological character of the correlated states. This work demonstrates that correlations and topology are not limited to mono and/or bilayer graphene, and points toward a general understanding of correlations in severallayer graphitic systems with a single rotated interface. 
Thursday, March 17, 2022 1:42PM  1:54PM 
T56.00012: Electrically modulated thermal emissivity in multilayer graphene devices Jonathan Habif, Stephen B Cronin, Arunkumar Jagannathan, Zhi Cai, Haley A Weinstein All warm bodies emit Planck radiation which is determined by object temperature, geometry, and material. These properties often result in deviation from a true “blackbody” emitter. Engineering the thermooptic properties of devices has been achieved both by development of novel materials as well as clever nanostructure patterning on device surfaces. In this work, we present experimental results from devices where the effective material emissivity can be modulated with an electrical voltage. We fabricate and characterize multilayer (~100 layers) graphene devices, by sandwiching an ionic liquidsoaked polymeric membrane between the graphene and a conductive gold electrode. When a voltage is placed across the device, the effective emissivity of the device is suppressed in the longwave infrared (LWIR) spectrum (i.e., 714µm), resulting in an apparent temperature decrease when measured with a thermal camera. We also report Raman spectra of the devices under normal operating conditions, which lend insight into the underlying mechanism of electrochemical doping and IR modulation. Additionally, we characterize the temporal and electrical emissivity change of these devices with a VOx camera. 
Thursday, March 17, 2022 1:54PM  2:06PM 
T56.00013: Invisible Nlayer graphene Kenichi Sasaki, Tomohiro Matsui, Masahiro Kamada, Takamoto Yokosawa, Taisuke Ochi The detection of graphene layers on Si substrates is a common task, but has the aspect of being a nontrivial scientific problem. In visual detection of graphene, the difference between the reflectance of light from the substrate and that from graphene on it must be sufficiently large for increasing their contrast. It is known that Si substrates with a SiO_{2} surface layer of a certain thickness have an advantage[1]. Namely, when the optical thickness of SiO_{2} is a quarter or three quarters of the light wavelength used, the reflectance from the substrates is minimized and the contrast takes a maximum. What happens to the contrast if the number of graphene layers on the substrates increases? 
Thursday, March 17, 2022 2:06PM  2:18PM 
T56.00014: Correlated states in ABC trilayer graphene/hBN moiré heterostructures Elena Bascones, María J Calderón, Alberto Camjayi Shortly after the discovery of correlated states and superconductivity in twisted bilayer graphene, correlated insulating states were also detected in another graphene based moiré heterostructures: the ABC trilayer aligned with hBN. The influence of the moiré on the correlated states has been questioned by the recent report of ferromagnetism and superconductivity in ABC trilayer graphene without the moiré pattern induced by hBN. Using Dynamical Mean Field Theory we have studied the signatures of Mott physics which can be induced by the moiré pattern on the trivial valence band at halffilling. In the talk we will discuss the doping and temperature dependent spectral weight reorganization characteristic of Mott correlations and absent in standard symmetry breaking transitions, and a competing antiferromagnetic state present very close to halffilling . 
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