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 B21: Moire Beyond Magic-Angle I |
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Sponsoring Units: DCMP Chair: Kevin Nuckolls, Princeton University Room: Room 213 |
Monday, March 6, 2023 11:30AM - 11:42AM |
B21.00001: The Cryogenic Quantum Twisting Microscope, Part II Alon Inbar, John Birkbeck, Jiewen Xiao, Takashi Taniguchi, Kenji Watanabe, shahal ilani The discovery of magic angle twisted bilayer graphene has highlighted the fantastic capability to dramatically modify the properties of materials by a small change of their twist angle. To date, however, twisted devices are generally fabricated with a fixed angle that cannot be modified once the device has been made. An in-situ “twistronics” apparatus that can bring into contact two van-der-Waals (vdW) layers and probe the hybrid interface with varying twist angle could be the ideal tool to explore correlated physics in a variety of such interfaces. Recently we have developed the Quantum Twisting Microscope (QTM)1, capable of performing such in-situ twisting. In the first part of this talk we focused on the momentum-resolving capabilities of this tool, where an insulating barrier between two vdW layers allows one to probe the unperturbed energy-momentum dispersion of the other. In this second part, we will focus on an orthogonal capability of the QTM to perform in-situ “twistronics” experiments. Here, the two vdW layers are brought into direct contact, and the transport properties of the emergent, hybridized interface are measured as the twist angle is continuously scanned. We recently demonstrated a proof-of-principle of this experiment at room temperature. In this talk we will describe the generalization of these experiments into cryogenic temperatures. |
Monday, March 6, 2023 11:42AM - 11:54AM |
B21.00002: The Cryogenic Quantum Twisting Microscope, Part I John Birkbeck, Alon Inbar, Jiewen Xiao, Kenji Watanabe, Takashi Taniguchi, shahal ilani Measuring the energy-momentum dispersion relation in quantum materials can provide key insights into their strongly correlated electronic phenomena. We have recently demonstrated a new type of a scanning probe microscope – the Quantum Twisting Microscope (QTM) – capable of measuring electrons in momentum space in a similar way to the way a scanning tunneling microscope (STM) measures electrons in real-space. The QTM is based on a van-der-Waals (vdW) heterostructure on a tip, which, when brought into contact with another vdW sample, allows electrons to tunnel into it at many locations simultaneously, and quantum coherently. This makes the QTM tip a scanning electronic interferometer. With an extra twist degree of freedom, this microscope becomes a momentum-resolving local scanning probe. The first version of our microscope operated at room temperature, and already there demonstrated quantum coherence at its tip, and the ability to image the dispersions of monolayer and twisted bilayer graphene1. Yet, even more exciting physics can emerge if the QTM could be generalized to cryogenic temperatures, where quantum mechanics and strong electronic correlations are at their prime. In the first part of this talk, we will present our advances in cryo-QTM momentum-resolved imaging. |
Monday, March 6, 2023 11:54AM - 12:06PM |
B21.00003: STM spectroscopy of a gate-switchable moiré quantum anomalous Hall insulator Canxun Zhang, Tiancong Zhu, Tomohiro Soejima, Salman A Kahn, Kenji Watanabe, Takashi Taniguchi, Alex K Zettl, feng wang, Michael P Zaletel, Michael F Crommie Twisting and stacking atomically-thin materials provides a versatile platform for investigating emergent quantum phases of matter driven by strong correlation and non-trivial topology. Novel phenomena such as correlated insulating states, unconventional superconductivity, and the quantum anomalous Hall (QAH) effect have been observed in different twisted moiré systems, but a full understanding of their underlying microscopic mechanisms remains a challenge. We have used scanning tunneling microscopy and spectroscopy to explore the interplay between correlation, topology, and local atomic structure in determining the behavior of a QAH insulator made from twisted monolayer-bilayer graphene (a sandwich of monolayer and bernal-stacked bilayer graphene with a small twist between them). We observe local spectroscopic signatures of correlated insulating states having total Chern number Ctot = +2 and -2 at ¾-filling of the conduction moiré mini-band and have characterized their evolution in an out-of-plane magnetic field. We have determined the relationship between topological behavior, local twist angle, and local hetero-strain, and show that Ctot can be switched between +2 and -2 via electrostatic gating only over a limited range of twist angle and strain. Electrical control of the Chern number results from a competition between the orbital magnetization of bulk bands and chiral edge states that is highly sensitive to distortion of the moiré superlattice. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B21.00004: Charge density texture in twisted multilayer graphene at low twist angles Jungho Daniel Choi, Jie Wang, Jennifer Cano In the chiral limit of the continuum model of twisted bilayer graphene (TBG), the band structure exhibits exactly flat bands at magic twist angles and the wave function exhibits an interesting nodal structure in real space. This work investigates how these properties persist when TBG is twisted away from magic angles and tuned away from the chiral model parameters to a more realistic limit. Specifically, we show that the charge density can exhibit an intra-cellular texture at smaller angles, which are distinct from the peaks seen at AA stacking points when near the first magic angle. Making use of an exact mapping to alternately twisted trilayer graphene (TTG), we show that the same charge density texture appears in TTG at larger angles, which provides a more viable experimental platform. We describe implications for future scanning tunnelling microscopy experiments. |
Monday, March 6, 2023 12:18PM - 12:30PM Author not Attending |
B21.00005: Testing the Nanotribology Theories on Twisted Bilayer Graphene Dogan Erbahar, Cem Celebi, Ozhan Unverdi, Semran Ipek, Dilara Ickecan, Eftal Gezer, Vesile Karakoyun, Tugba Sirin, Melike Gozek Friction is a ubiquitous force that appears in all length scales, and it has to be dealt with in a wide variety of circumstances ranging from daily life applications to high technology products. Depending on the circumstances it could be desirable or disadvantageous. In any case a solid theoretical understanding of the physical origin is most essential. However, since friction is not itself a fundamental force and instead arises from a combination of factors like inter-surface adhesion, surface roughness, surface deformation, contamination, etc. it is very challenging to calculate it from the first principles leaving aside developing a fundamental theory. The studies tackling this challenge mainly suffers from the fact that multiple variables come to play all at once during the experiments making it very difficult to pinpoint the individual contributions from different factors. One such study which was conducted by us previously showed that the charge distribution on the surface of a single layer graphene was dramatically affected from a minor topographical change in the substrate causing a big difference in the coefficient of friction. Here we will be presenting and discussing our preliminary results of our new research project where we have been studying the tribological properties of twisted bilayer graphene trying to exploit the fact that the surface periodicity of such systems is relatively easy to modify by a single parameter hence could contribute to the fundamental understanding of friction. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B21.00006: Weak localization in twisted double bilayer graphene Zhenxiang Gao Stacking two Bernal-stacked bilayer graphene sheet together with a twist angle θ makes a twisted double bilayer graphene (tDBG); adding top and back gates enables the Moire-modified band structure to be tuned independent of the filling of those bands. In tDBG devices with θ~1.2-1.3°, the first Moire conduction band is nearly flat and isolated fron neighboring bands over a narrow range of perpendicular displacement field, where correlated insulator and correlated metallic phases appear and where spin and/or valley symmetries are believed to be broken. Here, we present an investigation of low field magnetoresistance on tDBG devices down to 20mK. The data display a weak localization (WL) correction throughout most of the correlated metal phase, mapping out regions in which time reversal symmetry is intact; outside of the correlated metal phase, the WL correction is rarely seen. Modifications to the WL correction due to proximal WSe2 will also be discussed. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B21.00007: Correlated and topological states in twisted bilayer-trilayer graphene Anna Okounkova, Dacen Waters, Ellis Thompson, Esmeralda Arreguin-Martinez, Yahui Zhang, Matthew A Yankowitz, Takashi Taniguchi, Kenji Watanabe Moiré patterns have opened up many new avenues of study in van der Waals materials owing to their ability to generate flat bands with nontrivial topology. So far, the investigation of graphene-based moiré systems has been limited to twisted structures comprising only monolayers and bilayers of graphene. Here, we investigate a larger family of twisted M+N graphene multilayers, created by stacking and rotating M- and N-layer Bernal graphene sheets. We observe correlated and topological states with striking commonalities across many different structures in the ultra-thin limit (e.g., t1+2, t2+2, t1+3, t2+3, etc), owing to a close resemblance of their moiré flat bands. We find that twisted bilayer-trilayer graphene exhibits a particularly rich correlated phase diagram, manifesting a wide array of symmetry-broken states at partial filling of both the valence and conduction moiré minibands. We observe an anomalous Hall effect upon filling one electron per moiré unit cell, pointing to the emergence of orbital magnetism in a topological band. Additional correlated states emerge upon applying a perpendicular magnetic field. Overall, our work points towards a common understanding of the correlated phase diagram across a wide variety of twisted graphene multilayer structures. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B21.00008: Higher-Order Topological Phase in Twisted Bilayer Graphene Moon Jip Park We propose the twisted bilayer graphenes with large angles as higher-order topological insulators, hosting topological corner charges. At large commensurate angles, the intervalley scattering opens up the bulk gap and the corner states occur at half filling. Based on both first-principles calculations and analytic analysis, we show the striking results that the emergence of the corner states does not depend on the choice of the specific angles as long as the underlying symmetries are intact [1]. Finally, using a photonic crystal platform, we show that the strong coupling regime in a moiré superlattice can realize cascades of stable topological flat bands at large twist angles [2]. |
Monday, March 6, 2023 1:06PM - 1:18PM Author not Attending |
B21.00009: Intralayer and interlayer electron-phonon processes in twisted bilayer graphene investigated by resonance Raman spectroscopy MARCOS A PIMENTA, Pedro Venezuela, Eliel Silva, Marcus V Moutinho, Ariete Righi, Rafael N Gontijo, Thierry Michel, Matthieu Paillet, Po-Wen Chiu In this presentation, I will discuss the use of Raman spectroscopy to study electron-phonon |
Monday, March 6, 2023 1:18PM - 1:30PM |
B21.00010: Berry curvature dipole senses topological transition in twisted double bilayer graphene Subhajit Sinha, Pratap C Adak, Atasi Chakraborty, Kamal Das, Koyendrila Debnath, L. D. Varma Sangani, Kenji Watanabe, Takashi Taniguchi, Umesh V Waghmare, Amit Agarwal, Mandar M Deshmukh The topological phase of quantum materials can be characterized by the band-specific topological invariant Chern number. Often, systems undergo a transition between topological phases, and these transitions are hard to detect. Moiré systems host flat Chern bands. However, time-reversal symmetry dictates the Chern numbers from two valleys, namely, K and K', to be opposite, making the total Chern number CK + CK' = 0. In such cases, Berry curvature dipole (BCD) can be used as an indicator of the underlying topological transition of the valley Chern type, that is, a change in Z2=(CK – CK')/2. Unlike the quantum Hall effect or anomalous Hall effect, the approach based on the BCD does not require explicitly breaking the time-reversal symmetry. We reveal, using the nonlinear Hall (NLH) effect measurements in twisted double bilayer graphene (TDBG), that the BCD detects Z2 transition and changes its sign. 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 next-generation Berry curvature-based memory devices. Probing topological transitions, as we show, can be emulated in other 3D topological systems. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B21.00011: Anomalous Hall effect at half filling in twisted bilayer graphene Chun-Chih Tseng, Xuetao Ma, Zhaoyu Liu, Kenji Watanabe, Takashi Taniguchi, Jiun-Haw Chu, Matthew A Yankowitz Magic-angle twisted bilayer graphene (MA-tBLG) hosts a wealth of symmetry-broken states arising due to strong Coulomb interactions in the moiré flat bands. Orbital magnetic states, with spontaneously broken time-reversal symmetry at zero field, have been previously reported at odd integer filling factors (ν = +1 and +3) in a small number of devices, manifesting as an anomalous Hall effect (AHE). Here, we will discuss two tBLG devices with twist angles slightly away from the magic angle (0.96° and 1.2°). Surprisingly, both devices display an AHE at half filling (ν = +2 and -2, respectively), which has not been observed previously. These states are not anticipated in existing theoretical models owing to competing intervalley-coherent, spin-polarized, and/or valley Hall states with lower energy. We suggest that our observations at half filling likely correspond to valley polarized phases stabilized by the combination of the increased bandwidth away from the magic angle and symmetry-breaking substrate potentials from the boron nitride. We do not observe superconductivity in our devices, hinting at a possible antagonistic relationship between the two. Our results motivate further investigation of tBLG away from the strongly coupled limit. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B21.00012: Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces Aaron L Sharpe, Xiaoyu Wang, Joe Finney, Linsey Rodenbach, Connie L Hsueh, Kenji Watanabe, Takashi Taniguchi, Marc A Kastner, Oskar Vafek, David Goldhaber-Gordon, David Goldhaber-Gordon Uniaxial heterostrain in twisted bilayer graphene can have a profound effect on the bandstructure. In this talk, we will discuss the effects of strain in a twisted bilayer graphene device with a twist angle slightly above the magic angle as seen in both theory and transport. Features such as a non-saturating magnetoresistance are well described by our addition of uniaxial heterostrain into the Bistritzer-MacDonald model. We find that strain breaks the degeneracy of the three van Hove points, leading to broad range of densities where open Fermi surfaces exists, explaining the non-saturating magnetoresistance. Additionally, our theory also predicts a marked rotation of the electrical transport principal axes as a function of filling even for fixed strain and for rigid bands. Our results indicate that strain-induced effects may lead to similar phenomenology as interaction-induced nematic order. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B21.00013: Evidence of spin-valley texture in Twisted Double Bilayer Graphene Christopher Coleman, Manabendra Kuiri, Zhenxiang Gao, Joshua Folk Twisted van der Waals heterostructures are celebrated platforms for realizing flat electronic bands, in which electron-electron interactions can lead to correlated phases. Dual gated heterostructures composed of twisted double bilayer graphene have demonstrated a particularly rich landscape, exhibiting valley and spin polarized states, nematic order and spontaneous time reversal symmetry breaking (TRSB). In this work we report on an anomalous resistivity response to the application of magnetic fields parallel to the device plane, for the same gate voltage settings where orbital TRSB is observed: a narrow resistance peak is observed when crossing zero in-plane field, which may increase the sample resistance by up to a factor of 3. The feature is suppressed by just a few millitesla of in-plane field, but only weakly sensitive to out-of-plane field. The resistance peak height decreases with increasing temperature and is not observed above 500 mK. Such extreme sensitivity to in-plane field is not expected for an orbital magnet; these observations hint at an underlying spin-valley texture in TDBG. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B21.00014: Visualizing and manipulating chiral edge states in a graphene-based quantum anomalous Hall insulator Canxun Zhang, Tiancong Zhu, Salman A Kahn, Tomohiro Soejima, Kenji Watanabe, Takashi Taniguchi, Alex K Zettl, feng wang, Michael P Zaletel, Michael F Crommie Quantum anomalous Hall (QAH) insulators are topological phases of matter that host one-dimensional chiral edge-states that conduct electrical current unidirectionally, thus leading to quantized Hall conductance and dissipationless carrier transport. We have used scanning tunneling microscopy and spectroscopy to directly visualize and manipulate chiral edge states in a moiré QAH insulator made from twisted monolayer-bilayer graphene. We are able to control the local Chern number by manipulating the device carrier concentration, thus stabilizing adjacent domains having opposite Chern number and enabling the visualization of chiral edge states residing at domain interfaces that are independent of structural boundaries within our samples. The use of conventional back-gating to vary the carrier density enables chiral edge-states to be reversibly moved across the unbroken moiré landscape. Formation of tip-induced quantum dots provides more local control over carrier density and Chern number, thus allowing intentional creation of chiral edge states having reversible chirality. This approach to visualizing chiral edge states eliminates the complications of edge defects, dangling bonds, and trivial interface states that have plagued previous measurements. |
Monday, March 6, 2023 2:18PM - 2:30PM |
B21.00015: Electronic structure of lattice relaxed alternating twist tG-multilayer graphene: from few layers to bulk AT-graphite Nicolas Leconte, Jeil Jung, Youngju Park, Jiaqi An, Appalakondaiah Samudrala Alternating twist multilayer graphene systems are at the heart of recent research efforts on flat band superconductivity and therefore precise descriptions of their atomic and electronic structures are desirable. We present the electronic structure of AA’AA’. . . stacked alternating twist N-layer (tNG) graphene for N = 3, 4, 5, 6, 8, 10, 20 layers and bulk alternating twist (AT) graphite systems where the atomic structure is relaxed using a molecular dynamics simulation code. The low energy bands depend sensitively on the relative sliding between the layers but we show explicitly up to N = 6 that the highly symmetric AA′AA′. . . stacking is energetically preferred among all interlayer sliding geometries of each added layer, justifying why experimental devices consistently show results compatible with this geometry. It is found that lattice relaxations enhance electron-hole asymmetry, and leads to small reductions of the magic angle values with respect to analytical or continuum model calculations with ?xed tunneling strengths that we quantify from few layers to bulk AT-graphite. The twist angle error tolerance near the magic angles obtained by maximizing the density of states of the nearly flat bands expands progressively from 0.05? for t2G to up to 0.2? for AT-graphite, hence allowing a greater twist angle flexibility in multilayers. |
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