Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L14: Electronic Interactions in Twisted Bilayers |
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Sponsoring Units: DCMP Chair: Cory Dean Room: BCEC 153C |
Wednesday, March 6, 2019 11:15AM - 11:27AM |
L14.00001: Chiral SDW and d + id superconductivity in the magic-angle twisted bilayer-graphene Cheng-Cheng Liu, Li-Da Zhang, Wei-Qiang Chen, Fan Yang We model the newly synthesized magic-angle twisted bilayer-graphene superconductor with two px,y-like Wannier orbitals on the superstructure honeycomb lattice, where the hopping integrals are constructed via the Slater-Koster formulism by symmetry analysis. The characteristics exhibited in this simple model are well consistent with both the rigorous calculations and experimental observations. Van-Hove singularity and Fermi-surface nesting are found in the doping levels relevant to the correlated insulator and unconventional superconductivity revealed experimentally, base on which we identify the two phases as weak-coupling FS instabilities. Then, with repulsive Hubbard interactions turning on, we performed random-phase-approximation (RPA) based calculations to identify the electron instabilities. As a result, we find chiral d + id topological superconductivity bordering the correlated insulating state near half-filling, identified as noncoplanar chiral spin-density wave (SDW) ordered state, featuring quantum anomalous Hall effect. The phase-diagram obtained in our approach is qualitatively consistent with experiments. |
Wednesday, March 6, 2019 11:27AM - 11:39AM |
L14.00002: Multiple topological transitions in magic angle twisted bilayer graphene I Kasra Hejazi, Chunxiao Liu, Hassan Shapourian, Xiao Chen, Leon Balents The physics of twisted bilayer graphene (TBG) structures has attracted a lot of interest experimentally and theoretically. Attention was drawn to these systems largely by recent experimental works, which showed strong correlation effects in bilayer graphene systems with a twist angle of roughly 1°, the so-called magic angle. A continuum model for describing such systems is used in our study, and instead of observing flat bands only at the magic angle, we notice that the bands remain almost flat within a small range around the magic angle, where multiple topological transitions occur. The topological transitions are caused by creation and annihilation of Dirac points, which are transported within the moire Brillouin zone as the angle is varied around the magic angle. Furthermore, we propose an effective low energy six-band model near the Γ point, which we argue is the minimal model to explain the motion of the Dirac points around Γ as the angle is varied. These observations can also be exploited for explaining the experimental results regarding the response of the magic angle TBG systems to external magnetic field. |
Wednesday, March 6, 2019 11:39AM - 11:51AM |
L14.00003: Electron-phonon superconductivity and Landau levels in twisted bilayer graphene Biao Lian, Zhijun Wang, Fang Xie, Andrei B Bernevig In the first half of the presentation, we show the analytical calculation of the electron-phonon coupling in twisted bilayer graphene (TBG), which was recently experimentally observed to exhibit superconductivity around the magic twist angle $\theta\approx 1.05^\circ$. We show that phonon-mediated electron electron attraction at the magic angle is strong enough to induce a conventional intervalley pairing between graphene valleys $K$ and $K'$ with a superconducting critical temperature $T_c\sim1K$, in agreement with the experiment. In the second half of the talk, we show the existence of topological Landau levels at large magnetic fields due to nontrivial topology of the flat bands. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L14.00004: Magic angle twisted bilayer graphene: strong correlations and Landau level Gaurav Chaudhary, Allan MacDonald The electronic structure of twisted bilayer graphene (TBLG) features a peculiar Fermi velocity renormalization property which leads to vanishing Fermi-velocities near neutrality at certain "magic" twist angles. At the magic angle the system has strong correlations which have been shown experimentally to lead to Mott insulator and superconducting phases. I will discuss the Landau level structure of TBLG near magic angles, the relationship between electronic structure and experimentally observed Landau levels, and the relationship between magic angle behavior and the system's response to external magnetic fields. We show that the Landau level structure near neutrality is highly sensitive to self-energy modifications of magic angle electronic structure. |
Wednesday, March 6, 2019 12:03PM - 12:15PM |
L14.00005: High temperature quantum transport in 2D moiré superlattices Roshan Krishna Kumar, John Wallbank, Xi Chen, Gregory Auton, Matthew Holwill, Artem Mishchenko, Konstantin S Novoselov, Vladimir Falko, Andre Geim Van der Waals heterostructures based on moiré superlattices have become increasingly popular due to their large tunability unprecedented by any other solid-state system. In this talk, we review recent experimental results on high-temperature electron transport studies in heterostructures based on 2D moiré superlattices. Our experiments describe two fundamental phenomena of electron transport that manifest profoundly at high temperatures and are extremely sensitive to the twist-angle. First, we show that electron-electron collisions in graphene superlattices are dominated by umklapp processes [1]. They cause a giant increase in resistivity that grows rapidly upon decreasing the twist-angle between 2D layers, degrading the intrinsic mobility of graphene by more than an order of magnitude. Second, we show that moiré superlattices feature a novel class of quantum magnetoresistance oscillations [2] that differ fundamentally from the Shubnikov de Haas effect and all known quantum oscillatory phenomena. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L14.00006: Direct Imaging of Magnetic Structure in Twisted Bilayer Graphene with Scanning nanoSQUID-On-Tip Microscopy Marec Serlin, Charles Tschirhart, Hryhoriy Polshyn, Jiacheng Zhu, Martin E Huber, Andrea Young Bilayer graphene, rotationally faulted to ~1.1 degree misalignment, has recently been shown to host superconducting and resistive states associated with the formation of a flat electronic band. While numerous theories exist for the origins of both states, direct validation of these theories remains an outstanding experimental problem. Here, we focus on the resistive states occurring at commensurate filling (1/2, 1/4, and 3/4) of the two lowest superlattice bands. We test theoretical proposals that these states arise due to broken spin—and/or valley—symmetry by performing direct magnetic imaging with nanoscale SQUID-on-tip microscopy. This technique provides single-spin resolved magnetometry on sub-100nm length scales. I will present imaging data from our 4.2K nSOT microscope on graphite-gated twisted bilayers near the flat band condition and discuss the implications for the physics of the commensurate resistive states. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L14.00007: Multiple topological transitions in magic angle twisted bilayer graphene II Chunxiao Liu, Kasra Hejazi, Leon Balents Recent experiments have observed strongly correlated physics in twisted bilayer graphene (TBG) at very small angles θ0 = 1.05, along with nearly flat electron bands (FBs) at certain fillings. A good starting point to understand this is a continuum model [PNAS 108, 12233 (2011)] which successfully predicts the formation of FBs at θ0. Following this work, we investigate the low energy FB structure in the entire moiré Brillouin zone (mBZ) as twisting angle is changed; the effect of transverse lattice distortion is also considered. We notice that the bands remain almost flat within a small range around θ0, where multiple topological transitions occur. In addition to the previous understanding that the FBs are caused by the highly renormalized dispersion at K, we propose that there are other mBZ regions responsible for the low energy physics and the FBs. We trace the evolution of the Dirac points (DPs), which are very sensitive to the twist angle, and specify several processes of DP transfer within the mBZ. Furthermore, we study the effect of magnetic field to the system; comparison to the experimental results is given. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L14.00008: Role of lattice relaxations in magic angle graphene Stephen Carr, Shiang Fang, Ziyan Zhu, Efthimios Kaxiras Understanding the origin of correlated effects in twisted bilayer graphene (tBLG) first requires a complete single-particle picture of its low-energy electronic states. Previous models of tBLG have had either high accuracy (e.g. DFT, tight-binding supercells), or high twist angle resolution (e.g. continnum or k-dot-p models), but not both. We introduce an ab initio k-dot-p model that includes lattice relaxations which can exactly reproduce DFT tight-binding electronic band-structures, but with the ability to continously tune the twist angle. Inclusion of relaxation significantly changes the bandstructure near tBLG's first magic angle, and suppresses the appearance of the second magic angle. We find that a geometric interpretation of tBLG's relaxed atomic structure extends to it's low-energy electronic states, creating a comprehensive picture of both mechanical and electronic effects at small twist angle. |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L14.00009: Localized eigenstates in dodecagonal twisted bilayer graphene Moon Jip Park, Heeseung Kim, SungBin Lee Dodecagonal quasicrystalline order has been experimentally demonstrated in twisted bilayer graphene systems. In this talk, we report emergence of localized eigenstates in the dodecagonal quasicrystals without additional random disorder. That is to say, the non-periodic quasicrystalline order causes the localization. These localized states are evidenced by the scaling behaviors of inverse participation ratio and energy level statistics, which have been used to characterize Anderson localization phenomena. Using exact diagonalization method, we show that the localization can be driven by increasing the interlayer coupling strength. In addition, we will compare our result with the conventional two-dimensional Anderson localization transitions. |
Wednesday, March 6, 2019 1:03PM - 1:15PM |
L14.00010: Atmospheric Pressure Chemical Vapor Deposition of Twisted Bilayer Graphene on
Copper Foil Substrates Lucas Hanson, Nikhil M Tilak, Michael Altvater, Brian J Ellsworth, Eva Andrei Atmospheric pressure chemical vapor deposition (APCVD) of graphene bilayers on |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L14.00011: Strong Correlations in Magic Angle Twisted Bilayer Graphene Investigated by Scanning Tunneling Microscopy Alexander Kerelsky, Leo McGilly, Shaowen Chen, Matthew Yankowitz, Lede Xian, Dante Kennes, Angel Rubio, Cory R Dean, Abhay Pasupathy Recent results on twisted bilayer graphene near the “magic” angle of 1.1 degrees reveal gate-tunable superconducting and correlated insulating states generally attributed to the flat band emerging from the moire superlattice and hybridization between the two layers. To date, the investigation of these states has been limited to bulk probes, inherently hindered by overall sample disorder and inhomogeneity. Using gated scanning tunneling microscopy and spectroscopy (STS), we directly locally investigate twisted bilayer graphene around the magic angle. Many theories have been proposed as to the precise band structure which causes the insulating and superconducting states to emerge, but the question remains for instance as to the size of the gap at charge neutrality and at half filling of the moire superlattices. We show the real experimental L-DOS probed by STS at and near the magic angle with gate tunable doping. Our results show the size of the gap at charge neutrality as well as the half filling state which enables a better understanding of the correlation effects leading to the insulating and superconducting states. Additionally, we study the effects of the local inhomogeneity which likely leads to the variable results observed in transport. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L14.00012: Scanning tunneling microscopy of gated twisted bilayer graphene: Suppression of tunneling density of states due to electronic correlations Youngjoon Choi, Jeannette Kemmer, Harpreet Singh Arora, Robert Polski, Yiran Zhang, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge ‘Magic’ angle(~1.1°) twisted bilayer graphene(TBG) is predicted to host narrow, non-dispersive electronic bands, where the kinetic energy of electrons becomes much smaller than Coulomb interactions. Recent transport measurements in this system showed insulating behavior and superconductivity around half filling of the bands, key signatures of the strongly correlated electronic systems. But the detailed electronic structure and the many-body properties of this system are still not well-understood. We used scanning tunneling microscopy and spectroscopy to measure gated TBG with angles ranging from 0.9° to 2°. When the twist angle was close to the magic angle, at certain carrier densities, we observed strong suppression of the tunneling density of states at the Fermi level associated with electron-electron interactions. Our results may deepen the understanding of strongly correlated behavior in magic angle TBG. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L14.00013: Constructing low-energy models for the electronic structure of twisted bilayer graphene Zachary Goodwin, Arash A Mostofi, Johannes Lischner The recent, unexpected discovery of insulating and superconducting behaviour in magic angle twisted bilayer graphene (tBLG) has generated tremendous interest in theoretical communities to rationalise these observations. To study the angle dependent electronic properties, we have developed a tight binding model for tBLG. Importantly, the developed tight binding model accounts for atomic corrugation, and therefore, different interactions between AA and AB regions. Such asymmetry has been attributed to the isolation of bands near the Fermi energy, which has been experimentally observed. From the tight binding model, the Wannier orbitals and interaction matrix of these orbitals can be obtained. These calculations permit the construction of simplified Hamiltonians with no free parameters. Such downfolded Hamiltonians can be used in state-of-the-art functional renormalisation group methods to calculate the electronic phase diagram. The developed methodology naturally lends itself to systematic improvements and to other bilayer materials. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L14.00014: Intralayer and interlayer electron-phonon interactions in twisted bilayer graphene Marcos Pimenta, Ariete Righi, Eliel Gomes da Silva Neto, Andreij de Carvalho Gadelha, Leonardo Cristiano Campos, Marcus Moutinho, Pedro Venezuela, Po-Wen Chiu In this work we report on the possibility of using resonance Raman spectroscopy to distinguish intralayer and interlayer electron-phonon (el-ph) interactions in twisted bilayer graphene (TBG). This is experimentally attained by tuning the energy of the excitation photon and observing the resonances of the Raman modes in different samples of TBGs. In the intralayer process, the el-ph scattering occurs in a single graphene layer and the other layer imposes a periodic potential that back scatters the excited electron, whereas for the interlayer process the el-ph scattering occurs between states in the Dirac cones of adjacent graphene layers. Our methodology can be extended to study any kind of graphene-based heterostructure. [Eliel et al., Nature Comm. 9, 1221 (2018)] |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L14.00015: Lattice relaxation and energy band modulation in twisted bilayer graphene NGUYEN NAM, Mikito Koshino We theoretically study the lattice relaxation in the twisted bilayer graphene (TBG) and its effect on the electronic band structure. We develop an effective continuum theory to describe the lattice relaxation in general TBGs and obtain the optimized structure to minimize the total energy. At small rotation angles < 2 degrees, in particular, we find that the relaxed lattice drastically reduces the area of the AA stacking region and forms a triangular domain structure with alternating AB and BA stacking regions. We then investigate the effect of the domain formation on the electronic band structure. The most notable change from the nonrelaxed model is that an energy gap of up to 20 meV opens at the superlattice subband edges on the electron and hole sides. We also find that the lattice relaxation significantly enhances the Fermi velocity, which was strongly suppressed in the nonrelaxed model. |
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