Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q56: Twisted Bilayer Graphene Away from Magic Angle: Structure, Transport, PhononsRecordings Available
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Sponsoring Units: DCMP Chair: Linda Ye, Stanford University Room: Hyatt Regency Hotel -Burnham |
Wednesday, March 16, 2022 3:00PM - 3:12PM |
Q56.00001: Bottom Up Growth of Twisted Bilayer Graphene Rachel L Birchmier Moiré superlattices in twisted bilayer graphene have been shown to exhibit highly correlated electronic phases. Investigating these phases in graphene will provide the physics community with an accessible system to obtain information about unconventional superconductivity and correlated insulating phases. However, current twisted bilayer graphene devices can only be achieved through messy top-down construction methods. This results in small, disordered, and inconsistent devices inadequate for surface analysis. We report on a possible solution by combining UHV CVD and MBE methods to create clean bilayer graphene growth with varying twist angles grown on crystalline transition metal substrates. This growth process will aid in the creation of devices that can be characterized by Scanning Tunneling Microscopy. |
Wednesday, March 16, 2022 3:12PM - 3:24PM |
Q56.00002: Imaging topological edge states in twisted bilayer graphene Matthieu Fortin-Deschenes, Chao Ma, Rui Pu, Yanfeng Zhou, Fan Zhang, Xu Du, Fengnian Xia Stacking two 2D materials layers with a slight twist misalignment generates an additional larger periodic structure known as moiré pattern. These moiré bilayers have been shown to exhibit highly interesting physical phenomena, such as unconventional superconductivity in magic angle twisted bilayer graphene (t-BLG) [1], and novel moiré localized excitons transition metal dichalcogenide heterostructures [2]. Recently, we demonstrated that t-BLG has non-trivial topology at the moiré superlattice (SL) band gaps and that it exhibits a unique strong non-local resistance when Fermi-level is within these SL gaps [3]. Herein, by measuring the low temperature magneto-transport properties in MoRe/t-BLG/MoRe Josephson junctions (JJs), we were able to reconstruct the real-space current distribution in t-BLG, which allowed us to decipher the edge and bulk contributions and elucidate the physical origin of the non-local response in t-BLG. These results lay the groundwork to understand and control topological edge states in moiré heterostructures and bilayers. |
Wednesday, March 16, 2022 3:24PM - 3:36PM Withdrawn |
Q56.00003: Cryogenic near-field imaging of twist angle disorder in twisted bilayer graphene Niels Hesp, Petr Stepanov, Sergi Batlle, Daniel Rodan-Legrain, Iacopo Torre, Hitesh Agarwal, Roshan Krishna Kumar, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero, Frank Koppens Twisted bilayer graphene has emerged as a versatile platform to study strongly correlated phenomena and superconductivity. These properties only occur near the magic angle of 1.1 degree, and allow for little twist angle inhomogeneity. Recent works have shown the widespread twist angle disorder and strain in such samples, which are found to be a limiting factor for the existence of strongly correlated states. Yet, these works required the use of highly specialized experimental setups. Here we report on cryogenic (10 K) near-field photovoltage measurements, in which the photoresponse turns out to be a sensitive probe to map the twist angle within our device. Strengthened by nanoscale absorption measurements, we interpret our observations using the photothermoelectric effect, in which the resistive states at the full-filling carrier density act as sensitive probes of small twist angle variations. Our work demonstrates that cryogenic near-field photovoltage mapping is a powerful and accessible technique for studying the optical and optoelectronic properties of twisted bilayer graphene on the nanoscale. |
Wednesday, March 16, 2022 3:36PM - 3:48PM |
Q56.00004: Strain tunable magneto-transport study of twisted bilayer graphene heterostructures Chuankun Liu, Jameson G Berg, Ryuichi Tsuchikawa, Kenji Watanabe, Takashi Taniguchi, Vikram V Deshpande Twisting two single layer graphenes with small angle creates a moiré potential which can efficiently modify the electron band structure. At the first magic angle ~ 1.1o, the Coulomb potential dominates over the kinetic energy. The resultant two lowest flat bands host novel electronic phenomena, such as correlated insulating states, superconductivity, ferromagnetism, etc. At lower twist angles, higher order moiré bands and Hofstadter butterfly spectra of twisted bilayer graphene provide promising platforms for fascinating quantum phases and are highly unexplored. In our work, we fabricate twisted bilayer graphene heterostructures on flexible substrates which allows us to apply strain on the heterostructures. We explore twisted bilayer graphene over various twisting angles by magneto-transport measurements. Moiré superlattice area changes dramatically with the twisting angle. By observing the required magnetic field to induce one magnetic flux quantum per unit moiré cell, we found that strain is able to change the moiré superlattice area. Our work shows that strain plays an important role in modulating the band structure of twisted bilayer graphene. |
Wednesday, March 16, 2022 3:48PM - 4:00PM |
Q56.00005: Magnetophonon resonance and electron-phonon coupling in twisted bilayer graphene Matthew DeCapua, Jun Yan We use electronic Raman scattering at magnetic fields up to 9T to investigate the electron-phonon coupling in bilayer graphene with a relative twist between the two layers greater than the magic angle. We take advantage of the enhanced scattering at wavelengths resonant with the bright interlayer exciton state to detect electronic transitions between Landau levels in the Raman spectrum. We extract information on the electron-phonon coupling from observation of a splitting in the graphene Raman G band due to an avoided crossing where the phonon energy matches a Landau level transition. By applying an external gate voltage, we tune the occupation factor of the Landau levels and track the evolution of the magnetophonon resonance with helicity-resolved scattering. |
Wednesday, March 16, 2022 4:00PM - 4:12PM |
Q56.00006: Degradation of sound in moiré superlattices Hector Ochoa, Rafael M Fernandes The collective modes of twisted bilayer graphene and other moiré superlattices arise not from vibrations of a rigid crystal but from the relative displacement between the constituent layers. Despite their similarity to acoustic phonons, these modes, called phasons, are not protected by any conservation law. We show that phason modes become overdamped at low frequencies, reflecting that the moiré pattern relaxes via internal diffusive processes rather than by collective oscillations at long wavelengths. Disorder in the relative orientation between layers opens a gap in the phason dispersion, which displays a universal dependence on the twist-angle variance. Our results have important implications for the electronic properties of twisted moiré systems that are sensitive to the electron-phonon coupling, as well as for the low-temperature thermodynamic properties of these quasiperiodic lattices. |
Wednesday, March 16, 2022 4:12PM - 4:24PM |
Q56.00007: Momentum and spatially resolved view of multiple twisted bilayer graphene domains revealed through nano-focused angle resolved photoemission spectroscopy Ryan Muzzio, Henrique P Martins, Alfred Jones, Paulina E Majchrzak, Jeremy T Robinson, Simranjeet Singh, Kenji Watanabe, Takashi Taniguchi, Eli Rotenberg, Christopher Jozwiak, Aaron Bostwick, Søren Ulstrup, Jyoti Katoch The ability to tune the twist angle between constituent layers in two-dimensional material-based heterostructures offers a novel avenue for quantum material engineering and exploring fundamental physics. The resultant superlattices are known to induce moiré electronic bands, open gaps and produce flat bands whose location in energy depends on the twist angle. We will present our photoemission spectroscopy measurements of twisted bilayer graphene heterostructures with several twist angles using both micro- and nano-focused beam spots. The detailed four-dimensional (E,k,x,y) maps of the heterostructures are generated to unveil the complex spatially dependent electronic structure, which varies on the length scale of hundreds of nanometers. This work sheds light on the fundamental understanding of the complex interactions in twisted bilayer graphene as we pave the way for accessing the correlated phenomena at a large range of twist angles. |
Wednesday, March 16, 2022 4:24PM - 4:36PM |
Q56.00008: Twisted Bilayer Graphene Near Commensuration Michael Scheer, Kaiyuan Gu, Biao Lian It is well known that Twisted Bilayer Graphene (TBG) admits a continuum model for twist angles near $0$. In this talk, we derive a similar continuum model for TBG twisted near any commensurate angle. The model contains two additional parameters. One acts like an imaginary part for the AA-stacking parameter $w_0$ and the other provides an overall energy shift. For the first six distinct commensurate configurations (in order of the number of atoms per unit cell) we compute the model parameters. Additionally, we numerically observe flat bands at "magic angles" near each commensurate configuration. For at least the first nontrivial configuration (at which $\theta \approx 38.21^{\circ}$ or $21.79^{\circ}$) the energies and twist angles are on experimentally reasonable scales. |
Wednesday, March 16, 2022 4:36PM - 4:48PM |
Q56.00009: Negative differential conductance near Landau Levels in small-angle twisted bilayer graphene Zhenyuan Zhang, Shuang Wu, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei Negative differential conductance (NDC), described by decreasing current with increasing voltage, is an uncommon property arising due to nonlinear electronic response. Systems displaying NDC are of great interest because they are essential to the operation of electronic components such as amplifiers, switches, oscillators and rectifiers. In the tunneling spectroscopy of two-dimensional electron systems, NDC is revealed by resonant tunneling with extremely sharp features in the density of states. Here we report the observation of NDC in the tunneling spectroscopy of small-angle (0.7 °) twisted bilayer graphene. In the presence of perpendicular magnetic fields above 2T, NDC is observed near sharp and pronounced Landau level peaks. The gate and bias-voltage region of the NDC can be controlled by tuning the Landau level energy with a magnetic field. The differential conductance in the NDC regime is not simply proportional to the local density of states, but that it strongly depends on the transmission coefficient. This effect is especially pronounced in flat bands where the kinetic energy is suppressed as is the case of Landau levels or the flat band emerging at charge neutrality in twisted bilayer graphene. |
Wednesday, March 16, 2022 4:48PM - 5:00PM |
Q56.00010: Phonon Linewidth in Twisted Bilayer Graphene Shinjan Mandal, Indrajit Maity, Hulikal R Krishnamurthy, Manish Jain In a perfect crystal, the phonon linewidth is primarily determined by electron-phonon and phonon-phonon interactions. We have computed phonon lifetimes in twisted bilayer graphene as a function of twist angle, doping and temperature. Phonon modes are calculated using classical force fields and the electronic band structure using an atomistic tight-binding model. We use mode projected velocity autocorrelation functions for calculating the phonon-phonon contribution to the linewidth. This approach includes three, four and higher phonon-phonon interactions (up to all orders). The electron-phonon contribution to the phonon linewidth is calculated within the Migdal approximation. We provide estimates of the phonon linewidths that can be observed under experimental conditions in these systems. |
Wednesday, March 16, 2022 5:00PM - 5:12PM |
Q56.00011: Resistively-detected microwave resonance in magic-angle twisted bilayer graphene with WSe2 Erin Morissette, Jiangxiazi Lin, Andrew M Mounce, Mathias S Scheurer, Yahui Zhang, Jia Li, Zhi Wang, Song Liu, Daniel A Rhodes, Kenji Watanabe, Takashi Taniguchi, James C Hone The direct coupling of radio frequency (RF) fields with electrons enables a powerful method to probe the spin and isospin degrees of freedom associated with the flat energy band in two-dimensional moiré systems. Here we demonstrate resistively detected microwave resonance measured in magic-angle twisted bilayer graphene (tBLG). By creating an atomic interface between tBLG and a tungsten-diselenide (WSe2) crystal, proximity effect endows spin-orbit coupling (SOC) in the graphene layers, resulting in ferromagnetism at both quarter and half-filling and a rich system tunable by carrier density, temperature, perpendicular electric field, and parallel magnetic field [1]. In such a structure, we observed robust resonance features in the longitudinal resistance at microwave frequency that varies with both the in-plane and out-of-plane magnetic field. Based on the magnetic field-frequency dependence of the resonance feature, we are able to extract key parameters of material properties, such as the Ising and Rashba terms of SOC. Furthermore, the ability to detect microwave resonance using resistive measurement allows us to investigate the dynamics of spin waves and demonstrate magnon modes associated with spin and isospin orders. |
Wednesday, March 16, 2022 5:12PM - 5:24PM |
Q56.00012: Enhanced thermopower and large magneto-resistance in compensated electron-hole bands of twisted double bilayer graphene AYAN GHOSH, Souvik Chakraborty, Arup K Paul, Kenji Watanabe, Takashi Taniguchi, Unmesh Ghorai, Rajdeep Sensarma, Anindya Das Finding materials with large thermopower is crucial for the development of novel |
Wednesday, March 16, 2022 5:24PM - 5:36PM |
Q56.00013: Asymmetric correlated states in twisted monolayer-bilayer graphene Mohammed M Al Ezzi, Xingyu Gu, Alexandra Carvalho, Vladimir Falko, Kostya Novoselov, Antonio Castro Neto, Shaffique Adam In twisted monolayer-bilayer graphene massless and massive Dirac fermions mix together and lead to topological bands, asymmetric correlated states and superconductivity-like non-linear current-voltage characteristics. In this theoretical work, we first develop an analytical model to explain the observed asymmetry in formation of correlated states with respect to carrier density and displacement field [1]. Using the linearized gap equation method, we calculate the stability and critical temperature for different symmetry breaking phases, including spin density waves, charge density waves, and valley ordered phases. We compare our theoretical findings with available experimental data. |
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