Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P51: Graphene: Electronic Structure and Interactions: Moire patterns |
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Sponsoring Units: DCMP DMP Chair: Emilio Codecido, Ohio State Univ - Columbus Room: Mile High Ballroom 1D |
Wednesday, March 4, 2020 2:30PM - 2:42PM |
P51.00001: Fully tunable Magic Angle Twisted Bilayer Graphene Josephson Junction Daniel Rodan-Legrain, Yuan Cao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Superconductivity and correlated insulating states have recently been observed in ‘magic-angle’ twisted bilayer graphene (MATBG) heterostructures at twist angles close to 1.1 degrees, featuring nearly-flat bands owing to strong interlayer coupling. MATBG offers a new playground to study superconductivity and strongly correlated physics with an unprecedented degree of tunability. One of the elemental superconducting devices are Josephson Junctions, where two superconductors are coupled by a non-superconducting weak link. In this work, we exploit such tunability to create a fully tunable 2D Josephson Junction allowing to independently control the electronic state of the weak link and the rest of the junction via multiple electrostatic gates. The MATBG electrodes can thus be brought into a superconducting state, either in the hole regime or the electron regime, while the narrow junction at the center of the device can be tuned into any possible state of the phase diagram of MATBG (from metal, band insulator, charge neutrality, correlated insulator, superconductor). Such devices may pave the way towards new geometries in the study of strongly correlated systems, tunable superconducting qubits, and fully-integrated 2-dimensional electronics for the future nanodevice technology. |
Wednesday, March 4, 2020 2:42PM - 2:54PM |
P51.00002: Chiral twist on the high-Tc phase diagram in moiré heterostructures Yu-Ping Lin, Rahul Nandkishore
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Wednesday, March 4, 2020 2:54PM - 3:06PM |
P51.00003: Pairing in twisted double-bilayer graphene and related moiré superlattice systems Rhine Samajdar, Mathias Scheurer We present a systematic classification and analysis of possible pairing instabilities in graphene-based moiré superlattices. Motivated by recent experiments on twisted double-bilayer graphene showing signs of triplet superconductivity, we analyze both singlet and triplet pairing separately, and describe how these two channels behave close to the limit where the system is invariant under separate spin rotations in the two valleys, realizing an SU(2)+ × SU(2)- symmetry. Further, we discuss the conditions under which singlet and triplet can mix via two nearly degenerate transitions, and how the different pairing states behave when an external magnetic field is applied. We find that an approximate SU(2)+ × SU(2)- symmetry can generically account for the linear increase of the critical temperature with small magnetic fields, and we map out the possible forms of the phase diagram as a function of temperature and magnetic field. Finally, we also detail the differences in the classification when the additional microscopic or emergent symmetries relevant for twisted bilayer graphene and ABC trilayer graphene on hexagonal boron nitride are taken into account. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P51.00004: Twist angle homogeneity in twisted bilayer graphene devices studied with STM Tjerk Benschop, Vincent Stalman, Rosanne Wijgman, Maarten Leeuwenhoek, Petr Stepanov, Xiaobo Lu, Dmitri Efetov, Milan P. Allan Theoretic models that describe magic-angle twisted bilayer graphene strongly depend on how homogeneous the twist angle is. It is thus of high importance to characterize the (in)homogeneity experimentally, on the local scale. We use scanning tunneling microscopy to explore several non-encapsulated twisted bilayer graphene devices with different nominal twist angles. In this talk, I present quantitative results of the homogeneity on different length scales. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P51.00005: Tunable electron interactions in moiré graphene: extinction of Mott order in the presence of screening Ali Fahimniya, Dmitri Efetov, Petr Stepanov, Leonid Levitov Magic-angle twisted bilayer graphene (TBG) is a strongly-correlated system that hosts different ordered states, notably Mott-insulating and superconducting, stabilized by the electron-electron (e-e) and electron-phonon (el-ph) interactions. Understanding the individual roles of these interactions, as well as their interplay, is a central question of the moiré graphene physics. To that end, we explore the robustness of the Mott order in the presence of a screened e-e interaction, as is the case in currently studied heterostructures comprised of TBG, hexagonal boron nitride (hBN), and graphite layers. Screening of electron charges in TBG by image charges in graphite weakens e-e interactions as the hBN spacer width decreases. The suppression of the Mott order due to enhanced screening sets in at the hBN widths on the order of the Wannier orbital radius (~10nm) and is followed by complete extinction of the Mott order. At the same time, el-ph coupling strength remains unchanged. The experiments, too, show a drastic change of the Mott insulator phase upon varying hBN width but find that the superconducting phase survives in the absence of long-range e-e interactions. Comparison of theory and experiment sheds new light on the origin of Mott order and superconductivity in the TBG system. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P51.00006: Tunable bandwidths and gaps in twisted double bilayer graphene system on the verge of correlations Pratap Adak, Subhajit Sinha, Unmesh Ghorai, L. D. Varma Sangani, Kenji Watanabe, Takashi Taniguchi, Rajdeep Sensarma, Mandar M Deshmukh Recently, superlattices in graphene have gained much interest due to its ability to tune its band structure. In particular, it has been demonstrated that when two copies of monolayer graphene are stacked with a ‘magic’ angle twist between them, flat bands appear. Flat bands give birth to a plethora of electronic interaction governed phenomena like superconductivity, ferromagnetism, etc. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P51.00007: Observation of charging peaks near the flat band in magic-angle twisted bilayer Graphene Nikhil Tilak, Xinyuan Lai, Yuhang Jiang, Jinhai Mao, Mingyu Xu, Raquel de Almeida Ribeiro, Paul C Canfield, Eva Andrei Magic-angle twisted bilayer Graphene (MA-tBLG) has emerged as a highly tunable platform to study correlation physics. Numerous transport experiments have shown the existence of correlated insulating states, unconventional superconductivity and emergent ferromagnetism as the filling of the flat moire bands in MA-tBLG is tuned. More recently, several Scanning Tunneling Microscopy/Spectroscopy (STM/STS) experiments have also observed correlated-insulator like behavior, charge order and broken C3 symmetry in this system. Here, using Low temperature STM/STS measurements, we report the observation of sharp charging peaks which appear in the LDOS as the Fermi level is tuned close to the edges of the flat band. We perform a systematic study of these states to see their evolution in space using STS mapping. The observation of these states hints at the formation of local compressible regions surrounded by a gapped incompressible region because of tip induced band bending. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P51.00008: Experimental Observation of Tunable Chern insulator and Ferromagnetism in ABC-Trilayer Graphene on hBN Moiré Superlattice Guorui Chen, Aaron Sharpe, Eli J Fox, Yahui Zhang, Shaoxin Wang, Lili Jiang, Bosai Lv, Hongyuan Li, Kenji Watanabe, Takashi Taniguchi, Zhiwen Shi, Senthil Todadri, David Goldhaber-Gordon, Yuanbo Zhang, Feng Wang Haldane theorized that Chern insulators with integer QH effects could appear in lattice models with complex hopping parameters even at zero magnetic field. The ABC-trilayer graphene/hexagonal boron nitride (TLG/hBN) moiré superlattice provides an attractive platform to explore Chern insulators because it features nearly flat moiré minibands with a valley-dependent electrically tunable Chern number. Here we report the experimental observation of a correlated Chern insulator in a TLG/hBN moiré superlattice. We show that reversing the direction of the applied vertical electric field switches TLG/hBN's moiré minibands between zero and finite Chern numbers, as revealed by dramatic changes in magneto-transport behavior. The Hall resistance is well quantized at h/2e2, i.e. C = 2, for B > 0.4 T. The correlated Chern insulator is ferromagnetic, exhibiting significant magnetic hysteresis and a large anomalous Hall signal at zero magnetic field. Our discovery of a C = 2 Chern insulator at zero magnetic field should open up exciting opportunities for discovering novel correlated topological states, possibly with novel topological excitations, in nearly flat and topologically nontrivial moiré minibands. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P51.00009: Spin-triplet superconductivity and ferromagnetism in moire systems: A case study for twisted double bilayer graphene Yi-Ting Hsu, Fengcheng Wu Recent experiments have observed possible ferromagnetism and spin-triplet superconductivity in twisted double bilayer graphene that comprises a pair of AB-stacked bilayer graphene under a displacement field. We show that ferromagnetism and spin-triplet pairing can naturally be the dominant instabilities in such systems as a general consequence of the valley-dependent van Hove physics when the nesting is weak. Specifically, by performing a renormalization group analysis for all possible pairing, magnetic, and charge instabilities, we find ferromagnetism or spin-triplet pairing to be dominant throughout a broad parameter space of interactions and low-energy band structure properties we consider. Finally, we discuss the possibility of topological superconductivity and relevance to the experimental findings. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P51.00010: Ferromagnetism in twisted bilayer graphene Aaron Sharpe, Eli J Fox, Arthur W Barnard, Joe Finney, Kenji Watanabe, Takashi Taniguchi, Marc Kastner, David Goldhaber-Gordon When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. These enhanced interactions have led to the observation of interesting correlated electronic states, such as superconductivity, where metallic behavior would otherwise be expected. Here we present evidence that near three-quarters (3/4) filling of the conduction miniband these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. We suggest that the system is a Chern insulator. We further present magneto transport measurements in tilted field that may hold clues to the nature of the magnetism. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P51.00011: Ferromagnetism in narrow bands of moiré superlattices Cecile Repellin, Dong Zhihuan, Yahui Zhang, Senthil Todadri Many graphene moiré superlattices host narrow bands with non-zero valley Chern numbers. We provide analytical and numerical evidence for a robust spin and/or valley polarized insulator at total integer band filling in nearly flat bands of several different moiré materials. In the limit of a perfectly flat band, we present analytical arguments in favor of the ferromagnetic state substantiated by numerical calculations. Further, we numerically evaluate its stability for a finite bandwidth. We provide exact diagonalization results for models appropriate for ABC trilayer graphene aligned with hBN, twisted double bilayer graphene, and twisted bilayer graphene aligned with hBN. We also provide DMRG results for a honeycomb lattice with a quasi-flat band and non-zero Chern number, which extend our results to larger system sizes. We find a maximally spin and valley polarized insulator at all integer fillings, which can be destabilized by interaction-induced effective dispersive terms. These results still hold in the case of zero valley Chern number. We give an intuitive picture based on extended Wannier orbitals, and emphasize the role of the quantum geometry of the band, whose microscopic details may enhance or weaken ferromagnetism in moiré materials. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P51.00012: Transport experiments in twisted (double) bilayer graphene Peter Rickhaus, Folkert De Vries, Elias Marin Portoles, Giulia Zheng, Ming-Hao Liu, Jose Lado, John Wallbank, Sergey Slizovskiy, Marcin Kurpas, Riccardo Pisoni, Hiske Overweg, Yongjin Lee, Annika Kurzmann, Marius Eich, Chuyao Tong, Rebekka Garreis, Carolin Gold, Michele Masseroni, Klaus Richter, Kenji Watanabe, Takashi Taniguchi, Thomas Ihn, Klaus Ensslin We study signatures in electronic transport in twisted (double) bilayer graphene. Specifically, we are interested in the coupling and decoupling between single- and bilayer graphene layers as a function of twist angle. For large twist angles, the wavefunction of the layers are decoupled. Using twisted double bilayer graphene, we determine the intrinsic energy difference between inner and outer layers of the stack. Such a crystal field opens a gap in the groundstate [1]. Since the two layers are strongly capacitively coupled but their wavefunctions can be independently tuned, the geometric capacitance between the layers is heavily affected by the finite thickness of graphene, which we determine [2]. We also show transport data at small angles, where the layers are strongly coupled. At tiny twists, we detect a topological network [3] in twisted bilayer graphene and correlated states in twisted double bilayer graphene. Finally, we study the crossover between the coupled and decoupled regime, where the wavefunctions start to spread over all layers. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P51.00013: Colossal mid-infrared photoresponse in small-twist-angle bilayer graphene Bingchen Deng, Chao Ma, Qiyue Wang, Shaofan Yuan, Kenji Watanabe, Takashi Taniguchi, Fan Zhang, Fengnian Xia Small-twist-angle bilayer graphene has drawn much attention recently due to its exotic transport properties such as superconductivity and correlated insulating phase. The Brillouin zone folding and the formation of flat moiré bands significantly modify the density of states (DOS). Here we demonstrate the colossal and gate-tunable mid-infrared (5 ~ 12 μm) photoresponse leveraging the enhanced DOS and the emergence of superlattice-induced bandgaps. At liquid nitrogen temperature, we achieve a maximum extrinsic responsivity of 26 mA W-1 at 12 μm in a 1.81° twist angle bilayer graphene, when the Fermi level is tuned to the middle of the superlattice gap in the electron branch. Together with transport study, we reveal the bolometric origin of the photoresponse. Moreover, the colossal photoresponse critically relies on the formation of superlattice gaps as it is vanishingly small in ultrasmall twist angle (< 0.5°) device, where the gaps are absent in our measurement range. Our results show the desirable properties and promising applications of twisted bilayer graphene for mid-infrared optoelectronics. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P51.00014: Nano-photocurrent mapping of twisted bilayer graphene Sai Sunku, Alexander McLeod, Tobias Stauber, Hyobin Yoo, Dorri Halbertal, GuangXin Ni, Aaron Sternbach, Bor-Yuan Jiang, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Michael M Fogler, Dmitri Basov We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene. We show that the photocurrent as a function of carrier density changes sign at twist-induced insulating states. We use this property to identify domains of varying local twist angle in our sample. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Nano-photocurrent imaging can be broadly applied to moiré superlattices occurring in various other van der Waals heterostructures. |
Wednesday, March 4, 2020 5:18PM - 5:30PM |
P51.00015: Nano-optical study of Moire superlattice domains in twisted graphene heterostructures Dorri Halbertal, Nathan Finney, Sai Sunku, Shaowen Chen, Charles Zhang, Nicola Curreli, Matthew A Yankowitz, Alexander McLeod, GuangXin Ni, Alexander Kerelsky, Carmen Rubio Verdu, Leo McGilly, Abhay Narayan, Cory Dean, James C Hone, Dmitri Basov Twisted graphene heterostructure have attracted a considerable amount of attention, realizing a plethora of physical phenomena including strongly interacting correlated states, superconductivity, magnetism and more. In this work we used several nano-optical techniques to study the formation and reconstruction of domain walls in several twisted graphene heterostructures. The techniques are uniquely suited to provide a nanometric spatial map of the domains, over an entire flake scale, which allowed us to study domain formation and internal structure on all relevant scales, and to gain insight into the reconstruction problem in several system of interest, including the twisted bilayer graphene and the twisted double bilayer graphene. |
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