Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M42: Transport in Graphene MultilayersLive
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Sponsoring Units: DCMP Chair: Zhijian Xie, Peking Univ |
Wednesday, March 17, 2021 11:30AM - 11:42AM Live |
M42.00001: Hyperbolic enhancement of photocurrent patterns in minimally twisted bilayer graphene Sai Sunku, Dorri Halbertal, Tobias Stauber, Shaowen Chen, Alexander S McLeod, Andrey Rikhter, Michael Berkowitz, Chiu Fan Bowen Lo, Derick Gonzalez-Acevedo, James Hone, Cory Dean, Michael Fogler, Dmitri Basov Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene (TBG) have been intensely studied. At small twist angles θ, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride (hBN), which we use as a TBG substrate, is optically hyperbolic. Our results illustrate new capabilities of nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials. |
Wednesday, March 17, 2021 11:42AM - 11:54AM Live |
M42.00002: Strange metallic transport above correlated insulating states in twisted bilayer graphene Peter Cha, Aavishkar Patel, Eun-Ah Kim Recent experiments report ubiquitous observations of correlated insulating states at integer fillings of low energy bands, that give way to strange metal transport with T-linear resistivity upon heating, in magic-angle twisted bilayer graphene (MATBG). However, a theoretical framework that provides an understanding of this strange metal transport starting from the low energy correlated insulator physics of MATBG remains lacking. In this work, we focus on local tendencies for spontaneous symmetry breaking in the order parameters describing topological correlated insulating states. We consider the effect of the redirection of bulk current flow due to fluctuations of the order parameter landscape on transport. We find that thermal fluctuations in the landscape of heterogeneous order parameter configurations above the ordering transition lead to T-linear resistivity over a wide range of temperatures. Furthermore, we discuss the effects of such physics on the Hall resistivity at temperatures above the ordering transition. |
Wednesday, March 17, 2021 11:54AM - 12:06PM Live |
M42.00003: Transport study of interface effect between WSe2 and magic angle twisted bilayer graphene Jiangxiazi Lin, Zhi Wang, Erin Morissette, Kenji Watanabe, Takashi Taniguchi, Jia Li When two graphene crystals are rotated by the so-called magic angle, a moire flatband emerges where strong Coulomb correlation gives rise to a range of interesting phenomena such as the correlated insulators and superconductivity. The ground state properties in such a system is shown to be sensitively tunable by the encapsulating substrates. For example, aligning tBLG with the boron nitride substrate leads to intrinsic ferromagnetism, evidenced by the quantum anomalous Hall effect. In addition, close proximity with an insulating tungsten-diselendie (WSe2) crystal is shown to stabilize the superconducting phase over a wide range of twist angles. Here we report the influence of the WSe2 interface on correlated insulators in magic-angle tBLG. In such structures, transport measurements demonstrate unique tunability to the ground state properties, which is made possible by a dual-gated sample geometry. By exploring a multi-dimensional phase space as a function of carrier density, temperature, perpendicular electric field and parallel magnetic field, we show that the tBLG/WSe2 heterostructure offers a versatile platform to study the interplay between electron correlation, magnetism and topology. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M42.00004: Non-linear electron transport in two-dimensional moiré superlattices. Alexey Berdyugin, Na Xin, Shubhadeep Bhattacharjee, Piranavan Kumaravadivel, Haoyang Gao, Sergey Slizovskiy, M Holwill, Leonid ponomarenko, Minsoo Kim, Denis Bandurin, Yang Cao, Kenji Watanabe, Takashi Taniguchi, Vladimir Falko, Leonid Levitov, Roshan Krishna Kumar, Andre Geim Graphene based moire superlattices represent a new class of solid-state systems with exceptional electronic properties. In the linear response regime, their versatile properties have allowed access to rich plethora of phenomena including Hofstadter butterflies, strong electron correlations and even topological physics. In this work, we take the first steps to understand their behaviour in the non-linear regime. We perform high-field quantum transport measurements in a variety of graphene based moire superlattices, including monolayer graphene aligned to hBN and twisted bilayer graphene close to the magic angle (0.73 + 1.23 degree). In all the studied systems, we find non-linear transport is governed by the shift of the Fermi-surface within the superlattice mini-brillouin zones. In particular, we observe a set of sharp peaks in the differential resistivity that signals the on-set of electron-hole particle creation by the electric field. In addition, we observe a strongly non-linear Hall effect that correlates with the differential resistivity and even changes sign. Our work provides the first insights into non-linear electron transport phenomena in twisted graphene moire superlattices. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M42.00005: Carbon nanotube-graphene Coulomb drag in the linear and nonlinear transport regimes Laurel Anderson, Austin Kcon Cheng, Takashi Taniguchi, Kenji Watanabe, Philip Kim Double-layer drag experiments are an exceptionally sensitive tool for studying interactions in low-dimensional systems, since the drag response depends on long-range Coulomb interactions rather than particle exchange. We have measured 1D-2D drag resistance between a single carbon nanotube (CNT) and monolayer graphene separated by a few-layer boron nitride flake. At high temperatures, the drag response is symmetric under interchange of the drive and drag layers and resembles momentum transfer from drive to drag layer. This layer reciprocity is broken near the charge neutrality point of graphene at lower temperatures, which can be attributed to thermoelectric effects. The relations between the drag resistance and charge carrier density and temperature are not straightforwardly explained by existing theory. Intriguingly, the high-temperature drag response decays with distance from the CNT slower than expected for a diffusive current distribution, hinting at additional electron interactions coming into play in the graphene. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M42.00006: Unraveling the ground state of flat-band ABC-stacked trilayer graphene via quantum transport Felix Winterer, Fabian Geisenhof, Thomas Weitz Due to its flat electronic bands, rhombohedral (i.e. ABC) trilayer graphene is prone to symmetry broken states. However, it is challenging to access its transport properties experimentally due to the abundance of ABA stacked graphene, the known transition from ABC to ABA stacking under van-der-Waals dry processing or metal contact deposition. Prior to suspension we control the local stacking order of the flakes by near-field imaging to ensure its structural integrity. Here, we discuss our latest quantum transport data on high-quality dual-gated ABC-stacked trilayer graphene. For example, at a temperature of 7mK and magnetic field of down to 0.4 T we see a full lifting of all degeneracies of the lowest Landau level. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M42.00007: The impact of domain walls on quantum transport in bilayer graphene Fabian Rudolf Geisenhof, Felix Winterer, Thomas Weitz Bilayer graphene exhibits a rich variety of broken symmetry states due to its various internal degrees of freedom. Most important for their emergence is the cleanliness of the graphene, however, also the existence or absence of structural and electronic domain walls in bilayer graphene can heavily affect the quantum transport. Here, we present an extensive study on several dual-gated freestanding bilayer graphene devices, giving evidence that domains within a device change its transport properties significantly. Besides suppressing the spontaneous phase, domain walls have also a big impact on the electric field dependence of quantum Hall states. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M42.00008: Stacking and configuration dependent photocurrent in van der Waals heterostructures Ying Xiong, Likun Shi, Justin Song Conventional photocurernts at p-n junctions depend on the macroscopic built-in field and are typically independent of microscopic details of atomic configurations. Here we demonstrate stacking and configuration dependent shift photocurrent in van der Waals heterostructures. Such shift current arises in noncentrosymmetric materials as a result of real-space shift of electrons upon photo-excitation and is thus sensitive to the symmetry and atomic arrangement in the unit cell. We illustrate the stacking and configuration dependence in Bernal stacked bilayers of graphene and graphene-like 2D materials. The shift photocurrent switches sign when the stacking arrangement is changed from AB to BA or the interlayer potential switches direction. The direction of the photocurrent depends on the light polarisation and is transverse when the polarisation is aligned with the high-symmetry axes of the material. Furthermore, in the presence of strain, the photocurrent is nonzero even for unpolarised light. Our results provide a tool to determine the stacking arrangement, lattice orientation and strain in van der Waals heterostructures from optical responses. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M42.00009: Charge Density Waves in Twisted Double Bilayer Graphene Peter Rickhaus, Folkert K. de Vries, Jihang Zhu, Giulia Zheng, Elías Portolés, Annika Kurzmann, Michele Masseroni, Allan MacDonald, Thomas Ihn, Klaus Ensslin When twisted to angles near 1°, graphene multilayers provide a new window on electron correlation physics by hosting gate-tuneable strongly-correlated states, including insulators, superconductors, and unusual magnets. Here we report the discovery of a new member of the family,[1] density-wave states, in double bilayer graphene [2,3] twisted to 2.37°. At this angle the moiré states retain much of their isolated bilayer character, allowing their bilayer projections to be separately controlled by gates [4]. We use this property to generate an energetic overlap between narrow isolated electron and hole bands with good nesting properties. Our measurements reveal the formation of ordered states with reconstructed Fermi surfaces, consistent with density-wave states, for equal electron and hole densities. These states can be tuned without introducing chemical dopants, thus opening the door to a new class of fundamental studies of density-waves and their interplay with superconductivity and other types of order, a central issue in quantum matter physics. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M42.00010: Intrinsic crystal fields and electronic polarizabilities of multilayer graphene with and without twist Nikita Tepliakov, QuanSheng Wu, Oleg Yazyev We use an original approach combining ab initio calculations and Wannier function formalism to analyze intrinsic crystal fields in multilayer graphene of various stacking configurations. We find that Bernal- and rhombohedral-stacked multilayer graphene features a pronounced out-of-plane intrinsic polarization of the order of 10 mV/Å, which is directed towards the outer layers of the structure. Upon twisting the multilayers, the out-of-plane crystal field weakens significantly and reverses its direction. Twisted multilayer graphene also features a pronounced in-plane crystal field of the order of 10 meV, strongly modulated by the moiré pattern in these structures. We further include explicitly applied external electric field in our ab initio calculations and show that graphene without a twist has highly nonlinear electric-field screening, with its permittivity reaching a pronounced minimum of 2.0–2.5 at certain electric fields. On the contrary, twisted multilayer graphene is found to have a field-independent dielectric response. Overall, our study provides a highly accurate parametrization of intrinsic fields and dielectric properties of graphene multilayers and will enable accurate modelling of devices based on twisted bilayer graphene and other multilayer configurations. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M42.00011: Control of electron-electron interaction in graphene by proximity screening Minsoo Kim, Shuigang Xu, Alexey Berdyugin, Alessandro Principi, Sergey Slizovskiy, Na Xin, Piranavan Kumaravadivel, Wenjun Kuang, Matthew Hamer, Roshan Krishna Kumar, Roman Gorbachev, Kenji Watanabe, Takashi Taniguchi, Irina Grigorieva, Vladimir Falko, Marco Corrado Polini, Andre Geim Electron-electron (e-e) interactions play a critical role in many condensed matter phenomena, and it is tempting to gain control of them by changing the interactions’ strength. One possible approach is to place a studied electronic system in a proximity to a metal, which induces additional screening and hence suppresses electron interactions. So far, however, it was not possible to study experimentally the impact of the screening exerted by metallic or dielectric media on the e-e interaction in a crystal. Driving the electron system in graphene in the hydrodynamic regime, we can quantitatively access e-e scattering length lee and investigate how screening exerted by such metal gate changes viscous electron flow. Proximity screening is found to enhance lee and qualitatively changes its dependence on carrier density. Counterintuitively, the screening becomes important only at gate dielectrics’ thickness of a few nm, much smaller than a typical separation between electrons. Our theoretical analysis agrees well with the scattering rates extracted from measurements of electron viscosity in monolayer graphene and of umklapp e-e scattering in graphene superlattices. The results provide a guidance for future attempts to achieve screening of many-body phenomena in two-dimensional systems. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M42.00012: Realization of nearly dispersionless bands with strong orbital anisotropy from destructive interference in twisted bilayer MoS2 Lede Xian, Martin Claassen, Dominik Kiese, Michael M Scherer, Simon Trebst, Dante Kennes, Angel Rubio Recently, van der Waals 2D materials stacked with a twist between adjacent layers emerge as rich platforms for the study of various strongly correlated phenomena with high tunability. Here, we use an ab initio based approach to characterize the electronic properties of twisted bilayer MoS2. We report that, in marked contrast to twisted bilayer graphene, slightly hole-doped MoS2 realizes a strongly asymmetric px-py Hubbard model on the honeycomb lattice, with two almost entirely dispersionless bands emerging due to destructive interference. The origin of these dispersionless bands, is similar to that of the flat bands in the prototypical Lieb or Kagome lattices and co-exists with the general band flattening at small twist angle due to the Moire interference. We study the collective behavior of twisted bilayer MoS2 in the presence of interactions, and characterize an array of different magnetic and orbitally-ordered correlated phases, which may be susceptible to quantum fluctuations giving rise to exotic, purely quantum, states of matter. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M42.00013: Universal Moire Nematic Phase in Twisted Graphitic Systems Simon Turkel, Carmen Rubio Verdú, Larry Song, Lennart Klebl, Rhine Samajdar, Mathias Scheurer, Jorn W. F. Venderbos, Kenji Watanabe, Takashi Taniguchi, Hector Ochoa, Lede Xian, Dante Kennes, Rafael Fernandes, Angel Rubio, Abhay Narayan Graphene moiré superlattices display electronic flat bands. At integer fillings of these flat bands, energy gaps due to strong electron-electron interactions have been observed. Other correlation-driven phases in twisted graphitic systems at non-integer fillings have been proposed, but their presence remains unclear. Here, we report scanning tunneling microscopy (STM) measurements that reveal the existence of threefold rotational (C3) symmetry breaking in highly uniform twisted double bilayer graphene (tDBG). We demonstrate that this C3 symmetry breaking cannot be explained by extrinsic factors such as heterostrain and argue instead that it is a manifestation of an interaction driven electronic-nematic phase. This talk will focus on the experimental characterization of C3 symmetry breaking in tDBG and how it compares to symmetry broken phases observed in other moire systems such as magic angle twisted bilayer graphene. Considerations stemming from both microscopic and phenomenological modelling suggest that the observed nematic phase in tDBG emerges from interactions on the scale of the moire lattice. We expect, therefore, that similar nematic phases will be found in a broad range of moire systems with electronic flat bands. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M42.00014: Van Hove singularities and Lifshitz transitions in magic-angle twisted bilayer graphene zhenyuan zhang, Shuang Wu, Eva Andrei The moiré superlattice in magic angle twisted bilayer graphene gives rise to strong electron correlations when the Fermi level is aligned with its flat bands. We carried out magneto-transport and Hall density measurements which provide access to the Fermi surface topology and its evolution as a function of doping. As the Fermi level sweeps through the flat bands, we observe that the Fermi surface undergoes a sequence of Lifshitz transitions at integer moiré fillings where discontinuous changes in the Fermi surface topology are accompanied by van Hove singularities (VHS). We find that these VHS facilitate the emergence of correlation-induced gaps and topologically non-trivial sub-bands at fillings of 1, 2, 3 carriers per moiré cell. The non-trivial topology of these sub-bands is revealed in the presence of a magnetic field by the appearance of precisely quantized Hall plateaus with Chern numbers, C=3,2,1 respectively. |
Wednesday, March 17, 2021 2:18PM - 2:30PM On Demand |
M42.00015: Predicting Anomalous Quantum Confinement Effect in van der Waals Materials Francesca Tavazza, Kamal Choudhary Materials with van der Waals-bonding exhibit quantum confinement effect, in which the electronic bandgap of the three-dimensional (3D) form is lower than that of its two-dimensional (2D) counterpart. However, the possibility of an anomalous quantum confinement effect (AQCE) exists, where the bandgap trend is reversed. In this work, we computationally identify materials with AQCE. Using density functional theory (DFT), we compute ≈1000 OptB88vdW (semi-local functional), ≈50 HSE06 and ≈50 PBE0 (hybrid functional) bandgaps for bulk and their corresponding monolayers, in the JARVIS-DFT database. OptB88vdW identifies 65 AQCE materials, but the hybrid functionals only confirm such finding in 14 cases. Such materials are either hydroxides or oxide hydroxide compounds (AlOH2, Mg(OH)2, Mg2H2O3, Ni(OH)2, SrH2O3), Sb-halogen-chalcogenide compounds (SbSBr, SbSeI) or alkali-chalcogenides (RbLiS and RbLiSe). Electronic structure analysis shows that AQCE is often characterized by the lowering of the conduction band in the monolayer and related changes in the pz electronic orbital contribution. |
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