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 W38: 2D Moiré Materials: Transport and Electronic PropertiesFocus
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Sponsoring Units: DMP Chair: Aiming Yan, University of California, Santa Cruz; Fang Liu, Stanford University Room: Room 230 |
Thursday, March 9, 2023 3:00PM - 3:36PM |
W38.00001: Broken-symmetry states and valley ordering in twisted bilayer graphene/WSe2 heterostructures Invited Speaker: U Chandni Twisted bilayers of graphene (tBLG) offer a new platform where interlayer Coulomb interactions can be tailored conveniently. The formation of extremely flat bands at certain ‘magic’ angles, where the Coulomb energy exceeds the bandwidth, has led to the observation of correlated insulating states, superconductivity and other exotic states such as Chern insulators, orbital ferromagnets, and nematic phases. The dielectric environment of tBLG plays an important role in controlling electronic correlations within the flat bands. In this work, we have explored various facets of many-body correlations using a combination of magneto-transport and thermoelectric measurements in tBLG coupled to a layer of tungsten diselenide (WSe2). We observe states near half-integer band fillings 0.5 and ±3.5 at near-zero magnetic fields and a symmetry-broken Chern insulator at -0.5 emerging at high magnetic fields. The results can be explained using band structure calculations within a translational symmetry-broken supercell with twice the area of the original tBLG moiré cell, suggestive of a spin or charge density wave ground state. Furthermore, below half-filling, we report anomalous Hall effect with a giant coercive field, accompanied by a series of Lifshitz transitions that are highly tunable with carrier density and magnetic field. We infer that the observed valley ordering is favored by a Stoner-like instability, aided by van Hove singularities in the malleable bands. Overall, our results suggest that transition metal dichalcogenide- dielectrics can be a promising pathway to further understand and explore the nature of correlated phases in tBLG systems. |
Thursday, March 9, 2023 3:36PM - 3:48PM |
W38.00002: Twist-angle dependent proximity induced spin-orbit coupling in graphene/transition-metal dichalcogenide and graphene/topological insulator heterostructures Thomas Naimer, Klaus Zollner, Martin Gmitra, Jaroslav Fabian We investigate the proximity-induced spin-orbit coupling in twisted heterostructures of graphene/transition-metal dichalcogenides (MoS2, WS2, MoSe2, and WSe2) [1] as well as graphene/topological insulators (Bi2Se3 and Bi2Te3) from first principles. The strain in graphene, which is necessary to define commensurate supercells, is identified as the key factor affecting the band offsets and thus magnitudes of the proximity couplings. We establish that for biaxially strained graphene the band offsets between the Dirac point and the substrate bands vary linearly with strain, regardless of the twist angle. This relation allows to identify the apparent zero-strain band offsets and find a compensating transverse electric field correcting for the strain. The resulting corrected band structure is then fitted around the Dirac point to an established spin-orbit Hamiltonian, yielding the twist angle dependencies of the spin-orbit couplings. <!-- x-tinymce/html -->While for most structures a mix of Rashba and valley-Zeeman spin-orbit coupling is present, we also witness the emergence of Kane-Mele spin-orbit coupling in graphene/topological insulator structures at 30° twist angle. |
Thursday, March 9, 2023 3:48PM - 4:00PM |
W38.00003: Twist Angle-Dependent Spin-Orbit Proximity Effects and Charge-to-Spin Conversion in graphene/WSe2 Heterostructures Seungjun Lee, D. J. P. de Sousa, Young-Kyun Kwon, Fernando de Juan, Zhendong Chi, Felix Casanova, Tony Low The spin-orbit coupling (SOC) proximity effects of transition-metal dichalcogenides (TMDC) to graphene enrich the spin texture of Dirac states of graphene and thus lead to intriguing charge-to-spin conversion (CSC). Based on first-principles calculations and linear response theory, we investigate the twist angle dependence of proximity effects and charge-to-spin conversion (CSC) in graphene/WSe2 heterostructures. We found that both Rashba and valley-Zeeman SOCs strongly depend on the twist angle, and the induced staggered potential and the valley-Zeeman SOC are fully suppressed with 30° twisting due to symmetry constraints. As a result, the disorder-free spin Hall and Rashba-Edelstein CSC efficiencies are also shown to strongly depend on the twist angle, being simultaneously optimized near 30° twisting. In addition, symmetry breaking due to twisting gives rise to a novel unconventional Rashba-Edelstein effect, with non-equilibrium spin densities possessing spins parallel to the electric field direction. Our findings offer a new perspective on the spintronics of graphene/WSe2 systems, opening up opportunities for efficient control of CSC in Van der Waals heterostructures. |
Thursday, March 9, 2023 4:00PM - 4:12PM |
W38.00004: Ferroelectric switching at symmetry-broken interfaces by local control of dislocation networks Laurent Molino, Leena Aggarwal, Vladimir Enaldiev, Ryan Plumadore, Vladimir Falko, Adina A Luican-Mayer Semiconducting ferroelectric materials with low energy polarisation switching offer a platform for next-generation electronics such as ferroelectric field-effect transistors. Ferroelectric domains at symmetry-broken interfaces of transition metal dichalcogenide films provide an opportunity to combine the potential of semiconducting ferroelectrics with the design flexibility of two-dimensionalmaterial devices. Here, local control of ferroelectric domains in a marginally twisted WS2 bilayer is demonstrated with a scanning tunneling microscope at room temperature, and their observed reversible evolution understood using a string-like model of the domain wall network. We identify two characteristic regimes of domain evolution: (i) elastic bending of partial screw dislocations separating smaller domains with twin stacking and (ii) formation of perfect screw dislocations by merging pairs of primary domain walls. We also show that the latter act as the seeds for the reversible restoration of the inverted polarisation.These results open the possibility to achieve full control over atomically thin semiconducting ferroelectric domains using local electric fields, which is a critical step towards their technological use. |
Thursday, March 9, 2023 4:12PM - 4:24PM |
W38.00005: Evidence for Spatially Separated Flat Bands in a Graphene Double Moiré System Yimeng Wang, G W Burg, Kenji Watanabe, Takashi Taniguchi, Emanuel Tutuc The experimental advances in realizing two-dimensional moiré materials such as twisted bilayer graphene (TBG) has opened up new opportunities for flat band engineering. Correlated insulators and superconducting states have been demonstrated in TBGs when the twist angle is close to the magic angle. We demonstrate here a double moiré material in a twisted quadlayer graphene structure, where the twist angles between the first and second layers, as well as the third and fourth layers are small (1.2°-1.6°), while the angle between the second and third layer is large. The transport properties indicate the presence of two spatially separated moiré bands in the top and bottom small-angle TBGs formed by the top two layers and bottom two layers of the quadlayer, respectively. We observe clear resistance maxima when the top and bottom moiré bands are at charge neutrality and full-filling of the moiré Brillouin zone, controlled largely by the top and bottom gates, respectively. While neither top or bottom individual TBG shows correlated insulating states at half-filling of the moiré Brillouin zone, a finding consistent with their respective twist being away from the magic angle, surprisingly the double moiré structure exhibits correlated insulating states when both TBGs are at half moiré band fillings. The realization of spatially separated, independently tunable flat bands in this double moiré system provides insight into the evolution of the correlations between electrons when two moiré energy bands are closely spaced. |
Thursday, March 9, 2023 4:24PM - 4:36PM |
W38.00006: Tunable layer-contrasting electronic structure in a graphene moiré lattice Xueqiao Wang, Zhiren Zheng, Ziyan Zhu, Stephen T Carr, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Qiong Ma, Pablo Jarillo-Herrero The ability to create artificial superlattices from moiré patterns formed by van der Waals materials opens up a powerful platform to engineer controllable systems to study various problems in condensed matter physics. We identify an interesting asymmetric moiré system consisting of graphene only, by creating distinct moiré potentials on different layers of graphene. Transport data suggests multiple electronic systems with distinct characters coexist, with their interplay tunable by twist angles, temperature, and magnetic field. Such tuning knobs allow for the study of various emergent phenomena. |
Thursday, March 9, 2023 4:36PM - 4:48PM |
W38.00007: Tunable energy gap and the transitions between the insulating phases in dual-gated hBN/bilayer graphene superlattices at the charge neutrality point Takuya Iwasaki, Yoshifumi Morita, Kenji Watanabe, Takashi Taniguchi We report on the transport properties in hexagonal boron nitride (hBN)/bilayer graphene (BLG) moiré superlattice devices with top and bottom gates (dual gate) enabling individual modulation of the electric displacement field and the carrier density. By measuring the temperature and displacement field dependence of the resistivity at the charge neutrality point (CNP), we estimate the energy gap and demonstrate its tuning by the electric field. Even without an electric field, the alignment of BLG with hBN harbors an energy gap of ~1.4 meV in our device. It is found that the temperature dependence of the resistivity at moiré-induced satellite points is insensitive to an electric field, in contrast to that at the CNP. Under a perpendicular magnetic field, transitions between two insulating phases at the CNP are detected. In the phase diagram, there is no signature of a continuously vanishing gap even in the vicinity of the criticality. This result agrees with a scenario of the formation of a microscopic network of the two competing phases at the first-order transition. |
Thursday, March 9, 2023 4:48PM - 5:00PM Author not Attending |
W38.00008: Mapping the Density of States in Twisted Trilayer Graphene Rachel L Nieken, Takashi Taniguchi, Kenji Watanabe, John Schaibley, Brian J LeRoy Twisted trilayer graphene with mirror symmetry can be created by twisting the top and bottom layers by angles of θ and - θ with respect to the center layer. This system has been extensively studied because of the robust nature of the superconducting regime and the relative ease of creating the heterostructure due to the top and bottom layers locking into angular alignment. The additional Dirac cone of the system compared to twisted bilayer graphene changes the properties of the system and gives rise to more diverse phenomena. To explore the effects of the additional third layer we map the band structure for various twist angles in several different devices. Using scanning tunneling microscopy and spectroscopy at 4.5 K we obtain local density of states maps and dI/dV curves for these devices. |
Thursday, March 9, 2023 5:00PM - 5:12PM |
W38.00009: Phonon-limited resistivity of multilayer graphene systems Seth M Davis We calculate the theoretical contribution to the doping and temperature ($T$) dependence of electrical resistivity due to scattering by acoustic phonons in Bernal bilayer graphene (BBG) and rhombohedral trilayer graphene (RTG), as well as in twisted bilayer graphene (TBLG) near the magic angle. We focus on the role of nontrivial geometric features of the detailed, anisotropic $vex{k}cdotvex{p}$ band structures of these systems - e.g. Van Hove singularities, Lifshitz transitions, Fermi surface anisotropy, and band curvature near the gap - whose effects on transport have not yet been systematically studied. We find that these geometric features strongly influence the temperature and doping dependencies of the resistivity. In particular, the band geometry leads to a nonlinear $T$-dependence in the high-$T$ equipartition regime, complicating the usual $T^4$ to $T$ Bloch-Gr"{u}neisen crossover. Our focus on BBG and RTG is motivated by recent experiments in these systems that have discovered several exotic low-$T$ superconductivity proximate to complicated hierarchies of isospin-polarized phases. These interaction-driven phases are intimately related to the geometric features of the band structures, highlighting the importance of understanding the influence of band geometry on transport. While resolving the effects of the anisotropic band geometry on the scattering times requires nontrivial numerical solution, our approach is rooted in intuitive Boltzmann theory. We compare our results with recent experiment and discuss how our predictions can be used to elucidate the relative importance of various scattering mechanisms in these systems. |
Thursday, March 9, 2023 5:12PM - 5:24PM |
W38.00010: Strong gate-tunability of flat bands in multilayer graphene due to moiré encapsulation between hBN monolayers Robin Smeyers, Milorad V Milosevic, Lucian Covaci When using hBN as a substrate for graphene devices, the resulting moiré pattern generates secondary Dirac points. Recent experimental advances have enabled the possibility to precisely control the stacking and twist-angle of the individual layers within van Der Waals heterostructures. By encapsulating a multilayer graphene within aligned hBN sheets the controlled moiré stacking may offer even richer benefits. Using advanced tight-binding simulations on atomistically-relaxed heterostructures, we show that the gap at the secondary Dirac point can be opened in selected moiré-stacking configurations, and is independent of any additional vertical gating of the heterostructure. On the other hand, gating can broadly tune the gap at the principal Dirac point, and may thereby strongly compress the first moiré mini-band in width against the moiré-induced gap at the secondary Dirac point. We reveal that in hBN-encapsulated bilayer graphene this novel mechanism can lead to bands flatter than 10 meV under moderate gating, hence presenting a convenient pathway towards electronically-controlled strongly-correlated states on demand. |
Thursday, March 9, 2023 5:24PM - 5:36PM |
W38.00011: Correlated states and anomalous transport in twisted MoTe2 Jiaqi Cai, Eric Anderson, William G Holtzmann, Feng-ren Fan, Takashi Taniguchi, Kenji Watanabe, Wang Yao, Xiaodong Xu Twisted transition metal dichalcogenides(tTMDs) serve as the ideal platform to study strongly correlated two-dimensional systems due to their tunable bandwidth, simpler Hubbard band structure, and strong spin-orbital coupling. The 2H phase of MoTe2 has a smaller and direct bandgap in the intrinsic bilayer form, distinct from other TMDs. This makes MoTe2 an outstanding candidate for the transport study and engineering of emergent quantum states. Here we present a magnetotransport study of twisted MoTe2 with a twist angle range of 3.2-4.8 degrees. We observed multiple electric field tunable correlated states and anomalous magnetotransport, suggesting magnetic interaction associated with those correlated states. |
Thursday, March 9, 2023 5:36PM - 5:48PM |
W38.00012: Spatial and spectral signatures of intralayer moiré excitons in layered superlattices Medha Dandu, Jordan Hachtel, Mit H Naik, Patrick Hays, Sriram Sankar, Elyse Barre, Takashi Taniguchi, Kenji Watanabe, Steven G Louie, Felipe H da Jornada, Sefaatin Tongay, Peter Ercius, Archana Raja, Sandhya Susarla Stacking of individual monolayers of layered materials like transition metal dichalcogenides with precise control of twist angle offers an unconventional bottom-up approach to design artificial superlattices of nanoscale periodicity. The concomitant moiré patterns in such superlattices modulate the spectral contribution and the spatial distribution of the excitonic states. These twist-tunable moiré excitons typically probed by optical spectroscopy techniques lack nanoscale spatial resolution and hence it is imperative to correlate these spectral signatures with other suitable nanoscale characterization techniques [1]. In this study, we demonstrate spatially and spectrally resolved signatures of distinct intralayer moire excitons by employing monochromated electron energy loss spectroscopy (EELS) on prototypes of closely aligned WS2/WSe2 stacks. The distinct moiré exciton spectral signatures at cryogenic temperatures from EELS correlate quite well with the energy resonances of the corresponding reflection contrast spectra for both WSe2 and WS2. |
Thursday, March 9, 2023 5:48PM - 6:00PM |
W38.00013: Theoretical study of the Pauli violation ratio in magic-angle twisted multilayer graphene Darryl Foo, Mohammed Alezzi, Giovanni Vignale, Shaffique Adam Motivated by recent experimental studies on magic-angle twisted multilayer graphene [Park et al. Nature Materials 21, 877, 2022] who observed robust superconductivity with a critical magnetic field in excess of the Pauli limit, in this theoretical work we calculate how the upper bound on the Pauli violation ratio (i.e. the ratio between the critical magnetic field and the Pauli limit value) in this system depends on atomic relaxation, disorder and spontaneous emergence of ferroelectricity. We find that for realistic experimental parameters this ratio can be large, consistent with experimental observations. |
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