Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D51: Graphene: Electronic Structure and Interactions II; moire and topology |
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Sponsoring Units: DCMP DMP Chair: Avishai Benyamini, Columbia Univ Room: Mile High Ballroom 1D |
Monday, March 2, 2020 2:30PM - 2:42PM |
D51.00001: Correlated Insulators in Twisted Double Bilayer Graphene Gregory William Burg, Jihang Zhu, Takashi Taniguchi, Kenji Watanabe, Allan Macdonald, Emanuel Tutuc We present a combined experimental and theoretical study of twisted double bilayer graphene with twist angles near 1°. Consistent with moiré band structure calculations, we observe insulators at integer moiré band fillings one and three. Within windows of finite transverse electric fields, the first moiré conduction band is separated from neighboring bands, and we observe correlated insulators at 1/4, 1/2, and 3/4 band filling. The insulators at 1/4 and 3/4 filling emerge in a parallel magnetic field, whereas the resistance at half band filling is weakly dependent on parallel magnetic field. These findings suggest that correlated insulators are favored when a moiré flat band is spectrally isolated, with spin polarization at 1/4 and 3/4 band fillings, and valley polarization at 1/2 band filling. |
Monday, March 2, 2020 2:42PM - 2:54PM |
D51.00002: Correlated states in twisted trilayer graphene Shaowen Chen, Valerie Hsieh, Matthew A Yankowitz, Kenji Watanabe, Takashi Taniguchi, Cory Dean Twisted bilayer graphene near the “magic angle” of 1.1 degrees can host low energy flat bands. Here we show tunable band hybridization and correlated states in twisted trilayer graphene, realized by placing monolayer graphene on top of a bernal bilayer graphene. Twisted monolayer-bilayer (tML-BL) devices were fabricated in a dual gate geometry allowing independent tunability of the carrier density and perpendicular displacement field D. For twist angle between 1.4 and 2 degrees the low energy band structure mostly preserves the monolayer and bilayer subbands, with an energy offset tunable with displacement field. At D = 0 the charge neutrality point is semi-metallic with compensated electrons and holes. Mapping the Landau levels as a function of density, D and magnetic field reveals transition between subbands, with a gapped monolayer subband. For twist angle ~ 1.2 degrees, we observed evidence of a flat band and emergent correlated insulating states, which are asymmetrically tunable with displacement field due to the lack of mirror symmetry. |
Monday, March 2, 2020 2:54PM - 3:06PM |
D51.00003: Coexistence of ultraheavy and ultrarelativistic Dirac quasiparticles in sandwiched trilayer graphene. Chenyuan Li, Stephen Carr, Ziyan Zhu, Efthimios Kaxiras, Subir Sachdev, Alex Kruchkov Electrons in quantum materials exhibiting coexistence of flat bands piercing dispersive bands can give rise to strongly correlated phenomena, and are associated with unconventional superconductivity. It is known that in twisted trilayer graphene steep Dirac cones can coexist with band flattening, but the phenomenon is not stable under layer misalignments. Here we show that such a twisted sandwiched graphene (TSWG) - a three-layer van der Waals heterostructure with a twisted middle layer - can have very stable flat bands coexisting with Dirac cones near the Fermi energy when twisted to 1.5 degrees. These flat bands require a specific high-symmetry stacking order, and our atomistic calculations predict that TSWG always relaxes to it. Additionally, with external fields, we can control the relative energy offset between the Dirac cone vertex and the flat bands. Our work establishes TSWG as a new platform for research into strongly interacting phases, and topological transport beyond Dirac and Weyl semimetals. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D51.00004: Periodically strained graphene lattice: flat bands Slavisa Milovanovic, Misa Andelkovic, Lucian Covaci, Francois M Peeters The conditions for the appearance of flat bands in periodically buckled graphene systems is determined. We use a tight-binding model to calculate the band structure of periodically strained graphene lattice. Three different strain configurations are considered: 1) triangular pseudo-magnetic field (PMF) mode - which is the first two-dimensional buckling mode where flat bands have been observed, 2 ) hexagonal buckling mode - as a common out-of-plane buckling mode in case of stacking different hexagonal lattices, and 3 ) herringbone buckling mode - as the lowest energy configuration in the case of large biaxial strains. We examine the band flattening versus the period of the buckling and the strength of the deformation and give predictions for the necessary conditions to access the regime of correlated phases. Our simulations show that the triangular PMF configuration is the most favourable for the appearance of flat bands due to the PMF-induced electron confinement. Similarly as in the case of energy levels of graphene quantum dots in the magnetic field, we find that flat bands show the same dispersion versus the strength of the deformation with energy separation that is inversely proportional to the buckling period. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D51.00005: Experimental Discovery of High Dimensional Band Topology in Twisted Bilayer Graphene Chao Ma, Qiyue Wang, Scott Mills, Xiaolong Chen, Bingchen Deng, Shaofan Yuan, Cheng Li, Kenji Watanabe, Takashi Taniguchi, Du Xu, Fan Zhang, Fengnian Xia Recently twisted bilayer graphene (t-BLG) emerges as a strongly correlated physical platform near a magic twist angle, hosting the Mott-like insulating phases and unconventional superconducting behavior. Besides, band topology may be another critical element in strongly correlated twistronics. In this work, we performed a systematic nonlocal transport study and revealed the nontrivial high dimensional band topology in t-BLG. Pronounced nonlocal responses are observed both in the electron and hole superlattice gaps of t-BLG, which are robust to the interlayer electric field, twist angle, and edge termination. We elucidate that two high dimensional Z_{2} invariants characterize the topology of the moiré bands. Our findings provide a new perspective for understanding the emerging strongly correlated phenomena in twisted van der Waals heterostructures. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D51.00006: Flat bands and gaps in twisted double bilayer graphene Rodrigo Capaz, Francisco Culchac, Leonor Chico, Eric Suarez Morell We present electronic structure calculations of twisted double bilayer graphene (TDBG): A tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers. The surface effect competes with an energy upshift of localized states at inner layers, giving rise to the peculiar angle dependence of the band gap, which reduces at low angles. For these low twist angles, the TDBG develops flat bands, in which electrons in the inner layers are localized at the AA regions, as in twisted bilayer graphene. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D51.00007: Electric field tunable correlated insulating states and spin-polarized phase transitions in twisted bilayer-bilayer graphene Oriol Rubies-Bigorda, Yuan Cao, Daniel Rodan-Legrain, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Understanding how strongly correlated materials behave has been a challenge for the last decades. The recent discovery of superconductivity and correlated insulators in magic angle twisted bilayer graphene (MAGTB) has paved the way to realizing strongly correlated phases of matter using twisted van der Waals heterostructures. We report here on a new correlated platform, twisted bilayer-bilayer graphene (TBBG), which consists of two sheets of Bernal stacked bilayer graphene rotated by a small angle. This system exhibits tunable correlated insulating states at different commensurate fillings. In particular, these insulating states can be switched on and off if an electric displacement field is applied across the sample. The magnetic field response of the correlated insulators in TBBG points towards evidence of spin-polarized ground states. Besides, TBBG shows multiple flat bands near charge neutrality for small twist angles. This results into different correlated states that can be tuned by electric field and are located at half-filling of each flat band. This work contributes to the study of twist-angle and electric-field dependent correlated phases of matter. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D51.00008: Lifshitz transitions in magic-angle twisted bilayer graphene zhenyuan zhang, Shuang Wu, Eva Andrei In magic-angle twisted bilayer graphene, the flat band fosters the emergence of rich physics associated with strong correlations. Here we report on magneto-transport measurements of a twisted bilayer graphene device(1.17°±0.02°). From Hall measurements, we extract the Hall number n_{H}, which provides information about the Fermi surface topology as a function of doping within the flat band. At low filling, │n/n_{0}│< 2, we find n_{H} = n as expected for a system with closed Fermi pockets. Here n_{0} is the carrier density corresponding to one electron per moire cell. Upon approaching fillings of 2 carriers per moire cell, n_{H} follows the logarithmic dependence predicted for a Lifshitz transition at a van-Hove singularity. Interestingly on the high doping side of the transition the linear dependence is restored but with an offset, indicating that the Fermi surface shrinks by 2 carriers per moire cell and a correlation gap opens. Moreover,a singular dependence around ±3n_{0} is observed suggesting a Lifshitz transition but without a gap. As the field is increased above 4T a gap opens at n/n_{0 }= 3 signaled by the appearance of a Landau fan and quantized R_{xy} plateaus. At the same time n_{H} suggests yet another change in the Fermi surface topology. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D51.00009: Chern insulators at odd fillings in magic-angle twisted bilayer graphene Shuang Wu, zhenyuan zhang, Eva Andrei The interplay of magnetic field and the moire superlattice in magic-angle twisted bilayer graphene provides a rich playground for correlated electron physics. We report on magneto-transport measurements of a twisted bilayer graphene device(1.17°±0.02°). At high out-of-plane magnetic fields, we observe Landau fans at integer fillings with well quantized Hall plateaus: σ_{xy}=νe^{2}/h, corresponding to integer Chern numbers,ν∈Ζ. These are formally equivalent to the Chern bands of the Hofstadter butterfly formed by the single-particle spectrum of an electron in a periodic potential at high magnetic fields. At fillings of n/n_{0}=-1, we find ν=3, while at n/n_{0}=3, ν=1. Here, n is the total carrier density. n_{0} is the carrier density corresponding to one electron per moire cell. For these fillings, we find that the field dependence of the Hall number n_{H}(B) extracted from the Hall resistivity, is non-monotonic indicating the opening of a gap at high fields(B>4T). This finding is confirmed by measuring the field dependence of the thermal activation gaps for both in plane and out of plane magnetic fields. The linear field dependence of the gaps being consistent with Zeeman splitting, suggests the emergence of a spin-aligned ferromagnetic phase. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D51.00010: Nematicity in twisted bilayer graphene: impact of the moiré superlattice Rafael Fernandes, J.W.F. Venderbos Recent experiments have reported evidence that the threefold rotational symmetry of magic-angle twisted bilayer graphene is broken in several regions of the phase diagram, both in the normal state and in the superconducting state. Here, we theoretically investigate the coupling between electronic nematic and moiré superlattice degrees of freedom. Because the nematic transition is described in terms of a three-state Potts order parameter, we show that a true phase transition can still take place even in the presence of externally applied uniaxial strain. Moreover, the nature of the Potts-nematic transition is fundamentally altered by fluctuating strain modes associated with the domain walls separating AB/BA stacking regions of the triangular moiré superlattice. In particular, these fluctuations mediate an effective nemato-orbital coupling that not only renders the nematic transition mean-field and first-order, but also ties the orientation of the nematic director to certain soft directions in momentum space. Finally, we contrast our results to the more familiar case of Ising-nematic order in a tetragonal rigid lattice. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D51.00011: Z2 topology and edge states of twisted bilayer graphene Chao Ma, Qiyue Wang, Scott Mills, Xiaolong Chen, Bingchen Deng, Shaofan Yuan, Cheng Li, Kenji Watanabe, Takashi Taniguchi, Du Xu, Fan Zhang, Fengnian Xia Recently twisted bilayer graphene (tBLG) emerges as a new platform of strongly correlated electrons near a magic twist angle, which hosts many exciting phenomena such as the Mott-like insulating phase, unconventional superconducting behavior, and emergent Chern ferromagnetism. Besides the apparent significance of band flatness, band topology may be another critical element responsible for the strongly correlated phenomena (like in the fractional quantum Hall effect) yet receives much less attention. We demonstrate that, while an unusual symmetry of tBLG trivializes the Berry curvature, two Z2 invariants characterize the topology of the moiré Dirac bands. We further show that subtle gapless edge states can survive in the electron and hole superlattice gaps of tBLG and mediate universal nonlocal transport that are robust to the interlayer electric field, twist angle, and edge termination. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D51.00012: Topological Floquet engineering of twisted bilayer graphene Michael Sentef, Gabriel E. Topp, Gregor Jotzu, James W McIver, Lede Xian, Angel Rubio We investigate the topological properties of Floquet-engineered twisted bilayer graphene above the magic angle driven by circularly polarized laser pulses. Employing a full Moiré-unit-cell tight-binding Hamiltonian based on first-principles electronic structure, we show that the band topology in the bilayer, at twisting angles above 1.05 degrees, essentially corresponds to the one of single-layer graphene. However, the ability to open topologically trivial gaps in this system by a bias voltage between the layers enables the full topological phase diagram to be explored, which is not possible in single-layer graphene. Circularly polarized light induces a transition to a topologically nontrivial Floquet band structure with the Berry curvature analogous to a Chern insulator. Importantly, the twisting allows for tuning electronic energy scales, which implies that the electronic bandwidth can be tailored to match realistic driving frequencies in the ultraviolet or midinfrared photon-energy regimes. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D51.00013: Topological charge pumping by sliding moiré pattern Manato Fujimoto, Henri Koschke, Mikito Koshino We theoretically study the adiabatic topological charge pumping driven by interlayer sliding in the moiré superlattices. We show that, when we slide a single layer of the twisted bilayer system relatively to the other, a moiré pattern flow and a quantized transport of electrons occurs.[1] When the Fermi energy is in a spectral gap, the number of pumped charges in the interlayer sliding process is quantized to a sliding Chern number, which is related to the interlayer sliding degree of freedom and obeys a Diophantine equation analogous to the quantum Hall effect [2]. We apply the argument to the twisted bilayer graphene, and find that energy gaps above and below the nearly-flat bands has non-zero sliding Chern numbers. When the Fermi energy is in either of those gaps, the slide-driven topological pumping occurs perpendicularly to the sliding direction. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D51.00014: Artificial gauge field in Moiré superlattices Ce Shang, Adel Abbout, Xiaoning Zang, Udo Schwingenschloegl, Aurelien Manchon Moiré superlattices, originating from rotational alignment or/and lattice constants mismatch between the individual layers, open up new strategies for engineering electronic properties [1]. Gauge fields unveil one of the most ubiquitous concepts which describes many branches of physics, ranging from the standard model to the general theory of relativity. In recent years, artificial gauge field for engineered systems extend their proven quantum simulation abilities further, e.g., to quantum Hall physics or topological insulators [2,3]. Here, we propose a method to create a tunable, artificial gauge potential in a periodically modulated Moiré superlattice. By imprinting Peierls phase on the hopping parameters between neighboring lattice sites, we present the realization of a Haldane-like model and investigate the characterization of its topological band structure. As an application, we also provide a methodology to directly measure topological order in such system from a dynamical quench process. |
Monday, March 2, 2020 5:18PM - 5:30PM |
D51.00015: Multiple topological phases in graphene via magnetoelectric proximity effect Hiroyuki Takenaka, Shane Sandhoefner, Alexey Kovalev, Evgeny Y Tsymbal Topological antiferromagnetic (AFM) spintronics have been drawn special attention for the next generation of non-volatile, ultra-fast, and low-power memory and storage devices. The key to realize the spintronic applications is to manipulate the topological electronic states via switching the AFM order parameter. Here we propose a new approach to control the topological electronic states of a two-dimensional material by the proximity of a magnetoelectric antiferromagnet. Using density functional theory and tight-binding Hamiltonian approaches, we investigate an interface between graphene and AFM magnetoelectric Cr_{2}O_{3} (0001). Due to the proximity effect, the interface electronic structure exhibits non-trivial band gap openings in the graphene Dirac bands asymmetric between the K and K′ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) and, in addition, the spin-polarized valley Hall effect (VHE). The quantum anomalous Hall effect (QAHE), the valley-polarized QAHE and the quantum valley Hall effect emerge in graphene across a 180 magnetic domain wall in chromia. We theoretically demonstrate that these topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques. |
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