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 P42: Correlations in GrapheneLive
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Sponsoring Units: DCMP Chair: Oskar Vafek, Florida State Univ |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P42.00001: Strongly Correlated Holes in a MoSe2/WSe2 Moiré Superlattice Aidan Campbell, Mauro Brotons-Gisbert, Hyeonjun Baek, Kenji Watanabe, Takashi Taniguchi, Brian D Gerardot Recently TMD moiré superlattices have been shown to host hybrid excitonic states[1], localised quantum emitters[2] and tuneable correlated electronic states including Mott insulating and Wigner crystalline phases[3][4]. MoSe2/WSe2 heterostructures have been widely studied as a host of band-edge interlayer excitons. However, optical signatures of moiré flat bands in these structures have thus far proved elusive. We perform resonant absorption spectroscopy of intralayer excitons in a dual-gate tuneable angle-aligned MoSe2/WSe2 heterostructure at 4K. We see evidence for the formation of moiré minibands and, in the hole-doping regime, observe the WSe2 attractive and repulsive polarons exhibit doping dependent extra-ordinary g-factors and magnetic saturation behaviour[5]. We measure the magnetic phase diagram by tuning the fractional filling and observe peaks in the landé g-factor at 1/3, 2/3 and 1 hole per moiré site which we interpret as the formation of correlated states. Finally the peak position and intensity of the MoSe2 A-exciton are shown to be sensors of the filling factor of the WSe2 valence band. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P42.00002: Bounds on low energy spectral weight and superfluid stiffness in strongly correlated systems Nishchhal Verma, Tamaghna Hazra, Mohit Randeria We have previously obtained rigorous upper bounds [1] on the superfluid stiffness of multi-band systems with arbitrary interactions in any dimension. These results were then used to obtain upper bounds for the superconducting Tc of 2D systems and led to insightful results for strongly correlated systems like Twisted Bilayer Graphene, Monolayer FeSe/STO, and cold atoms across the BCS-BEC crossover. These bounds were expressed in terms of the optical spectral weight of all the bands; we now improve upon these results by projecting down to the active bands crossing the chemical potential. As a nontrivial example, we focus on the case of an isolated flat band near EF for which one cannot use the usual Peierls’ substitution and the projected interaction terms couple to the external vector potential. We show how the resulting low-energy optical spectral weight depends on the geometry of the band eigenstates (without making any mean-field approximations). We further derive exact upper bounds on the low-energy spectral weight and show that it is related to the Marzari-Vanderbilt localization function, and in some special cases, to the quantum metric. |
Wednesday, March 17, 2021 3:24PM - 3:36PM Live |
P42.00003: Local spectroscopic study of moiré band flattening and correlated gaps in magic-angle twisted bilayer graphene Hyunjin Kim, Youngjoon Choi, Cyprian Lewandowski, Yang Peng, Alex R Thomson, Kenji Watanabe, Takashi Taniguchi, Jason F. Alicea, Stevan Nadj-Perge Magic angle (~1.1°) twisted bilayer graphene (TBG) shows a wide range of correlated electronic states that originate from the flat moiré bands. Scanning tunneling spectroscopy experiments on TBG near the magic angle observed, a large separation between van Hove singularities and finite Dirac velocities signaling relatively large bandwidth that is not compatible with the continuum model expectations. While these previous measurements highlight the presence of electronic interactions, their impact on the band structure is still unclear. Here, we report scanning tunneling spectroscopy measurements of TBG across the area where the twist angle slowly changes from non-magic to magic-angle value. By systematically following the evolution of the moiré electronic bands, we study doping dependent band deformations. Surprisingly, we discover interaction-driven band flattening that can be also related to gap opening near half-filling. Our results provide crucial insights into the robustness of half-filling correlated phases. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P42.00004: Twisted Trilayer Graphene: a Precisely Tunable Platform for Correlated Electrons Ziyan Zhu, Stephen Carr, Daniel Massatt, Mitchell Luskin, Efthimios Kaxiras We introduce twisted trilayer graphene (tTLG) with two independent twist angles as an ideal system for the precise tuning of the electronic interlayer coupling to maximize the effect of correlated behaviors. As established by experiment and theory in the related twisted bilayer graphene system, van Hove singularities (VHS) in the density of states can be used as a proxy of the tendency for correlated behaviors. To explore the evolution of VHS in the twist-angle phase space of tTLG, we present a general low-energy electronic structure model for any pair of twist angles. We show that the basis of the model has infinite dimensions even at a finite energy cutoff and that no Brillouin zone exists even in the continuum limit. Using this model, we demonstrate that the tTLG system exhibits a wide range of magic angles at which VHS merge and the density of states has a sharp peak at the charge-neutrality point through two distinct mechanisms: the incommensurate perturbation of twisted bilayer graphene's flat bands or the equal hybridization between two bilayer moiré superlattices. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P42.00005: Theoretical study of correlated electronic states in twisted monolayer-bilayer graphene Mohammed Alezzi, Xingyu Gu, Alexandra Carvalho, Nimisha Raghuvanshi, Yanmeng Shi, Vladimir Falko, Kostya Novoselov, Antonio Helio Castro Neto, Shaffique Adam In twisted trilayer graphene with a graphene monolayer stacked and twisted on top of bilayer graphene, massless and massive Dirac fermions hybridize. This heterostructure gives rise to topological electronic bands that are a unique platform to explore symmetry-broken correlated electronic states [1,2,3]. 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. Using the linearized gap equation method, we then calculate the stability and critical temperature for different symmetry breaking phases, including spin density waves, charge density waves, and valley ordered phases. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P42.00006: Ultrafast Correlated Electron Dynamics in Finite Graphene Nanostructures Jan-Philip Joost, Niclas Schlünzen, Michael Bonitz Finite graphene nanoribbon (GNR) heterostructures host intriguing topological in-gap states [1]. These states may be localized either at the bulk edges or at the ends of the structure. In a previous work we showed that correlation effects play a key role in these systems, resulting in increased magnetic moments at the ribbon edges accompanied by a significant energy renormalization of the topological end states, even in the presence of a metallic substrate [2]. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P42.00007: Dipole-active collective excitations in moiré flat bands as a probe of the correlated insulating and superconducting states Ali Fahimniya, Cyprian Lewandowski, Leonid Levitov Collective plasma excitations in moiré flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional 2D plasmons, dispersing as k1/2 at low frequencies and plunging into particle-hole continuum at higher frequencies, the moiré plasmons pierce through the flat-band continuum and acquire a strong over-the-band character [1,2,3]. Due to the complex structure of the moiré superlattice (SL) unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in the SL Brillouin zone. Using a toy Hubbard model for the correlated insulating (CI) order in a flat band, we predict that these high-frequency modes become strongly dipole-active upon the system undergoing charge ordering, with the low-frequency modes gapped out within the CI gap. We also predict a similar behavior for the superconducting state. Strong dipole moments and sensitivity to charge order make these modes readily accessible by optical and microwave measurements, offering a convenient diagnostic of these states. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P42.00008: Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening Xiaoxue Liu, Zhi Wang, Kenji Watanabe, Takashi Taniguchi, Oskar Vafek, Jia Li In this talk, we report a novel device geometry, where the magic-angle twisted bilayer graphene (tBLG) [1-2] and a Bernal bilayer graphene (BLG) are separated by an insulating barrier with thickness of 3nm. The close proximity allows charge carriers from BLG to screen Coulomb interaction in the tBLG. Activation energy gap measurement shows that the correlated insulating states become less robust as Coulomb interaction is weakened by screening, demonstrating our ability to directly control the strength of electron correlation in the moire flatband. Most remarkably, transport measurements also demonstrate the significant influence Coulomb screening has on the robustness of the superconducting phase: at optimal doping, the stability of superconductivity is enhanced by the introduction of Coulomb screening [3]. Our results provide new insights into the nature of the superconducting phase in tBLG. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P42.00009: Floquet-Bloch Dynamics in Simple Systems Gayanath Fernando, Richard Geilhufe Fascinating changes in electronic properties such as topological and other phase |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P42.00010: Electron correlation and non-Fermi-liquid behavior in minimally twisted bilayer graphene Yang-Zhi Chou, Fengcheng Wu, Jay Sau We study the phenomena driven by electron-electron interactions in minimally twisted bilayer graphene (mTBLG) with a perpendicular electric field. The low-energy degrees of freedom in mTBLG are governed by a network of one-dimensional domain-wall states, described by two channels of linearly dispersing one-dimensional spin-1/2 fermions. We show that the interaction can realize a spin-gapped inter-channel charge density wave state at generic fillings. Moreover, we demonstrate that the finite-temperature resistivity features a non-Fermi-liquid behavior due to the (linearly dispersing) electrons scattering off the incoherent charge fluctuations in the domain-wall states. Our predictions are robust against the twist-angle disorder and can apply to other moir\'e systems that manifest topological domain-wall structures. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P42.00011: Correlation Effect in ABC-TLG and hBN Superlattice Revealed by Infrared Photocurrent Spectroscopy Jixiang Yang, Guorui Chen, Yahui Zhang, Qihang Zhang, Lili Jiang, Hongyuan Li, Bosai Lv, Kenji Watanabe, Takashi Taniguchi, Zhiwen Shi, Senthil Todadri, Yuanbo Zhang, feng wang, Long Ju The moiré superlattice formed by rhombohedral (ABC) stacked trilayer graphene on hexagonal boron nitride is a new playground for correlated physics, featuring tunable Mott insulators, superconductivity, and correlated Chern insulators. Here, we report a spectroscopy study of this system, using the Fourier-transform infrared (FTIR) photocurrent spectroscopy technique. We measured the interband optical transitions between moiré mini bands as a function of vertical displacement field tuned by gate voltages. A continuous evolution of bandgap and bandwidth was observed. We also observed the transitions across the Mott gap at 1/2 filling of the flat band, indicating an onsite Coulomb repulsion of ~20 meV. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P42.00012: Correlated insulator states in twisted bilayer graphene: Hartree-Fock studies Jianpeng Liu, Shihao Zhang, Xi Dai We discuss the correlated insulator states in twisted bilayer graphene (TBG) using two different types of Hartree-Fock (methods: in the first method all energy bands are included in the self-consistent calculations ,but some approximation is made for the Fock operators; in the other method, we project Coulomb interactions onto flat bands, and performed unrestricted Hartree-Fock calculations in the subspace of the flat bands. We have constructed the phase diagrams at even integer fillings of the flat bands in magic TBG, and discuss the experimental observables associated with different correlated insulator states. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P42.00013: Correlated states in the graphene moiré ABC trilayer-hBN Elena Bascones, Alberto Camjayi, Maria Jose Calderon Correlated insulating states resembling the ones in twisted bilayer graphene have been observed in the graphene moiré formed by an ABC trilayer aligned with the underlying boron nitride (ABC/TLG-hBN) [1]. In ABC/TLG-hBN the shape and the topology of the bands can be controlled by an electric field. The insulating states appear at several integer fillings for a specific sign of the electric field and the doping. The nature of these insulating states, in particular whether they are Mott insulating states is unknown. Starting from a Wannier function representation [2] we have studied the nature of the correlated states, including the doping and field dependence, with particular emphasis on their Mott character. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Live |
P42.00014: Electronic correlations in twisted bilayer graphene Maria Jose Calderon, Alberto Camjayi, Marcello Civelli, Marcelo Rozenberg, Elena Bascones The nature of the insulating and superconducting states appear upon doping magic angle twisted bilayer graphene (TBG) is not clear yet. To address their origin we have calculated [1] the interactions between the Wannier functions of the 8-orbital model proposed for TBG [2] as a function of twist angle and proximity of the gates. We will address the effects of these interactions on the correlated states and how they depend on the doping, the twist angle and the screening by the gates. [1] M.J. Calderón, E. Bascones arXiv:2007.16051. To be published in Phys. Rev. B. [2] Stephen Carr, et al, Phys. Rev. Research 1, 033072 (2019). |
Wednesday, March 17, 2021 5:48PM - 6:00PM Live |
P42.00015: Nematic and quantum valley Hall states in twisted bilayer-monolayer graphene systems Shihao Zhang, Jianpeng Liu The experimental discovery of correlated insulating states in twisted bilayer graphene drives the moiré two-dimensional materials system into a thriving area of condensed matter physics. Here, we use unrestricted Hartree-Fock variational method both in the original basis and in the projected-flat-band basis to study the correlated insulator and quantum anomalous Hall (QAH) states twisted monolayer-bilayer graphene. Our mean-field calculations reveal that the system can show the QAH state with |C|=2 Chern number at the 1/4 and 3/4 fillings under electric displacement field as an orbital Chern insulator, which is consistent with recent experimental results. When the twisted monolayer-bilayer graphene is set at the 1/2 filling under about -0.4 V/nm electric displacement field, quantum valley Hall effect will emerge in this moiré system. There is spin polarization but no valley polarization in this phase. The distributions of order parameters in the momentum space reveal that the system at this phase breaks the C3z rotational symmetry and orbital time-reversal symmetry. The quantum valley Hall effect features the valley-contrasting current loops circulating around the ABB region of the moiré pattern which shows the opposite orbital magnetization originated from different valleys. |
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