We present a theoretical study of single-particle and many-body properties of twisted bilayer WSe2. For single-particle physics, we calculate the band topological phase diagram and electron local density of states (LDOS), which are found to be correlated. By comparing our theoretical LDOS with those measured by scanning tunneling microscopy, we comment on the possible topological nature of the first moire valence band. For many-body physics, we construct a generalized Hubbard model on a triangular lattice based on the calculated single-particle moire bands. We show that a layer potential difference, arising, for example, from an applied electric field, can drastically change the noninteracting moire bands, tune the spin-orbit coupling in the Hubbard model, control the charge excitation gap of the Mott insulator at half-filling, and generate an effective Dzyaloshinskii-Moriya interaction in the effective Heisenberg model for the Mott insulator. Our theoretical results agree with transport experiments on the same system in several key aspects, and establish twisted bilayer WSe2 as a highly tunable system for studying and simulating strongly correlated phenomena in the Hubbard model.

}, doi = {10.1103/PhysRevResearch.2.033087}, author = {Pan, Haining and Wu, Fengcheng and Das Sarma, Sankar} } @article { ISI:000510176900007, title = {Collective Excitations of Quantum Anomalous Hall Ferromagnets in Twisted Bilayer Graphene}, journal = {Phys. Rev. Lett.}, volume = {124}, number = {4}, year = {2020}, month = {JAN 30}, pages = {046403}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We present a microscopic theory for collective excitations of quantum anomalous Hall ferromagnets (QAHF) in twisted bilayer graphene. We calculate the spin magnon and valley magnon spectra by solving Bethe-Salpeter equations and verify the stability of QAHF. We extract the spin stiffness from the gapless spin wave dispersion and estimate the energy cost of a skyrmion-antiskyrmion pair, which is found to be comparable in energy with the Hartree-Fock gap. The valley wave mode is gapped, implying that the valley polarized state is more favorable compared to the valley coherent state. Using a nonlinear sigma model, we estimate the valley ordering temperature, which is considerably reduced from the mean-field transition temperature due to thermal excitations of valley waves.}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.124.046403}, author = {Wu, Fengcheng and Das Sarma, Sankar} } @article {das_sarma_electron-phonon_2020, title = {Electron-phonon and electron-electron interaction effects in twisted bilayer graphene}, journal = {Ann. Phys.}, volume = {417}, year = {2020}, note = {Place: 525 B ST, STE 1900, SAN DIEGO, CA 92101-4495 USA Publisher: ACADEMIC PRESS INC ELSEVIER SCIENCE Type: Article}, month = {jun}, abstract = {By comparing with recently available experimental data from several groups, we critically discuss the manifestation of continuum many body interaction effects in twisted bilayer graphene (tBLG) with small twist angles and low carrier densities, which arise naturally within the Dirac cone approximation for the non-interacting band structure. We provide two specific examples of such continuum many body theories: one involving electron-phonon interaction and one involving electron-electron interaction. In both cases, the experimental findings are only partially quantitatively consistent with rather clear-cut leading-order theoretical predictions based on well-established continuum many body theories. We provide a critical discussion, based mainly on the currently available tBLG experimental data, on possible future directions for understanding many body renormalization involving electron-phonon and electron-electron interactions in the system. One definitive conclusion based on the comparison between theory and experiment is that the leading order 1-loop perturbative renormalization group theory completely fails to account for the electron-electron interaction effects in the strong-coupling limit of flatband moire tBLG system near the magic twist angle even at low doping where the Dirac cone approximation should apply. By contrast, approximate nonperturbative theoretical results based on Borel-Pade resummation or 1/N expansion seem to work well compared with experiments, indicating rather small interaction corrections to Fermi velocity or carrier effective mass. For electron-phonon interactions, however, the leading-order continuum theory works well except when van Hove singularities in the density of states come into play. (C) 2020 Elsevier Inc. All rights reserved.}, issn = {0003-4916}, doi = {10.1016/j.aop.2020.168193}, author = {Das Sarma, Sankar and Wu, Fengcheng} } @article { ISI:000529804100001, title = {Ferromagnetism and superconductivity in twisted double bilayer graphene}, journal = {Phys. Rev. B}, volume = {101}, number = {15}, year = {2020}, month = {APR 30}, pages = {155149}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We present a theory of competing ferromagnetic and superconducting orders in twisted double bilayer graphene. In our theory, ferromagnetism is induced by Coulomb repulsion, while superconductivity with intervalley equal-spin pairing can be mediated by electron-acoustic phonon interactions. We calculate the transition temperatures for ferromagnetism and superconductivity as a function of moire band filling factor, and find that superconducting domes can appear on both the electron and hole sides of the ferromagnetic insulator at half filling. We show that the ferromagnetic insulating gap has a dome shape dependence on the layer potential difference, which provides an explanation to the experimental observation that the ferromagnetic insulator only develops over a finite range of external displacement field. We also verify the stability of the half filled ferromagnetic insulator against two types of collective excitations, i.e., spin magnons and valley magnons.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.101.155149}, author = {Wu, Fengcheng and Das Sarma, Sankar} } @article {tenasini_giant_2020, title = {Giant anomalous {Hall} effect in quasi-two-dimensional layered antiferromagnet {Co1}/{3NbS2}}, journal = {Phys. Rev. Res.}, volume = {2}, number = {2}, year = {2020}, note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article}, month = {apr}, abstract = {The discovery of the anomalous Hall effect (AHE) in bulk metallic antiferromagnets (AFMs) motivates the search of the same phenomenon in two-dimensional (2D) systems, where a quantized anomalous Hall conductance can, in principle, be observed. Here we present experiments on microfabricated devices based on Co1/3NbS2, a layered AFM that was recently found to exhibit AHE in bulk crystals below the Neel temperature T-N = 29 K. Transport measurements reveal a pronounced resistivity anisotropy, indicating that upon lowering temperature the electronic coupling between individual atomic layers is increasingly suppressed. The experiments also show an extremely large anomalous Hall conductivity of approximately 400 S/cm, more than one order of magnitude larger than in the bulk, which demonstrates the importance of studying the AHE in small exfoliated crystals, less affected by crystalline defects. Interestingly, the corresponding anomalous Hall conductance, when normalized to the number of contributing atomic planes, is similar to 0.6 e(2)/h per layer, approaching the value expected for the quantized anomalous Hall effect. The observed strong anisotropy of transport and the very large anomalous Hall conductance per layer make the properties of Co1/3NbS2 compatible with the presence of partially filled topologically nontrivial 2D bands originating from the magnetic superstructure of the antiferromagnetic state. Isolating atomically thin layers of this material and controlling their charge density may therefore provide a viable route to reveal the occurrence of the quantized AHE in a 2D AFM.

}, doi = {10.1103/PhysRevResearch.2.023051}, author = {Tenasini, Giulia and Martino, Edoardo and Ubrig, Nicolas and Ghimire, Nirmal J. and Berger, Helmuth and Zaharko, Oksana and Wu, Fengcheng and Mitchell, J. F. and Martin, Ivar and Forro, Laszlo and Morpurgo, Alberto F.} } @article { ISI:000562320800002, title = {Higgs-like modes in two-dimensional spatially indirect exciton condensates}, journal = {Phys. Rev. B}, volume = {102}, number = {7}, year = {2020}, month = {AUG 25}, pages = {075136}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {Higgs-like modes in condensed-matter physics have drawn attention because of analogies to the Higgs bosons of particle physics. Here we use a microscopic time-dependent mean-field theory to study the collective mode spectra of two-dimensional spatially indirect exciton (electron-hole pair) condensates, focusing on the Higgs-like modes, i.e., those that have a large weight in electron-hole pair amplitude response functions. We find that in the low exciton density (Bose-Einstein condensate) limit, the dominant Higgs-like modes of spatially indirect exciton condensates correspond to adding electron-hole pairs that are orthogonal to the condensed pair state. We comment on the previously studied Higgs-like collective excitations of superconductors in light of this finding.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.102.075136}, author = {Xue, Fei and Wu, Fengcheng and MacDonald, A. H.} } @article {chou_hofstadter_2020, title = {Hofstadter butterfly and {Floquet} topological insulators in minimally twisted bilayer graphene}, journal = {Phys. Rev. Res.}, volume = {2}, number = {3}, year = {2020}, note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article}, month = {aug}, abstract = {We theoretically study the Hofstadter butterfly of a triangular network model in minimally twisted bilayer graphene. The band structure manifests periodicity in energy, mimicking that of Floquet systems. The butterfly diagrams provide fingerprints of the model parameters and reveal the hidden band topology. In a strong magnetic field, we establish that minimally twisted bilayer graphene realizes low-energy Floquet topological insulators (FTIs) carrying zero Chern number, while hosting chiral edge states in bulk gaps. We identify the FTIs by analyzing the nontrivial spectral flow in the Hofstadter butterfly, and by explicitly computing the chiral edge states. Our theory paves the way for an effective practical realization of FTIs in equilibrium solid-state systems.}, doi = {10.1103/PhysRevResearch.2.033271}, author = {Chou, Yang-Zhi and Wu, Fengcheng and Das Sarma, Sankar} } @article { ISI:000523412800006, title = {Mobius Insulator and Higher-Order Topology in MnBi2nTe3n+1}, journal = {Phys. Rev. Lett.}, volume = {124}, number = {13}, year = {2020}, month = {APR 3}, pages = {136407}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We propose MnBi2nTe3n+1 as a magnetically tunable platform for realizing various symmetry-protected higher-order topology. Its canted antiferromagnetic phase can host exotic topological surface states with a Mobius twist that are protected by nonsymmorphic symmetry. Moreover, opposite surfaces hosting Mobius fermions are connected by one-dimensional chiral hinge modes, which offers the first material candidate of a higher-order topological Mobius insulator. We uncover a general mechanism to feasibly induce this exotic physics by applying a small in-plane magnetic field to the antiferromagnetic topological insulating phase of MnBi2nTe3n+1, as well as other proposed axion insulators. For other magnetic configurations, two classes of inversion-protected higher-order topological phases are ubiquitous in this system, which both manifest gapped surfaces and gapless chiral hinge modes. We systematically discuss their classification, microscopic mechanisms, and experimental signatures. Remarkably, the magnetic-field-induced transition between distinct chiral hinge mode configurations provides an effective {\textquoteleft}{\textquoteleft}topological magnetic switch{{\textquoteright}{\textquoteright}}.

}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.124.136407}, author = {Zhang, Rui-Xing and Wu, Fengcheng and Das Sarma, Sankar} } @article { ISI:000542514200007, title = {Phonon scattering induced carrier resistivity in twisted double-bilayer graphene}, journal = {Phys. Rev. B}, volume = {101}, number = {24}, year = {2020}, month = {JUN 24}, pages = {245436}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {In this work we carry out a theoretical study of the phonon-induced resistivity in twisted double bilayer graphene (TDBG), in which two Bernal-stacked bilayer graphene devices are rotated relative to each other by a small angle theta. We show that at small twist angles (theta similar to 1 degrees) the effective mass of the TDBG system is greatly enhanced, leading to a drastically increased phonon-induced resistivity in the high-temperature limit where phonon scattering leads to a linearly increasing resistivity with increasing temperature. We also discuss possible implications of our theory on superconductivity in such a system and provide an order of magnitude estimation of the superconducting transition temperature.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.101.245436}, author = {Li, Xiao and Wu, Fengcheng and Das Sarma, S.} } @article { ISI:000576889300001, title = {Quantum geometry and stability of moire flatband ferromagnetism}, journal = {Phys. Rev. B}, volume = {102}, number = {16}, year = {2020}, month = {OCT 12}, pages = {165118}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {Several moire systems created by various twisted bilayers have manifested magnetism under flatband conditions leading to enhanced interaction effects. We theoretically study stability of moire flatband ferromagnetism against collective excitations, with a focus on the effects of Bloch band quantum geometry. The spin magnon spectrum is calculated using different approaches, including Bethe-Salpeter equation, single mode approximation, and an analytical theory. One of our main results is an analytical expression for the spin stiffness in terms of the Coulomb interaction potential, the Berry curvatures, and the quantum metric tensor, where the last two quantities characterize the quantum geometry of moire bands. This analytical theory shows that Berry curvatures play an important role in stiffening the spin magnons. Furthermore, we construct an effective field theory for the magnetization fluctuations and show explicitly that skyrmion excitations bind an integer number of electrons that is proportional to the Bloch band Chern number and the skyrmion winding number.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.102.165118}, author = {Wu, Fengcheng and Das Sarma, S.} } @article {wu_three-dimensional_2020, title = {Three-dimensional topological twistronics}, journal = {Phys. Rev. Res.}, volume = {2}, number = {2}, year = {2020}, note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article}, month = {apr}, abstract = {We introduce a theoretical framework for the concept of three-dimensional (3D) twistronics by developing a generalized Bloch band theory for 3D layered systems with a constant twist angle theta between successive layers. Our theory employs a nonsymmorphic symmetry that enables a precise definition of an effective out-of-plane crystal momentum, and also captures the in-plane moire pattern formed between neighboring twisted layers. To demonstrate topological physics that can be achieved through 3D twistronics, we present two examples. In the first example of chiral twisted graphite, Weyl nodes arise because of inversion-symmetry breaking, with theta-tuned transitions between type-I and type-II Weyl fermions, as well as magic angles at which the in-plane velocity vanishes. In the second example of a twisted Weyl semimetal, the twist in the lattice structure induces a chiral gauge field A that has a vortex-antivortex lattice configuration. Line modes bound to the vortex cores of the A field give rise to 3D Weyl physics in the moire scale. We also discuss possible experimental realizations of 3D twistronics.}, doi = {10.1103/PhysRevResearch.2.022010}, author = {Wu, Fengcheng and Zhang, Rui-Xing and Das Sarma, Sankar} } @article { ISI:000554826100006, title = {Topological superconductivity, ferromagnetism, and valley-polarized phases in moire systems: Renormalization group analysis for twisted double bilayer graphene}, journal = {Phys. Rev. B}, volume = {102}, number = {8}, year = {2020}, month = {AUG 3}, pages = {085103}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {Recent experiments have observed possible spin- and valley-polarized insulators and spin-triplet superconductivity in twisted double bilayer graphene, a moire structure consisting of a pair of Bernal-stacked bilayer graphene. Besides the continuously tunable bandwidths controlled by an applied displacement field and twist angle, these moire bands also possess Van Hove singularities near the Fermi surface and a field-dependent nesting which is far from perfect. Here we carry out a perturbative renormalization group analysis to unbiasedly study the competition among all possible instabilities in twisted double bilayer graphene and related systems with a similar Van Hove fermiology in the presence of weak but finite repulsive interactions. Our key finding is that there are several competing magnetic, valley, charge, and superconducting instabilities arising from interactions in twisted double bilayer graphene, which can be tuned by controlling the displacement field and the twist angle. In particular, we show that spin- or valley-polarized uniform instabilities generically dominate under moderate interactions smaller than the bandwidth, whereas p-wave spin-triplet topological superconductivity and exotic spin-singlet modulated paired state become important as the interactions decrease. Realization of our findings in general moire systems with a similar Van Hove fermiology should open up new opportunities for manipulating topological superconductivity and spin- or valley-polarized states in highly tunable platforms.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.102.085103}, author = {Hsu, Yi-Ting and Wu, Fengcheng and Das Sarma, S.} } @article {zhang_twist-angle_2020, title = {Twist-angle dependence of moire excitons in {WS2}/{MoSe2} heterobilayers}, journal = {Nat. Commun.}, volume = {11}, number = {1}, year = {2020}, note = {Place: HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY Publisher: NATURE RESEARCH Type: Article}, month = {nov}, abstract = {Moire lattices formed in twisted van der Waals bilayers provide a unique, tunable platform to realize coupled electron or exciton lattices unavailable before. While twist angle between the bilayer has been shown to be a critical parameter in engineering the moire potential and enabling novel phenomena in electronic moire systems, a systematic experimental study as a function of twist angle is still missing. Here we show that not only are moire excitons robust in bilayers of even large twist angles, but also properties of the moire excitons are dependant on, and controllable by, the moire reciprocal lattice period via twist-angle tuning. From the twist-angle dependence, we furthermore obtain the effective mass of the interlayer excitons and the electron inter-layer tunneling strength, which are difficult to measure experimentally otherwise. These findings pave the way for understanding and engineering rich moire-lattice induced phenomena in angle-twisted semiconductor van der Waals heterostructures. Here, the authors show that the properties of the moire excitons in twisted van der Waals bilayers of transition metal dichalcogenides are determined by the moire reciprocal lattice period, and can be controlled via twist-angle tuning.}, issn = {2041-1723}, doi = {10.1038/s41467-020-19466-6}, author = {Zhang, Long and Zhang, Zhe and Wu, Fengcheng and Wang, Danqing and Gogna, Rahul and Hou, Shaocong and Watanabe, Kenji and Taniguchi, Takashi and Kulkarni, Krishnamurthy and Kuo, Thomas and Forrest, Stephen R. and Deng, Hui} } @article {ISI:000473013000002, title = {Identification of superconducting pairing symmetry in twisted bilayer graphene using in-plane magnetic field and strain}, journal = {Phys. Rev. B}, volume = {99}, number = {22}, year = {2019}, month = {JUN 25}, pages = {220507}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We show how the pairing symmetry of superconducting states in twisted bilayer graphene can be experimentally identified by theoretically studying effects of externally applied in-plane magnetic field and strain. In the low-field regime, superconducting critical temperature T-c is suppressed by in-plane magnetic field B-parallel to in singlet channels, but is enhanced by weak B-parallel to in triplet channels, providing an important distinction. The in-plane angular dependence of the critical B-parallel to,B-c has a sixfold rotational symmetry, which is broken when strain is present. We show that anisotropy in B-parallel to,B-c generated by strain can be similar for s- and d-wave channels in moire superlattices. The d-wave state is pinned to be nematic by strain and consequently gapless, which is distinguishable from the fully gapped s-wave state by tunneling gap measurements.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.99.220507}, author = {Wu, Fengcheng and S. Das Sarma} } @article {ISI:000473132200039, title = {Orbital-flop Induced Magnetoresistance Anisotropy in Rare Earth Monopnictide CeSb}, journal = {Nat. Commun.}, volume = {10}, year = {2019}, month = {JUN 28}, pages = {2875}, publisher = {NATURE PUBLISHING GROUP}, type = {Article}, abstract = {The charge and spin of the electrons in solids have been extensively exploited in electronic devices and in the development of spintronics. Another attribute of electrons-their orbital nature-is attracting growing interest for understanding exotic phenomena and in creating the next-generation of quantum devices such as orbital qubits. Here, we report on orbital-flop induced magnetoresistance anisotropy in CeSb. In the low temperature high magnetic-field driven ferromagnetic state, a series of additional minima appear in the angle-dependent magnetoresistance. These minima arise from the anisotropic magnetization originating from orbital-flops and from the enhanced electron scattering from magnetic multidomains formed around the first-order orbital-flop transition. The measured magnetization anisotropy can be accounted for with a phenomenological model involving orbital-flops and a spin-valve-like structure is used to demonstrate the viable utilization of orbital-flop phenomenon. Our results showcase a contribution of orbital behavior in the emergence of intriguing phenomena.}, issn = {2041-1723}, doi = {10.1038/s41467-019-10624-z}, author = {Xu, Jing and Wu, Fengcheng and Bao, Jin-Ke and Han, Fei and Xiao, Zhi-Li and Martin, Ivar and Lyu, Yang-Yang and Wang, Yong-Lei and Chung, Duck Young and Li, Mingda and Zhang, Wei and Pearson, John E. and Jiang, Jidong S. and Kanatzidis, Mercouri G. and Kwok, Wai-Kwong} } @article {ISI:000463889200003, title = {Phonon-induced giant linear-in-T resistivity in magic angle twisted bilayer graphene: Ordinary strangeness and exotic superconductivity}, journal = {Phys. Rev. B}, volume = {99}, number = {16}, year = {2019}, month = {APR 9}, pages = {165112}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We study the effect of electron-acoustic phonon interactions in twisted bilayer graphene on resistivity in the high-temperature transport and superconductivity in the low-temperature phase diagram. We theoretically show that twisted bilayer graphene should have an enhanced and strongly twist-angle dependent linear-in-temperature resistivity in the metallic regime with the resistivity magnitude increasing as the twist angle approaches the magic angle. The slope of the resistivity versus temperature could approach one hundred ohm per kelvin with a strong angle dependence, but with a rather weak dependence on the carrier density. This higher-temperature density-independent linear-in-T resistivity crosses over to a T-4 dependence at a low density-dependent characteristic temperature, becoming unimportant at low temperatures. This angle-tuned resistivity enhancement arises from the huge increase in the effective electron-acoustic phonon coupling in the system due to the suppression of graphene Fermi velocity induced by the flat-band condition in the moire superlattice system. Our calculated temperature dependence is reminiscent of the so-called {\textquoteleft}{\textquoteleft}strange metal{{\textquoteright}{\textquoteright}} transport behavior except that it is arising from the ordinary electron-phonon coupling in a rather unusual parameter space due to the generic moire flat-band structure of twisted bilayer graphene. We also show that the same enhanced electron-acoustic phonon coupling also mediates effective attractive interactions in s, p, d, and f pairing channels with a theoretical superconducting transition temperature on the order of similar to 5 K near magic angle. The fact that ordinary acoustic phonons can produce exotic non-s-wave superconducting pairing arises from the unusual symmetries of the system.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.99.165112}, author = {Wu, Fengcheng and Hwang, Euyheon and S. Das Sarma} } @article {21371, title = {Phonon-induced giant linear-in-T resistivity in magic angle twisted bilayer graphene: Ordinary strangeness and exotic superconductivity}, journal = {Phys. Rev. B}, volume = {99}, year = {2019}, month = {Apr}, pages = {165112}, abstract = {We study the effect of electron-acoustic phonon interactions in twisted bilayer graphene on resistivity in the high-temperature transport and superconductivity in the low-temperature phase diagram. We theoretically show that twisted bilayer graphene should have an enhanced and strongly twist-angle dependent linear-in-temperature resistivity in the metallic regime with the resistivity magnitude increasing as the twist angle approaches the magic angle. The slope of the resistivity versus temperature could approach one hundred ohm per kelvin with a strong angle dependence, but with a rather weak dependence on the carrier density. This higher-temperature density-independent linear-in-T\ resistivity crosses over to a\ T4\ dependence at a low density-dependent characteristic temperature, becoming unimportant at low temperatures. This angle-tuned resistivity enhancement arises from the huge increase in the effective electron-acoustic phonon coupling in the system due to the suppression of graphene Fermi velocity induced by the flat-band condition in the moir{\'e} superlattice system. Our calculated temperature dependence is reminiscent of the so-called {\textquotedblleft}strange metal{\textquotedblright} transport behavior except that it is arising from the ordinary electron-phonon coupling in a rather unusual parameter space due to the generic moir{\'e} flat-band structure of twisted bilayer graphene. We also show that the same enhanced electron-acoustic phonon coupling also mediates effective attractive interactions in\ s,p,d, and\ f\ pairing channels with a theoretical superconducting transition temperature on the order of\ \~{}5\ K near magic angle. The fact that ordinary acoustic phonons can produce exotic non-s-wave superconducting pairing arises from the unusual symmetries of the system.

}, doi = {10.1103/PhysRevB.99.165112}, url = {https://link.aps.org/doi/10.1103/PhysRevB.99.165112}, author = {Wu, Fengcheng and Hwang, Euyheon and Das Sarma, Sankar} } @article {ISI:000467383900001, title = {Topological chiral superconductivity with spontaneous vortices and supercurrent in twisted bilayer graphene}, journal = {Phys. Rev. B}, volume = {99}, number = {19}, year = {2019}, month = {MAY 8}, pages = {195114}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We study d-wave superconductivity in twisted bilayer graphene and reveal phenomena that arise due to the moire superlattice. In the d-wave pairing, the relative motion (RM) of two electrons in a Cooper pair can have either d + id or d - id symmetry with opposite angular momenta. Due to the enlarged moire superlattice, the center-of-mass motion (COMM) can also carry a finite angular momentum while preserving the moire periodicity. By matching the total angular momentum, which has contributions from both the RM and the COMM, Cooper pairs with d + id and d - id RMs are intrinsically coupled in a way such that the COMM associated with one of the RMs has a spontaneous vortex-antivortex lattice configuration. Another phenomenon is that the chiral d-wave state carries spontaneous bulk circulating supercurrent. The chiral d-wave superconductors are gapped and also topological as characterized by an integer Chern number. Nematic d-wave superconductors, which could be stabilized, for example, by uniaxial strain, are gapless with point nodes.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.99.195114}, author = {Wu, Fengcheng} } @article {ISI:000459920900007, title = {Topological Insulators in Twisted Transition Metal Dichalcogenide Homobilayers}, journal = {Phys. Rev. Lett.}, volume = {122}, number = {8}, year = {2019}, month = {FEB 28}, pages = {086402}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We show that moire bands of twisted homobilayers can be topologically nontrivial, and illustrate the tendency by studying valence band states in +/- K valleys of twisted bilayer transition metal dichalcogenides, in particular, bilayer MoTe2. Because of the large spin-orbit splitting at the monolayer valence band maxima, the low energy valence states of the twisted bilayer MoTe2 at the +K (-K) valley can be described using a two-band model with a layer-pseudospin magnetic field Delta(r) that has the moire period. We show that Delta(r) has a topologically nontrivial skyrmion lattice texture in real space, and that the topmost moire valence bands provide a realization of the Kane-Mele quantum spin-Hall model, i. e., the two-dimensional time-reversal-invariant topological insulator. Because the bands narrow at small twist angles, a rich set of broken symmetry insulating states can occur at integer numbers of electrons per moire cell.}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.122.086402}, author = {Wu, Fengcheng and Lovorn, Timothy and Tutuc, Emanuel and Martin, Ivar and MacDonald, A. H.} } @article { ISI:000454178600013, title = {Theory of Phonon-Mediated Superconductivity in Twisted Bilayer Graphene}, journal = {PHYSICAL REVIEW LETTERS}, volume = {121}, number = {25}, year = {2018}, month = {DEC 17}, pages = {257001}, abstract = {We present a theory of phonon-mediated superconductivity in near magic angle twisted bilayer graphene. Using a microscopic model for phonon coupling to moire band electrons, we find that phonons generate attractive interactions in both s- and d-wave pairing channels and that the attraction is strong enough to explain the experimental superconducting transition temperatures. Before including Coulomb repulsion, the s-wave channel is more favorable; however, on-site Coulomb repulsion can suppress s-wave pairing relative to d wave. The pair amplitude varies spatially with the moire period, and is identical in the two layers in the s-wave channel but phase shifted by pi in the d-wave channel. We discuss experiments that can distinguish the two pairing states.}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.121.257001}, author = {Wu, Fengcheng and MacDonald, A. H. and Martin, Ivar} }