Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A43: 2D Materials Twistronics: Correlations and Moiré PhysicsInvited
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Sponsoring Units: DCMP Chair: Pablo Jarillo-Herrero, Massachusetts Inst of Tech-MIT Room: BCEC 210B |
Monday, March 4, 2019 8:00AM - 8:36AM |
A43.00001: Twisted Graphene Superlattices: Superconductivity, Electron Correlations and Beyond Invited Speaker: Yuan Cao The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system[1]. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice[2]. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity[3]. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. Since these discoveries, a large number of theoretical models have been proposed based on the magic-angle graphene superlattices and beyond. I will also discuss our data demonstrating nematic superconductivity as well as correlated states in other types of graphene superlattices. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids. |
Monday, March 4, 2019 8:36AM - 9:12AM |
A43.00002: Tuning flat bands in twisted bilayer graphene with pressure Invited Speaker: Matthew Yankowitz Twisted bilayer graphene (tBLG) with rotational mismatch of ~1.1°, referred to as the “magic angle,” has emerged as an exciting new platform to host strongly correlated electronic states due to its very flat low-energy bands. The electronic bandwidth – and consequentially the strength of the correlated states – is determined by an interplay between the separation of the Dirac cones of the two graphene layers in momentum space and the strength of the interlayer electronic coupling which hybridizes the bands. Here we demonstrate the capability to vary the bandwidth at fixed rotation angle by using hydrostatic pressure to modify the interlayer coupling [1]. For angles larger than the native magic angle, we can tune to the flat band condition with pressure, inducing correlated insulating states and superconductivity for both hole- and electron-type carriers. For a 1.27° twist angle, Tc increases above 3 K at ~1.3 GPa, and then diminishes with further pressure. This behavior is consistent with theoretical efforts to model the relationship between bandwidth and interlayer coupling strength, establishing layer compression as a viable route to engineering the energy scale of the superconductivity in this system. Improvements in device quality additionally allow us to resolve new sequences of quantum oscillations emerging from quarter-, half-, and three-quarters-filling of the moiré unit cell associated with the presence of new Fermi surfaces. |
Monday, March 4, 2019 9:12AM - 9:48AM |
A43.00003: Topology and Correlations in Magic Angle Bilayer Graphene Invited Speaker: Ashvin Vishwanath The recent discovery of superconductivity and Mott insulators in twisted bilayer graphene and related materials points to the importance of correlation effects. An additional interesting and potentially crucial ingredient is band topology inherited from the Dirac fermion dispersion of graphene. We will discuss our theoretical efforts to derive faithful tight binding models that captures both the symmetry and topology of the flat bands, as well as their energetics. We then discuss aspects of the Mott and superconducting phases and find that key aspects of the physics differ from previously studied correlated superconductors. |
Monday, March 4, 2019 9:48AM - 10:24AM |
A43.00004: Topologically Protected Helical States in Minimally Twisted Bilayer Graphene Invited Speaker: Brian LeRoy The ability to create arbitrary stacking configurations of layered two-dimensional materials has opened the way to the creation of designer band structures. Twisted bilayer graphene is one of the simplest examples of such a van der Waals heterostructure where the electronic properties of the composite material can be fundamentally tuned with twist angle. The angle between the two graphene layers in the heterostructure produces a moiré pattern which affects its electronic properties. Using scanning tunneling microscopy and spectroscopy, we have investigated these minimally twisted bilayer graphene heterostructures. As the twist angle between the layers decreases there are fundamental changes in the electronic properties. We have found that the degeneracy of the low energy bands increases at twist angles below about 1 degree. For even smaller twist angles, a series of domain walls form connecting the AA sites in the moiré pattern. When an electric field is applied to these very small twist angle samples, the AB and BA regions develop band gaps while topologically protected states emerge along the domain walls. In this talk, we will discuss the fabrication of these minimally twisted heterostructures as well as our latest scanning probe microscopy results. |
Monday, March 4, 2019 10:24AM - 11:00AM |
A43.00005: Quantum Simulation and Many-Body Physics in Moiré Bilayers Invited Speaker: Fengcheng Wu Moiré superlattices form in van der Waals bilayers with a small lattice mismatch or misalignment. The moiré pattern produces spatial modulation and can dramatically alter electronic properties. I will present theoretical proposals of using moiré bilayers as a quantum simulation platform to realize model Hamiltonians, and discuss many-body effects that are magnified when the moiré bands are nearly flat. In semiconducting transition metal dichalcogenide (TMD) heterobilayers, isolated flat moiré bands can be used to simulate Fermi-Hubbard model on a triangular lattice [1], in which parameters such as bandwidth, interaction strength, and band filling are widely tunable. When the two layers are formed from the same TMD, holes in ±K valleys move in a layer-pseudospin skyrmion texture in real space. The low-energy moiré bands then realize Kane-Mele model [2], providing a platform to study interplay between topology and correlation. Excitons in twisted TMD bilayers also experience moiré potential, which can be used to design topological exciton bands and to simulate Bose-Hubbard model [3]. Twisted bilayer graphene (TBLG) is a distinct example where the effective lattice theory does not simply map to any known model Hamiltonian. Motivated by the observed superconductivity, I will present a candidate theory of phonon-mediated superconductivity in TBLG [4]. Phonon fluctuations lead to both s-wave and d-wave pairings. New phenomena of the d-wave superconductivity in moiré pattern will be discussed. |
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