Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R14: 2D Materials (Metals, Superconductors, and Correlated Materials) -- Twisted Graphene IFocus
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Sponsoring Units: DMP DCOMP Chair: Philip Kim, Harvard University Room: BCEC 153C |
Thursday, March 7, 2019 8:00AM - 8:12AM |
R14.00001: A scanning tunneling study of twisted bilayer graphene at the first magic angle Yonglong Xie, Berthold Jaeck, Cheng-Li Chiu, Kyounghwan Kim, Yimeng Wang, Takashi Taniguchi, Kenji Watanabe, Emanuel Tutuc, Ali Yazdani Recently, bilayer graphene with small twist angle has been shown to be a valuable platform for correlated electron phenomena. Band structure calculations show that at certain twist angle values, labelled magic angles, the lowest band has a flat energy-momentum dispersion. Transport experiments find evidence for a correlated insulating phase when this flat band is exactly half filled. Furthermore, a small doping level away from half filling gives rise to a superconducting instability. In this talk, we will discuss how a scanning tunneling microscope can help to elucidate the electronic properties of twisted bilayer graphene close to the first magic angle. |
Thursday, March 7, 2019 8:12AM - 8:24AM |
R14.00002: Atomistic study of electron-phonon coupling in magic-angle twisted bilayer graphene Young Woo Choi, Hyoung Joon Choi We report strong electron-phonon coupling in magic-angle twisted bilayer graphene (MA-TBG) obtained from atomistic description of the system including more than 10,000 atoms in the moiré supercell. Electronic structure, phonon spectrum, and electron-phonon coupling strength λ are obtained before and after atomic-position relaxation both in- and out-of-plane. Obtained λ is very large for MA-TBG, with λ > 1 near the half-filling energies of the flat bands, while it is small (λ~0.1) for monolayer and unrotated bilayer graphene. Significant electron-hole asymmetry occurs in the electronic structure after atomic-structure relaxation, so λ is much stronger with hole doping than electron doping. Electron-phonon coupling is nearly isotropic and very weakly dependent on electron momentum, suggesting single-gap s-wave superconductivity. Relevant phonon energies are much larger than electron energy scale, going far beyond adiabatic limit. Our results provide fundamental understanding of electron-phonon interaction in MA-TBG, highlighting that it can contribute to rich physics of the system. |
Thursday, March 7, 2019 8:24AM - 8:36AM |
R14.00003: Band Degeneracy and Normal State Properties of Twisted Bilayer Graphene at the Magic Angle Cheng Li, Tin-Lun Ho Recently, superconductivity is discovered in twisted bilayer graphene at a small twist angle. Associated this superconducting state is a whole host of unusual normal state properties: |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R14.00004: Correlated Insulating and Superconducting Phases in Twisted Bilayer Graphene Shaowen Chen, Matthew Yankowitz, Hryhoriy Polshyn, Kenji Watanabe, Takashi Taniguchi, David E Graf, Andrea Young, Cory R Dean Bilayer graphene with ~ 1.1 degrees twist mismatch between the layers hosts a low energy flat band in which the Coulomb interaction is large relative to the bandwidth, promoting correlated insulating states at half band filling, and superconducting (SC) phases with dome-like structure neighboring correlated insulating states. Here we show measurements of a dual-graphite-gated twisted bilayer graphene device, which minimizes charge inhomogeneity. We observe new correlated phases, including for the first time a SC pocket near half-filling of the electron-doped band and resistive states at quarter-filling of both bands that emerge in a magnetic field. Changing the layer polarization with vertical electric field reveals an unexpected competition between SC and correlated insulator phases, which we interpret to result from differences in disorder of each graphene layer and underscores the spatial inhomogeneity like twist angle as a significant source of disorder in these devices [1]. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R14.00005: Electronic structure and optical properties of twisted multilayer graphene Adriana Lizeth Vela Peña, Marcus Moutinho, Francisco Culchac, Pedro Venezuela, Rodrigo Capaz We study the electronic and optical properties of twisted trilayer and tetralayer graphene structures using a combination of tight-binding and density-functional theory methods. Band structures, densities of states, and optical absorption spectra are calculated for a variety of layer stackings and twisting angles. Systematic trends of all properties are obtained and compared to the more well known case of twisted bilayer graphene. For trilayer and tetralayer structures, we find, respectively, two and three well-defined absorption peaks in the infrared/visible range that shift with twisting angle, in contrast to the single peak observed in bilayer graphene. In addition, systems containing Bernal-stacked layers present an extra peak in the infrared which is related to transitions between parabolic bands and does not shift with twisting angle. The observed trends may be used to identify the twisting angle and the number of layers in multilayer graphene samples. In particular, magic angles are predicted for the trilayer and tetralayer structures. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R14.00006: Wigner Crystallization in Twisted Bi-layer Graphene Bikash Padhi, Philip Phillips, Chandan Setty Study of strongly correlated phases took an interesting turn recently by the surprising discovery of high-Tc superconductivity in twisted sheets of graphene. The electronic properties of each graphene-layer can be described by non-interacting electrons. However, in a twisted bi-layer graphene (TBLG), close to the `magic angles', the kinetic energy of the electrons is heavily quenched. This causes interactions to dominate, paving way for strong correlation. In this talk, I disucss arXiv: 1804.01101 and 1810.00884. We first compute quasiparticle interaction energy and kinetic energy to obtain their ratio, $r_s$, which quantifies the extent of strong correlation. This ratio crossing unity already signals departure from a perturbative regime (or Fermi liquid behavior). For $r_s$ larger than 37, the system minimizes the strong Coulomb repulsion by forming electronic crystals, called Wigner crystals. We show that TBLG near magic angles exhibits $r_s$ much larger than this critical value, facilitating a hierarchy of metal-insulator transitions. Pressure enhances such crustallization. In light of Wigner crystallization we discuss various aspects of the recent experiments and show remarkable agreement with this scenario. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R14.00007: Emergent Flat band lattices in spatially periodic magnetic fields Muhammad Tahir, Hua Chen Motivated by the recent discovery of Mott insulating phase and unconventional superconductivity due to the flat bands in twisted bilayer graphene, we propose more generic ways of getting two-dimensional (2D) emergent flat band lattices using either 2D Dirac materials or ordinary electron gas subject to moderate periodic magnetic fields with zero spatial average. We provide simple formulas for the "magic ratios" between the field strength and its wavenumber for getting flat bands, and give an intuitive explanation for their origin by constructing coarse-grained lattice models. Our work provides new, flexible platforms for exploring interaction-driven phases in 2D systems with arbitrary superlattice symmetries. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R14.00008: Tuning the electronic structure and electron correlation in 2D twisted massive Dirac system: the case of twisted bilayer hBN Lede Xian, Dante Kennes, Nicolas Tancogne-Dejean, Massimo Altarelli, Angel Rubio Recent experiments have suggested that twisted bilayer graphene (TBG) near magic angles can be an idea platform for the study of strong correlation effects and unconventional superconductivity. Interestingly, our density functional theory (DFT) calculations show that flat bands also develop at both the top of the valence bands and bottom of the conduction bands in bilayer hBN with a twist. But different from the case of TBG systems, the band width of these flat bands decreases monotonically with twist angles and there is no appearance of any magic angles. We further use functional renormalization group (FRG) method to show that the flat band at the top of the valence bands can host exotic strong-correlated physics, such as the appearance of Mott insulator phase and unconventional superconductivity. Our study suggests that the gapless and massless characters of graphene is not essential in the observed strong-correlated phenomena and many more other 2D materials can also be explored in the form of twisted bilayer system for the study of unconventional superconductivity. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R14.00009: Tunable spin-polarized Mott insulator in twisted bilayer-bilayer graphene Xiaomeng Liu, Zeyu Hao, Kenji Watanabe, Takashi Taniguchi, Philip Kim Twisted bilayer graphene, forming a flatband at the magic angle, has become a new platform to study correlated electron states. Superconductivity and Mott insulator phase have been demonstrated in this system. However, the flatband only occurs at a specific twist angle, which is both hard to achieve and lacks tunablity to enable other emergent phenomena. Here we report experiments on twisted bilayer-bilayer graphene, where two Bernal-stacked bilayer graphene are placed together with a small twist angle. Due to the tunable band structure of bilayer graphene with displacement fields, we can achieve flatband structure in a large range of twist angles. At each twist angle, we observed Mott insulator phase at 1/2 filling in a specific range of displacement fields. Under a parallel magnetic field, additional insulating states emerge at 1/4 and 3/4 filling while the 1/2 filled insulating state become more robust. This suggest the 1/2 insulator is a spin-polarized state. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R14.00010: The nature of correlations in the insulating states of twisted bilayer graphene María José Calderón, José María Pizarro, Elena Bascones The recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the insulating behavior, the commensurability at which these states appear suggests they are Mott insulators. We focus on the insulating states with ±2 electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimental dependences on temperature and magnetic field if, as frequently assumed, only onsite (local) correlations are included. We argue that the inclusion of intersite correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments. The importance of these non-local correlations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott metal insulator transition. arXiv:1805.07303. |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R14.00011: On the nature of the correlated insulator states in twisted bilayer graphene Ming Xie, Allan MacDonald We use self-consistent Hartree-Fock calculations performed in the full π-band Hilbert space to assess the nature of the recently discovered correlated insulator states in magic-angle twisted bilayer graphene (MATBG). We show that at integer number of electrons per moiré period, the mean-field ground states break the combined two-fold rotation and time reversal symmetry $C_2T$ that protects the moiré-band Dirac points, inducing gaps and establishing valley projected bands that have non-zero Chern numbers. Broken spin/valley flavor symmetries then enable gapped states to form not only at neutrality but also at moiré band filling n = ±p/4, where p is the number of electrons per moiré period. We predict that the MATBG ground states at n = ±1/4 and n = ±3/4 have an anomalous Hall effect, and discuss the implications of our findings for theories of magic-angle superconductivity. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R14.00012: Electrostatic Modeling of Bilayer Graphene Band Structure for Scanning Tunneling Spectroscopy Gregory Holdman, Wyatt Behn, Zach Krebs, Keenan Smith, Benny Tock, Victor W Brar Bilayer graphene (BLG) is known to have a dynamic electronic structure including a continuously tunable bandgap, and correlated electron behavior under a variety of conditions. To better understand these phenomena, it is important to develop local probes that can directly determine how these effects manifest in the presence of defects and impurities. In many semiconducting or metallic systems, scanning tunneling spectroscopy (STS) can serve as such a local probe. However, STS necessarily applies a local electric field to the system it measures. For BLG, this field can alter the local band structure by breaking the symmetry of the two layers and simultaneously doping the surface. This dynamic band structure modification makes STS measurements of BLG difficult to interpret and prevents straightforward extraction of the material parameters. In this talk, we show how these effects can be modeled and understood by computing the expected voltage-dependent tunneling spectrum of a BLG sheet between two gate electrodes. We compare this model to STS data taken from BLG/SiO2 and BLG/h-BN systems under UHV conditions at a temperature of 4 K and show how to extract the BLG bandgap from STS measurements performed at different back-gate voltages. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R14.00013: Graphene quasicrystal Pilkyung Moon, Mikito Koshino, Young-Woo Son Quasicrystals have been used to study quantum states between the limits of periodic order and disorder. Recently, we reported that quasicrystals can be realized by stacking atomic layers at a specific configuration [1]. And, as a specific example of such designer quasicrystals, we reported a graphene quasicrystal composed of two graphene layers stacked at a rotation angle of 30°. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R14.00014: Flat bands due to twisted bilayer graphene: A coordinate transformation approach David Schmeltzer, Avadh Saxena We consider two-layer graphene with one layer twisted by a small angle θ. This can be investigated using a coordinate transformation given by an angle θ with respect to the untwisted layer. For the untwisted layer two sublattices are a(R) and b(R + δ) and in the twisted layer we have A(R') and B(R' + δ). We consider hopping between the two layers −ta,A and −ta,B . As a result, we obtain four eigenvalues. Two eigenvalues have low energy with vanishing dispersion as a function of the angle. The solution is based on the replacement of the twisted coordinate in the momentum space by a spinor transformation B'σ(k'(k)) = exp(iσ3θ/2) Bσ(k). In the presence of interaction the coordinates are affected by the transformation and thus contain the phase exp(iσ3θ/2). |
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