Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F40: Properties of Dirac Materials 
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Sponsoring Units: DCMP Chair: Madhab Neupane, Univ of Central Florida Room: LACC 501C 
Tuesday, March 6, 2018 11:15AM  11:27AM 
F40.00001: Abstract Withdrawn 12 inequivalent Dirac cones has been identified in the Brillouin zone of metal diboride monolayered ZrB2, divided in six groups of two nonsymmetryrelated Dirac cones with a slight electronpocket compensation. Contrary to group IV honeycombed Dirac materials, Dirac cones in monolayered ZrB2 are formed in the lines joining two high symmetry points, and are not shared with any neighbouring Brillouin zone. Firstprinciples calculations allowed to unveil the separate contribution of each dorbital of the transition metal atom, and their hybridization with the B atomic orbitals. Owing to the strong ZrB atoms hybridization, small changes in the bonding angle lead to one cone vanishing. Internal symmetry breaking leads to a selective Dirac cone disruption depending on the direction of applied distortion. 
Tuesday, March 6, 2018 11:27AM  11:39AM 
F40.00002: Chiral anomaly as origin of planar Hall effect in Weyl semimetals Sumanta Tewari, Snehasish Nandy, Girish Sharma, Arghya Taraphder In condensed matter physics, the term “chiral anomaly” implies the violation of the separate number conservation laws of Weyl fermions of different chiralities in the presence of parallel electric and magnetic fields. One effect of chiral anomaly in the recently discovered Dirac and Weyl semimetals is a positive longitudinal magnetoconductance (LMC). Here we show that chiral anomaly and nontrivial Berry curvature effects engender another striking effect in WSMs, the planar Hall effect (PHE). Remarkably, PHE manifests itself when the applied current, magnetic field, and the induced transverse “Hall” voltage all lie in the same plane, precisely in a configuration in which the conventional Hall effect vanishes. In this work we treat PHE quasiclassically, and predict specific experimental signatures for typeI and typeII Weyl semimetals that can be directly checked in experiments. 
Tuesday, March 6, 2018 11:39AM  11:51AM 
F40.00003: Persistence of Zero Landau Energies Protected from Vorticities of Opposite Chiral Weyl Nodes MingChien Hsu, Hsin Lin, ShinMing Huang The zero energy Landau level is a topological property of the Weyl nodes with linear dispersion. The Landau levels can change when various nearby Weyl nodes interact with each other through the coupling induced by the magnetic field. It is expected that these zero Landau levels may disappear under high field when the nodes are of opposite chiralities. We demonstrate that even for systems with opposite chiral Weyl nodes, different symmetry requirement can result in different conditions of these Landau levels. The chiralities of the Weyl nodes themselves are not enough to decide whether these zero energies can persist. It is the vorticities projected on the plane perpendicular to the magnetic field that decide the either survival or gaping out of the zero energies. Different ways of nodes connection required by the symmetries produce different number of vorticities on the plane. The gaping out of the zero energy levels turning the system into an insulator has no room for transport, while the persistence of zero energies still allows the chiral anomaly to transport electrons along the magnetic field direction. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F40.00004: Controlling multiphoton absorption in Dirac materials via pulse shaping Denis Gagnon, François FillionGourdeau, Joey Dumont, Steve MacLean In this theoretical work, we describe pulse shaping techniques for the control of electronhole pair production in graphene. Spectral optimization of short pulses is performed via differential evolution, a general purpose optimization algorithm. Differential evolution is combined with two different approaches to describe dynamics of charge carriers: Floquet theory for periodically driven systems and full timedependent simulations for pulses of finite duration. Starting from the Floquet picture, we discuss the effect of the spectral content of the pulse on the closing of dynamical gaps in graphene, corresponding to resonance suppression. In the more thorough case of timedependent calculations, we show that it is possible to vary the photoinduced carrier density by a factor of 4 for a fixed pulse energy. The potential of pulse shaping techniques for the control of scattering processes in Dirac materials is subsequently discussed. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F40.00005: Electronic and topological properties of 2D metalorganic frameworks Orlando Silveira, Erika Lima, Simone Alexandre, Helio Chacham We present a theoretical investigation of electronic and topological properties of several proposed and experimentally realized 2D hexagonal metalorganic frameworks (MOFs) through ab initio and tightbinding calculations. The band structures of these materials near the Fermi level are composed by a combination of flat and dispersive bands, with the dispersive bands possessing Dirac cones at the corners of the Brillouin zone. We show that the band structures of several of the investigated materials are strongly affected by the partial inclusion of exact exchange terms on the exchangecorrelation potential, primarily for those materials with a strong contribution of d electrons on the bands near the Fermi level. We also show that the presence of metal atoms on the structures of the MOFs lead to spinorbit coupling induced band gaps, and that the magnitude of these gaps can be enlarged by more than 5 times with appropriate choices of the metal centers. Finally, we show that the band structure of bilayer systems constructed by the stacking of the MOFs M_{3}C_{12}S_{12} is a realization of the Kane and Mele model for topological insulating graphene with a large spinorbit gap. 
Tuesday, March 6, 2018 12:15PM  12:27PM 
F40.00006: Electronic Branched Flow in Graphene and Dirac Materials Marios Mattheakis, George Tsironis, Efthimios Kaxiras Electronic flow in graphene and other Dirac materials is ultrarelativistic with limiting velocity the Fermi velocity. When the twodimensional material is placed on a substrate an additional potential is generated due to charged impurities; this weak random potential affects the Dirac dynamics. The resulting electronic flow coalesces into branches that merge and produce random local focusing and caustics. We investigate the onset of the electron caustic regime both analytically and numerically. We find that in the additional presence of a bias potential, electron caustics are generated at surface locations that scale with both the random and deterministic potential properties. In particular we find a scaling relationship that connects the caustic location to the external potentials that may be tested experimentally. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F40.00007: Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe$_2$ Suyang Xu, Qiong Ma, Huitao Shen, Valla Fatemi, Sanfeng Wu, TayRong Chang, Guoqing Chang, Andres Mier Valdivia, ChingKit Chan, Quinn Gibson, Kenji Watanabe, Takashi Taniguchi, Hsin Lin, Robert Cava, Liang Fu, Nuh Gedik, Pablo JarilloHerrero Recent experimental evidence for the quantum spin Hall (QSH) state in monolayer WTe$_2$ has bridged 2D materials and topological physics. While the realization of QSH has demonstrated the nontrivial topology of the electron wavefunctions of monolayer WTe$_2$, the geometrical properties of the wavefunction, such as the Berry curvatures, remain entirely unstudied. On the other hand, it has been increasingly recognized that the Berry curvature plays an important role in multiple areas of condensed matter physics including nonreciprocal electron transport, enantioselective optical responses, chiral polaritons and even unconventional superconductivity. Here we utilize midinfrared optoelectronic microscopy to investigate the Berry curvature in monolayer WTe$_2$. By optically exciting electrons across the inverted QSH gap, we observe an inplane circular photogalvanic current even under normal incidence. The sign of the photocurrent can be further switched by applying a perpendicular displacement field. Our observations suggest an electrically switchable Berry curvature dipole that arises from the nontrivial wavefunctions near the inverted gap edge, which opens the door for observing a wide range of quantum geometrical phenomena in both singleparticle and collective modes. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F40.00008: A Multiband TightBinding Model for Intact Dirac Cones in Graphene on Ni and Co Mayra Alejandra Peralta, Francisco Mireles When the AB sublattice symmetry in graphene is broken its linear bands of Dirac Fermions are destroyed and a parabolic dispersion arises [1]. Recent ARPES and SARPES measurements in graphene/Ni(Co) in the AC stacking configuration have shown to exhibit intact Dirac cones instead, as well as, spin separation in the graphene’s bands [2,3]. This would indicate that there is still some physics missing in the current models that describe these systems. In this work, we present an analytical multiorbital tightbinding model for a monolayer of graphene over a monolayer of Co(Ni). We take into account the spinorbit coupling of the intrinsic and Rashba type, as well as the magnetization of Ni(Co) through a StonerWohlfarth type coupling and consider hybridization of the d_{xy}, d_{xz}, d_{zy}, d_{3z2}_{}_{r2 }and d_{x2y2} orbitals of Co(Ni) atoms with those of graphene’s p_{z} orbitals within the SlaterKoster approximation. We show that under certain physical conditions, a gapless linear dispersion in the minority spin πbands can indeed arise in such systems. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F40.00009: Topological LC circuit and its realization in microstrip transmission lines Xiao Hu, Toshikaze Kariyado, Yuan Li, Yong Sun, Hong Chen

Tuesday, March 6, 2018 1:03PM  1:15PM 
F40.00010: Flat Building Blocks for Flat Silicene Masae Takahashi Silicene is the silicon equivalent of graphene, which is composed of a honeycomb carbon structure with one atom thickness and has attractive characteristics of a perfect twodimensional pconjugated sheet. However, unlike flat and highly stable graphene, silicene is sticky and thus unstable due to its crinkled structure. Flatness is important for stability, and to obtain perfect pconjugation, electrondonating atoms and molecules should not interact with the p electrons. The structural differences between silicene and graphene result from the differences in their building blocks, flat benzene and chairform hexasilabenzene. It is crucial to design flat building blocks for silicene with no interactions between the electron donor and porbitals. I report here our recent successful design of such building blocks with density functional theory calculations [M. Takahashi, Sci. Rep. 2017, 7, 10855]. Our fundamental concept is to attach substituents that have sphybrid orbitals and act as electron donors in a manner that it does not interact with the p orbitals. The honeycomb silicon molecule with BeH at the edge designed according to our concept, clearly shows the same structural, charge distribution, and molecular orbital characteristics as the corresponding carbonbased molecule. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F40.00011: Imaging a nematic electronic domain wall and its edge modes on the surface of bismuth Benjamin Feldman, Mallika Randeria, Hao Ding, Kartiek Agarwal, Huiwen Ji, Robert Cava, Siddharth Parameswaran, Shivaji Sondhi, Ali Yazdani Nematic electronic phases, whose wave functions spontaneously break the rotational symmetry of the underlying lattice, are susceptible to the formation of domains. In this talk, I will describe scanning tunneling microscope measurements that allow us to directly visualize the evolution of local nematic order across a domain wall on the surface of bismuth. Coulomb interactions in this material lift the degeneracy of six anisotropic hole valleys to produce nematic quantum Hall states [1]. Spatially resolved spectroscopy shows that the resulting exchange gap between Landau levels closes in the vicinity of the domain wall, where there is an abrupt switch in which valleys are occupied, as seen by imaging the orientation of anisotropic wave functions on either side. The data match well to theoretical simulations where the boundary is predicted to support counterpropagating valleypolarized edge modes, forming an extended topological defect. While we observe enhanced lowenergy conductance at the boundary, atomicscale defects allow for scattering between valleys that partially localizes the onedimensional electronic states. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F40.00012: Gate Tunable ThirdOrder Nonlinear Optical Response of Massless Dirac Fermions in Graphene Di Huang, Tao Jiang, Jinluo Cheng, Xiaodong Fan, Zhihong Zhang, Yuwei Shan, Yangfan Yi, Yunyun Dai, Lei Shi, Kaihui Liu, Changgan Zeng, Jian Zi, John Sipe, YuenRon Shen, Weitao Liu, Shiwei Wu Materials with massless Dirac fermions can possess exceptionally strong optical nonlinearity, which has already been explored by several experiments on graphene monolayer. However, the reported variation of the nonlinear optical coefficient by orders of magnitude is still not yet understood. A large part of the difficulty can be attributed to the lack of information on how doping affects the different nonlinear optical processes. In this talk, we will introduce our experimental study, in corroboration with theory, on third harmonic generation (THG) and fourwave mixing (FWM) in graphene that has its chemical potential tuned by iongel gating. THG was enhanced by ~30 times when pristine graphene was heavily doped, while differencefrequency FWM (DFM) appeared just the opposite. Moreover, the DFM was found to have a strong divergence toward degenerate FWM in undoped graphene, leading to a giant thirdorder nonlinearity. These truly amazing characteristics of graphene come from the gatecontrol of chemical potential, which selectively switches on and off photon resonant transitions that coherently contribute to the optical nonlinearity, and therefore can be utilized to develop graphenebased nonlinear optoelectronic devices. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F40.00013: Cooperlike instability in an excitonic insulator Baokai Wang, Robert Markiewicz, Alexander Balatsky, Arun Bansil We explore the crossover from weak to strong correlations in Dirac materials for the excitonic insulator transition in a paradigmatic family of ‘slow’ graphene, i.e. graphene with renormalized bands. Two different models are considered, contrasting a meanfield model with a BetheSalpeter calculation. Our analysis reveals competition between condensation of q=0 and q = (pi,0) excitons, with the latter splitting the Van Hove singularities of the graphene bands. 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F40.00014: Exotic Properties of Alloys of Carbon and Boron Nitride Michael Lucking, Humberto Terrones Considerable effort has been made to alloy graphene and boron nitride because of the potential to continuously tune the band gap over a large range. The tendency for the two materials to phase segregate has hampered such efforts. Another possibility is to make the alloy such that the carbon only sits on the B or the N sublattice. The resulting structures are not guaranteed to have a band gap due to the imbalance of boron and nitrogen atoms. Using first principles density functional calculations, we show the that such systems are host to interesting physics and deserve attention. Magnetic, excitonic, and topological properties of the resulting structures are discussed. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F40.00015: Electronic and Optical Properties of 2D Antimonene and Bismuthene Phases from ManyBody Ab initio Approaches 
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