Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H08: Electronic Structure: Topological Insulators and beyond 
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Sponsoring Units: DCMP Chair: Yang Zhang, Southeast University Room: LACC 153C 
Tuesday, March 6, 2018 2:30PM  2:42PM 
H08.00001: Modification of band structure of widebandgap crystals by nonmonochromatic electron/hole oscillations driven by highintensity ultrashort pulses Vitaly Gruzdev, Olga Sergaeva Electric field of highintensity ultrashort laser pulses distorts band structure of crystals. In widebandgap crystals, the fastest distortions are attributed to coherent laserdriven electron oscillations frequently analyzed under a monochromatic approximation. We report a theoretical study of nonperturbative bandstructure modification by ponderomotive energy of nonmonochromatic laserdriven electron/hole oscillations treated by a quasiclassical approximation. A slowlyvarying pulse envelope is assumed, but variations of field amplitude within each individual cycle are taken into account. Laserperturbed timedependent energymomentum relations are averaged over a single laser cycle and asymptotically expanded using a small parameter (ratio of cycle duration to pulse width) to obtain analytical formulae. A shift of the lasermodified energy bands with respect to each other along a quasimomentum direction parallel to laserpulse polarization and asymmetric (with respect to laserpulse peak) time evolution of effective band gap are predicted. Those effects substantially affect nonlinear absorption and are controlled by pulse phase, pulse width, peak intensity, and carrier wavelength. 
Tuesday, March 6, 2018 2:42PM  2:54PM 
H08.00002: Dirac Linenodes and Effect of Spinorbit Coupling in Nonsymmorphic Critical Semimetal MSiS (M=Hf, Zr) Cheng Chen, Xiang Xu, Juan Jiang, ShuChun Wu, yanpeng qi, LeXian Yang, Meixiao Wang, Yan Sun, Niels Schroeter, haifeng yang, Leslie Schoop, yangyang Lv, jian zhou, yanbin chen, shuhua yao, MingHui Lu, YangFang Chen, Claudia Felser, Binghai Yan, Zhongkai Liu, Yulin Chen Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na_{3}Bi and Cd_{3}As_{2}. More recently, new types of Dirac semimetals with robust Dirac linenodes have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angleresolved photoemission spectroscopy (ARPES), we explored the electronic structure of the nonsymmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac linenodes in parallel with the k_{z}–axis and their dispersions. Interestingly, along directions other than the linenodes in the 3D BZ, the bulk degeneracy is lifted by spinorbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac linenode semimetal family protected by nonsymmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds. 
Tuesday, March 6, 2018 2:54PM  3:06PM 
H08.00003: Towards photocathodes with elongated lifetimes and high quantum efficiency by passivating with two dimensional materials Gaoxue Wang, Ping Yang, Nathan Moody, Enrique Batista Photocathodes are important electron sources for Xray free electron laser (XFEL) and Xray energy recovery linacs (XERL), which generate brilliant, ultrafast, and coherent Xrays for exploration of matter with ultrahigh resolutions in both space and time. Quantum efficiency (QE) and operational lifetimes are two of the key figures of merit of photocathodes performance. Whereas alkalibased semiconducting photocathodes display high QE in the visible light spectrum, their lifetimes are shorter than their metallic counterparts due to the sensitivity of alkalibased surfaces to the residual gases in the vacuum chamber. Herein, a novel approach is proposed to protect alkalibased photocathodes with two dimensional (2D) materials. It is found that a monolayer of BN can maintain the QE of semiconducting photocathodes in addition to protecting the surfaces. On the other hand, a monolayer of graphene or molybdenum disulfide (MoS_{2}) suppresses the QE. Analyses of the electronic structure reveal that the induced dipole moments at the interfaces of photocathodes and 2D materials play central roles in affecting the work function of the coated photocathodes, thus their QE. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H08.00004: Energy and MomentumResolved Characterization of Optical Transitions in Bi_{2}Se_{3} Alexandre Gauthier, Hadas Soifer, Costel Rotundu, Alexander Kemper, Shuolong Yang, Hongyu Xiong, Makoto Hashimoto, Donghui Lu, Patrick Kirchmann, Jonathan Sobota, ZhiXun Shen Lightmatter interactions are at the heart of many physical phenomena including photocurrents in the topological insulator Bi_{2}Se_{3}. However, it is not clear whether these photocurrents stem from topological surface states or bulk bands. Time and angleresolved photoemission spectroscopy is used to probe the structure and dynamics of photocurrents in unoccupied electronic states of Bi_{2}Se_{3}. Here we demonstrate the use of a “time mapping” analysis to measure which initial states below the Fermi level couple to which unoccupied states via optical excitations, in a bandresolved manner. We measure the time following optical excitation for unoccupied states to reach maximum population as a function of energy and momentum; faster rise times indicate direct excitation from initial occupied states. Time mapping analysis displays promise for improving our understanding of optoelectronic effects including photocurrent generation. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H08.00005: Discovery of topological nodalline fermionic phase in a magnetic material GdSbTe Md Mofazzel Hosen, Gyanendra Dhakal, Klauss Dimitri, Pablo Maldonado, Alex Aperis, Firoza Kabir, Christopher Sims, Peter Riseborough, Peter Oppeneer, Dariusz Kaczorowski, Tomasz Durakiewicz, Madhab Neupane Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. Most of the discovered topological semimetals are nonmagnetic and conserve timereversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodalline semimetallic state in GdSbTe using angleresolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the nodalline state along the ΓM direction of the Brillouin zone (BZ). Interestingly, our experimental data show a robust Dirac like state at the X point of the BZ both below and above the magnetic transition temperature (T_{N} = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H08.00006: Insitu strain engineering of the Dirac surface states in Bi_{2}Se_{3} films David Flötotto, Yang Bai, Y H Chan, Peng Chen, Xiaoxiong Wang, C. Z Xu, Eric Mittemeijer, James Eckstein, TaiChang Chiang A controlled manipulation of the bulk band gap and spinpolarized Dirac surface states of topological insulators is of great fundamental importance and relevant to novel device applications. A promising pathway involves the application of strain, which alters the interatomic lattice spacing and thus induces corresponding changes in the electronic band structure. By performing angleresolved photoemission spectroscopy (ARPES) and Xray diffraction (XRD) measurements during insitu tensile tests of ultrathin epitaxial Bi_{2}Se_{3} films on flexible substrates we demonstrate that the band structure of the prototypical topological insulator Bi_{2}Se_{3} can be reversibly tuned insitu by means of elastic strain. In accordance with our first principle calculations, the Dirac point reversibly shifts to larger binding energies with increasing tensile strain as a result of the decreasing inter quintuplelayer distance. Our study is an important step forward towards using strain as an insitu tool for tailoring of the functional properties of topological materials and opens new routes for a momentumresolved quantification of straininduced bandstructure changes. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H08.00007: Anomalous dielectric response of inversionsymmetric systems with the π Zak phase Yusuke Aihara, Motoaki Hirayama, Shuichi Murakami In the modern theory of polarization, the charge polarization is written in terms of the Zak phase. When the system is invariant under space inversion, the Zak phase is either 0 or π, and when it is π, it indicates existence of halffilled edge states. We expect that such halffilled edge states are sensitive to external electric field, and they exhibit novel electric response. For this purpose, we consider the onedimensional SSH model, which can have either π or 0 Zak phase by tuning the model parameters. We calculate the electric polarization under the electric field, and we find singular nonlinear response of the electric polarization when the Zak phase is π. We confirm that this anomalous phenomenon is caused by the edge states. We also find this anomalous polarization in higherdimensional systems with π Zak phase when the dispersion of edge states is negligible. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H08.00008: Longlived Photostriction in Croconic Acid Thin Film Xuanyuan Jiang, Xiao Wang, Andy Clark, Le Yu, Anthony DiChiara, Xuemei Cheng, Xiaoshan Xu Croconic Acid (CA) shows ferroelectricity due to proton displacement between molecules. Based on photovoltaic and converse piezoelectric effect the photostriction effect can appear in CA crystals. In order to study this photostriction effect, we carried out pumpprobe experiments on CA thin film using timeresolved Xray diffraction. The results of this experiment show a lattice expansion under laser exposure, but the anisotropy of this lattice expansion is different than the anisotropy for thermal expansion. In particular, the largest lattice expansion happens along polarization direction instead of weakly bonded direction. After the laser is turned off, the lattice distortion persists for approximately 20 minutes, indicating a metastable state. The appearance of this photostriction may be due to the reduction of depolarization field under the distribution of excited electrons. And the existence of a longlived metastable state may be due to the polycrystalline thin film and single domain in small crystallites. This result helps us unveil the real relation between proton displacement and polarization switching, which should promote the development of protontransfer ferroelectric organics. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H08.00009: The Photoemission Threshold Rate Harold Hjalmarson, Weng Chow, Christopher Moore, Mathew Hopkins, Taisuke Ohta Photoemission is an important process in electrical breakdown involving a surface. The rate at which electrons are emitted from a surface has contributions from bulk and surface effects. The bulk effects involve three steps: absorption of a photon, transport of an electron and emission of that electron. The surface process is often regarded as one step that involves absorption and emission. In this talk, these contributions are combined to produce a method that includes both the photoemission from very energetic photons and also photons whose energies are near the threshold energy for photoemission. This method allows the use of approximate electronic structure methods in which the surface effects are included using a perturbation method. Results will be compared with data to illustrate the features. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H08.00010: Theoretical optical properties of highly birefringent compounds Eric Shirley In this work, the layered compound orpiment (As_{2}S_{3}), elemental boron and orthorhombic sulfur, polytetrafluoroethylene (PTFE) and ammonium nitrate are examined regarding their optical properties and extremely large birefringence effects. A comparison of observed and theoretical optical properties will be made. The theoretical results are obtained using the BetheSalpeter equation to treat the equation of motion of the excited electronhole pair. This work can serve as a test of the peculiar optical properties and the ability to predict them from first principles. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H08.00011: A topological phase transition in the Lanthanum Monopnictides as observed with ARPES Tom Nummy, HungYu Yang, Haoxiang Li, Samantha Jaszewski, Mykola Abramchuk, Fazel Tafti, Daniel Dessau The family of binary Lanthanum monopnictides, LaBi, LaSb, and LaAs have attracted a great deal of attention as they display an unusual “extreme” magnetoresistance (XMR) that is not well understood. Possible mechanisms involving nontrivial topology of band structure have been proposed. In this talk, I will present ARPES measurements from LaBi, LaSb, and LaAs which show a clear topological phase transition within the chemical family. The persistence of XMR on either side of this phase transition is incompatible with proposed mechanisms for XMR which involve Z_{2} topological order as a prerequisite. 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H08.00012: How the local density of states and total photoemission spectra change due to a pumped electric field Mona Kalthoff, Goetz Uhrig, James Freericks Singleband models of noninteracting electrons can be solved exactly in the presence of an electric field pump (for a pump/probe experiment) by employing the Peierls substitution. Because this transformation simply relabels momentum wavevectors as a function of time, the instantaneous spectrum is unchanged by the pump pulse. But the local density of states and the total photoemission spectrum both display significant time dependence due to the field. This occurs for two reasons: first, the timedependent change of the momentum label changes the relationship between momentum and bandstructure, which, in turn, changes the density of states and second, the timedependent wavefunctions affect the density of states through the equation of motion for the Green's functions. We describe how these effects determine what appears as a fielddressed bandstructure which changes with time. For example, these effects can create spectra outside of the range of the instantaneous bandstructure. We describe what impact this has on the interpretation of timeresolved photoemission spectra. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H08.00013: Applications of fully selfconsistent GWΓ method for realistic materials. Andrey Kutepov In this talk I will show on a few examples that diagrammatic steps beyond GW approximation are manageable for realistic materials without the need to apply traditional simplifications such as local (onsite) approximation, or static (frequency independent) approximation for the effective interaction, or introduction of models with parametric description. The approach is based on selfconsistent solving of Hedin’s equations with nontrivial vertex function. Different selfconsistent diagrammatic extensions of GW method will be briefly introduced and the applications will be demonstrated using as examples the electron gas, a few metals, and a number of semiconductors/insulators. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H08.00014: Electronic Structure Dynamics in Semimetallic TMD Mo_{1x}W_{x}Te_{2} using Time and AngleResolved Photoemission Spectroscopy Conrad Stansbury, Nicholas Dale, Clara Nyby, Daniel Rhodes, Sam Ciocys, Gregory Affeldt, Aaron Lindenberg, Alessandra Lanzara In time and angleresolved photoemission spectroscopy (trARPES), direct measurements can be made of both the electronic structure dynamics, and the dynamics of interaction between electronic and structural degrees of freedom in a material. Here we discuss trARPES experiments on the semimetallic transition metal dichalcogenide Mo_{1x}W_{x}Te_{2} and examine the dynamics of the topological surface states in these materials. The relationship between the dynamics of these electronic features and the material's structural features will be discussed. 
Tuesday, March 6, 2018 5:18PM  5:30PM 
H08.00015: Electronic Structure and optical properties of Th doped MgF_{2} Martin Pimon, Johannes Gugler, Peter Mohn

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