Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P07: Electronic Structure of Semiconductors: Theory and Spectra 
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Sponsoring Units: FIAP Chair: Jun Yan, Univ of Mass  Amherst Room: LACC 153B 
Wednesday, March 7, 2018 2:30PM  2:42PM 
P07.00001: Use and Misuse of Optical Dispersion Formulas for the IR Index of Silicon William Karstens , David Smith Interference measurements of the IR refractive index of silicon conflict with prism measurements by an order of magnitude more than their precision. This arises from erroneous use of classical dispersion formulas for polar optical glass to analyze measurement on homopolar silicon. We show that many of these dispersion formulas, often considered empirical, follow from the KramersKronig (KK) relations:1) Binomial expansion of the KK integrand gives the index as a series of absorptionspectrum moments leading to the Cauchy, Briot and Conrady formulas; 2) Expansion of the absorption spectrum in deltafunctions yields the Sellmeier, DrudeKetteler and Hartmann expressions. In these formulas the contribution from electronic polarization is explicit, but phonon terms are often obscure. In two specific cases for silicon, confusion of electronic and phonon excitation energies in the LiSellmeier formula and misunderstanding of the Cauchy and Briot expansions in the BaumeisterEdwards analysis distorted the original data. The latter are now lost precluding reanalysis. Hence, we recommend use of prism data until new interference measurements are made. 
Wednesday, March 7, 2018 2:42PM  2:54PM 
P07.00002: Luminescent Emission from 1s, 2s, 3s and 4s Excitons of Monolayer WSe_{2} in High Magnetic Fields ShaoYu Chen , Zhengguang Lu , Thomas Goldstein , Kenji Watanabe , Takashi Taniguchi , Dmitry Smirnov , Jun Yan The strong Coulomb interaction in 2DTMDCs provides an outstanding platform for investigating the Rydberg exciton physics. In this study, we observed for the first time excitonic luminescence of up to the 4s excited state in ultrahigh quality BN/1LWSe_{2}/BN heterostructures in a strong outofplane magnetic field. By analyzing the diamagnetic shift of the ground and excited exciton states, the radii of the 2s and 3s excitons are found to be 3.4 and 6 times larger comparing with the 1s exciton. Moreover, the Zeeman splitting measurement shows the monotonic increase of g factor from 4.4 to 4.8 for 1s to 3s, indicating existence of nontrivial differences of magnetic moment and effective mass between different Rydberg exciton states. Complementary to PLE and absorption measurements in literature, our results provide an alternative method to optically investigate tightlybound electronhole pairs and strong Coulomb interaction in 2D excitonic systems. 
Wednesday, March 7, 2018 2:54PM  3:06PM 
P07.00003: Structural Phase Transformations in Photoexcited Transition Metal Chalcogenide Monolayers Studied Using Combined PumpProbe Experiments and NonAdiabatic Molecular Dynamics Simulations Aravind Krishnamoorthy , MingFu Lin , Clemens Weninger , Rajiv Kalia , Aiichiro Nakano , Fuyuki Shimojo , Uwe Bergmann , Priya Vashishta Optical control of structural phases in two dimensional chalcogenides is a promising route for precise functionalization of these materials for electronic, optical and catalytic applications. In this study, we use ab initio time dependent density functional theory simulations supported by ultrafast electron diffraction measurements to understand the electronic structure of electronicallyexcited MoTe_{2} crystals as well as the resulting atomic dynamics responsible for transformation between the H and T’ crystal structures. Specifically, we identify a nesting of the excitedstate Fermi surface that leads to softening of phonon modes at the Brillouin zone boundary. This modulation of the ionic potential energy surface exposes a low activationenergybarrier pathway for the HT’ phase transformation in this family of materials. This example of excitationdriven phase transformation has important advantages over other reported phase transformation mechanisms that rely on thermal and chemical driving forces or electrondoping. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P07.00004: FirstPrinciples Investigation of Nonlinear Optical Properties of 2D Materials Hua Wang , Xiaofeng Qian Understanding the interaction between 2D materials and strong electromagnetic field is important for the development of novel devices for optical spectroscopy and imaging, sensors and information technology. Here we present our recent studies of nolinear optical responses in 2D materials using our inhouse developed nonlinear optical response codes based on firstprinciples densityfunctional theory. We will show that nanomaterials especially 2D materials exhibit large second and third order nonlinear optical responses, including second harmonic generation, shift current, circular photogalvanic current, and third harmonic generation. Their physical origin is closely related to topological quantity of intraband and interband Berry connection as well as electron/hole group velocity. The present results demonstrate the great potential of lowdimensional materials for nonlinear optical device and sensing applications. References: 1. Giant Optical Second Harmonic Generation in TwoDimensional Multiferroics. Nano Letters 17, 50275034 (2017). 2. Twodimensional multiferroics in monolayer group IV monochalcogenides. 2D Materials 4, 015042 (2017). 3. Third Harmonic Generation in 2D Materials. in preparation (2017). 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P07.00005: Inversion symmetry in methylammonium lead iodided single crystals probed by second harmonic optical anisotropy Tejas Deshpande , John Harter , Kyle Frohna , Wei Peng , Bradford Barker , Jeffrey Neaton , Steven Louie , Oman Bakr , David Hsieh , Marco Bernardi Single crystals of the methylammonium lead iodide perovskite (CH_{3}NH_{3}PbI_{3}) are promising candidates for solar cells due to their long (tens of microns) photocarrier diffusion lengths.^{[1]} It has been proposed that such long diffusion lengths are a consequence of bulk static Rashba fields resulting from an absence of bulk inversion symmetry at room temperature.^{[2]} Here we directly probe the point group symmetries of CH_{3}NH_{3}PbI_{3} using high sensitivity optical second harmonic rotational anisotropy measurements. We show that contrary to previous proposals, the bulk crystallographic structure of tetragonal CH_{3}NH_{3}PbI_{3} at room temperature is centrosymmetric with a space group of I4/mcm. These results are consistent with our density functional theory calculations, which establish the absence of a large bulk Rashba effect and show that incorrect structural relaxations are responsible for the previously predicted large Rashba effect. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P07.00006: Thermal plasmon Frequency for tunable extrinsic Dirac structures Godfrey Gumbs , Andrii Iurov , Danhong Huang , Ganesh Balakrishnan Analytic expressions have been obtained for the chemical potential of a variety of extrinsic (doped) gapped Dirac materials. We also created a reliable piecewiselinear model for the densityofstates in molybdenum disulfide, which demonstrates good agreement with previously obtained numerical results. A decrease of chemical potential with increasing temperature due to enhanced thermal populations of an upper subband in silicene was also shown. In MoS$_2$, the chemical potential is found to cross a zeroenergy point at sufficiently high temperatures because of the broken symmetry with respect to electron and hole states. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P07.00007: Optical properties of a highlyexcited excitonpolariton condensate Ryo Hanai , Peter Littlewood , Yoji Ohashi We theoretically investigate nonequilibrium effects on optical properties of a highlyexcited excitonpolariton condensate. In this system, it has been theoretically predicted that a negative energy branch of the Bogoliubov dispersion (the socalled “ghost branch”) appear in photoluminescence, as a direct consequence of the quantum depletion of the Bosecondensate. However, in most experiments, the ghost branch is absent. In this work, by extending the combined generalized random phase approximation with the HartreeFockBogoliubov theory to the nonequilibrium Keldysh formalism, we show that nonequilibrium effects strongly suppress the visibility of the ghost branch. Our results are in qualitative agreement with experiments, where we see a blue shift of the condensate emission, appearance of the diffusive Goldstone mode, as well as the suppression of dispersive profile of the branch in the photoluminescence. Our results indicate that quantum depletion is strongly suppressed by the drivendissipative nature of a nonequilibrium excitonpolariton condensate. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P07.00008: Exciton fission in monolayer transition metal dichalcogenide semiconductors Alexander Steinhoff , Matthias Florian , Malte Rösner , Gunnar Schönhoff , Tim Wehling , Frank Jahnke

Wednesday, March 7, 2018 4:06PM  4:18PM 
P07.00009: Effective BondOrbital Model of IIINitride Wurtzite Structures Based on Modified Interaction Parameters of Zinc Blende Structures FuChen Hsiao , YiaChung Chang , John Dallesasse A simple theoretical method for deducing the effective bondorbital model (EBOM) of IIInitride wurtzite semiconductors from the zinc blende structure is presented. In the EBOM for a zinc blende structure one slike and three plike bond orbitals are assumed. The set of EBOM parameters are obtained by fitting the band energies at high symmetry points in reciprocal space with the firstprinciple calculation. It is shown that the band structures of the wurtzite structure can be produced by using the EBOM parameters derived for the zinc blende structure defined in the rotated frame by adding correction to threecenter overlap integrals resulting from the rotation of atomic positions, which transfers the lattice symmetry from Td to C3v. Details of the parametrization of the EBOM Hamiltonian suitable for bulk IIInitride wurtzite semiconductors are presented. The band structures of bulk GaN, AlN, and InN with a zinc blende and wurtzite structure calculated in the same set of EBOM parameters with correction of threecenter overlap integrals are presented and compared with firstprinciple calculations. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P07.00010: A firstprinciples view of pyroelectricity Jian Liu , Sokrates Pantelides The pyroelectric effect is the response of the spontaneous polarization with respect to the temperature fluctuation. It impacts a wide range of applications. Pyroelectricity arises from the primary effect (at constant external strain) and the secondary effect (thermal expansion alters piezoelectric polarization). Here a computational route from first principles is developed. Calculations are made for the pyroelectric coefficients of wurtzite GaN and ZnO. An excellent agreement with experimental data is found. In these bulk materials, we reveal the crucial role of the primary pyroelectricity arising from the electronic redistribution induced by atomic thermal vibrations. For 2D materials, we demonstrate outofplane pyroelectricity in the recently synthesized Janus MoSSe monolayer and inplane pyroelectricity in the theoretically stable groupIV monochalcogenide GeS monolayer. For GeS monolayer, it is notable that the secondary pyroelectricity is significant, and hence applying strains can dramatically tune its intrinsic pyroelectricity. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P07.00011: Developing StructureProperty Relations in MetalInsulator Transition Materials Emily Schueller , Julia Zuo , Stephen Wilson , Ram Seshadri Materials which undergo a metalinsulator transition a rapid change in resistivity upon temperature, pressure, or composition have potential for the next generation of electronic switching devices. The underlying mechanisms which drive metalinsulator transitions are of interest both fundamentally in the field of condensedmatter physics and practically for the design of highperformance electronic materials. We develop a link between crystal structure and macroscopic electronic properties to realize new metalinsulator transition materials. We start by examining the AB_{4}Q_{8} defect spinel system, where A is Al, Ga, Ge, B is Ti, V, Nb, Mo, Ta, and Q is S/Se. These materials, with molecularorbitallike clusters, start at a cubic F43m structure at room temperature and then distort to R3m or Imm2 at low temperatures.^{1} Detailed analysis of the local structure, through pairdistribution function analysis, and the average structure, through highresolution diffraction, can provide insights into subtle links between structure and electronic behavior. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P07.00012: Electronic Raman scattering due to spinorbit splitoff band in tensilelystrained Ge Tomoyuki Matsushita , Chang Yang , Yuhsuke Yasutake , Susumu Fukatsu Electronic Raman scattering is an inelastic twophoton process involving electronic transitions rather than elementary excitations like phonons. Ge exhibits ERS spectra dominated by the lighthole valence subband when excited by the 1064nm light. Here we attempt to observe the straininduced development of the ERS due to the spinorbit splitoff (SO) valence subband, which is invisible otherwise. To this end, a subns pulse laser was used, which helped discriminate the relevant transition as a pronounced spectral peak with negative circular polarization, as opposed to the cw or fs excitations. A similar negativehelicity peak was confirmed for the ERS due to the lighthole subband as expected. We therefore infer that the otherwise inexplicable negative helicity near the direct bandedge in unstrained Ge is most likely due to the ERS involving SO. Based on these, an argument is invoked which takes account of the electronelectron scattering as the microscopic mechanism of an enhanced ERS in the strained multivalleyed Ge where the intervalley scattering of electrons in the conduction band plays a pivotal role. The possibility of a nearedge Raman gain will be discussed in light of the absorption characteristics. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P07.00013: Crossover from Trionhole Complex to Exciton Polaron for ndoped Twodimensional Semiconductors YiaChung Chang , ShiueYuan Shiau , Monique Combescot We study the excitation spectra of ndoped twodimensional semiconductors due to the absorption of a photon. Because of the interaction between the photocreated exciton and the Fermi sea electrons, exotic new states can emerge. We show that an excitonsinglepair complex can exist by only keeping single electronhole pair excitations in the Fermi sea. This 4body complex behaves like a Fermisea hole weakly bound to a trion for k_{F} a_{x} << 1, where k_{F} is the Fermi wavevector and a_{x} the exciton Bohr radius. The oscillator strength of photoabsorption associated with this trionhole bound state increases as k_{F} increases. As k_{F} continues to increase, the feature of trionhole bound state becomes unrecognizable, while the exciton state dressed by scattered electronhole pairs (which can be interpreted as an excitonpolaron) becomes more pronounced. The evolution of the excitation spectra of these 4particle coupled states (one exciton and one Fermisea electronhole pair) as k_{F} increases reveals the crossover from trionhole resonance to excitonpolaron resonance, which is associated with the internal reconfiguration of the 4body complex in the presence of the Fermi sea. 
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