Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A27: Atomic Structure, Lattice Properties, and Phase Transitions 
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Sponsoring Units: FIAP Chair: Nihar Pradhan Room: 404 
Monday, March 2, 2020 8:00AM  8:12AM 
A27.00001: Electric FieldInduced MetaltoInsulator Phase Transition in FewLayered MoSe_{2} Nihar Pradhan, Carlos Garcia, Bhaswar Chakrabarti, Jawnaye Nash, Christina S Miller, Dharmaraj Raghavan, Alamgir Karim, Liliana Stan, Ralu Divan, Daniel Rosenmann, Anirudha Sumant, Stephen A McGill The MetalInsulator phase transition (MIT) is one of the most interesting phenomena to study particularly in twodimensional transitionmetal dichalcogendes (TMDCs). A few recent studies^{1,2} have indicated a possible MIT on MoS_{2} and ReS_{2}, but the nature of the MIT is still enigmatic due to the interplay between charge carriers and disorder in 2D systems. We will present a potential MIT in fewlayered MoSe_{2} FETs based on fourterminal conductivity measurements. Conductivities measured in multiple samples strongly demonstrate the insulatingtometalliclike phase transition when the charge carrier density increased above a critical threshold. The nature of the phase transition will be discussed with an existing theoretical model. 
Monday, March 2, 2020 8:12AM  8:24AM 
A27.00002: Partial Switching in Phase Change Material Sb_{2}Te_{3} Nanowires to an Intermediate Resistance State by Microwave injection Pok Lam Tse Microwave (MW) induced resistance responses on Sb_{2}Te_{3} nanowire (NW) samples were observed at about 3GHz in the MW frequency sweeps from 0 to 4GHz. The stepwise shifts in resistance occurred for varies samples with Ti/Au or Nb electrodes. The magnitude of electrical resistance change depended on the initial resistance of the NW samples and it appeared to be a crystalline transition into an intermediate state before changed to amorphous. Temperature dependences of NW samples at saturated resistance state of 10^{8}Ω 10^{9}Ω exhibited semiconductor property while pristine NW sample at 500 Ω range had metallic property. 
Monday, March 2, 2020 8:24AM  8:36AM 
A27.00003: Understanding the Phase Behaviour of Pyrochlore Bi_{2}Sn_{2}O_{7} Warda Rahim, Jonathan M. Skelton, Aron Walsh, David Scanlon Bi_{2}Sn_{2}O_{7} exists in a number of polymorphic forms, with an α to β transition occurring at ≈400 K and β to γ transition occurring above 900 K.^{1} The structural model for γ is undisputed (Fd3m) but there has been a controversy over the structural models of α and β, with two models existing for α, one with 352 atoms per cell (P1C1)^{2} and other with 88 atoms per cell (C1c1),^{3 }and recently β belonging to space group Aba2 has been reported.^{3} We perform DFT lattice dynamics calculations using Phonopy^{4} starting from γ, and map out the potential energy surfaces^{5} spanned by imaginary mode eigenvectors, with the aim of elucidating the lowest energy structure. This approach successfully takes us from γ to the new structural model suggested for α, and also shows that β is a thermal average of a lower symmetry structure. The success of the method highlights the strength of abintio lattice dynamics in predicting the dynamically stable structural model of a compound and can speed up the exploration of different structures for solidstate applications. 
Monday, March 2, 2020 8:36AM  8:48AM 
A27.00004: Electron Transfer in Contact Electrification Benjamin J Kulbago, James Chen, Jun Liu, Thomas G Thundat Contact causes a weak interaction at the interface of two materials. This interaction transforms the surface of the lattice into a perturbed state. This perturbed surface is modeled as a series of dipoles. These dipoles change the surface state of both materials and contribute to a modified work function. The surface dipoles also induce a potential field at the interface, which drives electron transfer across the gap between the materials. A tribopair of Si and SiO2 is used to demonstrate how the modified work function and potential field drive electron transfer, as de scribed by Schrodinger’s equation. The results will also be compared to experimental data. 
Monday, March 2, 2020 8:48AM  9:00AM 
A27.00005: Density Functional Investigation of Phase Transition in Silicon Nanomembranes Joel Ambriz Ponce, Evan M Macintosh, William D Parker Silicon is an important material at the center of the microelectronic systems industry, and nanomembranes of silicon have a wide range of material, mechanical, optical, and device applications. Recent experimental advances have achieved nanometerscale flatness, and membrane properties have been found to depend directly on thickness. Using an atomistic slab model, we simulate the electrons with density functional theory in order to investigate the pressure phase transition of silicon in the membrane from semiconducting diamond phase to metallic betatin phase. We vary exchangecorrelation approximation, comparing to bulk values in each approximation, and we calculate the electronic and vibrational properties of the membranes. 
Monday, March 2, 2020 9:00AM  9:12AM 
A27.00006: UltraHigh Mechanical Flexibility of 2D Silicon Telluride Romakanta Bhattarai, Xiao Shen Silicon telluride (Si_{2}Te_{3}) is a twodimensional material with a unique variable structure where the silicon atoms form SiSi dimers to fill the “metal” sites between the Te layers. The SiSi dimers have four possible orientations: three inplane and one outofthe plane directions. The structural variability of Si_{2}Te_{3} allows unusual properties, especially mechanical properties. Firstprinciples density functional theory calculations are performed to determine the critical strain of monolayer Si_{2}Te_{3}. The results show that Si_{2}Te_{3} can sustain a critical uniaxial tensile strain up to 38% with a breaking stress of 8.63 N/m along the direction of Si dimer, making Si_{2}Te_{3} the most flexible 2D material reported. Because of the high flexibility, a large strain can be used to tune the band structure, and the bandgap can be reduced by up to 1.5 eV. With increasing strain, the bandgap undergoes an unusual indirectdirectindirectdirect transition. We also find that the uniaxial strain can effectively control the orientation of Si dimers, which may be beneficial for certain applications. 
Monday, March 2, 2020 9:12AM  9:24AM 
A27.00007: Entanglement entropy of skeletal regions Alex Vigeant Entanglement entropy (EE) provides deep insights about quantum matter. For most groundstates and spatial regions, the EE is dominated by the celebrated area law. However, much less is known about the EE of skeletal regions, i.e. regions without volume such as a line embedded in 2D. When the boundary and volume coincide, there is little understanding about what is encoded in the EE. Here, we study this question using a 2D harmonic lattice model that leads to a relativistic massless scalar (conformal field theory) at low energy. We find new universal contributions to the EE as a function of the geometry of the skeletal region. We also discuss the field theory interpretation, and outline general open questions. 
Monday, March 2, 2020 9:24AM  9:36AM 
A27.00008: Origin of unexpectedly low thermal conductivity in AMg_{2}X_{2} (A = Mg, Ca, Yb, X = Sb, Bi ) Jingxuan Ding, Tyson LaniganAtkins, Mario Calderon Cueva, Alexandra Zevalkink, Arnab Banerjee, Olivier Delaire Thermoelectric (TE) materials enable direct conversion of heat into electrical energy. The conversion efficiency is inversely proportional to the thermal conductivity, which is dominated by phonons in semiconductors. Zintl compounds AMg_{2}X_{2} constitute a class of new TE with excellent performance in ntype Mg_{3}(Sb,Bi)_{2} alloys, with zT up to 1.6 reported so far. Mg_{3}X_{2} exhibits very low lattice thermal conductivity (~1 W/m/K at 300K), comparable with PbTe and Bi_{2}Te_{3}, despite a much lighter average ionic mass. We report on inelastic neutron scattering (INS) and firstprinciples studies of the lattice dynamics of AMg_{2}X_{2}. INS provided the temperature dependent phonon density of states. Extra peaks were found at low frequency in Mg_{3}X_{2} compared to CaMg_{2}X_{2} or YbMg_{2}X_{2}, possibly originating from softer lowfrequency TA phonons. Considerably stronger softening with temperature is also observed in Mg_{3}X_{2}. The anharmonic effects were examined with firstprinciples simulations, including ab initio molecular dynamics. We present our analysis of the thermal conductivity based on INS and simulations. 
Monday, March 2, 2020 9:36AM  9:48AM 
A27.00009: Anharmonic Effects on Phonon Eigenvectors and S(Q,E) in Quantum Paraelectric SrTiO_{3} Xing He, Dipanshu Bansal, Barry Winn, Songxue Chi, Lynn A Boatner, Olivier Delaire The quantum paraelectric behavior and strongly anharmonic lattice dynamics of SrTiO_{3} have attracted interest for decades. Inelastic neutron scattering (INS) measurements of SrTiO_{3 }reveal an anomalous evolution of transverse acoustic (TA) phonon intensity with temperature as the transverse optic (TO) mode softens. This reflects the incipient ferroelectric (FE) instability near the quantum critical point and couplings between TA and TO phonons. The experimental trends are confirmed and rationalized using DFT simulations including anharmonic renormalization. By analyzing the temperaturedependent force constants (FC) and eigenvectors, it is found that the structure factors of phonon modes change dramatically with temperature, as a direct consequence of the anharmonicity in this system. We identify that changing Ti and O eigenvectors, originating from FC changes in the TiO bonds, are responsible for these striking observations. These results establish how temperaturedependent phonon intensities from INS can provide direct insights into the behavior of phonon eigenvectors, and show how firstprinciples simulations can rationalize such anharmonic effects. 
Monday, March 2, 2020 9:48AM  10:00AM 
A27.00010: Covalencydriven Structural Evolution in the Polar Pyrochlore Series Cd_{2}Nb_{2}O_{7–x}S_{x} Daniel HickoxYoung, Geneva Laurita, Samra Husremovic, James Rondinelli Pyrochlores have attracted considerable interest lately due to their ability to host a variety of interesting phenomena, often derived from their geometrically frustrated interpenetrating sublattices. The triangular arrangement of cations tends to disfavor the coordinated displacements necessary to realize polar structures, but there are a few materials which overcome this frustration and exhibit ferroelectricity. Here we examine the origin and nature of the distortion mechanisms in the oxysulfide series Cd_{2}Nb_{2}O_{7x}S_{x}. Using density functional theory, group theoretical methods, and diffraction techniques, we characterize changes in the phase transition under sulfur substitution. We ultimately identify the role of covalent bonding on the CdX’ sublattice in shifting the ferroelectric phase transition from proper (x=0) to improper (x=0.25) and eventual elimination (x=0.7). This work sheds light on the origin of polar distortion mechanisms in the pyrochlore structure family, in addition to providing insight into the offcentering mechanisms of complex oxides in general. 
Monday, March 2, 2020 10:00AM  10:12AM 
A27.00011: Relationship between multiple degrees of freedom and oxygen interstitial ordering in rare earth ferrites Yang Zhang, Wenbing Wang, Wandong Xing, Rong Yu, Jing Zhu Oxygen interstitials and vacancies, as two kinds of common defects in oxide system, usually play an important role in tuning the microstructure and properties of materials. To understand the tuning mechanism resulting from oxygen doping, the precise positions of these point defects and interaction between them and with other degrees of freedom need to be solved. Here, we report how such information can, for the first time, be obtained from the rare earth ferrite LuFe_{2}O_{4+x }by highresolution electron microscopy, atomicresolution spectroscopy and DFT calculations. The oxygen interstitial ordering and related lattice, charge, spin ordering form a new modulation structure, described by the same modulation vector. Meanwhile, we prove the modulation structure can be affected by changing the oxygen interstitial ordering. The interaction between oxygen interstitials and multiple degrees of freedom presented here provide a direct insight into tuning mechanism of oxygen doping in nonstoichiometric oxide systems. 
Monday, March 2, 2020 10:12AM  10:24AM 
A27.00012: First Principle Study of Phase Change Properties of Ag and Indoped SbTe compound as Phase Change Material Hanjin Park, YoungKyun Kwon, Dasol Kim, MannHo Cho Ag and In doped SbTe binary compounds (AIST) are widely attracted as base material of phase change random access memory (PCRAM) with its fast recrystallization time (ps scale) and less energy consumption in the phase transition than GeTeSb_{2}Te_{3} pseudobinary compounds (group 1) which are most famous as phase change material (PCM). Although AIST has better phase change behavior than group 1, it has much less investigated such as local structural environments or roles of Ag and In atoms. To reveal phase change properties and local atomic behavior in AIST, we performed molecular dynamics (MD) simulations using ab initio density functional theory. AgInSb_{18}Te_{4} is selected as an exemplary model configuration. We identify its crystalline phase with various atomic configurations and evaluating their total energy. Amorphous phase, a representative expanded structure to be a 2×2×2 supercell, is constructed with MD simulation which mimics the meltquenching experiment process. We explore the phase change mechanism by evaluating the radial distribution function, angle distribution function, solid angle distribution, and order parameter, and find that the In atom plays an important role not only in the phase transition process but also in retaining or stabilizing amorphous phase. 
Monday, March 2, 2020 10:24AM  10:36AM 
A27.00013: Structural orderbydisorder in framework materials Gian GuzmanVerri, Peter B Littlewood Systems which do not order at absolute zero due to a large ground state degeneracy can generate new correlations through thermal or quantum fluctuations and attain longrange order. Here, the internal energy of different ground states is the same, but their free energy can differ because the fluctuations around them give a different entropic weighting to each ground state. This is known as orderbydisorder (ObD) and has proven useful in understanding a wide variety of observed ordering phenomena in magnetic systems such as spin ices and frustrated antiferromagnets^{. }Outside magnetism, however, the ramifications of ObD physics remain largely unexplored. In this talk, we propose an ObD mechanism is at play in the observed structural transitions in a class of framework materials (MF_{3} metal triflourides), which display soft manifold lattice dynamics with an inability to go through a symmetry breaking distortion, providing the required ground state degeneracy analogous to that of frustrated magnetism. 
Monday, March 2, 2020 10:36AM  10:48AM 
A27.00014: Electrical properties of ferroelectric 1,4diaminobutane zinc formate crystals under high hydrostatic pressure. Anna Szeremeta, Andrzej Nowok, Adam Sieradzki, Miroslaw Maczka, Sebastian Pawlus 1,4diaminobutane zinc formate is an example of metal organic frameworks, MOF, which possesses a nicolitetype structure. This compound is characterized by ordered pore structure, formed by a specific arrangement of atoms formed by coordinating bonds between zinc atoms and formate ligands. Due to its multiferroic properties this compound as well as other MOFs can be applied in memory components. 

A27.00015: Cubic and Tetragonal Perovskites from the Random Phase Approximation Fanhao Jia, Georg Kresse, Cesare Franchini, Peitao Liu, Jian Wang, Alessandro Stroppa, Wei Ren Evaluating many body correlation effects beyond the commonly applied local or semilocal density functionals has received tremendous attention over the past few years. Using the random phase approximation (RPA) to describe the correlation energy combined with the exact exchange energy, we have investigated twenty cubic ABO_{3}type perovskites and three prototypical ferroelectric (tetragonal) perovskites. A quantitative analysis and comparison of the performance of various local and semilocal exchangecorrelation functionals (XCFs) shows that the inclusion of dynamical correlation effects allows for an excellent account of the structure and energetics of complex ABO_{3}type oxides. 
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