Bulletin of the American Physical Society
85th Annual Meeting of the APS Southeastern Section
Volume 63, Number 19
Thursday–Saturday, November 8–10, 2018; Holiday Inn at World’s Fair Park, Knoxville, Tennessee
Session F01: Condensed Matter IV |
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Chair: Peizhi Mai, Oak Ridge National Lab Room: Holiday Inn Knoxville Downtown Summit |
Friday, November 9, 2018 11:00AM - 11:12AM |
F01.00001: Structure identification at extreme conditions Jennifer Niedziela, Andrew Miskowiec, Ashley Shields We are investigating low-crystallinity, amorphous uranium oxides. One compound has stoichiometry of ~UO3.5 (a-UO3). Powder XRD shows broad peaks commensurate with extreme finite-size broadening or amorphous character. To understand the a-UO3 structure, we use density functional theory with a genetic algorithm structure prediction mechanism to identify stable structures of non-stoichiometric uranium oxides. The lowest energy predicted structure with stoichiometry UO3.5 contains a peroxide bridge. The presence of peroxide bridges is compatible with a study of comparable material using neutron pair-distribution function techniques. We attempted to crystallize a-UO3 using a diamond anvil cell and Raman spectroscopy. We observed highly anharmonic responses to pressure, but no long range order. Computational studies on the low-energy structure from the genetic algorithm show commensurate responses to the dynamical observations, along with anomalous changes in local bonding character. Here we discuss anharmonic responses to pressure coupled with computational modeling to shed light on the use of extreme conditions for structure identification. |
Friday, November 9, 2018 11:12AM - 11:24AM |
F01.00002: Slow Dynamics of the Fredkin Spin Chain Khagendra Adhikari, Kevin Stuart David Beach The dynamical behavior of many-particle systems is characterized by the lifetime of quasi-particles or excitations. Observables of any non-conserved quantity decay exponentially, but those of a conserved quantity relax to equilibrium with a power law ( τ ∼ 1/Δ ~ Lz ). Such decay process are associated with a dynamical exponent (e.g., z = 1 for the ballistic spread of quasi-particles and z = 2 for diffusion) that relates the spread of correlations in space and time. We present numerical results for the Fredkin model---a quantum spin chain with an unusual three-body interaction term---which exhibits a dynamical exponent z ≈ 3. We discuss our efforts to make a reliable, quantitative estimate of z and to explain the very slow dynamics in terms of a random walk executed by the excitation in Monte Carlo time. |
Friday, November 9, 2018 11:24AM - 11:36AM |
F01.00003: Ultrafast Processes in Weyl semimetals Fatemeh Nematollahi, Vadym Apalkov, Mark I Stockman We theoretically study the electron dynamics of topological Weyl semimetals in both linear and circularly ultrafast optical pulse. We use the effective model in the reciprocal space near the Weyl points. We present the results for TaAs. For linear optical pulse, the electron dynamics for each set of Weyl point is different and strongly depends on the polarization direction of the pulse. In circularly polarized ultrafast pulse, we predict that the electron dynamics is coherent and highly irreversible. For a pulse propagating in the z-direction, the conduction band (CB) population in reciprocal space not only depends on the profile of the dipole matrix elements but also depends on the band gap. For small band gaps, the system behaves similarly to graphene with a localized dipole matrix near (kx,ky)=(0,0) point which causes a large CB population along the separatrix which is defined as a mirror-symmetric of the electron trajectory in the reciprocal space. However, for large kz, the system is similar to the gapped graphene with delocalized interband dipole matrix, and therefore the CB population is not confined within a narrow region. We also show that both linear and circularly polarized pulse causes an electrical current and net charge transfer through the system during the pulse.
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Friday, November 9, 2018 11:36AM - 11:48AM |
F01.00004: High-pressure diffraction of Prussian blue analogues Daniel M Pajerowski We present recent work on high pressure powder diffraction (HPPD) of Prussian blue analogues (PBAs) using X-rays and neutrons up to (and above) 1 GPa of applied pressure. These PBAs are nominally face-centered cubic materials of metals linked by cyanide. Interest in PBAs has been in varied from magnetism, to fundamentals of the structure, and more recently as cathode materials for energy storage. Indeed, incorporation into devices (such as a cathode) requires understanding of response to strain, for which HPPD is one useful tool. |
Friday, November 9, 2018 11:48AM - 12:00PM |
F01.00005: Probing Transient Nanoscale Electric and Magnetic Resonances of Dielectric Silicon Metasurfaces Uddhab Tiwari, Kannatassen Appavoo Controlling optical electromagnetic fields at nanoscale has gained interest over last decade as they improve device efficiency in energy, telecommunication and medical sector. Recently, it was demonstrated that dielectric nanostructures can also create strong electric and magnetic near fields similar to plasmonic nanostructures due to their Mie resonances. Unlike their metallic counterparts that can sustain mostly electric responses while having high ohmic losses, dielectric metasurfaces display rich optical features with low parasitic loss. Here we study the steady-state and femtosecond optical properties of dielectric silicon (Si) metasurfaces to understand how structural changes of the unit cell structure affect its overall optical performance. Si metasurfaces are fabricated using electron beam lithography and their scattering and absorption (extinction) properties are studied using white-light broadband spectroscopy. Furthermore, we perform ultrafast broadband transient absorption spectroscopy on these large arrays by exciting near (900 nm) and well above (350 nm) the Si bandgap to demonstrate how modulating the electron density dynamically modifies these electric and magnetic resonances. |
Friday, November 9, 2018 12:00PM - 12:12PM |
F01.00006: Study of charge transfer of Fe3+ in Ga2O3 by photo-induced electron paramagnetic resonance Suman Bhandari, M. E. Zvanut We investigate charge transfer of a point defect, Fe3+, by electron paramagnetic resonance (EPR) spectroscopy in Ga2O3 single crystal. Two Czochralski grown Ga2O3 single crystals: one doped with 1018 cm-3 Mg and 1017 cm-3 unintentional Fe, and the other with 5 x 1017 cm-3 Fe were studied by photo-induced EPR. The technique detects a specific charge state of a point defect such as Fe3+ and the intensity of EPR signal is proportional to the amount of Fe3+, therefore, a change in the EPR intensity suggests a change in charge state of the point defect. Steady state photo-EPR was performed by illuminating the crystals with light of 0.8-4.9 eV. The amount of Fe3+ decreased at approximately 2.5 eV for the Mg- and Fe-doped crystals. The observed threshold is surprisingly higher than the Fe-defect level, which is reported to be within 1.0 eV of band edges. Therefore, rather than direct ionization, we suggest that the amount of Fe3+ may decrease via two-step process: ionization of a defect Ir3+, which is reported to be at 2.25 eV below conduction band edge, and capture of the free electron by Fe3+. However, we note that Fe3+ acts as a compensator in Ga2O3, so one should not disregard the possibility that the 2.5 eV mid-gap threshold represents the Fe-defect level. |
Friday, November 9, 2018 12:12PM - 12:24PM |
F01.00007: Photoinduced Magneto-Structural Interplay at the Interface of Core@Shell Nanoparticles John M Cain, Ashley Felts, Daniel Talham, Mark W Meisel Core@shell heterostructures of Prussian blue analogues (PBAs), namely RbxCo[Fe(CN)6]y@KaNi[Cr(CN)6]b, show a photoinduced decrease in magnetization of the non-photoactive KaNi[Cr(CN)6]b shell when irradiated with white light below Tc = 70 K [1,2]. This decrease is magnetomechanical in origin, where the photoinduced volume changes of the core and the resulting change of strain in the shell reduce the shell magnetization. An initial model provided an estimate of the strain depth in the shell [2], and now this model has been extended to comprehensively describe data from three new sets of core@shell heterostructures (one set = one RbCoFe PBA core and three different KNiCr PBA shells). Increasing the core size from ≈ 125 nm to ≈ 575 nm modifies the shell strain depth from ≈ 45 nm to ≈ 110 nm. The assumption of a rigid core fails, and PXRD data show a quantitative model for the strain depth must also account for the effect of the strain induced in the core [2,3]. Proposed SANS studies of the shell magnetization will be described. [1] M.F. Dumont et al., Inorg. Chem. 50 (2011) 4295. [2] A.C. Felts et al., J. Phys. Chem. C 120 (2016) 5420. [3] A.C. Felts et al., J. Am. Chem. Soc. 140 (2018) 5814. |
Friday, November 9, 2018 12:24PM - 12:36PM |
F01.00008: Relaxation dynamics in magnetic skyrmions with quenched disorder Bart J Brown, Uwe Claus Tauber, Michel Pleimling Magnetic skyrmions are topologically protected spin textures of nanometer size found in certain chiral magnets. Skyrmions can be moved by very low current densities which makes them ideal for applications in spintronics such as data storage devices and logic gates. A thorough understanding of the relaxation processes for systems of interacting skyrmions far from equilibrium could prove invaluable in real world applications. We use a particle based model derived from Thiele's approach to study the relaxation dynamics of two-dimensional skyrmions subject to quenched disorder and a time-dependent Gaussian noise. The particle model differs most notably from similar models which describe vortices in type-II superconductors by the addition of the Magnus force which always acts perpendicular to the forces in the plane. Previously, it was shown that the interplay between the Magnus force, repulsive skyrmion-skyrmion interaction and Gaussian noise yields different regimes during non-equilibrium relaxation. In this work, we focus on the effects of randomly distributed attractive defects on the relaxation process. |
Friday, November 9, 2018 12:36PM - 12:48PM |
F01.00009: Modified Law of Cooling: Continuum and Discreet Effect 'Kale Oyedeji, Ronald E. Mickens We derive a modified law of cooling (and heating) which extends the standard Newton’s law of cooling/heating. A critical feature of our formulation is the inclusion of both continuum and discreet physical aspects of the system. One consequence of our mathematical equation ( a first-order, nonlinear differential equation) is that the temperature reaches its equilibrium value at a finite time. We also show that this equation has an exact solution which can be explicitly calculated in terms of elementary functions. |
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