297 research outputs found
Surface versus bulk characterization of the electronic inhomogeneity in a VO_{2} film
We investigated the inhomogeneous electronic properties at the surface and
interior of VO_{2} thin films that exhibit a strong first-order metal-insulator
transition (MIT). Using the crystal structural change that accompanies a VO_{2}
MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate
the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature
dependence of the p was very closely correlated with the dc
conductivity near the MIT temperature, and fit the percolation theory
predictions quite well: (p - p_{c})^{t} with t = 2.00.1
and p_{c} = 0.160.01. This agreement demonstrates that in our VO
thin film, the MIT should occur during the percolation process. We also used
surface-sensitive scanning tunneling spectroscopy (STS) to investigate the
microscopic evolution of the MIT near the surface. Similar to the XRD results,
STS maps revealed a systematic decrease in the metallic phase as temperature
decreased. However, this rate of change was much slower than the rate observed
with XRD, indicating that the electronic inhomogeneity near the surface differs
greatly from that inside the film. We investigated several possible origins of
this discrepancy, and postulated that the variety in the strain states near the
surface plays an important role in the broad MIT observed using STS. We also
explored the possible involvement of such strain effects in other correlated
electron oxide systems with strong electron-lattice interactions.Comment: 27 pages and 7 figure
Classical Spin Models with Broken Continuous Symmetry: Random Field Induced Order and Persistence of Spontaneous Magnetization
We consider a classical spin model, of two-dimensional spins, with continuous
symmetry, and investigate the effect of a symmetry breaking unidirectional
quenched disorder on the magnetization of the system. We work in the mean field
regime. We show, by numerical simulations and by perturbative calculations in
the low as well as in the high temperature limits, that although the continuous
symmetry of the magnetization is lost, the system still magnetizes, albeit with
a lower value as compared to the case without disorder. The critical
temperature at which the system starts magnetizing, also decreases with the
introduction of disorder. However, with the introduction of an additional
constant magnetic field, the component of magnetization in the direction that
is transverse to the disorder field increases with the introduction of the
quenched disorder. We discuss the same effects also for three-dimensional
spins.Comment: 12 pages, 12 figures, RevTeX
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
Atomistic simulation of light-induced changes in hydrogenated amorphous silicon
We employ ab initio molecular dynamics to simulate the response of
hydrogenated amorphous silicon to light exposure (Staebler-Wronski effect). We
obtain improved microscopic understanding of PV operation, compute the motion
of H atoms, and modes of light-induced degradation of photovoltaics. We clarify
existing models of light-induced change in aSi:H and show that the Hydrogen
collision model of Branz3 is correct in essentials.Comment: 10 pages, 3 figures, to be published in J. Phys. Cond matt. (Letter
Infrared and Raman spectra of LiV2O5 single crystals
The phonon dynamics of LiV2O5 single crystals is studied using infrared and
Raman spectroscopy techniques. The infrared-active phonon frequencies and
dielectric constants are obtained by oscillator fitting procedure of the
reflectivity data measured at room temperature. The Raman scattering spectra
are measured at room temperature and at T=10 K in all nonequivalent polarized
configurations. The assignment of the phonons is done by comparing the infrared
and Raman spectra of LiV2O5 and NaV2O5. The factor-group-analysis of the LiV2O5
crystal symmetry and of its constituent layers is performed to explain the
symmetry properties of the observed modes. We concluded that layer symmetry
dominates in the vibrational properties of this compound.Comment: 10 pages, 5 figure
Optimal Packings of Superballs
Dense hard-particle packings are intimately related to the structure of
low-temperature phases of matter and are useful models of heterogeneous
materials and granular media. Most studies of the densest packings in three
dimensions have considered spherical shapes, and it is only more recently that
nonspherical shapes (e.g., ellipsoids) have been investigated. Superballs
(whose shapes are defined by |x1|^2p + |x2|^2p + |x3|^2p <= 1) provide a
versatile family of convex particles (p >= 0.5) with both cubic- and
octahedral-like shapes as well as concave particles (0 < p < 0.5) with
octahedral-like shapes. In this paper, we provide analytical constructions for
the densest known superball packings for all convex and concave cases. The
candidate maximally dense packings are certain families of Bravais lattice
packings. The maximal packing density as a function of p is nonanalytic at the
sphere-point (p = 1) and increases dramatically as p moves away from unity. The
packing characteristics determined by the broken rotational symmetry of
superballs are similar to but richer than their two-dimensional "superdisk"
counterparts, and are distinctly different from that of ellipsoid packings. Our
candidate optimal superball packings provide a starting point to quantify the
equilibrium phase behavior of superball systems, which should deepen our
understanding of the statistical thermodynamics of nonspherical-particle
systems.Comment: 28 pages, 16 figure
Raman spectrum and lattice parameters of MgB2 as a function of pressure
We report Raman spectra and synchrotron x-ray diffraction measurements of
lattice parameters of polycrystalline MgB2 under hydrostatic pressure
conditions up to 15 GPa. An anomalously broadened Raman band at 620 cm-1 is
observed that exhibits a large linear pressure shift of its frequency. The
large mode damping and Gruneisen parameter indicate a highly anharmonic nature
of the mode, broadly consistent with theoretical predictions for the E2g
in-plane boron stretching mode. The results obtained may provide additional
constraints on the electron-phonon coupling in the system.Comment: 3 pages, 3 figure
Superdipole Liquid Scenario for the Dielectric Primary Relaxation in Supercooled Polar liquids
We propose a dynamic structure of coupled dynamic molecular strings for
supercooled small polar molecule liquids and accordingly we obtain the
Hamiltonian of the rotational degrees of freedom of the system. From the
Hamiltonian, the strongly correlated supercooled polar liquid state is
renormalized to a normal superdipole (SD) liquid state. This scenario describes
the following main features of the primary or a-relaxation dynamics in
supercooled polar liquids: (1) the average relaxation time evolves from a high
temperature Arrhenius to a low temperature non-Arrhenius or super-Arrhenius
behavior; (2) the relaxation function crosses over from the high temperature
exponential to low temperature non-exponential form; and (3) the temperature
dependence of the relaxation strength shows non-Curie features. According to
the present model, the crossover phenomena of the first two characteristics
arise from the transition between the superdipole gas and the superdipole
liquid. The model predictions are quantitatively compared with the experimental
results of glycerol, a typical glass-former.Comment: 40 pages, 3 figure
Long term aging of Selenide glasses: Evidence of sub-Tg endotherms and pre-Tg exotherms
Long term aging is studied on several families of chalcogenide glasses
including the Ge-Se, As-Se, Ge-P-Se and Ge-As-Se systems. Special attention is
given to the As-Se binary, a system that displays a rich variety of aging
behavior intimately tied to sample synthesis conditions and the ambient
environment in which samples are aged. Calorimetric (Modulated DSC) and Raman
scattering experiments are undertaken. Our results show all samples display a
sub-Tg endotherm below Tg in glassy networks possessing a mean coordination
number r in the 2.25 < r < 2.45 range. Two sets of AsxSe1-x samples aged for 8
years were compared, set A consisted of slow cooled samples aged in the dark,
and set B consisted of melt quenched samples aged at laboratory environment.
Samples of set B in the As concentration range, 35% < x < 60%, display a pre-Tg
exotherm, but the feature is not observed in samples of set A. The aging
behavior of set A presumably represents intrinsic aging in these glasses, while
that of set B is extrinsic due to presence of light. The reversibility window
persists in both sets of samples but is less well defined in set B. These
findings contrast with a recent study by Golovchak et al., which finds the
onset of the reversibility window moved up to the stoichiometric composition (x
= 40%). Here we show that the upshifted window is better understood as
resulting due to demixing of As4Se4 and As4Se3 molecules from the backbone,
i.e., Nanoscale phase separation (NSPS). We attribute sub-Tg endotherms to
compaction of the flexible part of networks upon long term aging, while the
pre-Tg exotherm to NSPS. Finally, the narrowing and sharpening of the
reversibility window upon aging is interpreted as the slow 'self-organizing'
stress relaxation of the phases just outside the Intermediate phase.Comment: In press - J. of Physics: Condensed Matte
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