2,729 research outputs found
Strain dependence of bonding and hybridization across the metal-insulator transition of VO2
Soft x-ray spectroscopy is used to investigate the strain dependence of the
metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p
hybridization are observed across the transition, and are linked to the
structural distortion. Furthermore, although the V-V dimerization is
well-described by dynamical mean-field theory, the V-O hybridization is found
to have an unexpectedly strong dependence on strain that is not predicted by
band theory, emphasizing the relevance of the O ion to the physics of VO2
Experimental and theoretical cross sections for positron scattering from the pentane isomers.
Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present. (C) 2016 AIP Publishing LLC
Mixed Weyl Symbol Calculus and Spectral Line Shape Theory
A new and computationally viable full quantum version of line shape theory is
obtained in terms of a mixed Weyl symbol calculus. The basic ingredient in the
collision--broadened line shape theory is the time dependent dipole
autocorrelation function of the radiator-perturber system. The observed
spectral intensity is the Fourier transform of this correlation function. A
modified form of the Wigner--Weyl isomorphism between quantum operators and
phase space functions (Weyl symbols) is introduced in order to describe the
quantum structure of this system. This modification uses a partial Wigner
transform in which the radiator-perturber relative motion degrees of freedom
are transformed into a phase space dependence, while operators associated with
the internal molecular degrees of freedom are kept in their original Hilbert
space form. The result of this partial Wigner transform is called a mixed Weyl
symbol. The star product, Moyal bracket and asymptotic expansions native to the
mixed Weyl symbol calculus are determined. The correlation function is
represented as the phase space integral of the product of two mixed symbols:
one corresponding to the initial configuration of the system, the other being
its time evolving dynamical value. There are, in this approach, two
semiclassical expansions -- one associated with the perturber scattering
process, the other with the mixed symbol star product. These approximations are
used in combination to obtain representations of the autocorrelation that are
sufficiently simple to allow numerical calculation. The leading O(\hbar^0)
approximation recovers the standard classical path approximation for line
shapes. The higher order O(\hbar^1) corrections arise from the noncommutative
nature of the star product.Comment: 26 pages, LaTeX 2.09, 1 eps figure, submitted to 'J. Phys. B.
Limits to Poisson's ratio in isotropic materials
A long-standing question is why Poisson's ratio v nearly always exceeds 0.2
for isotropic materials, whereas classical elasticity predicts v to be between
-1 to 1/2. We show that the roots of quadratic relations from classical
elasticity divide v into three possible ranges: -1 < v <= 0, 0 <= v <= 1/5, and
1/5 <= v < 1/2. Since elastic properties are unique there can be only one valid
set of roots, which must be 1/5 <= v < 1/2 for consistency with the behavior of
real materials. Materials with Poisson's ratio outside of this range are rare,
and tend to be either very hard (e.g., diamond, beryllium) or porous (e.g.,
auxetic foams); such substances have more complex behavior than can be
described by classical elasticity. Thus, classical elasticity is inapplicable
whenever v < 1/5, and the use of the equations from classical elasticity for
such materials is inappropriate.Comment: Physical Review B, in pres
On the Hall Effect in Ferromagnetics
With precise measurements at magnetic 6elds well above those required for saturation, it is shown thatthe Hall electric 6eld per unit current density consists of two distinct parts. Its value averaged betweenpoles is given by ROH+R&M, where H and j/I are the magnetizing force and average intensity of magnetization,respectively. in the sample.The value Eo=—0.611X10 ~ volt-cm/amp. -oersted indicates 1.16 s-band electrons with no d-bandconduction, 0.6 s-band electrons with 30 percent d-band conduction, or something intermediate betweenthese two.The value R&=-74.9X10 "volt-cm/amp. -oersted suggests the existence of a very large average magnetic6eld acting on the conduction electrons. An explanation for this is suggested
Non-Fermi Liquids in the Extended Hubbard Model
I summarize recent work on non-Fermi liquids within certain generalized
Anderson impurity model as well as in the large dimensionality () limit of
the two-band extended Hubbard model. The competition between local charge and
spin fluctuations leads either to a Fermi liquid with renormalized
quasiparticle excitations, or to non-Fermi liquids with spin-charge separation.
These results provide new insights into the phenomenological similarities and
differences between different correlated metals. While presenting these
results, I outline a general strategy of local approach to non-Fermi liquids in
correlated electron systems.Comment: 30 pages, REVTEX, 14 figures included. To appear in ``Non Fermi
Liquid Physics'', J. Phys: Cond. Matt. (1997
The Kondo Resonance in Electron Spectroscopy
The Kondo resonance is the spectral manifestation of the Kondo properties of
the impurity Anderson model, and also plays a central role in the dynamical
mean-field theory (DMFT) for correlated electron lattice systems. This article
presents an overview of electron spectroscopy studies of the resonance for the
4f electrons of cerium compounds, and for the 3d electrons of V_2O_3, including
beginning efforts at using angle resolved photoemission to determine the
k-dependence of the resonance. The overview includes the comparison and
analysis of spectroscopy data with theoretical spectra as calculated for the
impurity model and as obtained by DMFT, and the Kondo volume collapse
calculation of the cerium alpha-gamma phase transition boundary, with its
spectroscopic underpinnings.Comment: 32 pages, 11 figures, 151 references; paper for special issue of J.
Phys. Soc. Jpn. on "Kondo Effect--40 Years after the Discovery
Stripes, Vibrations and Superconductivity
We propose a model of a spatially modulated collective charge state of
superconducting cuprates. The regions of higher carrier density (stripes) are
described in terms of Luttinger liquids and the regions of lower density as a
two-dimensional interacting bosonic gas of d_{x^2-y^2} hole pairs. The
interactions among the elementary excitations are repulsive and the transition
to the superconducting state is driven by decay processes. Vibrations of the
CCS and the lattice, although not participating directly in the binding
mechanism, are fundamental for superconductivity. The superfluid density and
the lattice have a strong tendency to modulation implying a still unobserved
dimerized stripe phase in cuprates. The phase diagram of the model has a
crossover from 1D to 2D behavior and a pseudogap region where the amplitude of
the order parameters are finite but phase coherence is not established. We
discuss the nature of the spin fluctuations and the unusual isotope effect
within the model.Comment: 51 pages, 20 figures. Post-March Meeting version: New references are
added, some of the typos are corrected, and a few new discussions are
include
The Structure, Dynamics and Electronic Structure of Liquid Ag-Se Alloys Investigated by Ab Initio Simulation
Ab initio molecular-dynamics simulations have been used to investigate the
structure, dynamics and electronic properties of the liquid alloy Ag(1-x)Se(x)
at 1350 K and at the three compositions x=0.33, 0.42 and 0.65. The calculations
are based on density-functional theory in the local density approximation and
on the pseudopotential plane-wave method. The reliability of the simulations is
confirmed by detailed comparisons with very recent neutron diffraction results
for the partial structure factors and radial distribution functions (RDF) of
the stoichiometric liquid Ag2Se. The simulations show a dramatic change of the
Se-Se RDF with increasing Se content. This change is due to the formation of Se
clusters bound by covalent bonds, the Se-Se bond length being almost the same
as in pure c-Se and l-Se. The clusters are predominantly chain-like, but for
higher x a large fraction of 3-fold coordinated Se atoms is also found. It is
shown that the equilibrium fractions of Se present as isolated atoms and in
clusters can be understood on a simple charge-balance model based on an ionic
interpretation. The Ag and Se diffusion coefficients both increase with Se
content, in spite of the Se clustering. An analysis of the Se-Se bond dynamics
reveals surprisingly short bond lifetimes of less than 1 ps. The changes in the
density of states with composition arise directly from the formation of Se-Se
covalent bonds. Results for the electronic conductivity obtained using the
Kubo-Greenwood approximation are in adequate agreement with experiment for
l-Ag2Se, but not for the high Se contents. Possible reasons for this are
discussed.Comment: 14 pages, Revtex, 14 Postscript figures embedded in the tex
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