911 research outputs found
Electrodynamics near the Metal-to-Insulator Transition in V3O5
The electrodynamics near the metal-to-insulator transitions (MIT) induced, in
V3O5 single crystals, by both temperature (T) and pressure (P) has been studied
by infrared spectroscopy. The T- and P-dependence of the optical conductivity
may be explained within a polaronic scenario. The insulating phase at ambient T
and P corresponds to strongly localized small polarons. Meanwhile the T-induced
metallic phase at ambient pressure is related to a liquid of polarons showing
incoherent dc transport, in the P-induced metallic phase at room T strongly
localized polarons coexist with partially delocalized ones. The electronic
spectral weight is almost recovered, in both the T and P induced metallization
processes, on an energy scale of 1 eV, thus supporting the key-role of
electron-lattice interaction in the V3O5 metal-to-insulator transition.Comment: 7 pages, 5 figure
Infrared phonon dynamics of multiferroic BiFeO3 single crystal
We discuss the first infrared reflectivity measurement on a BiFeO3 single
crystal between 5 K and room temperature. The 9 predicted ab-plane E phonon
modes are fully and unambiguously determined. The frequencies of the 4 A1
c-axis phonons are found. These results settle issues between theory and data
on ceramics. Our findings show that the softening of the lowest frequency E
mode is responsible for the temperature dependence of the dielectric constant,
indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.Comment: 5 pages (figures included
Vacuum Gas Tungsten Arc Welding
A two-year program investigated vacuum gas tungsten arc welding (VGTAW) as a method to modify or improve the weldability of normally difficult-to-weld materials. After a vacuum chamber and GTAW power supply were modified, several difficult-to-weld materials were studied and key parameters developed. Finally, Incoloy 903 weld overlays were produced without microfissures
Topology of amorphous tetrahedral semiconductors on intermediate lengthscales
Using the recently-proposed ``activation-relaxation technique'' for
optimizing complex structures, we develop a structural model appropriate to
a-GaAs which is almost free of odd-membered rings, i.e., wrong bonds, and
possesses an almost perfect coordination of four. The model is found to be
superior to structures obtained from much more computer-intensive tight-binding
or quantum molecular-dynamics simulations. For the elemental system a-Si, where
wrong bonds do not exist, the cost in elastic energy for removing odd-membered
rings is such that the traditional continuous-random network is appropriate.
Our study thus provides, for the first time, direct information on the nature
of intermediate-range topology in amorphous tetrahedral semiconductors.Comment: 4 pages, Latex and 2 postscript figure
Pressure-tuning of the c-f hybridization in Yb metal detected by infrared spectroscopy up to 18 GPa
It has been known that the elemental Yb, a divalent metal at mbient pressure,
becomes a mixed-valent metal under external pressure, with its valence reaching
~2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the
evolution of microscopic electronic states associated with the valence
crossover in Yb at external pressures up to 18 GPa. The measured infrared
reflectivity spectrum R(w) of Yb has shown large variations with pressure. In
particular, R(w) develops a deep minimum in the mid-infrared, which shifts to
lower energy with increasing pressure. The dip is attributed to optical
absorption due to a conduction c-f electron hybridization state, similarly to
those previously observed for heavy fermion compounds. The red shift of the dip
indicates that the - hybridization decreases with pressure, which is
consistent with the increase of valence.Comment: 2 pages, to appear in J. Phys. Soc. Jpn. Supp
Optical evidence for a spin-filter effect in the charge transport of
We have measured the optical reflectivity of
as a function of temperature between 1.5 and 300
and in external magnetic fields up to 7 . The slope at the onset of the
plasma edge feature in increases with decreasing temperature and
increasing field but the plasma edge itself does not exhibit the remarkable
blue shift that is observed in the binary compound . The analysis of
the magnetic field dependence of the low temperature optical conductivity
spectrum confirms the previously observed exponential decrease of the
electrical resistivity upon increasing, field-induced bulk magnetization at
constant temperature. In addition, the individual exponential magnetization
dependences of the plasma frequency and scattering rate are extracted from the
optical data.Comment: submitted to Phys. Rev. Let
Dynamics of Bulk vs. Nanoscale WS_2: Local Strain and Charging Effects
We measured the infrared vibrational properties of bulk and nanoparticle
WS in order to investigate the structure-property relations in these novel
materials. In addition to the symmetry-breaking effects of local strain,
nanoparticle curvature modifies the local charging environment of the bulk
material. Performing a charge analysis on the \emph{xy}-polarized E
vibrational mode, we find an approximate 1.5:1 intralayer charge difference
between the layered 2H material and inorganic fullerene-like (IF)
nanoparticles. This effective charge difference may impact the solid-state
lubrication properties of nanoscale metal dichalcogenides.Comment: 6 pages, 5 figure
Magneto-optical evidence of double exchange in a percolating lattice
Substituting by in ferromagnetic leads to a percolation
limited magnetic ordering. We present and discuss magneto-optical data of the
series, based on measurements of the reflectivity
from the far infrared up to the ultraviolet, as a function of
temperature and magnetic field. Via the Kramers-Kronig transformation of
we extract the complete absorption spectra of samples with
different values of . The change of the spectral weight in the Drude
component by increasing the magnetic field agrees with a scenario based on the
double exchange model, and suggests a crossover from a ferromagnetic metal to a
ferromagnetic Anderson insulator upon increasing -content at low
temperatures.Comment: 10 pages, 3 figure
Structural, electronic, and dynamical properties of amorphous gallium arsenide: a comparison between two topological models
We present a detailed study of the effect of local chemical ordering on the
structural, electronic, and dynamical properties of amorphous gallium arsenide.
Using the recently-proposed ``activation-relaxation technique'' and empirical
potentials, we have constructed two 216-atom tetrahedral continuous random
networks with different topological properties, which were further relaxed
using tight-binding molecular dynamics. The first network corresponds to the
traditional, amorphous, Polk-type, network, randomly decorated with Ga and As
atoms. The second is an amorphous structure with a minimum of wrong (homopolar)
bonds, and therefore a minimum of odd-membered atomic rings, and thus
corresponds to the Connell-Temkin model. By comparing the structural,
electronic, and dynamical properties of these two models, we show that the
Connell-Temkin network is energetically favored over Polk, but that most
properties are little affected by the differences in topology. We conclude that
most indirect experimental evidence for the presence (or absence) of wrong
bonds is much weaker than previously believed and that only direct structural
measurements, i.e., of such quantities as partial radial distribution
functions, can provide quantitative information on these defects in a-GaAs.Comment: 10 pages, 7 ps figures with eps
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