226 research outputs found
Phonon dispersion of Mo-stabilized γ-U measured using inelastic x-ray scattering.
We have measured the room-temperature phonon spectrum of Mo-stabilized γ-U. The dispersion curves show unusual softening near the H point, q = [1/2, 1/2, 1/2], which may derive from the metastability of the γ-U phase or from strong electron-phonon coupling. Near the zone center, the dispersion curves agree well with theory, though significant differences are observed away from the zone center. The experimental phonon density of states is shifted to higher energy compared to theory and high-temperature neutron scattering. The elastic constants of γ-UMo are similar to those of body-centered cubic elemental metals
Quantization of Superflow Circulation and Magnetic Flux with a Tunable Offset
Quantization of superflow-circulation and of magnetic-flux are considered for
systems, such as superfluid He-A and unconventional superconductors, having
nonscalar order parameters. The circulation is shown to be the anholonomy in
the parallel transport of the order parameter. For multiply-connected samples
free of distributed vorticity, circulation and flux are predicted to be
quantized, but generically to nonintegral values that are tunably offset from
integers. This amounts to a version of Aharonov-Bohm physics. Experimental
settings for testing these issues are discussed.Comment: 5 two-column pages, ReVTeX, figure available upon request (to
[email protected]
On the Theory of Metal-Insulator Transitions in Gated Semiconductors
It is shown that recent experiments indicating a metal-insulator transition
in 2D electron systems can be interpreted in terms of a simple model, in which
the resistivity is controlled by scattering at charged hole traps located in
the oxide layer. The gate voltage changes the number of charged traps which
results in a sharp change in the resistivity. The observed exponential
temperature dependence of the resistivity in the metallic phase of the
transition follows from the temperature dependence of the trap occupation
number. The model naturally describes the experimentally observed scaling
properties of the transition and effects of magnetic and electric fields.Comment: 4 two-column pages, 4 figures (included in the text
Pressure dependent intermediate valence behavior in YbNiGa and YbNiIn
We report a comprehensive structural and valence study of the intermediate
valent materials YbNiGa and YbNiIn under pressures up to 60 GPa.
YbNiGa undergoes a smooth volume contraction and shows steady increase in
Yb-valence with pressure, though the Yb-valence reaches saturation around 25
GPa. In YbNiIn, a change in pressure dependence of the volume and a peak
in Yb-valence suggest a pressure induced electronic topological transition
occurs around 10-14 GPa. In the pressure region where YbNiIn and
YbNiGa possess similar Yb-Yb spacings the Yb-valence reveals a
precipitous drop. This drop is not captured by density-functional-theory
calculations and implies that both the lattice degrees of freedom and the
chemical environment play an important role in establishing the valence of Yb
Theory of "ferrisuperconductivity" in
We construct a two component Ginzburg-Landau theory with coherent pair motion
and incoherent quasiparticles for the phase diagram of .
The two staggered superconducting states live at the Brillouin zone center and
the zone boundary, and coexist for temperatures at concentrations
. We predict below
appearance of a charge density wave (CDW) and Be-sublattice distortion. The
distortion explains the SR relaxation anomaly, and Th-impurity mediated
scattering of ultrasound to CDW fluctuations explains the attenuation peak.Comment: 4 pages, 4 eps figures, REVTe
Avalanche dynamics, surface roughening and self-organized criticality - experiments on a 3 dimensional pile of rice
We present a two-dimensional system which exhibits features of self-organized
criticality. The avalanches which occur on the surface of a pile of rice are
found to exhibit finite size scaling in their probability distribution. The
critical exponents are = 1.21(2) for the avalanche size distribution and
= 1.99(2) for the cut-off size. Furthermore the geometry of the avalanches
is studied leading to a fractal dimension of the active sites of =
1.58(2). Using a set of scaling relations, we can calculate the roughness
exponent = 0.41(3) and the dynamic exponent = 1.56(8). This result is compared with that obtained from a power
spectrum analysis of the surface roughness, which yields = 0.42(3) and
= 1.5(1) in excellent agreement with those obtained from the scaling
relations.Comment: 7 pages, 8 figures, accepted for publication in PR
Electron-Assisted Hopping in Two Dimensions
We have studied the non-ohmic effects in the conductivity of a
two-dimensional system which undergoes the crossover from weak to strong
localization with decreasing electron concentration. When the electrons are
removed from equilibrium with phonons, the hopping conductivity depends only on
the electron temperature. This indicates that the hopping transport in a system
with a large localization length is assisted by electron-electron interactions
rather than by the phonons.Comment: 5 pages, 4 figure
Pressure-induced suppression of ferromagnetism in the itinerant ferromagnet LaCrSb
We have performed an extensive pressure-dependent structural, spectroscopic,
and electrical transport study of LaCrSb. The ferromagnetic phase (T =
120 K at p = 0 GPa) is fully suppressed by p = 26.5 GPa and the Cr-moment
decreases steadily with increasing pressure. The unit cell volume decreases
smoothly up to p = 55 GPa. We find that the bulk modulus and suppression of the
magnetism are in good agreement with theoretical predictions, but the Cr-moment
decreases smoothly with pressure, in contrast to steplike drops predicted by
theory. The ferromagnetic ordering temperature appears to be driven by the
Cr-moment.Comment: 8 pages, 8 figure
Experiments in vortex avalanches
Avalanche dynamics is found in many phenomena spanning from earthquakes to
the evolution of species. It can be also found in vortex matter when a type II
superconductor is externally driven, for example, by increasing the magnetic
field. Vortex avalanches associated with thermal instabilities can be an
undesirable effect for applications, but "dynamically driven" avalanches
emerging from the competition between intervortex interactions and quenched
disorder constitute an interesting scenario to test theoretical ideas related
with non-equilibrium dynamics. However, differently from the equilibrium phases
of vortex matter in type II superconductors, the study of the corresponding
dynamical phases - in which avalanches can play a role - is still in its
infancy. In this paper we critically review relevant experiments performed in
the last decade or so, emphasizing the ability of different experimental
techniques to establish the nature and statistical properties of the observed
avalanche behavior.Comment: To be published in Reviews of Modern Physics April 2004. 17 page
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