29 research outputs found
Theory of a Higher Order Phase Transition: Superconducting Transition in BKBO
We describe here the properties expected of a higher (with emphasis on the
order fourth) order phase transition. The order is identified in the sense
first noted by Ehrenfest, namely in terms of the temperature dependence of the
ordered state free energy near the phase boundary. We have derived an equation
for the phase boundary in terms of the discontinuities in thermodynamic
observables, developed a Ginzburg-Landau free energy and studied the
thermodynamic and magnetic properties. We also discuss the current status of
experiments on and other based superconductors,
the expectations for parameters and examine alternative explanations of the
experimental results.Comment: 18 pages, no figure
Optically induced coherent intra-band dynamics in disordered semiconductors
On the basis of a tight-binding model for a strongly disordered semiconductor
with correlated conduction- and valence band disorder a new coherent dynamical
intra-band effect is analyzed. For systems that are excited by two, specially
designed ultrashort light-pulse sequences delayed by tau relatively to each
other echo-like phenomena are predicted to occur. In addition to the inter-band
photon echo which shows up at exactly t=2*tau relative to the first pulse, the
system responds with two spontaneous intra-band current pulses preceding and
following the appearance of the photon echo. The temporal splitting depends on
the electron-hole mass ratio. Calculating the population relaxation rate due to
Coulomb scattering, it is concluded that the predicted new dynamical effect
should be experimentally observable in an interacting and strongly disordered
system, such as the Quantum-Coulomb-Glass.Comment: to be published in Physical Review B15 February 200
Absence of Persistent Magnetic Oscillations in Type-II Superconductors
We report on a numerical study intended to examine the possibility that
magnetic oscillations persist in type II superconductors beyond the point where
the pairing self-energy exceeds the normal state Landau level separation. Our
work is based on the self-consistent numerical solution for model
superconductors of the Bogoliubov-deGennes equations for the vortex lattice
state. In the regime where the pairing self-energy is smaller than the
cyclotron energy, magnetic oscillations resulting from Landau level
quantization are suppressed by the broadening of quasiparticle Landau levels
due to the non-uniform order parameter of the vortex lattice state, and by
splittings of the quasiparticle bands. Plausible arguments that the latter
effect can lead to a sign change of the fundamental harmonic of the magnetic
oscillations when the pairing self-energy is comparable to the cyclotron energy
are shown to be flawed. Our calculations indicate that magnetic oscillations
are strongly suppressed once the pairing self-energy exceeds the Landau level
separation.Comment: 7 pages, revtex, 7 postscript figure
Electron Dephasing in Mesoscopic Metal Wires
The low-temperature behavior of the electron phase coherence time,
, in mesoscopic metal wires has been a subject of controversy
recently. Whereas theory predicts that in narrow wires should
increase as as the temperature is lowered, many samples exhibit
a saturation of below about 1 K. We review here the experiments
we have performed recently to address this issue. In particular we emphasize
that in sufficiently pure Ag and Au samples we observe no saturation of
down to our base temperature of 40 mK. In addition, the measured
magnitude of is in excellent quantitative agreement with the
prediction of the perturbative theory of Altshuler, Aronov and Khmelnitskii. We
discuss possible explanations why saturation of is observed in
many other samples measured in our laboratory and elsewhere, and answer the
criticisms raised recently by Mohanty and Webb regarding our work.Comment: 14 pages, 3 figures; to appear in proceedings of conference
"Fundamental Problems of Mesoscopic Physics", Granada, Spain, 6-11 September,
200
Superconducting fluctuation corrections to ultrasound attenuation in layered superconductors
We consider the temperature dependence of the sound attenuation and sound
velocity in layered impure metals due to superconducting fluctuations of the
order parameter above the critical temperature. We obtain the dependence on
material properties of these fluctuation corrections in the hydrodynamic limit,
where the electron mean free path is much smaller than the wavelength of sound
and where the electron collision rate is much larger than the sound frequency.
For longitudinal sound propagating perpendicular to the layers, the open Fermi
surface condition leads to a suppression of the divergent contributions to
leading order, in contrast with the case of paraconductivity. The leading
temperature dependent corrections, given by the Aslamazov-Larkin, Maki-Thompson
and density of states terms, remain finite as T->Tc. Nevertheless, the
sensitivity of new ultrasonic experiments on layered organic conductors should
make these fluctuations effects measurable.Comment: 13 pages, 6 figures. Accepted for PRB. Added discussion on incoherent
interlayer tunneling and other small modifications suggested by referee
Frequency behavior of Raman coupling coefficient in glasses
Low-frequency Raman coupling coefficient of 11 different glasses is
evaluated. It is found that the coupling coefficient demonstrates a universal
linear frequency behavior near the boson peak maximum and a superlinear
behavior at very low frequencies. The last observation suggests vanishing of
the coupling coefficient when frequency tends to zero. The results are
discussed in terms of the vibration wavefunction that combines features of
localized and extended modes.Comment: 8 pages, 9 figure