149 research outputs found
Fluctuation conductivity in superconductors in strong electric fields
We study the effect of a strong electric field on the fluctuation
conductivity within the time-dependent Ginzburg-Landau theory for the case of
arbitrary dimension. Our results are based on the analytical derivation of the
velocity distribution law for the fluctuation Cooper pairs, from the Boltzmann
equation. Special attention is drawn to the case of small nonlinearity of
conductivity, which can be investigated experimentally. We obtain a general
relation between the nonlinear conductivity and the temperature derivative of
the linear Aslamazov-Larkin conductivity, applicable to any superconductor. For
the important case of layered superconductors we derive an analogous relation
between the small nonlinear correction for the conductivity and the
fluctuational magnetoconductivity. On the basis of these relations we provide
new experimental methods for determining both the lifetime constant of
metastable Cooper pairs above T_c and the coherence length. A systematic
investigation of the 3rd harmonic of the electric field generated by a harmonic
current can serve as an alternative method for the examination of the
metastable Cooper-pair relaxation time.Comment: 18 pages, REVTeX, submitted to Phys. Rev.
Mode-locking in ac-driven vortex lattices with random pinning
We find mode-locking steps in simulated current-voltage characteristics of
ac-driven vortex lattices with {\it random} pinning. For low frequencies there
is mode-locking above a finite ac force amplitude, while for large frequencies
there is mode-locking for any small ac force. This is correlated with the
nature of temporal order in the different regimes in the absence of ac drive.
The mode-locked state is a frozen solid pinned in the moving reference of
frame, and the depinning from the step shows plastic flow and hysteresis.Comment: 4 pages, 4 figure
Fluctuation-dissipation theorem and flux noise in overdamped Josephson junction arrays
The form of the fluctuation-dissipation theorem for a resistively shunted
Josephson juction array is derived with the help of the method which
explicitely takes into acoount screening effects. This result is used to
express the flux noise power spectrum in terms of frequency dependent sheet
impedance of the array. The relation between noise amplitude and parameters of
the detection coil is analysed for the simplest case of a single-loop coil.Comment: ReVTeX, 8 page
Photothermal Activation of Shallow Dopants Implanted in Silicon
Dopant impurities were implanted at high dose and low energy (10 15 cm -2 , 0.5-2.2 keV) into double-side polished 200 mm diameter silicon wafers and electrically activated to form p-n junctions by 10 s anneals at temperatures of 1,025, 1,050, and 1,075°C by optical heating with tungsten incandescent lamps. Activation was studied for P, As, B, and BF 2 species implanted on one wafer side and for P and BF 2 implanted on both sides of the wafer. Measurements included electrical sheet resistance (Rs) and oxide film thickness. A heavily boron-doped wafer, which is optically opaque, was used as a hot shield to prevent direct exposure to lamp radiation on the adjacent side of the test wafer. Two wafers with opposing orientations with respect to the shield wafer were annealed for comparison of exposure to, or shielding from, direct lamp illumination. Differences in sheet resistance for the two wafer orientations ranged from 4% to 60%. n-Type dopants implanted in p-type wafers yielded higher Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been reduced. p-Type dopants implanted in ntype wafers yielded lower Rs when the implanted surface was exposed to the lamps, as though the effective temperature had been increased. Effective temperature differences larger than 5°C, which were observed for the P, B, and BF 2 implants, exceeded experimental uncertainty in temperature control
Mode-locking in driven vortex lattices with transverse ac-drive and random pinning
We find mode-locking steps in simulated current-voltage characteristics of
driven vortex lattices with {\it random} pinning when an applied ac-current is
{\it perpendicular} to the dc-current. For low frequencies there is
mode-locking only above a non-zero threshold ac force amplitude, while for
large frequencies there is mode-locking for any small ac force. This is
consistent with the nature of {\it transverse} temporal order in the different
regimes in the absence of an applied ac-drive. For large frequencies the
magnitude of the fundamental mode-locked step depends linearly with the ac
force amplitude.Comment: 4 pages, 4 figures, .tar.gz fil
Strong Pinning and Plastic Deformations of the Vortex Lattice
We investigate numerically the dynamically generated plastic deformations of
a 3D vortex lattice (VL) driven through a disorder potential with isolated,
strong pinning centers (point-like or extended along the field direction). We
find that the VL exhibits a very peculiar dynamical behavior in the plastic
flow regime, in particular, topological excitations consisting of three or four
entangled vortices are formed. We determine the critical current density
and the activation energy for depinning in the presence of a finite
density of strong pinning centers.Comment: 12 pages, TeX type, Postscript figure
Direct observation of the washboard noise of a driven vortex lattice in a high-temperature superconductor, Bi2Sr2CaCu2Oy
We studied the conduction noise spectrum in the vortex state of a
high-temperature superconductor, Bi2Sr2CaCu2Oy, subject to a uniform driving
force. Two characteristic features, a broadband noise (BBN) and a narrow-band
noise (NBN), were observed in the vortex-solid phase. The origin of the large
BBN was determined to be plastic motion of the vortices, whereas the NBN was
found to originate from the washboard modulation of the translational velocity
of the driven vortices. We believe this to be the first observation ofComment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Large Predicted Self-Field Critical Current Enhancements In Superconducting Strips Using Magnetic Screens
A transport current distribution over a wide superconducting sheet is shown
to strongly change in a presence of bulk magnetic screens of a soft magnet with
a high permeability. Depending on the geometry, the effect may drastically
suppress or protect the Meissner state of the sheet through the enhancement or
suppression of the edge barrier critical current. The total transport current
in the magnetically screened Meissner state is expected to compete with the
critical current of the flux-filled sheet only for samples whose critical
current is initially essentially controlled by the edge barrier effect.Comment: 6 figure
Mode locking of vortex matter driven through mesoscopic channels
We investigated the driven dynamics of vortices confined to mesoscopic flow
channels by means of a dc-rf interference technique. The observed mode-locking
steps in the -curves provide detailed information on how the number of rows
and lattice structure in the channel change with magnetic field. Minima in flow
stress occur when an integer number of rows is moving coherently, while maxima
appear when incoherent motion of mixed and row configurations is
predominant. Simulations show that the enhanced pinning at mismatch originates
from quasi-static fault zones with misoriented edge dislocations induced by
disorder in the channel edges.Comment: some minor changes were made, 4 pages, 4 figures, accepted for
publication in Phys. Rev. Let
Temperature and Frequency Dependence of Complex Conductance of Ultrathin YBa2Cu3O7-x Films: A Study of Vortex-Antivortex Pair Unbinding
We have studied the temperature dependencies of the complex sheet conductance
of 1-3 unit cell (UC) thick YBa2Cu3O7-x films sandwiched between semiconducting
Pr0.6Y0.4Ba2Cu3O7-x layers at high frequencies. Experiments have been carried
out in a frequency range between: 2 - 30 MHz with one-spiral coil technique,
100 MHz - 1 GHz frequency range with a new technique using the spiral coil
cavity and at 30 GHz by aid of a resonant cavity technique. The real and
imaginary parts of the mutual-inductance between a coil and a film were
measured and converted to complex conductivity by aid of the inversion
procedure. We have found a quadratic temperature dependence of the kinetic
inductance, L_k^-1(T), at low temperatures independent of frequency, with a
break in slope at T^dc_BKT, the maximum of real part of conductance and a large
shift of the break temperature and the maximum position to higher temperatures
with increasing frequency. We obtain from these data the universal ratio
T^dc_BKT/L_k^-1(T^dc_BKT) = 25, 25, and 17 nHK for 1-, 2- and 3UC films,
respectively in close agreement with theoretical prediction of 12 nHK for
vortex-antivortex unbinding transition. The activated temperature dependence of
the vortex diffusion constant was observed and discussed in the framework of
vortex-antivortex pair pinning.
PACS numbers: 74.80.Dm, 74.25.Nf, 74.72.Bk, 74.76.BzComment: PDF file, 10 pages, 6 figures, to be published in J. Low Temp. Phys.;
Proc. of NATO ARW: VORTEX 200
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