6 research outputs found
Reduction of the Superfluid Density in the Vortex-Liquid Phase of Bi2Sr2CaCu2Oy
In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was
measured in the mixed state at 40.8 GHz.The surface reactance, which is
proportional to the real part of the effective penetration depth, increased
rapidly just above the first-order vortex-lattice melting transition field and
the second magnetization peak field.This increase is ascribed to the decrease
in the superfluid density rather than the loss of pinning.This result indicates
that the vortex melting transition changes the electronic structure as well as
the vortex structure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Experimental Study of the Inductance of Pinned Vortices in Superconducting YBa2Cu3O7-d Films
Using a two-coil mutual inductance method, we have measured the complex
resistivity, rho_v(T,Be), of pinned vortices in c-axis pulsed laser deposited
YBa2Cu3O7-d films with magnetic field Be applied perpendicular to the film. At
low frequencies, (<100 kHz), rho_v is inductive and is inversely proportional
to the Labusch parameter, the average vortex pinning force constant, kappa_exp.
The observed weakening of kappa_exp with Be is consistent with a simple model
based on linear pinning defects. Adding classical thermal fluctuations to the
model in a simple way describes the observed linear T dependence of rho_v,
below ~15 K and provides reasonable values for the effective radius (.3 nm to
>.8 nm) of the defects and the depth of the pinning potential. The success of
this model implies that thermal supercurrent (phase) fluctuations have their
full classical amplitude down to 5 K for frequencies below the characteristic
depinning frequency. To date, no sufficient theory exists to explain the data
between ~15 K and the vortex glass melting temperature.Comment: 31 pages, 8 figures. Subm. to PR
Coherence lengths and anisotropy in MgB2 superconductor
Field and temperature microwave measurements have been carried out on MgB2
thin film grown on Al2O3 substrate. The analysis reveals the mean field
coherence length xi_{MF} in the mixed state and a temperature independent
anisotropy ratio gamma_{MF} = xi_{MF}^{ab} / xi_{MF}^c approximately 2. At the
superconducting transition, the scaling of the fluctuation conductivity yields
the Ginzburg-Landau coherence length with a different anisotropy ratio
gamma_{GL} = 2.8, also temperature independent.Comment: submitted to PR
Critical State Flux Penetration and Linear Microwave Vortex Response in YBa_2Cu_3O_{7-x} Films
The vortex contribution to the dc field (H) dependent microwave surface
impedance Z_s = R_s+iX_s of YBa_2Cu_3O_{7-x} thin films was measured using
suspended patterned resonators. Z_s(H) is shown to be a direct measure of the
flux density B(H) enabling a very precise test of models of flux penetration.
Three regimes of field-dependent behavior were observed: (1) Initial flux
penetration occurs on very low field scales H_i(4.2K) 100Oe, (2) At moderate
fields the flux penetration into the virgin state is in excellent agreement
with calculations based upon the field-induced Bean critical state for thin
film geometry, parametrized by a field scale H_s(4.2K) J_c*d 0.5T, (3) for very
high fields H >>H_s, the flux density is uniform and the measurements enable
direct determination of vortex parameters such as pinning force constants
\alpha_p and vortex viscosity \eta. However hysteresis loops are in
disagreement with the thin film Bean model, and instead are governed by the low
field scale H_i, rather than by H_s. Geometric barriers are insufficient to
account for the observed results.Comment: 20 pages, LaTeX type, Uses REVTeX style files, Submitted to Physical
Review B, 600 dpi PostScript file with high resolution figures available at
http://sagar.physics.neu.edu/preprints.htm
Peak effect in a superconducting DyBa2Cu3O7-y film at microwave frequencies
We report the observation of a peak in the microwave (9.55 GHz) surface
resistance in an epitaxial DyBa2Cu3O7-y superconducting film in magnetic fields
(parallel to the c axis) ranging between 0.2 to 0.9 Tesla. Such a peak is
absent in the measurements done in zero-field. The temperature and field
dependence of the peak suggests that this peak could be associated with the
peak effect phenomenon reflecting the order-disorder transformation in the
flux-line lattice. A strong dependence of this peak effect at frequencies close
to the depinning frequency of the flux line lattice is observed.Comment: 1 text, 4 figures (all postscript) to be published in Phys. Rev.