Superconductivity

In previous work on quantum phase transition in granular superconductors, where mean-field theory was used, an assumption was made that the order parameter as a function of the mean field is a convex up function. Though this is not always the case in phase transitions, this assumption must be verified, what is done in this article.

We report on measurements of the angular dependence of the irreversibility temperature T irr (θ) in YB a 2 C u 3 O 7−δ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of T irr (θ) we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.

Spontaneous nucleation and the consequent penetration of vortices into thin superconducting films and wires, subjected to a magnetic field, can be considered as a nonlinear stage of primary instability of the current-carrying superconducting state. The development of the instability leads to the formation of a chain of vortices in strips and helicoidal vortex lines in wires. The boundary of instability was obtained analytically. The nonlinear stage was investigated by simulations of the time-dependent generalized Ginzburg-Landau equation.

We use the three-dimensional Josephson junction array system as a model for studying the temperature dependence of the c-axis resistivity of high temperature superconductors, in the presence of an external magnetic field H applied in the c-direction. We show that the temperature at which the dissipation becomes different from zero corresponds to a percolation transition of the vortex lattice. In addition, the qualitative features of the resistivity vs. temperature curves close to the transition are obtained starting from the geometrical configurations of the vortices. The results apply to the cases H greater than 0 and H=0.

We calculate the effective charge for multimagnon infrared (IR) absorption assisted by phonons in the parent insulating compounds of cuprate superconductors and the spectra for two-magnon absorption using interacting spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys. Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a two-magnon virtual bound state, and one phonon plus higher multimagnon absorption processes. The virtual bound state consists of a narrow resonance occurring when the magnon pair has total momentum close to (π,0) .

Local magnetic relaxation measurements in YBa 2 Cu 3 O 7−x crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy U pl , which decreases with field, drops below the elastic (collective) creep activation energy, U el , which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa 2 Cu 3 O 7−x .

Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both n -type and p -type. In Nd 2 CuO 4−y these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at ∼ 1000 cm −1 . The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm −1 . Most of the above infrared features are shown to survive in the metallic phase of Nd 2−x Ce x Cu0 4−y , Bi 2 Sr 2 CuO 6 , and YBa 2 Cu 3 O 7−y , where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results.

It is shown that, within a Ginzburg-Landau (GL) formalism, the superconducting fluctuation is insulating at zero temperature even if the fluctuation dynamics is metallic (dissipative). Based on this fact, the low temperature behavior of the H c2 -line and the resistivity curves near a zero temperature transition are discussed. In particular, it is pointed out that the neglect of quantum fluctuations in data analysis of the dc resistivity may lead to an under-estimation of the H c2 values near zero temperature.

A renormalization group theory for a system consisting of coupled superconducting layers as a model for typical high-temperature superconducters is developed. In a first step the electromagnetic interaction over infinitely many layers is taken into account, but the Josephson coupling is neglected. In this case the corrections to two-dimensional behavior due to the presence of the other layers are very small. Next, renormalization group equations for a layered system with very strong Josephson coupling are derived, taking into account only the smallest possible Josephson vortex loops. The applicability of these two limiting cases to typical high-temperature superconductors is discussed. Finally, it is argued that the original renormalization group approach by Kosterlitz is not applicable to a layered system with intermediate Josephson coupling.

We report the first energy loss spectrum obtained with a geometrically metastable type I superconducting tin strip irradiated by the beta-emission of S-35. (Nucl. Instr. Meth. A, in press)