Quasiparticle contribution to heat carriers relaxation time in DyBa2_2Cu3_3O7−x_{7-x} from heat diffusivity measurements

It is shown that the controversy on phonons or electrons being the most influenced heat carriers below the critical temperature of high-T$_c$ superconductors can be resolved. Electrical and thermal properties of the same DyBa$_2$Cu$_3$O$_{7-x}$ monodomain have been measured for two highly different oxygenation levels. While the oxygenated sample DyBa$_2$Cu$_3$O$_{7}$ has very good superconducting properties ($T_c=90$ K), the DyBa$_2$Cu$_3$O$_{6.3}$ sample exhibits an insulator behavior. A careful comparison between measurements of the {\bf thermal diffusivity} of both samples allows us to extract the electronic contribution. This contribution to the relaxation time of heat carriers is shown to be large below $T_c$ and more sensitive to the superconducting state than the phonon contribution.Comment: 13 pages, 6 figure

Influence of a low magnetic field on the thermal diffusivity of Bi-2212

The thermal diffusivity of a Bi-2212 polycrystalline sample has been measured under a 1T magnetic field applied perpendicularly to the heat flux. The magnetic contribution to the heat carrier mean free path has been extracted and is found to behave as a simple power law. This behavior can be attributed to a percolation process of electrons in the vortex lattice created by the magnetic field.Comment: 10 pages, 3 figures; to be published in Phys. Rev.

The Flux-Line Lattice in Superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.Comment: Review paper for Rep.Prog.Phys., 124 narrow pages. The 30 figures do not exist as postscript file

A.C. susceptibility and magnetization of high-Tc superconductors: Critical state model for the intergranular region

A.C. susceptibility and magnetization measurements on a sintered YBa2Cu3O7 12y sample are presented. The measured magnetization is separated into two terms: one depending on the grains, the other on the intergranular region magnetization. For intergranular region we intend a desordered array of holes surrounded by a shell whose thickness is \u3bb at the surface of the grains connected through Josephson junctions. At these holes the fluxoids are pinned. In presence of a fluxoid gradient, a net macroscopic magnetization current results from the sum of all the microscopic components, as in the traditional type II superconductors. The contribution of the intergranular region to the magnetization is well explained in terms of the critical state model, if a critical current density strongly dependent on the magnetic field is assumed

Thermoelectric and thermomagnetic effects in the mixed state analysis of the thermal angle

In this paper thermoelectric and thermomagnetic coefficients in the mixed state are discussed. Besides the contribution of the Abrikosov vortices, also the role of the vortex-antivortex pairs and of the quasi-particles unbounded and localized in the vortex core is considered. The vortex-antivortex contribution to the Seebeck and Nernst effects is calculated in detail. To study the role of normal excitation we propose to measure the thermal angle, alpha(th) that represents the direction of the vortex velocity. In fact, taking the unbounded quasiparticles into account, or not, as the temperature varies, alpha(th) shows an important difference in behavior, easy to detect experimentally. Measurements of the Seebeck effect and Nernst effect on a Bi(2223) tape are presented and the thermal-angle behavior is discussed. The experimental data are in agreement with the models that consider the unbounded quasiparticles contribution, but some caution must be taken as to the role of the Josephson vortices

Supercurrent lengthscale in sintered YBCO and critical state model.

Magnetization measurements on sintered YBa2Cu3O7 12y in two geometries (slab and cylinder) show the coexistence of two transport mechanisms: intergranular and intragranular currents. Both mechanisms can be explained in the framework of the critical state model. Using a simple law for the critical current versus magnetic field dependence, it is possible to calculate the magnetization and the in-phase and out-of-phase AC susceptibility and the agreement with the experimental data is good. Magnetization measurements on tubular samples have been performed too; these measurements, together with \u3c7AC measurements allow to determine the dimensions of magnetized regions

Fully automated apparatus for thermal diffusivity measurements on HTSC in high magnetic field

This paper describes an apparatus to measure the thermal diffusivity of high temperature superconductors in an extended range of temperature (10-300 K) and magnetic field (up to 8 T). Heat losses are taken into account by adopting an ac technique. Effects introduced by power dissipated on the heater and thermal contacts between thermocouples and sample are considered. In fact the presence of a thermocouple relaxation time not negligible in respect to the sample relaxation time causes an error in the thermal diffusivity evaluation (up to 13% in a sample with poor thermal contact). We have estimated that, also at the higher temperatures, frequencies over 0.03 Hz keep the thermal losses low enough; in this case the poor thermal contact of the thermocouples can be a major problem. All of the experiment is computer controlled. The accuracy of the measurement, mainly determined by evaluation of the sample length, is 10%, while the sensitivity to the relative variations is much higher (0.1%)

Magnetization decay and pinning energy in a BSCCO single crystal: a comparison between different methods of measurement

The magnetization decay in a BSCCO single crystal was measured by two different methods: in the first way the decay is observed in the time after stopping the magnetic field, in the second one various hysteresis loops are performed at different field sweep rate. Using this two methods of measurements we investigate both flux creep regime both the thermally assisted flux flow regime and, in the creep regime, we calculated the energy pinning Uo. The two methods give different values: an explanation for this discrepancy is proposed

A new technique to obtain a fast thermocouple sensor for thermal diffusivity measurements in an extended temperature range

An improved ac method to measure the thermal diffusivity in an extended temperature range is presented. The basic idea is to solder thermocouple wires directly on the sample, so that the sample itself acts as a thermocouple junction and the relaxation time for the temperature measurement becomes negligible. This new technique allows one to measure smaller samples and to increase the frequency of measurement so that the thermal losses could in principle be completely neglected. Finally, by the particular geometry a concurrent measurement of the electrical resistivity is obtained