H.-C. Ri

Seebeck and Nernst effect in the mixed state of slightly oxygen deficient YBaCuO

Abstract For an epitaxial c -axis oriented slightly oxygen deficient YBaCuO film, showing a positive Seebeck coefficient around 100 K, the Nernst effect has been measured in the mixed state for magnetic fields up to 9 T. The transport entropy calculated in the usual way is about an order of magnitude smaller than for a similar but fully oxygenated film with a negative Seebeck coefficient near 100 K. This can be explained by the enhanced decoupling of the CuO 2 planes in the slightly oxygen deficient material, resulting in a strong departure of the vortices (“pancake vortices”) from the standard Abrikosov type.

Flux-flow induced Nernst effect in superconducting YBaCuO films

Abstract The thermal force due to a temperature gradient results in a flux motion from the hot to the cold end of a superconductor and in the generation of the Nernst voltage VN perpendicular to the direction of the temperature gradient and magnetic field. Measurements of VN are expected to contribute significantly to the understanding of the physics of flux flow and flux pinning. We report on the first observation of the flux-flow induced Nernst effect in a high-Tc superconductor, using polycrystalline YBaCuO films.

Thermoelectric and thermomagnetic effects in the mixed state of HTSC's

Abstract Whereas flux line diffusion in the mixed state of HTSCs driven by the Lorentz force has been studied extensively, the thermal diffusion phenomena have received much less attention. In the mixed state, thermal diffusion of the quasiparticles and of the flux lines result in the Seebeck effect and the Nernst effect, respectively. Measurements of these effects yield the transport entropy of the diffusing species. We discuss the recent results obtained mostly for YBaCuO epitaxial films or single crystals, and emphasize the points where thermomagnetic experiments can discriminate between different models for dissipation.

Nernst, Seebeck, and hall effect in the mixed state of anisotropic superconductors — effect of vortex-antivortex excitations

Abstract In the mixed state of superconductors the motion of the vortex structure due to a temperature gradient generates the Nernst and Seebeck effect. The first is essentially the result of the thermal force acting directly on the vortex lines. The second effect is primarily due to the thermal diffusion of quasiparticles which is compensated by a backflow supercurrent. We show that the presence of free vortex-antivortex excitations does not contribute to the measured transverse Nernst electric field but to the longitudinal Seebeck electric field. The opposite is true for the respective Hall components.

Thermal diffusion of quasiparticles and flux quanta in the mixed state of high-temperature superconductors

Abstract In the mixed state of the copper-oxide superconductors the motion of the vortex structure due to a temperature gradient generates the Nernst and Seebeck effect. The first effect is essentially the result of the thermal force acting directly on the vortex lines. The second effect is primarily due to the thermal diffusion of quasiparticles which is compensated by a backflow supercurrent. In the mixed state both effects exist over the same temperature range over which the electric resistivity remains finite.

Nernst effect in high-Tc superconducting films

We report measurements of the Nernst Effect in polycrystalline and epitaxial YBaCuO films. Such studies represent an important complement to Lorentz force experiments, since the thermal force is not subjected to spatial redistribution due to inhomogeneous flux pinning or damping. The Nernst effect also appears interesting for probing the superconducting fluctuation regime.

Comparative study of the magnetoresistivity of YBa2Cu3O7−δ and Bi2Sr2Ca Cu2O8+x epitaxial thin films in the fluktuation regime

Abstract We measured the magnetoresistivity of high quality, c-axis oriented epitaxial YBa 2 Cu 3 O 7− δ and Bi 2 Sr 2 Ca Cu 2 O 8+ x (BSCCO) thin films in the fluctuation regime above the critical temperature. The measurements have been performed in magnetic fields up to 12 Tesla applied parallel and perpendicular to the CuO 2 -planes. The experimental data was analysed in the framework of the Lawrence-Doniach model with the appropriate clean limit expressions for the Aslamazov-Larkin- and Maki-Thompson- fluctuation conductivities. Orbital as well as Zeeman contributions to the magnetoconductivity have been taken into account. With the thermodynamic critical temperature T c , the coherence lengths parallel and perpendicular to the ab-plane ξ ab and ξ c , respectively, and the product of the mean free path l and the phase breaking time τ φ as fitting parameters, good agreement between experiment and theory was found for YBCO, whereas for BSCCO no convincing fit could be obtained. Fitting the data for YBCO we obtained T c = 90.2 K , ξ ab = 13.8 A , ξ c = 3.1 A , and (lτ gf ) 100 K ≅ 1.5 × 10 −22 sm . These values are close to those derived from measurements of the transport energy of magnetic flux lines via the Nernst effect at temperatures below T c . For BSCCO we obtained T c ≅ 90 K and ξ ab ≅ 24 to 30 A. The very small coherence length in c-direction ξ c ≤ 0.4 A reflects the quasi 2-dimensional nature of this material.

Thermal diffusion of quasiparticles and vortices in high-temperature superconductors

Abstract In the mixed state of high-temperature superconductors (HTSC) vortex motion under the influence of the Lorentz force of an applied transport or shielding current has been studied in a large number of experiments. Recently, an increasing amount of work has concentrated also on the transport effects due to the thermal force acting on the quasiparticles and vortices in an applied temperature gradient, resulting in the Nernst and Seebeck effect. We summarize the highlights of these latter experiments. In particular, we address the following subjects: origin of the Seebeck effect in the mixed state, transport entropy of the vortex structure in strongly anisotropic HTSCs, Hall contribution to the Nernst and Seebeck effect, and measurements in the fluctuation regime. From the temperature dependence of the transport energy of the vortices the slope ϑ H c2 /ϑ T near T c , the coherence length and the Ginzburg-Landau parameter can be determined.