In this work we present recent experimental studies of heterostructures composed of ferromagnetic La_{0.67}Sr_{0.33}MnO₃ (LSMO) and superconducting YBa₂Cu₃O₇ (YBCO) multilayers grown by dc sputtering method. The transport and magnetic properties of the large set of heterostructures were investigated in a wide temperature range from 2 to 400 K and in fields up to 12.5 T. The resistivity and the upper critical field were measured for samples with the plane active dimensions 1.5× 5 mm². The Nernst effect was studied in the mixed state revealing the characteristic temperature dependence of the Nernst signal which was correlated with variation of the upper critical field. We have observed an influence of the magnetic LSMO layer on the superconducting properties of the adjacent YBCO layer which consists in a substantial reduction of the superconducting critical temperature and a small lowering of the upper critical field.
The thermopower of the junction between normal conductor and s-wave superconductor has been investigated. For this purpose we have analyzed in detail a simple generalization of the Blonder-Tinkham-Klapwijk theory by taking into account explicitly an energy dependence of the density of states near the Fermi level. Both linear and nonlinear thermopowers have been calculated for 3D free electron gas, 3D Fermi liquid, and the case with Van Hove singularity in the vicinity of the Fermi level. In the linear regime, for all models, the thermopower as function of temperature has a clear maximum with its position and the value depending strongly on the junction barrier strength. In the nonlinear regime, we have found very large values of the thermopower (up to 8 k_{B}/e) and strongly asymmetric behavior with respect to the change of the temperature gradient sign.
The thermoelectric power (S) of M_3C_{60} (M = K, Rb) alkali intercalated fullerides is theoretically investigated by considering the Mott expression within parabolic band approximation to reveal the electron diffusive thermoelectric power (S_{c}^{diff}). We follow the Fermi energy as electron parameter and S_{c}^{diff} discerned linear temperature dependence. S infers a change in slope above transition temperature and becomes almost linear above 70 K for M_3C_{60} alkali intercalated fullerides. As a next step, the phonon drag thermoelectric power (S_{ph}^{drag}) is computed within relaxation time approximation when thermoelectric power is limited by scattering of phonons from defects, grain boundaries, phonons and electrons as carriers. It is noticed that the S_{ph}^{drag} of K_3C_{60} is anomalous and it is an artifact of strong phonon-electron and -phonon scattering mechanism. The thermoelectric power within relaxation time approximation has been taken into account ignoring a possible energy dependence of the scattering rates. Behaviour of S(T) is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between carrier diffusion and phonon drag contributions in M_3C_{60} (M = K, Rb) alkali intercalated fullerides.
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