Sub-gap conductance at a large area junction with a rough interface of a ferromagnet and a high-T$_{C}$ superconductor is superimposed by multiple peaks which is not expected from an ideal point contact Andreev reflection process. We demonstrate this
phenomenon by measuring resistance as a function of bias voltage of a Co/Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-delta}$ junction with contact area 50 x 70 $mu$ $m^{2}$ at various temperatures. In order to analyze such Andreev reflection data, the interface is assumed to have random potentials which can create local electric fields. The Blonder-Tinkham-Klapwijk theory is modified with the inclusion of a broadening parameter due to finite life time effects of quasi particles. An additional voltage drop due to local electric fields at the rough interface has been included in terms of an extra energy shift which may be related to the asymmetry of normalized resistance data. Spin polarization has been introduced for the ferromagnet. The presented model explains the multi-peak nature and asymmetry of Andreev reflection data experimentally observed at large area junctions. Extension of the model also interprets the experimentally observed anomalous enhancement of resistance peaks in the sub-gap region which may result from crossing the critical current limit across the junction.
We report temperature dependent Andreev reflection measurements of Co/ Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-delta}$ (YBCO) heterostructure samples with junction areas of 1 $mu$m diameter. Modelling of the 5-70 K conductivity data according to a modified Blon
der-Tinkham-Klapwijk theory yields a spin polarization in Co film amounting to 34% which is almost constant up to 70 K. The YBCO films have been grown by pulsed laser deposition on sapphire substrates. The Co films are deposited by thermal evaporation on YBCO. The film is characterized by powder X-ray diffraction measurements which shows YBCO is grown in (001) direction.The critical current density, 5 x 10$^{6}$ A/cm$^{2}$, in YBCO remains nearly constant after deposition of Co at zero field and 77 K.
Electrical transport and specific heat properties of Nd_{1-x}Pb_{x}MnO_{3} single crystals for 0.15 < x 0.5 have been studied in low temperature regime. The resistivity in the ferromagnetic insulating (FMI) phase for x < 0.3 has an activated characte
r. The dependence of the activation gap Delta on doping x has been determined and the critical concentration for the zero-temperature metal-insulator transition was determined as x_{c} ~ 0.33. For a metallic sample with x=0.42, a conventional electron-electron (e-e) scattering term proportional T^{2} is found in the low-temperature electrical resistivity, although the Kadowaki-Woods ratio is found to be much larger for this manganite than for a normal metal. For a metallic sample with x=0.5, a resistivity minimum is observed for x= 0.5. The effect is attributed to weak localization and can be described by a negative T^{1/2} weak-localization contribution to resistivity for a disordered three-dimensional electron system. The specific heat data have been fitted to contributions from free electrons (gamma), spin excitations (beta_{3/2}), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The value of gamma is larger than for normal metals, which is ascribed to magnetic ordering effects involving Nd. Also, the Schottky-like anomaly appears broadened and weakened suggesting inhomogeneous molecular fields at the Nd-sites.
