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Sub-gap conductance at a large area junction with a rough interface of aferromagnet and a high-T$_{C}$ superconductor is superimposed by multiple peakswhich is not expected from an ideal point contact Andreev reflection process.We demonstrate this phenomenon by measuring resistance as a function of biasvoltage of a Co/Y$_{1}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ junction with contactarea 50 x 70 $\mu$ $m^{2}$ at various temperatures. In order to analyze suchAndreev reflection data, the interface is assumed to have random potentialswhich can create local electric fields. The Blonder-Tinkham-Klapwijk theory ismodified with the inclusion of a broadening parameter due to finite life timeeffects of quasi particles. An additional voltage drop due to local electricfields at the rough interface has been included in terms of an extra energyshift which may be related to the asymmetry of normalized resistance data. Spinpolarization has been introduced for the ferromagnet. The presented modelexplains the multi-peak nature and asymmetry of Andreev reflection dataexperimentally observed at large area junctions. Extension of the model alsointerprets the experimentally observed anomalous enhancement of resistancepeaks in the sub-gap region which may result from crossing the critical currentlimit across the junction.

Andreev reflections at large ferromagnet/high-T_C superconductor area junctions with rough interface