PYROELECTRIC VOLTAGES IN YBCO THIN-FILMS

The pyroelectric voltage V-p observed earlier only in YBCO single crys tals was revealed in thin films of this material. The temperature beha viour of V-p was found to exhibit such specific features as the peak j ust above T-c and the sign reversal at a higher temperature implying a sharp change in the spontaneous polarization P-s near T-c. Applicatio n of an external magnetic field with the aim of clarifying a possible role of the superconducting transition in the P-s discontinuity showed a surprisingly strong effect on the pyroelectric peak involving a pro nounced decrease in its height. At the same time, the expected shift o f the peak position toward low temperatures is not observed. The tempe rature dependences of P-s considered simultaneously with data on behav iour of the V-p peak in magnetic field point to the preservation of P- s below T-c in the regions responsible for the pyroelectric effect. Th e origin of the P-s(T) discontinuity can be attributed to a structural instability of the YBCO near T-c. (C) 1997 Published by Elsevier Scie nce Ltd