Electrical response of superconduciing YBs2CU307_& to light

peratures above the critical temperature Tc. At such tem­ peratures any exotic mechanisms are presumably absent, so only the bolometric effect is present. Dividing the voltage by the temperature coefficient of the normal-state voltage gives the temperature rise. If one is careful to keep the illumina­ tion geometry and power level identical, a similar temper­ ture rise will be produced in the illuminated superconductor at a temperature just below Te. (The lattice thermal proper­ ties vary slowly, and the electronic contribution to the ther­ mal properties is a small fractional effect near lOOK.) The assumption of similar thermal properties is only correct for a small range of temperatures around Te. however, since for large fractional temperature changes significant changes in the thermal conductivity and heat capacity of the film and substrate will occur. Thus this technique can be confidently applied only to films with a sharp transition to fully super­ conducting behavior. Broader transitions 2 make evaluation of the possible bolometric contribution more difficult.