Measurement of Microwave-Enhanced Energy Gap in Superconducting Aluminum by Tunneling

Al-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$-Al tunnel junctions were used to measure large increases in the energy gap of superconducting aluminum films in the presence of 10-GHz microwave radiation. When the microwave power was increased at constant temperature, enchancement occurred only for temperatures at which twice the equilibrium energy gap exceeded the photon energy. When the temperature was increased at constant microwave power, enhancement was observed at higher temperatures.