Non-Equilibrium Superconductivity in Aluminium Tunnel Junctions by Self-Injection and Millimeter Wave Radiation

We have studied arrays of superconducting tunnel junctions made of aluminium, with and without an applied high frequency field. The radiation frequency was around 73 GHz, which corresponded to 85% of the energy gap frequency. For sufficiently high critical current densities ( j c ⩾ 20 A/cm 2 ) these A1 junctions exhibited non-equilibrium effects. This was manifested in a heavy back-bending of the current-voltage characteristic in the gap region. The gap suppression could be up to 16% of the energy gap. A back-bending of the curve could also be observed at the onset of the photon induced step. From the gap reduction we could estimate an effective quasi-particle relaxation time to about 650 ns at an assessed film temperature of 0.48K. This can be compared to a theoretical value of 1400 ns at the same temperature. A kink in the current-voltage characteristic was seen at an injection rate of the order of the value predicted from the T * model of a non-equilibrium superconductor. A strong r.f. field caused pair breaking and gap reduction. Smaller r.f. powers, however, gave a gap enhancement and we observed up to about 5% increases of the gap as compared to the suppressed value caused by the tunnel current injection without mm waves.