Tunnel studies of niobium films containing excess quasiparticles

The instability of the nonequilibrium state in superconducting niobium films is studied for wide-band (eV>3δ) and narrow-band (δ>>eV-3δ) sources of quasiparticles injected into the film. For a wide-band source, the critical instability current I/sub i/ was found to vary anomalously with the helium bath temperature. The pulse technique was used to measure the local heating temperature T/sub l/ in the neighborhood of a tunnel contact. The variation of I/sub i/ with T/sub l/ is not anomalous and is consistent with the theoretical temperature dependence #betta#/sub c/(T) of the critical power [V. F. Elesin, V. E. Kondrashov, and A. S. Sukhikh, Sov. Phys. Solid State 21, 1861 (1979)]. Instability is found for a narrow-band source for the contacts having the lowest resistance. The instability current in this case decreases with falling temperature, in qualitative agreement with the theory of threshold instability in a superconductor