Abstract
Inhomogeneous energy-gap states in nonequilibrium Pb-In alloy films have been studied under high-energy injection of quasiparticles via tunneling. Two types of configuration of double tunnel junctions (Pb-In-Pb-In-Pb) have been used in the experiment: In the first configuration, ${T}_{\mathrm{I}}$, geometrical dimensions of both injectors and detectors of the junctions were changed precisely and quasicontinuously. The second configuration, ${T}_{\mathrm{II}}$, consisted of a long inline injector and very narrow multiple detectors. For samples with ${T}_{\mathrm{I}}$ configuration, coexistence of two different energy gaps was observed only in samples with dimensions above a critical sample size of about 25 \ifmmode\times\else\texttimes\fi{} 25 \ensuremath{\mu}${\mathrm{m}}^{2}$, while in samples with dimensions below the critical size a first-order transition from the superconducting state with a broad but reduced energy gap to the normal state was observed. The results indicate that the inhomogeneous energy-gap state possesses a critical characteristic dimension with a macroscopic size in nonequilibrium superconductors. Spatial structures of the inhomogeneous gap state were measured by using samples with ${T}_{\mathrm{II}}$ configuration. The spatial structure was found to be affected dominantly by the macroscopic sample boundary. These inhomogeneous gap states are discussed by current theories of nonequilibrium instabilities.
Published Version
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