Distribution Function in Nonequilibrium States of a Superconductor

We report the first measurements of the distribution function $f$ of quasiparticles in a nonequilibrium superconductor. For a wide range of quasiparticle-injection energies, $f\left(E\right)\mathrm{}\simeq {[1+\exp (\frac{(E-{\mu }^{*})}{k{T}^{*}}\left)\right]}^{-1\mathrm{}}$, where ${\mu }^{*}$ and $T^{*}$ depend on the injection energy. Novel results include a negative ${\mu }^{*}$, a $T^{*}$ greater than the transition temperature, and a totally unanticipated bump in $f$ at an energy of $3\Delta$, where $\Delta$ is the energy gap. This last result shows that detailed balance is violated in nonequilibrium states.