Temporal relaxation of photoinduced nonequilibrium in niobium

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Abstract
Transient photoimpedance responses (TPR) of niobium thin films are examined in the normal, transition, and superconducting states. The niobium sample’s impedance is modulated by the photoabsorption of 300-fs, ∼2-eV laser pulses with 0.002–1 μJ fluence. The samples, biased with a dc current, exhibit transient voltage signals corresponding to the photoinduced impedance transient. The TPR-signal amplitude and temporal dependence are analyzed in terms of the electron-photon, electron-phonon, electron-electron interactions and phonon trapping. Data interpretation is facilitated by comparing measurements of the TPR signal in all three states. Typically, the normal- and transition-state TPR signals are bolometric and are a manifestation of resistive impedance changes. At low laser fluences and in the zero-resistance superconducting state, the TPR signal is primarily due to kinetic inductance changes produced by the nonequilibrium quasiparticle generation and recombination processes strongly influenced by phonon trapping in the sample. At higher fluence, the niobium samples undergo temporary phase transitions that lead to more complicated TPR-signal responses, which we also analyze.