Quasiparticle energy relaxation in the cuprate superconductors

At high flux-flow velocities in the mixed state of a type-II superconductor the nonequilibrium distribution of the quasiparticles is shifted to higher energies. As shown theoretically by Larkin and Ovchinnikov, a distinct signature of this nonequilibrium effect is an electronic instability, resulting in a sharp kink in the current-voltage characteristic at a critical vortex velocity nu(phi)*. From measurements of nu(phi)* the quasiparticle-energy relaxation rate tau(epsilon)(-1) can be found. We have measured this instability point for epitaxial c axis oriented films of La1.85Sr0.15CuO4-x and YBa2CU3O7-delta. In both cuprates the observed temperature dependence of r, well fitted with the function tau(epsilon)(-1) = a exp[-2 Delta(T)/k(B)T], suggesting that the electron-electron recombination process, perhaps in combination with the emission of another excitation, is crucial. As an example of a low-T-c, superconductor we have also studied amorphous Mo3Si films and found that tau(epsilon)(-1) can be fitted by a function similar to that we have used for the cuprates, again indicating the importance of the electron-electron interaction.