Effect of electron-beam irradiation on superconducting films

Scanning electron microscopy performed at cryogenic conditions can be used for the two-dimensional display of various properties of superconducting thin-film samples. The Rothwarf-Taylor phenomenological equations are used to treat the spatial and temporal decay of the perturbation generated by the electron-beam irradiation. In the limit of high phonon trapping in the superconducting film, the spatial extension of the perturbed area determining the resolution of this imaging method is given by the thermal healing length. In the case of low phonon trapping, however, the healing length is given by the quasiparticle diffusion length. Also calculated is the characteristic time scale of the temporal decay after switching off the electron-beam irradiation. Modulating the electron beam at frequencies high compared to the inverse of this characteristic time can reduce the healing length considerably below the low-frequency value.