Nature of the granular structure in low-temperature helium-irradiated YBaCuO thin films

We report low-temperature irradiation experiments with helium ions (190 keV) on thin YBa2Cu3O7 films 500 nm thick deposited on MgO (100 orientation) substrates. The temperature dependence of the resistance was measured in situ. We analyze the relation between the irradiation-induced damage, the increase in the normal resistance and the decrease of the critical superconducting temperature. Our analysis includes the result of a recent transmission electron microscopy study demonstrating the existence of an irradiation-induced orthorhombic to tetragonal transition. The change of the shape of the transition curve under irradiation is interpreted in terms of the superconductivity of inhomogeneous media. © 1989.