Influence of He Ion bombardment at medium energies on the vortex instability at high current densities observed in YBaCuO films

Abstract

Isothermal current-voltage (I - V) characteristics of c-axes oriented YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films taken in different external magnetic fields {mu}{sub 0}H, exhibit voltage jumps at a well defined current density J{sup *}(T, H) pointing to a specific instability of the vortex system. Interpreted as Larkin- Ovchinnikov (LO)-instability, this behavior allows to extract the inelastic scattering time {tau}{sub in} of quasiparticles. In the present work, the LO-analysis is applied to study the effect of additional defects on {tau}{sub in}. For this purpose, YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films were irradiated at 77K with increasing fluences {phi} of 350 keV He{sup +} ions. The I - V characteristics were determined in the temperature range 45K < T < 80K and in magnetic fields up to 5 Tesla. While J{sup *} as well as the superconducting transition temperature T{sub c} are significantly decreased by the ion bombardment, the critical vortex velocity as deduced from LO-theory is much less affected. If the temperature dependence of the extracted scattering times {tau}{sub in} is rescaled by plotting versus the reduced temperature t = T/T{sub c}({phi}), the results for all ion fluences fall on one common curve within the error bars. This leads to the conclusion thatmore » a universal quasiparticle density n{sub N}(t) dominates the scattering rate 1/{tau}{sub in}.« less