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-axis orientedYBa ₂ CU ₃ O _7−δ films taken in different external magnetic fields μ_oH, exhibit voltage jumps at a well defined current density J*(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 T_in of quasiparticles. In the present work, the LO-analysis is applied to study the effect of additional defects on T_in. For this purpose,YBa ₂ CU ₃ O _7−δ films were irradiated at 77K with increasing fluences Φ of 350 ke V He⁺ ions. The I-V characteristics were determined in the temperature range 45K < T < 80K and in magnetic fields up to 5 Tesla. While J* as well as the superconducting transition temperature T_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 Γ_in is rescaled by plotting versus the reduced temperature t = T/T_c(Φ), the results for all ion fluences fall on one common curve within the error bars. This leads to the conclusion that a universal quasiparticle density n_N(t) dominates the scattering rate 1/T_in.