Heavy ion irradiation effects on the high-frequency properties of YBCO and Nb3Sn thin films

Abstract

High-energy heavy-ion irradiation is known to produce effective vortex pinning centers in the high-$T_c$ cuprate superconductors, as amorphous columnar tracks. However, while the beneficial effects on pinning has been well established through dc and low-frequency characterizations, the same analysis in the high-frequency regime is far from complete. Even less investigated are the effects of heavy ion irradiation on the microwave properties of metallic low-$T_c$ superconducting films. Here, we report on the effects of 1.15 GeV Pb irradiation on the high frequency properties of YBa${}_2$Cu${}_3$O${}_{7-x}$ (YBCO) and Nb${}_3$Sn thin films. The microwave analysis, performed in the range 7-8 GHz, allows obtaining the fundamental properties of both the materials, as the London penetration depth and gap values, and of the main pinning parameters, through the determination of the Campbell length by measurements in dc magnetic fields up to 4 T. GeV heavy-ion irradiation confirmed to be extremely effective for YBCO also in the high frequency regime, enhancing both the pinning constant and the depinning frequency, thus pushing the critical current density to about 30% of the depairing current density. On the other hand, the discontinuous but correlated defects produced in Nb${}_3$Sn was found to be ineffective to enhance the pinning properties (the pinning constant in fact decreases), while the observed increment of the depinning frequency is ascribed to the reduction of the vortex viscosity, in turn due to the growth of the normal state resistivity.