Upper critical field and related properties of superconducting amorphous alloys Zr-Si

Abstract

The upper critical fieldH c2 has been measured for the superconducting amorphous alloys Zr100−x Si x withx=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current densityJ p , pinning force densityF p , and flux flow resistivity ρ f , reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely lowJ p of 1 A/cm2 andF p of 1.5×104 N/m3 ath (≡H/H c2)=0.5 andt (≡T/T c )=0.5. The temperature dependence ofH c2 is well fitted by a curve of the Werthamer-Helfand-Hohenberg theory with Maki parameter α=1.7−1.5 and spin-orbit interaction parameter λso=1−2.4. These results indicate that the Pauli spin-paramagnetic effect plays an important role in the low-temperature behavior ofH c2 in these low-T c and high-H c2 amorphous superconductors. Corrections of the electron-phonon coupling (λep=0.6) to the Pauli spin-paramagnetic limiting field lead to a reduced importance of the spin-orbit interaction effect, with λso less than 1.0. A discussion is given of the structural inhomogeneities in an amorphous phase, which are sensitively reflected in the strength of the fluxoid line pinning interaction (J p ,F p ) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, thatH c2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowedH c2 values.