The upper critical field Hc2 has been measured for the superconducting amorphous alloys Zr100-xSix with x=13, 15, and 17 over a wide temperature range down to 50 mK. Measurements for related properties, such as the depinning current density Jp, pinning force density Fp, and flux flow resistivity ρf, reveal that these systems are well characterized as typically soft and dirty type II superconductors with an extremely low Jp of 1 A/cm2 and Fp of 1.5×104 N/m3 at h (≡H/Hc2)=0.5 and t (≡T/Tc)=0.5. The temperature dependence of Hc2 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 of Hc2 in these low-Tc and high-Hc2 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 (Jp, Fp) and are intimately related to the nonintrinsic anomaly, observed in many amorphous alloys, that Hc2 depends linearly on temperature even at low temperatures, exceeding the theoretically allowed Hc2 values.
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