Abstract
The Kogan-Nakagawa (KN) theory is improved to describe a magnetic-field-enhanced superconductivity by incorporating spin paramagnetism and spin orbit scattering. The present theory explains the critical-temperature (Tc)-enhancement above the zero-field Tc under parallel magnetic fields observed in layered thin films of Au/Ge except for the exact temperature (T) dependence of the critical field (Hc). The conditions for the occurrence of Tc-enhancement are also discussed as a function of dirtyness. In the vicinity of the critical condition for Tc-enhancement, the theory yields a three-valued character in Hc vs T relation and deviates from the experiment. In contrast to the KN theory, no Tc-enhancement is expected in the dirty limit because of the spin paramagnetic effect.
