Spin Paramagnetic Effect on Magnetic-Field-Enhanced Superconductivity in Layered Thin Films of Au/Ge

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 ( T c )-enhancement above the zero-field T c under parallel magnetic fields observed in layered thin films of Au/Ge except for the exact temperature ( T ) dependence of the critical field ( H c ). The conditions for the occurrence of T c -enhancement are also discussed as a function of dirtyness. In the vicinity of the critical condition for T c -enhancement, the theory yields a three-valued character in H c vs T relation and deviates from the experiment. In contrast to the KN theory, no T c -enhancement is expected in the dirty limit because of the spin paramagnetic effect.