Theoretical description of nearly discontinuous transition in superconductors with paramagnetic depairing

Based on a theoretical argument and Monte Carlo simulations of a Ginzburg-Landau model derived microscopically, it is argued that, in type-II superconductors where both the paramagnetic and orbital depairings are important, a strong first-order transition (FOT) at H_c2 expected in the mean field (MF) approximation never occurs in real systems and changes due to the fluctuation into a crossover. The present result explains why a nearly discontinuous crossover at H_c2 with no intrinsic hysteresis is observed only in a clean superconducting material with a singlet pairing and a high condensation energy such as CeCoIn₅.