Layered Superconductor in a Magnetic Field: Breakdown of the Effective Mass Model

We theoretically study the upper critical magnetic fields at zero temperature in a quasi-two-dimensional (Q2D) superconductor in the parallel and perpendicular fields, Hc2∥(0) and Hc2⊥(0), respectively. We find that Hc2∥(0)≈0.75|dHc2∥/dT|TcTc and that Hc2⊥(0)≈0.75|dHc2⊥/dT|TcTc, where dHc2∥/dT|Tc and dHc2⊥/dT|Tc are the corresponding Ginzburg-Landau slopes of the upper critical magnetic fields. Our results demonstrate the breakdown of the so-called effective mass model in Q2D case and may be partially responsible for the experimentally observed deviations from the effective mass model in a number of layered superconductors, including MgB₂.