Reentrant Orbital Effect against Superconductivity in the Quasi-Two-Dimensional Superconductor NbS2

We derive an integral equation for the superconducting gap, which takes into account the quantum nature of electron motion in a parallel magnetic field in a quasi-two-dimensional superconductor in the presence of a nonzero perpendicular field component. By comparison of our theoretical results with the recent experimental data obtained on NbS₂, we show that the orbital effect against superconductivity partially destroys superconductivity in the so-called Ginzburg-Landau area of this quasi-two-dimensional conductor, as expected. Nevertheless, at relatively high magnetic fields, $H ∼eq 15$ T, the orbital effect starts to improve the Fulde-Ferrell-Larkin-Ovchinnikov phase in NbS₂, due to the quantum nature of electron motion in a parallel magnetic field. In our opinion, this is the clearest demonstration that the orbital effect against superconductivity in a parallel magnetic field has a reentrant nature.