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, $H \simeq 15$ 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.