Superconductivity beyond Pauli's limit in bulk NbS2: Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state

Reentrant orbital effect against superconductivity in the quasi-two-dimensional superconductor NbS$_2$

“…These allow us to show that, indeed, in the magnetic fields range H ≃ 15 T , quantum effects are very strong and completely suppress the orbital effect against superconductivity. As a result, the FFLO phase appears with the transition temperature value like for a pure 2D superconductor, which satisfies the experimental situation in NbS 2 [31]. In our opinion, this is the first firm demonstration of a reversible nature of the orbital effect against superconductivity [8][9][10][11][12][13][14][15][16][17].…”

“…Recently, the FFLO phase has been found by Lortz and collaborators in the Q2D compound NbS 2 in a parallel magnetic filed [31]. The peculiarity of this work is that at relatively low magnetic fields (i.e., in the Ginzburg-Landau (GL) area [1]) the orbital effect of the field partially destroys superconductivity but, at high magnetic fields, everything looks like there is no any orbital effect against superconductivity.…”

“…On the other hand, if H c2 (0) tries to destroy superconductivity, then quantum effects of electron motion in a magnetic field can, in principle, restore it as the Reentrant Superconducting (RS) phase [8][9][10][11][12][13][14][15][16][17]. Although there are numerous experimental results [18][19][20][21][22][23][24][25][26][27][28][29][30][31], confirming the existence of the FFLO phase in several Q2D superconductors, there exist only a few experimental works [2][3][4][5], where the presumably RS phase revives in ultrahigh magnetic fields due to quantum effects of electron motion in a magnetic field in one compound -UBe 2 . On the other hand, the above mentioned unique RS phenomenon has been theoretically predicted for a variety of Fermi surfaces: for Q1D [8][9][10], for isotropic 3D [11], and for Q2D superconductors [12][13][14][15][16][17].…”

“…The latter is derived, for the best of our knowledge, for the first time. We use comparison of these equations with experimental data [31] to extract the so-called GL coherence lengths and in-plane Fermi velocity. These allow us to show that, indeed, in the magnetic fields range H ≃ 15 T , quantum effects are very strong and completely suppress the orbital effect against superconductivity.…”

“…since the experiments in Ref. [31] are done both for the parallel (2) and the slightly inclined (8) fields. For our calculations, it is convenient to choose the following gauge, where the vector-potential of the magnetic field (8) depends only on coordinate x:…”