New Superconducting Phases in Field-Induced Organic Superconductorλ−(BETS)2FeCl4

We derive the parallel upper critical field, $H_{c2\mathrm{}}$, as a function of the temperature $T$ in quasi-2D organic compound $\lambda -(\mathrm{BETS}{)}_2\mathrm{FeCl}{}_4$, accounting for the formation of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state. To further check the 2D LOFF model, we propose to study the $H_{c2\mathrm{}}\left(T\right)$ curve at low $T$ in tilted fields, where the vortex state is described by the high Landau level functions characterized by the index $n$. We predict a cascade of first-order transitions between vortex phases with different $n$, between phases with different types of the symmetry at given $n$ and the change of the superconducting transition from the second order to the first order as $\mathrm{FeCl}{}_4$ ions are replaced partly by $\mathrm{GaCl}{}_4$ ions.