High magnetic field properties of quasi-two-dimensional organic superconductors

Investigations of quasi-two-dimensional (2D) organic charge-transfer salts in high magnetic fields have deepened our understanding of these materials. Comprehensive studies of e.g. magnetic quantum oscillations or angular-dependent transport properties allowed to unravel details of the electronic band-structure. In some cases, deviations from the usually adopted Fermi-liquid picture for metals were observed. For β´´-(BEDT-TTF)₂SF₅CH₂CF₂SO₃ e.g., strong indications for an incoherent electronic transport across the layers and a field-induced insulating state were found. Strong magnetic fields as well influence the superconducting state, usually by driving the super¬conductor from the Shubnikov into the normal state and thereby suppressing the critical temperature in a well-known monotonous fashion. The 2D organic superconductors are, however, good candidates for exhibiting in addition to the Meissner and Shubnikov phase the so-called FFLO state, named after Fulde, Ferrell, Larkin, and Ovchinnikov, when applying the magnetic field exactly parallel to the conducting layers. In this case, the orbital critical field is much larger than the Pauli-limiting field for these singlet superconductors. In the FFLO state, superconductivity can survive even at magnetic fields above the Pauli limit by sacrificing parts of the superconducting volume with a spatially oscillating order parameter. Our recent high-resolution specific-heat experiments in parallel fields up to 28 T for κ-(BEDT-TTF)₂Cu(NCS)₂ gave strong evidence for the existence of the FFLO state. For fields above 21 T, the superconducting transition becomes first order signalling that the Pauli limit is reached. Below about 3 K, the upper critical field increases sharply and a second first-order transition appears within the superconducting phase. With additional magnetic-torque measurements up to 32 T we were able to follow the phase diagram down to 50 mK. The well-known electronic band-structure parameters allow a thorough theoretical estimate of the Pauli-limited superconducting phase diagram.