Magnetic field–temperature phase diagram of the quasi-two-dimensional organic superconductor λ-(BETS)₂GaCl₄ studied via thermal conductivity

The thermal conductivity κ of the quasi-two-dimensional (Q2D) organic superconductor λ-(BETS)₂GaCl₄ [BETS = bis(ethylenedithio)tetraselenfulvalene] was studied in the magnetic field H applied parallel to the Q2D plane. The phase diagram determined from this bulk measurement shows notable dependence on the sample quality. In dirty samples the upper critical field H_c2 is consistent with the Pauli paramagnetic limiting, and a sharp change is observed in κ(H) at H_c2‖. In contrast, in clean samples H_c2(T) shows no saturation towards low temperatures and the feature in κ(H) is replaced by two slope changes reminiscent of second-order transitions. The peculiarity was observed below ∼0.33T_c and disappeared on field inclination to the plane when the orbital suppression of superconductivity became dominant. This behavior is consistent with the formation of a superconducting state with spatially modulated order parameter in clean samples.