The Effect of Disorder on the FFLO State

One of the criteria for stabilizing the FFLO state, a type of inhomogeneous superconducting state, is a mean free path for the carriers that is greater than the superconducting coherence length. By increasing the amount of disorder in a material, thereby decreasing the mean free path, the properties of the FFLO state should change or the FFLO state should not form. We have studied two organic superconducting materials κ-(ET)₂Cu[N(CN)₂]Br and β”- (ET)₄ [(H₃O)Ga (C₂O₄)₃]·C₆H₅NO₂ . The κ material has an ordering transition which is quenched or annealed based on the cooling rate of the sample. The β” material crystalizes with different degrees of disorder as measured by changes in the superconducting transition temperature. We have studied the FFLO state in both of these materials paying particular attention to the dependance of disorder. The motivation of studying disorder arose when we were unable to find evidence of FFLO state in some samples, and yet our critical fields measurements were consistent with previous data where the FFLO state was found. Among other properties, the FFLO lower transition field, the paramagnetic limit, H_p, increases, as disorder increases. The increase in Hp is expected due to increased spin-orbit scattering as predicted by the WHH theory. We will present examples of these phase diagram changes by measuring rf penetration depth with a tunnel diode oscillator at low temperatures, in a pulsed magnetic field.