Resistance and magnetic torque measurements are reported in a layered organic superconductor, $\beta ”-{(\mathrm{BEDT}-\mathrm{TTF})}_4\left[\left({\mathrm{H}}_3\mathrm{O}\right)\mathrm{Ga}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ with $T_c=4.8$ K, where BEDT-TTF stands for bis(ethylenedithio)tetrathiafulvalene. Because of the large anion between the BEDT-TTF conducting layers, the superconductivity of this salt is highly anisotropic. In magnetic fields parallel to the conducting layers for $T=0.4$ K, the magnetic torque shows a large diamagnetic signal associated with hysteresis up to $\sim 21$ T, suggesting the upper critical field $H_{c2}\gtrsim 21$ T at 0.4 K. The large reduction of the diamagnetic signal is observed above 16 T, which shows a Fulde and Ferrell and Larkin and Ovchinnikov (FFLO) phase transition. For $T=0.5$ K, the interlayer resistance has nonzero value in a wide field region up to $H_{c2}$, arising from the Josephson vortex dynamics. Successive dips in the second derivative curves of the resistance are observed between 16 T and $H_{c2}$, which are ascribed to the commensurability effect between the Josephson vortex lattice and the order parameter oscillation of the FFLO phase. The commensurability effect is observed only in nearly parallel fields, showing that the FFLO phase is stable in a very limited field angle region. The temperature-field phase diagram is determined.

• Received 13 January 2018

DOI:https://doi.org/10.1103/PhysRevB.97.144505