Superconducting Pairing Symmetry in a Hole-doped Organic Superconductor kappa-(ET)4Hg2.89Br8 probed by muSR

Organic superconductors (SCs) generally have fixed band filling as 1/2 or 1/4. But, kappa-(ET)4Hg2.89Br8 (k-HgBr) is an exceptional one because 11%-hole doping electronic state from the 1/2-filled band is realized, while ET dimers form the triangular lattice. In the metallic state under pressure, a non-Fermi liquid behavior was observed in the low-pressure range and gradually changed to a Fermi-liquid one with a critical pressure Pc~0.5 GPa. This metallic state undergoes SC with a broad transition width at 4.3 K. The upper critical field in the perpendicular configuration, Bc2, under low-pressure obviously exceeds Pauli limit, Bp. There is no example of that Bc2 exceeds Bp among organic SCs which may indicate the nature of triplet superconductivity. As the first step, we would like to clarify whether triplet SC is realized in k-HgBr at ambient pressure down to 0.3 K by zero-field muSR.