muSR Study on the Spin Dynamics of Hole-doped Organic Metal kappa-(ET)4Hg3-dCl8, d=22%

The hole-doped organic metal kappa-(ET)4Hg3-dX8; X = Br, Cl are the exceptional carrier-doped metal among half-filled organics, besides ET dimers (S=½) tend to arrange triangular lattice. The k-HgBr compound undergoes a superconductor while the k-HgCl compound transitions to the insulator and becomes a superconductor under pressure. k-HgBr is proposed as a doped quantum spin liquid (QSL) from its spin susceptibility. Whereas the lattice of k-HgCl is less triangular from ZF-muSR we found a plateau and broad peak at 0.1 K in the temperature dependence of muon relaxation rate. The LF scan at several temperatures down to 0.3 K distinguished the behavior at low- and high-temperature regions just at the plateau around 6.5 K. The plateau itself coincides with the metal-insulator transition. Finer LF-scan below 6.5 K down to the base-temperature of the DR fridge will finalize the characterization of the ground state and spin dynamics of k-HgCl which we proposed involving a quantum critical metal at a higher temperature with charge fluctuations and quantum spin liquid.