Superconductivity nearby quantum critical region in hole-doped organic strange metal κ-(ET)4Hg3-δBr8, δ=11%

Abstract

Abstract The hole-doped organic superconductor κ-(ET)4Hg3-δBr8, (κ-HgBr), where δ=11% and ET=bis(ethylenedithio)tetrathiafulvalene, has been the key to bridge the knowledge gap between half-filled organics and doped cuprate systems. Nonetheless, the isotropic triangular lattice of ET dimers of κ-HgBr is responsible for the magnetic susceptibility and its superconductivity. We have measured zero-field (ZF) muon spin relaxation-rotation (µ+SR) in κ-HgBr showing the ZF-µ+SR relaxation rate from temperature around 10 K down to 0.3 K is temperature-independent. This is consistent with a superconducting state that preserved time-reversal symmetry. There was almost no change in the maximally 100 Oe of transverse-field-µ+SR time spectra, at 0.3 K and above superconducting temperature, T c~4.6(3) K. This suggests that the in-plane London penetration depth, λbc, is longer than a μm order, while we estimate the lower limit of the lower critical field, H c1, to be 30 Oe, although, however, the measurement using another geometric setup is necessary to determine the absolute value of λbc. These could be an indication of a strong-coupling superconductor. A possible mechanism of preserved time-reversal Cooper pairing formation from strong-coupling non-FL metal with geometrical frustration is discussed.