Magnetothermopower study of the quasi-two-dimensional organic conductor α − ( BEDT − TTF ) 2 KHg ( SCN ) 4

We have used a low-frequency magneto-thermopower (MTEP) method to probe the high-magnetic-field ground-state behavior of $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4}$ along all three principal crystallographic axes at low temperatures. The thermopower tensor coefficients ${(S}_{\mathrm{xx}}{,S}_{\mathrm{yx}},$ and ${S}_{\mathrm{zz}})$ have been measured to 30 T, beyond the anomalous low-temperature, field-induced transition at 22.5 T. We find a significant anisotropy in the MTEP signal, and also observe large quantum oscillations associated with Landau quantization. The anisotropy indicates that the ground-state properties are clearly driven by mechanisms that occur along specific directions for the in-plane electronic structure. Both transverse and longitudinal magnetothermopower show asymptotic behaviors in the field, which can be explained in terms of magnetic breakdown of compensated closed orbits.