Nonlinear and spin-dependent transport in correlated electron insulators at ultra low temperatures

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We have measured the dielectric constant of the molecular crystal theta-(BEDT-TTF)_2CsZn(SCN)_4, which is a correlated electron insulator, at ultra low temperatures in a wide frequency range. Ruling out the effect of the sample-electrode interface, we found a large anisotropy in the dielectric constant, which is consistent with its nonlinear conductivity, and obtained dynamical properties of electron-hole excitations. We have also investigated the transport properties under electron-spin-resonance conditions to clarify the origin of a giant magnetoresistance of the molecular crystal.