Low-temperature vortex liquid states induced by quantum fluctuations in the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂

We report the transport properties in the vortex liquid states induced by quantum fluctuations at low temperature in the layered organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂. A steep drop of the resistivity observed below about 1 K separates the liquid state into two regions. In the low-resistance state at lower temperature, a finite resistivity with weak temperature dependence persists down to 100 mK at least. The finite resistivity in the vortex state at T ≃ O K indicates the realization of quantum vortex liquid assisted by the strong quantum fluctuations instead of the thermal one. A possible origin for separating these liquid states is a remnant vortex melting line at the original position, which is obscured and suppressed by the quantum fluctuations. A nonlinear behavior of the in-plane resistivity appears at large current density in only the low-resistance state, but not in another vortex liquid state at higher temperature, where the thermal fluctuations are dominant. The transport properties in the low-resistance state are well understood in the vortex slush concept with a short-range order of vortices. Thus the low-resistance state below 1 K is considered to be a novel quantum vortex slush state.