Quantum liquid of vortices in the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

We present magnetic torque measurements on the quasi-two-dimensional organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) at low temperature T/T-c similar or equal to 0.01. The irreversible field H-irr lies far below the upper critical field H-c2 even at T similar or equal to 0. In the magnetic field region, H(irr)less than or equal to H less than or equal to H-c2, a vortex liquid state exists even at T similar or equal to 0, resulting from the quantum fluctuations instead of the contribution of the thermal one at higher temperature. This finding is explained as the quantum vortex liquid state driven by the quantum fluctuations from the different theoretical models. In the liquid state, de Haas-van Alphen oscillations are observed, and H-c2 is defined by the appearance of the additional damping on the oscillation amplitude. [S0163-1829(98)11017-2].