Quantum vortex melting and phase diagram in the layered organic superconductor κ -(BEDT-TTF)2Cu(NCS ) 2

Resistance and magnetic torque measurements have been performed to investigate vortex phases for a layered organic superconductor κ -(BEDT-TTF_{) 2}Cu (NCS_{) 2} [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], which is modeled as stacks of Josephson junctions. At 25 mK, the out-of-plane resistivity increases at 0.6 T, has a step feature up to 4 T, and then increases again, whereas the in-plane resistivity monotonically increases above 4 T. The results show that both pancake vortices (PVs) and Josephson vortices (JVs) are in solid phases for μ₀H <0.6 T, but only JVs are in a liquid phase for 0.6 <μ₀H <4 T. For μ₀H >4 T, both PVs and JVs are in liquid phases. These melting transitions are predominantly induced by quantum fluctuations (not by thermal fluctuations). In the magnetic torque curves, the irreversibility transition is clearly observed, roughly corresponding to the melting transition of the PVs but no anomaly is found at the JV melting transition. The detailed vortex phase diagram is determined in a wide temperature region.