Magnetic torque of κ-(BEDT-TTF)₂Cu(NCS)₂ single crystals

The magnetic torque of κ-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field direction θ with respect to the a^*-axis under constant magnetic fields, H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found at θ_c1 near the H∥bc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed at θ_c2 and θ_c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component of H giving each cusp is kept constant as {Mathematical expression} i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to the bc-plane H_cp1, H_cp2, and H_cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.