First-order vortex phase transition in κ-type BEDT-TTF organic superconductors

We measured the local magnetization using a micro-Hall probe in a magnetic field perpendicular to the conduction layer to investigate the mixed state of the quasi-two-dimensional organic superconductors κ-(BEDT-TTF)2Cu(NCS)2. A sharp magnetization jump was observed in the vicinity of the irreversible temperature (Tirr) below ∼80 Oe, demonstrating the first-order phase transition. Meanwhile, much broader step structure above higher magnetic field than 80 Oe appears below Tirr. The magnetic field regime of 80–100 Oe is likely to be connected with the second peak line in the magnetic field–temperature phase diagram.