Novel superconductivity in high magnetic fields

Shinya Uji, Taichi Terashima
Quantum Properties Group


Abstract

Under special conditions, a superconducting state where the order parameter oscillates in real space, the so-called FFLO state, is theoretically predicted to exist near the upper critical field, as first proposed by Fulde and Ferrell, and Larkin and Ovchinnikov. We have performed systematic measurements of the interlayer resistance in high magnetic fields to 45 T in the two dimensional magnetic-field-induced organic superconductor λ-(BETS)2FeCl4, where BETS=bis(ethylenedithio)tetra-selenafulvalene. The resistance is found to show characteristic dip structures in the superconducting state. The results are consistent with pinning interactions between the vortices penetrating the insulating layers and the order parameter of the FFLO state. This gives strong evidence for an oscillating order parameter in real space.



<Fig. 1> Two dimensional organic superconductor λ-(BETS)2FeCl4.


<Fig. 2> Magnetic field dependence of resistance at low temperatures in λ-(BETS)2FeCl4. The field is applied parallel to the c-axis. Characteristic dip structures in the resistive transitions are evident, which are ascribed to commensurability effect between the vortices penetrating the insulating layers and the order parameter of the FFLO state. <Fig. 3> Phase diagram. FFLO state are stabilized near the Hc2 boundary in the superconducting state. The wave-lengh of the order parameter is also shown.