Orbital-Induced Transformation of FFLO into Abrikosov-like States

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state can emerge in superconductors for which the orbital critical field exceeds the Pauli limit. Quasi-two-dimensional (2D) organic superconductors show clear evidence for the occurrence of this state with modulated superconducting order parameter [1]. However, the inevitable presence of orbital effects leads to deviations of the order parameter from the original FFLO prediction and to experimental signatures not well understood, so far. Here, we present angular-resolved specific-heat data of the 2D organic superconductor k-(ET)₂Cu(NCS)₂, focusing on the orbital suppression of the high-field FFLO state. Rotating the field away from the in-plane orientation leads to an increase of orbital effects and changes the nature of the transition from second to first order. Before orbital effects finally suppress the FFLO superconductivity, the specific-heat data reveal a transition to a further superconducting state. Our observations are in line with theoretical predictions of a successive conversion of the FFLO into an Abrikosov-like order parameter of higher-order Landau levels by increasing orbital contributions. The Abrikosov-like and FFLO states compete in 2D systems and might represent a general phenomenology of 2D Pauli-limited superconductors [2].

[1] J. Wosnitza, Ann. Phys. 530, 1700282 (2018).

[2] T. Kotte et al., Phys. Rev. B 106, L060503 (2022).