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A. BUZDIN - University of Bordeaux

Revealing the FFLO phase by the in-plane critical field anisotropy in layered superconductors

There are strong experimental evidences of the FFLO (Fulde-Ferrell-Larkin-Ovchinnikov) state formation in layered organic superconductors in parallel magnetic field.
However the direct proofs of its existence are still elusive. The presented analysis show that due to the interplay between the orbital effect and the FFLO modulation the in-plane critical
field anisotropy drastically changes at the transition to the FFLO state. The predicted very
special type of the angular dependence of the superconducting onset temperature may serve for
unambiguous identification of the FFLO modulation. The vector potential of a parallel
magnetic field produces the modulation of the interlayer coupling between adjacent layers.
Moreover, in some cases the period of this modulation can coincide with the period of the
FFLO modulation of the superconducting order parameter. Such resonance condition results in
the cusps on the temperature and in-plane angular dependencies of the upper critical field, Hc2.
Remarkably, the proposed signature of the FFLO state is directly based on the main feature of
the FFLO state, the spatial modulations of the order parameter.