Vortex tilt modulus in Fulde-Ferrell-Larkin-Ovchinnikov state

Vortex tilt response in a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex lattice is studied as a probe reflecting the spatial structure of this state. In quasi-two-dimensional materials under a parallel magnetic field, the tilt modulus E₂ of the nodal planes in the FFLO state modulating along the field decreases as the paramagnetic effect is effectively enhanced, and this reduction of E₂, in turn, reduces the vortex tilt modulus. This reduction, more remarkable in higher fields or in more two-dimensional-like systems, of vortex tilt modulus upon entering the FFLO state may be one origin of an anomalous anisotropic reduction of sound velocity detected in an ultrasound measurement in CeCoIn₅.