Fulde-Ferrell-Larkin-Ovchinnikov State in Perpendicular Magnetic Fields in Strongly Pauli-Limited Quasi-Two-Dimensional Superconductors

TL;DR

This paper investigates how the nesting effect influences the stability of the FFLO state in strongly Pauli-limited quasi-two-dimensional superconductors under perpendicular magnetic fields, revealing conditions where the effect is notably strong.

Contribution

It demonstrates that the nesting effect can significantly stabilize the FFLO state in perpendicular fields when interlayer transfer energy is small, challenging previous assumptions.

Findings

Nesting effect enhances FFLO stability in perpendicular fields with small interlayer transfer.

Interlayer states with warped Fermi surfaces exhibit high critical fields in perpendicular orientation.

Results explain similar high-field phase regions in CeCoIn5 and FeSe.

Abstract

We examine the Fermi-surface effect called the nesting effect for the FFLO state in strongly Pauli-limited Q2D superconductors, focusing on the effect of 3D factors, such as interlayer electron transfer, interlayer pairing, and off-plane magnetic fields including those perpendicular to the most conductive layers. We examine the systems with a large Maki parameter so that the orbital pair-breaking effect is negligible, except for the locking of the direction of the FFLO vector q in the field direction.It is known that the nesting effect for the FFLO state can be strong in QLD systems in which the orbital pair-breaking effect is suppressed by applying the mag. field parallel to the layers. Hence, it has sometimes been suggested that the nesting effect may hardly enhance the stability of the FFLO state for perpendicular fields. We illustrate that, contrary to this view, the nesting effect can strongly stabilize the FFLO state for perpendicular fields as well as for parallel fields when tz is small so that the Fermi surfaces are open in the kz-direction, where tz denotes the interlayer transfer energy. In particular, the nesting effect in perpendicular fields can be strong in interlayer states. For example, in systems with cylindrical Fermi surfaces warped by tz /= 0, interlayer states with Dlt_k prop sin k_z exhibit mu_e Hc=1.65 Dlt_a0 for perpendicular fields, which is much larger than typical values for parallel fields, such as mu_e Hc=Dlt_s0 of the s-wave state and mu_e Hc = 1.28 Delta_d0 of the d-wave state in cylindrical systems with tz=0. The present result could potentially provide a physical reason why the areas in the phase diagrams occupied by the high-field phases for the perpendicular and parallel fields are of the same order in CeCoIn5 and FeSe.