Instability of square vortex lattice in d-wave superconductors is due to paramagnetic depairing

TL;DR

This paper investigates how paramagnetic effects influence vortex lattice structures in d-wave superconductors, revealing destabilization of square lattices and emergence of reentrant rhombic phases at high magnetic fields.

Contribution

It demonstrates that paramagnetic depairing causes structural transitions in vortex lattices, including destabilization of square lattices and the appearance of reentrant rhombic phases, in d-wave superconductors.

Findings

Square vortex lattice destabilized with increasing magnetic field.

Reentrant rhombic lattice appears at higher fields.

Fermi surface anisotropy induces additional structural transitions near Hc2.

Abstract

Effects of the paramagnetic depairing on structural transitions between vortex lattices of a quasi two dimensional d-wave superconductor are examined. It is found that, in systems with Maki parameter $\alpha_{\rm M}$ of order unity, a square lattice induced by a d-wave pairing is destabilized with {\it increasing} fields, and that a reentrant rhombic lattice occurs in higher fields. Further, a weak Fermi surface anisotropy competitive with the pairing symmetry induces another structural transition near $H_{c2}$. These results are consistent with the structure changes of the vortex lattice in CeCoIn$_5$ in ${\bf H} \parallel c$ determined from recent neutron scattering data.