Rashba-controlled emergence and reversal of critical-field anisotropy in p-wave magnet--superconductor hybrids

Abstract

We show that tuning Rashba spin-orbit coupling can change the in-plane field direction along which an $s$-wave superconductor is most susceptible to Zeeman depairing. We consider a two-dimensional $s$-wave superconductor subject to an odd-parity exchange field induced by a $p$-wave magnet. Without Rashba coupling, the out-of-plane exchange field enhances the in-plane critical field but leaves its angular response exactly isotropic, despite the anisotropic magnetic spin texture. Minimizing the Bogoliubov--de Gennes grand potential over uniform singlet pairing, we find that increasing Rashba coupling first generates and then reverses the principal-axis critical-field contrast, switching the minimum-critical-field direction by $90^{\circ}$. Equal principal-axis fields at the intervening compensation point coexist with residual angular modulation. We interpret the reversal through competing momentum-dependent depairing contributions from the magnetic and Rashba spin textures, while the on-site singlet pairing channel remains unchanged. Direction-dependent discontinuous collapses of the order parameter further shape the critical-field curves. These results identify a tunable directional paramagnetic response within the uniform-pairing sector. In heterostructures with sufficient electrostatic tunability of interfacial Rashba coupling, this reversal would enable gate control of the easy-depairing axis without reorienting the magnetic texture.