Emergent Antiferromagnetism in D-wave Superconductor with Strong Paramagnetic Pair-Breaking

TL;DR

This paper theoretically demonstrates that strong paramagnetic pair-breaking in a d-wave superconductor induces both FFLO superconducting states and incommensurate antiferromagnetic order, explaining phenomena observed in CeCoIn_5.

Contribution

It reveals that paramagnetic pair-breaking can induce antiferromagnetic order within the superconducting phase, providing a new understanding of high-field low-temperature phases in unconventional superconductors.

Findings

AFM order appears below Hc2 in the superconducting phase.

The AFM order coexists with FFLO modulation in CeCoIn_5.

PPB-induced AFM order explains quantum critical fluctuations near Hc2.

Abstract

It is theoretically shown that, in the four-fold symmetric d-wave superconducting phase, a paramagnetic pair-breaking (PPB) enhanced sufficiently by increasing the applied magnetic field induces not only the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state but also an incommensurate antiferromagnetic (AFM) order with Q-vector parallel to a gap node. This AFM ordering tends to occur only below H_{c2} at low temperatures, i.e., in the presence of a nonvanishing superconducting energy gap $\Delta$ rather than in the normal phase. Through a detailed study on the resulting AFM order and its interplay with the FFLO spatial modulation of $\Delta$, it is argued that the strange high field and low temperature (HFLT) superconducting phase of CeCoIn_5 is a coexisting phase of the FFLO and incommensurate AFM orders, and that this PPB mechanism of an AFM ordering is also the origin of the AFM quantum critical fluctuation which has occurred close to H_{c2}(0) in several unconventional superconductors including CeCoIn_5.