Microscopic evidence for field-induced magnetism in CeCoIn$_5$

TL;DR

This study provides microscopic NMR evidence of field-induced static magnetism in CeCoIn$_5$, revealing local moments in vortex cores and challenging the FFLO phase interpretation.

Contribution

It presents the first NMR evidence of static spin moments in CeCoIn$_5$ under high magnetic fields near the superconducting transition.

Findings

Discontinuous Knight shifts across the first-order transition.

Dramatic increase in magnetic linewidths indicating static moments.

Line broadening inconsistent with FFLO phase expectations.

Abstract

We present NMR data in the normal and superconducting states of CeCoIn$_5$ for fields close to $H_{\rm c2}(0)=11.8$ T in the $ab$ plane. Recent experiments identified a first-order transition from the normal to superconducting state for $H> 10.5$ T, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2) and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and high-field phase the broad NMR lineshapes suggest ordered local moments, rather than a long wavelength quasiparticle spin density modulation expected for an FFLO phase.