Field Dependence of the Ground State in the Exotic Superconductor CeCoIn$_5$: a Nuclear Magnetic Resonance Investigation

TL;DR

This study uses $^{115}$In NMR measurements to explore the magnetic field dependence of the ground state in CeCoIn$_5$, revealing a transition from increased quasiparticle density to a possible FFLO phase at high fields.

Contribution

First NMR investigation showing field-dependent changes in the ground state of CeCoIn$_5$, including evidence for a transition to a modulated superconducting phase.

Findings

Spin susceptibility increases as √H₀ below 10 T.

Identification of a new superconducting phase above 10 T.

Observation of a first-order transition to the normal state.

Abstract

We report $^{115}$In nuclear magnetic resonance (NMR) measurements in CeCoIn$_5$ at low temperature ($T \approx 70$ mK) as a function of magnetic field ($H_0$) from 2 T to 13.5 T applied perpendicular to the $\hat c$-axis. NMR line shift reveals that below 10 T the spin susceptibility increases as $\sqrt{H_0}$. We associate this with an increase of the density of states due to the Zeeman and Doppler-shifted quasiparticles extended outside the vortex cores in a d-wave superconductor. Above 10 T a new superconducting state is stabilized, possibly the modulated phase predicted by Fulde, Ferrell, Larkin and Ovchinnikov (FFLO). This phase is clearly identified by a strong and linear increase of the NMR shift with the field, before a jump at the first order transition to the normal state.