Magnetic field induced lattice anomaly inside the superconducting state of CeCoIn$_5$: evidence of the proposed Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This study uses high magnetic field magnetostriction measurements on CeCoIn$_5$ to identify anomalies associated with the transition to the FFLO superconducting state, revealing a second order-like feature near the upper critical field.

Contribution

It provides experimental evidence supporting the existence of the FFLO state in CeCoIn$_5$ through detailed magnetostriction analysis and identification of a distinctive high-field anomaly.

Findings

Sharp discontinuity at the upper critical field $B_{c2}$

Second order-like feature along the c-axis in high field and low temperature

Anomaly only when magnetic field is within 20° of the ab-planes

Abstract

We report high magnetic field linear magnetostriction experiments on CeCoIn$_5$ single crystals. Two features are remarkable: (i) a sharp discontinuity in all the crystallographic axes associated with the upper superconducting critical field $B_{c2}$ that becomes less pronounced as the temperature increases; (ii) a distinctive second order-like feature observed only along the c-axis in the high field (10 T $ \lesssim B \leq B_{c2}$) low temperature ($T \lesssim$ 0.35 K) region. This second order transition is observed only when the magnetic field lies within 20$^o$ of the ab-planes and there is no signature of it above $B_{c2}$, which raises questions regarding its interpretation as a field induced magnetically ordered phase. Good agreement with previous results suggests that this anomaly is related to the transition to the Fulde-Ferrel-Larkin-Ovchinnikov superconducting state.