Anisotropic effect of Cd and Hg doping on Pauli limited superconductor CeCoIn$_5$

TL;DR

This study examines how Cd and Hg doping affect the superconducting transition and high field-low temperature state of CeCoIn$_5$, revealing anisotropic suppression of the FFLO state and supporting its non-magnetic origin.

Contribution

It provides new insights into the anisotropic effects of Cd and Hg doping on the superconducting and FFLO states in CeCoIn$_5$, highlighting impurity sensitivity and the non-magnetic nature of the high field-low temperature phase.

Findings

Cd doping suppresses the crossover temperature T0 for H∥[100] but not for H∥[001].

Hg doping washes out the FFLO anomaly at x=0.00048.

The high field-low temperature state is sensitive to impurities and likely non-magnetic.

Abstract

We investigated the effect of Cd and Hg doping on the first order superconducting (SC) transition and the high field-low temperature SC state of CeCoIn$_5$ by measuring the specific heat of CeCo(In$_{\rm 1-x}$Cd$_{\rm x}$)$_5$ with x=0.0011, 0.0022 and 0.0033 and CeCo(In$_{\rm 1-x}$Hg$_{\rm x}$)$_5$ with x=0.00016, 0.00032, and 0.00048 at temperatures down to 0.1 K and fields up to 14 T. Cd substitution rapidly suppresses the cross-over temperature $T_{\rm 0}$, where the superconducting transition changes from second to first order, to $T$=0 K with x=0.0022 for $H\parallel$ [100], while it remains roughly constant up to x=0.0033 for $H\parallel$ [001]. The associated anomaly of the proposed FFLO state in Hg-doped samples is washed out by x=0.00048, while remaining at the same temperature, indicating high sensitivity of that state to impurities. We interpret these results as supporting the non-magnetic, possibly FFLO, origin of the high field - low temperature state in CeCoIn$_5$.