Anomalous doping effect on the superconducting state in CeCoIn$_5$ at high magnetic fields

TL;DR

This study explores how slight electron and hole doping in CeCoIn$_5$ affects its high-field superconducting state, revealing contrasting effects on critical temperature and field, and supporting the superconducting nature of the high-field Q-state.

Contribution

It provides new insights into doping effects on CeCoIn$_5$, showing that hole doping can enhance $H_{c2}$ while electron doping suppresses both $T_c$ and $H_{c2}$, and links the high-field Q-state to superconductivity.

Findings

Hole doping increases $H_{c2}$ up to a maximum at 0.2% doping.

Electron doping decreases both $T_c$ and $H_{c2}$ monotonically.

The high-field Q-state is correlated with superconductivity.

Abstract

We investigated the effect of electron and hole doping on the high-field low-temperature superconducting state in CeCoIn$_5$ by measuring specific heat of CeCo(In$_{\rm 1-x}$M$_{\rm x}$)$_5$ with M=Sn, Cd and Hg and $x$ up to 0.33% at temperatures down to 0.1\,K and fields up to 14\,T. Although both Cd- and Hg-doping (hole-doping) suppresses the zero-field $T_c$ monotonically, $H_{c2}$ increases with small amounts of doping and has a maximum around $x$=0.2% (M=Cd). On the other hand, with Sn-doping (electron-doping) both zero-field $T_c$ and $H_{c2}$ decrease monotonically. The critical temperature for the high-field low-temperature superconducting state (so called {\it Q}-state) correlates with $H_{c2}$ and $T_c$, which we interpret in support of the superconducting origin of this state.