Evolution of Structure and Superconductivity in Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study investigates how cobalt substitution affects the structural phase transition and superconductivity in Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$, revealing a dome-shaped superconducting phase diagram and suggesting a non-phononic pairing mechanism.

Contribution

It provides the first detailed phase diagram of Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ showing a continuous evolution of superconductivity and structural suppression, highlighting a different behavior from similar systems.

Findings

Superconducting transition temperature $T_c$ peaks at 2.3 K near $x=0.083$

Structural transition is suppressed at $x extgreater0.133$

Heat capacity jump indicates strong coupling superconductivity.

Abstract

The effects of Co-substitution on Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x\leq 0.251$) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi$_2$As$_2$ exhibits a first order tetragonal to triclinic structural phase transition at $T_s=137 K$ upon cooling, and enters a superconducting phase below $T_c=0.7 K$. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\geq0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3 K$ at the structural critical point $x=0.083$ and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi$_2$As$_2$ substitutional studies, which show an abrupt change in $T_c$ at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ exhibits a dome-like phase diagram centered around the first-order critical point. Together with an anomalously large heat capacity jump $\Delta C_e/\gamma T\sim 2.2$ at optimal doping, the smooth evolution of $T_c$ in the Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ system suggests a mechanism for pairing enhancement other than phonon softening.