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

Abstract

The effects of Co-substitution on Ba(Ni${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($0\le x\le 0.251$) 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 $T_s=137K$ upon cooling, and enters a superconducting phase below $T_c=0.7K$. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\ge 0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3K$ 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.

arXiv comment7 pages, 8 figures

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