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

Chris Eckberg; Limin Wang; Halyna Hodovanets; Hyunsoo Kim; Daniel Campbell; Peter Zavalij; Phil M. Piccoli; Johnpierre Paglione

doi:10.1103/PhysRevB.97.224505

paper

Evolution of Structure and Superconductivity in Ba(Ni $_{1 - x}$ Co $_{x}$ ) $_{2}$ As $_{2}$

arXiv:1802.08743 · doi:10.1103/PhysRevB.97.224505

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 $x \geq 0.133$ . 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 $Δ C_{e} / γ T \sim 2.2$ 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.

7 pages, 8 figures

Evolution of Structure and Superconductivity in Ba(Ni1−x​Cox​)2​As2​

Abstract

Evolution of Structure and Superconductivity in Ba(Ni $_{1 - x}$ Co $_{x}$ ) $_{2}$ As $_{2}$