Copper Doping of BaNi$_{2}$As$_{2}$: Giant Phonon Softening and Superconductivity Enhancement

TL;DR

Copper doping in BaNi₂As₂ induces a phase transition that significantly enhances superconductivity through phonon softening, with a notable increase in transition temperature from 0.6 K to over 3 K.

Contribution

This study reveals that copper doping causes giant phonon softening and superconductivity enhancement in BaNi₂As₂, a behavior distinct from iron-based superconductors.

Findings

Superconducting transition temperature increases from 0.6 K to over 3 K with copper doping.

Phonon softening is identified as the mechanism for superconductivity enhancement.

Structural phase transition from triclinic to tetragonal correlates with increased $T_c$.

Abstract

The effects of copper doping on the structural and superconducting phase transitions of Ba(Ni$_{1-x}$Cu$_{x}$)$_{2}$As$_{2}$ were studied by examining the resistivity, magnetic susceptibility, and specific heat. We found an abrupt increase in the superconducting transition temperature $T_{\rm c}$ from 0.6 K in the triclinic phase with less copper ($x$ $\leq$ 0.16) to 2.5-3.2 K in the tetragonal phase with more copper ($x$ $>$ 0.16). The specific-heat data suggested that doping-induced phonon softening was responsible for the enhanced superconductivity in the tetragonal phase. All of these observations exhibited striking similarities to those observed in the phosphorus doping of BaNi$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ [K. Kudo et al., Phys. Rev. Lett. 109, 097002 (2012).], which markedly contrast the behavior of phosphorus and copper doping of the iron-based superconductor BaFe$_{2}$As$_{2}$.