Giant phonon softening and enhancement of superconductivity by phosphorus doping of BaNi ₂As ₂

The effects of phosphorus doping on the structural and superconducting phase transitions of BaNi ₂(As _1-xP _x) ₂ were studied. The specific heat, resistivity, and magnetic susceptibility were measured. The results revealed an abrupt increase in the superconducting transition temperature (T _c) from 0.6 K in the triclinic phase (space group P1̄) with less phosphorus (x≤0.067) to 3.3 K in the tetragonal phase (space group I4/mmm) with more phosphorus (x≥0.067). Our data analysis suggests that a doping-induced softening related to an in-plane Ni and As(P) phonon mode is responsible for the enhanced superconductivity in the tetragonal phase.