Strong-Coupling Superconductivity in BaPd2As2 Induced by Soft Phonons in the ThCr2Si2-type Polymorph

Abstract

The specific heat of two polymorphs of BaPd 2 As 2 was measured. The ThCr 2 Si 2 -type polymorph (space group I4/mmm, D 17 4h , No. 139) is a previously reported superconductor with a transition temperature T c ≃ 3.5 K, while the CeMg 2 Si 2 -type polymorph (P4/mmm, D 1 4h , No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr 2 Si 2 -type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg 2 Si 2 -type. Moreover, a large specific-heat jump at T c indicated that the superconductivity of ThCr 2 Si 2 -type is a strong-coupling type, which is likely derived from soft phonons.