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

## TL;DR

This study investigates two polymorphs of BaPd2As2, revealing that the ThCr2Si2-type exhibits strong-coupling superconductivity likely driven by soft phonons, unlike the non-superconducting CeMg2Si2-type.

## Contribution

It demonstrates that soft phonons in the ThCr2Si2-type polymorph induce strong-coupling superconductivity, providing insight into phonon-mediated pairing mechanisms.

## Key findings

- ThCr2Si2-type has low Debye temperature indicating soft phonons
- Superconductivity in ThCr2Si2-type is strong-coupling
- CeMg2Si2-type remains a normal metal down to 1.8 K

## Abstract

The specific heat of two polymorphs of BaPd2As2 was measured. The ThCr2Si2-type polymorph (space group I4/mmm, $D_{4h}^{17}$, No. 139) is a previously reported superconductor with a transition temperature Tc $\simeq$ 3.5 K, while the CeMg2Si2-type polymorph (P4/mmm, $D_{4h}^{1}$, No. 123) is a normal metal and does not exhibit superconductivity down to 1.8 K. Our results revealed that the ThCr2Si2-type has an anomalously low Debye temperature, indicative of soft phonons, compared to the CeMg2Si2-type. Moreover, a large specific-heat jump at Tc indicated that the superconductivity of ThCr2Si2-type is a strong-coupling type, which is likely derived from soft phonons.