Multigap superconductivity in iron-free pnictide BaPd₂As₂ revealed by local magnetization and SNS-Andreev spectroscopy

We present experimental study of the iron-free palladium based analogue of the 122 iron arsenide. Single crystals of BaPd₂As₂ with sharp superconducting transition were studied by two distinct techniques: local magnetization and Break-Junction. Local magnetization was used to obtain temperature dependence of the first critical field (H _c1) from T _c down to 10 mK. Extracted data was fitted with various models to obtain order parameter value and draw a conclusion about its potential symmetry. The H _c1(T) data fitting with theoretical models suggested either a single s-wave gap with high anisotropy factor or two s-wave gaps. Multiple Andreev Reflections spectroscopy was used to determine the quantity of condensates and their order parameter amplitudes. The experimental results of Multiple Andreev Reflections showed the presence of two gap-like features. Studying Andreev spectra temperature evolution from 1.6 K up to T _c we’ve extracted the gap temperature dependencies. Further fitting proved the presence of two superconducting condensates. The obtained evidence for two gap superconductivity in iron-free analogue of 122 iron arsenide family is reported for the first time. Despite the two gap superconducting state, the characteristic ratio of the large gap value is almost equal to that typical for Bardeen-Cooper-Schrieffer theory; this result shows that BaPd₂As₂ is a conventional superconductor unlike BaFe₂As₂.