Superconducting and Normal State Properties of APd2As2 (A = Ca, Sr, Ba) Single Crystals

TL;DR

This study investigates the structural, magnetic, and superconducting properties of APd2As2 single crystals (A = Ca, Sr, Ba), revealing bulk superconductivity in CaPd2As2 and SrPd2As2 with conventional s-wave pairing, and analyzing their heat capacity and anisotropic behavior.

Contribution

It provides detailed characterization of APd2As2 compounds, demonstrating bulk superconductivity in Ca and Sr variants and analyzing their superconducting gap structure using the alpha-model.

Findings

CaPd2As2 and SrPd2As2 are bulk superconductors below 1.27 K and 0.92 K.

BaPd2As2 shows only trace superconductivity.

CaPd2As2 exhibits an anisotropic s-wave superconducting gap.

Abstract

The synthesis and crystallography, magnetic susceptibility chi, magnetization M, specific heat Cp, in-plane electrical resistivity rho and in-plane magnetic penetration depth measurements are reported for single crystals of APd2As2 (A = Ca, Sr, Ba) versus temperature T and magnetic field H. The crystals were grown using PdAs self-flux. CaPd2As2 and SrPd2As2 crystallize in a collapsed body-centered tetragonal ThCr2Si2-type structure (I4/mmm), whereas BaPd2As2 crystallizes in the primitive tetragonal CeMg2Si2-type structure (P4/mmm), in agreement with literature data. The rho(T) data exhibit metallic behavior for all three compounds. Bulk superconductivity is reported for CaPd2As2 and SrPd2As2 below Tc = 1.27 and 0.92 K, respectively, whereas only a trace of superconductivity is found in BaPd2As2. No other phase transitions were observed. The chi(T) and M(H) data reveal anisotropic diamagnetism in the normal state, with chi_c > chi_ab for CaPd2As2 and BaPd2As2, and chi_c < chi_ab for SrPd2As2. The normal and superconducting state data indicate that CaPd2As2 and SrPd2As2 are conventional type-II nodeless s-wave electron-phonon superconductors. The electronic superconducting-state heat capacity data for CaPd2As2, which has an extremely sharp heat capacity jump at Tc, are analyzed using our recent elaboration of the alpha-model of the BCS theory of superconductivity, which indicates that the s-wave gap in this compound is anisotropic in momentum space.