Superconductivity in layered ZrP2−xSex with PbFCl-type structure

Abstract

We performed a systematic study of the crystal structure, physical properties, and electronic structure of PbFCl-type ZrP2−xSex (0.3 ≤ x ≤ 0.9). We successfully synthesized single-phase polycrystalline samples for the Se substitution range of 0.4 ≤ x ≤ 0.8. The crystal structure of the compound is characterized by the alternate stacking of a two-dimensional P square net and a Zr–(P1−xSex) network. ZrP2−xSex exhibits a dome-like superconductivity phase diagram and has a maximum superconducting transition temperature (Tc) of 6.3 K for x ≈ 0.6. Resistivity and Hall measurements indicated that electron–phonon scattering plays a dominant role and that electron-type carriers dominate charge transport. Specific heat measurements confirmed that ZrP2−xSex exhibits bulk superconductivity. Further, the value of the specific heat jump at Tc (ΔC/γTc ≈ 1.35) is in keeping with the BCS weak-coupling model. These facts suggest a rather conventional pairing mechanism in ZrP2−xSex. The x dependence of Tc can be explained on the basis of the density of states (DOS) for x ≤ 0.7, whereas the decrease in Tc with an increase in the DOS for x = 0.8 needs further investigation. One possible reason for the suppression of superconductivity is that the PbFCl-type structure becomes unstable for x ≥ 0.8. The results of electronic structure calculations agree reasonably well with those of the experimental observations, suggesting that the Zr_d band plays a primary role in determining the physical properties. Further, the calculations predict a significant change in the Fermi-surface topology for x ≥ 0.8; this is a probable reason for the decrease in Tc as well as the instability of the PbFCl-type structure.