Electronic structure, lattice dynamics and magnetism of new ThXAsN (X=Fe,Co,Ni) superconductors: A First Principles Study

Abstract

In this work, we present a comparative study of mechanical properties, electronic structure, magnetism and lattice dynamics of three different systems namely ThFeAsN, ThCoAsN and ThNiAsN which posses same crystal structure. Experimentally it is well established that ThFeAsN and Th-NiAsN show superconducting properties. However, ThCoAsN compound is not experimentally synthesized. We evaluate the elastic constants of all these systems and it turns out that, these systems are mechanically stable. We found substantial differences in the electronic structures of these three compounds in terms of density of states, band structures and Fermi surfaces. We also present the band structure of ThFeAsN in the anti-ferromagnetic ground state which appears to be the theoretical ground state of the system. Role of spin-orbit coupling in the electronic structures of these three systems is also discussed. For ThFeAsN system, spin-orbit coupling primarily influence the hole pockets around Γ point, where the largest superconducting gaps are usually found. Low energy electronic structures of the other two systems are hardly affected by spin-orbit coupling. Our phonon calculation reveals that all the systems including the ThCoAsN system are dynamically stable. Phonon dispersion relations indicate that optical modes of all the three systems are almost the same but there are significant variations in the lower frequency manifold consisting of mixed modes. We also evaluate the electron-phonon coupling constants as well as superconducting transition temperatures for all three systems within Eliashberg formalism. Our electron-phonon calculation predicts a Tc of 5.4 K for the ThCoAsN system. But the evaluated electron-phonon coupling constant for ThFeAsN, give rise to a Tc < 1 K as compared to the experimental Tc of 30 K. However, the value of Tc is increased considerably (∼15 K) when we take into account the amplifying effects of anti-ferromagnetic spin density wave order and out of plane soft phonon modes.