Modulation Of Structure And Superconductivity Of Zirconium Nitride Under Strain

Transition metal nitrides with excellent physical and chemical properties（such as high melting point,high hardness,and high wear resistance）have attracted a large number of concerns in recent years,finding wide-ranging applications as ultrahigh temperature ceramics,ultrawear-resistant materials,cutting tools,optoelectronic devices and so on.Among them,zirconium nitride possesses a high degree of structural uniformity and superior flank wear resistance.In addition,ZrN hosts superconductivity with the highest superconducting critical temperature（T_c）of 10.0 K among the IVB transition-metal nitrides,which has attracted considerable interest for probing the inherent mechanisms and finding potential applications.Strain engineering is an effective approach to modulate material properties and enhance functional performance,which has been widely adopted in advanced materials research and processing.In recent years,extensive efforts to explore strain-induced effects have been directed toward modulating superconducting properties of diverse metallic compounds,such as MgB₂ thin films,SrTiO₃ thin films and dense H₃S solids.It has been found,however,that strain engineering has a less profound influence on the superconductivity for these compounds because of relatively weak structural and stress response.In contrast,hard or super-hard materials exhibit stronger ability to sustain larger structural changes and the resulting modulation of electronic and lattice vibrational behaviors,thus allowing larger degrees of modulations in their electronic band structures and electron-phonon coupling（EPC）.Our recent work shows that a striking closure of the electronic band gap occurs in diamond,induced by extraordinary structural changes via versatile strain engineering,inducing unexpected robust superconducting state,producing a T_c up to 2.4-12.4 K for the Coulomb pseudopotentialμ^*=0.15-0.05.These results show that strain engineering can serve as a highly effective tool to induce superconductivity in other non-superconductive materials,and this approach also holds great promise for enhancing superconducting critical temperature for existing superconductors via further increasing the electronic density of states and inducing lattice softening that raises electron-phonon coupling.It is thus desirable to explore the enhancement of superconductivity via strain engineering in wider varieties and ranges of materials to better describe the mechanisms and establish general rules governing physical conditions and properties important to fundamental understanding and practical applications.In this paper,we carried out systematic and in-depth calculation of stress-strain relations for ZrN using the first-principles method to examine the stress responses of electronic and superconducting properties under a variety of loading conditions and obtained the following innovative results:1.The theoretical ideal strength for ZrN is 28.4 GPa,25.3 GPa and 17.0 GPa under tensile,pure shear and Vickers shear loading conditions,which is closely related to the strong covalent bond properties.2.In order to examine the effect of strain on electronic and superconducting behaviors,we have examined the phonon dispersion curves,electronic band structures,and superconducting critical temperature of ZrN at equilibrium and under selected prominent tensile,pure shear and Vickers shear deformation modes.It is found that the applied stress has a significant influence on the superconductivity,especially the[001]tensile strain and（111）[-1-12]shear strain.3.The theoretical results show that the enhancement of superconducting critical temperature stems from the increase of electronic density of states near the Fermi surface and the softening of phonon vibrations,which triggers stronger electron-phonon interaction than that under hydrostatic pressure.