Enhancement of superconductivity in Zr₂S₂C arising from phonon softening on transition from bulk to monolayer

Abstract

We report a new compound, Zr2S2C, belonging to the transition metal
carbo-chalcogenide (TMCC) family. Through first-principles calculations, our
analysis of phonon dispersion spectra indicates that the compound is dynamically
stable in both bulk and monolayer forms. We systematically investigated
the electronic structure, phonon dispersion, and electron-phonon coupling
driven superconducting properties in bulk and monolayer Zr2S2C. The results
demonstrate the metallic character of bulk Zr2S2C, with a weak electron-phonon
coupling strength (λ) of 0.41 and superconducting critical temperature (Tc) of
∼3 K. The monolayer Zr2S2C has an enhanced λ of 0.62 and Tc of ∼6.4 K.
The increased λ value in the monolayer results from the softening of the acoustic
phonon mode. We found that when biaxial strain is applied, the low energy
acoustic phonon mode in monolayer becomes even softer. This softening leads to
a transformation of the Zr2S2C monolayer from its initial weak coupling state (λ
= 0.62) to a strongly coupled state, resulting in an increased λ value of 1.33.
Consequently, the superconducting critical temperature experiences a twofold
increase. These findings provide a theoretical framework for further exploration
of the layered two-dimensional TMCC family, in addition to offering valuable
insights.