Electron-phonon mediated superconductivity in transition metal based two-dimensional materials

Abstract

In this thesis, we present a comprehensive study of electron-phonon mediated superconductivity in transition metal-based two-dimensional (2D) materials using first-principles calculations and anisotropic Migdal-Eliashberg formalism [1,2]. Motivated by the increasing interest in 2D superconductors due to their tunable electronic properties, reduced dimensionality, and potential applications in quantum devices [3–8], we explore the electron-phonon mediated superconductivity in two-dimensional materials. Exploring superconductivity in two-dimensional materials offers a rich foundation for investigating phenomena such as unconventional pairing mechanisms, topological superconductivity, and quantum phase transitions—all of which are profoundly influenced by the reduced dimensionality and strong electronic correlations [9–11]. These fundamental insights go hand in hand with technological applications, particularly in nano superconducting devices. By harnessing external controls—such as gating, strain engineering, or advanced material synthesis—researchers can systematically induce or enhance superconductivity in 2D systems, enabling device miniaturization and opening new frontiers in quantum technologies [12–15]. In this context, our work focuses on discovering novel transition metal-based two-dimensional superconductors and probing their underlying quantum phenomena. We first investigate monolayer tungsten monofluoride (WF), where phonon dispersion calculations confirm its dynamical stability. Using anisotropic Migdal-Eliashberg theory within the EPW framework, we find that pristine WF exhibits a weak electron-phonon coupling constant (λ = 0.49)andasuperconducting transition temperature (Tc) of 2.6 K. Interestingly, we identify a VanHovesingularity (VHS)belowtheFermilevel. UponholedopingtoshifttheFermileveltothe VHS, λ increases to 0.93, enhancing Tc to 11 K. Further inclusion of full-bandwidth Eliashberg effects boosts Tc to 17.2 K, establishing WF as a strong candidate for tunable superconductivity. Next, we report on a newly predicted member of the transition metal carbo-chalcogenide (TMCC) family: Zr2S2C. Our phonon band structures analysis confirms its stability in bulk and monolayer forms. While the bulk phase shows weak coupling superconductivity (λ = 0.41, Tc ≈ 3 K), the monolayer version exhibits enhanced λ = 0.62 and Tc ≈ 6.4 K, primarily due to the softening of acoustic phonon modes. Under biaxial strain, the λ value increases to 1.33, driving the monolayer into a strong coupling regime and nearly doubling its Tc. These findings reveal the sensitivity of superconductivity to lattice and phonon structure in 2D TMCC systems. We then explore superconductivity in functionalized MXenes, which offer remarkable chemical tunability. Specifically, we examine V2C, and Nb2C MXenes functionalized with hydrogen, fluorine, and their mixture. While pristine V2C is non-superconducting, functionalization induces phonon softening and modifies the electronic density of states, leading to finite electron-phonon coupling and the emergence of superconductivity. The mixed-functionalized V2C shows the highest Tc due to the synergistic effects of low-energy ZA mode softening and high-frequency hydrogen vibrations. We extend this analysis to Nb2C, where mixed functionalization results in Tc = 9.2 K, exceeding other reported functionalized Nb2C phases, thus confirming the effectiveness of chemical design strategies. Finally, motivated to realize coexisting superconducting and topological states, we investigate monolayer CrH2 in two crystallographic symmetries: hexagonal P-6m2 and trigonal P-3m1. Phonon and mechanical stability criteria validate both structures’s stability, while Z2 invariant calculations reveal nontrivial topology in the P-6m2 phase. Migdal-Eliashberg analysis predicts superconductivity in both phases, with Tc ∼ 11K (P-6m2) and Tc ∼ 8K (P-3m1), attributed to Cr-d orbital interactions with low-energy phonons. Our findings offer a promising foundation for further exploration of co-existence of topological and superconducting states in monolayer hydrides and their experimental realization. Overall, our results establish design principles for inducing and enhancing superconductivity in 2D materials through electronic tuning, strain engineering, and surface functionalization. The thesis contributes to the broader effort of identifying new 2D superconductors with high tunability and multifunctional properties, and it provides theoretical guidance for experimental investigations targeting topological and strongly correlated superconducting phases.