In the past decade, 2D materials have provided rich physical insights into superconductivity (SC), e.g., Ising pairing, quantum criticality and interface-caused high transition temperature (Tc). It is intriguing when 2D superconductors coexist with topological properties, which is promising for topological superconductivity (TSC), large electron-phonon coupling (EPC) at surface/edge , topological phonon mediated SC , etc.
2D van der Waals(vdW) superconductors, in particular, are of great interest and advantage in future applications. Typical vdW materials possess odd atomic layers, such as TMDs (N = 3), MXenes and MA2Z4 family (N = 7). Here we propose a material prototype MX (N = 4) for achieving 2D vdW superconductors (Figure 1), where M is transition-metals and X can be halogens (e.g., F, Cl) or chalcogens (e.g. S, Se). The MX family has middle M-M layers as conducting layers, and outmost X atoms to maintain a good vdW-feature.
In our work published on npj Computational Materials, we selected transition-metal monohalides MX (M = Zr, Mo; X = F, Cl) to demonstrate the prospects of MX family as 2D superconductors with relatively high-Tc and emerging topological properties. Based on first-principles calculations, we revealed 2D MX (M = Zr, Mo; X = F, Cl) have strong EPC (λ > 1) and remarkable Tc (5.9-12.4 K) due to the rich mechanisms of phonon softening. Anisotropic Migdal-Eliashberg theory shown the superconducting gap (i.e., single-gap or two-gap) depends on orbital components of the Fermi surface. Besides SC, we found 2D MX (M = Zr, Mo; X = F, Cl) are encoded with nontrivial Z2 topological invariants, with MoCl and MoF as TSC candidates. We also found there exist Dirac phonons at the Brillouin zone boundaries of ZrCl and MoCl. The distinctive w-shape (i.e., two-dip like) phononic edge states indicate a underlying edge-enhanced EPC in 1D zigzag ribbons. A further study of the Janus Zr2FCl (M2XY), which has the inversion breaking with respect to ZrCl, shown the interesting chiral phonon  related SC. The acoustic chiral modes at K/K' valleys seem to accelerate the so-called rotonlike softening , leading to an enhanced Tc = 13.2 K.
Our results provide opportunities for the interplay between 2D SC and topological electrons/phonons & chiral phonons in a single material platform. The proposed MX family with N = 4 may become a rising class of vdW materials, deserving the attention from experimentalists.
For more details on our work, please see: https://doi.org/10.1038/s41524-022-00871-y
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