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 (T_c). 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 [1], topological phonon mediated SC [2], 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-T_c 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 T_c (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 [3] related SC. The acoustic chiral modes at K/K' valleys seem to accelerate the so-called rotonlike softening [4], leading to an enhanced T_c = 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

References

[1] Li, R. et al. Underlying Topological Dirac Nodal Line Mechanism of the Anomalously Large Electron-Phonon Coupling Strength on a Be (0001) Surface. Phys. Rev. Lett. 123, 136802 (2019).

[2] Di Miceli, D.; Setty, C.; Zaccone, A. Theory of superconductivity mediated by topological phonons. Phys. Rev. B 106, 054502 (2022).

[3] Zhang, L. & Niu Q. Chiral Phonons at High-Symmetry Points in Monolayer Hexagonal Lattices. Phys. Rev. Lett. 115, 115502 (2015).

[4] Kishine, J., Ovchinnikov A. S. & Tereshchenko A. A. Chirality-Induced Phonon Dispersion in a Noncentrosymmetric Micropolar Crystal. Phys. Rev. Lett. 125, 245302 (2020).