Multiple anharmonic charge density wave analysis in monolayer Weyl Semimetal SnP.*

In this work we analyze the stability and CDW phase transitions of Sn4P3 and its potentially exfolliable monolayer SnP. We confirmed the stability of bulk Sn4P3 both using density functional theory (DFT) calculations and synthesizing it experimentally. The experiments confirm the layer structure of the compound, suggesting the structure may be synthesized down to the monolayer by exfoliation or chemical means. Despite the stability of the bulk counterpart, the monolayer presents unstable phonons at Γ, K and M, which may lead to three possible charge density waves. We study the occurrence of these three CDWs by analyzing the phonons with a non-variational treatment of anharmonicity [1,2]. Interestingly, the ground-state structure would be unnoticed without anharmonic effects, which are very strong in the system. All three CDWs lead to metastable insulator phases. Ultimately we performed a symmetry indicators [3,4] based topological analysis, showing how under strain the phase driven by the active phonon in the K is topologically non-trivial.

[1] Errea, I., Calandra, M. & Mauri, F. Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: Application to platinum and palladium hydrides. Phys. Rev. B 89, 064302 (2014).

[2] Bianco, R., Errea, I., Paulatto, L., Calandra, M. & Mauri, F. Second-order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: theory and stochastic implementation. Phys. Rev. B 96, 014111 (2017).

[3] C. Po, A. Vishwanath, and H. Watanabe, Symmetry-based indicators of band topology in the 230 spacegroups, Nature Communications 8, 50 (2017).

[4] B. Bradlyn, L. Elcoro, J. Cano, M. G. Vergniory, Z. Wang, C. Felser, M. I. Aroyo, and B. A. Bernevig. Topological quantum chemistry. Nature 547, 298 (2017).