Switching of topological phase transition from semiconductor to ideal Weyl states in Cu$_2$SnSe$_3$ family of materials

TL;DR

This work shows that a symmetry-independent topological phase transition from a trivial semiconductor to an ideal Weyl semimetal can be achieved by bandgap closure driven by spin-orbit coupling and chemical doping in non-centrosymmetric Cu2SnSe3-family materials. Through first-principles DFT, Wannier-based tight-binding, and surface-state calculations, the authors identify four Weyl points near the Fermi level and robust Fermi arcs, addressing the limitations of symmetry-based routes. They demonstrate mechanisms to tune into the WSM phase via SOC strengthening (or simulated SOC scaling) and chemical substitution (Si↔Ge, S↔Te), with concrete doping thresholds x ≈ 0.4–0.5. The work provides a practical, symmetry-preserving pathway to engineer Weyl fermions in semiconductors and offers a pristine platform to study anomalous transport in WSMs.

Abstract

The exploration of topological phase transition (TPT) mechanisms constitutes a central theme in quantum materials research. Conventionally, transitions between Weyl semimetals (WSMs) and other topological states rely on the breaking of time-reversal symmetry (TRS) or precise manipulation of lattice symmetry, thus constraints the available control strategies and restrict the scope of viable material systems. In this work, we propose a novel mechanism for TPT that operates without TRS breaking or lattice symmetry modification: a class of semiconductors can be directly transformed into an ideal WSM via bandgap closure. This transition is driven by chemical doping, which simultaneously modulates the band gap and enhances spin-orbit coupling (SOC), leading to band inversion between the valence and conduction bands and thereby triggering the TPT. Using first-principles calculations, we demonstrate the feasibility of this mechanism in the Cu$_2$SnSe$_3$ family of materials, where two pairs of Weyl points emerge with energies extremely close to the Fermi level. The bulk Fermi surface becomes nearly point-like, while the surface Fermi surface consists exclusively of Fermi arcs. This symmetry-independent mechanism bypasses the constraints of conventional symmetry-based engineering, and also offers an ideal platform for probing the anomalous transport properties of WSMs.

Figures (8)

• Figure 1: (a) Crystal structure of Cu$_2$SnSe$_3$ series materials with $Cc$ space group. (b) Corresponding BZ and high-symmetry points.
• Figure 2: DFT band structures of nine materials along high-symmetry paths, with SOC effects fully included.
• Figure 3: DFT band structures of Cu$_2$SnSe$_3$ and Cu$_2$GeSe$_3$ along one Weyl point W$_1$ (left column) and their corresponding Fermi surfaces (right column), with SOC included. Both materials exhibit four point-like Fermi surfaces near the BZ center (computed at 5 meV below the Fermi energy to enable clearer visualization), each arising from a Weyl cone.
• Figure 4: The Berry curvature of Cu$_2$SnSe$_3$ in the $\mathbf{k}$ plane contains four Weyl points, projected onto the $k_x$ - $k_y$ plane. The black and green dots denote Weyl points with chirality $+$1 and $-$1, respectively.
• Figure 5: The first and second rows display the surface Fermi surfaces of Cu$_2$SnSe$_3$ and Cu$_2$GeSe$_3$ with opened (001) surface, while the first and second columns showing the contributions from bulk layers and surface layers, respectively. The two rightmost columns present corresponding magnified views.