Observation of a structurally driven, reversible topological phase transition in a distorted square net material

Topological materials hold immense promise for exhibiting exotic quantum phenomena, yet achieving controllable topological phase transitions remains challenging. Here, we demonstrate a structurally driven, reversible topological phase transition in the distorted square net material GdPS, induced via in situ potassium dosing. Using angle-resolved photoemission spectroscopy and first principles calculations, we demonstrate a cascade of topological phases in the sub-surface P layer: from a large, topologically trivial band gap to a gapless Dirac cone state with a 2 eV dispersion, and finally to a two-dimensional topological insulator as inferred from theory. This evolution is driven by subtle structural distortions in the first P layer caused by potassium adsorption, which in turn contribute to the band gap closure and topological phase transition. Furthermore, the ability to manipulate the topology of a sub-surface layer in GdPS offers a unique route for exploring and controlling topological states in bulk materials.

💡 Research Summary

In this work the authors demonstrate a reversible, structurally driven topological phase transition in the layered material GdPS, achieved by in‑situ potassium (K) dosing. GdPS consists of alternating Gd‑S‑Gd slabs and phosphorus (P) layers. In the pristine crystal the P layers are not perfect square nets; instead they form quasi‑one‑dimensional arm‑chair chains, which increase the P‑P bond angle from 90° to about 100.5°. This distortion breaks the C2v symmetry that would otherwise protect a Dirac crossing, opening a sizable bulk band gap of roughly 0.74 eV at the Y point, as confirmed by angle‑resolved photoemission spectroscopy (ARPES) and density‑functional theory (DFT) calculations.

The experimental protocol involves sequential K deposition cycles (60 s each, 6.1 A current). After approximately four to five cycles—corresponding to roughly one monolayer of K coverage—the ARPES spectra reveal a progressive narrowing of the valence‑conduction gap, culminating in a complete closure at a critical dosing level. At this point a linear, gapless Dirac cone spanning more than 2 eV appears at the Y point. Photon‑energy‑dependent measurements show no dispersion along k_z, confirming the surface‑like nature of this Dirac state. Further K dosing drives a band inversion: the formerly valence band moves above the conduction band, a hallmark of a topological transition.

First‑principles slab calculations reproduce the experimental evolution. When K is added without allowing the lattice to relax, the bands simply shift downward without gap reduction, indicating that pure electron doping cannot close the gap. By contrast, allowing the P layer to relax under the influence of K leads to a reduction of the P‑P bond angle to ~98°, a partial restoration of the square‑net geometry, and consequently a band inversion. The calculated Z₂ invariant changes from 0 (trivial) to 1 (non‑trivial) for the topmost P layer, while the bulk GdPS remains topologically trivial throughout. Edge‑state calculations on a monolayer model show conducting states bridging the inverted bulk gap, confirming the emergence of a two‑dimensional topological insulator confined to the first P layer.

Reversibility is demonstrated by annealing the K‑dosed sample, which removes K atoms and restores the original large trivial gap. Momentum‑distribution‑curve analysis of the Y‑Γ‑Y direction shows a systematic reduction of the separation between the two Y‑point pockets with increasing K coverage, providing direct momentum‑space evidence of an in‑plane lattice contraction in the first P layer.

The authors emphasize that the driving force of the transition is the structural distortion of the P layer induced by K adsorption, not the electrostatic Stark effect or simple charge transfer. Calculations of two limiting scenarios—(i) K dosing without structural relaxation and (ii) retaining only the K‑induced lattice distortion after K removal—support this conclusion: only the latter yields a band inversion. Consequently, the work establishes a clear causal link between a subtle lattice parameter change (≈2.5° reduction in bond angle) and a dramatic topological phase change.

Overall, the study provides a compelling example of how surface‑adsorbate‑induced lattice engineering can be used to toggle topological phases in a buried two‑dimensional layer of a three‑dimensional crystal. The ability to reversibly switch between a trivial insulator, a gapless Dirac semimetal, and a 2D topological insulator by a simple dosing/annealing cycle opens new pathways for dynamic control of topological states, with potential implications for quantum devices, spintronic applications, and the broader field of topological materials engineering.