Material Shifts State on Demand, Paving the Way for Controllable Quantum Devices

Topological materials represent a burgeoning field with the potential to revolutionise materials science, but realising practical control over their topological properties remains a significant hurdle. Xian P. Yang, Chia-Hsiu Hsu, and Gokul Acharya, alongside et al., from Princeton University, Nanyang Technological University, and the University of Arkansas, now report the observation of a reversible topological phase transition in the distorted square net material GdPS. Their research details how in situ potassium dosing induces this transition, demonstrated through angle-resolved photoemission spectroscopy and corroborated by first-principles calculations. This work is significant because it reveals a cascade of topological changes within the material’s sub-surface phosphorus layer, progressing from a trivial band gap to a Dirac cone state and ultimately a two-dimensional topological insulator, all driven by subtle structural distortions induced by potassium adsorption. The ability to manipulate topology in this manner offers a novel pathway for exploring and controlling topological states within bulk materials.

Potassium-induced topological phase transitions in layered GdPS are observed with increasing carrier density

Scientists have unveiled a structurally driven, reversible topological phase transition in the quantum material GdPS, achieved through precise potassium dosing. This breakthrough addresses a significant challenge in the field of topological materials, the ability to dynamically control and manipulate their exotic electronic properties.
The research details a cascade of topological phases emerging within the sub-surface phosphorus layer of GdPS, transitioning from a large band gap indicative of a topologically trivial state, through a gapless Dirac cone displaying a 2 eV dispersion, and culminating in a two-dimensional topological insulator as predicted by theoretical models. This carefully orchestrated evolution is not driven by simple electronic doping, but by subtle, quantifiable distortions in the crystal lattice induced by the adsorbed potassium.

The work demonstrates that in situ potassium deposition induces a reversible topological phase transition in GdPS, a distorted square net material. Employing angle-resolved photoemission spectroscopy and first principles calculations, researchers observed a progression of phases within the phosphorus layer.
Initially, a substantial band gap characterised the material, indicative of a topologically trivial state. Subsequent potassium dosing induced a band gap closure and the emergence of Dirac cones at the Γ point.

Electronic structure characterisation via angle-resolved photoemission spectroscopy and potassium doping reveals significant band modifications

Angle-resolved photoemission spectroscopy served as the primary experimental technique for characterizing the electronic structure of GdPS throughout the topological transition. Measurements were conducted using 81 eV light to probe the Γ-X-Y plane, corresponding to kz = 0, revealing the Fermi surface and band dispersions.

Initial ARPES data of pristine GdPS displayed a small pocket at the Y point and deeper binding energy features, consistent with first principles calculations. Detailed band dispersion mapping along the Γ-Y-Γ direction confirmed a substantial band gap of approximately 0.74 eV at the Y point, directly attributable to the lattice distortion within the phosphorus layers.

The study then employed in situ potassium dosing to induce a reversible topological phase transition. Potassium deposition was performed in cycles lasting 60 seconds each, utilizing a 6.1A applied current to control the dosage. Band evolution was monitored with ARPES after each cycle, revealing a systematic reduction in the band gap at the Y point.

Electron and hole pockets moved closer together with increasing potassium coverage, while a flat band remained relatively stable. Crucially, continued potassium dosing led to band inversion, signifying the emergence of a two-dimensional topological insulator state. This transition was not simply a rigid band shift, but rather a consequence of subtle, quantifiable lattice distortions in the first phosphorus layer.

First principles calculations corroborated the ARPES findings, accurately capturing the observed band dispersions and validating the structural origin of the topological change. The work demonstrates a unique pathway for manipulating topological states in bulk materials through surface-driven structural modifications.

Potassium doping drives topological phase transitions and band inversion in GdPS subsurface phosphorus layers, resulting in enhanced thermoelectric properties

Angle-resolved photoemission spectroscopy and theoretical calculations reveal a cascade of topological phases in the sub-surface phosphorus layer of GdPS induced by potassium dosing. Initial measurements demonstrate a large, topologically trivial band gap exceeding 2 eV, which evolves into a gapless Dirac cone state with a dispersion of 2 eV.

Subsequent potassium dosing ultimately leads to the formation of a two-dimensional topological insulator phase, as predicted by theory. Band inversion was visualized through the analysis of momentum distribution curves, utilizing incident light with linear vertical and horizontal polarizations to selectively excite hole and electron bands.

These measurements confirm a complete electronic phase transition at the Y point, progressing from a large band gap to full band closure and subsequent band inversion. Slab model calculations, focusing on the first phosphorus layer, corroborate the experimental findings and demonstrate a rigid downward shift in the Fermi level, consistent with donor-like defects on the sample surface.

Following potassium dosing, slab calculations reveal band inversion near the Y point for the first phosphorus layer, while bulk states remain largely unchanged. Although a nontrivial band gap arises from this band crossing, its small size prevented resolution with ARPES; however, density functional theory calculations confirm its topological nature.

The topological phase transition primarily occurs within the first phosphorus layer, with minimal influence from bulk bands, as evidenced by surface-only monolayer models accurately reproducing the full slab calculations. The monolayer model demonstrates a Z2 value of 0 before potassium dosing and 1 after, signifying the topological transition.

Local density of states calculations within the 2D band gap of the first phosphorus layer reveal edge states connecting the valence and conduction bands, confirming the formation of a topological insulator phase. The bulk band gap persists throughout potassium dosing, maintaining a topologically trivial bulk phase. Potassium dosing induces a structural distortion in the first phosphorus layer, decreasing the P-P bonding angle from 100.5° to 98.0°, which directly drives the observed topological phase transition.

Potassium-induced topological phase transition in subsurface phosphorus of GdPS alters its electronic properties

Researchers have demonstrated a reversible topological phase transition induced by potassium dosing in the distorted square net material, GdPS. Through angle-resolved photoemission spectroscopy and theoretical calculations, a progression of topological states was observed within the phosphorus sub-surface layer.

This evolution begins with a substantial, topologically trivial band gap, transitions through a gapless Dirac cone exhibiting a dispersion of over 2 electron volts, and culminates in a two-dimensional topological insulator state. The observed changes are fundamentally driven by structural distortions in the initial phosphorus layer resulting from potassium adsorption, rather than solely by electron doping.

This structural modification induces band gap closure and the emergence of topological characteristics. The ability to manipulate the topology of a sub-surface layer within a bulk material provides a novel approach to controlling topological states and exploring their potential. GdPS therefore represents a promising material for investigating and harnessing tunable topological phase transitions for future device applications.

The authors acknowledge that their analysis primarily focuses on the first phosphorus layer due to the dominance of its signal in the spectroscopic measurements. This limits a complete understanding of topological behaviour in deeper layers. Future research could investigate the extent to which these topological changes propagate into the bulk material and explore the potential for manipulating multiple sub-surface layers. The reversible and structurally driven nature of this transition establishes GdPS as a valuable platform for fundamental studies and potential technological advancements in topological materials.