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Mesoscale variations of chemical and electronic landscape on the surface of Weyl semimetal Co$_3$Sn$_2$S$_2$ visualized by ARPES and XPS

Sudheer Anand Sreedhar, Matthew Staab, Mingkun Chen, Robert Prater, Zihao Shen, Giuseppina Conti, Ittai Sidilkover, Zhenghong Wu, Eli Rotenberg, Aaron Bostwick, Chris Jozwiak, Hadas Soifer, Slavomir Nemsak, Sergey Y. Savrasov, Vsevolod Ivanov, Valentin Taufour, Inna M. Vishik

TL;DR

Co3Sn2S2 exhibits variable surface terminations on a single cleave, including a mesoscale disordered intermediate region that hosts distinct S-2p surface states linked to Sn vacancy densities and altered ARPES features near $E_F$. The authors integrate spatially resolved ARPES and XPS with DFT slab calculations and SEESSA simulations, plus unsupervised ML (UMAP, k-means) to map chemistry and electronics across the surface. They show that intermediate-region spectra cannot be expressed as a simple linear mix of the pure terminations and identify a characteristic vertical ARPES feature associated with disorder, establishing heuristics to identify surface disorder and its impact on topological surface states. This framework provides a practical approach to study how variable surface disorder influences Fermi arcs in Weyl semimetals and can be extended to other materials with similar surface variability.

Abstract

The multiple crystalline terminations in magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ display distinct topological and trivial surface states, which have successfully been distinguished experimentally. However, a model of pure terminations is known to be inadequate because these surfaces exhibit a high degree of spatial heterogeneity and point disorder. Here we perform a spectromicroscopy study of the surface chemistry and surface electronic structure using photoemission measurements in combination with first-principles calculations of core levels. We identify an intermediate region with properties distinct from both the sulfur and tin terminations, and demonstrate that the spectral features in this region can be associated with a disordered termination with a varying density of surface tin vacancies. This work establishes heuristics for identifying variable surface disorder using photoemission, an important prerequisite to experimentally establishing the behavior of momentum-space topological surface features subject to variable surface disorder on a single cleave.

Mesoscale variations of chemical and electronic landscape on the surface of Weyl semimetal Co$_3$Sn$_2$S$_2$ visualized by ARPES and XPS

TL;DR

Co3Sn2S2 exhibits variable surface terminations on a single cleave, including a mesoscale disordered intermediate region that hosts distinct S-2p surface states linked to Sn vacancy densities and altered ARPES features near . The authors integrate spatially resolved ARPES and XPS with DFT slab calculations and SEESSA simulations, plus unsupervised ML (UMAP, k-means) to map chemistry and electronics across the surface. They show that intermediate-region spectra cannot be expressed as a simple linear mix of the pure terminations and identify a characteristic vertical ARPES feature associated with disorder, establishing heuristics to identify surface disorder and its impact on topological surface states. This framework provides a practical approach to study how variable surface disorder influences Fermi arcs in Weyl semimetals and can be extended to other materials with similar surface variability.

Abstract

The multiple crystalline terminations in magnetic Weyl semimetal CoSnS display distinct topological and trivial surface states, which have successfully been distinguished experimentally. However, a model of pure terminations is known to be inadequate because these surfaces exhibit a high degree of spatial heterogeneity and point disorder. Here we perform a spectromicroscopy study of the surface chemistry and surface electronic structure using photoemission measurements in combination with first-principles calculations of core levels. We identify an intermediate region with properties distinct from both the sulfur and tin terminations, and demonstrate that the spectral features in this region can be associated with a disordered termination with a varying density of surface tin vacancies. This work establishes heuristics for identifying variable surface disorder using photoemission, an important prerequisite to experimentally establishing the behavior of momentum-space topological surface features subject to variable surface disorder on a single cleave.

Paper Structure

This paper contains 5 sections, 8 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Results
  4. Discussion
  5. Conclusions

Figures (8)

  • Figure 1: Pure terminations of Co$_3$Sn$_2$S$_2$: ARPES. (a)-(b) ARPES spectra and (c)-(d) corresponding DFT slab calculation along the high-symmetry $\Gamma-K$ direction on the Sn (a,c) and S (b,d) termination, respectively. (e)-(f) constant energy maps at $E_F$ for the Sn and S termination and (g)-(h) corresponding DFT-slab calculation on the Sn (e),(g) and S (f),(h) terminations, respectively. DFT colors heavily emphasize bands with strong Sn (blue) or S (red) surface character. Weyl points momenta are shown as green and pink dots in panel (g) and (h).
  • Figure 2: S$-2p$ core levels on pure terminations. (a)-(b) S$-2p$ core levels on the Sn- and S- terminations. (c)-(d) SESSA simulation of the S$-2p$ XPS spectra at the two terminations using DFT-calculated binding energy shifts and attenuation based on the crystal structure supplement. Binding energy shifts indicated by vertical dashed lines and corresponding labels. Near-surface atomic planes with sulphur (yellow), tin (gray) and cobalt (pink) atoms. A photoemitting sulphur atom is marked red to disambiguate the emitter atom from the surface atom.
  • Figure 3: Spatial dependence of S$-2p$ core levels. (a) Spatial map of S$-2p$ S-termination feature, found by integrating in energy window $160.8\pm0.02$ eV at each measurement position. (b) UMAP 3D performed on the same spatial core level data set (c) k-means clustering of the spatial core levels into 3 clusters representing S termination (blue), Tin Termination (Red) and Intermediate (Int.) region (black)
  • Figure 4: Correlation between local S$-2p$ core levels and ARPES spectra. Scatter plots of XPS and ARPES euclidean difference metrics for the full data set (a), as well as S (b), Sn (c), and mixed (d) cluster subsets. $r$ is the Pearson correlation coefficient.
  • Figure 5: UMAP on spatially resolved photoemission data. (a) 2D UMAP on spatial ARPES data. (b) 2D UMAP on spatial S-2p spectra measured at the same spatial positions.
  • ...and 3 more figures