The two conduction bands of monolayer CrSBr on Au

TL;DR

The paper investigates how an ultraflat metal interface influences the electronic structure and symmetry-protected features of a monolayer CrSBr. Using angle-resolved photoemission spectroscopy on CrSBr supported by template-stripped Au, the study reveals substantial substrate-driven charge transfer that fills conduction-band states and strongly renormalizes the band gap, yielding an estimated monolayer gap of ~1.31 eV. Crucially, two conduction bands are observed with a ~30 meV splitting at the X point (and at Γ), signaling substrate-induced breaking of glide-mirror symmetry that would otherwise enforce degeneracies in a freestanding monolayer. These results demonstrate that metal-2D material interfaces can fundamentally alter band topology, with important implications for device integration and the interpretation of symmetry-protected electronic features in 2D magnets.

Abstract

We report the electronic structure of monolayer CrSBr exfoliated onto mica template-stripped gold substrates. Angle-resolved photoemission spectroscopy reveals charge transfer from the substrate, populating the conduction band of monolayer CrSBr, accompanied by a pronounced reduction in the quasiparticle band gap. Furthermore, we observe two separate conduction bands that exhibit a splitting at the X point. This indicates a breaking of glide-mirror symmetry, which in the bulk or in a free-standing monolayer protects the band degeneracies at the Brillouin zone boundary. Our results demonstrate that ultraflat gold substrates do more than modify carrier densities and screening: they can lift symmetry-protected degeneracies and thus fundamentally reshape the band topology of 2D materials.

Figures (3)

• Figure 1: Characterisation of bulk-like and monolayer regions of CrSBr on TSG. (a) Schematic of the TSG approach. (b) SPEM map, highlighting the 1 ML and bulk regions from which the data in Fig. 2 are measured. (c) Optical image. (d) Grayscale image of the blue channel only, which is the most sensitive to the thinnest regions. (e) AFM height profile, and (f) optical intensity within the region shown in (g), corresponding to the black rectangle in (c). (h) Estimate of the magnitude of sample charging during ARPES, as a function of position.
• Figure 2: Evolution of the band gap in the monolayer limit. (a) ARPES spectra measured with $hv$=80 eV on the bulk-like region. The VBM is identified by a bright feature at the $\Gamma_{0\bar{1}}$ point in the second Brillouin zone. (b) Equivalent measurement on the monolayer region, highlighting the relatively weak spectral weight on the conduction band states. (c) EDCs from the $\Gamma_{0\bar{1}}$ points, with arrows showing the features associated with the VBMs in the bulk and monolayer cases. The inset shows a fit to the EDC at $\Gamma_{00}$, from which the CBM is extracted. (d) Comparison of the band gap of bulk and monolayer CrSBr on mica-TSG and also the Au(111) substrate via the KISS method reported by Bianchi et al.Bianchi2023ChargeTransfer.
• Figure 3: Dimensionality crossover as a function of conduction band filling. (a) ARPES spectra on the 1 ML region along $\Gamma-\rm{X}$. (b) Constant energy contour at $-0.12$ eV, corresponding to the CBM of the lower conduction band. (c) Constant energy contour at $E_F$. (d) Dispersion along $\Gamma-\rm{X}$ with an EDC at $\rm{X}$ highlighting contributions from two electron bands. (e) EDC at $\rm{X}$ showing a splitting of 30 meV. (f) Schematic band structure evolution with conduction band filling. (g) Schematic band structures. (h) Density functional theory calculation of freestanding monolayer showing CB degeneracy at $\rm{X}$ enforced by glide-mirror symmetry. (i) Structural representation of monolayer CrSBr on Au(111). The two Cr sites would be equivalent in a freestanding monolayer via the glide-mirror symmetry, but become inequivalent due to the substrate.