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Revealing the (111) surface electronic structure of epitaxially grown Na$_2$KSb photocathode

N. Yu. Solovova, V. A. Golyashov, S. V. Eremeev, S. Yu. Priobrazhenskii, S. P. Lebedev, A. A. Lebedev, V. S. Rusetsky, O. E. Tereshchenko

TL;DR

The paper addresses the limited understanding of the electronic structure of multil-alkali antimonides by achieving crystalline epitaxial growth of $Na_2KSb$ on graphene-coated SiC(0001) and conducting ARPES measurements complemented by DFT. It identifies surface-state bands arising from multiple surface terminations, notes a spin-orbit splitting of about $0.8$ eV, and places the Fermi level near the mid-gap with a gap of approximately $1.4$ eV. Crucially, crystalline order persists after Cs/Sb activation, enabling future spin-resolved ARPES studies and providing a pathway to rationally tailor $Na_2KSb(Cs)$ photocathodes and their negative electron affinity properties. Overall, the work establishes a robust platform for momentum-resolved investigations of multialkali photocathode surfaces and their spin-polarized emission characteristics.

Abstract

A recent study has established the Na$_2$KSb(Cs) photocathode as a highly efficient emitter of spin-polarized electrons. However, the electronic structure of alkali antimonides remains poorly understood. In this work, we report the first crystalline epitaxial growth of Na$_2$KSb films, achieved via chemical vapor deposition (CVD) on a graphene-coated SiC(0001) substrate. The high crystalline quality of these films enabled a direct investigation of the material's electronic structure using angle-resolved photoemission spectroscopy (ARPES). By comparing the experimental results with density functional theory (DFT) calculations, we have identified dispersive surface states originating from different terminations of the Na$_2$KSb(111) surface. Furthermore, we demonstrate that the crystalline order of the film is preserved following its activation via the deposition of Cs and Sb. This finding opens a pathway for investigating the electronic structure of multialkali Na$_2$KSb(Cs) photocathodes and for rationally improving their properties.

Revealing the (111) surface electronic structure of epitaxially grown Na$_2$KSb photocathode

TL;DR

The paper addresses the limited understanding of the electronic structure of multil-alkali antimonides by achieving crystalline epitaxial growth of on graphene-coated SiC(0001) and conducting ARPES measurements complemented by DFT. It identifies surface-state bands arising from multiple surface terminations, notes a spin-orbit splitting of about eV, and places the Fermi level near the mid-gap with a gap of approximately eV. Crucially, crystalline order persists after Cs/Sb activation, enabling future spin-resolved ARPES studies and providing a pathway to rationally tailor photocathodes and their negative electron affinity properties. Overall, the work establishes a robust platform for momentum-resolved investigations of multialkali photocathode surfaces and their spin-polarized emission characteristics.

Abstract

A recent study has established the NaKSb(Cs) photocathode as a highly efficient emitter of spin-polarized electrons. However, the electronic structure of alkali antimonides remains poorly understood. In this work, we report the first crystalline epitaxial growth of NaKSb films, achieved via chemical vapor deposition (CVD) on a graphene-coated SiC(0001) substrate. The high crystalline quality of these films enabled a direct investigation of the material's electronic structure using angle-resolved photoemission spectroscopy (ARPES). By comparing the experimental results with density functional theory (DFT) calculations, we have identified dispersive surface states originating from different terminations of the NaKSb(111) surface. Furthermore, we demonstrate that the crystalline order of the film is preserved following its activation via the deposition of Cs and Sb. This finding opens a pathway for investigating the electronic structure of multialkali NaKSb(Cs) photocathodes and for rationally improving their properties.
Paper Structure (10 sections, 3 figures)

This paper contains 10 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Experimental and computational details
  3. UHV equipment and measurements
  4. DFT calculations
  5. Results and discussion
  6. Na$_2$KSb epitaxial growth
  7. Electronic properties
  8. Chemical composition
  9. Conclusions
  10. Acknowledgments

Figures (3)

  • Figure 1: (a) Scheme of the Na$_2$KSb(Cs) growth process; (b) Change in photocurrent during one growth cycle of Na$_2$KSb; (c) Change in photocurrent during activation of the Na$_2$KSb surface; (d) Spectral dependence of the photocurrent measured on Na$_2$KSb and Na$_2$KSb(Cs); LEED pattern observed on the surface (e) of graphene/SiC(0001) substrate, $E=80$ eV (f) Na$_2$KSb films $E=34$ eV; (g) the contribution of two different domains to the LEED pattern (f) is highlighted in blue and yellow; (h) LEED pattern observed after the Na$_2$KSb surface activation.
  • Figure 2: (a) ARPES spectrum of Na$_2$KSb measured at 77 K in the $\bar{\mathrm K}-\bar{\Gamma}-\bar{\mathrm K}$ direction. (b) Smoothed second derivative of the spectrum shown in (a). (c) Direct comparison between the ARPES data from (b) and the calculated band structure (f). (d) Calculated bulk band structure of Na$_2$KSb. (e) Ball-and-stick model of the Na$_2$KSb(111) crystal structure. (f) DFT-calculated surface electronic structure of Na$_2$KSb(111).
  • Figure 3: XPS spectra of the Na-$2s$, K-$2p$ and Sb-$3d$ core levels for Na$_2$KSb. The dashed lines indicate the positions of the elemental reference lines.