Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of $AB$-Stacked Crystals

We investigate the electronic structure of $2H$-$\mathrm{N}\mathrm{b}{\mathrm{S}}_2$ and $h$-$\mathrm{B}\mathrm{N}$ by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the $k_z=\pi \mathrm{/}c$ plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk one due to partially broken nonsymmorphic symmetries.