Fragile flat-band surface state in ternary transition metal chalcogenides NbIrTe₄

Ternary transition metal chalcogenides have been predicted to exhibit both topological Weyl semimetal properties in bulk crystals and quantum spin-hall insulating behavior in their monolayer form[1]. NbIrTe₄ features an orthorhombic lattice structure (T_d-phase) with broken inversion-symmetry similar to WTe₂, with a topological band structure predicted to host 16 Weyl points in the Brillouin zone[2]. Here we present a recent scanning tunneling microscopy study of the surface electronic structure of NbIrTe₄, together with ARPES measurements and DFT calculations. On one of the two inequivalent surfaces, we observe a prominent, narrow surface-state peak – with a FWHM around 10 mV – located about 8 meV below the Fermi level. This is in excellent agreement with our ARPES and DFT results, and corresponds to a flat band across the quasi-1D rows, as observed in previous ARPES experiments[3]. This sharp surface-state feature vanishes in tunnelling spectroscopy in areas where the top layer was shifted, disrupting the symmetry and polarization at the surface.