Electronic structure of the LaB₆ (001)-surface

Lanthanum hexaboride (LaB₆) is the first compound among the rare earth hexaborides. Due to its low work function and high melting point, LaB₆ is widely used for thermionic electron emission. Recently, the LaB₆ (001)-cleavage plane has been investigated with scanning tunneling microscopy (STM) and spectroscopy (STS). Our data shows a mainly 2×1 reconstructed surface. We rationalize the constant current topographies and differential conductance spectra by simulations based on density functional theory (DFT). Our simulations of a LaB₆ (001)-surface, terminated by chains of lanthanum ions, show, that in STM measurements with positive bias voltage the d-orbitals of the topmost lanthanum ions are addressed. In contrast, at small negative bias voltages, a broad feature in the local density of states below the Fermi energy could be successfully resolved by both STS and DFT and is traced back to surface boron orbitals. Our study shows the necessity of surface calculations in order to understand the experimental data in detail and can be applied to various model systems.