Scanning tunneling spectroscopy of rare earth hexaborides

The surface physics of rare earth hexaborides is of great interest for both technological applications and fundamental research. For example, LaB₆ is a widely used electron emitter due to its low work function and the 4f-electron system SmB₆ is a Kondo insulator with proposed topologically protected surface states. In order to systematically investigate the influence of 4f-electrons on the hexaboride surface physics, we performed scanning tunneling microscopy (STM) on the (001)-cleavage planes of LaB₆ [1] and PrB₆. We found that on both crystal surfaces a chain-like (2x1) reconstruction is present. Furthermore, we found uniform terminations on PrB₆. By performing slab calculations within the framework of density functional theory (DFT) we rationalize this reconstruction as parallel chains of rare earth ions on top of a B₆ framework. Using scanning tunneling spectroscopy (STS) we find a spectral feature below E_F, which can be described as boron dangling-bond state. For PrB₆, an additional contribution is visible in the spectra, which we tentatively attribute to the 4f-electrons.
[1] P. Buchsteiner and F. Sohn et al., arXiv:1907.08096, to be published in Phys. Rev. B.