Density Functional Theory Simulations of Rare-Earth Hexaborides: Bulk and Surface Studies
by Florian Sohn
Date of Examination:2021-10-25
Date of issue:2021-12-13
Advisor:Prof. Dr. Peter E. Blöchl
Referee:Prof. Dr. Peter E. Blöchl
Referee:Prof. Dr. Stefan Kehrein
Persistent Address:
http://dx.doi.org/10.53846/goediss-8983
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Abstract
English
Rare-earth hexaborides are boron-rich solid state compounds and belong to the strongly correlated electron materials. The physics of the rare-earth hexaborides is strongly influenced by the filling of the 4f atomic shells of the rare-earth constituents. Moreover, their unique range of physical properties make the rare-earth hexaborides interesting for technological applications. Lanthanum hexaboride is an excellent thermionic electron emitter, one reason being its extraordinarily low work function. The rare-earth hexaborides also received considerable attention for possible applications in nanotechnology. Although the rare-earth hexaborides have been studied for decades, a thorough understanding of the correlation physics and also the surface physics is lacking. This thesis is concerned with investigations of rare-earth hexaborides with density functional theory (DFT), a popular technique for ab-initio material simulations. In the DFT computational scheme an approximation for the exchange-correlation energy functional is introduced, for which commonly the local density approximation or the generalized gradient approximation are employed. However, many functionals based on these approximations yield poor results in DFT simulations of materials with strong electronic correlations. A prime example are the transition metal oxides where DFT calculations give qualitatively incorrect results. Various approaches exist to improve upon the results of DFT simulations of strongly correlated materials, including LDA+U, DFT+DMFT, and hybrid functionals. In this thesis, we use DFT with the PBE0r hybrid functional for electronic structure calculations on the light rare-earth hexaborides LaB6, CeB6, PrB6, and NdB6 and explore the applicability of the PBE0r hybrid-functional approach. The PBE0r hybrid functional is based on the Perdew-Burke-Ernzerhof functional of the generalized gradient approximation and mixes the exchange part of the Perdew-Burke-Ernzerhof functional with a portion of exact exchange. The exact exchange contribution is computed as a sum of on-site terms from each atomic site, which are obtained in a local orbital basis. We also present the results of our DFT simulations of the LaB6 (001) cleavage plane. In scanning tunneling microscopy experiments, atomically ordered areas of the cleavage plane appear chainlike (2x1)-reconstructed. We show that these chainlike structures correspond to chains of La ions on top of a B6 layer. For such a (2x1)-reconstructed area, the differential tunneling conductance from scanning tunneling spectroscopy has a feature below the Fermi level. Our DFT calculations show that this surface resonance is mainly composed of dangling bonds of the topmost B6 octahedra and dxy orbitals of the terminal lanthanum ions.
Keywords: density functional theory; PBE0r hybrid functional; rare-earth hexaborides; strongly correlated electron systems; 4f electrons; electronic structure; lanthanum hexaboride surface; scanning tunneling microscopy; scanning tunneling spectroscopy
