Bulk electronic structure of lanthanum hexaboride ( LaB6 ) by hard x-ray angle-resolved photoelectron spectroscopy

Abstract

In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB 6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB 6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB 6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike" electronic band structure of rare-earth hexaborides and strongly correlated materials.

Hard x-ray angle-resolved photoemission from a buried high-mobility electron system

“…Beyond recording the angle-integrated valence-band spectrum, we also employed hard x-ray angle-resolved photoelectron spectroscopy (HARPES), which has been successfully applied to a few prototypical sample materials like W and Ga 1−x Mn x As [41,42], Ga 1−x Mn x P [43], Mo and TiTe 2 [44], and LaB 6 [45]. A prerequisite for the application of HARPES 125137-4 is that the number of direct transitions excited in the photoemission process is sufficiently high to allow for a mapping of the momentum-resolved band structure.…”

Hard x-ray angle-resolved photoemission from a buried high-mobility electron system

“…Beyond recording the angle-integrated valence-band spectrum, we also employed the novel technique of HARPES, which has been successfully applied so far only to a few prototypical sample materials like, e.g, W and Ga 1−x Mn x As [37,38], Ga 1−x Mn x P [39], Mo and TiTe 2 [40], and LaB 6 [41]. A prerequisite for the application of HARPES is that the number of direct transitions excited in the photoemission process is sufficiently high to allow for a mapping of the momentum-resolved band structure.…”

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

“…Indeed, samples of YB6 of increasing Tc have been confirmed by 89 Y nuclear magnetic resonance (NMR) to also have increasing Ef-DOS, and YB6 is known through tunneling and DC magnetization [33] , point-contact spectroscopy [34] , and electronic Raman scattering [35] , as well as DFT [36] , to have moderate to strong EPC depending on the sample. While experimental and computational studies have been conducted to characterize the Fermi level electronic structure [3,12,[37][38][39][40][41] of YB6 and LaB6 and their phonon modes [41][42][43][44][45][46][47][48] contributing to EPC, the linkage of the two -how frontier electronic eigenstates physically couple to these lattice vibrations -has yet to be established, as has been done with other prominent phonon-mediated superconductors. [26,27,49] Prior studies of our group have quantitatively and qualitatively explained properties of metal hexaborides through the lens of chemical bonding [6,9,50] , and in this study, we extend this capability to superconducting hexaborides by analyzing changes in this bonding under phonon-induced lattice deformations.…”