Fermi Surface Modeling of Light‐Rare‐Earth Hexaborides using Positron Annihilation Spectroscopy

Abstract

2D angular correlation of the positron annihilation radiation (2D‐ACAR) spectra are measured for LaB6 along high‐symmetry directions and compared with first‐principles calculations based on density functional theory (DFT). This allows the modeling of the Fermi surface in terms of ellipsoid electron pockets centered at X‐points elongated along the Σ axis (Γ−M direction). The obtained structure is in agreement with quantum oscillation measurements and previous band structure calculations. For the isostructural topologically nontrivial SmB6, the similar ellipsoids are connected through necks that have significantly smaller radii in the case of LaB6. A theoretical analysis of the 2D‐ACAR spectra is also conducted for CeB6 including the on‐site repulsion U‐correction to the local density approximation (LDA+U) of the DFT. The similarities of the 2D‐ACAR spectra and the Fermi surface projections of these two compounds allow to infer that both LaB6 and CeB6 are topologically trivial correlated metals.

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

“…Computational methods: Density functional theory (DFT) calculations were performed using the Vienna Ab-initio Software Package (VASP). [67,68] Due to its excellent agreement with positron annihilation spectroscopy experiment in terms of the Fermi surface [40] , the versatile PW91 exchange-correlation functional [69,70] , of the generalized gradient approximation (GGA) family of functionals, was used without any Hubbard U correction on d or f orbitals. Use of a van-der-Waals correction via the PBE-D3 formalism (D3 damping parameters for PW91 are not yet available) was found to have no effect on band structures or energy differences, additionally showing expected matching between the PW91 and PBE GGA functionals.…”

“…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.…”

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

“…Computational methods: Density functional theory (DFT) calculations were performed using the Vienna Ab-initio Software Package (VASP). [67,68] Due to its excellent agreement with positron annihilation spectroscopy experiment in terms of the Fermi surface [40] , the versatile PW91 exchange-correlation functional [69,70] , of the generalized gradient approximation (GGA) family of functionals, was used without any Hubbard U correction on d or f orbitals. Use of a van-der-Waals correction via the PBE-D3 formalism (D3 damping parameters for PW91 are not yet available) was found to have no effect on band structures or energy differences, additionally showing expected matching between the PW91 and PBE GGA functionals.…”

“…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.…”

Integrated anti-electronics for positron annihilation spectroscopy

“…Slow-positron spectroscopy is at the forefront of material science, allowing, among others, the investigation of electronic structure [1,2], fatigue in metals [3,4], the presence of defects [5,6], and the size and distribution of pores [7]. These applications would benefit from high-resolution three-dimensional spectroscopic reconstruction of the sample features.…”