Comparative study of rare earth hexaborides using high resolution angle-resolved photoemission

Abstract

Strong electron correlations in rare earth hexaborides can give rise to a variety of interesting phenomena like ferromagnetism, Kondo hybridization, mixed valence, superconductivity and possibly topological characteristics. The theoretical prediction of topological properties in SmB6 and YbB6, has rekindled the scientific interest in the rare earth hexaborides, and high-resolution ARPES has been playing a major role in the debate. The electronic band structure of the hexaborides contains the key to understand the origin of the different phenomena observed, and much can be learned by comparing the experimental data from different rare earth hexaborides. We have performed high-resolution ARPES on the (001) surfaces of YbB6, CeB6 and SmB6. On the most basic level, the data show that the differences in the valence of the rare earth element are reflected in the experimental electronic band structure primarily as a rigid shift of the energy position of the metal 5d states with respect to the Fermi level. Although the overall shape of the d-derived Fermi surface contours remains the same, we report differences in the dimensionality of these states between the compounds studied. Moreover, the spectroscopic fingerprint of the 4f states also reveals considerable differences that are related to their coherence and the strength of the d-f hybridization. For the SmB6 case, we use ARPES in combination with STM imaging and electron diffraction to reveal time dependent changes in the structural symmetry of the highly debated SmB6(001) surface. All in all, our study highlights the suitability of electron spectroscopies like high-resolution ARPES to provide links between electronic structure and function in complex and correlated materials such as the rare earth hexaborides.

Nesting-driven multipolar order in CeB6 from photoemission tomography

“…This Fermi surface agrees with previous measurements for CeB 6 (refs 27 , 28 , 29 , 30 ). The ellipsoids are typical for the hexaborides in general 28 29 . Their orbital character is composed of extended Ce 5 d states with admixtures of localized Ce 4 f near the Fermi energy ( E F ; ref.…”

“…The electronic structure derived here by soft X-ray photoemission from various cleavage planes is consistent with previous low photon energy studies from the (100) plane 28 29 . Neupane et al reported a strong k -dependent renormalization around the Γ point concluded from the deviation of the experimental data and the bandstructure calculation in this region and enhanced quasiparticle intensity.…”

“…Third, hexaborides are subject to surface reconstructions and possess surface states, which might mask the bulk electronic structure 23 24 25 26 . Previous ARPES studies on CeB 6 are therefore sparse 27 28 29 . The Fermi surface has also been studied by de-Haas–van Alphen measurements.…”

Additional energy scale in SmB6 at low-temperature

“…As discussed below, this small peak is most likely related to the indirect tunneling into the localized Γ (2) 8 states. Compared to delocalized f -states, such localized f -states may give rise to only small anomalies in spectroscopy measurements [43].…”

SmB6electron-phonon coupling constant from time- and angle-resolved photoelectron spectroscopy

“…Despite the observation of a gap opening at the Fermi level by spectroscopic measurements, transport experiments show signs of residual conductivity [4]. The origin of this residual conductivity has puzzled the scientific community until the recent discovery of low-temperature metallic surface states [5], of potential, albeit debated, topological character [6][7][8][9][10][11][12][13][14].…”