Absence of X-Point Band Overlap in Divalent Hexaborides and Variability of the Surface Chemical Potential

Abstract

Angle-resolved photoemission measurements of divalent hexaborides reveals a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. While the global ARPES band structure and gap size observed are consistent with the results of bulk-sensitive soft x-ray absorption and emission boron K-edge spectroscopy, the surface-sensitive photoemission measurements also show a variation with cation, surface and time of the position of the surface chemical potential in the band structure.KEYWORDS: hexaboride, CaB 6 , EuB 6 , angle resolve photemission §1. Introduction Great interest in the divalent hexaborides has been generated recently by the discovery of high Curie temperature weak-moment ferromagnetism (FM) in La-doped CaB 6 1) and by exotic theoretical models to explain the unusual magnetism, e.g. that it represents the ground state of a dilute electron gas 2) or of a doped excitonic insulator.3) The starting point of most thinking about the divalent hexaborides, and central to the excitonic instability model, is the presumed existence of a band overlap between the top of the boron valence states and the bottom of the cation d-conduction band at the Xpoint of the simple cubic Brillouin zone appropriate to these materials. Without such overlap stoichiometric divalent hexaborides would be insulators. Band overlap is predicted by band structure calculations 4, 5) and de Haas van Alphen (dHvA) and Shubnikov de Hass (ShD) data 6) have been interpreted in this semi-metal framework.Our early angle-resolved photoemission spectroscopy (ARPES) studies of EuB 6 and SrB 6 synchrotron measurements predating the FM discovery, showed, contrary to the band-overlap picture, an isolated X-point electron pocket separated from the X-point boron valence band maximum by a gap >1 eV. However, motivated by the prior theory results, by apparently firm evidence for bulk band overlap from dHvA and SdH data discussed below and by certain surface sensitive aspects of our ARPES data, we interpreted 7) the observed gap as a property only of the surface region probed in ARPES. It was reported essentially simultaneously 8) that a new band calculation including a GW self energy predicts CaB 6 to have an X-point bandgap of 0.8 eV, similar to what we measured, thereby motivating us to reexamine our early ARPES results as showing a bulk property. 9)We have recently presented new ARPES measurements and complementary bulk-sensitive soft x-ray emission and absorption 9, 10) that demonstrate conclusively the existence of a bulk band gap at the X-point and a global band structure in good agreement with the GW band calculation. This experimental result rules out all mechanisms for FM requiring an X-point band overlap and also forces the consideration of boron vacancies as the origin of the conduction band carriers observed in nominally stoichiometric divalent material, and possibly 11) also the origin of the magnetic moments. It is important that ARPES data showing onl...

Insights from angle-resolved photoemission spectroscopy on the metallic states ofYbB6(001):E(k)dispersion, temporal changes, and spatial variation

“…1(d) could then be the ν = 1 and 2 quantum-well states of the bulk conduction band [44,45], which has shifted below E F and is subject to the confinement potential. In the following, we argue that the (k x ,k y ) contours of the metallic states are reminiscent of those measured from the conduction band of other divalent hexaborides, which are known to be subject to downward band bending [37,46], we present experimental evidence that the observed states are indeed confined in the near-surface region, and we will point out a possible origin for the confinement potential via analysis of the Yb 4f line shape. Figure 2 presents the Fermi surface contours and reveals the dimensionality of the states at X.…”

“…[41,42], our data lead us to conclude that these states do not possess the dispersion characteristics expected for edge states in a 3D topological insulator, correlated or otherwise. Although we cannot exclude the possibility that these two-dimensional states correspond to true surface states of trivial character, the similarity of their (k x ,k y ) fingerprint with the contours of the bulk conduction band seen in other band-bent divalent hexaborides [37,46] and the observation of two, concentric electron pockets around [ Fig. 1(d)] point toward a likely origin in quantum-well states from confinement of the bulk conduction band.…”

“…3, we focus on a different characteristic that has not been reported in the recent ARPES studies of YbB 6 [41][42][43], but it has been reported previously for EuB 6 [37,46], namely that there is a pronounced time dependence in the ARPES data, resulting in a gradual energy shift and modification of the surface-related features in YbB 6 . Figure 3(a) presents a wide k -range Fermi surface of YbB 6 (001).…”

“…Figure 2 presents the Fermi surface contours and reveals the dimensionality of the states at X. [37,46], and they have been attributed to the bulk conduction band. Nevertheless, comparison of Figs.…”

“…The issue as to whether holelike and electronlike bands cross near the X-point in k-space so as to enable the formation of an excitonic insulator [33][34][35][36] was an important part of the debate at that time. From a combination of ARPES and bulk-sensitive, k-resolved, resonant inelastic x-ray scattering, a gap between the valence and conduction bands at the X-point of more than 1 eV was concluded for the divalent hexaborides of Ca, Sr, and the rare earth Eu [37]. Going back to the claims for a mixed-valent-rather than divalent-character of YbB 6 [28], they were based on the results of x-ray electron spectroscopies that found the Yb valence to be 2.2 [31].…”