Surface mixed valence in Sm and SmB6

Abstract

Surface-sensitive photoelectron measurements reveal bulk-to-surface shifts of the Sm 4f ' level which imply inhomogeneous valence mixing on the surface of Sm and Sm86. The surface valence fraction is estimated to be the same for both materials. The measurements take advantage of a large resonant enhancement of 4f electron emission due to 4d~4f photon absorption, and detailed spectra showing this phenomena are presented. It is shown that the 4d hole in the 4d -+4f absorption process stabilizes the 4f state by -4-7 eV. Exposure of Sm films to oxygen is found to eliminate, rather than increase, the emission from the surface 4f state, showing that the 4f state does not arise from oxygen contamination. Observed variations 6 6 in Sm film spectra are described, including the finding in some films of an unexplained photoemission peak 2.4 eV below the Fermi level. Sm86 also displays a broad band of Auger emission when a boron 1s core hole is created, and this is ascribed to electrons in the boron 2p bonding band. Various trends in 4d and 4f binding energies for Sm and Sm86 are pointed out and discussed.

Nature of the5fstates in actinide metals

“…This structure could arguably be due to surface effects from the surface sensitivity of UPS; however, it is probable that the spectrum contains both surface and bulk contributions. The small peak at 1.8 eV has been interpreted by Johansson (1984) and Mårtenson et al (1987) as being due to a divalent (spd 2 ) surface layer, similar to observations made in Sm (Lang and Baer, 1979;Allen et al, 1980). Studies of AmN, AmSb, and Am 2 O 3 in relation to Am metal conclude that the peak at 1.8 eV is attributed to a well-screened channel of photoemission from the 5f 6 ground state, while the large peak at 2-3 eV is due to the poorly-screened channel (Gouder et al, 2005).…”

Correlated Topological Insulators with Mixed Valence

“…Firstly, the XPS measurements find quite strong 4f multiplet peaks, indicating strong atomic nature of 4f electrons in SmB 6 (in other words, most of the 4f electrons are not envolved in the formation of energy bands) [10,11]. Secondly, both transport [30][31][32] and optical [33][34][35][36] measurements reveal the formation of a small gap only for temperature below 50 K. The above two features imply that the f electrons in SmB 6 have both localized and itinerant natures and the correct description of its electronic structure should include both of them.…”

“…(i) The x-ray photoelectron spectroscopy (XPS) and X-ray absorption spectra (XAS) contain peaks from both divalent and trivalent multiplets with comparable spectral weight [10][11][12], indicating the valence of Sm or Yb to be close to 2.5.…”

Two-component analysis of the 4f multiplet of samarium hexaboride

“…The (occupied) Hubbard 25 bands, as being measured by photoemission spectroscopy, appear near E F and at higher binding energy E B ≈ 7 eV as a characteristic multiplet for the 4f 6 → 4f 5 and 4f 5 → 4f 4 transitions, respectively [8,21,22,23]. Since a Hubbard band lies near E f , SmB 6 belongs to the mixed valence regime of the Anderson model [17,24] where charge fluctuations occur even in low-energy excitations [24,25].…”