Photoemission Evidence for Valence Fluctuations and Kondo Resonance in YbAl₂

Abstract

We use hard x-ray photoemission spectroscopy (HAXPES) to investigate the electronic structure of YbAl2, for which the Yb valence has not been consistently reported to date. The bulk sensitivity and the analytical simplicity provided by the Yb 3d core-level HAXPES allow a reliable determination of the mean valence of Yb ions. For YbAl2, it is evaluated to be +2.20, which remains nearly unchanged below 300 K. The Kondo resonance peak with an extremely high Kondo temperature (above 2000 K) is clearly identified in the valence-band spectra. The results indicate that a coherent Kondo state can be robust even in a nearly divalent system.

Strongly hybridized electronic structure of YbAl2: An angle-resolved photoemission study

“…2(b). The peak position and width obtained were 0.171 and 0.155 eV, respectively, which are consistent with those in the hard and soft x-ray photoemission spectra 14 . Additional fine structures can be seen at E B ∼1.09 eV and 1.25 eV, which might be derived from the subsurface and surface 4f 7/2 states, respectively 21 .…”

“…This contrasts with the Ce case, in which the valence fluctuation is restricted near +3 since the Ce 4f spin-orbit splitting is smaller than the 4f bandwidth. In the case of YbAl 2 , the Yb valence is nearly divalent +2.2 as a consequence of the strong c-f hybridization 14 . With decreasing hybridization strength toward Yb 3+ , the resonance peak will move closer to E F and become sharper, providing a well-defined Kondo resonance peak 31,32 .…”

Electronic structure of YbFe$_4$Al$_8$ antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

“…It is not uncommon to put the same material into both categories based on similar experimental results, where a well-known example is YbAl 3 [12][13][14]. A valence band, similar to the one observed for YbFe 4 Al 8 [15], consisting of contributions from Yb 2+ and Yb 3+ has been seen previously for both materials classified as valence fluctuating, such as YbCu 2 Si 2 [16], YbAl 2 [17,18], YbB 12 [19], Yb 2 Pt 6 Ga 15 and Yb 2 Pt 6 Al 15 [20], as well as for materials of intermediate valence, such as YbFe 4 Sb 12 [21], YbInCu 4 [22], YbNi 0.8 Al 4.2 [23], YbRh 2 Si 2 [11], Yb 4 Ga 24 Pt 9 [24], and Yb 2 Si 2 Al [10]. Since our experimental analysis is based solely on XPS measurements, we cannot unambiguously resolve the membership of the compound under consideration, YbFe 4 Al 8 , in the class of fluctuating or intermediate valence materials.…”

Similar temperature scale for valence changes in Kondo lattices with different Kondo temperatures

“…4 results for YbAl 2 . This system presents a valence of about 2.2 and a characteristic valence fluctuation energy of the order of 2500 K 32 . The experimental RXES spectra at 3 and 300 K lie on top of each other.…”