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Phys Rev Lett. 2010 Jun 18;104(24):247201. Epub 2010 Jun 14.

Strong valence fluctuation in the quantum critical heavy fermion superconductor β-YbAlB4: a hard x-ray photoemission study.

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Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan.


Electronic structures of the quantum critical superconductor β-YbAlB4 and its polymorph α-YbAlB4 are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb 3d core level spectra, the values of the Yb valence are estimated to be ∼2.73 and ∼2.75 for α- and β-YbAlB4, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show Yb2+ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in β-YbAlB4.

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