Journal
PHYSICAL REVIEW LETTERS
Volume 104, Issue 24, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.104.247201
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Funding
- JSPS KAKENHI [21684019]
- MEXT of Japan [17071003]
- Innovative Areas Heavy Electrons'' [21102507]
- Kurata Grant
- Grants-in-Aid for Scientific Research [17071003, 20102003] Funding Source: KAKEN
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Electronic structures of the quantum critical superconductor beta-YbAlB4 and its polymorph alpha-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 similar to 2.73 and similar to 2.75 for alpha- and beta-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 beta-YbAlB4.
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