We found the first empirical evidence of "strange metal" emerging due to strong "quantum valence fluctuation". 

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At a quantum critical point (QCP),  a (2nd order) transition temperature to an ordered phase is suppressed to absolute zero. Strong quantum fluctuations around QCP often result in novel metallic states such as non-Fermi liquid, unconventional superconductivity and so on. This is why QCP and quantum criticality (QC) have been extensively studied in condensed matter physics. 

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So far, the most of the studies have been focusing on "magnetic" fluctuation, which arises at border of magnetic ordered phases. Most prototypical systems are found in 4f heavy fermion intermetallic compounds. Thanks to the small characteristic energy scale, 4f electrons can be fine tuned from localized magnetic moment state to itinerant heavy fermions state by changing external parameters such as magnetic field and (chemical) pressure.  On the other hand, there are several attempts to go beyond the "conventional" idea. One of the interesting possibilities is the valence QC. Normally, a valence transition is first order. However, if the critical end point is suppressed down to T = 0 or even negative temperature, it has been suggested that the associated strong quantum fluctuation may lead to a breakdown of Fermi liquid.  

 

Here we found the first empirical evidence in a Yb-based mixed valence compound a-YbAlB4. Interestingly, only a tiny (1.4%) chemical substitution of Al site with Fe induces a sharp valence crossover. Moreover, at the same time, it exhibits a pronounced non-Fermi liquid behavior, which are characterized by a diverging effective mass and unusual T/B scaling in magnetization. Our further comprehensive studies indicate that a magnetism cannot be a main player for the observed QC. Instead, our results strongly suggest the realization of a valence QC.  

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For more detail, please see K. Kuga, Y. Matsmoto et al.,  Sci. Adv. 4, eaao3547 (2018).

Press release is available here (in Japanese).