Discovery of quantum criticality in an intermediate valence compound

The journal Nature Communications has published an unexpected discovery of quantum criticality in the intermediate valence compound α-YbAlB4, caused by anisotropic hybridization. The authors of the paper are researchers from Japan, Germany and the USA, and among the main authors is a member of our NMR group M. S. Grbić.

In α-YbAlB4, the Yb ions have an intermediate valence of +2.75. Compounds from the so-called intermediate valence class are usually characterized by behavior typical of metals; e.g. isotropicity, a decrease in resistance with cooling, and high stability when a magnetic field or pressure is applied. This stability is typically provided by the conduction electrons within the compound, as they are the ones that try to screen the intermediate valence of Yb that occurs at relatively high orbital hybridization energies (in this compound, this is at T₀ ≈ 200 K). Therefore, it was expected that it would be difficult to achieve any new properties under standard laboratory conditions.

However, the properties of the α-YbAlB₄ compound change when a small magnetic field (less than 5 T) is applied. Using a number of techniques, this work shows how conduction electrons hybridize with the ytterbium f-orbital in a very anisotropic manner, leaving "blind spots" where the hybridization is not present at all. Such a complex situation in α-YbAlB₄ enables the phenomenon of quantum criticality to occur, where electrons, due to the action of quantum fluctuations, begin to "forget" their properties.

This discovery is new as it was previously thought that compounds with intermediate valences were uninteresting in modern science, while on the other hand, the phenomenon of quantum criticality has been one of the most interesting topics in science for the past 25 years.

You can read the full article at the link: https://www.nature.com/articles/s41467-022-29757-9