https://doi.org/10.1140/epja/s10050-025-01653-y
Review
Shear viscosity and hierarchical structure of matter
1
Department of Physics, Chuo University, Bunkyo, 112-8551, Tokyo, Japan
2
Department of Physics, Osaka Metropolitan University, 3-3-138, Sugimoto-cho, Sumiyoshi-ku, 558-8585, Osaka, Japan
3
Physics Program, Hiroshima University, 739-8526, Higashi-Hiroshima, Hiroshima, Japan
4
International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM2), Hiroshima University, 739-8526, Higashi-Hiroshima, Hiroshima, Japan
5
Department of Physics, Keio University, 223-8522, Yokohama, Kanagawa, Japan
a
dkagamihara119@g.chuo-u.ac.jp
Received:
4
June
2025
Accepted:
19
July
2025
Published online:
31
July
2025
This article deals with the shear viscosity and effects of inter-particle correlations from the viewpoint of the hierarchical structure of matter. Although the change in hierarchical levels, which means that one kind of dominant degrees of freedom transitions into another, does not necessarily accompany the phase transition, and then we cannot expect a clear boundary between them in general, we propose that the shear viscosity is a useful probe to detect it: the shear viscosity takes a minimum around the boundary of the change in hierarchies. The minimum of the shear viscosity itself has attracted much attention and has widely been investigated from various research fields, ranging from low-energy condensed matter physics to high-energy nuclear physics, due to the so-called Kovtun–Son–Starinets (KSS) conjecture (which predicts the existence of a universal lower bound (KSS bound) of this quantity). We discuss a viscosity minimum and the KSS bound in two examples: quark matter in high-energy physics and an ultracold Fermi gas in low-energy physics.
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© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2025
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
