Mysterious behavior of quantum liquid elucidated, a world first

Quantum liquids are macroscopic ensembles of interacting particles dense enough for quantum statistics to manifest itself. For fermions, it is known that, around equilibrium, all the quantum liquids can be universally described within a single theory, so called Landau Fermi liquid theory. The central idea is that they can be treated as an ensemble of free "quasi-particles." This conceptual framework has been applied to many physical systems, such as liquid helium 3, normal metals, heavy fermions, neutron stars, and cold gases, where their properties in the linear-response regime have been successfully described by the theory. However, non-equilibrium properties beyond this regime have still to be established and remain a key issue of many-body physics.

Kensuke Kobayashi, Meydi Ferrier, Tomonori Arakawa, Tokuro Hata, Ryo Fujiwara at Osaka University in collaboration with Akira Oguri at Osaka City University and Rui Sakano at the University of Tokyo together with Raphaëlle Delagrange, Raphaël Weil, Richard Deblock at Laboratoire de Physique des Solides, CNRS, Univ. Paris-Sud, Université Paris Saclay show a precise experimental demonstration of Landau Fermi-liquid theory extended to the non-equilibrium regime in a 0-D system. Combining transport and sensitive current noise measurements, they have identified the SU(2) and SU(4) symmetries of quantum liquid in a carbon nanotube tuned in the Kondo regime. They find that, while the electronic transport is well described by the free quasi-particle picture around equilibrium just as the Fermi liquid theory tells us, a two-particle scattering process due to residual interaction shows up in the non-equilibrium regime. The result, in perfect agreement with theory, provides a strong quantitative experimental background for further developments of the many-body physics. Moreover, they discovered a new scaling law for the effective charge, signaling as-yet-unknown universality in the non-equilibrium regime.

This achievement will open up a new way to explore quantum many-body physics through fluctuations, which stands on firm ground even out of equilibrium beyond the conventional Landau Fermi liquid theory. The newly discovered universality would trigger a vast theoretical effort.