Universality of non-equilibrium fluctuations in strongly correlated quantum liquids

• Published in: Nature physics 2016-03, Vol.12 (3), p.230-235
• Main Authors: Ferrier, Meydi, Arakawa, Tomonori, Hata, Tokuro, Fujiwara, Ryo, Delagrange, Raphaëlle, Weil, Raphaël, Deblock, Richard, Sakano, Rui, Oguri, Akira, Kobayashi, Kensuke
• Format: Article
• Language: English
• Subjects: 639/766/119/995; 639/766/119/999; 639/766/483/1139; 639/925/357/1017; Atomic; Charge; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Fermi liquids; Fluctuation; Fluctuations; Fluid dynamics; Heavy fermion systems; Helium; Helium-3; letter; Liquids; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Physics; Quantum physics; Symmetry; Theoretical; Transport
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys3556

Quantum liquids at equilibrium follow Fermi liquid theory, but less is known about non-equilibrium conditions. Carbon nanotubes, which exhibit universal scaling behaviour, provide a testbed for many-body physics beyond equilibrium. Interacting quantum many-body systems constitute a fascinating research field because they form quantum liquids with remarkable properties and universal behaviour 1 . In fermionic systems, such quantum liquids are realized in helium-3 liquid, heavy fermion systems 1 , neutron stars and cold gases 2 . Their properties in the linear-response regime have been successfully described by the theory of Fermi liquids 1 . The idea is that they behave as an ensemble of non-interacting ‘quasi-particles’. However, non-equilibrium properties have still to be established and remain a key issue of many-body physics. Here, we show a precise experimental demonstration of Landau Fermi liquid theory extended to the non-equilibrium regime in a zero-dimensional system. Combining transport and ultra-sensitive current noise measurements, we have unambiguously identified the SU(2) (ref.  3 ) and SU(4) (refs  4 , 5 , 6 , 7 , 8 ) symmetries of a quantum liquid in a carbon nanotube tuned in the universal Kondo regime. Whereas the free quasi-particle picture is found valid around equilibrium 9 , an enhancement of the current fluctuations is detected out of equilibrium and perfectly explained by an effective charge induced by the residual interaction between quasi-particles 8 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . Moreover, an as-yet-unknown scaling law for the effective charge is discovered, suggesting a new non-equilibrium universality. Our method paves a new way to explore the exotic nature of quantum liquids out of equilibrium through their fluctuations in a wide variety of physical systems 18 .