Posted: Jan 25, 2016 
Mysterious behavior of quantum liquid elucidated, a world first
(Nanowerk News) In cooperation with researchers from Osaka City University and the University of Tokyo, researchers at Osaka University, through their precise measurement of current fluctuations in quantum liquids in an artificial atom created by nanotechnology, succeeded in elucidating theoreticallypredicted behavior of quantum liquid in a nonequilibrium regime (Nature Physics, "Universality of Nonequilibrium Fluctuations in Strongly Correlated Quantum Liquids").


Fig1: (left) In a single particle system, the behavior of the particle is well understood by solving the Schrödinger equation. Here the particle possesses wave nature characterized by the de Broglie wave length. (right) In a many particle system, on the other hand, the particles interact each other in a quantum mechanical way and behave as if they are "liquid". This is called quantum liquid whose properties are very different from that of the single particle case.

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 “quasiparticles”.

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 linearresponse regime have been successfully described by the theory. However, nonequilibrium properties beyond this regime have still to be established and remain a key issue of manybody 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. ParisSud, Université Paris Saclay show a precise experimental demonstration of Landau Fermiliquid theory extended to the nonequilibrium regime in a 0D 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 quasiparticle picture around equilibrium just as the Fermi liquid theory tells us, a twoparticle scattering process due to residual interaction shows up in the nonequilibrium regime. The result, in perfect agreement with theory, provides a strong quantitative experimental background for further developments of the manybody physics. Moreover, they discovered a new scaling law for the effective charge, signaling asyetunknown universality in the nonequilibrium regime.

This achievement will open up a new way to explore quantum manybody 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.
