Universality of Non-equilibrium Fluctuations in Strongly Correlated Quantum Liquids

TL;DR

This paper experimentally demonstrates the extension of Landau Fermi-liquid theory to non-equilibrium regimes in strongly correlated quantum liquids, revealing universal behaviors and new scaling laws through transport and noise measurements in carbon nanotubes.

Contribution

It provides the first precise experimental validation of non-equilibrium Fermi-liquid theory in a 0-D quantum system, identifying symmetries and universal scaling laws.

Findings

Confirmation of SU(2) and SU(4) symmetries in quantum liquids

Measurement of Wilson ratios R=1.9 and R=1.35 for different symmetries

Discovery of a new scaling law for effective charge in non-equilibrium

Abstract

Interacting quantum many-body systems constitute a fascinating playground for researchers since they form quantum liquids with correlated ground states and low-lying excitations, which exhibit universal behaviour. In fermionic systems, such quantum liquids are realized in helium-3 liquid, heavy fermion systems, neutron stars and cold gases. Their properties in the linear-response regime have been successfully described by the theory of Fermi liquids. However, non-equilibrium properties beyond this regime 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 0-D system. Combining transport and ultra-sensitive current noise measurements, we have unambiguously identified the SU(2) and SU(4) symmetries of quantum liquid in a carbon nanotube tuned in the universal Kondo regime. We find that, while the electronic transport is well described by the free quasi-particle picture around equilibrium, a two-particle scattering process due to residual interaction shows up in the non-equilibrium regime. By using the extended Fermi-liquid theory, we obtain the interaction parameter "Wilson ratio" $R=1.9 \pm 0.1$ for SU(2) and $R=1.35 \pm 0.1$ for SU(4) as well as the corresponding effective charges, characterizing the quantum liquid behaviour. This result, in perfect agreement with theory, provides a strong quantitative experimental background for further developments of the many-body physics. Moreover, we discovered a new scaling law for the effective charge, signalling as-yet-unknown universality in the non-equilibrium regime. Our method to address quantum liquids through their non-equilibrium noise paves a new road to tackle the exotic nature of quantum liquids out-of-equilibrium in various physical systems.