Neutrons uncover new density waves in fermion liquids

Fermion liquids are composed of strongly interacting fermion particles, a group that includes quarks, electrons, protons and neutrons. They are common in nature and are found in atomic nuclei, metals, semiconductors and neutron stars.

They are also one of two types of quantum liquid used in a field known as 'many-body physics' to model and explain the complex interplay between atoms, and even sub-atomic particles, that is governed by quantum mechanics. Fermion particles are defined by their adherence to the Pauli Exclusion Principle, which states that no two identical fermions can exist in the same energetic state, making fermion systems particularly complicated. As a result, whilst the other types of quantum liquid, composed of bosons such as gluons and photons, are well understood in terms of their underlying physics, fermion liquids remain more mysterious.

As part of this on-going investigation a team of researchers from the CNRS (Centre National de la Recherche Scientifique) in France, Aalto University in Finland, Oak Ridge National Laboratory and SUNY University at Buffalo in the US, and Johannes Kepler University in Austria carried out the first direct investigation of these very short wave-length elementary excitations in a fermion liquid by inelastic neutron scattering. In their study, funded in part by the MICROKELVIN collaboration, the neutrons were focused on a one atom-thick layer of helium-3, a much rarer version of helium on Earth than the helium-4 used in balloons and airships which acts like a Fermi liquid at temperatures close to absolute zero.

Using this scattering technique the scientists were able to observe high-frequency, very short wave-length density waves known as zero-sound mode oscillations. The results from the scattering experiments revealed the zero-sound modes to be far longer lived in this two-dimensional fluid than those seen in bulk liquids during previous experiments at the ILL, where they were strongly damped.

The discovery of these oscillations in a fermion helium liquid is particularly interesting as it is thought that if this type of high-frequency density oscillation were to be seen in another fermion liquid, composed of electrons, this could be a mechanism for high-temperature superconductivity. Once the team have completed their investigation of the properties of the helium system, their next step is to extend this understanding to electron liquids.

Dr. Henri Godfrin, Director of research at CNRS, based at the Institut Néel, a leading laboratory for fundamental research in condensed matter physics: "People working with electron systems will be very interested to see if this property exists in their own systems and this finding suggests it is entirely possible. This is an important discovery in the field of quantum fluids which has direct consequences in other areas of many-body physics, for instance in understanding the makeup of metals and the physics behind neutron stars."