Scientists
working at the Institut Laue-Langevin, a leading center for neutron
science in Grenoble, France, have carried out the first investigation of
2D fermion liquids using neutron scattering, and discovered a new type
of very short wavelength density wave. The team believe their discovery,
published in Nature,
will interest researchers looking at electronic systems, since high
temperature superconductivity could result from this type of density
fluctuations.
Fermi
liquids are composed of strongly interacting fermion particles, a group
that includes quarks, electrons, protons and neutrons. They are common
in nature, found in atomic nuclei, metals, semiconductors, and neutron
stars.
They
are also one of two types of quantum liquid used to model and explain
the complex interplay between atoms or even sub-atomic particles that is
governed by quantum mechanics in a field known as ‘many-body physics’.
Fermion
particles are defined by their adherence to the Pauli Exclusion
Principle that 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 like 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
Institut Néel (Centre national de la recherche scientifique and
Université J. Fourier) in France and Aalto University in Finland
(Microkelvin Collaboration), Oak Ridge National Laboratory and SUNY
University at Buffalo in the US, 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, the neutrons were focused on a one
atom thick layer of helium-3, a much rarer version of helium on Earth
than helium-4 that is 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
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 during previous experiments at the ILL in bulk liquids,
where they were strongly damped.
The
discovery of these oscillations in a fermion helium liquid is
particularly interesting as it’s thought that if this type of high
frequency density oscillation is 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, particularly in understanding the makeup of metals
and the physics behind neutron stars.”