SPring-8, the large synchrotron radiation facility

The Fermi surface is an important concept for understanding the properties of metals because it describes how electrons move. The X-rays produced at the SPring-8 synchrotron can be used to probe the electrons in metals and measure the Fermi surface.

A team led by Prof. Stephen Dugdale of the HH Will Physics Laboratory (University of Bristol, UK) in collaboration with Japan Synchrotron Radiation Institute (SPring-8, Japan), Oak Ridge National Laboratory (USA), Polish Academy of Science (Poland), the University of Warwick (UK), Cardiff University (UK), and the DMSC-European Spallation Source (Denmark), has succeeded in measuring the extreme smearing of the Fermi surface in an alloy composed of equal amounts of the elements Ni, Fe, Co and Cr.

An alloy is a combination of at least two different elements. Normally, there is one principal element (e.g. Cu) to which a small amount of something else is added, for example adding Sn to make bronze. In a class of materials known as High Entropy Alloys, each constituent element has the same concentration, and there are typically at least four different elements present. This class of materials has proven to be interesting because they have some unexpected and technologically important properties, such has being very strong at low temperature. It is important to understand how the electrons behave in these alloys, if further progress is to be made in exploiting their properties.

High-resolution X-ray Compton scattering experiments were performed on beamline BL08W at SPring-8, allowing the researchers to directly visualise the impact of compositional disorder on the Fermi surface. In most metals at low temperature, the Fermi surface is a sharp discontinuity between occupied and unoccupied states, but here the maximal disorder smears this discontinuity out across a significant fraction of the Brillouin zone. This smearing can be related to the distance the electrons can typically travel before being scattered. In most metals, electrons can travel very long distances (even up to a cm), but the team were able to show that the electrons in this equiatomic NiFeCoCr alloy would barely make it to the next atom before being scattered.

Beyond their technological relevance, these materials may also help shed light on fundamental physics. Owing to the similar effects of chemical disorder and strong electron-electron on transport, these alloys may also be helpful in understanding the behaviour of electrons in strongly correlated materials such as high-temperature superconductors.

This work was published in Physical Review Letter and selected as an editor's suggestion.

Publication Information
Title: Extreme Fermi Surface Smearing in a Maximally Disordered Concentrated Solid Solution
Authors: Hannah C. Robarts, Thomas E. Millichamp, Daniel A. Lagos, Jude Laverock, David Billington, Jonathan A.Duffy, Daniel O’Neill, Sean R. Giblin, Jonathan W. Taylor, Grazyna Kontrym-Sznajd, Małgorzata Samsel-Czekała, Hongbin Bei, Sai Mu, German D. Samolyuk, G. Malcolm Stocks, and Stephen B. Dugdale
Journal: Physical Review Letters (Editor's Suggestion)
DOI: 10.1103/PhysRevLett.124.046402