Shining light on superconductivity

Superconductivity refers to the ability of some materials to conduct electricity without any resistance. So far, superconductivity has only been achieved at extremely low temperatures, and the search is on to discover a material that can superconduct possibly even at room temperature. One of the main obstacles to progress in this field is that we do not understand the currently available superconductors on the particle level. Publishing in Nature Communications, EPFL scientists have used a unique spectroscopic technique to understand unconventional behavior of a metal’s electrons, which may hold the key to its ability to superconduct.

Quasiparticles and Fermi liquids

When an electric current passes through a material, the moving electrons are hindered by the material’s own particles, as well as by interactions with each other. The overall result is a “slowing down” in their movement and a loss of energy, which we interpret as electrical resistance. In order to describe this array of complicated interactions, physicists use the concept of the quasiparticle, which was introduced by the famous physicist Lev Landau (1908–1968).

The quasiparticle is not an actual particle. Rather it is a virtual package made up of an electron plus its interactions with all other electrons. A quasiparticle has the same charge and spin as an electron, but the momentum and energy are renormalized depending on the electron interactions.

The quasiparticle concept enables us to describe how electrons interact inside a metal. When that metal is cooled down to low temperatures (e.g. to achieve superconductivity), the entire process of interacting electrons can be described by a theoretical model called a Fermi liquid, named after Enrico Fermi (1901–1954), another famous physicist. In essence, a Fermi liquid describes the situation where electrons within a metal at low temperatures interact weakly. However, if the electrons interact more strongly, the Fermi liquid system may break down and become a so-called non-Fermi liquid. It is this unconventional behavior that may lie behind the equally unconventional nature of superconductivity.

Isolating and measuring electrons

An EPFL research team consisting of Johan Chang, Martin Månsson, and Joël Mesot used a state-of-the-art spectroscopic technique in order to measure how individual electrons behave in a superconducting metal. The technique, called ARPES, essentially involves shining a light onto the metal’s surface at a low temperature (15K), which causes it to absorb energy and emit quasiparticles. By using ARPES, the researchers were then able to measure the momentum and energy of each emitted quasiparticle.

The study showed that the metal changed from a Fermi liquid-type system to a non-Fermi liquid type but did so in a gradual fashion. It was previously expected that this change happened instantaneously, but the team’s data show that quasiparticles with specific direction of momentum display Fermi-liquid behavior, while along other directions non-Fermi liquid quasiparticles were observed.

The study offers a fundamental glimpse into the unconventional metallic state of superconductors, and the electron interactions that constitute the foundation of superconductivity. By gaining a deeper understanding of this phenomenon, it might be possible to begin designing candidates for new and improved high-temperature superconductors.

This work represents a collaboration between EPFL, the Paul Scherrer Institute, Kungliga Tekniska Högskolan (Sweden), Université de Lyon, the University of Bristol and ETH Zürich.

Reference

J. Chang, M. Månsson, S. Pailhès, T. Claesson, O. J. Lipscombe, S. M. Hayden, L. Patthey, O. Tjernberg & J. Mesot. 2013 Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2−xSrxCuO4. Nature Communications 4, Article number: 2559.