X-ray Induced Quasiparticles: New Window on Unconventional Superconductivity

For the first time, a new type of particle has been created that can help explain the electron interactions responsible for high-temperature superconductivity. In many materials, the electrons interactions can be assumed to be independent of other electrons, resulting in straightforward approaches to understanding the origins of the material properties. However, when the electrons interact strongly with one another – as they do in superconductors – a more complex approach is required. Physicists have developed the concept of “quasiparticles” to describe the electrons in these systems. Quasiparticles have renormalized mass and finite lifetimes, but otherwise retain the basic characteristics of electrons. Characterizing quasiparticles is essential to understanding the fundamental physics but their short lifetimes make unambiguous characterization difficult and interpretation controversial. In this research, resonant x-ray scattering was used to create a new class of neutral quasiparticle with significantly longer lifetimes.

The Impact

The creation and observation of neutral quasiparticles with long lifetimes will help scientists understand the corresponding charged quasiparticles believed to be responsible for the phenomenon of high-temperature superconductivity in a class of materials called cuprates. Because superconductors can conduct electricity with near 100% efficiency and exclude magnetic fields, they are used in high power electromagnets, MRI machines, and accelerators. Potential future uses with improved high temperature performance include the electrical grid, transportation, and expanded use in motors.

Summary

Copper-based superconductors (cuprates) are among a class of materials whose high temperature superconducting properties are assumed to be determined by quasiparticles, formed by interactions among electrons. Quasiparticle characterization is essential to understanding how to control and improve the superconductivity. In this research, resonant x-ray radiation of an iridium-based material (Sr₂IrO₄) was used to create charge-neutral quasiparticles. These quasiparticles have far weaker interactions with the lattice and lattice defects and therefore have much longer lifetimes. Such quasiparticles were clearly observed for the first time in this investigation headed by scientists at Argonne National Laboratory. The quasiparticle dispersion was mapped along with that of the magnetic excitations (magnons) also present in this class of materials. Because Sr₂IrO₄ shares many structural, magnetic, and electronic features with the cuprate superconductors, study of these neutral quasiparticles in the iridates may provide new insights into the mechanism responsible for high-temperature superconductivity in the cuprates. Furthermore, the new links to cuprates revealed by this work strengthen theoretical predictions that iridates such as Sr₂IrO₄, if properly doped with mobile carriers, could produce another class of unconventional superconductors.

Funding

DOE Office of Science, Basic Energy Sciences, including support of the X-ray scattering at the Advanced Photon Source, an Office of Science User Facility.

Publications

J. Kim, M. Daghofer, A.H. Said, T. Gog, J. van den Brink, G. Khaliullin, B.J. Kim, “Excitonic quasiparticles in a spin–orbit Mott insulator.” Nature Communications 5, 4453, 2014. [DOI: 10.1038/ncomms5453]