Symmetric solution

An exotic uranium material might provide a solution to a symmetry puzzle that has vexed scientists for three decades.

So-called "hidden order" was first seen in materials chilled to 17.5 Kelvin and is a subtle effect changing the electrical and magnetic properties of a material but has not been understood since its discovery in 1985. There have been almost one thousand papers discussing "hidden order", according to Girsh Blumberg of Rutgers University. Working with colleagues at Los Alamos National Laboratory in New Mexico and a team at Leiden University led by John Mydosh who discovered the original phenomenon, new insights into hidden order have now been revealed.

Changes in order are what make liquid crystals, magnetic materials and superconductors work and perform useful functions. The researchers have worked with a crystalline material containing uranium, ruthenium and silicon - URu₂Si₂ - that has unprecedented purity. It is the electron orbitals unique to uranium that make it useful in this experiment rather than its radioactivity. At 17.5 K, uranium's orbitals in adjacent crystal layers flip to become mirror images of each other, whereas above that temperature they are the same. The team used polarization resolved Raman spectroscopy to reveal this broken "mirror symmetry" phenomenon.

Blumberg points out that it was Rutgers scientists that predicted this particular form of hidden order. "In this field, it's rare to have such predictive power," he says, noting that Gabriel Kotliar developed a computational technique that led to the prediction of the hidden order symmetry. Kristjan Haule and Kotliar applied this technique to predict the changes in electron orbitals that Hsiang-Hsi Kung and Blumberg detected.

This particular material becomes a superconductor at 1.5 K although its chemical constituents and this extreme cold requirement make it impractical for any real-world technological application, the insights into materials structure and properties could point the way to the development of novel materials that have such hidden properties at high temperatures.

Leiden collaborator, Mydosh, is enthusiastic that the new work is a step forward in the field he pioneered. "The work of Blumberg and his team is an important and viable step towards the understanding of hidden order," Mydosh says. "We are well on our way after 30 years towards a solution."

"The Chirality Density Wave is a new, never discussed before, order parameter," Blumberg told Materials Today. "In that sense, this is a new form of material," he adds and quotes from his paper: "Such order with alternating left and right handed states at the uranium sites for neighboring basal planes has no modulation of charge or spin, and does not couple to tetragonal lattice, hence it is hidden to all probes but the scattering of A2g symmetry." [Blumberg et al., 2015; DOI: 10.1126/science.1259729]

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".