Physicists peel back secret to 'hidden order'

Physicists at Rutgers have made a major step in answering a 30-year-old question about the structure of certain materials.

Girsh Blumberg and Kristjan Haule, professors in the Department of Physics and Astronomy, are part of a team that has spent years searching for the “hidden order” that has evaded scientists for decades, working with a unique material called uranium-ruthenium-2-silicon-2 (URu2Si2).

“Our proposal was one out of many," Haule said. "Even our colleagues here at Rutgers had their own theories. There are probably a hundred theories for this material. And, of course, it is very hard to figure out which proposal is right.”

Every material has an “order,” or type of symmetry that it possesses, he said. The symmetry determines how the molecules within the material align themselves and may reveal some of the properties of the material.

The researchers took the uranium material and brought it down to 17.5 Kelvin, or extremely close to absolute zero, said Hsiang-Hsi Kung, a student in the Graduate School of New Brunswick. At this temperature, the material undergoes a “phase transition,” breaking its symmetry. 

An example of this is iron going from being nonmagnetic to magnetic below 770 degrees Celsius, Kung said.

“For 30 years, no one seemed to be able to see what symmetry was broken,” he said. “A few years ago, here at Rutgers, there was a development in the theory (and) people used high powered computers to calculate, in theory, the structure and low temperature physics of this material.”

To find evidence for their theory of the “hidden order,” the researchers used a technique called Raman spectroscopy, a field of expertise for Blumberg. The group shined light on a crystal and measured the energy of the light that reflected off of it, Kung said.

The reflected light intensity data was plotted against the wavenumber, proportional to the energy of the reflected light, and this graph provides evidence for the broken order. The information found from the uranium material could be applied to materials similar to it, Haule said.

“The energy lost to the crystal is not random: it actually reflects the underlying physics of your crystal,” Kung said.

The researchers’ findings suggest that there is a break in the order, but there is no definitive explanation as to what it is yet. The researchers raised the possibility that the broken order could be something related to magnetism, such as a chirality density wave, but also could very well be something completely different, he said.

In their paper, the physicists propose that something new emerges once the symmetry is broken, called “handedness.” They say that the uranium electron orbitals could become symmetric similar to your left and right hand. They would not be translationally or rotationally symmetric when superimposedonto one another, he said.

The uranium used in this experiment is a “correlated material,” or one that has complex, connected behavior, Haule said. Materials that are similar to the URuSi2 would behave similarly to it, making this finding vital to future researchers.

“Correlated materials are becoming very important for technology in future devices," he said. "It turns out that similar materials are very susceptible to small changes of magnetic fields, electric fields, pressure and so on."

Transistors in computers work by having their electric fields switch from one state to another. Understanding how materials similar to those used in transistors undergo their phase changes will make it possible to improve on this technology in the future, he said.

Their research further shows that magnetism and the chirality density wave for materials are closely related, but it is not clear how, Kung said. Many things relying on magnetism, from hard drives to credit cards, can easily be damaged by magnets. 

Nicholas Buchinski, a first-year student in the School of Arts and Sciences, said that this new order could complicate ideas physicists previously thought they understood. Alternatively, it could prove extremely useful for describing the work they do.

“But that’s the beauty of physics, isn’t it?” he said. “We’re never bored as long as we have questions to answer.”

Editor's Note: A previous version of this article said Professors Girsh Blumberg and Kristjan Haule worked with the compound uranium-ruthenium-silicon-2 (URuSi2).