Professors to solve compound mystery


The work of two University professors and an MIT post-doctoral student could solve the 30-year-old mystery of the behavior of electrons in a compound of uranium, rubidium and silicon.

Dubbed the “hastatic order,” the condensed matter theorists’ theory may lead to the understanding and development of new materials.

When scientists first discovered URu2Si2, they were surprised to find that it did not follow the rules of increasing entropy, or disorder, in the compound, said Premala Chandra, a professor in the department of Physics and Astronomy.

“The electrons were organizing themselves … but the question was how … with every tool in the system we weren’t able to figure it out,” she said. “A whole set of experimental probes were performed on this material.”

She said she knew an order in the compound had to exist.

“[There were] many, many theories, but the question was an outstanding mystery,” she said.

Piers Coleman, professor in the Department of Physics and Astronomy, said he worked with Chandra and a graduate student in the early 2000s to study the inexplicable behavior they termed “the hidden order problem.”

He said they wanted to know how to characterize and to discover the strange organization of electrons.

   Rebecca Flint, postdoctoral researcher at MIT, said she joined the professors to search for an explanation for this behavior, which only occurred at negative 428 Fahrenheit.

Their theory depends on the concept of broken symmetry, Chandra said. An example of broken symmetry is water freezing into ice. A drop of water is spherically symmetrical, but a snowflake usually has a six-fold symmetry.

The hastatic order the researchers discovered concerns not geometric symmetry, but time-reversal symmetry. Flint, a University alumna, said in even-numbered states, objects rotate through 360 degrees, or time reverse twice, to return to their original states.

States with an odd number of electrons, like URu2Si2, break symmetry instead. Its symmetry breakage is unique because of the uranium atom, Flint said. The strong uranium nucleus pulls and keeps two electrons around it instead of one.

She said because of this, the compound does not reverse its state the way that typical compounds do.

“[The mixture of electrons] with two states with different numbers of electrons, behave[s] like a single electron,” Flint said.

Coleman said somehow, the rubidium and silicon electrons obtained the properties of these uranium atoms.

The hybridization of the electrons’ behavior is hastatic order. Chandra said her and Coleman’s son, a University alumnus, named the order for the Latin word for spear.

 “Before they invented the javelin, they had the hasta. It is the predecessor, the root, of a javelin. So [hastatic order] is like a root of magnetism. It is an early or more primitive version of magnetism, or the magnet itself,” Coleman said.

The theory sparked worldwide interest, Coleman said. It was published in the science journal Nature last month as a rare theory paper, a contrast to most of their publications that generally focus on experimental papers.

 “No one has ever seen [this] as a macroscopic order parameter before,” he said.

Once experiments prove the theory is correct, Coleman said he, Chandra and Flint will take their discovery to the next step.

“One of the things we’re very curious looking at next is the relationship between the order, if it is indeed hastatic, to the superconductor order,” Coleman said.

Chandra said it is too early to predict the practical applications for the discovery.

Since the compound contains radioactive uranium, it would not be a compound used in a gadget like an iPhone, Coleman said.

Instead, he said their hope is that once the theory is approved and further studied, they can find hastatic order in other materials that may be useful in the future.

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