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Research in MAGLAB verifies new phase of matter in material

Strange behavior of ytterbium dodecaboride better understood.

The three most-common phases of matter are well known: liquid, solid, gas. But under certain extreme conditions, matter exhibits much more bizarre behavior.

Working with ytterbium dodecaboride (YbB12) under extreme conditions, a research team at the National High Magnetic Field Laboratory’s Pulsed Field Facility, or MAGLAB, in Los Alamos recently measured and identified unusual behavioral properties, especially in how the material can carry electrical charges, that reflect a new phase of matter.

“The odd behavior of ytterbium dodecaboride has puzzled scientists in the field for several years,” said John Singleton, research team member. “Understanding the behavior better, we now realize that we’re looking at a new phase of matter.”

Understanding strange behavior
The compound YbB12 has unusual properties. Using the Duplex 2 and other magnets at the Pulsed Field Facility, the research team sent electric current through the blackish crystal under magnetic fields of up to 75 Tesla and at temperatures as low as 0.4 Kelvin (minus 459 degrees Fahrenheit). The 75 Tesla equates to about 50 times the magnetic pull of an MRI machine, or 750 times the pull of a refrigerator magnet.

Within the 50 millisecond-long magnet pulse in those conditions, the team found that YbB12’s resistance and magnetization oscillate in the high magnetic fields, behavior usually seen only in metals. And the material’s thermal properties, including its ability to conduct heat, look just like the properties of a metal. Metals are good conductors of heat because electrons in metals can move around easily, carrying heat efficiently from one place to another.

But unlike most metals, in the experiment’s conditions YbB12 doesn’t possess excellent electrical conductivity; in fact, the material acts as an electrical insulator, meaning that electricity does not flow freely through it.

“The behavior of YbB12 in these conditions is as if the electrons in the material have hidden their electrical charge,” said Singleton. “While it looks like the material should be conducting electricity, its electrical resistance actually increases to very large values as temperature decreases.”

To understand this behavior, the experiments included the first measurement in YbB12 of a quantity called the Hall effect, which counts the number of mobile charges in a material. The combined data show that during the experiment the electrons in YbB12 reorganize into two liquids that cohabit but don’t mix much. The research team concluded that the observed properties of YbB12 under high magnetic fields are due to collisions between the electrons in the two liquids, allowing them to mix a little and influence each other’s behavior.

“The liquids in the YbB12 under these conditions could be thought of as similar to the oil and vinegar in a vinaigrette dressing,” said Singleton. “They don’t mix much, but the overall properties of the dressing — its flavor — depend on both being present at the same time.”

One liquid in the YbB12 under high magnetic fields is made up of electrons that behave in many ways like those in ordinary metals. The liquid can carry heat, for example. However, these electrons interact with each other and with the ions in YbB12 in a way that makes them act as if they have no charge.

The second liquid in YbB12 under those conditions is made up of a much smaller number of electrons that can carry electrical charge. But those electrons behave in an exotic manner previously seen only in unusual substances referred to as “strange metals,” such as high-temperature superconductors, which are materials that lose electrical resistance at low temperatures (albeit it at higher temperatures than classical superconductor materials).

Insight into physics and the future of devices
Future research on YbB12 and associated compounds will test if the properties seen in the current research occur across a class of materials. That understanding will contribute to an understanding of so-called quantum materials, which may also inform how to build new devices useful for everything from communication to computing.

“We’re always looking for new physics, trying to access more complicated behavior by working with new compounds in which quantum mechanics is writ large,” said Singleton. “We can increase our understanding of quantum materials. That insight may also ultimately benefit technology in significant ways.”

Paper: “Hall Anomaly, Quantum Oscillations and Possible Lifshitz Transitions in Kondo Insulator YbB12: Evidence for Unconventional Charge Transport,” by Ziji Xiang, et al, in Physical Review X.

Funding: The research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences program “Science of 100 T.” The National High Magnetic Field Laboratories Pulsed Field Facility is supported by the National Science Foundation and the Department of Energy.”

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