Saturday, October 8, 2022

Mysterious transition discovered in an electronic crystal

Thermal span in a layered compound promises applications in next-generation electrical switches.

Physicists at MIT have recently discovered hysteretic transition in a layered compound called EuTe4. Hysteresis is a phenomenon where the response of a material to a perturbation, such as a temperature change, depends on the history of the material.

The hysteresis covers a giant temperature range of over 400 kelvins within the compound, setting a record among crystalline solids.

For this study, scientists used advanced facilities in the United States and China. The high-speed charged particles in a circular track generate brilliant light sources. Focusing the intense light on EuTe4 unveils its internal structure.

Postdoc Baiqing Lyu said, “In EuTe4, we instead found an extensive temperature range for the hysteresis over 400 kelvins. The actual number could be much larger, as this value is limited by the capabilities of current experimental techniques. This finding immediately caught our attention. Our combined experimental and theoretical characterization of EuTe4 challenges conventional wisdom on the type of hysteretic transitions that can occur in crystals.”

The material’s electrical resistance is one of the manifestations of hysteretic behavior. Scientists need to either cool or warm the crystals of EuTe4 to vary the electrical resistivity of the material by orders of magnitude.

Graduate student Alfred Zong Ph.D. ’20 from the Gedik lab said, “The value of resistivity at a given temperature, say at room temperature, depends on whether the crystal used to be colder or hotter. This observation indicates that the electrical property of the material somehow has a memory of its thermal history, and microscopically the properties of the material can retain the traits from a different temperature in the past.”

“Such ‘thermal memory’ may be used as a permanent temperature recorder. For example, by measuring the electrical resistance of EuTe4 at room temperature, we immediately know what the coldest or the hottest temperature the material has experienced in the past is.”

You said, “Several oddities were found in hysteresis. For example, unlike other phase transitions in crystals, they did not observe any modification in the electronic or lattice structure across the large temperature range.”

“The absence of microscopic change looks peculiar to us. Adding to the mystery, unlike other hysteretic transitions that sensitively depend on the rate of cooling or warming, the hysteresis loop of EuTe4 appears unaffected by this factor.”

Zong said, “At room temperature, electrons in a EuTe4 crystal spontaneously condense into regions with low and high densities, forming a secondary electronic crystal on top of the original periodic lattice. We believe the oddities associated with the giant hysteresis loop may be related to this secondary electronic crystal, where different layers of this compound exhibit disordered movement while establishing the long-range periodicity.”

Scientists are further planning to find other ways to induce the metastable states in EuTe4. This could allow scientists to control its electrical properties in technologically valuable ways.

The team includes scientists from Stanford University, SLAC National Accelerator Laboratory, University of California at Berkeley, Argonne National Laboratory, Cornell University, Clemson University, Moscow Institute of Physics and Technology, Russian Academy of Sciences, University of Leipzig, Peking University, Songshan Lake Materials Laboratory, Shanghai Advanced Research Institute at the Chinese Academy of Sciences, and Hong Kong University of Science and Technology.

Journal Reference:

  1. B. Q. Lv et al. Unconventional Hysteretic Transition in a Charge Density Wave. DOI: 10.1103/PhysRevLett.128.036401
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