Mysterious Transition in Layered Compound Could Be Useful for Advanced Electrical Switches and Nonvolatile Memory

Many materials experience a phase transition, such as a metal to a superconductor, or liquid water to ice when there is a change in temperature.

Mysterious Transition in Layered Compound Could Be Useful for Advanced Electrical Switches and Nonvolatile Memory
A cartoon illustrates a hysteresis—when the value of a physical property lags behind changes in the effect causing it—during hiking, with different uphill and downhill paths. Image Credit: Xinyue Lu

Occasionally, a supposed hysteresis loop accompanies such a phase transition, so that the transition temperatures are varied based on whether the material is warmed up or cooled down.

In a recent paper in Physical Review Letters, an international research team directed by MIT physics professor Nuh Gedik found a rare hysteretic transition in a layered compound known as EuTe4, where the hysteresis encompasses a large temperature range of more than 400 kelvins.

This large thermal span not only is a record-breaking occurrence among crystalline solids but also promises to launch a new type of transition in materials that comprise a layered structure.

These findings would develop a new platform for fundamental study on hysteretic behavior in solids across extreme temperature ranges.

The many metastable states existing inside the massive hysteresis loop provide sufficient opportunities for researchers to skillfully control the electrical property of the material, which can find application in advanced electrical switches or nonvolatile memory, a type of computer memory that preserves data when switched off.

Scientists include postdoc Baiqing Lyu and graduate student Alfred Zong PhD ’20 from the Gedik lab, as well as 26 others from 14 institutions worldwide.

The experimental works conducted in this study utilized modern synchrotron facilities in the United States and China, where vivid light sources are generated by rapidly-moving charged particles in a kilometer-long circular track, and the powerful light is aimed at EuTe4 to reveal its inner structure.

Gedik and his team also worked together with a team of theorists including Professor Boris Fine and A. V. Rozhkov from Germany and Russia, both of whom helped to assimilate several pieces of the puzzle in experimental observations into a stable picture.

Hysteresis and Thermal Memory

Hysteresis is an occurrence where the reaction of a material to a perturbation, such as a temperature change, is contingent on the history of the material. A hysteresis specifies that the system is stuck in some local but not universal minimum in the energy landscape.

In crystalline solids defined by long-range order, that is, where there is a periodic design of an atomic arrangement over the whole crystal, hysteresis normally happens over a moderately narrow temperature range, from a few to tens of kelvins in the majority of cases.

In EuTe4, we instead found an extremely wide 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, and our combined experimental and theoretical characterization of EuTe4 challenges conventional wisdom on the type of hysteretic transitions that can occur in crystals.

Baiqing Lyu, Postdoctoral Researcher, Gedik Lab, MIT

One exhibition of the hysteretic behavior is in the material’s electrical resistance. By warming up or cooling down crystals of EuTe4, the scientists could differ their electrical resistivity by orders of magnitude.

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 to us 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.

Alfred Zong PhD ’20, Graduate Student, Gedik Lab, MIT

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 is the coldest or the hottest temperature the material has experienced in the past,” Zong added.

Oddities Found

The scientists also discovered numerous oddities in the hysteresis. For example, in contrast to other phase transitions in crystals, they did not notice any alteration in the electronic or lattice structure across the massive temperature range.

“The absence of microscopic change looks really peculiar to us,” adds Lyu, “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.”

One clue to the scientists is the way electrons are oriented in EuTe4. “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,” elucidates Zong. “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.” 

The layered nature of EuTe4 is crucial in this explanation of the hysteresis. The weak interaction between the secondary crystals in different layers enables them to move relative to each other, hence creating many metastable configurations in the hysteresis loop.

Baiqing Lyu, Postdoctoral Researcher, Gedik Lab, MIT

The following step is to plan ways, other than varying the temperature, to trigger these metastable states in EuTe4. This will allow researchers to exploit its electrical properties in technologically beneficial ways.

We can produce intense laser pulses shorter than one-millionth of one-millionth of a second. The next goal is to trick EuTe4 into a different resistive state after shining a single flash of light, making it an ultrafast electrical switch that can be used, for instance, in computing devices.

Nuh Gedik, Study Lead and Physics Professor, MIT

Besides MIT’s scientists, other study authors are associated with 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.

This research received support mainly from the U.S. Department of Energy. Further support for the MIT scientists was provided by the U.S. National Science Foundation, the Gordon and Betty Moore Foundation, the U.S. Army Research Office, and the Miller Institute; other co-authors were backed by the National Natural Science Foundation of China, and the National Key Research and Development Program of China.

Journal Reference:

Lv, B. Q., et al. (2022) Unconventional Hysteretic Transition in a Charge Density Wave. Physical Review Letters.


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