Scientists Discover Mysterious Transition in an Electronic Crystal |  MIT News

Scientists Discover Mysterious Transition in an Electronic Crystal | MIT News

As the temperature changes, many materials undergo a phase transition, such as liquid water in ice or a metal in a superconductor. Sometimes, a so-called hysteresis loop accompanies such a phase change, so that the transition temperatures are different depending on whether the material is cooled or heated.

In a new newspaper in Physical Review Lettersa global research team led by MIT physics professor Nuh Gedik has discovered an unusual hysteretic transition in a layered compound called EuTe4, where the hysteresis covers a gigantic temperature range of over 400 kelvins. This wide thermal range not only breaks the record for crystalline solids, but also promises to introduce a new type of transition in materials that possess a layered structure. These results would create a new platform for fundamental research on hysteretic behavior in solids in extreme temperature ranges. Furthermore, the numerous metastable states residing within the giant hysteresis loop provide ample opportunities for scientists to exquisitely control the electrical properties of the material, which may find application in next-generation electrical switches or non-volatile memory, a type computer memory that retains data when turned off.

Researchers include postdoctoral Baiqing Lyu and graduate student Alfred Zong PhD ’20 from the Gedik lab, as well as 26 others from 14 institutions around the world. Experimental work performed in this paper used state-of-the-art synchrotron facilities in the United States and China, where bright light sources are generated by rapidly moving charged particles in a kilometer-long circular track, and bright light is focused on EuTe4 to reveal its internal structure. Gedik and his group also collaborated with a team of theorists including Professor Boris Fine and AV Rozhkov from Germany and Russia, who both helped to integrate many pieces of the puzzle in the experimental observations into a coherent framework.

Hysteresis and thermal memory

Hysteresis is a phenomenon in which the response of a material to a perturbation, such as a change in temperature, depends on the history of the material. A hysteresis indicates that the system is trapped in a local but not global minimum in the energy landscape. In crystalline solids characterized by a long-range order, i.e. where there is a periodic pattern of an atomic arrangement over the entire crystal, hysteresis typically occurs in a fairly narrow temperature range, from a few to tens of kelvins in most part of the cases.

“In EuTe4instead, we found an extremely wide temperature range for hysteresis above 400 kelvins, “says Lyu.” The actual number could be much larger, as this value is limited by the capabilities of current experimental techniques. This discovery immediately attracted our attention and our combined experimental and theoretical characterization of EuTe4 challenges conventional wisdom about the kind of hysteretic transitions that can occur in crystals. “

One manifestation of hysteretic behavior is in the electrical resistance of the material. By cooling or heating the EuTe crystals4the researchers were able to vary their electrical resistivity by orders of magnitude.

“The value of the resistivity at a given temperature, let’s say at room temperature, depends on whether the crystal was colder or warmer,” explains Zong. “This observation tells us that the electrical property of the material has somehow a reminder of its thermal history, and microscopically the properties of the material can retain traits of a different temperature in the past. This “thermal memory” can 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 hottest temperature the material has ever experienced in the past ”.

Oddities found

The researchers also found several oddities in hysteresis. For example, unlike other phase transitions in crystals, they did not observe any changes in the electronic or lattice structure over the wide temperature range. “The absence of microscopic changes seems very peculiar to us,” adds Lyu, “Adding to the mystery, unlike other hysteretic transitions that depend significantly on the rate of cooling or heating, the hysteresis loop of EuTe4 it seems unaffected by this factor “.

One clue to the researchers is the way the electrons are arranged in EuTe4. “At room temperature, electrons in an EuTe4 the crystals spontaneously condense in regions of low and high density, forming a secondary electron crystal above the original periodic lattice, “explains Zong.” We believe that the oddities associated with the giant hysteresis loop may be related to this secondary electron crystal, where several layers of this compound show disordered movements as they establish long-range periodicity. ”

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

The next step is to devise ways, in addition to modifying the temperature, to induce these metastable states in EuTe4. This will allow scientists to manipulate its electrical properties in technologically useful ways.

“We can produce intense laser pulses shorter than a millionth of a millionth of a second,” says Gedik. “The next goal is to cheat EuTe4 in a different resistive state after turning on a single flash of light, making it an ultra-fast electrical switch that can be used, for example, in computing devices. “

In addition to MIT researchers, other authors of the paper are associated with Stanford University, SLAC National Accelerator Laboratory, University of California at Berkeley, Argonne National Laboratory, Cornell University, Clemson University, the Institute of Physics and Technology in Moscow, Russian Academy of Sciences, Leipzig University, 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 was mainly supported by the United States Department of Energy. Additional support for MIT researchers was provided by the US National Science Foundation, the Gordon and Betty Moore Foundation, the US Army Research Office and the Miller Institute; other co-authors were supported by the National Natural Science Foundation of China and the National Key Research and Development Program of China.


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