A mysterious transition in a layered compound could be useful for advanced electrical switches and nonvolatile memory

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

A cartoon illustrates hysteresis – when the value of a physical property lags behind changes in the effect that causes it – during hiking, with different paths up and down. Image Credit: Xinyue Lu

Sometimes an assumed hysteresis loop accompanies such a phase transition, so that the transition temperatures vary depending on whether the material is warmed or cooled.

In a recent article by Physical examination lettersan international research team led by MIT physics professor Nuh Gedik has discovered a rare hysteretic transition in a layered compound known as EuTe4where the hysteresis encompasses a wide temperature range of more than 400 Kelvin.

This wide thermal range is not only a record-breaking event among crystalline solids, but also promises to initiate a new kind of transition in materials that include a layered structure.

These results would develop a new platform for the fundamental study of the hysteretic behavior of solids in extreme temperature ranges.

The many metastable states existing inside the massive hysteresis loop provide researchers with enough opportunities to skillfully control the electrical property of the material, which may find application in advanced electrical switches or non-volatile memory, a type of computer memory that retains data when turned off.

The scientists include postdoc Baiqing Lyu and graduate student Alfred Zong PhD ’20 from the Gedik Lab, along with 26 others from 14 institutions around the world.

The experimental work in this study used modern synchrotron facilities in the United States and China, where bright light sources are generated by charged particles moving rapidly in a circular track one kilometer long, and the powerful light is intended for EuTe4 reveal its internal structure.

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

Hysteresis and thermal memory

Hysteresis is an event where the reaction of a material to a disturbance, such as a change in temperature, depends on the history of the material. A hysteresis indicates that the system is stuck in a local but not universal minimum of the energy landscape.

In crystalline solids defined by long-range order, i.e. when there is a periodic design of an atomic arrangement over the entire crystal, hysteresis normally occurs 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 above 400 kelvins. The actual number could be much larger, as this value is limited by the capabilities of current experimental techniques. This discovery immediately caught our attention, and our combined experimental and theoretical characterization of EuTe4 challenges conventional wisdom about the type of hysteretic transitions that can occur in crystals.

Baiqing Lyu, Postdoctoral Researcher, Gedik Lab, MIT

One exhibit of the hysteretic behavior is in the electrical resistance of the material. By heating or cooling EuTe crystals4scientists could differ their electrical resistivity by orders of magnitude.

The value of the resistivity at a given temperature, say at room temperature, depends on whether the crystal was colder or hotter. This observation tells us that the electrical property of the material has some kind of memory of its thermal history, and under the microscope the properties of the material can retain traits of a different temperature in the past.

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

Such a “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 experienced in the past,” Zong added.

Oddities found

Scientists have also discovered many quirks in hysteresis. For example, unlike other phase transitions in crystals, they did not notice any alteration in the electronic or lattice structure in the massive temperature range.

“The lack of microscopic change seems really peculiar to us,” adds Lyu, “Adding to the mystery, unlike other hysteretic transitions that are sensitively dependent on the rate of cooling or heating, EuTe’s hysteresis loop4 does not appear to be affected by this factor.

A clue for scientists is how electrons are oriented in EuTe4. “At room temperature, the electrons in an EuTe4 the crystal spontaneously condenses in low and high density regions, forming a secondary electron crystal on top of the original periodic lattice,” explains Zong. “We think the quirks associated with the giant hysteresis loop may be related to this secondary electronic crystal, where different layers of this compound exhibit disordered motion while establishing the long-range periodicity.”

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

Baiqing Lyu, Postdoctoral Researcher, Gedik Lab, MIT

The next step is to plan ways, other than temperature variation, to trigger these metastable states in EuTe4. This will allow researchers to exploit its electrical properties in a technologically advantageous way.

We can produce intense laser pulses shorter than a millionth of a millionth of a second. The next objective is to deceive EuTe4 into a different resistive state after emitting a single flash of light, making it an ultra-fast electrical switch that can be used, for example, in computing devices.

Nuh Gedik, study leader and professor of physics, MIT

In addition to the MIT scientists, other study authors are associated with Stanford University, SLAC National Accelerator Laboratory, University of California, 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, Advanced Research Institute of Shanghai of the Chinese Academy of Sciences and Hong Kong University of Science and Technology.

This research received support primarily from the US Department of Energy. Additional support for MIT scientists 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.

Journal reference:

Level, BQ, et al. (2022) Unconventional hysteretic transition in a charge density wave. Physical examination letters. doi.org/10.1103/PhysRevLett.128.036401.

Source: https://mit.edu