As basic science teaches us, changes in temperature can result in phase transitions in materials – like when water solidifies as ice in the freezing cold.
However, in some cases the temperature that triggers the change is different depending on whether the material is cooling down or warming up. This is known as a hysteresis loop, and researchers think they’ve discovered a weird and entirely new example of this phenomenon.
It’s not a transition you’re likely to see in everyday life, requiring a layered compound crystalline solid called EuTe4, huge temperature ranges, and a kilometer-long track for firing fast-moving charged particles deployed to create brilliant laser light.
Through such a lab setup, scientists spotted that the hysteresis loop for EuTe4 covered a giant temperature range of at least 400 Kelvins – far more than the usual range for a crystalline solid like this, which would usually only be in the tens of Kelvins at most.
“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,” says physicist Baiqing Lyu from the Massachusetts Institute of Technology (MIT).
The research got curiouser and curiouser from there. There was no change in the electronic or lattice structure in the material across the temperature range that was measured, which again isn’t how phase transitions in crystals should work.
While it’s early days for this discovery, the team does have a few ideas about what might be happening: the particular way electrons are arranged in EuTe4 causes a secondary electronic crystal to form, and it could be that as this second layer moves and shifts, it creates different configurations in the hysteresis loop.
Further experiments showed that the researchers were able to significantly vary the electrical resistance of the material by cooling down or warming up the crystals – another indication of something strange and unexpected going on.
“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,” says physicist Alfred Zong from MIT.
“Such ‘thermal memory’ may be used as a permanent temperature recorder.”
This opens up a whole host of possibilities. One of the ways this could be used by scientists is to measure the electrical resistance of EuTe4 at room temperature, and from there deduce the coldest or hottest temperature the material has previously experienced, because of this ‘thermal memory’.
According to the team, the work done here could be expanded further to look at other solids and how they change when exposed to extreme temperature ranges. It could be particularly promising in terms of getting more control over materials used in switches and memory in computers.
First though, further research is needed. The researchers suspect that there’s more to discover beyond the 400 Kelvin range – that was simply as far as their setup would allow them to go. After more analysis, hysteresis might also be controlled by other ways besides changing the temperature.
“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,” says physicist Nuh Gedik from MIT.
The research has been published in Physical Review Letters.
