Physicists cooled TmVO4 from 5 to 2.36 K using the elastocaloric effect, a phenomenon in which stretching or compressing a material leads to a decrease in its temperature. The scientists also recorded the nonlinear nature of the process: elastocaloric cooling reached its maximum at an initial sample temperature of 3.6 K. The results of the study were published in Physical Review Applied.
To achieve temperatures of a few kelvins or lower for macroscopic amounts of matter, physicists typically use one of two tried-and-true methods: liquid helium cooling or adiabatic nuclear demagnetization. The former requires an expensive isotope of helium that is extremely rare in nature, while the latter requires bulky, complex equipment that generates strong magnetic fields, which is why scientists are constantly looking for new ways to achieve ultracold temperatures.
The elastocaloric effect is one of the alternative candidates for cryogenic technologies. The essence of this effect is that during adiabatic (i.e. without heat exchange with the environment) stretching or compression, the material loses some of its internal energy, thereby cooling. However, there are two problems today: firstly, researchers have not yet developed methods for measuring the elastocaloric effect at near-zero temperatures, and secondly, not many materials with suitable properties have been discovered.
Mark Zic from Stanford University, together with colleagues from the USA, demonstrated cooling in the ultra-low temperature region using thulium vanadate, which exhibits elastocaloric properties due to the Jahn-Teller effect: under asymmetric deformation, the ground-state doublet of the Tm3+ ion splits, absorbing energy.
To do this, the physicists grew TmVO4 single crystals and placed them between the plates of a strain gauge, which, when voltage was applied, caused the samples to deform in two different directions, creating an asymmetric curvature of the crystal lattice. Before each new experiment, the scientists first measured the initial temperature of the material using a ruthenium dioxide sensor and set the initial deformation (in most experiments, this was compression of -2.7 × 10-3), and then transmitted a pulse of about a second to the strain gauge, which deformed the sample by stretching or compressing it.
As a result, the physicists noticed that when the deformation increased, the cooling of the sample also increased, but when reaching a value of -1.8 × 10-3 (the minus sign means compressive deformation), the cooling of the material noticeably decreases. At the same time, the elastocaloric effect was practically absent at the initial temperature of thulium vanadate at 8 Kelvin and reached its maximum at 3.6 Kelvin. The maximum experimental cooling that the scientists recorded was equal to approximately 2.64 Kelvin (the sample cooled from 5 to 2.36 Kelvin) with a tensile deformation of about 1.8 × 10-3. To assess the efficiency of the developed cooler, the researchers calculated its volumetric specific power - it was 0.34 watts per cubic centimeter for a sample weighing 0.19 milligrams.
Since the elastocaloric properties of the material arise due to the Jahn-Teller effect, the authors of the work also proposed using nuclear magnetic resonance and Raman scattering to search for new candidates and test their cryogenic capabilities.
We wrote earlier about how physicists used another exotic method – “dark” states of particles – and cooled a cloud of molecules to a record low temperature.