Â鶹AV

Researchers selected for JST FOREST in the FY2022

Establishment of New Technologies about Simpler, Lower-cost Cryogenic Environments for Broader Scientific Advancement

My specialty is condensed matter physics, with a particular focus on experiments into material properties in cryogenic environments. I study, for instance, magnetism, heat capacity, magnetic field effect, and electrical conductivity in the cryogenic temperature of 0.1¨C4 Kelvin, close to absolute zero temperature (0 on the Kelvin scale, equal to -273.15¡ãC). My goal is to unlock the mysteries of physical phenomena observed in materials at cryogenic temperatures.

Most of the materials we study are compounds containing rare-earth elements. These are located on the upper row in the ¡°island¡± made by two rows at the bottom of the periodic table. One common feature of rare-earth elements is that they have f electrons which occupy the f orbits in various electronic orbits in atoms. The feature of f electrons is that they are easily magnetized at cryogenic temperatures. My focus of particular interest is a compound composed of rare-earth ytterbium (Yb; atomic number 70), copper, and nickel. It conducts heat well even at cryogenic temperatures because of the intermetallic compounds.

In general, the lower temperature in materials, the more its atoms order. At microscopic level, magnetism emerges when the spin which is equal to a small magnet in each individual atom is aligned in a direction. At high temperature, the spins driven by the high thermal energy dynamically sway. In contrast, the lower the temperature, the smaller the thermal fluctuation, so the spins tend to align in a uniform direction, just as some magnets align themselves in a direction when placed side by side. As a result, the cooler the material, the stronger its magnetism becomes, and the true nature of the material¡ªpreviously masked by thermal fluctuations¡ªis revealed.

Our project, ¡°the Development of Ytterbium-based Magnetic Refrigerants for Generating Temperatures Below 1 Kelvin¡±, which was accepted for the Japan Science and Technology Agency¡¯s Fusion Oriented Research for Disruptive Science and Technology, seeks to utilize these physical properties in cryogenic environments. I am driven by a desire to find simpler, lower-cost methods of achieving cryogenic temperatures close to absolute zero.

The most common way to cool a material to near absolute zero is to use liquid helium-4. Since liquid helium-4 has a temperature of 4.2 Kelvin, a substance immersed in the liquid will be cooled down close to 4.2 Kelvin. Just as evaporation of sweat cools the body, forced vaporization of liquid Helium-4 by the external pump can remove further heat and lower temperatures to around 1.5 Kelvin. Lowering the temperature below that, however, is more difficult. Temperatures below 1 Kelvin can be achieved using extremely expensive helium-3, an isotope helium-4. Especially, generating temperatures below 0.1 Kelvin requires a dilution refrigerator, which is expensive and bulky. Because of these barriers, limited laboratories have the wherewithal to cool materials down to below 1 Kelvin at present.

That is why my research has been focused on trying to establish simpler and lower-cost methods to create cryogenic environments around 0.1 Kelvin.
Thereby, I am trying to apply the technique named as magnetic refrigeration to the ytterbium-based compounds. Cooling by magnetic refrigeration is performed as below. At first, a material magnetized by a strong magnetic field in advance are thermally insulated from its surroundings. After that, when the magnetic field is switched off, the temperature of the material goes down.

For instance, when the atoms that make up a material are held in a fixed, stationary state in a certain location and then released so they can move freely, they take the heat of their surroundings when they go. The reason why a spray can gets cool when using is that the degrees of freedom of gas particles inside the can increase. In magnetic refrigeration, when the spin fixed by magnetic field freely move by switching off the magnetic field, the temperature of the materials goes down.

Magnetic refrigeration requires substances whose spins do not freeze even at cryogenic temperatures. Our experiments have shown that the compounds constructed by ytterbium, copper and nickel is a very promising compound. After cooling this ytterbium-based compound with 2.7 gram down to 1.8 Kelvin by vaporizing liquid helium-4 using a simple apparatus, we successfully cooled it to 0.17 Kelvin by magnetic refrigeration. In another experiment, we successfully maintain a temperature below 0.3 Kelvin for more than three hours by using the same substance with 53 grams.

Magnetic refrigeration has a drawback that substances cooled by magnetic refrigeration gradually warm back up. We are working hard as part of our JST-supported research to find ways to overcome this weakness.

Technology to create the cryogenic environments is demanded in the various science like developments of quantum computer, study of superconductivity, and so on.  I hope that our JST-supported research will lead to the establishment of technologies that contribute to the furtherance of scientific research.