A research group comprising doctoral student Tomoyuki Tani (at the time of research), at the Keio University Graduate School of Science and Technology, School of Fundamental Science and Technology, Professor Keiya Shirahama, and research associate Yusuke Nago, at the Keio University Faculty of Science and Technology, Department of Physics, found that liquid helium confined in a nanoporous glass, a material with nanometer-sized sponge-like pores, undergoes superfluid phase transition of 4-dimensional XY type. This means that although it is spatially 3-dimensional, a substance that exhibits 4-dimensional phase transition has actually been found.
Phase transitions are classified into categories called "universality class", and ordinary liquid helium belongs to the 3-dimensional XY universality class. It is also known that thin helium films exhibit a 2-dimensional XY Berezinskii–Kosterlitz–Thouless (BKT) superfluid transition. From previous studies, it was known that helium in nanoporous materials exhibit 4-dimensional XY quantum phase transition at absolute zero. The present study showed for the first time that superfluid transition at finite (non-zero) temperatures is also 4-dimensional. Theoretically, 4-dimensional phase transition provides the simplest example of the phase transitions, and its experimental discovery will make significant contributions to our understanding of phase transition phenomena.
The outcomes of this research were published in the Journal of the Physical Society of Japan in March 2021. It was selected as an Editor's Choice paper by the same journal and was also featured on their English website JPS Hot Topics.