Two-dimensional quantum critical behavior of Boson gas revealed.

News 2024/05/15

A joint team of researchers from the Department of Quantum Materials (Prof.Takagi) at the Max Planck Institute for Solid State Research and the University of Tokyo has experimentally demonstrated for the first time the distinct behavior of an ideal two-dimensional (2D) dilute Bose gas using a quantum magnet. Certain types of magnets near the saturation magnetic field can be viewed as an ensemble of dilute bosons. What is unique here is that we can precisely tune the boson density just by changing the magnetic field. At low-temperatures, such a characteristic of bosons leads to an intriguing phenomenon known as the Bose-Einstein condensation (BEC). In the pure 2D case, however, bosons turn to an exotic quasi-condensate through a topological phase transition described by the dissociation of vortex pairs, known as the Berezinskii-Kosterlitz-Thouless (BKT) transition. In this work, the international research team clarified that thermodynamic properties, specific heat and magnetization, at the saturation magnetic field are surprisingly well described as a 2D Bose gas in the dilute limit, using the honeycomb Heisenberg magnetic material YbCl3. Furthermore the boson-boson repulsion takes a considerably small value, which evidences the presence the logarithmic correction unique to 2D. With decreasing the magnetic field from the saturation field, the quantum critical 2D Bose gas was found to experience a 3D BEC, marginally produced by an extremely small interlayer coupling. The demonstration of such a distinct 2D nature of Bose gas establishes YbCl3 as a unique playground of further explorations for novel phenomena of 2D Bose gas.

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