Discovery of Decoupling Phenomena in a Quantum Spin Liquid
—Spins Isolated from a Lattice at Extremely Low Temperatures and in Strong Magnetic Fields—
National Institute for Materials Science (NIMS)
School of Engineering, The University of Tokyo
For the first time in the world, NIMS and The University of Tokyo jointly observed “spin-lattice decoupling,” a phenomenon in which spins are isolated from a lattice due to weakened interactions between the spin and lattice systems. This observation was made in a quantum spin liquid state, a peculiar magnetic state occurring at extremely low temperatures.
(“Spin-lattice decoupling in a triangular-lattice quantum spin liquid,” Takayuki Isono, Shiori Sugiura, Taichi Terashima, Kazuya Miyagawa, Kazushi Kanoda &
Shinya Uji; Nature Communications, volume 9, Article number: 1509 (2018) doi:
10.1038/s41467-018-04005-1)
Abstract
- For the first time in the world, NIMS and The University of Tokyo jointly observed “spin-lattice decoupling,” a phenomenon in which spins are isolated from a lattice due to weakened interactions between the spin and lattice systems. This observation was made in a quantum spin liquid state, which appears in a peculiar magnetic material.
- At low temperatures, atoms and molecules are frozen, resulting in a stable and ordered state. For example, liquid water transforms into ice. Quantum spin liquids strangely deviate from this principle—electron spins strongly fluctuate and are not frozen even at extremely low temperatures. In theories, unique particles “spinons” move around freely within a material, which is called “quantum liquid state”, and this state is expected to show various unusual physical phenomena. The organic material κ-(BEDT-TTF)2Cu2(CN)3 is known to form a quantum spin liquid, which is not electrically conductive (electrons in the material remain motionless). Despite these properties, the specific heat and magnetic susceptibility indicated the existence of free-moving spins within it. On the other hand, the thermal conductivity suggested that the spins within it are completely motionless. These inconsistent results could not be explained for many years.
- The joint research group synthesized high-purity single crystals of κ-(BEDT-TTF)2Cu2(CN)3 and precisely measured the magnetocaloric effect at temperatures as low as 0.1 kelvin and magnetic fields as strong as 17 tesla. As a result, the group discovered that the heat flow from the electron spin system to the lattice system rapidly ceased at extremely low temperatures and in strong magnetic fields. These results indicate “spin-lattice decoupling,” a phenomenon in which spins are isolated from a lattice. This is caused by weakened interaction between the spin and lattice systems in a quantum spin liquid state. Our results explain the long-standing inconsistencies between the studies on specific heat, magnetic susceptibility and thermal conductivity in κ-(BEDT-TTF)2Cu2(CN)3.
- The spin-lattice decoupling phenomena discovered in this study are of great interest to science. In future studies, we hope to gain a full understanding of the mechanisms of the phenomena, which could be applied to the development of new magnetic refrigeration technology and a magnetic switch of heat conduction.
- This project was carried out jointly by a research group led by Takayuki Isono (Postdoctoral Researcher, NIMS) and Shinya Uji (Deputy Director-General of the Research Center for Functional Materials, NIMS) and a research group led by Professor Kazushi Kanoda (School of Engineering, The University of Tokyo).
- This study was published in the online British scientific journal Nature Communications on April 17, 2018.
Figure. (Left) Electron spins in a material. Purple circles and orange arrows represent atoms and the spins of their electrons, respectively. At high temperatures, the electron spins undergo thermal fluctuations and orient in various directions (spin liquid state). When the material is cooled, the thermal fluctuations of the electron spins are suppressed and the spins are ordered (spin solid state). The diagram assumes an antiferromagnetic interaction between the spins.
(Right) Schematic crystal structure of the organic material κ-(BEDT-TTF)2Cu2(CN)3 in a quantum spin liquid state. Pairs of flat BEDT-TTF organic molecules (dimers)—each encircled in purple in the diagram—possess a spin. On a triangular lattice, when the orientation of two spins is fixed, the third spin orientation is unpredictable. This phenomenon, known as geometrical frustration, has been thought to be the main reason of the extraordinary spin liquid state in which spins move around even at extremely low temperatures.
