Materials change their form between three states — solid, liquid, and gas — depending on factors such as temperature and pressure. Among those states, a phase transition does not necessarily occur between liquid and gas, and they can continuously transform from the one to the other.
With regard to spins, microscopic magnets associated with electrons in magnetic solids, one may consider three similar states. The solid corresponds to, for instance, a ferromagnetic state in which all the spins point the same direction, while the gas corresponds to a paramagnetic state in which the spin directions are random. On the other hand, for the spin liquid, although the concept of the “quantum spin liquid” was proposed in analogy with liquid helium that does not solidify even down to the lowest temperatures, its existence and nature have remained as an enigma for a long time.
Assistant Professor Joji Nasu in the Department of Physics at the Tokyo Institute of Technology (University of Tokyo Graduate School of Engineering Department of Applied Physics at the time of research) and Assistant Professor Masafumi Udagawa and Associate Professor Yukitoshi Motome in the Department of Applied Physics in the University of Tokyo Graduate School of Engineering have discovered that there is always a phase transition between quantum spin liquids and a paramagnetic state: they cannot continuously transform into each other, contrary to the common belief.
This was shown by large-scale numerical simulations for a theoretical spin model called the Kitaev model. The transition they discovered is of new type, not explained by the conventional theory for phase transitions. The researchers also clarified that the novel transition is regarded as a change of the topological nature of the system.
The discovery urges a reconsideration of recent experimental studies that suggest the existence of quantum spin liquids by the absence of phase transitions. Also, it will have an impact on the field of quantum information, in which topological nature is used for data processing.
Source: University of Tokyo