# Theory of undiscovered elementary particle applied to topological insulator

## Prediction of a novel phase transition inspired by axion search

Kavli Institute for the Physics and Mathematics of the Universe

Institute for Solid State Physics

2012/07/11

In elementary particle theory, there exists a paradox called the strong CP problem. A theoretical solution to this problem has been proposed that requires the existence of an elementary particle, the axion. The axion has also attracted attention in recent years as a candidate for “dark matter,” which is deemed a necessary component of cosmological theory.

However, researchers have been unable to detect the axion. As it is highly difficult to observe directly, researchers have instead tried to observe phenomena that would indicate the existence of the axion, and experiments to this end are taking place.

On the other hand, it has been pointed out that an excitation corresponding to the axion appears in equations describing a class of substances called topological insulators, the theory of which has attracted attention recently in the field of condensed matter physics. Although the scale of the physics is quite different in elementary particle physics and in condensed matter physics, they share a universal mathematical language: quantum field theory.

In this research, Kavli IPMU Senior Research Fellow Hirosi Ooguri and Professor Masaki Oshikawa of the Institute for Solid State Physics have announced a new axion research discovery in condensed matter physics based on advances in theoretical elementary particle physics research.

The researchers have shown theoretically that axion electrodynamics possesses an instability in a strong electric field, which leads to a phase transition. Beyond the transition, the electric field is screened above a certain field strength and excess energy converted to a magnetic field. This phenomenon was unknown until now. Furthermore, they pointed out the possibility of observation of this phenomenon experimentally in the solid state, using topological and other insulators.

This new discovery is the result of the accumulation of research knowledge in both these fields and was made possible by the cooperation of the Kavli IPMU and the Institute for Solid State Physics, both at the Kashiwa campus. Such on-campus interdisciplinary exchanges are expected to stimulate many more great research achievements in the future.

Department release/press release

#### Paper

Hirosi Ooguri, Masaki Oshikawa,

“Instability in magnetic materials with dynamical axion field”,

*Physical Review Letters* April 20, 2012 Online, arXiv:1112.1414v2 [cond-mat.mes-hall]

Article link

#### Links

Kavli Institute for the Physics and Mathematics of the Universe

The Institute for Solid State Physics

#### Related information

Hirosi Ooguri, Masaki Oshikawa, *Interview: “Particle theory meets condensed matter physics: A dream from the combination of axion and topological insulators*, KAGAKU, 2012, July, Iwanami Shoten.