Dirac electrons were predicted by P. Dirac and discovered by A. Geim, both of whom were awarded the Nobel Prize in Physics in 1933 and in 2010, respectively. Dirac electrons behave like photons rather than electrons, for they are considered to have no mass and in materials they move with light velocity. Because of such differences from standard electrons, they are expected to add unprecedented electronic properties to materials. For example, they could be applied to electronic devices to perform computation and communication with an extraordinary efficiency and low energy consumption.
To develop such technology, we must first understand the net properties and effects of Dirac electrons. But they generally coexist with standard electrons in materials, which prevents unambiguous observation and measurement. In the present study, Naito et al. discovered a method enabling selective observation of the Dirac electrons in materials. Using electron spin resonance, to directly observe unpaired electrons in materials to distinguish differences in character, the research group established amethod to determine their scope of action in the materials and their energies. The latter information contains how rapidly they move, namely the velocity. This information requires a four-dimensional world, for it consists of positions (x, y, z) and energy (E). The present research group has described it in an easy-to-understand scheme.
These findings, have taken the understanding Dirac electrons a step forward. We now know that their velocity is anisotropic and it depends on their direction and location, instead of the constant velocity of light.