Material scientists have recently given considerable attention to a quantum state of matter called topological material. These materials behave as electronic insulators on the inside and conductors on the outside.
To date, many kinds of topological materials are known, each with unique physical properties that originate from Dirac fermions (DF), which govern properties other than the standard fermions (SF) of electrons.
Particles With Elusive Behavior
Dirac electrons refer to particles that move in materials at the speed of light. They were first predicted by mathematician Paul Dirac and discovered by physicist Andre Geim. Since they are considered to have no mass, they behave like photons rather than electrons.
These particles exhibit different properties from standard electrons, so they are expected to contribute unprecedented electronic properties of materials. For instance, they can be applied to electronic devices to aid communication and computation with remarkable efficiency and low energy consumption.
In order to develop such technology, experts must first understand the net properties and effects of Dirac electrons. However, these particles generally coexist with standard electrons in materials, making it difficult to conduct unambiguous observation and measurement.
Direct observation of Dirac electrons is demanding, partly because some need high pressure to realize such band structures and partly due to the extremely high-accurate energy required to observe the band structures directly.
Understanding Dirac Electrons
A new method discovered by scientists from Ehime University has finally made selective observation of Dirac electrons in materials possible. The study, led by Ryuhei Naito, is discussed in the paper "Nearly three-dimensional Dirac fermions in an organic crystalline material unveiled by electron spin resonance."
Naito and his colleagues used electron spin resonance to directly observe unpaired electrons in materials to distinguish differences in character. The team also established a method of determining their scope of action in the materials and their energies.
The latter is defined by how rapidly they move or velocity. This information needs a four-dimensional world of positions (x, y, z) and energy (E). The research team considers this as an easy-to-understand scheme.
The experiment involves analyzing an organic crystalline material. Before measuring their physical properties, all single crystals were checked using X-ray oscillation photographs. The researchers focused on the crystal quality and orientation of crystallographic axes.
The Vienna Ab initio Simulation Package (VASP) was used to calculate the electronic band structures of the materials derived from the X-ray diffraction experiments in the team's previous work. It was found that the conducting properties of the crystals are qualitatively sensitive to the dimensions of the electronic systems. Moreover, the DF systems of any dimension exhibit conducting behavior that is not exhibited by SFs under any condition.
The research's findings help experts understand Dirac electrons a step further. The experiment made them realize that the velocity of Dirac electrons is anisotropic. They also found that this velocity depends on the particle's direction and location instead of the constant velocity of light.
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