An artist's impression of the cone-shaped structure of Dirac electronic systems. (Toshio Naito/Ehime University)An elusive behavior of electrons has finally been isolated from more mundane electron activity in a real-world material.
A team of physicists led by Ryuhei Oka of Ehime University has measured what are known as Dirac electrons in a superconducting polymer called bis(ethylenedithio)-tetrathiafulvalene. These are electrons that exist under conditions that effectively make them massless, allowing them to behave more like photons and oscillate at the speed of light.
This discovery, the researchers say, will allow a better understanding of topological materials – quantum materials that behave as an electronic insulator on the inside and conductor on the outside.
Superconductors, semiconductors, and topological materials are all growing in relevance, not least for their potential applications in quantum computers. But there is a lot we still don't know about these materials and the way they behave.
Dirac electrons refer to common old electrons under extraordinary conditions which require a dose of special relativity for quantum behaviors to be understood. Here, the overlap of atoms puts some of their electrons into a strange space that allows them to jump around materials with excellent energy efficiency.
Formulated from the equations of the theoretical physicist Paul Dirac nearly a century ago, we now know they're out there – they've been detected in graphene, as well as other topological materials.
In order to harness the potential of Dirac electrons, however, we need to understand them better, and this is where physicists run into a snag. Dirac electrons coexist with standard electrons, which means detecting and measuring one type is very hard to do unambiguously.
Oka and colleagues found a way to do this by leveraging a property called electron spin resonance. Electrons are charged particles that spin; this rotating distribution of charge means they each exhibit a magnetic dipole. So, when a magnetic field is applied to a material, it can interact with the spins of any unpaired electrons therein, altering their spin state.
This technique can allow physicists to detect and observe unpaired electrons. And, as Oka and the other researchers found, it can also be used to directly observe the behavior of Dirac electrons in bis(ethylenedithio)-tetrathiafulvalene, distinguishing them from standard electrons as different spin systems.
The team found that, in order to fully understand it, the Dirac electron needs to be described in four dimensions. There's the standard three spatial dimensions, the x, y, and z axes; and then there's the energy level of the electron, which constitutes a fourth dimension.
"As 3D band structures cannot be depicted in a four-dimensional space," the researchers explain in their paper, "the analysis method proposed herein provides a general way to present important and easy-to-understand information of such band structures that cannot be obtained otherwise."
By analyzing the Dirac electron based on these dimensions, the researchers were able to figure out something we didn't know before. Their speed of their motion isn't constant; rather, it's dependent on temperature and magnetic field angle within in the material.
This means that we now have another piece of the puzzle that helps us understand the behavior of Dirac electrons – one that may aid in harnessing their properties in future technology.
The team's research has been published in Materials Advances.
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