Correlation Effect of Massless Electrons

Investigation of interaction effect that produces a wide variety of electronic states

Enhancement of velocity of massless Dirac electrons in organic salt, α-(ET)2I3 This figure represents the relationship between the energy of electrons in crystal (vertical axis; units in million electron volts, or meV) and wave number (horizontal axis; units in angstrom). Enhancement of the velocity of electrons in strong correlations (represented by gradient of the cone's slope) has position dependence compared with the non-interacting case (in translucent gray). The rainbow color spectra show the distribution of electrons with strong signals (shown in red, concentrated in the steep region) and weak signals (shown in blue, present in region with gentle slope), within the crystal. © 2016 Michihiro Hirata.

University of Tokyo researchers have revealed both theoretically and experimentally the anomalous behaviors of massless Dirac electrons, a special type of electron population, by means of nuclear magnetic resonance (NMR) measurements, an experimental technique that allows detailed observation of electron behavior.

It is well known that an electron at rest in a vacuum has a finite mass. On the other hand, in a crystal, an electron has an apparent mass (effective mass), which can take a variety of values depending on the crystal structure, elemental composition, and so on. Under special conditions in a solid, an electron acts as if it had zero mass. This type of particle is called a massless Dirac electron, and its novel physical properties are an active research frontier in both basic and applied sciences. After first discovering massless Dirac electrons in graphene about 10 years ago, they were found in the surface of topological insulators (materials that are conductors on the surface but insulators internally) and similar materials, and further in molecular crystals. The science of Dirac materials has spread rapidly and is an important topic of research.

Since the effect of electric and magnetic interactions on massless Dirac electrons will be quite different from that on electrons with a finite mass in normal metals and semiconductors, peculiar characteristics and behaviors are expected from populations of massless Dirac electrons. Indeed, in graphene, anomalous enhancement of velocity of electrons as a result of electron interaction has been observed. However, the diversity of electron population characteristics in Dirac materials is a largely unexplored area because of weak interactions between particles in graphene.

A collaborative research group including Professor Kazushi Kanoda and Assistant Professor Kazuya Miyagawa at the University of Tokyo Graduate School of Engineering and their colleagues used both NMR and theoretical calculations to investigate an organic Dirac material that has stronger interaction between electrons (termed “electron correlation”) than graphene. They found for the first time, both theoretically and experimentally, increased electron velocity due to strong electron repulsion, and the emergence of ferrimagnetism due to alignment of electron spin against a magnetic field in a portion of the electrons.

“Our study revealed for the first time experimentally that Dirac electron populations exhibit significantly larger variation in collective behavior than had previously been known,” says Kanoda. He continues, “This work opens the door to investigate the diversity of massless Dirac electrons focusing on ‘electronic and magnetic interaction.’ “