Seminar: Aktuelle Probleme der Theoretischen Physik
Sprecher: Koudai Sugimoto
It is known that, in narrow-gap semiconductors or semimetals, pairs of electrons and holes (excitons) are spontaneously formed and go into a condensed state with macroscopic phase coherence. This state is referred to as an excitonic condensation phase and was predicted half a century ago, but actual materials of this phase is not known. Recent angle-resolved photoemission spectroscopy experiments have suggested that Ta2NiSe5 may be in the excitonic phase accompanied by the structural transition. However, a direct evidence for the excitonic phase transition is still lacking.
With the three-chain model describing Ta2NiSe5, we have suggested the way of experiments to determine whether Ta2NiSe5 is in excitonic condensation phase. In the excitonic phase of Ta2NiSe5, microscopic quantum interferences appear in ultrasonic attenuation rate and NMR relaxation rate. Moreover, the hybridization of conduction band and valence band by the excitonic condensation leads to negative diamagnetic susceptibility. These measurements can be powerful tools to investigate the excitonic condensation.
In addition, we have performed optical-conductivity calculations based on the density functional theory (DFT) in the normal phase of Ta2NiSe5, and compared with the spectra observed in experiments. The calculated spectra are well consistent with the experiments in visible-light and ultraviolet-light energy regions. However, in infrared-light energy region, a characteristic spectral peak observed in experiments is not obtained from the DFT calculation. We show that this peak structure can be reproduced by introducing strong intraorbital Coulomb interactions leading to the excitonic condensation at low temperature.
 K. Sugimoto, T. Kaneko, and Y. Ohta, Phys. Rev. B 93, 041105(R) (2016).
 K. Sugimoto and Y. Ohta, Phys. Rev. B 94, 085111 (2016).
 K. Sugimoto, S. Nishimoto, T. Kaneko, and Y. Ohta, in preparation.