<Advanced Energy Conversion Division> Clean Energy Conversion Research Section

3-1. Research Activities in 202

Theory of exciton thermal radiation in semiconducting single-walled carbon nanotubes

Spectral control of thermal radiation is an essential strategy for highly efficient and functional utilization of thermal radiation energy. Among the various proposed methods, quantum confinement in low-dimensional materials is promising because of its inherent ability to emit narrowband thermal radiation. Here, we theoretically investigate thermal radiation from one-dimensional (1D) semiconductors characterized by the strong quantum correlation effect due to the Coulomb interaction. We derive a simple and useful formula for the emissivity, which is then used to calculate the thermal radiation spectrum of semiconducting single-walled carbon nanotubes as a representative of 1D semiconductors. The calculations show that the exciton state, which is an electron–hole pair mutually bound by the Coulomb interaction, causes enhancement of the radiation spectrum peak and significant narrowing of its linewidth in the near-infrared wavelength range. The theory developed here will be a firm foundation for exciton thermal radiation in 1D semiconductors, which is expected to lead to new energy harvesting technologies

Multiple Exciton Generation by a Single Photon in Single-Walled Carbon Nanotubes

Multiple-exciton generation in single-walled carbon nanotubes is investigated theoretically. We show that multiple excitons can be directly generated by a single photon through resonant coupling with multiexciton states. Further, the theoretically predicted threshold energy for this process is consistent with recent experimental results. Our calculations clarify the elementary processes of multiple-exciton generation in single-walled carbon nanotubes

Landau damping: instability mechanism of superfluid Bose gases moving in optical lattices

We investigate Landau damping of Bogoliubov excitations in a dilute Bose gas moving in an optical lattice at finite temperatures. Using a 1D tight-binding model, we explicitly obtain the Landau damping rate, the sign of which determines the stability of the condensate. We find that the sign changes at a certain condensate velocity, which is exactly the same as the critical velocity determined by the Landau criterion of superfluidity. This coincidence of the critical velocities reveals the microscopic mechanism of the Landau instability. This instability mechanism is also consistent with the recent experiment suggesting that a thermal cloud plays a crucial role in breakdown of superfluids, since the thermal cloud is also vital in the Landau damping process. We also examine the possibility of simultaneous disappearance of all damping processes.Comment: 9 pages, 5 figure