Femtosecond Light Field-Induced Electronic Structure Changes and Spin-Polarized Current Generation in Topological Insulators

by Dr Yasushi Shinohara (NTT Basic Research Laboratories)

Friday 12 Jul 2024, 14:00 → 15:00 Europe/Zurich

OVGA/200

With recent advancements in optical science and technology, it has become possible to apply a slowly oscillating, high-amplitude electric field for just a few femtoseconds. Using such light to irradiate materials and control them with the electric field of the light wave, femtosecond (petahertz) electronics are garnering significant attention. From this perspective, schemes to control the electronic structure and electron dynamics of materials at ultrafast speeds, represented by Floquet theory, have been actively researched. In this talk, we would like to introduce theoretical research on ultrafast gap renormalization due to many-body effects and ultrafast generation of spin-polarized currents.

 Existing theories and experimental studies on gap renormalization have focused on discussing changes in the gap after light irradiation. With high-intensity excitation, many electron-hole pairs are generated in the material, leading to a reduction in the band gap. In this study, by solving the time-dependent Hatree-Fock theory, we investigated how the band gap changes while the electric field is being applied [1]. As a result, we found that when an electric field of 15.6 V/nm is applied to an insulator like quartz, a band gap change of about 10% appears in an extremely short time of 25 fs.
 The generation of spin-polarized currents is often reported in the field of spintronics under quasi-equilibrium steady states generated by steady electric fields. In this study, we applied the idea of light wave-driven currents to materials with spin textures and researched light wave-driven spin-polarized currents. We showed that when an ultrashort pulse wave irradiates the electronic state on the surface of a three-dimensional topological insulator, spin-polarized currents are generated. By changing the time waveform of the irradiation electric field, we clarified through simulations that the direction of the spin-polarized current can be altered or nullified.

[1] Yasushi Shinohara, Haruki Sanada, Katsuya Oguri, Enhancement of Zener tunneling rate via electron-hole attraction within a time-dependent quasi-Hartree-Fock method, Phys. Rev. B 108 (2023) 134309.