> Nakamura and her colleagues have now developed their technique to measure vortex motion in two dimensions, this time in the iron-based superconductor FeSe0.5Te0.5, or FST. The researchers fabricated a 38-nm-thick film of FST and cooled it below its superconducting transition temperature (16.5 K). A coil generated a magnetic field, which produced vortices in the film. Additionally, the magnetic field induced a shielding current, which traced out a several-millimeter-diameter circle on the film.
> The researchers irradiated the film with a 20-picosecond infrared (0.3-terahertz) pulse and detected a second harmonic in the spectrum of the pulse that emerged from the sample, as in the previous experiments. But here they detected both polarizations of the emitted light, parallel and perpendicular to the incident pulse’s polarization.
> Using a roughly 1-mm-wide beam, they probed a region near the ring of the field-induced shielding current and then analyzed the transmitted waveforms in order to reconstruct the motion of a typical vortex residing in that location. The team found an oscillating, roughly parabolic trajectory rather than a straight line. This shape resulted from the interaction between the vortex, which is magnetic, and the shielding current. Discovering this motion was the most exciting part of the work, Nakamura says. “It felt like we were looking directly into the 2D motion of the vortex.”
> The researchers irradiated the film with a 20-picosecond infrared (0.3-terahertz) pulse and detected a second harmonic in the spectrum of the pulse that emerged from the sample, as in the previous experiments. But here they detected both polarizations of the emitted light, parallel and perpendicular to the incident pulse’s polarization.
> Using a roughly 1-mm-wide beam, they probed a region near the ring of the field-induced shielding current and then analyzed the transmitted waveforms in order to reconstruct the motion of a typical vortex residing in that location. The team found an oscillating, roughly parabolic trajectory rather than a straight line. This shape resulted from the interaction between the vortex, which is magnetic, and the shielding current. Discovering this motion was the most exciting part of the work, Nakamura says. “It felt like we were looking directly into the 2D motion of the vortex.”
"Picosecond trajectory of two-dimensional vortex motion in FeSe0.5Te0.5 visualized by terahertz second harmonic generation" (2024) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.13...