Figure 1

Far-infrared measurement of fluxon mass. (a) Sketch of the magneto-optical setup. The continuous laser beam is split by a linear wire-grid polarizer; the reflected part is monitored using a pyroelectric detector to keep trace of unavoidable power fluctuations, whereas the transmitted part proceeds toward the sample. The retarder converts the light from linear to circular polarization. The propagation of the circularly polarized beam and the magnetic field are perpendicular to the film surface, as detailed in panel (b). (b) Vortices in the film (gray circles) control the transmittance of the sample via the following mechanism: The electric field of the laser light drives the supercurrent. The Magnus force accelerates vortices in the direction perpendicular to the supercurrent; in reaction, the vortex motion affects the supercurrent and, thus, the transmittance. In the sketch, the electric field in the sample, as well as the vortices, rotate clockwise. If the light frequency is close to the cyclotron frequency of vortices, the motion of vortices is resonantly enhanced, leading to the observed dichroism. The extent of the cyclotron motion is strongly exaggerated; in fact, the fluxon circulates on a radius of less than \(10^{-12}\) m at the strongest laser line. (c) Transmittance of the YBa\(_2\)Cu\(_3\)O\(_{7-\delta }\) superconducting sample, normalized to the normal-state transmittance \({{\mathcal {T}}}_N\) at 100 K and plotted for two circular polarizations versus temperature. The dichroism is clearly visible below 70 K in a magnetic field of 10 T; in a zero field, no dichroism appears. (d) Transmittance ratio \(\mathcal{T}_+/{{\mathcal {T}}}_-\) measured in several applied magnetic fields plotted versus temperature. The data were obtained using a 312 \(\mu\)m laser line (\(6.1\times 10^{12}\) rad/s). Above the critical temperature, the dichroism is absent, showing that the normal-state Hall component is negligible.