ABSTRACT
The Higgs mode in superconductors, i.e., the collective amplitude mode of the order parameter, does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. Contrary to this common understanding, here, we demonstrate that if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at ω=2Δ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.
ABSTRACT
Techniques using electroencephalography (EEG)-based brain computer interfaces (BCIs) have been developed and are eagerly anticipated as novel interfaces for controlling power wheelchairs. In addition to the BCIs, smart glass technology has been developed. In our previous study, we propose a prototype of an intuitive control system for a power wheelchair; this system comprises a simple EEG recorder, smart glass, and a microcomputer. Using this system, the power wheelchair moves straight ahead when a user concentrates, stops when the user blinks, and turns left or right when the user tilt his/her neck to the left or right, respectively. A ß/α ratio as an indicator of the concentration and blinks are detected from raw EEG waves, and the tilting of the neck is detected by acceleration sensors in the smart glass. In this study, we proposed a new control system which display the visual feedback of the ß/α ratio on the smart glass to induce user's concentration. Our results show that during the experiment, the system successfully worked and induced for the ß/α ratio in specific concentrating states.