Observation of single vortices by magneto-optical imaging

We have improved our magneto-optical imaging system and succeeded in observing single vortices in a type-II superconductor. In order to maximize the quality of differential magneto-optical images, noises from vibration and drift of the cryostat are suppressed by fixing the pumping line for He-flow and by using a cryostat with reduced thermal contraction of the cold stage. To obtain large signals from single vortices, we minimized the distance between the sample surface and the garnet film by utilizing the thermal contraction of Apiezon grease. All these improvements made it possible to observe single vortices in NbSe2 single crystals using magneto-optical imaging at low magnetic inductions (∼1 G).