Vortex lines nearly parallel to the surface of a single-crystal superconductor disk in a perpendicular field

Abstract

Using a contact-free ultrasonic method we obtain strong experimental evidence that vortex lines penetrating into a type-II single-crystal superconductor disk in a perpendicular dc magnetic field lie during the virgin magnetization stage nearly parallel to the surface over a substantial area of the sample. This phenomenon is very different from the predictions of various existing theoretical models in which the vortex lines are inclined at significant angles to the disk plane. We demonstrate that this phenomenon continues until the applied field is increased to a magnitude that correlates with the lower critical field ${H}_{c1}.$