Vortex arrangement in an ultrathin superconducting bilayer disc

A direct current superconducting quantum interference device (SQUID) fixed on a recently developed superconducting bilayer disc is effective for studying fractional magnetic quantum and fractional vortices. However, such devices cannot magnetically image fractional vortices. To overcome this limitation, we compared the bilayer SQUID signal with and without a pinhole to experimentally investigate the vortex arrangement. We measured the amount of flux passing through the SQUID ring via the SQUID signal phase shift, which reflected the vortex locations. We measured the voltages of a series of 100 SQUIDs in which the bilayer discs were present under each SQUID. When we applied a constant bias current of 5.5 μA and swept the external magnetic field, a voltage appeared when the amount of magnetic flux passing through the SQUID ring became close to half of the flux quantum. A sharp voltage peak was obtained for the bilayers with a pinhole, whereas these peaks were broader for the bilayers without a pinhole. We evaluated the amount of the flux originating from the vortices trapped in the bilayer discs and estimated the vortex locations from the peak position. When the amount of flux originating from the trapped vortices increased, the bilayer discs without a pinhole exhibited reduced amounts of flux originating from the trapped vortices by approximately 15% compared to the bilayer discs with a pinhole. When no pinhole was present, the vortex locations varied inside the SQUID ring. The results indicated that the vortices were located at the same position for all 100 bilayer discs with a pinhole; therefore, we concluded that the pinholes must trap the vortices.