No defects needed for flux penetration in a type-II superconductor. When the conference length is smaller than the penetration depth (up to a factor of sqrt(2))

No defects needed for flux penetration in a type-II superconductor. When the conference length is smaller than the penetration depth (up to a factor of sqrt(2)), flux vortices can nucleate as soon as the surface magnetic field gets above the lower critical field Bc1.