Abstract
The gradient of the phase of superconducting order parameter diverges at the center of a quantized flux (fluxon). Hence, the superconducting order parameter should have a zero value at the center to avoid the divergence of the superconducting current density at that point. This is the physical reason for the normal core in the fluxon. Energy is dissipated when fluxons are forced to move. The microscopic model of Bardeen and Stephen shows that the energy dissipation occurs mostly inside the normal core due to the motion of normal electrons driven by the electric field induced by the flux motion. However, the normal core causes the flux pinning interaction with inhomogeneities through which the energy dissipation can be avoided by stopping the flux motion.
