Neutron studies of the flux-line lattice in high temperature superconductors

Abstract

Abstract In a type II superconductor, such as the Nb or the high temperature superconductors, there are two critical magnetic fields, HC1 and HC2. When a sample is cooled through the critical temperature for superconductivity, TC in a magnetic field of less than HC1 all magnetic flux is expelled from the material-the Meissner-Ochsenfeld effect. HC2 is the maximum magnetic field which can be applied to the superconductor without driving the sample back into the normal state. At intermediate magnitudes of magnetic field, HC1<H<HC2 a superconducting sample is in the mixed state where magnetic flux still exists within the bulk of the material. In this mixed state, the magnetic flux enters as “flux-lines”; each one of these objects has a normal core which is surrounded by a circulating supercurrent flow and contains a total amount of flux equal to h/2e. At low fields, these flux-lines are independent, but in higher fields their magnetic fields and supercurrents overlap. In response to this interaction, the flu...