Deviations from the London model in superconductors

Abstract: Using Small-Angle Neutron Scattering (SANS) we have studied the flux line lattices of the first heavy-fermion superconductor to be discovered, CeCu2Si2, the well-known high-temperature superconductor YBa2Cu3O7, along with Ca-doped variants of this compound, and several topological superconductor candidates that have attracted a lot of attention in the last few years, such as PbTaSe2, Au2Pb and β-Bi2Pd.Unconventional superconductors, with pairing mechanisms and properties that are not accounted for within BCS theory, are a subject of continuing interest. One example is heavy-fermion materials, in which the charge carriers respond as if they had a mass many times that of a free electron. We find that CeCu2Si2 shows a strong increase of scattered intensity from the flux lines as the applied magnetic field is increased towards the edge of the superconducting phase. In the standard theories, this intensity should drop steadily towards zero. This indicates that the superconductivity in this material is not destroyed by the same mechanism as in most all other superconductors but is instead destroyed by Pauli paramagnetic effects (PPE), which act to break up the Cooper pairs by favouring parallel alignment of the spins, increasing the relative strength of the magnetization in the vortex cores. This behaviour had previously been observed in CeCoIn5. To better understand the behaviour of CeCu2Si2 and other Pauli limited superconductors, we present a physically-based expression for the size and magnetization of the vortex cores in Pauli-limited superconductors.For many years, high-temperature superconductors have attracted significant attention as they may be key for the use of superconductors in our everyday life. In the high-Tc superconductors, like YBa2Cu3O7−δ (YBCO), the doping level can be used to tune the observed properties. Usually, δ is the doping control parameter in YBCO, which varies the occupancy of the oxygen chains running along the b-axis of the crystal. The (d-wave) superconductivity in YBCO develops primarily from the CuO2 planes, but s-wave superconductivity also develops from the chains, which can be seen in the distortion of the vortex lattice. We study the temperature and field dependence of this anisotropy and SANS intensity from the vortex lattice up to 25 T using a newly developed formula for SANS time-of-flight (TOF) experiments. Furthermore, we can dope the yttrium site with calcium instead, resulting in a higher hole concentration than fully over-doped YBCO, pushing the field scale down closer to the magnetic fields we can apply in experiment. We include in this work some of the specific effects of doping YBCO with calcium and how it lowers the characteristic critical fields.Topological materials have attracted a lot of attention due to their novel quantum states and their possible applications to quantum computation. A key property is the appearance of Majorana bound states in the vortex core. When the Bogoliubov quasiparticles associated with the superconducting state can be constructed as superpositions of electron and hole states such that the bound quasiparticle is its own anti-quasiparticle (i.e. a Majorana particle), in the bulk, these quasiparticles are dispersive Majorana fermions, and in the vortex core they lead to bound states that obey non-Abelian statistics, and can (in theory) be used as qubits. In this thesis we report SANS experiments performed on three different topological superconductor candidates, PbTaSe2, Au2Pb and β-Bi2Pd. For the latter, we discuss the anisotropy of the vortex lattice and the temperature dependence of the SANS intensity by rotating the sample with respect to the magnetic field direction and shed some light on the superconducting gap nature of β-Bi2Pd.