Small Angle Neutron Scattering Studies of the Flux Line Lattices in the Borocarbide Superconductors

Morten Ring Eskildsen

    Research output: Book/ReportPh.D. thesis

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    Abstract

    This thesis describes small angle neutron scattering studies of the flux line lattice (FLL) in the borocarbide superconductors with stoichiometry RNi2B2C, where R is one of Y, Dy, Ho, Er, Tm and Lu. The “pure” compounds YNi2B2C, ErNi2B2C, TmNi2B2C and LuNi2B2C, as well as Y0.75Lu0.25Ni2B2C and Co doped Lu(Ni1−xCox)2B2C with x = 1.5 − 9% was studied. Of the materials studied ErNi2B2C and TmNi2B2C orders magnetically, the coexistence of superconductivity and magnetism being one of the main motivations for these studies. Three main conclusions can be derived from the experiments reported in this thesis.
    1. Existence of a low field hexagonal to square symmetry transition of the FLL, ubiquitous to the superconducting borocarbides, magnetic and non-magnetic alike. This symmetry transition is due to a four-fold anisotropy of the Fermi surface in the borocarbides. The superconducting screening currents around each flux line are distorted away from the circular flow pattern of an isotropic system, and towards a square. The square flow patterns manifest themselves by inducing a transition to a square FLL as the field is increased, with a transition onset field determined by the range of the non-local electrodynamics.
    2. A static disordering of the FLL in YNi2B2C and LuNi2B2C. In these materials one observes a well ordered FLL, with a longitudinal correlation length exceeding 100 flux line spacings. As the applied field is increased the longitudinal correlation length, or equivalent, the “straightness” of the flux lines, increases with field up to H/Hc2 ∼ 0.2. Above this field the FLL correlation length slowly starts to fall off, in contradiction to a theoretical model, which predicts a monotonically increasing correlation length up to the onset of the peak effect. The origin of the crossover to a less ordered FLL is not resolved, but it is speculated that this is driven by the shear properties of the FLL.
    3. The existence of complex interactions between the magnetic state and the
    FLL in TmNi2B2C. The FLL in this material undergoes two symmetry transitions in addition to the low field hexagonal to square transition present in all the borocarbides, and described above. As the applied field is raised the square FLL first undergoes a rhombic distortion and then a discontinuous transition into a hexagonal symmetry. The FLL symmetry transitions are closely linked to changes in the magnetic structure. In zero field the magnetic structure is a transverse modulated spin density wave. As the field is increased the magnetic structure is modified, signalled by the appearence of additional neutron scattering reflections. In addition, the FLL reflectivity shows distinct peaks as the thulium ions orders magnetically at TN and across the field driven magnetic transition. Furthermore the field induced magnetic structure is dependent on the field- and temperature history in the superconducting phase, giving rise to hysteresis. No explanation for this behaviour exists at the present.
    Original languageEnglish
    PublisherRisø National Laboratory
    Number of pages84
    ISBN (Print)87-550-2472-6
    ISBN (Electronic)87-550-2473-4
    Publication statusPublished - 1998
    SeriesDenmark. Forskningscenter Risoe. Risoe-R
    Number1084(EN)
    ISSN0106-2840

    Note re. dissertation

    This thesis was submitted in partial fulfilment of the requirements to obtain a Ph. D. in physics at the University of Copenhagen.

    Keywords

    • Risø-R-1084(EN)
    • Risø-R-1084

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