Spontaneous Symmetry-Breaking and the Vortex Lattice: Surprising Results from a Conventional Superconductor

M. Laver Birmingham University, UK

The magnetic vortex lattice of Type-II superconductors, as a quasi-2D assembly of rod-like structures, is in itself an interesting system to explore. Moreover the characteristics of the vortex lattice (VL) often reflect properties of the underlying superconducting crystal. Here recent research is presented that uses the SANS technique to observe the VL morphology in single-crystals of pure niobium, a 'conventional' superconductor. A surprising, and beautiful, variety of new VL structures is observed when the magnetic field is applied parallel to a fourfold-symmetry crystal axis; this induces frustration between the superconducting crystal symmetry and the hexagonal VL coordination expected in a purely isotropic situation. All of the VL phases observed spontaneously break some crystallographic symmetry -- some even have the lowest possible symmetry of a 2D Bravais lattice. This is quite unlike the situation in high-Tc or borocarbide materials where the VL structures observed are locked to the crystal structure of the superconductor.

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