In condensed matter physics the vortex lattice of type-II superconductors provides a prototype for studying the effect of random pinning on elastic systems. One important problem is to understand the structural transition from a theoretically predicted ordered Bragg glass phase to a disordered vortex liquid phase. Defects, such as screw dislocations, should play an important role in mediating this order-disorder transition. It is important to confirm the existence of these defect structures in the vortex lattice.
Experimentally it is difficult to probe the detailed defect structures inside the vortex lattice. Traditional methods can only provide information on the surface configuration or an averaged picture of the bulk behavior of the vortex lattice. Here we use a novel high-resolution neutron diffraction technique to probe the angular orientation of the lattice planes as the flux lines traverse the atomic crystal. Our results provide structural evidence for screw dislocations inside the vortex lattice. The anisotropic defect structure in the underlying atomic lattice serves as a symmetry breaking field for the vortex lattice. The strong dependence of the vortex lattice structure on the growth procedure reveals that the system is metastable and can be perturbed through thermal cycling to a possible ground state. We measure the structure of the vortex lattice under different applied magnetic fields and temperatures to study the interplay between vortex-vortex interaction, vortex-atomic lattice interaction, and thermal fluctuations.
This high-resolution neutron diffraction technique opens up a new way in studying the detailed structure of the vortex lattice. Our results suggest that the vortex lattice in low temperature superconductors with anisotropic defects in the atomic lattice could be an excellent candidate for exploring the entangled flux liquid phase.