Vortex lattice reorientation and anisotropy in MgB2--effects of two-band superconductivity

We present small-angle neutron scattering (SANS) measurements of the vortex lattice (VL) in single crystal MgB2--a two-band superconductor. Bragg diffraction of neutrons visualizes the structure and orientation of the VL while the scattered intensity probes the super-carrier density. Superconductivity in the π-band is rapidly suppressed with increasing field with a corresponding decrease in scattered intensity and a re-orientation of the VL between 0.5 and 0.9 T. Both these observations are consistent with superconductivity in the π-band being weaker than in the σ-band (Δπ<Δσ), and a changing influence of different parts of the Fermi surface determining the VL orientation. SANS measurements of the VL for fields applied at an angle to the c-axis allow the penetration depth anisotropy, γλ, to be estimated