Title: Anisotropy and multiband superconductivity in Sr2RuO4,
Abstract: Despite numerous studies the exact nature of the order parameter in
superconducting Sr2RuO4 remains unresolved. We have extended previous
small-angle neutron scattering studies of the vortex lattice in this material
to a wider field range, higher temperatures, and with the field applied close
to both the <100> and <110> basal plane directions. Measurements at high field
were made possible by the use of both spin polarization and analysis to improve
the signal-to-noise ratio. Rotating the field towards the basal plane causes a
distortion of the square vortex lattice observed for H // <001>, and also a
symmetry change to a distorted triangular symmetry for fields close to <100>.
The vortex lattice distortion allows us to determine the intrinsic
superconducting anisotropy between the c-axis and the Ru-O basal plane,
yielding a value of ~60 at low temperature and low to intermediate fields. This
greatly exceeds the upper critical field anisotropy of ~20 at low temperature,
reminiscent of Pauli limiting. Indirect evidence for Pauli paramagnetic effects
on the unpaired quasiparticles in the vortex cores are observed, but a direct
detection lies below the measurement sensitivity. The superconducting
anisotropy is found to be independent of temperature but increases for fields >
1 T, indicating multiband superconductvity in Sr2RuO4. Finally, the temperature
dependence of the scattered intensity provides further support for gap nodes or
deep minima in the superconducting gap.