Simultaneous evidence for Pauli paramagnetic effects and multiband superconductivity inKFe2As2by small-angle neutron scattering studies of the vortex lattice

Abstract

Despite numerous studies the exact nature of the order parameter in superconducting ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ 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 $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}110\ensuremath{\rangle}$ 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 $\mathbit{H}\ensuremath{\parallel}\ensuremath{\langle}001\ensuremath{\rangle}$ and also a symmetry change to a distorted triangular symmetry for fields close to $\ensuremath{\langle}100\ensuremath{\rangle}$.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 $\ensuremath{\sim}60$ at low temperature and low to intermediate fields. This greatly exceeds the upper critical field anisotropy of $\ensuremath{\sim}20$ 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 $\ensuremath{\gtrsim}1$ T, indicating multiband superconductvity in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. Finally, the temperature dependence of the scattered intensity provides further support for gap nodes or deep minima in the superconducting gap.