Small-angle neutron scattering study of the mixed state of Yb₃Rh₄Sn₁₃

Using the small angle neutron scattering (SANS) technique we investigated the vortex lattice (VL) in the mixed state of the stannide superconductor Yb₃Rh₄Sn₁₃. We find a single domain VL of slightly distorted hexagonal geometry for field strengths between 350 and 18 500 G and temperatures between T=0.05 and 6.5 K. We observe a clear in-plane rotation of the VL for different magnetic field directions relative to the crystallographic axes. We also find that the hexagonal symmetry of the VL is energetically favorable in Yb₃Rh₄Sn₁₃ for external fields oriented along axes of different symmetries: twofold [110], threefold [111], and fourfold [100]. The observed behavior is different from other conventional and unconventional superconductors. The superconducting state is characterized by an isotropically gapped order parameter with an amplitude of Δ(0)=1.57±0.05 meV. At the lowest temperatures, the field dependence of the magnetic form factor in our material reveals a London penetration depth of λ_L=2508±17 Å and a Ginzburg coherence length of ξ=100±1.3 Å, i.e., it is a strongly type-II superconductor, κ=λ_L/ξ=25.