SANS study of the vortex lattice PrOs$_{4}$Sb$_{12}$

We report on the magnetic field ($H$) dependence of the form factor $|F|^2$ of the vortex lattice (VL) in PrOs$_{4}$Sb$_{12}$ obtained by small angle neutron scattering for $H$ applied along the crystallographic $a$-axis. PrOs$_{4}$Sb$_{12}$ is the fist Pr based heavy fermion superconductor (SC) with a critical temperature $T_c$ of 1.85 K and a $H_{c2}$ of 2.5 T Although PrOs$_{4}$Sb$_{12}$ crystallizes in a filled Skutterudite structure with a cubic lattice, this structure has a tetrahedral point group symmetry. In consequence with the application of a magnetic field along the $c$-direction, the $a$ and $b$-axis are no longer equivalent. Measurements of the angular dependence of the thermal conductivity in PrOs$_{4}$Sb$_{12}$ suggest a change of the superconducting order parameter deep inside the SC phase diagram. The thermal conductivity a low fields is compatible with a SC gap structure with a two fold symmetry, while at higher field it changes to a fourfold symmetry.