In plane vortex lattice of the heavy Fermion superconductor CeCoIn5

We present a small angle neutron scattering study of the vortex lattice (VL) in the unconventional superconductor CeCoIn$_5$ for magnetic fields $H$ applied in the crystallographic $ab$-plane. CeCoIn$_5$ is a $d_{x^2-y^2}$ superconductor with the lines nodes of the superconducting gap oriented along the $[110]$ direction. At low fields $H$ applied parallel to $[100]$, and $[110]$, we observed a distorted hexagonal VL with an opening angle of about 80$^{\circ}$. However, for $[110]$ we find a reorientation transition for fields above 9~T where the vortex lattice is rotated by 90$^{\circ}$. The $H$-dependence of the form factor of the vortex lattice is similar to what we observed previously along the $c$-axis: The scattering intensity increases with increasing $H$ due to Pauli paramagnetism. For fields where the transition into the SC state becomes first order, we observe a broadening of the VL.