  We report experimental evidence of strong orientational coupling between the
crystal lattice and the vortex lattice in a weakly pinned Co-doped NbSe2 single
crystal through direct imaging using low temperature scanning tunneling
microscopy/spectroscopy. At low fields, when the magnetic field is applied
along the six-fold symmetric c-axis of the NbSe2 crystal, the equilibrium
configuration of the vortex lattice is preferentially aligned along the basis
vectors of the crystal lattice. The orientational coupling between the vortex
lattice and crystal lattice becomes more pronounced as the magnetic field is
increased. We show that this coupling enhances the stability of the
orientational order of the vortex lattice, which persists even in the
disordered state at high fields where dislocations and disclinations have
destroyed the topological order.
