  The work presented in my doctoral thesis is an experimental investigation of
the nature of order to disorder transition of vortex lattice in a Type-II
superconductor namely Co-intercalated 2H-NbSe2 using scanning tunnelling
spectroscopy and complimentary ac susceptibility measurements. We have shown by
performing real space imaging across the magnetic field driven peak effect that
the vortex lattice in a 3-dimensional, weakly pinned Type-II superconductor
disorders through two distinct topological phase transitions. Clear evidence of
superheating and supercooling with different topological property of the
metastable states from the actual ground states indicated the nature of both
the phase transformations to be of first order. We have also demonstrated the
role of underlying crystalline lattice symmetry by showing that it aligns the
vortex lattice along preferential directions which is the lattice vectors of
the crystal lattice itself. This underlying coupling also has profound
implication in the order disorder transition indicating the significant role of
vortex lattice-crystal lattice interaction in vortex phase diagram which was
earlier being neglected.
