Disorder-induced two-step melting of vortex matter in Co-intercalated NbSe2 single crystals

Disorder-induced melting where the increase in positional entropy created by random pinning sites drives the order-disorder transition in a periodic solid provides an alternate route to the more conventional thermal melting. Here, using real-space imaging of the vortex lattice through scanning tunneling spectroscopy, we show that, in the presence of weak pinning, the vortex lattice in a type-II superconductor disorders through two distinct topological transitions. Across each transition, we separately identify metastable states formed through superheating of the low-temperature state or supercooling of the high-temperature state. Comparing crystals with different levels of pinning we conclude that the two-step melting is fundamentally associated with the presence of random pinning which generates topological defects in the ordered vortex lattice.