  Topological Weyl semimetals (TWSs) are exotic crystals possessing emergent
relativistic Weyl fermions connected by unique surface Fermi-arcs (SFAs) in
their electronic structures. To realize the TWS state, certain symmetry (such
as the inversion or time reversal symmetry) must be broken, leading to a
topological phase transition (TPT). Despite the great importance in
understanding the formation of TWSs and their unusual properties, direct
observation of such a TPT has been challenging. Here, using a recently
discovered magnetic TWS Co3Sn2S2, we were able to systematically study its TPT
with detailed temperature dependence of the electronic structures by
angle-resolved photoemission spectroscopy. The TPT with drastic band structures
evolution was clearly observed across the Curie temperature (TC = 177 K),
including the disappearance of the characteristic SFAs and the recombination of
the spin-split bands that leads to the annihilation of Weyl points with
opposite chirality. These results not only reveal important insights on the
interplay between the magnetism and band topology in TWSs, but also provide a
new method to control their exotic physical properties.
