  Topological phases of matter exhibit phase transitions between distinct
topological classes. These phase transitions are exotic in that they do not
fall within the traditional Ginzburg-Landau paradigm but are instead associated
with changes in bulk topological invariants and associated topological surface
states. In the case of a Weyl semimetal this phase transition is particularly
unusual because it involves the creation of bulk chiral charges and the
nucleation of topological Fermi arcs. Here we image a topological phase
transition to a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ with changing
composition $x$. Using pump-probe ultrafast angle-resolved photoemission
spectroscopy (pump-probe ARPES), we directly observe the nucleation of a
topological Fermi arc at $x_c \sim 7\%$, showing the critical point of a
topological Weyl phase transition. For Mo dopings $x < x_c$, we observe no
Fermi arc, while for $x > x_c$, the Fermi arc gradually extends as the bulk
Weyl points separate. Our results demonstrate for the first time the creation
of magnetic monopoles in momentum space. Our work opens the way to manipulating
chiral charge and topological Fermi arcs in Weyl semimetals for transport
experiments and device applications.
