  Black holes formed in dense star clusters, where dynamical interactions are
frequent, may have fundamentally different properties than those formed through
isolated stellar evolution. Theoretical models for single star evolution
predict a gap in the black hole mass spectrum from roughly $40-120\,M_{\odot}$
caused by (pulsational) pair-instability supernovae. Motivated by the recent
LIGO/Virgo event GW190521, we investigate whether black holes with masses
within or in excess of this "upper-mass gap" can be formed dynamically in young
star clusters through strong interactions of massive stars in binaries. We
perform a set of $N$-body simulations using the CMC cluster-dynamics code to
study the effects of the high-mass binary fraction on the formation and
collision histories of the most massive stars and their remnants. We find that
typical young star clusters with low metallicities and high binary fractions in
massive stars can form several black holes in the upper-mass gap and often form
at least one intermediate-mass black hole. These results provide strong
evidence that dynamical interactions in young star clusters naturally lead to
the formation of more massive black hole remnants.
