  Constraints in the dynamics of quantum many-body systems can dramatically
alter transport properties and relaxation timescales even in the absence of
static disorder. Here, we report on the observation of such constrained
dynamics arising from the distinct mobility of two species in the
one-dimensional mass-imbalanced Fermi-Hubbard model, realized with ultracold
ytterbium atoms in a state-dependent optical lattice. By displacing the trap
potential and monitoring the subsequent dynamical response of the system, we
identify suppressed transport and slow relaxation with a strong dependence on
the mass imbalance and interspecies interaction strength, consistent with
eventual thermalization for long times. Our observations demonstrate the
potential for quantum simulators to provide insights into unconventional
relaxation dynamics arising from constraints.
