  Topological crystalline insulators (TCIs) can exhibit unique, quantized
electric phenomena such as fractional electric polarization and
boundary-localized fractional charge. This quantized fractional charge is the
generic observable for identification of TCIs that lack robust spectral
features, including ones having higher-order topology. It has been predicted
that fractional charges can also manifest where crystallographic defects
disrupt the lattice structure of TCIs, potentially providing a bulk probe of
crystalline topology. However, this capability has not yet been confirmed in
experiment since measurements of charge distributions in TCIs have not been
accessible until recently. Here, we experimentally demonstrate that
disclination defects can robustly trap fractional charges in TCI metamaterials,
and show that this trapped charge can indicate non-trivial higher-order
crystalline topology even in the absence of any spectral signatures.
Furthermore, we uncover a connection between the trapped charge and the
existence of topological bound states localized at these defects. We test the
robustness of these topological features when the protective crystalline
symmetry is broken, and find that a single robust bound state can be localized
at each disclination alongside the fractional charge. Our results conclusively
show that disclination defects in TCIs can robustly trap fractional charges as
well as topological bound states, and moreover demonstrate the primacy of
fractional charge as a probe of crystalline topology.
