  Robust boundary states epitomize how deep physics can give rise to concrete
experimental signatures with technological promise. Of late, much attention has
focused on two distinct mechanisms for boundary robustness - topological
protection, as well as the non-Hermitian skin effect. In this work, we report
the first experimental realizations of hybrid higher-order skin-topological
effect, in which the skin effect selectively acts only on the topological
boundary modes, not the bulk modes. Our experiments, which are performed on
specially designed non-reciprocal 2D and 3D topolectrical circuit lattices,
showcases how non-reciprocal pumping and topological localization dynamically
interplays to form various novel states like 2D skin-topological, 3D
skin-topological-topological hybrid states, as well as 2D and 3D higher-order
non-Hermitian skin states. Realized through our highly versatile and scalable
circuit platform, theses states have no Hermitian nor lower-dimensional analog,
and pave the way for new applications in topological switching and sensing
through the simultaneous non-trivial interplay of skin and topological boundary
localizations.
