  The direct current (d.c.) conductivity and emergent functionalities at
ferroelectric domain walls are closely linked to the local polarization
charges. Depending on the charge state, the walls can exhibit unusual d.c.
conduction ranging from insulating to metallic-like, which is leveraged in
domain-wall-based memory, multi-level data storage, and synaptic devices. In
contrast to the functional d.c. behaviors at charged walls, their response to
alternating currents (a.c.) remains to be resolved. Here, we reveal a.c.
characteristics at positively and negatively charged walls in ErMnO3,
distinctly different from the response of the surrounding domains. By combining
voltage-dependent spectroscopic measurements on macroscopic and local scales,
we demonstrate a pronounced non-linear response at the electrode-wall junction,
which correlates with the domain-wall charge state. The dependence on the a.c.
drive voltage enables reversible switching between uni- and bipolar output
signals, providing conceptually new opportunities for the application of
charged walls as functional nanoelements in a.c. circuitry.
