  The electrical control of a material's conductivity is at the heart of modern
electronics. Conventionally, this control is achieved by tuning the density of
mobile charge carriers. A completely different approach is possible in Mott
insulators such as Ca$_2$RuO$_4$, where an insulator-to-metal transition (IMT)
can be induced by a weak electric field or current. This phenomenon has
numerous potential applications in, e.g., neuromorphic computing. While the
driving force of the IMT is poorly understood, it has been thought to be a
breakdown of the Mott state. Using in operando angle-resolved photoemission
spectroscopy, we show that this is not the case: The current-driven conductive
phase arises with only a minor reorganisation of the Mott state. This can be
explained by the co-existence of structurally different domains that emerge
during the IMT. Electronic structure calculations show that the boundaries
between domains of slightly different structure lead to a drastic reduction of
the overall gap. This permits an increased conductivity, despite the persistent
presence of the Mott state. This mechanism represents a paradigm shift in the
understanding of IMTs, because it does not rely on the simultaneous presence of
a metallic and an insulating phase, but rather on the combined effect of
structurally inhomogeneous Mott phases.
