Topological currents in the bulk in the absence of gapless states
We provide evidence that, alongside symmetry protected edge states, two-dimensional topological phases also support bulk currents. These currents are activated by local potential gradients in the bulk, while all parts of the system are adiabatically connected to the same phase. To understand their origin one can view the bulk of a homogeneous topological insulator as a perfectly entangled state of pairs of edge-like currents, adding up to a zero net flow. Potential gradients strain those states, progressively disentangling the hidden currents through a transfer of population. This produces a localised bulk current that is transverse to the strain, even when the potential is always below the energy gap, where one expects only edge currents to appear. Bulk currents are topologically protected and behave like edge currents under external influence, such as temperature or local disorder. The resilience and the tuning of bulk currents with local potentials makes them an appealing medium for technological applications.