A direct current (dc) between two electrodes is normally linked to the flow of the electrons in response to an applied bias voltage between those leads.
However, during the last years many of us have been actively pursuing new ways of generating a current even at zero bias voltage. One of the most popular alternatives is the use of time-dependent potentials to produce a quantum pump.
But there is yet another more subtle way which (1) does not rely on time-dependent potentials and (2) can be built-in the band structure of a system. This way consists on crafting one-way states able to carry directional transport of either charge, valley or spin.
If this interests you take a look at this paper:
Crafting zero-bias one-way transport of charge and spin L. E. F. Foa Torres, V. Dal Lago, and E. Suárez Morell
Physical Review B 93, 075438 (2016).