Generating directional spin currents usually requires breaking time-reversal symmetry — a magnetic field, magnetic ordering, or spin-orbit coupling. Some external or internal agent must distinguish spin-up from spin-down. Without it, both spin species propagate identically, and no net spin current flows.
This paper shows that spatial asymmetry alone can do the job. On a one-dimensional fermionic ring, spin-independent potential barriers with asymmetric shapes generate spin-resolved circulating currents. The barriers treat both spins identically at each point in space, but the asymmetry of the barrier profile creates different transmission coefficients for clockwise and counterclockwise propagation. Combined with the Pauli exclusion principle, this directional asymmetry separates the spins.
The effect can be enhanced or reversed by tuning the barrier geometry to resonant conditions. At specific shapes, the spin current peaks; at others, it reverses direction. The control parameter is the spatial profile of a purely scalar potential — no magnetic ingredients required.
The result separates two things usually conflated: spin selection (choosing one spin over another) and time-reversal breaking (distinguishing forward from backward). Spatial asymmetry breaks the latter without touching the former, and the combination of broken spatial symmetry with quantum statistics produces the spin selection as a consequence.