Fig. 2: Point tunneling contact.
From: Coherent spin transport through helical edge states of topological insulator
Non-magnetic point contact between the helical ring and the spinful wire. Different spin projections (shown by blue (a) and red color (b)) do not mix at the contact and correspond to electrons propagating along the helical edge in the opposite directions. Scattering on this contact is described by the spin-independent real amplitudes t and r obeying t2 + r2 = 1.
