Spin-Momentum Locking on a Topological Insulator Edge

This simulator visualizes the defining feature of a quantum spin Hall (2D topological) insulator: a pair of counter-propagating, spin-locked edge channels circling an insulating bulk. Right-moving electrons carry spin up, left-moving electrons carry spin down — a rigid correspondence dictated by time-reversal symmetry. Drop non-magnetic impurities on the edge and watch electrons sail through untouched, exactly as Kramers' theorem predicts; drop magnetic impurities instead and watch them locally break time-reversal symmetry, flip spins, and backscatter electrons into the opposite channel, degrading the quantized edge conductance. Switch the sample to a trivial insulator and the protected channel disappears entirely — there is nothing left to protect.