Topological Insulator Surface States — Spin-Momentum Locking

The conducting surface of a 3D topological insulator hosts a single Dirac cone of states whose spin is rigidly locked perpendicular to momentum. This simulator renders that Dirac cone with a real gapped/gapless dispersion E(k) = ±√[(ħv_F k)² + m²], draws the spin texture around a tunable Fermi circle, and fires electron wavepackets at a fixed impurity to show — quantitatively — why non-magnetic disorder cannot backscatter a helical Dirac electron while a magnetic impurity that breaks time-reversal symmetry opens a gap and switches backscattering back on.