Topological Insulator — Z₂ Invariant

2D BHZ model (HgTe/CdTe quantum well): bulk-edge correspondence
Z₂ = ?
Mass M (topo param) -1.0
Spin-orbit coupling B 1.00
Rashba α 0.20
Stripe size Ny 30
The BHZ (Bernevig-Hughes-Zhang) model describes HgTe/CdTe quantum wells: H(k) = d₀(k)I + d(k)·σ. Topological phase: when |M/B|<2, Z₂=1 (topological insulator), otherwise Z₂=0 (trivial). The Z₂ invariant is computed from time-reversal-invariant momenta (TRIM). The bulk-edge correspondence guarantees helical edge states: spin-up moves right on top, spin-down on bottom. These states are protected from backscattering by time-reversal symmetry.