Kitaev chain topological phase iff |μ| < 2t — gap-closing transition at |μ| = 2t

Bogoliubov-de Gennes spectrum of the Kitaev chain at momentum k: E(k) = ±√((μ + 2t cos k)² + 4Δ² sin² k). Bulk gap closes only when both terms vanish: k = 0 and μ = −2t, or k = π and μ = 2t (and Δ ≠ 0 required so the sin²-term never…