code: def e(self, k, n, τ, σ):
          
          d = self._dichalcogenide
          at, Δ, λ = d.at, d.Δ, d.λ
          sqrt = numpy.sqrt
          α = τ * σ
          return 0.5 * (λ * α + n * sqrt((2 * at * k)**2 + (Δ - λ * α)**2))
docs: Band energy.
  
          :param k: Momentum :math:`k`.
          :type k: float
  
          :param n: Band index :math:`n = ±1`.
          :type n: int
  
          :param τ: Valley index :math:`τ = ±1`.
          :type τ: int
  
          :param σ: Spin index :math:`σ = ±1`.
          :type σ: int
  
          :return: :math:`E^n_{τ σ} (k)`
          :rtype: float