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

def e(self, k, n, τ, σ):
        """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
        """
        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))