The superconducting parameter :math:`Δ_\\mathbf{k}`
        as a function of the energy :math:`E`.

        :return: :math:`Δ(E)`
        :rtype: func

def Δk(self):
        """The superconducting parameter :math:`Δ_\\mathbf{k}`
        as a function of the energy :math:`E`.

        :return: :math:`Δ(E)`
        :rtype: func
        """
        Δc, Δv = self.Δc, self.Δv
        cos = self._energy.trig('cos θ')
        return lambda e: (1 / 2) * ((Δc + Δv) + (Δc - Δv) * cos(e))