Allowed range for the variable :math:`ξ = E - μ`.

        :return: :math:`(\\left|Δμ\\right| - 2 λ, \\left|Δμ\\right|)`
        :rtype: tuple

def ξ_bounds(self):
        """Allowed range for the variable :math:`ξ = E - μ`.

        :return: :math:`(\\left|Δμ\\right| - 2 λ, \\left|Δμ\\right|)`
        :rtype: tuple
        """
        λ, Δμ = self._dichalcogenide.λ, self.Δμ
        return (abs(Δμ) - 2 * λ, abs(Δμ))