docs: 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))