Plot the conductivity in function of Fermi level. Semi-log plot

        Args:
            temp: the temperature
            xlim: a list of min and max fermi energy by default (0, and band
                gap)
            tau: A relaxation time in s. By default none and the plot is by
               units of relaxation time

        Returns:
            a matplotlib object

def plot_conductivity_mu(self, temp=600, output='eig',
                             relaxation_time=1e-14, xlim=None):
        """
        Plot the conductivity in function of Fermi level. Semi-log plot

        Args:
            temp: the temperature
            xlim: a list of min and max fermi energy by default (0, and band
                gap)
            tau: A relaxation time in s. By default none and the plot is by
               units of relaxation time

        Returns:
            a matplotlib object
        """
        import matplotlib.pyplot as plt
        cond = self._bz.get_conductivity(relaxation_time=relaxation_time,
                                         output=output, doping_levels=False)[
            temp]
        plt.figure(figsize=(9, 7))
        plt.semilogy(self._bz.mu_steps, cond, linewidth=3.0)
        self._plot_bg_limits()
        self._plot_doping(temp)
        if output == 'eig':
            plt.legend(['$\\Sigma_1$', '$\\Sigma_2$', '$\\Sigma_3$'])
        if xlim is None:
            plt.xlim(-0.5, self._bz.gap + 0.5)
        else:
            plt.xlim(xlim)
        plt.ylim([1e13 * relaxation_time, 1e20 * relaxation_time])
        plt.ylabel("conductivity,\n $\\Sigma$ (1/($\\Omega$ m))", fontsize=30.0)
        plt.xlabel("E-E$_f$ (eV)", fontsize=30.0)
        plt.xticks(fontsize=25)
        plt.yticks(fontsize=25)
        plt.tight_layout()
        return plt