Meyer-Neldel rule (MNR) : primarily used in materials science and condensed matter physics; the MNR is often stated as the plot of the logarithm of the preexponential factor against activation energy is linear: σ σ ( T ) = σ σ 0 exp ⁡ ⁡ ( − − E a k B T ) where ln σ 0 is the preexponential factor, E a is the activation energy, σ is the conductivity, and k B is Boltzmann's constant, and T is temperature.