Flow of covalent solids at low temperatures

An empirical correlation shows that the activation energy for plastic flow in covalent solids equals twice the energy gap. This provides evidence that the flow rate is controlled by the rate of tunneling of bonding electrons into nonbonding states under the influence of the applied stress. The model yields an explicit expression for the critical shear stress that contains no adjustable parameters, and which agrees quantitatively with observations for silicon. At finite temperatures tunneling is assisted by high-frequency phonons. An explicit equation for the temperature dependence is derived that contains no disposable parameters and which conforms quite well with observations.