Enhanced thermoelectric efficiency near a conductivity threshold

We calculate the thermopower and efficiency of the thermoelectric effect in systems which have a conductivity threshold (as function of the electronic energy). For example, disordered metals near the mobility edge (i.e., at the metal-insulator transition). Near such a threshold the thermopower and thermoelectric efficiency (i.e., figure of merit) are enhanced. The figure of merit ZT can be made much larger than unity, however at the price of low power output. In the presence of minimal lattice heat conductivity we calculate ZT~6 near the threshold. The enhancement effect is universal in the sense that it ensues from a competition between two physical mechanisms which sustain the steady-state thermopower. Any system that exhibits an electrical conductivity threshold and low lattice thermal conductivity is a good candidate for observing such an enhancement