Thermoelectric transport in perfectly conducting channels in quantum spin Hall systems

Thermoelectric transport of two-dimensional quantum spin Hall systems are theoretically studied in narrow ribbon geometry. We find that at high temperature electrons in the bulk states dominate. However, by lowering temperature, the ``perfectly conducting'' edge channels becomes dominant, and a bulk-to-edge crossover occurs. Correspondingly, by lowering temperature, the figure of merit first decreases and then will increase again due to edge-state-dominated thermoelectric transport.