  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. 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.
