Bulk quantum Hall effect: Evidence that surface states play a key role

It has been suggested recently that the α-(BEDT-TTF${)}_2$MHg(SCN${)}_4$ salts (M=K and Tl) exhibit a bulk quantum Hall effect at high fields [Harrison et al., Phys. Rev. Lett. 77, 1576 (1996)]. We have examined experimentally predictions concerning the transport of current under such conditions. We find that the high field Shubnikov–de Haas oscillations exhibit a complicated temperature dependence, which is not evident in de Haas–van Alphen measurements on the same samples. These differences may be explained in terms of the competition between transport via the bulk and via the surface of the sample.