Bulk quantum Hall effect in η-Mo₄O₁₁

We report the observation of a bulk quantum Hall effect in the quasi-two-dimensional conductor η-Mo₄O₁₁. The Hall resistance exhibits well-defined quantized plateaus, coincident with pronounced minima in the diagonal resistance. We propose a model involving (i) tiny quasi-two-dimensional electron and hole pockets, left over from an imperfectly nested charge-density wave, and (ii) the exchange of carriers between these mobile states and the localized charge-density-wave condensate. Together with a magnetic field, these factors result in well-separated bands of mobile states superimposed on a background continuum of localized states - the key ingredients for a quantum Hall effect. We also discuss the implications of these findings in the light of recent predictions concerning chiral metallic surface states in bulk quantum Hall systems.