Bulk Quantum Hall Effect in η-Mo4O11

We have observed a quantum Hall effect in the bulk quasi-two-dimensional conductor η-Mo 4 -O 11 . The Hall resistance exhibits well defined plateaux, coincident with pronounced minima in the diagonal resistance. We present data for several different samples and contact geometries, and discuss a possible mechanism for the quantum Hall effect in this system. We also discuss the implications of these findings in the light of recent predictions concerning chiral metallic surface states in bulk quantum Hall systems.