QUANTUM LIMIT AND ANOMALOUS FIELD-INDUCED INSULATING BEHAVIOR IN ETA-MO4O11

The quasi-two-dimensional metal eta-Mo4O11 undergoes two successive ch arge-density-wave transitions at 109 and 30 K, with corresponding chan ges in the electronic structure. Measurements of the magnetoresistance , Hall effect, and magnetization, in magnetic fields of up to 50 T and as a function of temperature and pressure, have been performed to det ermine the ground-state electronic structure. Several distinct quantum oscillations are observed in the low-field magnetoresistance, corresp onding to very small closed Fermi surfaces, all of which reach the qua ntum limit by 20 T. Analysis of the quantum oscillation amplitudes ind icate very low carrier effective masses (m<0.1m(e)) associated with e ach of the closed two-dimensional Fermi-surface pockets. At higher fie lds (B>20 T) we see a crossover from (semi-) metallic to semiconductin g behavior, which we associate with a field-induced electron- and hole -band inversion, resulting in a clear gap at the Fermi-energy. Measure ments of this energy gap allow an independent determination of the car rier effective masses, which are in excellent agreement with the value s obtained from the analysis of the low-field (B<15 T) quantum oscilla tions. We find that the transport and thermodynamic properties of the field-induced insulating state are highly anomalous. In particular, we discuss the Hall effect and the origin of an additional quantum oscil lation at high magnetic fields (B>20 T).