Correlation-enhanced effective mass of two-dimensional electrons in Mg _xZn_1-xO/ZnO heterostructures

We performed combined magnetotransport and cyclotron resonance experiments on two-dimensional electron systems confined in the Mg_xZn _1-xO/ZnO heterostructures over a wide range of carrier densities, from 1.9 to 12×1011cm^-2 (3.5â‰r _sâ‰10, where r_s is the Wigner-Seitz radius). As the carrier density was reduced, the transport mass mtr* was strongly enhanced. In marked contrast, the effective masses determined from the cyclotron resonance mCR* were found to be independent of the carrier density and <?format ?>as large as the bulk effective mass. The large enhancement of mtr*, which exceeds mCR* by ∼60%, at the lowest carrier density with r_s∼10 is purely attributed to the strong electron correlation.