Correlation-Enhanced Effective Mass of Two-Dimensional Electrons in MgxZn1-xO/ZnO Heterostructures

We performed combined magnetotransport and cyclotron resonance experiments on two-dimensional electron systems confined in the MgxZn1-xO/ZnO heterostructures over a wide range of carrier densities, from 1.9 to 12 x 10(11) cm(-2) (3.5 less than or similar to r(s) less than or similar to 10, where r(s) is the Wigner-Seitz radius). As the carrier density was reduced, the transport mass m(tr)* was strongly enhanced. In marked contrast, the effective masses determined from the cyclotron resonance m(CR)* were found to be independent of the carrier density and as large as the bulk effective mass. The large enhancement of m(tr)*, which exceeds m(CR)* by similar to 60%, at the lowest carrier density with r(s) similar to 10 is purely attributed to the strong electron correlation.