Correlation-enhanced effective mass of two-dimensional electrons in Mg(x)Zn(1-x)O/ZnO heterostructures.

We performed combined magnetotransport and cyclotron resonance experiments on two-dimensional electron systems confined in the Mg(x)Zn(1-x)O/ZnO heterostructures over a wide range of carrier densities, from 1.9 to 12 × 10(11) cm(-2) (3.5 </~ r(s) </~ 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 ~ 60%, at the lowest carrier density with r(s) 10 is purely attributed to the strong electron correlation.