Enhanced quantum oscillatory magnetization and nonequilibrium currents in an interacting two-dimensional electron system in MgZnO/ZnO with repulsive scatterers

Torque magnetometry at low temperature and in high magnetic fields $B$ is performed on MgZnO/ZnO heterostructures incorporating high-mobility two-dimensional electron systems. We find a sawtoothlike quantum oscillatory magnetization $M\left(B\right)$, i.e., the de Haas-van Alphen (dHvA) effect. At the same time, nonequilibrium currents and unexpected spikelike overshoots in $M$ are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptorlike scatterers give rise to a magnetic thaw down effect which enhances the dHvA amplitude beyond the electron-electron interaction effects being present in the MgZnO/ZnO heterostructures.

Figure 1

Oscillatory part of the magnetization of a MgZnO/ZnO heterostructure at $\alpha =52.0^{\circ }$ and $T=0.28$ K. At filling factor $\nu =1$, the data exhibits the dHvA effect as well as large NECs, which change sign upon changing the sweep direction (arrows). In the inset, experimental data (light line) as well as model calculations [21] for the ideal (dashed line) and real (dotted line) 2DES are shown ($3.0<B_{\perp }<4.0\mathrm{T}$).

Figure 2

Angular dependencies of $M\left(B\right)$ near (a) $\nu =3$ and (b) $\nu =2$. Curves are shifted in the vertical direction for clarity. (c) dHvA amplitudes per electron $\Delta M_{\text{e},\nu }=\Delta M_{\nu }/N$ as a function of $\alpha$. Values for $\nu =1$ were extracted from curves shown in Figs. 1 and 3. (d) Thermodynamic energy gaps $\Delta E_{\nu }$ as a function of $\alpha$. The line denotes the energy gap expected for $\nu =2$ in a disorder-free 2DES at $T=0$.

Figure 3

Temperature dependence of $M\left(B\right)$ near (a) $\nu =1$ at $\alpha =36.8^{\circ }$ and (b) $\nu =2$ at $\alpha =62.0^{\circ }$ measured for down-sweeping $B$. From top to bottom the temperature varies from $T=0.28\mathrm{to}1.6$ K. For $T\le 0.9$ K NECs are present at $\nu =1$ in (a). Going from higher to lower temperatures, the field position of the maximum nonequilibrium signal in (a) moves to larger $B$ by $\Delta B_{\mathrm{neq}}$. A shift $\Delta B_{\mathrm{eq}}$ is seen also for the equilibrium signal in (b). (c) $\Delta B_{\mathrm{neq}}/B_{\nu =1}$ (squares) and $\Delta B_{\mathrm{eq}}/B_{\nu =2}$ (triangles) as a function of $T$. At low $T$, $\Delta B_{\mathrm{neq}}$ increases linearly with decreasing $T$ as indicated by a linear fit (black line). (d) Spikelike equilibrium overshoot of $M\left(B\right)$ at $\nu =1$ and $T=1.6$ K.