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

Abstract

Torque magnetometry at low temperature and in high magnetic fields B is performed on a MgZnO/ZnO heterostructure incorporating a high-mobility two-dimensional electron system. We find a sawtooth-like quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, unexpected spike-like overshoots in M and non-equilibrium currents are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptor-like 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 heterostructure. PACS numbers: Valid PACS appear hereOxide heterostructures have generated tremendous interest in recent years [1,2]. Two-dimensional electron systems (2DESs) formed therein exhibit remarkable properties such as superconductivity [3] and magnetism [4] or the fractional quantum Hall effect (QHE) [5,6]. MgZnO/ZnO-based heterostructures are outstanding in that 2DESs of small carrier density n s exhibit extremely high mobilities µ at low temperature T [5,6]. At the same time, the electron-electron interaction parameter r s ∝ n −0.5 s [7] is large allowing for electron correlation effects at oxide interfaces in an applied magnetic field B [6,8,9]. Still, the quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect reflecting the ground state properties of such 2DESs has not yet been explored. Since the discovery of the dHvA effect in Bi more than eight decades ago it has been argued that disorder reduces peak-to-peak amplitudes ∆M via broadening of the quantized Landau levels E j (j = 0, 1, 2, ...) [10][11][12]. In contrast, electron-electron interaction effects are known to enhance ∆M [13]. The two counteracting effects are however not easy to distinguish in a balancing situation. Sometimes the dHvA effect has been obscured in the QHE regime even completely by extremely large non-equilibrium currents (NECs) [14]. The NECs are induced near integer filling factors ν = hn s /(eB ⊥ ) at low T when the longitudinal resistivity ρ xx takes a vanishingly small value and are believed to be limited only by breakdown of the QHE [14]. Independent transport experiments on GaAs-based heterostructures have evidenced that, strikingly, minima in ρ xx and plateaus in the Hall resistivity ρ xy could be displaced away from integer ν towards smaller ν due to repulsive scatterers [15][16][17][18][19]. This phenomenon has not yet been resolved in M (B), and a clear experimental manifestation of the underlying asymmetric density of states (DOS) in a ground state property is still lacking.In this Letter, we report torque magnetometry on the equilibrium dHvA effect and NECs of a high-mobility 2DES residing at a MgZnO/ZnO heterointerface. We observe dHvA amplitudes ∆M at filling factors ν = 1 and 2 that are significantly enhanced over the expected values in the single-particle picture. Addressing a regime 0.28 K < T < 1.6 K we observe T -dependent shifts of ...