Matthias Baenninger

Low-temperature collapse of electron localization in two dimensions.

We report direct experimental evidence that the insulating phase of a disordered, yet strongly interacting two-dimensional electron system becomes unstable at low temperatures. As the temperature decreases, a transition from insulating to metal-like transport behavior is observed, which persists even when the resistivity of the system greatly exceeds the quantum of resistivity h/e2. The results have been achieved by measuring transport on a mesoscopic length scale while systematically varying the strength of disorder.

Local transport in a disorder-stabilized correlated insulating phase

We report the experimental realization of a correlated insulating phase in two-dimensional (2D) GaAs/AlGaAs heterostructures at low electron densities in a limited window of background disorder. This has been achieved at mesoscopic length scales, where the insulating phase is characterized by a universal hopping transport mechanism. Transport in this regime is determined only by the average electron separation, independent of the topology of background disorder. We have discussed this observation in terms of a pinned electron solid ground state, stabilized by the mutual interplay of disorder and Coulomb interaction.

Highly Enhanced Thermopower in Two-Dimensional Electron Systems at Millikelvin Temperatures

We report experimental observation of an unexpectedly large thermopower in mesoscopic two-dimensional (2D) electron systems in GaAs/AlGaAs heterostructures at sub-Kelvin temperatures and zero magnetic field. Unlike conventional nonmagnetic high-mobility 2D systems, the thermopower in our devices increases with decreasing temperature below 0.3 K, reaching values in excess of 100 microV/K, thus exceeding the free electron estimate by more than 2 orders of magnitude. With support from a parallel study of the local density of states, we suggest such a phenomenon to be linked to intrinsic localized states and many-body spin correlations in the system.

Colossal nonsaturating linear magnetoresistance in two-dimensional electron systems at a GaAs/(Al,Ga)As heterointerface

Engineering devices with a large electrical response to magnetic field is of fundamental importance for a range of applications such as magnetic field sensing and magnetic read heads. We show that a colossal nonsaturating linear magnetoresistance (NLMR) arises in two-dimensional electron systems hosted in a GaAs/AlGaAs heterostructure in the strongly insulating regime. When operated at high source-drain bias, the magnetoresistance of our devices increases almost linearly with magnetic field, reaching nearly 10 000% at 8 T, thus surpassing many known nonmagnetic materials that exhibit giant NLMR. The temperature dependence and mobility analysis indicate that the NLMR has a purely classical origin, driven by nanoscale inhomogeneities. A large NLMR combined with small device dimensions makes these systems an attractive candidate for on-chip magnetic field sensing.

Evidence of gate-tunable topological excitations in two-dimensional electron systems

We report experimental observations of a new mechanism of charge transport in two-dimensional electron systems (2DESs) in the presence of strong Coulomb interaction and disorder. We show that at low enough temperature the conductivity tends to zero at a nonzero carrier density, which represents the point of essential singularity in a Berezinskii-Kosterlitz-Thouless-like transition. Our experiments with many 2DESs in GaAs/AlGaAs heterostructures suggest that the charge transport at low carrier densities is due to the melting of an underlying ordered ground state through proliferation of topological defects. Independent measurement of low-frequency conductivity noise supports this scenario.

Magnetic Field Induced Instabilities In Localized Two-Dimensional Electron Systems

We report density dependent instabilities in the localized regime of mesoscopic two-dimensional electron systems (2DESs) with intermediate strength of background disorder. They are manifested by strong resistance oscillations induced by high perpendicular magnetic fields B-perpendicular to. While the amplitude of the oscillations is strongly enhanced with increasing B-perpendicular to, their position in density remains unaffected. The observation is accompanied by an unusual behavior of the temperature dependence of resistance and activation energies. We suggest the interplay between a strongly interacting electron phase and the background disorder as a possible explanation.

Quantisation of hopping magnetoresistance prefactor in strongly correlated two-dimensional electron systems

We report an universal behaviour of hopping transport in strongly interacting mesoscopic two-dimensional electron systems (2DES). In a certain window of background disorder, the resistivity at low perpendicular magnetic fields follows the expected relation p(B ⊥ ) = ρ B exp(αB 2 ⊥ .) The prefactor ρ B decreases exponentially with increasing electron density but saturates to a finite value at higher densities. Strikingly, this value is found to be universal when expressed in terms of absolute resistance and and shows quantisation at R B ≈ h/e 2 and R Β ≈ 1/2 h/e 2 . We suggest a strongly correlated electronic phase as a possible explanation.

Large linear magnetoresistance in a GaAs/AlGaAs heterostructure

We report non-saturating linear magnetoresistance (MR) in a two-dimensional electron system (2DES) at a GaAs/AlGaAs heterointerface in the strongly insulating regime. We achieve this by driving the gate voltage below the pinch-off point of the device and operating it in the non-equilibrium regime with high source-drain bias. Remarkably, the magnitude of MR is as large as 500% per Tesla with respect to resistance at zero magnetic field, thus dwarfing most non-magnetic materials which exhibit this linearity. Its primary advantage over most other materials is that both linearity and the enormous magnitude are retained over a broad temperature range (0.3 K to 10 K), thus making it an attractive candidate for cryogenic sensor applications.

Topological excitations in semiconductor heterostructures

Topological defects play an important role in the melting phenomena in two-dimensions. In this work, we report experimental observation of topological defect induced melting in two-dimensional electron systems (2DES) in the presence of strong Coulomb interaction and disorder. The phenomenon is characterised by measurement of conductivity which goes to zero in a Berezinskii-Kosterlitz-Thouless like transition. Further evidence is provided via low-frequency conductivity noise measurements.

Low‐Temperature Breakdown of the Insulating Phase in Two‐Dimensional Electron Systems

We present experimental evidence of a breakdown of localization at low temperatures in strongly interacting two‐dimensional electrons systems with moderate background disorder. The observation is manifested by a transition from an activated transport behavior at high temperature to a metal‐like (dρ/dT>0) behavior at low temperature T ≲1 K. The results have been achieved in mesoscopic two‐dimensional electron systems (2DES), where the disorder strength was systematically varied.