Magnetic Field Induced Instabilities in Localised Two-Dimensional Electron Systems

We report density dependent instabilities in the localised regime of mesoscopic two-dimensional electron systems (2DES) with intermediate strength of background disorder. They are manifested by strong resistance oscillations induced by high perpendicular magnetic fields B_{\perp}. While the amplitude of the oscillations is strongly enhanced with increasing B_{\perp}, their position in density remains unaffected. The observation is accompanied by an unusual behaviour 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.Comment: 5 pages, 4 figure

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 $\rho(B_\perp) = \rho_{\rm{B}}\exp(\alpha B_\perp^2)$. The prefactor $\rho_{\rm{B}}$ 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 $R_{\rm{B}}\approx h/e^2$ and $R_{\rm{B}}\approx 1/2$ $h/e^2$. We suggest a strongly correlated electronic phase as a possible explanation.Comment: 5 pages, 3 figures, Proceedings of EP2DS 17, Reference adde

Interaction effects and disorder in two-dimensional electron systems

This thesis reports an experimental study of transport on a mesoscopic lengthscale in the localised regime of two-dimensional electron systems (2DES) with varying disorder. Devices with dimensions of a few microns were fabricated from modulation doped GaAs/ AlGaAs heterostructures, where the strength of disorder was tuned by changing the width of the undoped spacer layer separating the 2DES from the charged dopants, which are the main source of disorder in these systems. The main motivation of the experiments was to study the interplay between electron-electron interactions and short-range disorder at low electron densities, while avoiding the impact of long-range charge inhomogeneities that are usually present in this regime. Indeed, several new observations have been achieved with this approach: Chapter 5 reports an universal behaviour of hopping magnetoresistance, with evidence of the average hopping distance being equal to the average electronelectron separation, and a quantisation of the hopping prefactor in units of the quantum of resistance h/e2. Chapter 6 discusses the temperature dependence of resistance. The main result is an apparent temperature driven metal-to-insulator transition with a crossover from activated transport at high temperatures to metallic transport at low temperatures. This observation persists to resistivities of several hundred times the quantum of resistance. In chapter 7 a new kind of resistance oscillations is reported, which appear as a function of electron density when a strong perpendicular magnetic field is applied. A strongly amplified pick-up of the Shubnikov-de Haas oscillation and a modification of t he quantum Hall effect are reported in chapter 8. Furthermore, a new technique for measuring the electron density in mesoscopic 2DES is presented in chapter 4.Digitisation of this thesis was sponsored by Arcadia Fund, a charitable fund of Lisbet Rausing and Peter Baldwin

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/e(2). 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 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 mutual interplay of disorder and Coulomb interaction.Comment: 4+delta pages, 4 figures, To appear in the Physical Review B (Rapid Comm

Distribution of preoperative angle alpha and angle kappa values in patients undergoing multifocal refractive lens surgery based on a positive contact lens test

Purpose: To assess the preoperative objective angle alpha and angle kappa measurements of patients deciding to undergo multifocal refractive lens surgery based on a subjective positive multifocal contact lens test (MCLT). Methods: Retrospective, consecutive case series. Alpha and kappa angles were measured using the iTrace aberrometer. All patients also performed a 1-week MCLT. Only patients with a positive MCLT underwent surgery. Visual outcome (UCVA) was obtained in the 1-year follow-up. We assessed the preoperative distribution of angle values within MCLT positive and negative patient groups. Results: Two hundred seventeen eyes (111 patients) were included. Mean age was 56.4 years (SD 5.6) and 46.9% were female. In 71 eyes (38 patients), MCLT was positive. Of them, 12 eyes (17%) had an angle alpha and angle kappa ≥ 0.5mm. Of 146 eyes (73 patients) who refrained from surgery due to a negative MCLT, 71 eyes (48.6%) had both angles small (<0.5mm). In the 1-year follow-up, UCVA improved by 0.68 logMAR (SD 0.51; p<0.001) from baseline. Eyes with both small angle alpha and kappa sizes improved by 0.78 logMAR (SD 0.56), as did eyes with high (≥0.5mm) angle sizes (0.82 logMAR (SD 0.53). UCVA of eyes (n=24) with high alpha but low kappa sizes improved less (-0.31 logMAR (SD 0.13; p=0.019)). Conclusion: Four out of five patients with a positive MCLT also had correspondingly small angle values. One-half of patients with low preoperative angle values refrained from surgery due to a negative MCLT result. One-year visual acuity improvement was substantial and independent from angle sizes. Keywords: Angle alpha; Angle kappa; Decision-making; Multifocal intraocular lens; Refractive lens surgery

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 mu V/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 non-saturating linear magnetoresistance in two-dimensional electron systems at a GaAs/AlGaAs 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 non-saturating 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 Tesla, thus surpassing many known non-magnetic 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 a new and attractive candidate for on-chip magnetic field sensing.Comment: Main text contains 5 pages, 4 figures; supplementary information is include