New Insights into the Plateau-Insulator Transition in the Quantum Hall Regime

We have measured the quantum critical behavior of the plateau-insulator (PI) transition in a low-mobility InGaAs/GaAs quantum well. The longitudinal resistivity measured for two different values of the electron density follows an exponential law, from which we extract critical exponents kappa = 0.54 and 0.58, in good agreement with the value (kappa = 0.57) previously obtained for an InGaAs/InP heterostructure. This provides evidence for a non-Fermi liquid critical exponent. By reversing the direction of the magnetic field we find that the averaged Hall resistance remains quantized at the plateau value h/e^2 through the PI transition. From the deviations of the Hall resistance from the quantized value, we obtain the corrections to scaling.Comment: accepted proceedings of EP2DS-15 (to be published in Physica E

Quantum criticality, particle-hole symmetry, and duality of the plateau-insulator transition in the quantum Hall regime

We report new experimental data on the plateau-insulator transition in the quantum Hall regime, taken from a low mobility InGaAs/InP heterostructure. By employing the fundamental symmetries of the quantum transport problem we are able to disentangle the universal quantum critical aspects of the magnetoresistance data (critical indices and scaling functions) and the sample dependent aspects due to macroscopic inhomogeneities. Our new results and methodology indicate that the previously established experimental value for the critical index (kappa = 0.42) resulted from an admixture of both universal and sample dependent behavior. A novel, non-Fermi liquid value is found (kappa = 0.57) along with the leading corrections to scaling. The statement of self-duality under the Chern Simons flux attachment transformation is verified.Comment: 4 pages, 2 figure

The effects of macroscopic inhomogeneities on the magneto transport properties of the electron gas in two dimensions

In experiments on electron transport the macroscopic inhomogeneities in the sample play a fundamental role. In this paper and a subsequent one we introduce and develop a general formalism that captures the principal features of sample inhomogeneities (density gradients, contact misalignments) in the magneto resistance data taken from low mobility heterostructures. We present detailed assessments and experimental investigations of the different regimes of physical interest, notably the regime of semiclassical transport at weak magnetic fields, the plateau-plateau transitions as well as the plateau-insulator transition that generally occurs at much stronger values of the external field only. It is shown that the semiclassical regime at weak fields plays an integral role in the general understanding of the experiments on the quantum Hall regime. The results of this paper clearly indicate that the plateau-plateau transitions, unlike the the plateau-insulator transition, are fundamentally affected by the presence of sample inhomogeneities. We propose a universal scaling result for the magneto resistance parameters. This result facilitates, amongst many other things, a detailed understanding of the difficulties associated with the experimental methodology of H.P. Wei et.al in extracting the quantum critical behavior of the electron gas from the transport measurements conducted on the plateau-plateau transitions.Comment: 20 pages, 9 figure

Transparent dynamic insulation: a novel system combining ventilation and insulation for sustainable greenhouse applications

Greenhouse horticulture cultivates vegetables, fruits, and flowers in protected glass houses. Approximately 65%-85% of the total energy in greenhouses is used for heating. To reach the climate goals energy reduction is needed. Energy savings cannot only be achieved by improving the insulation value of the greenhouse covers since also the relative humidity of indoor air needs to stay below a critical humidity level to avoid fungus growth. Therefore ventilation is required with an average air change rate (ACH) of 2.5/h. This is often realized by (partly) opening and closing the glass greenhouse cover thereby losing part of the heat. In this paper, we focus on a novel method that improves the thermal insulation of transparent top covers and/or walls while ensuring sufficient ventilation: transparent dynamic insulation (TDI). TDI is based on a concept called Dynamic Insulation, also known as a "breathing wall": infiltrating air flows through the multi-layer insulation panel. While doing this, the incoming air takes up part of the heat and uses this for pre-heating. Thereby it reduces the overall heat transfer value (U-value) and provides pre-warmed, fresh air flow into the greenhouse. The main difference compared to “traditional” dynamic insulation is that its design is transparent for solar irradiation and therefore can be used for greenhouses. In this paper, we show experimental results on a TDI prototype panel using a hot box apparatus with an ACH=2.5/h and prescribed temperature differences between indoor and outdoor of ΔT=10C and ΔT=20C. It is shown that the measured overall U-value for the TDI panel is about 19% lower than for double glass and more than a factor 2 lower than single-layered Hortiplus glass. Therefore it is concluded that TDI is a promising approach to reducing energy consumption in greenhouse horticulture while ensuring sufficient air refreshment for moisture control