Unexpected features of branched flow through high-mobility two-dimensional electron gases

GaAs-based two-dimensional electron gases (2DEGs) show a wealth of remarkable electronic states, and serve as the basis for fast transistors, research on electrons in nanostructures, and prototypes of quantum-computing schemes. All these uses depend on the extremely low levels of disorder in GaAs 2DEGs, with low-temperature mean free paths ranging from microns to hundreds of microns. Here we study how disorder affects the spatial structure of electron transport by imaging electron flow in three different GaAs/AlGaAs 2DEGs, whose mobilities range over an order of magnitude. As expected, electrons flow along narrow branches that we find remain straight over a distance roughly proportional to the mean free path. We also observe two unanticipated phenomena in high-mobility samples. In our highest-mobility sample we observe an almost complete absence of sharp impurity or defect scattering, indicated by the complete suppression of quantum coherent interference fringes. Also, branched flow through the chaotic potential of a high-mobility sample remains stable to significant changes to the initial conditions of injected electrons.Comment: 22 pages, 4 figures, 1 tabl

Single-electron transport driven by surface acoustic waves: moving quantum dots versus short barriers

We have investigated the response of the acoustoelectric current driven by a surface-acoustic wave through a quantum point contact in the closed-channel regime. Under proper conditions, the current develops plateaus at integer multiples of ef when the frequency f of the surface-acoustic wave or the gate voltage Vg of the point contact is varied. A pronounced 1.1 MHz beat period of the current indicates that the interference of the surface-acoustic wave with reflected waves matters. This is supported by the results obtained after a second independent beam of surface-acoustic wave was added, traveling in opposite direction. We have found that two sub-intervals can be distinguished within the 1.1 MHz modulation period, where two different sets of plateaus dominate the acoustoelectric-current versus gate-voltage characteristics. In some cases, both types of quantized steps appeared simultaneously, though at different current values, as if they were superposed on each other. Their presence could result from two independent quantization mechanisms for the acoustoelectric current. We point out that short potential barriers determining the properties of our nominally long constrictions could lead to an additional quantization mechanism, independent from those described in the standard model of 'moving quantum dots'.Comment: 25 pages, 12 figures, to be published in a special issue of J. Low Temp. Phys. in honour of Prof. F. Pobel

Spatially resolved manipulation of single electrons in quantum dots using a scanned probe

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage independent contribution.Comment: 4 pages, 4 figure

Optimizing nitrogen rates in Camelina sativa

Non-Peer ReviewedCamelina is a new oilseed crop to western Canada with potential applications in cosmetics, human nutrition, and biofuel. Nitrogen recommendations for camelina production in Western Canada aren’t available. Field studies were conducted in 2008 and 2009 for 10 site years at locations in western Canada to determine the effect of nitrogen rate on seed yield. Depending on the experiment, nitrogen rates ranged from 0 to 200 kg ha-1. The join point (N rate at which yields plateau) for camelina were 111 to 116 kg ha-1, which is similar to other Brassica oilseed species

Optimizing N fertilizer rates for seed yield in Camelina sativa and Brassica carinata

Non-Peer Reviewe

Optimizing seeding rates and plant densities for Camelina sativa

Non-Peer ReviewedCamelina is a relatively new oilseed crop to western Canada. Currently there is little research available for camelina seeding rates in western Canada. A field study was conducted in 2007 and 2008 for 9 site years at locations in western Canada to determine the effect of seeding rate on various agronomic aspects of camelina like yield, days to maturity, lodging, and plant height. The seeding rates used were 12, 25, 50, 100, 200, 400, 800, and 1600 seeds m-2. Camelina exhibited a plastic yield response to seeding rate. Maximum yield was reached at 450-500 seeds m-2. Small yield increases were seen with seeding rates greater than 100 seeds m-2. Days to maturity decreased by up to seven days as seeding rate increased. The optimum seeding rate was approximately 500 seeds m-2 because of the positive effect on maturity, plant height, and yield. 500 seeds m-2 would supply enough seeds to produce an acceptable plant density under poor seeding conditions to help prevent crop failure due to poor emergence or seedling mortality

Intersubband Electron Interaction in 1D-2D Junctions

We have shown that the electron transport through junctions of one-dimensional and two-dimensional systems, as well as through quantum point contacts, is considerably affected by the interaction of electrons of different subbands. The interaction mechanism is caused by Friedel oscillations, which are produced by electrons of the closed subbands even in smooth junctions. Because of the interaction with these oscillations, electrons of the open subbands experience a backscattering. The electron reflection coefficient, which describes the backscattering, has a sharp peak at the energy equal to the Fermi energy and may be as high as about 0.1. This result allows one to explain a number of available experimental facts.Comment: 5 pages, 3 figure