Validation of three-component wind lidar sensor for traceable highly resolved wind vector measurements

Conventional monostatic wind lidar (light detection and ranging) systems are well-established wind speed remote sensing devices in the field of wind energy that provide reliable measurement results for flat terrain and homogeneous wind fields. These conventional wind lidar systems use a common transmitting and receiving unit and become unacceptably inaccurate as the wind fields become increasingly inhomogeneous due to their spatial and temporal averaging procedure (large measurement volume) that is inherent to the monostatic measurement principle. The new three-component fiber laser-based wind lidar sensor developed by the Physikalisch-Technische Bundesanstalt (PTB) uses one transmitting unit (fiber laser) and three receiving units to measure the velocity vector of single aerosols in a spatially highly resolved measurement volume (with diameter d and length l) in heights from 5&thinsp;m (d=300&thinsp;µm, l=2&thinsp;mm) to 250&thinsp;m (d=14&thinsp;mm, l=4&thinsp;m) with a resolution of about 0.1&thinsp;m&thinsp;s−1. Detailed comparison measurements with a 135&thinsp;m high wind met mast and a conventional lidar system have proven that the high spatial and temporal resolution of the new, so-called bistatic lidar leads to a reduced measurement uncertainty compared to conventional lidar systems. Furthermore, the comparison demonstrates that the deviation between the bistatic lidar and the wind met mast lies well within the measurement uncertainty of the cup anemometers of the wind met mast for both homogeneous and inhomogeneous wind fields. At PTB, the aim is to use the bistatic wind lidar as a traceable reference standard to calibrate other remote sensing devices, necessitating an in-depth validation of the bistatic lidar system and its measurement uncertainty. To this end, a new, specially designed wind tunnel with a laser Doppler anemometer (LDA) as flow velocity reference has been erected on a platform at a height of 8&thinsp;m; this allows the new wind lidar to be positioned below the wind tunnel test section to be validated for wind vector measurements that are traceable to the SI units. A first validation measurement within the wind tunnel test section is presented, showing a deviation between the bistatic lidar system and the LDA clearly below 0.1&thinsp;%.</p

Skipping orbits and enhanced resistivity in large-diameter InAs/GaSb antidot lattices

We investigated the magnetotransport properties of high-mobility InAs/GaSb antidot lattices. In addition to the usual commensurability features at low magnetic field we found a broad maximum of classical origin around 2.5 T. The latter can be ascribed to a class of rosetta type orbits encircling a single antidot. This is shown by both a simple transport calculation based on a classical Kubo formula and an analysis of the Poincare surface of section at different magnetic field values. At low temperatures we observe weak 1/B-periodic oscillations superimposed on the classical maximum.Comment: 4 pages, 4 Postscript figures, REVTeX, submitted to Phys Rev

Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells

Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si quantum wells we determine the carrier-density-dependence of the magnetic susceptibility. Assuming weak interaction we evaluate the density of states at the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11 cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the mobility from q_TF yields good agreement with experimental values justifying the approach. The decrease in D(E_F) is explained by potential fluctuations which lead to tail states that make screening less efficient and - in a positive feedback - cause an increase of the potential fluctuations. Even in our high mobility samples the fluctuations exceed the electron-electron interaction leading to the formation of puddles of mobile carriers with at least 1 micrometer diameter.Comment: 4 pages, 3 figure

Edge-Magnetoplasmon Wave-Packet Revivals in the Quantum Hall Effect

The quantum Hall effect is necessarily accompanied by low-energy excitations localized at the edge of a two-dimensional electron system. For the case of electrons interacting via the long-range Coulomb interaction, these excitations are edge magnetoplasmons. We address the time evolution of localized edge-magnetoplasmon wave packets. On short times the wave packets move along the edge with classical E cross B drift. We show that on longer times the wave packets can have properties similar to those of the Rydberg wave packets that are produced in atoms using short-pulsed lasers. In particular, we show that edge-magnetoplasmon wave packets can exhibit periodic revivals in which a dispersed wave packet reassembles into a localized one. We propose the study of edge-magnetoplasmon wave packets as a tool to investigate dynamical properties of integer and fractional quantum-Hall edges. Various scenarios are discussed for preparing the initial wave packet and for detecting it at a later time. We comment on the importance of magnetoplasmon-phonon coupling and on quantum and thermal fluctuations.Comment: 18 pages, RevTex, 7 figures and 2 tables included, Fig. 5 was originally 3Mbyte and had to be bitmapped for submission to archive; in the process it acquired distracting artifacts, to upload the better version, see http://physics.indiana.edu/~uli/publ/projects.htm