Plasmon attenuation and optical conductivity of a two-dimensional electron gas

In a ballistic two-dimensional electron gas, the Landau damping does not lead to plasmon attenuation in a broad interval of wave vectors q << k_F. Similarly, it does not contribute to the optical conductivity \sigma (\omega, q) in a wide domain of its arguments, E_F > \omega > qv_F, where E_F, k_F and v_F are, respectively, the Fermi energy, wavevector and velocity of the electrons. We identify processes that result in the plasmon attenuation in the absence of Landau damping. These processes are: the excitation of two electron-hole pairs, phonon-assisted excitation of one pair, and a direct plasmon-phonon conversion. We evaluate the corresponding contributions to the plasmon linewidth and to the optical conductivity.Comment: 8 pages, 4 figures; final form, misprints correcte

Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?

The STEREO/WAVES instrument has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.Comment: In press in Solar Physics, 13 pages, 5 figure

The Kuiper Belt and Other Debris Disks

We discuss the current knowledge of the Solar system, focusing on bodies in the outer regions, on the information they provide concerning Solar system formation, and on the possible relationships that may exist between our system and the debris disks of other stars. Beyond the domains of the Terrestrial and giant planets, the comets in the Kuiper belt and the Oort cloud preserve some of our most pristine materials. The Kuiper belt, in particular, is a collisional dust source and a scientific bridge to the dusty "debris disks" observed around many nearby main-sequence stars. Study of the Solar system provides a level of detail that we cannot discern in the distant disks while observations of the disks may help to set the Solar system in proper context.Comment: 50 pages, 25 Figures. To appear in conference proceedings book "Astrophysics in the Next Decade

Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era

We report on the γ-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) γ-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 ± 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 ± 0.14, and the softest one is 2.51 ± 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size ≲0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (≃1044 erg s-1) constitutes only a small fraction (∼10-3) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude. © 2011. The American Astronomical Society

Supported growth of polycrystalline silicon sheet on low-cost ceramic, carbon, or reusable substrate. Quarterly report No. 1, September 26-December 31, 1979

The overall objective of this program is to identify and develop high-throughput, supported-growth methods for producing low-cost, large-area polycrystalline silicon sheet on ceramic, carbon, or reusable substrates. The first method being investigated is the SCIM coating technique. (SCIM is an acronym for Silicon Coating by Inverted Meniscus.) With this technique, a low-cost ceramic substrate is silicon-coated in a continuous manner by passing the substrate over a molten silicon meniscus which is contained in a narrow fused-silica trough. During this reporting period, several mullite substrates were silicon-coated with this method. The best coatings were approximately 100 ..mu..m thick with large columnar grains up to 0.5 cm wide and several cm long. Structural characterization of these coatings shows that these large grains are heavily twinned with boundaries perpendicular to the surface of the layer. Minority-carrier diffusion length measurements made on photodiodes fabricated from these coatings indicate diffusion lengths of about 25 ..mu..m. The short-circuit current densities of the diodes were about 23 mA/cm/sup 2/, with open-circuit voltages of approximately 0.49 V. With the substrates positioned horizontally as they pass over the meniscus trough, instability usually occurs, causing a buildup of molten silicon immediately downstream from the trough. This solicon eventually spills over onto key parts of the coater. By tilting the substrate at angles of 10, 15, and 20/sup 0/, stability can be achieved. A theoretical study of meniscus shapes conducted during this reporting period suggests that with an appropriate trough design and the right pressure and substrate height, a SCIM-coating with horizontal substrates should also be possible

Silicon on ceramic process. Silicon sheet growth development for the Large-Area Silicon Sheet Task of the Low-Cost Silicon Solar Array Project. Annual report No. 2, September 17, 1976--September 19, 1977

The objective of this research program is to investigate the technical and economic feasibility of producing solar-cell-quality sheet silicon by coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon from the melt. In the past year significant progress was made in all areas of the program. The physical and chemical properties of the standard mullite refractory used for the majority of the coating runs (McDanel MV20 and Coors S1SI) have been characterized. A number of experimental compositions have been identified and procured from Coors. Characterization of the standard compositions revealed that the thermal expansion of mullite depends on both relative amounts of glass phase and on the impurity level in the glass. Since the thermal expansion in mullite exceeds that of silicon, the silicon coating should be in a state of compression. This was confirmed by x-ray measurements. After modifying and cleaning the dip-coating facility, silicon on ceramic (SOC) solar cells were fabricated which demonstrate that the SOC process can produce silicon of solar cell quality. SOC cells having 1 cm/sup 2/ active areas demonstrated measured conversion efficiencies as high as 7.2 percent. Typical open-ciruit voltages (V/sub oc/) and short-circuit current densities (J/sub sc/) were 0.51 volt and 20 mA/cm/sup 2/, respectively. Since the active surface of these solar cells is a highly reflective ''as-grown'' surface, one can expect improvement in J/sub sc/ after an anti-reflection (AR) coating is applied. Results of an economic analysis of the SOC process are presented

Silicon-on ceramic process. Silicon sheet growth and device development for the large-area silicon sheet and cell development tasks of the low-cost solar array project. Quarterly report No. 12, April 2, 1979-June 29, 1979

The objective of this research program is to investigate the technical and economic feasibility of producing solar-cell-quality sheet silicon. We plan to do this by coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon from the melt. During the quarter, significant progress was demonstrated in several areas: (1) a 10-cm/sup 2/ cell having 9.9 percent conversion efficiency (AM1, AR) was fabricated; (2) the Honeywall-sponsored SCIM coating development succeeded in producing a 225-cm/sup 2/ layer of sheet silicon (18 inches x 2 inches); and (3) 100 ..mu..m-thick coatings at pull speed of 0.15 cm/sec wer$obta9ned, although apoproximately 50 percent of the layer exhibited dendritic growth. Other results and accomplishments during the quarter are reported in detail. (WHK

Structural characterization and ferroelectric properties of strontium barium niobate (Sr xBa1-xNb2O6) thin films

Strontium barium niobate (SBN) thin films of good quality were deposited on Pt/Ti/SiO2/Si substrate using a polymeric resin containing metallic ions. Films were crystallized at different temperatures and for different duration of time. The structure of these films was studied using X-ray diffraction. The coexistence of SrNb2O6 (SN) and SBN was observed in films crystallized at 700 °C. The amount of SN decreases when the crystallization time increases. Ferroelectric properties were determined for films crystallized at 700 °C for 1 and 5 h. For SBN film crystallized at 700 °C for 1 h, the remanent polarization (Pr) and the coercive field (Ec) were 2.6 muC/cm² and 71.9 kV/cm, respectively. For the film crystallized at 700 °C for 5 h these parameters were Pr = 1.1 muC/cm² and Ec = 50.5 kV/cm