80 research outputs found
The role of Ar III in quantitative spectroscopy on hot argon plasmas
We perform quantitative optical emission spectroscopy on the hot core of the cathode region of a free-burning arc in argon under atmospheric pressure. As the peak temperatures in the centre of the discharge exceed 22 000 K we are able to observe three spectra of argon (Ar I, Ar II and Ar III) and the continuum emission. We report on some inconsistencies concerning the evaluation of Ar III radiation in both line and continuum emission. These are caused by erroneous data in the literature and common misconceptions about the influence of Ar III on the plasma emission. We discuss the impact of this fact on published data
On Generalizations of Network Design Problems with Degree Bounds
Iterative rounding and relaxation have arguably become the method of choice
in dealing with unconstrained and constrained network design problems. In this
paper we extend the scope of the iterative relaxation method in two directions:
(1) by handling more complex degree constraints in the minimum spanning tree
problem (namely, laminar crossing spanning tree), and (2) by incorporating
`degree bounds' in other combinatorial optimization problems such as matroid
intersection and lattice polyhedra. We give new or improved approximation
algorithms, hardness results, and integrality gaps for these problems.Comment: v2, 24 pages, 4 figure
Approximating the minimum directed tree cover
Given a directed graph with non negative cost on the arcs, a directed
tree cover of is a rooted directed tree such that either head or tail (or
both of them) of every arc in is touched by . The minimum directed tree
cover problem (DTCP) is to find a directed tree cover of minimum cost. The
problem is known to be -hard. In this paper, we show that the weighted Set
Cover Problem (SCP) is a special case of DTCP. Hence, one can expect at best to
approximate DTCP with the same ratio as for SCP. We show that this expectation
can be satisfied in some way by designing a purely combinatorial approximation
algorithm for the DTCP and proving that the approximation ratio of the
algorithm is with is the maximum outgoing degree of
the nodes in .Comment: 13 page
Shot noise in resonant tunneling through a zero-dimensional state with a complex energy spectrum
We investigate the noise properties of a GaAs/AlGaAs resonant tunneling
structure at bias voltages where the current characteristic is determined by
single electron tunneling. We discuss the suppression of the shot noise in the
framework of a coupled two-state system. For large bias voltages we observed
super-Poissonian shot noise up to values of the Fano factor .Comment: 4 pages, 4 figures, accepted for Phys. Rev.
Terahertz photoresponse of AlInSb/InSb/AlInSb quantum well structures
We have studied the photoresponse (transmission and photoconductivity of Corbino-shaped devices) of structures with InSb quantum wells (AlInSb barriers). To characterize the devices, the Shubnikov-de Haas (SdH) effect up to magnetic fields B of 7 T and current-voltage (I-V) characteristics at various magnetic fields were measured. Some of the samples showed clearly resolvable SdH oscillations. The I-V curves showed pronounced nonlinearities. The phototransmission and the photoconductivity at various terahertz (THz) frequencies were measured around 2.5 THz generated by a p-Ge laser. From the cyclotron resonance (transmission measurements) we deduced a cyclotron mass of 0.022m0. We also performed photoconductivity measurements on Corbino-shaped devices in the THz frequency range. Oscillations of the photoconductivity with maxima near the minima of the conductivity in the dark were observed. Thus, these devices are potentially suitable for the detection of THz radiation
The Amazon Tall Tower Observatory (ATTO): Overview of pilot measurements on ecosystem ecology, meteorology, trace gases, and aerosols
The Amazon Basin plays key roles in the carbon and water cycles, climate change, atmospheric chemistry, and biodiversity. It has already been changed significantly by human activities, and more pervasive change is expected to occur in the coming decades. It is therefore essential to establish long-term measurement sites that provide a baseline record of present-day climatic, biogeochemical, and atmospheric conditions and that will be operated over coming decades to monitor change in the Amazon region, as human perturbations increase in the future. The Amazon Tall Tower Observatory (ATTO) has been set up in a pristine rain forest region in the central Amazon Basin, about 150 km northeast of the city of Manaus. Two 80 m towers have been operated at the site since 2012, and a 325 m tower is nearing completion in mid-2015. An ecological survey including a biodiversity assessment has been conducted in the forest region surrounding the site. Measurements of micrometeorological and atmospheric chemical variables were initiated in 2012, and their range has continued to broaden over the last few years. The meteorological and micrometeorological measurements include temperature and wind profiles, precipitation, water and energy fluxes, turbulence components, soil temperature profiles and soil heat fluxes, radiation fluxes, and visibility. A tree has been instrumented to measure stem profiles of temperature, light intensity, and water content in cryptogamic covers. The trace gas measurements comprise continuous monitoring of carbon dioxide, carbon monoxide, methane, and ozone at five to eight different heights, complemented by a variety of additional species measured during intensive campaigns (e.g., VOC, NO, NO2, and OH reactivity). Aerosol optical, microphysical, and chemical measurements are being made above the canopy as well as in the canopy space. They include aerosol light scattering and absorption, fluorescence, number and volume size distributions, chemical composition, cloud condensation nuclei (CCN) concentrations, and hygroscopicity. In this paper, we discuss the scientific context of the ATTO observatory and present an overview of results from ecological, meteorological, and chemical pilot studies at the ATTO site. © Author(s) 2015
The role of Ar III in quantitative spectroscopy on hot argon plasmas
We perform quantitative optical emission spectroscopy on the hot core of the cathode region of a free-burning arc in argon under atmospheric pressure. As the peak temperatures in the centre of the discharge exceed 22 000 K we are able to observe three spectra of argon (Ar I, Ar II and Ar III) and the continuum emission. We report on some inconsistencies concerning the evaluation of Ar III radiation in both line and continuum emission. These are caused by erroneous data in the literature and common misconceptions about the influence of Ar III on the plasma emission. We discuss the impact of this fact on published data
Determination of log Poct values of chloro-substituted benzenes, toluenes and anilines by high-performance liquid chromatography on ODS-silica
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