Software for Acoustic Rendering

SLAB is a software system that can be run on a personal computer to simulate an acoustic environment in real time. SLAB was developed to enable computational experimentation in which one can exert low-level control over a variety of signal-processing parameters, related to spatialization, for conducting psychoacoustic studies. Among the parameters that can be manipulated are the number and position of reflections, the fidelity (that is, the number of taps in finite-impulse-response filters), the system latency, and the update rate of the filters. Another goal in the development of SLAB was to provide an inexpensive means of dynamic synthesis of virtual audio over headphones, without need for special-purpose signal-processing hardware. SLAB has a modular, object-oriented design that affords the flexibility and extensibility needed to accommodate a variety of computational experiments and signal-flow structures. SLAB s spatial renderer has a fixed signal-flow architecture corresponding to a set of parallel signal paths from each source to a listener. This fixed architecture can be regarded as a compromise that optimizes efficiency at the expense of complete flexibility. Such a compromise is necessary, given the design goal of enabling computational psychoacoustic experimentation on inexpensive personal computers

Local structural order in the disordered vanadium tetracyanoethylene room-temperature molecule-based magnet

Journal ArticleWe determined the vanadium oxidation state and local coordination environment in disordered samples of magnetic V[TCNE]x(x=2) prepared by chemical vapor deposition (CVD). Systematic studies of the x-ray absorption near-edge structure (XANES) in this material and reference compounds show that V ions have a valence state near 2+. Extended x-ray absorption fine structure (EXAFS) analysis shows that vanadium ions are coordinated by 6.04±0.25 nitrogen atoms at a room-temperature average distance of 2.084(5)A(o). The local environment is well defined with a distribution of V-N bond lengths comparable to that commonly found in ordered compounds. This distribution is mostly vibrational in origin, with static contributions being at least four fold smaller. The small disorder in V-N distances is a consequence of strong binding between V and TCNE, with an effective local force constant of k=87 N/m. This strong bonding leads to strong nearest neighbor coupling, which for the extended structure of V[TCNE]x with six N nearest neighbors results in magnetic ordering above room temperature. The strong V-N bonding explains in part the insoluble nature of this compared to other molecule-based magnets. The room-temperature XANES and EXAFS results for the CVD-prepared samples are compared to those for V[TCNE]x prepared as a powder from CH2Cl2 solvent, which has a similar magnetic ordering temperature but a magnetization that is more strongly temperature dependent. This comparison suggests that coordination of 6 nitrogens around each V(II) with little variation in the V-N distances is important for achieving the high magnetic ordering temperature of 400 K associated with samples made by both the CVD and CH2Cl2 solution methods

Optical properties of cesium tetracyanoquinodimethanide, Cs2(TCNQ)3

Journal ArticleRoom-temperature polarized reflectance measurements have been made on cesium tetracyanoquinodimethanide, Cs2(TCNQI3, over the frequency range between the far infrared and the near ultraviolet. The optical properties of the compound were obtained by Kramers-Kronig analysis. These properties are dominated by vibrational features at low frequencies and by electronic excitations at high frequencies. The observed vibrational features include ordinary intramolecular modes and "anomalous" infrared activity of the totally symmetric vibrations

Optical and electron-energy-loss studies of the monomeric and dimeric phases of decamethylferrocenium tetracyanoquinodimethanide, (DMeFc)(TCNQ)

Journal ArticleThe optical properties of the two crystallographic phases of 1:l decaniethylferrocenium tetracyanoauinodimethanide, (DMeFc)(TCNQ), have been measured from 0.1 to 10 eV. One phase consists of isolated paramagnetic TCNQ anion monomers while the other contains isolated diamagnetic dimers. The spectrum of the monomeric phase exhibits a strong localized monomer exciton which is not normally observed in solid TCNQ salts, while the dimeric phase shows a charge-transfer excitation as well as a shifted local exciton. From the frequency- dependent conductivity of the dimeric phase the effective on-site Coulomb interaction and the transfer matrix element are measured to be 1.0 and 0.27 eV, respectively. The infrared absorption spectrum of the dimeric phase shows an unusual activity of the symmetric phonon modes due to the interaction of these modes with the radical electron, whereas in the monomeric phase only normally infrared active phonons are observed. In electron-energy-loss measurements an anomalous momentum dependence of the line shape of the monomeric exciton was observed. This result is attributed to a dielectric effect caused by the decrease in strength of local excitons with increasing momentum

Cluster glass state and photoinduced effects on the freezing dynamics in KxCo[Fe(CN)6]y.ZH2O (x-0.16,y-0.72, z-4.4)

Journal ArticleThe magnetic properties and photoinduced magnetization of a Prussian blue analog, KxCo[Fe(Cn)6]y•ZH2O (x-0.16, y-0.72, z-4.4) were systematically studied. The frequency dependence of the linear ac susceptibility, the irreversibility in the field-cooled/ zero-field-cooled magnetization (MFC /MZFC), and the relaxation of MZFC suggest a cluster glass behavior. Illumination with red light leads to an increase in the magnetic irreversibility. While MFC is significantly increased after illumination, MZFC at low temperatures is decreased. The observed photoinduced magnetic effects are explained within a cluster glass model. The photoinduced increase in the concentration of spins leads to a shift of dynamics toward longer length and time scales, resulting in freezing of spin clusters at a higher temperature. © 2000 American Institute of Physics

The Closest Damped Lyman Alpha System

A difficulty of studying damped Lyman alpha systems is that they are distant, so one knows little about the interstellar medium of the galaxy. Here we report upon a damped Lyman alpha system in the nearby galaxy NGC 4203, which is so close (v_helio = 1117 km/s) and bright (B_o = 11.62) that its HI disk has been mapped. The absorption lines are detected against Ton 1480, which lies only 1.9' (12 h_50 kpc) from the center of NGC 4203. Observations were obtained with the Faint Object Spectrograph on HST (G270H grating) over the 2222-3277 Angstrom region with 200 km/s resolution. Low ionization lines of Fe, Mn, and Mg were detected, leading to metallicities of -2.29, -2.4, which are typical of other damped Lyman alpha systems, but well below the stellar metallicity of this type of galaxy. Most notably, the velocity of the lines is 1160 +- 10 km/s, which is identical to the HI rotational velocity of 1170 km/s at that location in NGC 4203, supporting the view that these absorption line systems can be associated with the rotating disks of galaxies. In addition, the line widths of the Mg lines give an upper limit to the velocity dispersion of 167 km/s, to the 99% confidence level.Comment: 4 pages LaTeX, including 1 figure and 1 table, uses emulateapj.sty. Accepted for publication by Astrophysical Journal Letter

Cosmic Filaments in Superclusters

Large-scale structure calculations show that modest overdensity filaments will connect clusters of galaxies and these filaments are reservoirs of baryons, mainly in gaseous form. To determine whether such filaments exist, we have examined the UV absorption line properties of three AGNs projected behind possible filaments in superclusters of galaxies; the AGNs lie within 3 Mpc of the centerlines of loci connecting clusters. All three lines of sight show absorption in Ly\alpha, Ly\beta, or/and OVI at redshifts within about 1300 km/s of the nearby galaxy clusters that would define the closest filaments. For one AGN, the absorption line redshifts are close to the emission line redshift of the AGN, so we cannot rule out self-absorption for this object. These absorption line associations with superclusters are unlikely to have occurred by chance, a result consistent with the presence of cosmic filaments within superclusters.Comment: 16 pages, 7 eps figures, Accepted for publication in Ap