Backward diode composed of a metallic and semiconducting nanotube

The conditions necessary for a nanotube junction connecting a metallic and semiconducting nanotube to rectify the current are theoretically investigated. A tight binding model is used for the analysis, which includes the Hartree-Fock approximation and the Green's function method. It is found that the junction has a behavior similar to the backward diode if the gate electrode is located nearby and the Fermi level of the semiconducting tube is near the gap. Such a junction would be advantageous since the required length for the rectification could be reduced.Comment: 4 pages, RevTeX, uses epsf.st

Correlation equalities and upper bounds for the transverse Ising model

Starting from an exact formal identity for the two-state transverse Ising model and using correlation inequalities rigorous upper bounds for the critical temperature and the critical transverse field are obtained which improve effective results.Comment: 8 pages, 1 figur

UV observations of the galaxy cluster Abell 1795 with the optical monitor on XMM-Newton

We present the results of an analysis of broad band UV observations of the central regions of Abell 1795 observed with the optical monitor on XMM-Newton. As have been found with other UV observations of the central regions of clusters of galaxies, we find evidence for star formation. However, we also find evidence for absorption in the cD galaxy on a more extended scale than has been seen with optical imaging. We also report the first UV observation of part of the filamentary structure seen in H$\alpha$, X-rays and very deep U band imaging. The part of the filament we see is very blue with UV colours consistent with a very early (O/B) stellar population. This is the first direct evidence of a dominant population of early type stars at the centre of Abell 1795 and implies very recent star formation at the centre of this clusterComment: 6 pages, 3 figures accepted by A&A Letter

The Origin of Color Gradients in Early-Type Systems and Their Compactness at High-z

In this Letter, we present mean optical+NIR color gradient estimates for 5080 early-type galaxies (ETGs) in the grizYJHK wavebands of the Sloan Digital Sky Survey (SDSS) plus UKIRT Infrared Deep Sky Survey (UKIDSS). The color gradient is estimated as the logarithmic slope of the radial color profile in ETGs. With such a large sample size, we study the variation of the mean color gradient as a function of waveband with unprecedented accuracy. We find that (i) color gradients are mainly due, on average, to a metallicity variation of about -0.4dex per decade in galaxy radius; and (ii) a small, but significant, positive age gradient is present, on average, in ETGs, with the inner stellar population being slightly younger, by ~0.1dex per radial decade, than the outer one. Also, we show that the presence of a positive mean age gradient in ETGs, as found in the present study, implies their effective radius to be smaller at high z, consistent with observations.Comment: 4 pages, 2 color figures, accepted for publication in the Astrophysical Journal Letter

Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex

We carry out three-dimensional MHD simulations of star formation in turbulent, magnetized clouds, including ambipolar diffusion and feedback from protostellar outflows. The calculations focus on relatively diffuse clouds threaded by a strong magnetic field capable of resisting severe tangling by turbulent motions and retarding global gravitational contraction in the cross-field direction. They are motivated by observations of the Taurus molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an ordered large-scale magnetic field, as well as elongated condensations that are generally perpendicular to the large-scale field. We find that stars form in earnest in such clouds when enough material has settled gravitationally along the field lines that the mass-to-flux ratios of the condensations approach the critical value. Only a small fraction (of order 1% or less) of the nearly magnetically-critical, condensed material is turned into stars per local free-fall time, however. The slow star formation takes place in condensations that are moderately supersonic; it is regulated primarily by magnetic fields, rather than turbulence. The quiescent condensations are surrounded by diffuse halos that are much more turbulent, as observed in the Taurus complex. Strong support for magnetic regulation of star formation in this complex comes from the extremely slow conversion of the already condensed, relatively quiescent C$^{18}$O gas into stars, at a rate two orders of magnitude below the maximum, free-fall value. We analyze the properties of dense cores, including their mass spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap

X-ray Spectroscopy of the Cluster of Galaxies Abell 1795 with XMM-Newton

The initial results from XMM-Newton observations of the rich cluster of galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken by the European Photon Imaging Cameras (EPIC) show a temperature drop at a radius of $\sim 200$ kpc from the cluster center, indicating that the ICM is cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra extracted from the cluster center can be described by an isothermal model with a temperature of $\sim 4$ keV. The volume emission measure of any cool component ($<1$ keV) is less than a few % of the hot component at the cluster center. A strong OVIII Lyman-alpha line was detected with the RGS from the cluster core. The O abundance and its ratio to Fe at the cluster center is 0.2--0.5 and 0.5--1.5 times the solar value, respectively.Comment: Accepted: A&A Letters, 2001, 6 page

Disorder-induced phonon self-energy of semiconductors with binary isotopic composition

Self-energy effects of Raman phonons in isotopically disordered semiconductors are deduced by perturbation theory and compared to experimental data. In contrast to the acoustic frequency region, higher-order terms contribute significantly to the self-energy at optical phonon frequencies. The asymmetric dependence of the self-energy of a binary isotope system $m_{1-x} M_x$ on the concentration of the heavier isotope mass x can be explained by taking into account second- and third-order perturbation terms. For elemental semiconductors, the maximum of the self-energy occurs at concentrations with $0.5<x<0.7$, depending on the strength of the third-order term. Reasonable approximations are imposed that allow us to derive explicit expressions for the ratio of successive perturbation terms of the real and the imaginary part of the self-energy. This basic theoretical approach is compatible with Raman spectroscopic results on diamond and silicon, with calculations based on the coherent potential approximation, and with theoretical results obtained using {\it ab initio} electronic theory. The extension of the formalism to binary compounds, by taking into account the eigenvectors at the individual sublattices, is straightforward. In this manner, we interpret recent experimental results on the disorder-induced broadening of the TO (folded) modes of SiC with a $^{13}{\rm C}$-enriched carbon sublattice. \cite{Rohmfeld00,Rohmfeld01}Comment: 29 pages, 9 figures, 2 tables, submitted to PR

Color Gradients in Early-Type Galaxies in Clusters at the Redshift from 0.37 to 0.56

Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$) are examined by using the archival deep imaging data of Wide Field Planetary Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color gradients are compared with the two model gradients to examine the origin of the color gradients. In one model, a color gradient is assumed to be caused by a metallicity gradient of stellar populations, while in the other one, it is caused by an age gradient. Both of these model color gradients reproduce the average color gradient seen in nearby ellipticals, but predict significantly different gradients at a redshift larger than $\sim$0.3. Comparison between the observed gradients and the model gradients reveals that the metallicity gradient is much more favorable as the primary origin of color gradients in elliptical galaxies in clusters. The same conclusion has been obtained for field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the primary origin of the color gradients in elliptical galaxies does not depend on galaxy environment.Comment: 23 pages LaTeX, 5 PostScript figures, accepted for publication in The Astronomical Journa

X-ray Measurements of the Gravitational Potential Profile in the Central Region of the Abell 1060 Cluster of Galaxies

X-ray spectral and imaging data from ASCA and ROSAT were used to measure the total mass profile in the central region of Abell 1060, a nearby and relatively poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the spatial response of the X-ray telescope, show an isothermal distribution of the intra-cluster medium (ICM) within at least $\sim$ 12' (or $160h_{70}^{-1}$ kpc; $H_0 = 70 h_{70}$ km s$^{-1}$Mpc$^{-1}$) in radius of the cluster center. The azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a central excess above an isothermal $\beta$ model. The ring-sorted ASCA GIS spectra and the radial surface brightness distribution from the ROSAT PSPC were simultaneously utilized to constrain the gravitational potential profile. Some analytic models of the total mass density profile were examined. The ICM density profile was also specified by analytic forms. The ICM temperature distribution was constrained to satisfy the hydrostatic equilibrium, and to be consistent with the data. Then, the total mass distribution was found to be described better by the universal dark halo profile proposed by Navarro, Frenk, and White (1996;1997) than by a King-type model with a flat density core. A profile with a central cusp together with a logarithmic radial slope of $\sim 1.5$ was also consistent with the data. Discussions are made concerning the estimated dark matter distribution around the cluster center.Comment: 32 pages. Accepted: ApJ 2000, 35 pages, Title was correcte