Numerical study of the one-dimensional quantum compass model

The ground state magnetic phase diagram of the one-dimensional quantum compass model (QCM) is studied using the numerical Lanczos method. A detailed numerical analysis of the low energy excitation spectrum is presented. The energy gap and the spin-spin correlation functions are calculated for finite chains. Two kind of the magnetic long-range orders, the Neel and a type of the stripe-antiferromagnet, in the ground state phase diagram are identified. Based on the numerical analysis, the first and second order quantum phase transitions in the ground state phase diagram are identified.Comment: 6 pages, 8 figures. arXiv admin note: text overlap with arXiv:1105.211

Enhancement of the Deuteron-Fusion Reactions in Metals and its Experimental Implications

Recent measurements of the reaction d(d,p)t in metallic environments at very low energies performed by different experimental groups point to an enhanced electron screening effect. However, the resulting screening energies differ strongly for divers host metals and different experiments. Here, we present new experimental results and investigations of interfering processes in the irradiated targets. These measurements inside metals set special challenges and pitfalls which make them and the data analysis particularly error-prone. There are multi-parameter collateral effects which are crucial for the correct interpretation of the observed experimental yields. They mainly originate from target surface contaminations due to residual gases in the vacuum as well as from inhomogeneities and instabilities in the deuteron density distribution in the targets. In order to address these problems an improved differential analysis method beyond the standard procedures has been implemented. Profound scrutiny of the other experiments demonstrates that the observed unusual changes in the reaction yields are mainly due to deuteron density dynamics simulating the alleged screening energy values. The experimental results are compared with different theoretical models of the electron screening in metals. The Debye-H\"{u}ckel model that has been previously proposed to explain the influence of the electron screening on both nuclear reactions and radioactive decays could be clearly excluded.Comment: 22 pages, 12 figures, REVTeX4, 2-column format. Submitted to Phys. Rev. C; accepte

Spatial study with the VLT of a new resolved edge-on circumstellar dust disk discovered at the periphery of the rho Ophiuchi dark cloud

We report the discovery in NIR with SofI at the NTT of a resolved circumstellar dust disk around a 2MASS source at the periphery of the rho Ophiuchi dark cloud. We present follow-up observations in J, H, and Ks-band obtained with ISAAC at the VLT, under 0.4"-seeing conditions, which unveil a dark dust lane oriented East-West between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 2.15" (300 AU at 140 pc), and a width of 1.2" (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (Av=2.1\pm2.6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called ``Pixel NIR Color Mapping'', which allows to visualize directly the NIR colors of the nebula pixels. Thanks to this method we identify a ridge, 0.3" (40 AU at 140 pc) to the north of the dark lane and parallel to it, which displays a NIR color excess. This ridge corresponds to an unusual increase of brightness from J to Ks, which is also visible in the NTT observation obtained 130 days before the VLT one. We also find that the northern nebula shows ~3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i=86\pm1 deg, correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar asymmetrical NIR color properties of this object.Comment: 11 pages, 8 figures, 2 tables. Accepted for publication in ApJ (vol. 586, March 20, 2003). Table 2 appears now correctly in the postscript versio

Chandra Orion Ultradeep Project: Observations and Source Lists

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.Comment: 52 pages, 11 figures, 12 tables. Accepted for publication in ApJS, special issue dedicated to Chandra Orion Ultradeep Project. A version with high quality figures can be found at http://www.astro.psu.edu/users/gkosta/COUP_Methodology.pd

Understanding the DayCent model: Calibration, sensitivity, and identifiability through inverse modeling

AbstractThe ability of biogeochemical ecosystem models to represent agro-ecosystems depends on their correct integration with field observations. We report simultaneous calibration of 67 DayCent model parameters using multiple observation types through inverse modeling using the PEST parameter estimation software. Parameter estimation reduced the total sum of weighted squared residuals by 56% and improved model fit to crop productivity, soil carbon, volumetric soil water content, soil temperature, N2O, and soil NO3− compared to the default simulation. Inverse modeling substantially reduced predictive model error relative to the default model for all model predictions, except for soil NO3− and NH4+. Post-processing analyses provided insights into parameter–observation relationships based on parameter correlations, sensitivity and identifiability. Inverse modeling tools are shown to be a powerful way to systematize and accelerate the process of biogeochemical model interrogation, improving our understanding of model function and the underlying ecosystem biogeochemical processes that they represent

Electron propagation in crossed magnetic and electric fields

Laser-atom interaction can be an efficient mechanism for the production of coherent electrons. We analyze the dynamics of monoenergetic electrons in the presence of uniform, perpendicular magnetic and electric fields. The Green function technique is used to derive analytic results for the field--induced quantum mechanical drift motion of i) single electrons and ii) a dilute Fermi gas of electrons. The method yields the drift current and, at the same time it allows us to quantitatively establish the broadening of the (magnetic) Landau levels due to the electric field: Level number k is split into k+1 sublevels that render the $k$th oscillator eigenstate in energy space. Adjacent Landau levels will overlap if the electric field exceeds a critical strength. Our observations are relevant for quantum Hall configurations whenever electric field effects should be taken into account.Comment: 11 pages, 2 figures, submitte

Shear-banding in a lyotropic lamellar phase, Part 1: Time-averaged velocity profiles

Using velocity profile measurements based on dynamic light scattering and coupled to structural and rheological measurements in a Couette cell, we present evidences for a shear-banding scenario in the shear flow of the onion texture of a lyotropic lamellar phase. Time-averaged measurements clearly show the presence of structural shear-banding in the vicinity of a shear-induced transition, associated to the nucleation and growth of a highly sheared band in the flow. Our experiments also reveal the presence of slip at the walls of the Couette cell. Using a simple mechanical approach, we demonstrate that our data confirms the classical assumption of the shear-banding picture, in which the interface between bands lies at a given stress $\sigma^\star$. We also outline the presence of large temporal fluctuations of the flow field, which are the subject of the second part of this paper [Salmon {\it et al.}, submitted to Phys. Rev. E]