InAs/InP single quantum wire formation and emission at 1.5 microns

Isolated InAs/InP self-assembled quantum wires have been grown using in situ accumulated stress measurements to adjust the optimal InAs thickness. Atomic force microscopy imaging shows highly asymmetric nanostructures with average length exceeding more than ten times their width. High resolution optical investigation of as-grown samples reveals strong photoluminescence from individual quantum wires at 1.5 microns. Additional sharp features are related to monolayer fluctuations of the two dimensional InAs layer present during the early stages of the quantum wire self-assembling process.Comment: 4 pages and 3 figures submitted to Applied Physics Letter

Radiation Magnetohydrodynamics In Global Simulations Of Protoplanetary Disks

Our aim is to study the thermal and dynamical evolution of protoplanetary disks in global simulations, including the physics of radiation transfer and magneto-hydrodynamic (MHD) turbulence caused by the magneto-rotational instability. We develop a radiative transfer method based on the flux-limited diffusion approximation that includes frequency dependent irradiation by the central star. This hybrid scheme is implemented in the PLUTO code. The focus of our implementation is on the performance of the radiative transfer method. Using an optimized Jacobi preconditioned BiCGSTAB solver, the radiative module is three times faster than the MHD step for the disk setup we consider. We obtain weak scaling efficiencies of 70% up to 1024 cores. We present the first global 3D radiation MHD simulations of a stratified protoplanetary disk. The disk model parameters are chosen to approximate those of the system AS 209 in the star-forming region Ophiuchus. Starting the simulation from a disk in radiative and hydrostatic equilibrium, the magnetorotational instability quickly causes MHD turbulence and heating in the disk. For the disk parameters we use, turbulent dissipation heats the disk midplane and raises the temperature by about 15% compared to passive disk models. A roughly flat vertical temperature profile establishes in the disk optically thick region close to the midplane. We reproduce the vertical temperature profile with a viscous disk models for which the stress tensor vertical profile is flat in the bulk of the disk and vanishes in the disk corona. The present paper demonstrates for the first time that global radiation MHD simulations of turbulent protoplanetary disks are feasible with current computational facilities. This opens up the windows to a wide range of studies of the dynamics of protoplanetary disks inner parts, for which there are significant observational constraints.Comment: Accepted to A&

Study of TeV variability of Mrk 421 from 3 years of monitoring with the Milagro Observatory

The Milagro experiment was a TeV gamma-ray observatory designed to continuously monitor the overhead sky in the 0.1-100 TeV energy range. It operated from 2000 and 2008 and was characterized by a large field of view ($\sim$ 2 sr) and a high duty cycle ($\geq$ 90$\%$). Here we report on the long-term monitoring of the blazar Mrk 421 with Milagro over the period from September 21, 2005 to March 15, 2008. We present a study of the TeV variability of the source and provide upper limits for the measured flux for different time scales, ranging from one week up to one year.Comment: 3 pages, 2 figures. Paper to appear in the Proceedings of the Thirteen Marcel Grossmann Meeting, held in Stockholm, Sweden, on July 2012, World Scientific, Singapore, in pres

Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study

Motivated by the recent realization of graphene sensors to detect individual gas molecules, we investigate the adsorption of H2O, NH3, CO, NO2, and NO on a graphene substrate using first-principles calculations. The optimal adsorption position and orientation of these molecules on the graphene surface is determined and the adsorption energies are calculated. Molecular doping, i.e. charge transfer between the molecules and the graphene surface, is discussed in light of the density of states and the molecular orbitals of the adsorbates. The efficiency of doping of the different molecules is determined and the influence of their magnetic moment is discussed.Comment: 6 pages, 6 figure

Suspensions Thermal Noise in the LIGO Gravitational Wave Detector

We present a calculation of the maximum sensitivity achievable by the LIGO Gravitational wave detector in construction, due to limiting thermal noise of its suspensions. We present a method to calculate thermal noise that allows the prediction of the suspension thermal noise in all its 6 degrees of freedom, from the energy dissipation due to the elasticity of the suspension wires. We show how this approach encompasses and explains previous ways to approximate the thermal noise limit in gravitational waver detectors. We show how this approach can be extended to more complicated suspensions to be used in future LIGO detectors.Comment: 28 pages, 13 figure

Numerical Implementation of a Critical State Model for Soft Rocks

This paper details the basic tasks for the numerical implementation of a simple elasto-plastic critical state model for bonded materials (i.e. soft rocks-hard soils) into the finite element program SNAC developed at the University of Newcastle in Australia. The first task described focusses on the derivation of the incremental constitutive relationships used to represent the mechanical response of a bonded/cemented material under saturated conditions. The second task presents how these stress-strain relations can be numerically integrated using an explicit substepping scheme with automatic error control. The third task concentrates on the verification of the substepping algorithm proposed. The model used to represent the saturated mechanical response of a bonded material combines the modified Cam clay with the constitutive relationships for cemented materials proposed in Gens & Nova (1993), but incorporates some flexibility on the degradation law adopted. The role of suction and other relevant aspects of unsaturated behaviour are also discussed at the end of the paper