MeV oxygen ion implantation induced compositional intermixing in AlAs/GaAs superlattices

We present in this letter an investigation of compositional intermixing in AlAs/GaAs superlattices induced by 2 MeV oxygen ion implantation. The results are compared with implantation at 500 keV. In addition to Al intermixing in the direct lattice damage region by nuclear collision spikes, as is normally present in low-energy ion implantation, Al interdiffusion has also been found to take place in the subsurface region where MeV ion induced electronic spike damage dominates and a uniform strain field builds up due to defect generation and diffusion. Uniform compositional intermixing of the superlattices results after subsequent thermal annealing when Al interdiffusion is stimulated through recovery of the implantation-induced lattice strain field, the reconstruction and the redistribution of lattice defects, and annealing of lattice damage

Effects of low energy electron irradiation on formation of nitrogen-vacancy centers in single-crystal diamond

Exposure to beams of low energy electrons (2 to 30 keV) in a scanning electron microscope locally induces formation of NV-centers without thermal annealing in diamonds that have been implanted with nitrogen ions. We find that non-thermal, electron beam induced NV-formation is about four times less efficient than thermal annealing. But NV-center formation in a consecutive thermal annealing step (800C) following exposure to low energy electrons increases by a factor of up to 1.8 compared to thermal annealing alone. These observations point to reconstruction of nitrogen-vacancy complexes induced by electronic excitations from low energy electrons as an NV-center formation mechanism and identify local electronic excitations as a means for spatially controlled room-temperature NV-center formation

Influence of substrate temperature on lattice strain field and phase transition in MeV oxygen ion implanted GaAs crystals

A detailed study of the influence of substrate temperature on the radiation-induced lattice strain field and crystalline-to-amorphous (c-a) phase transition in MeV oxygen ion implanted GaAs crystals has been made using channeling Rutherford backscattering spectroscopy, secondary ion mass spectrometry, and the x-ray rocking curve technique. A comparison has been made between the cases of room temperature (RT) and low temperature (LT) (about 100 K) implantation. A strong in situ dynamic annealing process is found in RT implantation at a moderate beam current, resulting in a uniform positive strain field in the implanted layer. LT implantation introduces a freeze-in effect which impedes the recombination and diffusion of initial radiation-created lattice damage and defects, and in turn drives more efficiently the c-a transition as well as strain saturation and relaxation. The results are interpreted with a spike damage model in which the defect production process is described in terms of the competition between defect generation by nuclear spikes and defects diffusion and recombination stimulated by electronic spikes. It is also suggested that the excess population of vacancies and their complexes is responsible for lattice spacing expansion in ion-implanted GaAs crystals

Asymptotic Search for Ground States of SU(2) Matrix Theory

We introduce a complete set of gauge-invariant variables and a generalized Born-Oppenheimer formulation to search for normalizable zero-energy asymptotic solutions of the Schrodinger equation of SU(2) matrix theory. The asymptotic method gives only ground state candidates, which must be further tested for global stability. Our results include a set of such ground state candidates, including one state which is a singlet under spin(9).Comment: 51 page

The nature of the long time decay at a second order transition point

We show that at a second order phase transition, of \phi^4 like system, a necessary condition for streched exponential decay of the time structure factor is obeyed. Using the ideas presented in this proof a crude estimate of the decay of the structure factor is obtained and shown to yield stretched exponential decay under very reasonable conditions.Comment: 7 page

Lexicographic choice functions without archimedeanicity

We investigate the connection between choice functions and lexicographic probabilities, by means of the convexity axiom considered by Seidenfeld, Schervisch and Kadane (2010) but without imposing any Archimedean condition. We show that lexicographic probabilities are related to a particular type of sets of desirable gambles, and investigate the properties of the coherent choice function this induces via maximality. Finally, we show that the convexity axiom is necessary but not sufficient for a coherent choice function to be the infimum of a class of lexicographic ones

Discretizing Gravity in Warped Spacetime

We investigate the discretized version of the compact Randall-Sundrum model. By studying the mass eigenstates of the lattice theory, we demonstrate that for warped space, unlike for flat space, the strong coupling scale does not depend on the IR scale and lattice size. However, strong coupling does prevent us from taking the continuum limit of the lattice theory. Nonetheless, the lattice theory works in the manifestly holographic regime and successfully reproduces the most significant features of the warped theory. It is even in some respects better than the KK theory, which must be carefully regulated to obtain the correct physical results. Because it is easier to construct lattice theories than to find exact solutions to GR, we expect lattice gravity to be a useful tool for exploring field theory in curved space.Comment: 17 pages, 4 figures; references adde

Hall effect in quasi one-dimensional organic conductors

We study the Hall effect in a system of weakly coupled Luttinger Liquid chains, using a Memory function approach to compute the Hall constant in the presence of umklapp scattering along the chains. In this approximation, the Hall constant decomposes into two terms: a high-frequency term and a Memory function term. For the case of zero umklapp scattering, where the Memory function vanishes, the Hall constant is simply the band value, in agreement with former results in a similar model with no dissipation along the chains. With umklapp scattering along the chains, we find a power-law temperature dependance of the Hall constant. We discuss the applications to quasi 1D organic conductors at high temperatures.Comment: Proceedings of the ISCOM conference "Sixth International Symposium on Crystalline Organic Metals, Superconductors, and Ferromagnets", Key West, Florida, USA (Sept. 2005), to be plublished in the Journal of Low Temperature Physic

RHESSI and SOHO/CDS Observations of Explosive Chromospheric Evaporation

Simultaneous observations of explosive chromospheric evaporation are presented using data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Coronal Diagnostic Spectrometer (CDS) onboard SOHO. For the first time, co-spatial imaging and spectroscopy have been used to observe explosive evaporation within a hard X-ray emitting region. RHESSI X-ray images and spectra were used to determine the flux of non-thermal electrons accelerated during the impulsive phase of an M2.2 flare. Assuming a thick-target model, the injected electron spectrum was found to have a spectral index of ~7.3, a low energy cut-off of ~20 keV, and a resulting flux of >4x10^10 ergs cm^-2 s^-1. The dynamic response of the atmosphere was determined using CDS spectra, finding a mean upflow velocity of 230+/-38 km s^-1 in Fe XIX (592.23A), and associated downflows of 36+/-16 km s^-1 and 43+/-22 km s^-1 at chromospheric and transition region temperatures, respectively, relative to an averaged quiet-Sun spectra. The errors represent a 1 sigma dispersion. The properties of the accelerated electron spectrum and the corresponding evaporative velocities were found to be consistent with the predictions of theory.Comment: 5 pages, 4 figures, ApJL (In Press

Evaluation of bias-correction methods for ensemble streamflow volume forecasts

Ensemble prediction systems are used operationally to make probabilistic streamflow forecasts for seasonal time scales. However, hydrological models used for ensemble streamflow prediction often have simulation biases that degrade forecast quality and limit the operational usefulness of the forecasts. This study evaluates three bias-correction methods for ensemble streamflow volume forecasts. All three adjust the ensemble traces using a transformation derived with simulated and observed flows from a historical simulation. The quality of probabilistic forecasts issued when using the three bias-correction methods is evaluated using a distributions-oriented verification approach. Comparisons are made of retrospective forecasts of monthly flow volumes for a north-central United States basin (Des Moines River, Iowa), issued sequentially for each month over a 48-year record. The results show that all three bias-correction methods significantly improve forecast quality by eliminating unconditional biases and enhancing the potential skill. Still, subtle differences in the attributes of the bias-corrected forecasts have important implications for their use in operational decision-making. Diagnostic verification distinguishes these attributes in a context meaningful for decision-making, providing criteria to choose among bias-correction methods with comparable skill