Neutron and muon-induced background studies for the AMoRE double-beta decay experiment

AMoRE (Advanced Mo-based Rare process Experiment) is an experiment to search a neutrinoless double-beta decay of $^{100}$Mo in molybdate crystals. The neutron and muon-induced backgrounds are crucial to obtain the zero-background level (<$10^{-5}$ counts/(keV$\cdot$kg$\cdot$yr)) for the AMoRE-II experiment, which is the second phase of the AMoRE project, planned to run at YEMI underground laboratory. To evaluate the effects of neutron and muon-induced backgrounds, we performed Geant4 Monte Carlo simulations and studied a shielding strategy for the AMORE-II experiment. Neutron-induced backgrounds were also included in the study. In this paper, we estimated the background level in the presence of possible shielding structures, which meet the background requirement for the AMoRE-II experiment

Mechanism of enhanced light output in InGaN-based microlight emitting diodes

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output

The Exchange Gate in Solid State Spin Quantum Computation: The Applicability of the Heisenberg Model

Solid state quantum computing proposals rely on adiabatic operations of the exchange gate among localized spins in nanostructures. We study corrections to the Heisenberg interaction between lateral semiconductor quantum dots in an external magnetic field. Using exact diagonalization we obtain the regime of validity of the adiabatic approximation. We also find qualitative corrections to the Heisenberg model at high magnetic fields and in looped arrays of spins. Looped geometries of localized spins generate flux dependent, multi-spin terms which go beyond the basic Heisenberg model.Comment: 13 pages, 8 figure

InGaN nano-ring structures for high-efficiency light emitting diodes

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated

Reheating and turbulence

We show that the ''turbulent'' particle spectra found in numerical simulations of the behavior of matter fields during reheating admit a simple interpretation in terms of hydrodynamic models of the reheating period. We predict a particle number spectrum $n_{k}\propto k^{-\alpha}$ with $\alpha \sim 2$ for $k\to 0.$Comment: 10 pages, one figure included in tex

Simplification of Manipulator Dynamic Model for Nonlinear Decoupled Control

This paper presents the development of simplified manipulator dynamic models which satisfy the desired steady-state error specification in the joint-variable space or in the Cartesian space under a nonlinear decoupled controller. The formulae which relate the tracking errors of joint variables in the joint-variable space or the manipulator hand in the Cartesian space to the dynamic modeling errors are first developed. Using these formulae, we derive the maximum error tolerance for each dynamic coefficient of the equations of motion. Then each simplified dynamic coefficient of the equations of motion can be expressed as a linear combination of the product terms of sinusoidal and polynomial basis functions. To illustrate the approach, a computer simulation has been carried out to obtain two simplified dynamic models of a Stanford robot arm which satisfy the specified error tolerances in the joint-variable space and in the Cartesian space under respective nonlinear decoupled controllers. Finally, to measure the time complexity of simplified models, the number of mathematical operations in terms of multiplication and addition for computing the joint torques is tabulated and discussed with the parallel computation result of Newton-Euler equations of motion

Generalized Approach for the Control of Constrained Robot Systems

This paper presents a generalized approach for controlling various cases of the constrained robot system. To accomplish specific tasks successfully by a constrained robot system, both the constraint forces/torques and the motion of the manipulator end-effector must be specified and controlled. Using the Jacobian matrix of the constraint function, the generalized coordinates of the constrained robot system can be partitioned into two sets; this leads to partitioning the constrained robot system into two subsystems. The constraint forces/torques in each subsystem can be decomposed into two components: the motion-independent and the motion-dependent forces/torques. Using the constraint function in the Cartesian space, the motion-independent forces/torques can be expressed by a generalized multiplier vector and the Jacobian matrix of the constraint function. The motion-dependent forces/torques can be determined by the motion of the manipulator end-effector, the motion-independent forces/torques, and other known quantities. This decomposition of the constraint robot system into subsystems leads to the design of a nonlinear decoupled controller with a simple structure, which takes the constraints into consideration for controlling the constrained robot system. Applying the proposed nonlinear decoupled controller to each subsystem and using the relation between the motion-independent forces/torques in the subsystems, we can show that both the errors in the manipulator end-effector motion and the constraint forces/torques approach zero asymptotically. Typical examples of the constrained robot systems are analyzed and discussed

Fluctuation of Gaps in Hadronization at Phase Transition

Event-by-event fluctuations of hadronic patterns in heavy-ion collisions are studied in search for signatures of quark-hadron phase transition. Attention is focused on a narrow strip in the azimuthal angle with small $\Delta y$. The fluctuations in the gaps between particles are quantified by simple measures. A scaling exponent $\alpha$ is shown to exist around $T_c$. An index $\xi$ is shown to characterize the critical fluctuation; it is a numerical constant $\xi=0.05\pm0.01$. All the measures considered in this gap analysis are experimentally observable. Whether or not the theoretical predictions, based on simulations using 2-dimensional Ising model, are realistic for heavy-ion collisions, analysis of the experimental data suggested here should be carried out, since the existence of a scaling behavior is of interest in its own right.Comment: 10 pages LaTex + 8 eps figure

Particle Number Fluctuations in Relativistic Bose and Fermi Gases

Particle number fluctuations are studied in relativistic Bose and Fermi gases. The calculations are done within both the grand canonical and canonical ensemble. The fluctuations in the canonical ensemble are found to be different from those in the grand canonical one. Effects of quantum statistics increase in the grand canonical ensemble for large chemical potential. This is, however, not the case in the canonical ensemble. In the limit of large charge density a strongest difference between the grand canonical and canonical ensemble results is observed.Comment: 13 pages, 6 figure