Search results for "Computational physics"
showing 10 items of 725 documents
External Noise Effects in Silicon MOS Inversion Layer
2013
The effect of the addition of an external source of correlated noise on the electron transport in silicon MOS inversion layer, driven by a static electric field, has been investigated. The electron dynamics is simulated by a Monte Carlo procedure which takes into account non-polar optical and acoustic phonons. In our modelling of the quasi-two-dimensional electron gas, the potential profile, perpendicular to the MOS structure, is assumed to follow the triangular potential approximation. We calculate the changes in both the autocorrelation function and the spectral density of the velocity fluctuations, at different values of noise amplitude and correlation time. The findings indicate that, t…
High precision astrometry with closure constraints: The triplet 1803+784/1928+738/2007+777
1997
Abstract Improvements in the technique of high precision differential astrometry in VLBI based on the use of triangles of radio sources are reported. Such geometry provides new constraints by taking advantage of “closure”. Results on the relative separations (ranging from 4.6 to 6.8°) of the sources in the triangle 1803+784/1928+738/2007+777 from ionosphere-free delays are shown and “sky closure” verified. A comparison with previous results on the pair 1928+738/2007+777 is also presented.
The 35-d modulation of the X-ray emission of Her X-1 in the framework of the SOD model: results of a three-dimensional SPH simulation
1994
Several models have been proposed to explain the 35-d X-ray periodicity observed in Her X-1. We present the results of six three-dimensional quasi-polytropic smoothed particle hydrodynamics (SPH) simulations of the tilted, twisted accretion disc of Her X-1 carried out in the light of Roberts' slaved orienting disc model (SOD) with the intention of finding some limits to the inclination of the rotation axis of the secondary and to the value of the polytropic index γ. These results show that a γ value between 1.05 and 1.1 and an inclination angle φ of the order of 45° are the most suitable for enabling the SOD model to work in three-dimensional space. The simulated disc is rather small and th…
Linestrengths of the ν2 and ν4 bands of 12CH4 and 13CH4
1989
Absorption spectra recorded on the high-resolution Fourier transform spectrometer at Kitt Peak National Observatory/National Solar Observatory were used to measure individual line strengths of the nu(2) and nu(4) bands of (C-12)H4 and (C-13)H4. The measurements were used to obtain expressions that could be used to correctly predict individual line strengths through five orders of magnitude of absorption strength and high values of J-prime. Transition strengths were modeled using the dyad formalism of two interacting bands and a seven-term second-order dipole-moment expansion. The successful fitting of these data indicates that the method can be used to model measurements of 2- to 5-percent …
Analysis of Cylindrical Dielectric Resonators in Rectangular Cavities Using a State-Space Integral-Equation Method
2006
In this letter, a state-space integral-equation method in the s-domain has been employed for the accurate analysis of rectangular cavities loaded with cylindrical dielectric resonators. The dielectric obstacles have been treated in terms of their polarization equivalent charge and current densities. The dielectric resonator can be placed at any arbitrary position inside the cavity. The presented technique allows to calculate in a very efficient way a large number of solenoidal modes. The resonant frequencies of dielectric-loaded cavities are calculated and compared with data from literature and a commercial finite element method software, showing a good agreement
The Computer Code for Calculations of the Positron Distribution in a Layered Stack Systems
2014
In this paper we present experimental and theoretical studies of the implantation pro le of positrons emitted fromNa radionuclide into layered sample. The measured pro le for the nonsymmetrical, around the positron emitter, stack of aluminium and silver foils was measured using the depth scanning of implantation pro le technique. For the description of the obtained results two theoretical approaches were applied. The rst one was the Monte Carlo simulation using the GEANT4 tool kit. The generated pro le does not reproduce accurately the experimental pro le. We proposed the multiple scattering model based on the fact that a positron due to backscattering can travel as an energetic particle se…
A Ge(Li)Ge(Li) sum-peak (summing coincidence) spectrometer
1970
Abstract The sum-peak spectrometer (also called the integral-bias summing coincidence spectrometer) arrangement earlier developed with NaI(Tl) detectors is extended to Ge(Li) detectors. The integral-bias method in sorting sums of coincident pulse amplitudes is replaced by a set of pulse-height selection windows, which simply and more effectively aid in the analysis of sum-peak complexities associated with the symmetric linear summing procedure employed. A large part of the original information lost in summing of the pulse amplitudes can be retained by simultaneous sorting of the total spectrum into suitable subgroups. The arrangement represents in effect a real-time totalizing spectrometer …
Highly efficient multichannel spin-polarization detection.
2011
Since the original work by Mott, the low efficiency of electron spin polarimeters, remaining orders of magnitude behind optical polarimeters, has prohibited many fundamental experiments. Here we report a solution to this problem using a novel concept of multichannel spin-polarization analysis that provides a stunning increase in efficiency by 4 orders of magnitude. This improvement was demonstrated in a setup using a hemispherical electron energy analyzer. An imaging setup proved the principal capability of resolving more than ${10}^{5}$ data points in parallel.
Generalized Langevin dynamics: construction and numerical integration of non-Markovian particle-based models.
2018
We propose a generalized Langevin dynamics (GLD) technique to construct non-Markovian particle-based coarse-grained models from fine-grained reference simulations and to efficiently integrate them. The proposed GLD model has the form of a discretized generalized Langevin equation with distance-dependent two-particle contributions to the self- and pair-memory kernels. The memory kernels are iteratively reconstructed from the dynamical correlation functions of an underlying fine-grained system. We develop a simulation algorithm for this class of non-Markovian models that scales linearly with the number of coarse-grained particles. Our GLD method is suitable for coarse-grained studies of syste…
Simultaneous time-optimal control of the inversion of two spin-12particles
2010
We analyze the simultaneous time-optimal control of two-spin systems. The two noncoupled spins, which differ in the value of their chemical offsets, are controlled by the same magnetic fields. Using an appropriate rotating frame, we restrict the study to the case of opposite shifts. We then show that the optimal solution of the inversion problem in a rotating frame is composed of a pulse sequence of maximum intensity and is similar to the optimal solution for inverting only one spin by using a nonresonant control field in the laboratory frame. An example is implemented experimentally using nuclear magnetic resonance techniques.