AstroGK: Astrophysical gyrokinetics code

The gyrokinetic simulation code AstroGK is developed to study fundamental aspects of kinetic plasmas and for applications mainly to astrophysical problems. AstroGK is an Eulerian slab code that solves the electromagnetic gyrokinetic-Maxwell equations in five-dimensional phase space, and is derived from the existing gyrokinetics code GS2 by removing magnetic geometry effects. Algorithms used in the code are described. The code is benchmarked using linear and nonlinear problems. Serial and parallel performance scalings are also presented.     

Flow transitions resembling bifurcations of the logistic map in simulations of the baroclinic rotating annulus

We present evidence for a sequence of bifurcations in simulations of the differentially heated baroclinic rotating annulus, similar to bifurcations of the logistic map. The Met. Office / Oxford Rotating Annulus Laboratory Simulation (MORALS) code is used to construct a detailed numerical regime diagram for the annulus, and the distribution of regimes in parameter space is discussed. The bifurcations are observed in a sequence of runs at high temperature forcing, identified by Poincaré sections of the dominant temperature mode amplitude time series. Higher order return maps and predictions using quadratic fits to the data are used to verify this result, and Lyapunov exponents are calculated to identify and quantify the chaotic parts of the sequence.     

VALIS: A split-conservative scheme for the relativistic 2D Vlasov–Maxwell system

An accurate treatment of the relativistic Vlasov–Maxwell system is of fundamental importance to a broad range of plasma physics topics, including laser–plasma interaction, transport in solar and magnetospheric plasmas and magnetically confined plasmas. This paper introduces VALIS: an algorithm for the numerical solution of the Vlasov–Maxwell system in two spatial dimensions and two momentum dimensions.     

Anisotropy easy axes alignment of deposited Co nanoclusters

We report on a method to orient the easy axes of ferromagnetic nanoclusters during their deposition, with the use of a magnetic field supplied by a set of permanent magnets. This field is applied close to the substrate where the preformed clusters are deposited. The efficiency is demonstrated on an assembly of 4 nm Co clusters produced by a sputtering and gas-condensation technique. The magnetization of the deposited clusters measured at 6 K exhibits an overall in-plane alignment of the easy axes in the direction of the field applied during deposition, H_D. Using a Stoner-Wohlfarth model, it is estimated that half of the clusters easy axes lie within 35^° from H_D.     

Polymorphism and high-temperature conductivity of Ln2M2O7 (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores

We present a comparative analysis of the order─disorder transitions in Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) pyrochlore-like compounds and solid solutions existing in the Ln₂O₃─MO₂ systems. In the range ~ 600─1200 °C, Ln₂Ti₂O₇ (Ln = Sm─Lu) and Ln₂Zr₂O₇ (Ln = Sm─Gd) undergo ordering transitions, F? → PI → P, which culminate in the formation of an ideal pyrochlore structure, P, existing between 1100 and 1300 °C. Above 1300 °C, Ln₂Ti₂O₇ (Ln = Gd─Lu), Ln₂Zr₂O₇ (Ln = Sm─Gd) and Ln₂Hf₂O₇ (Ln = Eu─Tb) exist as oxygen-ion-conducting phases, PII, disordered in both the oxygen and cation sublattices. Ionic conductivity data for Ln₂(M_2 ─ xLn_x)O_7 ─ δ (Ln = Sm─Lu; M = Ti, Zr, Hf; x = 0, 0.096) synthesized at 1600-1670 °C indicate that the highest conductivity in these systems is typically offered by nominally stoichiometric (Ln:M = 1:1), disordered Ln₂M₂O₇ (Ln = Sm─Lu; M = Ti, Zr, Hf) pyrochlores containing anti-structure pairs (Ln_M^' + M_Ln^•) and oxygen vacancies (V_O^••) on the 48f (O2) site. The highest conductivity of Yb₂Ti₂O₇, in which the cations have the smallest radii among the lanthanides and Group IVa metals, seems to be due to the increased role of the geometric factor in the Ln₂Ti₂O₇ (Ln = Sm-Lu) pyrochlores with predominantly covalent metal─oxygen bonding M-O (Ti-O). The ion transport parameters in these materials are determined primarily by the relationship between the sizes of the mobile oxygen ions and conduction channels.     

Influenza epidemic spread simulation for Poland — a large scale, individual based model study

In this work a construction of an agent based model for studying the effects of influenza epidemic in large scale (38 million individuals) stochastic simulations, together with the resulting various scenarios of disease spread in Poland are reported. Simple transportation rules were employed to mimic individuals’ travels in dynamic route-changing schemes, allowing for the infection spread during a journey. Parameter space was checked for stable behaviour, especially towards the effective infection transmission rate variability. Although the model reported here is based on quite simple assumptions, it allowed to observe two different types of epidemic scenarios: characteristic for urban and rural areas. This differentiates it from the results obtained in the analogous studies for the UK or US, where settlement and daily commuting patterns are both substantially different and more diverse. The resulting epidemic scenarios from these ABM simulations were compared with simple, differential equations based, SIR models — both types of the results displaying strong similarities. The pDYN software platform developed here is currently used in the next stage of the project employed to study various epidemic mitigation strategies.     

Approximate tolerance limits for Zipf–Mandelbrot distributions

Zipf–Mandelbrot distributions are commonly used to model natural phenomena where the frequency of an event’s occurrence is inversely proportional to its rank based on that frequency of occurrence. This discrete distribution typically exhibits a large number of rare events; however, it may be of interest to obtain reasonable limits that bound the majority of the number of different events. We propose the use of statistical tolerance limits as a way to quantify such a bound. The tolerance limits are constructed using Wald confidence limits for the Zipf–Mandelbrot parameters and are shown through a simulation study to have coverage probabilities near the nominal levels. We also calculate Zipf–Mandelbrot tolerance limits for two real datasets and discuss the associated computer code developed for the R programming language.     

High-sensitivity transient spectroscopy using tunable diode lasers

Experimental techniques have been developed to monitor transient infrared absorptions using lead-salt tunable diode lasers. The techniques are easily implemented, yield sensitivities which are limited by detector noise at 10^–5 level of absorbance, and have a response time on the order of one microsecond. The transient absorption detection techniques are high frequency versions of the sweep integration technique pioneered by Jennings [Appl. Opt.19, 2695 (1980)]. TDL modulation rates of 100 kHz and 500 kHz allow for absorption sampling rates of 200 kHz and 1 MHz, respectively. In order to reproducibly achieve near-detector-noise-limited sensitivities for 100 kHz TDL modulation rates, an automated analog subtraction circuit has been developed which removes the effects of minor TDL power variations. At the 500 kHz modulation rate, digital filtering techniques are used to remove the effects of this power variation.     

Selfconsistent calculation of the 11- and 33-amplitude in the low-energy π-N-scattering

A self-consistent calculation of the masses of the nucleon and the 33-resonance, theπN-coupling constant and the resonance width is made. By using a simplified version of theN/D-method the consistency equations are reduced to algebraic equations. Therefore selfconsistency can be obtained at once for both partial waves. In order to suppress the high energy contributions a second subtraction at threshold is made. The thereby introduced subtraction constants are connected with the 11- and 33-scattering lengths which are known from experiment.     

Next-to-leading-order results for five, six, and seven jets in electron-positron annihilation

We present next-to-leading order corrections in the leading color approximation for jet rates in electron-positron annihilation up to seven jets. The results for the two-, three-, and four-jet rates agree with known results. The NLO jet rates have been known previously only up to five jets. The results for the six- and seven-jet rate are new. The results are obtained by a new and efficient method based on subtraction and numerical integration.     

Electroweak corrections to neutralino and chargino decays into a W-boson in the (N)MSSM

We present the complete electroweak one-loop corrections to the partial widths for two-body decays of a chargino (neutralino) into a W-boson and a neutralino (chargino). We perform the calculation for the minimal and the next-to-minimal supersymmetric standard model using an on-shell renormalization scheme. Particular attention is paid to the question of gauge invariance which is achieved using the so-called pinch technique. Furthermore we show that these corrections show a strong parameter dependence and usually are in the range of 1–10 percent if the neutralino involved is a higgsino- or wino-like state. However, in case of a bino-like or a singlino-like neutralino the corrections can go up to 50% and more. Moreover we present the public program CNNDecays performing these calculations.     

Efficient calculation of two-dimensional periodic and waveguide acoustic Green's functions

New representations and efficient calculation methods are derived for the problem of propagation from an infinite regularly spaced array of coherent line sources above a homogeneous impedance plane, and for the Green's function for sound propagation in the canyon formed by two infinitely high, parallel rigid or sound soft walls and an impedance ground surface. The infinite sum of source contributions is replaced by a finite sum and the remainder is expressed as a Laplace-type integral. A pole subtraction technique is used to remove poles in the integrand which lie near the path of integration, obtaining a smooth integrand, more suitable for numerical integration, and a specific numerical integration method is proposed. Numerical experiments show highly accurate results across the frequency spectrum for a range of ground surface types. It is expected that the methods proposed will prove useful in boundary element modeling of noise propagation in canyon streets and in ducts, and for problems of scattering by periodic surfaces.     

Spectral editing of weakly coupled spins using variable flip angles in PRESS constant echo time difference spectroscopy: Application to GABA

A general in vivo magnetic resonance spectroscopy editing technique is presented to detect weakly coupled spin systems through subtraction, while preserving singlets through addition, and is applied to the specific brain metabolite γ-aminobutyric acid (GABA) at 4.7 T. The new method uses double spin echo localization (PRESS) and is based on a constant echo time difference spectroscopy approach employing subtraction of two asymmetric echo timings, which is normally only applicable to strongly coupled spin systems. By utilizing flip angle reduction of one of the two refocusing pulses in the PRESS sequence, we demonstrate that this difference method may be extended to weakly coupled systems, thereby providing a very simple yet effective editing process. The difference method is first illustrated analytically using a simple two spin weakly coupled spin system. The technique was then demonstrated for the 3.01 ppm resonance of GABA, which is obscured by the strong singlet peak of creatine in vivo. Full numerical simulations, as well as phantom and in vivo experiments were performed. The difference method used two asymmetric PRESS timings with a constant total echo time of 131 ms and a reduced 120° final pulse, providing 25% GABA yield upon subtraction compared to two short echo standard PRESS experiments. Phantom and in vivo results from human brain demonstrate efficacy of this method in agreement with numerical simulations.     

Stark-enhanced diode-laser spectroscopy of formaldehyde using a modified Herriott-type multipass cell

The combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is used to suppress time dependent background signals, which in general limit spectrometer performance during measurements. A rapid background subtraction scheme, in which the external electric field was turned off on alternate scans, as well as a double modulation experiment show drift free, white noise limited characteristics up to integration times of more than 1000 s. This exceeds the generally obtained spectrometer stability by about one order of magnitude. PACS 82.80.Gk; 42.62.Fi; 07.88.+y; 33.55.Be     

QCD corrections in an asymmetric momentum subtraction scheme

Numerical results for higher order QCD corrections for (a) e⁺e^?-annihilation, (b) para-quarkonium decay and (c) deep-inelastic scattering are presented in the recently proposed asymmetric momentum subtraction renormalization scheme (AMOS). The corrections are comparable to those in the symmetric momentum subtraction scheme (SMOS) which has yielded the smallest numbers so far. It is also pointed out that AMOS is theoretically more appealing than SMOS.     

Chaotic-to-ordered state transition of cathode-sheath instabilities in DC glow discharge plasmas

Transition from chaotic to ordered state has been observed during the initial stage of a discharge in a cylindrical DC glow discharge plasma. Initially it shows a chaotic behavior but increasing the discharge voltage changes the characteristics of the discharge glow and shows a period subtraction of order 7 period → 5 period → 3 period → 1 period, i.e. the system goes to single mode through odd cycle subtraction. On further increasing the discharge voltage, the system goes through period doubling, like 1 period → 2 period → 4 period. On further increasing the voltage, the system goes to stable state through two period subtraction, like 4 period → 2 period → stable.     

Si(111)解理面氧吸附后Si L2,3VV俄歇谱形的分析

对Si(111)解理清洁表面及其在氧吸附后测得的SiL_2,3VV俄歇谱进行数字积分、背底扣除及退自卷积后,得到了解理清洁表面在氧吸附前后的部分跃迁态密度。分析这两者之间的差别,结合别人的实验及理论计算结果表明:氧同时以分子形式及原子形式进行吸附,在吸附过程中,还同时形成氧化硅的价态。 关键词：