Asymmetry Term Research Articles

Electronic properties of a biased graphene bilayer

We study, within the tight-binding approximation, the electronic properties of a graphenebilayer in the presence of an external electric field applied perpendicular to the system—abiased bilayer. The effect of the perpendicular electric field is included through aparallel plate capacitor model, with screening correction at the Hartree level.The full tight-binding description is compared with its four-band and two-bandcontinuum approximations, and the four-band model is shown to always be a suitableapproximation for the conditions realized in experiments. The model is applied toreal biased bilayer devices, made out of either SiC or exfoliated graphene, andgood agreement with experimental results is found, indicating that the model iscapturing the key ingredients, and that a finite gap is effectively being controlledexternally. Analysis of experimental results regarding the electrical noise and cyclotronresonance further suggests that the model can be seen as a good starting point forunderstanding the electronic properties of graphene bilayer. Also, we study the effect ofelectron–hole asymmetry terms, such as the second-nearest-neighbour hopping energiest′ (in-plane) andγ4 (inter-layer), andthe on-site energy Δ.

Isoscalar giant resonances in the Sn nuclei and implications for the asymmetry term in the nuclear-matter incompressibility

We have investigated the isoscalar giant resonances in the Sn isotopes using inelastic scattering of 386-MeV alpha-particles at extremely forward angles, including 0deg. We have obtained completely "background-free inelastic-scattering spectra for the Sn isotopes over the angular range 0deg--9deg and up to an excitation energy of 31.5 MeV. The strength distributions for various multipoles were extracted by a multipole decomposition analysis based on the expected angular distributions of the respective multipoles. We find that the centroid energies of the isoscalar giant monopole resonance (ISGMR) in the Sn isotopes are significantly lower than the theoretical predictions. In addition, based on the ISGMR results, a value of K_{tau} = -550 \pm 100 MeV is obtained for the asymmetry term in the nuclear incompressibility. Constraints on interactions employed in nuclear structure calculations are discussed on the basis of the experimentally-obtained values for K_(infty) and K_(tau).

Peak fitting and identification software library for high resolution gamma-ray spectra

A new gamma-ray spectral analysis software package is under development in our laboratory. It can be operated as a stand-alone program or called as a software library from Java, C, C++ and MATLAB TM environments. It provides an advanced graphical user interface for data acquisition, spectral analysis and radioisotope identification. The code uses a peak-fitting function that includes peak asymmetry, Compton continuum and flexible background terms. Peak fitting function parameters can be calibrated as functions of energy. Each parameter can be constrained to improve fitting of overlapping peaks. All of these features can be adjusted by the user. To assist with peak identification, the code can automatically measure half-lives of single or multiple overlapping peaks from a time series of spectra. It implements library-based peak identification, with options for restricting the search based on radioisotope half-lives and reaction types. The software also improves the reliability of isotope identification by utilizing Monte-Carlo simulation results.

Are There Smaller Leverage Effects in Less-Developed Markets? Evidence from an Oil Exporting Country

Problem statement: This study uses daily data from the Tehran Stock Market (TSM) to illustrate the nature of stock market volatility in an undeveloped and young stock market. Although most studies suggest that a negative shock to stock prices will generate more volatility than a positive shock of equal magnitude but there is no evidence of asymmetric effect in TSM. Determine the nature of stock market volatility in an oil exporting country. Approach: Trading in Tehran Stock Market (TSM) is based on orders sent by the brokers. The data consist of 2375 daily observations of the closing value of the Tehran stock market from 3/30/1998 to 5/04/2007. Our empirical finding shows that the unconditional variance is 0.18 but visual inspections of the time series suggests that volatility of the stock return rate displays the clustering phenomenon associated with GARCH processes. Results: The estimation and test results for all models suggest that the leverage effect term, γ, is not significant at 5% level. Although, in Asym. CARCH model based on normal distribution for errors, the estimated coefficient on the asymmetry term is -0.066 with a z-statistics of -1.749 recognized as significant at 10% level, but it has the wrong sign. It seems that good news and bad news has the same effect on stock prices in TSM, a result that is contradictory to other studies for developed countries. Conclusion: The estimated models containing TARCH, EGARCH, asymmetric CARCH and PARCH with different assumptions on error distributions suggest no strong and significant asymmetric effect. There are some reasons for this finding: (1) In Iran with Islamic laws, debt contracts are illegal or at least not enforced and Iranian firms do not have any financial leverage. As a result, we would expect to find smaller leverage effects in volatility in Iran than in the United States, for example. In deed the institutional differences with western financial markets manifest themselves in different return characteristics. (2) Stock prices in the TSM by regulation and intervention cannot exceed from some range. The strong serial correlation in returns necessitating long lags in the mean equations is possibly due to such regulations. (3) The history of TSM is very short compared to other stock markets and the information flow in this market is very slow. The estimated coefficients on the expected risk (as a measure of the risk-return tradeoff) are not significant. These findings suggest that the TSM is not efficient.

Isospin dependent global nucleon-nucleus optical model at intermediate energies

A global nucleon-nucleus optical potential for elastic scattering has been produced which replicates experimental data to high accuracy and compares well with other recently formulated potentials. The calculation that has been developed describes proton and neutron scattering from target nuclei ranging from carbon to nickel and is applicable for projectile energies from 30 to 160 MeV. With these ranges it is suitable for calculations associated with experiments performed by exotic beam accelerators. The potential is also isospin dependent and has both real and imaginary isovector asymmetry terms to better describe the dynamics of chains of isotopes and mirror nuclei. An analysis of the validity and strength of the asymmetry term is included with connections established to other optical potentials and charge-exchange reaction data. An on-line observable calculator is available for this optical potential.

Improved Formula for (n,3He) Fusion Reactions Cross Sections Using Optical Model

Non-elastic cross-sections have been calculated by using optical model for (n, 3He) reactions at 14–15 MeV energy. The new empirical formula including optical model non-elastic effects by fitting two parameters for the (n, 3He) reaction cross-sections have been suggested. The excitation function character and reaction Q-values depending on the asymmetry term effect for the (n, 3He) reaction have been investigated. The obtained cross-section formula with new coefficients has been compared with the experimental data and the other fitting formulae existed in the literature and discussed. It has seen that the fit of new formula in this paper is greatly improved with the experimental data.

The Study of (n,d) Reaction Cross Sections for New Evaluated Semi-Empirical Formula Using Optical Model

Nuclear fusion can be one of the most attractive sources of energy from the viewpoint of safety and minimal environmental impact. The neutron scattering cross sections data have a critical importance on fusion reactor (and in the fusion–fission hybrid) reactors. So, the study of the systematic of (n,d) etc., reaction cross sections is of great importance in the definition of the excitation function character for reaction taking place on various nuclei at energies up to 20 MeV. In this study, non-elastic cross-sections have been calculated by using optical model for (n,d) reactions at 14–15 MeV energy. The excitation function character and reaction Q-values depending on the asymmetry term effect for the (n,d) reaction have been investigated. New coefficients have been obtained and the semi-empirical formulas including optical model non-elastic effects by fitting two parameters for the (n,d) reaction cross-sections have been suggested. The obtained cross-section formulas with new coefficients have been compared with the available experimental data and discussed.

New Evaluated Semi-Empirical Formula Using Optical Model for 14–15 MeV (n, t) Reaction Cross Sections

In the next century the world will face the need for new energy sources. Nuclear fusion can be one of the most attractive sources of energy from the viewpoint of safety and minimal environmental impact. Fusion will not produce CO2 or SO2 and thus will not contribute to global warming or acid rain. Achieving acceptable performance for a fusion power system in the areas of economics, safety and environmental acceptability, is critically dependent on performance of the blanket and diverter systems which are the primary heat recovery, plasma purification, and tritium breeding systems. Tritium self-sufficiency must be maintained for a commercial power plant. The hybrid reactor is a combination of the fusion and fission processes. For self-sustaining (D-T) fusion driver tritium breeding ratio should be greater than 1.05. So working out the systematics of (n, t) reaction cross-sections are of great importance for the definition of the excitation function character for the given reaction taking place on various nuclei at energies up to 20 MeV. In this study, we have calculated non-elastic cross-sections by using optical model for (n, t) reactions at 14–15 MeV energy. We have investigated the excitation function character and reaction Q-values depending on the asymmetry term effect for the (n, t) reaction cross-sections. We have obtained new coefficients for the (n, t) reaction cross-sections. We have suggested semi-empirical formulas including optical model nonelastic effects by fitting two parameters for the (n, t) reaction cross-sections at 14–15 MeV. We have discussed the odd–even effect and the pairing effect considering binding energy systematic of the nuclear shell model for the new experimental data and new cross-sections formulas (n, t) reactions developed by Tel et al. We have determined a different parameter groups by the classification of nuclei into even–even, even–odd and odd–even for (n, t) reactions cross-sections. The obtained cross-section formulas with new coefficients have been discussed and compared with the available experimental data.

The effect of inbreeding on fluctuating asymmetry of wing veins in two laboratory strains of Drosophila melanogaster

Many authors have proposed that inbreeding destabilizes developmental processes. This destabilization may be reflected by increased fluctuating asymmetry (FA) in inbred compared to relatively outbred populations, but many studies have failed to find such differences. We measured the left and right wings of a large number of individual Drosophila melanogaster from two genetically distinct populations to estimate changes in FA caused by inbreeding. The large sample size and experimental design allowed removal of potentially confounding directional asymmetry (DA) and measurement error terms. Trait means in the two populations were essentially unchanged by inbreeding (less than 0.5% smaller in both populations). Inbred lines showed higher signed FA variances (16 and 38% higher, significantly so in one population) and higher unsigned FA means (3.7 and 13.2%, significantly increased in one population). Significant DA was found in both populations, although the pattern differed between populations. DA did not change due to inbreeding.

Correlation between electron spin relaxation time and hetero-interface roughness in (1 1 0)-oriented GaAs/AlGaAs multiple-quantum wells

The correlation between the electron spin relaxation time τ s and the hetero-interface roughness in (1 1 0)-oriented GaAs/AlGaAs multiple-quantum wells (MQWs) was investigated using growth interruption during molecular beam epitaxy growth. τ s was characterized using polarization- and time-resolved PL measurements, and the hetero-interface roughness was evaluated by the full-width-at-half-maximum of the PL spectra at 4 K. It was found that τ s in the MQWs with smooth interfaces became longer and the temperature dependence of τ s changed from τ s ∼ T 0.6– T 0.9 when the interface roughness was reduced using growth interruption. These results indicate that the structure inversion asymmetry term of the D’yakonov-Perel’ mechanism was suppressed in the MQWs with smooth hetero-interfaces.

Edge turbulence and SOL transport in tokamaks

Some key topics in tokamak edge plasma transport and turbulence are reviewed. Multi-device results reveal a new paradigm of scrape-off layer (SOL) transport. Radial transport is driven by intermittency throughout the SOL, in between edge localized modes (ELMs) in H-mode, and comprised of plasma filaments that are generated near the last closed flux surface likely by interchange instability. The filaments travel radially at speeds of ∼1 km/s into the SOL and have a poloidal size of 1–3 cm in most devices. The radial transport in the SOL is poloidally asymmetric, by factors of 2–5, causing a pressure peak in the low field side. This asymmetry and other neo-classical terms, such as Pfirsch–Schlüter currents, are found to drive strong SOL flows. The intermittent particle flux, is 20% of the total, including ELMs, at low collisionality, becoming 70% of total at high collisionality. Numerical and analytical models can reproduce the scaling of intermittency with collisionality as well as many details of the filament dynamics in the SOL.

Application of asymmetry depending empirical formulas for ( p, nα) reaction cross-sections at 24.8 and 28.5 MeV incident energies

In this study, we have investigated the asymmetry term effect for the ( p, nα) reaction cross-sections and we have obtained new coefficients for the ( p, nα) reactions at 24.8 and 28.5 MeV energies. We have suggested empirical formulas including the new coefficients found by fitting two parameters for proton induced reactions. The coefficients were determined by least-squares fitting method. The obtained cross-section formulas with new coefficients have been discussed and compared with the available experimental data.

Calculation of 14–15 MeV (n, d) Reaction Cross Sections Using Newly Evaluated Empirical and Semi-empirical Systematics

In this study, we have investigated the asymmetry term effect for the (n, d) reaction cross sections at 14–15 neutron incident energy. It has been discussed the odd–even effect and the pairing effect considering binding energy systematic of the nuclear shell model for the new experimental data and new cross section formulae developed by Tel et al. for (n, d) reactions. We have determined different parameter groups by the classification of nuclei into even–even, even–odd and odd–even for (n, d) reactions cross sections. The obtained empirical and semi-empirical formulae by fitting two parameters for (n, d) reactions were given. By using the new cross sections formulae for (n, d) reactions the obtained results have been discussed and compared with the available experimental data.

Investigation of 14–15 MeV (n, t) Reaction Cross-sections by Using New Evaluated Empirical and Semi-empirical Systematic Formulas

In the hybrid reactor, tritium self-sufficiency must be maintained for a commercial power plant. For self-sustaining (D-T) fusion driver tritium breeding ratio should be greater than 1.05. Working out the systematics of (n, t) reaction cross-sections are of great importance for the definition of the excitation function character for the given reaction taking place on various nuclei at energies up to 20 MeV. In this study we have investigated asymmetry term effect for the (n, t) reaction cross-sections at 14–15 neutron incident energy. It has been discussed the odd–even effect and the pairing effect considering binding energy systematic of the nuclear shell model for the new experimental data and new cross-sections formulas (n, t) reactions developed by Tel et al. We have determined a different parameter groups by the classification of nuclei into even–even, even–odd and odd–even for (n, t) reactions cross-sections. The obtained empirical and semi-empirical formulas by fitting two parameter for (n, t) reactions were given. All calculated results have been compared with the experimental data and the other semi-empirical formulas.

Isotopic Dependence of the Giant Monopole Resonance in the Even-ASn112–124Isotopes and the Asymmetry Term in Nuclear Incompressibility

The strength distributions of the giant monopole resonance (GMR) have been measured in the even-A Sn isotopes (A=112-124) with inelastic scattering of 400-MeV alpha particles in the angular range 0 degrees -8.5 degrees . We find that the experimentally observed GMR energies of the Sn isotopes are lower than the values predicted by theoretical calculations that reproduce the GMR energies in 208Pb and 90Zr very well. From the GMR data, a value of Ktau = -550 +/- 100 MeV is obtained for the asymmetry term in the nuclear incompressibility.

Investigation of the pairing effect using newly evaluated empirical studies for 14–15 MeV neutron reaction cross sections

The asymmetry term effects for the cross sections of ($n$, charged particle) and ($n,2n$) reactions at 14--15 MeV neutron incident energy have been investigated. The effects of pairing and odd-even nucleon numbers in new data and in the formula of Tel et al. [J. Phys. G. 29, 2169 (2003)] are discussed. We have determined three different parameters groups by the classification of nuclei into even-even, even-odd, and odd-even ($n,d$) reactions. In addition, since there are not enough experimental data available, we have considered two different parameters groups by the classification of nuclei into odd-$A$ and even-$A$ ($n,t$) reaction cross sections. The empirical formulas with two parameters for the evaluation of the ($n,d$) and ($n,t$) reactions cross sections are discussed in the present study.

Neutron and Proton Transverse Emission Ratio Measurements and the Density Dependence of the Asymmetry Term of the Nuclear Equation of State

Recent measurements of preequilibrium neutron and proton transverse emission from (112,124)Sn+(112,124)Sn reactions at 50 MeV/A have been completed at the National Superconducting Cyclotron Laboratory. Free nucleon transverse emission ratios are compared to those of A=3 mirror nuclei. Comparisons are made to Boltzmann-Uehling-Uhlenbeck (BUU) transport calculations and conclusions concerning the density dependence of the asymmetry term of the nuclear equation of state at subnuclear densities are made. Comparison to BUU model predictions indicate a density dependence of the asymmetry energy that is closer to a form in which the asymmetry energy increases as the square root of the density for the density region studied. A coalescent-invariant analysis is introduced as a means of reducing suggested difficulties with cluster emission in total nucleon emission.

Two-photon effects in lepton-antilepton pair photoproduction from a nucleon target using real photons

We consider the production of a lepton-antilepton pair by real photons off a hadronic target. The interference of one and two-photon exchange amplitudes leads to a charge asymmetry term that may be calculated explicitly in the large-$t$ limit in terms of hadronic distribution amplitudes. A rather compact expression emerges for the leading order asymmetry at fixed angle in the center-of-mass of the lepton pair. The magnitude appears sizeable and is approximately independent of the pair mass in the asymptotic limit.

Least-action principle for envelope functions in abrupt heterostructures

We apply the envelope function approach to abrupt heterostructures starting with the least action principle for the microscopic wave function. The interface is treated nonperturbatively, and our approach is applicable to mismatched heterostructure. We obtain the interface connection rules for the multiband envelope function and the short-range interface terms which consist of two physically distinct contributions. The first one depends only on the structure of the interface, and the second one is completely determined by the bulk parameters. We discover new structure inversion asymmetry terms and new magnetic energy terms important in spintronic applications.

Fluctuations and Isospin Equilibration in Heavy Ion Reactions

The Boltzmann–Uehling–Uhlenbeck (BUU) approach with density fluctuations was used to study the density dependence of the asymmetry term in the nuclear equation of state (EOS). The isospin diffusion and equilibration process was investigated. The isotopic ratios of light particles were not sensitive enough to distinguish between asy-stiff and asy-soft EOS. However, the isospin ratio seems to be more sensitive to the density dependence of the asymmetry term in the nuclear equation of state and it favors a more asy-stiff equation of state.