铒离子注入碳化硅的射程和退火行为研究

用300—500 keV能量的铒(Er)离子注入碳化硅(6H-SiC)晶体中,利用卢瑟福背散射技术研究了剂量为5×10¹⁵ cm^-2 的Er离子注入6H-SiC晶体的平均投影射程R_p和射程离散ΔR_p,将测出的实验值和TRIM软件得到的理论模拟值进行了比较,发现R_p的实验值与理论值符合较好,ΔR_p的实验值和理论值差别大一些 关键词： 离子注入 投影射程和射程离散 退火行为 卢瑟福背散射技术     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代,制备了La_2/3Ca_1/3Mn_1-xCo_xO₃ (0≤x≤0.15) 系列样品并研究了体系的结构和输运特性.结果表明,在替代范围内,样品呈现很好的单相结构,各晶格参数随替代量的增大而减小;Co替代导致体系出现电输运反常,具体表现为在居里温度T_C以下电阻-温度曲线的二次金属-绝缘转 关键词： Mn位替代 双峰现象 自旋结构 磁电阻效应     

Overall values of conventional discrepancy indices for crystals with heavy atoms. Part I—Results for a triclinic crystal when the model consists of the heavy atoms and a part of light atoms

The joint probability density functions of the normalized structure amplitudes of the structure and the model (i.e.,y _N andy _p ^c ) are derived for triclinic crystals containing heavy atoms (1, 2 and many) by taking the model to consist of the heavy atoms and a part of the light atoms in the unit cell. These functions are derived for the two cases where the model is completely correct (i.e., the related case) and where the model is completely wrong (i.e., the unrelated case) in terms of the fractional contributions to the local mean intensity from the heavy atoms and all known atoms (i.e., σ _1h ^/2 and σ ₁ ² ) as parameters. These functions are then used to obtain the theoretical local values of 〈y _N〉 and 〈|y _N ⁿ − σ ₁ ⁿ (y _P ^c ) ⁿ |〉,n=1, 2. A method of using these results to compute the theoretical overall values ofR(F) andR(I) for the related and unrelated cases is briefly described. A comparison of the observed values of these indices with their theoretical values for the related and unrelated cases would help in determining the correctness of the proposed trial structure. Contribution No. 548     

Small angle neutron scattering from oxygen precipitates in silicon

The formation of silicon oxide precipitates from Czochralski grown silicon depends on the time and temperature of the heat treatment as well as on the initial content of interstitially dissolved oxygen. Samples containing between 5×10¹⁷ O_i/cm³ and 13×10¹⁷ O_i/cm³ have been heated at 750° C for 96 h. SiO₂ precipitates of various shape and size have been obtained and investigated by means of small angle neutron scattering (SANS) in the Q-range 0.05 Å^–1<Q<0.2 Å^–1. The obtained SANS patterns reveal a typical anisotropy of their intensity distribution, which splits into a central peak at Q<0.1 Å^–1 due to the shape of the individual particles and a number of weak intensities for large Q-values, originating from a correlation between defects, possibly between the precipitates. While these correlation peaks in the SANS patterns are seen best for rather low values of about (5–7)×10¹⁷ O_i/cm³ oxygen content, the central peak anisotropy is most pronounced for higher values of ca 10×10¹⁷ O_i/cm³. The integrated intensity of the central peak increases with increasing initial oxygen content. For comparison, untreated samples of the same initial oxygen content do not reveal any anisotropic SAN scattering or a broadened central peak beam.     

Chemical hardness-based prediction of the surface tension and enthalpy changes of sublimation for alkali halides

A new theoretical route employing the concept of chemical hardness has been developed to predict the surface tension γ and the changes of the standard enthalpies (CSEs) of sublimation Δ_sH⁰ of alkali halides. The values of these quantities have been calculated by means of the ratios η_M/V^1/3_m where η_M and V_m are the molecular hardness and molecular volume, respectively. The obtained results have been compared with those of previous theoretical models as well as with experimental data.     

Determination of thermoluminescence kinetic parameters of thulium doped lithium calcium borate

For the first time kinetic parameters of thulium doped Lithium calcium borate (LCB) Thermoluminescence (TL) material are reported here. Irradiated LCB:Tm³⁺ powder has revealed two intense TL glow peaks one at 510 (peak 1) and the other at 660 K (peak 2). Activation energy (E), frequency factor (s) and order of kinetics (b) of these peaks were determined by various heating rate (VHR), initial rise (IR), and peak shape (PS) methods. The trap depth and frequency factor determined for peaks 1 and 2 of LCB:Tm phosphor using VHR and IR methods are in good agreement. The average activation energy of peaks 1 and 2 obtained by these methods is 1.62 and 1.91 eV respectively. The frequency factors of peaks 1 and 2 are in the range of 10^13–16 and 10^12–14 sec⁻¹ respectively. The E and s values estimated using the glow peak shape dependent parameters are relatively less compared to the values obtained from other methods. The large difference in these values is due to the complex nature of the glow curves. The order of the kinetics process for complex glow curve peaks could not be assigned on the basis of shape parameters alone but T_m response on absorbed dose is to be considered for final confirmation. Glow peaks 1 and 2 of LCB:Tm³⁺ obey first and general order kinetics respectively.     

Isotopic effects of the N(2D) + H2 → NH + H reaction: a quantum time-dependent wave packet investigation

Based on the potential energy surface reported by Li and co-workers (J. Comput. Chem. 34 1686–1696 (2013)), the dynamics calculations of N(²D)?+?H₂(v ₀?=?0, j ₀?=?0) reaction and its isotopic variants HD and D₂ are studied using time-dependent wave packet method in the collision energy range of 0.01–1.0?eV. Dynamics properties such as reaction probability, differential cross section, and integral cross section are studied at state-to-state level of theory. Present values are compared with available theoretical and experimental results. The results indicate that the integral cross sections of N(²D)?+?D₂ reaction are in general good agreement with the experimental data at collision energy below 0.15?eV. The rotational state-resolved integral cross sections of N(²D)?+?H₂/HD/D₂ reactions are compared with experimental values for the first time, with the obtained values being in good agreement with the experimental data.     

Semiclassical approximation for the specific heat of non-crystalline solids at intermediate temperatures

The origin of the broad maximum in the specific heat (c _P) of glassy systems in the c _P/T ³ representation, in correspondence with the boson peak, has been described from a semiclassical perspective. A new model, based in the Debye approximation, taking into account the anharmonic vibrations of correlated atoms, is proposed. The theoretical predictions are shown to be in good agreement with the experimental data obtained in several glass samples in the temperature range 2–80 K.     

Optical spectra and spin-Hamiltonian parameters of trivalent ytterbium in lead tungstate

By using crystal-field theory, the optical spectra and spin-Hamiltonian parameters (abbr. SH parameters, i.e. the anisotropic g factors g _∥, g _⊥, and hyperfine structure constants A _∥, A _⊥) of ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes in the tetragonal PbWO₄ are calculated. The theoretical results agree well with the experimental values. The crystal-field parameters and the signs of the hyperfine structure constants for both ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes are determined. The validities of the theoretical results are discussed.     

Diffusion of sodium in rubidium chloride

The diffusion of Na²²Cl in RbCl was measured in the temperature range 377–707°C by the tracer-sectioning technique. The activation energy is 2.06± 0.02 and 0.59 ± 0.01 eV in the intrinsic and extrinsic regions, respectively. The temperature dependence of the correlation factor, as deduced from isotope-effect measurements, is 0.19 eV in the intrinsic region and -0.05 eV in the extrinsic region. When a theoretical value for the Na^+- vacancy binding energy is used, values of ½h_f =1.22, eV and h_m= 0.55 eV are obtained where h_f is the energy of formation, and h_m the energy of motion, of a cation vacancy. These values are not in agreement with the calculation by Tosi and Doyama.     

Role of localized excitons in the stimulated emission in ultra-thin CdSe/ZnSe/ZnSSe single quantum-well structures

Photo-pumped lasing properties have been investigated in a CdSe/ZnSe/ZnSSE single quantum wells (SQWs) with the well-layer thickness (L_W) of 1, 2 and 3 monolayer (ML). At 20 K, the laser threshold for the SQW withL_W = 1 ML was the lowest in spite of the smallest active layer thickness. The carrier (exciton) sheet density at the threshold (n)_thwas estimated to be as low as 7 × 10¹⁰cm⁻², which is well below Mott's screening density. Time-resolved photoluminescence has revealed that the localized biexciton band, whose peak energy agrees with the lasing peak, appeared on the low-energy side of the main PL peak at this level of carrier concentrations. Theoretical calculation has also shown that the localized biexciton recombination has to be taken into account for the lasing process. On the contrary, then_thvalues of the SQWs with 2 and 3 ML are 1 order of magnitude larger than that of the SQW with 1 ML. This may be due to the smaller oscillator strength of both localized excitons and localized biexcitons because of the larger inhomogeneous broadening, resulting in an increased carrier density for achieving optical gain sufficient to overcome the reflection losses.     

Nuclear vertex constants and asymptotic normalization coefficients for the tritium nucleus

The properties of the nuclear vertex constant for virtual triton decay to a deuteron and a neutron (T → d + n) are investigated along with the properties of the asymptotic normalization coefficient defined for the triton wave function and related to this constant. These quantities are calculated numerically on the basis of an equation that relates the asymptotic normalization coefficient to the triton effective radius ρ _T, which was introduced in the present study. The values of G _T² = 1.244(68) fm and C _T² = 2.958(162) found from our calculations are in good agreement with experimental and theoretical estimates obtained for these quantities in other studies. Physical properties of the triton virtual state are also discussed.     

Heavy-flavor production in <Emphasis Type="Italic">pp</Emphasis> and heavy-ion collisions in QCD up-to-LHC energies

Charm production in pp collisions is considered in the framework of perturbative QCD. The values of two parameters, the charm quark mass m _c and the QCD scale μ², are determined from the comparison of the theoretical calculations with experimental data. The RHIC data on charm and beauty production are compared with the k _T -factorization approach predictions and with standard NLO QCD. The calculated results underestimate the STAR Collaboration data. The role of possible nuclear effects is discussed. The predictions for LHC energies are also given.     

Algebraic method of determining bond length of perovskite compounds doped with Mn

An algebraic method is proposed for calculating the exponent n and constant K in the power law 10Dq=KR⁻ⁿ. By using this method, the values of n and K for the cubic perovskites KMnF₃ and RbMnF₃ are derived and then used to determine the true Mn²⁺–F⁻ bond length R in crystals of KMgF₃, KZnF₃, RbCdF₃, and CsCaF₃ doped with Mn²⁺ from the reported optical spectra. The results obtained are in good agreement with those derived from the electron paramagnetic resonance data and the extended X-ray absorption fine structure techniques. Thus, as a new theoretical method, it is valuable and convenient for the determination of the bond lengths.     

Comparison of the Seltzer Coefficient C 1 to Experimental Data

Experimental Coulomb isotope shifts δE _Coul from K _α transitions, and radius differences δ〈r ²〉 _eμ measured by electron scattering and muonic atom X-rays were used to derive ‘experimental’ coefficients C _1,exp for 54 isotope pairs of 18 elements from Mo to U. A χ²-analysis shows that these experimental coefficients are – on average – 3.5% lower than the theoretical C ₁ values calculated by Seltzer, or more precisely: C _1,exp=0.965(± 0.014)×C ₁. The need for more accurate theoretical calculations is stressed, and consequences of this deviation are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microphase separation in weakly charged hydrophobic polyelectrolytes

Aqueous solutions of a well-defined poly(N-isopropylacrylamide-co-sodium 2-acrylamido-methylpropanesulfonate) (NIPAM/NaAMPS in a 95/5 molar ratio) have been investigated by means of small-angle neutron scattering (SANS) and rheological experiments as a function of temperature ( 25^° C T 60^° C) and polymer concentration ( 0.5wt% C 12wt%). The solutions remain optically transparent and isotropic over the whole temperature range, in contrast with the homopolyNIPAM which precipitates above its lower critical solution temperature (LCST = 32^° C). Upon addition of salt, the systems undergo a micro-macrophase separation. At temperatures above 45^° C, the SANS spectra exhibit a sharp peak at a scattering wave vector, q _max, which increases slightly with temperature. At high temperature ( T∼ 60^° C), the scattered intensity follows a power law I(q) ∼q ^-4 in the asymptotic regime, characteristic of two-density media with sharp interfaces, and q _max is found to vary with polymer concentration as q _max∼C ^0.22. Estimates of the typical sizes give values between 40 ? and 200 ?. These results provide a strong evidence of a thermally induced microphase separation, which is corroborated by the very sharp increases of the viscosity (over 2 decades) and of the stress relaxation time of the solutions, occurring in the temperature range where the scattering peak is observed. The results are discussed and compared with the theoretical models proposed for weakly charged polyelectrolytes in a poor solvent. Received 1 October 2001     

Mechanisms of neutrinoless double-beta decay: A comparative analysis of several nuclei

The neutrinoless double beta decay of several nuclei that are of interest from the experimental point of view (⁷⁶Ge, ⁸²Se, ¹⁰⁰Mo, ¹³⁰Te, and ¹³⁶Xe) is investigated on the basis of a general Lorentzinvariant effective Lagrangian describing physics effects beyond the Standard Model. The half-lives and angular-correlation coefficients for electrons are calculated for various decay mechanisms associated, in particular, with the exchange of Majorana neutrinos, supersymmetric particles (with R-parity violation), leptoquarks, and right-handed W _R bosons. The effect of theoretical uncertainties in the values of relevant nuclear matrix elements on decay features is considered.     

Hyperfine Interactions of Short-Lived β Emitters in Pd

The Knight shifts K and the spin-lattice relaxation time T ₁ for the short-lived β-emitters ¹²B and ¹²N implanted into Pd have been measured by means of the β-NMR method. The results show that K depends on temperature. The obtained values of experimental K were compared with theoretical values derived with the KKR method.     

Radiative lifetimes of 7d, 8d <Superscript>1</Superscript>D<Subscript>2</Subscript> excited states of Hg I

Radiative lifetimes of 7d, 8d ¹ D ₂ excited states of Hg I are measured using pulsed two-photon excitation from the ground [Xe]5d ¹⁰6s ^{2 1} S ₀ mercury state, detecting the decay of the laser-induced fluorescence. The results are compared with theoretical values, obtained by means of a Hartree-Fock single configuration method, taking into account electron configuration interaction. The radiative lifetime value dependence on the effective principal quantum number for the nd ¹ D ₂ series is analyzed and compared with the quantum defect dependence. Received 25 February 2000 and Received in final form 26 July 2000