Systematic trends in the line strengths of E1 transitions in the oxygen isoelectronic sequence

Line strengthsS for the dipole allowed transitions within then=2 complex of the oxygen isoelectronic sequence have been fitted in the formZ ² S=A+B/(Z −C), whereZ is the nuclear charge of a particular ion. The constantsA, B andC are determined by using a non-linear least square method. The data forS are taken from the configuration interaction calculations which included internal, semi-internal and all external type correlations for ions in the rangeZ=8 − 25. It is shown that the values ofA obtained from the fit for all the transitions are in excellent accord with the ab-initio values obtained in the hydrogenic limitZ → ∞ provided near degeneracy effects are included in the ground state multiplet 1s²2s²2p^{4 1}S.     

Interface optical anisotropy in a heterostructure with different cations and anions

A theory of optical anisotropy and quantum-confined Pockels effect in CA/C′A′(001) quantum-well structures with different cations and anions, i.e. for C≠C′ and A≠A′, has been developed. The theory is based on a generalized effective-mass method, in which the boundary conditions for the envelope functions were constructed taking into account the mixing of heavy-and light-hole states under normal incidence of the hole on the interface. It is shown that an absorption anisotropy in interband transitions occurs for different mixing coefficients t _l−h in the boundary conditions for the right-hand (A-C′) and left-hand (A′-C) interfaces. An analysis is made of the interface contribution to the anisotropy induced by an external electric field for coinciding and different band offsets at the interfaces. The microscopic sp ³ s* tight-binding model is used to estimate the difference between the t _l−h(A-C′) and t _l−h(A′-C) coefficients. Fiz. Tverd. Tela (St. Petersburg) 40, 1925–1931 (October 1998)     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

<Emphasis Type="Italic">Z</Emphasis>′ signals from the LEP2 data

The LEP2 data on e ⁺ e ⁻ → e ⁺ e ⁻, μ⁺μ⁻, and τ ⁺ τ ⁻ processes are fitted to estimate possible signals of the Abelian Z′ boson. In the many-parametric fits, four independent parameters must be fitted if the derived already low-energy relations between the Z′ couplings to the standard-model fermions are taken into consideration. No signals are found for the complete LEP2 data set for these processes. In the fit of the backward bins, the hint at the 1.25σ C.L. is detected. The Z′ couplings to the vector and axial-vector lepton currents are constrained. Comparisons with the one-parameter fits are performed. The text was submitted by the authors in English.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Observational constraint on generalized Chaplygin gas model

We investigate observational constraints on the generalized Chaplygin gas (GCG) model as the unification of dark matter and dark energy from the latest observational data: the Union SNe Ia data, the observational Hubble data, the SDSS baryon acoustic peak and the five-year WMAP shift parameter. The result is obtained that the best-fit values of the GCG model parameters with their confidence level are A _s=0.73_−0.06^+0.06 (1σ) _−0.09^+0.09 (2σ), α=−0.09_−0.12^+0.15 (1σ) _−0.19^+0.26 (2σ). Furthermore, in this model, we can see that the evolution of equation of state (EOS) for dark energy is similar to quiessence, and its current best-fit value is w _0de=−0.96 with the 1σ confidence level −0.91≥w _0de≥−1.00.     

Collisional processes near the CH B 2Σ- v′=0,1 predissociation limit in laser-induced fluorescence flame diagnostics

Excitation and dispersed laser-induced fluorescence spectra of CH B ²Σ^-v^′=0,1 in methane flames are analyzed using rotational relaxation models to investigate their applicability for flame diagnostics. The existence of non-predissociative and highly predissociative rotational levels in the same vibrational state provides a unique scenario to test the effects of rotational relaxation in laser-induced fluorescence measurements. Using a statistical power gap law for rotational relaxation modeling, we find that the levels with collision-free lifetimes as short as 100 ps have apparent fluorescence yields larger than expected because of the extent of rotational relaxation at atmospheric pressure. Also, vibrational (v^′=1 to v^′=0) and electronic energy transfer (B ²Σ^-v^′=1 to A ²Δ) are competitive, and together are half the value for the total collisional removal rate from CH B ²Σ^-v^′=0. The measured electronic energy transfer branching ratio into A (v^′=0-3) depends on the initial rotational level pumped, and energy gap considerations can be used to explain these propensities. The combination of measurements and model calculations finds the excitation of the CH B ²Σ^- v^′=1,N^′=8 level a good candidate for laser-induced fluorescence quantitative measurements in flames at atmospheric pressure. Received: 24 September 1999 / Published online: 7 June 2000     

Tails in harnesses

We consider space- and time-uniformd-dimensional random processes with linear local interaction, which we call harnesses and which may be used as discrete mathematical models of random interfaces. Their components are rea random variablesa _s ^t , wheres ∈ Z ^d andt=0, 1, 2.,... At every time step two events occur: first, every component turns into a linear combination of itsN neighbors, and second, a symmetric random i.i.d. “noise”v is added to every component. For any σ ∈Z _d ⁺ define Δ_σ a′ _s as follows. If σ=(0,...,0), σ=(0,...,0), Δ_σ a _s ^t =a _s ^t . Then by induction, wheree _i is thed-dimensional vector, whoseith component is one and other components are zeros. Denote |σ| the sum of components of σ. Call a real random variable ϕ symmetric if it is distributed as −ϕ. For any symmetric random variable ϕpower decay or P-decay is defined as the supremum of thoser for which therth absolute moment of ϕ is finite. Convergence a.s., in probability and in law whent→∞ is examined in terms of P-decay(v): Ifd=1, σ=0 ord=2, σ=(0,0), Δ_σ a _s ^t diverges. In all the other cases: If P-decay(v)<(d+2)/(d+|σ|), Δ_σ a _s ^t diverges; if P-decay(v)>(d+2)/(d+|σ|), Δ_σ a _s ^t , converges and P-decay(ν) For any symmetric random variable ϕexponential decay or E-decay is defined as the supremum of thoser for which the expectation of exp(|x|^r) is finite. Let E-decay(v)>0. Whenever Δ_σ a _s ^t converges (that is, ifd>2 or |σ|>0: Ifd>2, E-decay(lima _s ^t )=min(E-decay(v),d+2/2); if |σ|=1, E-decay (lim Δ_σ a _s ^t )=min(E-decay(ν),d+2); if |σ| ⩾, E-decay (lim Δ_σ a _s ^t )=E-decay(ν).     

Some remarks on the ionic conductivity in NaY dehydrated zeolite

On the basis of experimental measurements of the electrical conductivity of the Nafaujasite zeolites (NaY), treated under vacuum up to 673 K for 24 hours, we clearly demonstrate that the behavior of the measured conductivity σ_ac of the dehydrated zeolite NaY, over an interval of high frequency change, may be described by a power-law function: σ_ac=A▹^s. The exponent s, in this case, should be considered as temperature and frequency dependent when the parameter A is a temperature dependent function. On the other hand, when considering the measured conductivity as a sum of two terms (σ_ac=σ₀+σ′(▹)) resulting from the contributions of the dc and the ac components respectively, we find that one of them obeys the Arrhenius law while the other can be expressed as A▹^s. Parameter s is practically frequency independent when the frequency of the applied electrical field is higher than a characteristic value ▹_c A comparison with the measurements performed on NaY dehydrated at 435 K is also included. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Release and uptake of oxygen in mixed-conducting SOFC cathode materials measured by temperature-programmed methods

Oxygen uptake and release in A_1−aA′_aB_1−bB′_bO_3−δ perovskite-type oxides with A=La, Pr; A′=Sr; B=Fe, Mn, B′=Co was examined by means of temperature-programmed oxidation (TPO) and temperature-programmed reduction (TPR). The investigated powders were prepared by spray-drying and were annealed at 900 to 1300 °C. In the TPR profiles, two processes of oxygen release from these materials were observed. The first TPR peak was regarded as surface adsorption, while the second one was considered to result from oxygen bulk diffusion. The oxygen volumes involved and the activation energy for the oxygen diffusion were determined from the TPO and TPR profiles and varied between 0.53 and 2.2 eV for the ferrites and manganites, respectively. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Optical absorption spectrum of Cr3+ in natural ruby

Two sharp line-like bands calledN andR lines on the red side, a close doublet (B lines) on the violet side and two broad bands are observed for natural ruby. At liquid air temperature the splitting ofR line was found and also three sharp-bands calledR, R′ andB lines are identified with spin-forbidden transitions of² E,² T ₁ and² T ₂. The two broad bands calledU band andY band are assigned accordingly to the spin-allowed transitions⁴ T ₂ and⁴ T ₁ respectively. The observed bands of natural ruby have been attributed to Cr³⁺ ion in an octahedral environment with trigonal distortion. The crystal field parameters which gave a good fit to the observed band positions areB=732 cm⁻¹,C=4.25B,Dq=1830 cm⁻¹,V=−1996 cm⁻¹ andλ=34 cm⁻¹.     

Positron scattering from hydrocarbons

The total cross sections for positron impact on hydrocarbons have been calculated using the additivity rule in which the total cross section for a molecule is the sum of the total cross section for the constituent atoms. The energy range considered is from a few eV to several thousand eV. The total cross sections for positron impact on an atom are calculated by employing a complex spherical potential which comprises of a static, polarization and an absorption potential. We have good agreement with the experimental results for hydrocarbons for positron energy ⩾100 eV. Our results also agree with the available calculations for CH₄ and C₂H₂ which employed full molecular wavefunctions beyond 100 eV. Our absorption cross sections also agree with molecular wave-function calculations for C₂H₂ and CH₄ beyond 100 eV. We have shown the Bethe plots fore ⁺−C ande ⁺−H scattering systems and Bethe parameters have been extracted. We have fitted the cross section for positron impact on hydrocarbons in the formσ _t(C _n H _m )=naE ⁻ b+mcE ⁻ d in the energy range 300–5000 eV wherea=195.0543,b=0.7986,c=371.1757 andd=1.1379 withE in eV andσ _t in 10⁻¹⁶ cm².     

Electrical properties of nickel-doped arsenic trisulphide

Results of temperature and frequency dependent a.c. conductivity of pure and nickel-doped a-As₂S₃ are reported. The a.c. conductivity of pure As₂S₃ obeys a well-known relationship: σ_ac ∝ω ^s. Frequency exponents is found to decrease with increasing temperature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As₂S₃. But a different behaviour of a.c. conductivity has been observed for the nickel doped As₂S₃. At higher temperatures, distinct peaks have been observed in the plots of temperature dependence of a.c. conductivity. The frequency dependent behaviour of a.c. conductivity (σ_ac ∝ω ^s) for nickeldoped As₂S₃ is similar to pure As₂S₃ at lower temperatures. But at higher temperatures, ln σ_ac vs lnf curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D ^0′) in the band gap. Single polaron hopping is expected to occur between theseD ^0‘ andD ⁺ states. Furthermore, allD ⁺,D ^0′ pairs are assumed to be equivalent, having a fixed relaxation time at a given temperature. The contribution of this relaxation to a.c. conductivity is found to be responsible for the observed peak in the plots of temperature dependence of a.c. conductivity for nickel-doped As₂S₃. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explained by using CBH and “simple pair” models. Theoretical results obtained by using these models, have been found to be in agreement with the experimental results.     

Testing of QED-Theory and Precise Measurements of the Rydberg Series for the He-Like Multicharged Ions

The wavelengths of the 1snp ¹ P ₁−1s ^{2 1} S ₀ transitions in He-like Mg XI, F VIII (n= 4–8) and Al XII (n=6,9) have been calculated in the framework of the 1/Z expansion method including relativistic effects and QED contributions. It is found that QED corrections to the ground-state ionization energy are significant at the present level of experimental accuracy. This revised version was published online in July 2006 with corrections to the Cover Date.     

The search for cosmic strangelets with the supersonic concorde and with JACEE's circumpolar balloon flight in Antarctica

The search for cosmic strangelet nuclei was carried out by two experiments with emulsion chambers. A balloon-borne JACEE emulsion chamber was flown at 3.5 g/cm² for 200 h in Antarctica (JACEE-10 experiment) and the Concorde flights were made by ECHOS at an atmospheric depth of 110 g/cm² between Paris and New York. No nuclei withZ⩾30 survived after traversing 60–120 g/cm² of the detector materials in the JACEE instruments. No evidence for a long mean free path were found in the zenith angle distribution forZ/β⩾26 nuclei. The exposure factor used by the JACEE was 72 m²hsr. The intensity upperbounds,I⩽(2.2–9.7)×10⁻²/m²h sr, were obtained for strangelets having an atmospheric attenuation length of 220−50 g/cm², which corresponds to the case for mass numberA=100–10000 andZ/β > 13. Concorde experiments (ECHOS) used both a thin and a thick emulsion chamber. The total exposure was 209 m² h sr and no candidates with chargeZ⩾30 were found. The largest track hadZ/β=28.6±1.29 withβ ∼ 1. Nuclei observed with charge 13⩽Z⩽30 were consistent with the survival intensity of ordinary nuclei. The flux bounds from the ECHOS experiments were I⩽(2.1–5.0) x 10⁻²/m²h for strangelets with mass number 100⩽A⩽1000.     

Heat capacity of MnBr2·4H2O near the antiferromagnetic transition temperature

Heat capacity of MnBr₂·4H₂O has been measured in the critical region around the Neel temperature. The data can be fitted, over a restricted range of |t|⩽10⁻², to the asymptotic power law. The critical exponents and the amplitudesA andA′ are not consistent with any theoretic predictions. However when scaling constraints are imposed, their values agree with the parameters of Ising model. Corrections to scaling are necessary to extend the range of the fit to |t|>10⁻². The correction terms are asymmetric giving −1·15±0·25 as the ratio of the amplitudes of the lowest order correction terms,D andD′ above and belowT _c. This value is in agreement with the recent predictions of the renormalisation group theory.     

ESR study of exchange coupled pairs in copper diethyldithiocarbamate

ESR investigations on exchange coupled pairs of Cu ions in single crystals of Cu(dtc)₂, isomorphously diluted with the corresponding diamagnetic zinc salt, are reported. The spin Hamiltonian parameters for the coupled species (S=1) are:g _‖=2.1025,g ₊=2.031,A=75.1×10⁻⁴ cm⁻¹,B=14.8×10⁻⁴,D=276.0×10⁻⁴ cm⁻¹ andE=46.7×10⁻⁴ cm⁻¹. While theg andA tensors show tetragonal symmetry, the zeor-field splitting tensor is rhombic and has principal axes different from those of theg andA tensors. Intensity measurements made down to 4.2 K indicate that the exchange is ferromagnetic with |FFF| ∼ 10 cm⁻¹. Direct dipole-dipole interaction appears to be the major contribution to the zero-field splitting. A calculation on the distributed point dipole model shows that dipolar interaction is considerably modified by the high covalency of the Cu-S bond and accounts for the rhombic nature of the tensor. The possible exchange mechanisms in Cu(dtc)₂—direct exchange and superexchange through the bridging sulphurs—are discussed.     

Transport properties of hexanoyl chitosan-based gel electrolyte

Films of hexanoyl chitosan-based polymer electrolyte were prepared by the technique of solution casting. The effect of plasticizer on the transport properties of hexanoyl chitosan:lithium trifluoromethanesulfonate (LiCF₃SO₃) electrolytes have been investigated. The plasticizer used was ethylene carbonate (EC). The highest room temperature conductivity achieved in the EC-plasticized hexanoyl chitosan-based electrolytes is 2.75×10⁻⁵ S cm⁻¹. The Rice and Roth model was used to explain the variations in the dc conductivity observed. The exponent, s, in Jonscher’s universal power law equation σ(ω)=σ _dc+Aω ^s , was analyzed as a function of temperature for the sample containing 30 wt% of EC. The analysis suggests that the conduction mechanism follows that proposed by the overlapping large polaron tunneling model.     

Oxygen ion diffusion in perovskite-type oxides determined by permeation and by relaxation measurements

Oxygen ion diffusion in oxides of the perovskite-type structure was determined by two methods of measurement: i) stepwise change of temperature or oxygen partial pressure of the gaseous environment of a ceramic sample, and measurement of the relaxation of electrical conductivity or oxygen content, ii) oxygen permeation through a gas dense oxide ceramic pellet from air to argon. The experimental problems and sources of errors, calculation methods of chemical diffusion coefficients and oxide ionic conductivities from these two modes of investigation are discussed. The comparison of relaxation and permeation measurements on 10 oxides of the type A_1−aA′_aM_1−bM′_bO_3−σ (A=La, Pr, Ce; A′=Sr, Ca; M=Mn, Fe; M′=Co, Ni) show advantages of the relaxation method in relation to the experimental performance and possible errors.     

Search for Stueckelberg <Emphasis Type="Italic">Z</Emphasis>′ at LHC

We present the discovery potential for heavy Z′ gauge bosons in the Z′→e ⁺ e ⁻ decay channel at the Large Hadron Collider. The analysis focuses on the direct search for the massive Z′ bosons predicted by the Stueckelberg extension of the Standard Model. Using signal and background simulated events, and taking into account the basic parameters of the Compact Muon Solenoid detector, the discovery reach for the Stueckelberg Z′ is found to be in the range between 800 and 900 GeV/c² for an integrated luminosity of 30 fb⁻¹.