Molecular dynamics analysis of impurity ions behavior in β″-Al2O3

The effects of foreign impurity ions in the conduction plane on the β″-Al₂O₃ lattice have been analyzed by molecular dynamics. As impurity ions, K and Ca ions were chosen and Na sites in the conduction plane were replaced with these ions. Results are summarized as follows: (1) The β″-Al₂O₃ lattice expands perpendicular to the conduction plane when K ions are doped; (2) Ca ions do not contribute to expansions of the β″-Al₂O₃ lattice; (3) both K and Ca ions reduce the mean square displacement of Na ions, which can be attributed to decreased Na ion diffusion.     

Correlation between structural and optical properties of ion beam synthesized β-FeSi2 precipitates in Si

The structural and optical properties of β-FeSi₂ precipitates produced by ion beam synthesis have been investigated by transmission electron microscopy, photoluminescence (PL) analysis and near infrared transmission measurements. The PL spectrum of β-FeSi₂ precipitates in a dislocation free sample has been observed to consist of a sharp line at 1.54 μm and a weak peak at 1.46 μm. Optical transmission measurements showed a direct band gap about 0.8 eV smaller than in continuous β-FeSi₂ film. Calculation of the electronic bands of β-FeSi₂ for different values of the lattice parameters indicates that this reduction can be ascribed to band distortion provided by the lattice strain.     

XPS analysis of β-AlF3 phases with Al successively substituted by Mg to be used for heterogeneously catalyzed Cl/F exchange reactions

A new method of static charge referencing was applied in a XPS study of β-AlF₃ phases with aluminum successively substituted by magnesium. This class of compounds is characterized by strong Lewis acid sites, which are assumed to be the catalytically active sites for Cl/F exchange reactions. In order to adjust their strengths Mg was introduced into the β-AlF₃ lattice. At low amounts of Mg the resulting samples were found to be highly catalytically active in Cl/F exchange reactions. 20 nm gold particles were deposited on the sample surface to provide a static charge reference. With the help of this procedure binding energies (BE) of photo peaks as well as kinetic energies (KE) of Auger electron emission peaks are presented in relation to the Au4f_7/2 BE reference. Furthermore, a detailed analysis of the relaxation behavior of the Mg nucleus lead to the conclusion that, at low Mg contents, strong Lewis acidic Mg cations in Mg_xF_y clusters act as the catalytically active sites. At high Mg contents, small MgF₂ like crystallites are formed in the disturbed β-AlF₃ lattice, which are catalytically inactive as MgF₂.     

On long-range order in low-dimensional lattice-gas models of nematic liquid crystals

The problem of the orientational ordering transition for lattice-gas models of liquid crystals is discussed in the low-dimensional case d = 1, 2. For isotropic short-range interactions, orientational long-range order at finite temperature is excluded for any packing of molecules on the lattice Z^d; on the other hand, for reflection-positive long-range isotropic interactions, we prove the existence of an orientational ordering transition for high packing (μ > μ₀) and low temperatures (β > β_c(μ)).     

Renormalization group invariant matrix elements of ΔS=2 and ΔI=3/2 four-fermion operators without quark masses

We introduce a new parameterization of four-fermion operator matrix elements which does not involve quark masses and thus allows a reduction of systematic uncertainties. In order to simplify the matching between lattice and continuum renormalization schemes, we express our results in terms of renormalization group invariant B-parameters which are renormalization-scheme and scale independent. As an application of our proposal, matrix elements of ΔI=3/2 and SUSY ΔS=2 operators have been computed. The calculations have been performed using the tree-level improved Clover lattice action at two different values of the strong coupling constant (β=6/g²=6.0 and 6.2), in the quenched approximation. Renormalization constants and mixing coefficients of lattice operators have been obtained non-perturbatively. Using lowest order χPT, we also obtain ππ|O₇|K^NDR_I=2=(0.11±0.02) GeV⁴ and ππ|O₈|K^NDR_I=2=(0.51±0.05) GeV⁴ at μ=2 GeV.     

Incorporating dynamical light quarks in lattice QCD at finite temperature

Using the pseudo-fermion Monte Carlo method we investigate the chiral and the deconfinement phase transitions in the SU(3) gauge theory with dynamical staggered fermions on a 6³ × 2 lattice. The energy density of the gluonic sector ε_G, the average thermal Wilson loop |L| and the order parameter are found to have a rapid variation in the same range of β (=6/g²). The variation is similar to that observed in the quenched theory but is at a smaller value of β.     

Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2−xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).     

Enhancement of the low field magnetoresistance by grain boundary modification in Sr2FeMoO6+δ

We study the magnetic and electric properties of polycrystalline Sr₂FeMoO_6+δ with controlled oxygen content, for nominal values δ=0 and 0.04. While the magnetization and lattice parameters are the same in both samples, the resistivity and the magnetoresistance are greatly enhanced in the δ=0.04 sample. This behavior is related to the formation of an impurity (insulating) phase at the grain boundaries, thus reinforcing the intergrain tunneling barrier.     

Interpolating between Ising, XY-, and non-linear σ-models

The β-functions of O(N)-symmetric non-linear σ-models on the lattice were recently discovered to be non-monotonic for N 3. We explain the non-monotonic behaviour as a non-perturbative lattice effect by relating it to the Kosterlitz-Thouless transition of the XY-model. We also relate the latter transition to the phase transition of the Ising model. These relationships are established by interpolating between the O(N)- and the O(N − 1)-symmetric non-linear σ-models by suppression of the Nth component of the N-vector field with a mass term. A critical line in the coupling-mass plane connects the critical point of the Ising model (N = 1) with the critical point of the XY-model (N = 2). This line extends towards the region of non-monotonic behaviour of the β-function of the O(3)-symmetric model. The nature of the transition lines is also investigated.     

Electron spin resonance of irradiated ferroelectric tri-glycine sulfate

A preliminary analysis of the electron spin resonance spectra of ferroelectric tri-glycine sulfate single crystals, which were subjected to γ-ray damage indicated that: (a) the principal stable radical Produced by γ-ray damage is NH₃CH-; (b) the orientations of the radicals are fixed in the crystal lattice; (c) the approximate values of the H_c, the three H_N, and the N Fermi coupling constants are 21.0, 14.5 and 3.2 G respectively; (d) the NH₃ group is rotating in the ferroelectric phase.     

27Al NMR study in ZrNiAl

We have studied the microscopic properties of the hexagonal ZrNiAl, a model compound for a wide family of intermetallic compounds crystallizing in this type of structure, by using ²⁷Al NMR spectroscopy. We have investigated the lineshape of static and MAS NMR spectra as a function of magnetic field strength (4.7–9.4 T) and temperature (5–300 K). Our data indicate that the ²⁷Al NMR spectra result from a combined effect of quadrupole and anisotropic shift interactions. The ²⁷Al nuclei are in an environment characterized by the quadrupole coupling constant e²qQ/h of 3.3 MHz, asymmetry parameter η_Q of 0.42, isotropic shift δ_iso of 393 ppm, shift anisotropy δ_anis = δ_zz − (δ_xx + δ_yy)/2 of 150 ppm, and asymmetry factor η_S of 0.5. They are found to be temperature independent. The spin–lattice relaxation rate measured at 7.05 T is proportional to the temperature with T₁T = 135 s K. The mechanisms responsible for observed values of δ_iso, δ_anis, T₁T, and the enhanced Korringa constant are discussed.     

Low-temperature structure of Fe-Ni alloys

The Debye temperature (θ_M) of Fe-Ni alloys was obtained from measurements of the X-ray integrated intensity and the electrical resistivity at low temperatures from 4 to 300 K. A decrease of θ_M, implying a lattice softening effect, was found by lowering the temperature or increasing the iron concentration in the Invar region.     

Radiative decay of ρ and φ mesons in a chiral unitary approach

We study the ρ⁰ and φ decays into π⁺π⁻γ, π⁰π⁰γ and φ into π⁰ηγ using a chiral unitary approach to deal with the final state interaction of the MM system. The final state interaction modifies only moderately the large momenta tail of the photon spectrum of the ρ⁰→π⁺π⁻γ decay. In the case of φ decay the contribution to π⁺π⁻γ and π⁰π⁰γ decay proceeds via kaonic loops and gives a distribution of ππ invariant masses in which the f₀(980) resonance shows up with a very distinct peak. The spectrum found for φ→π⁰π⁰γ decay agrees with the recent experimental results obtained at Novosibirsk. The branching ratio for φ→π⁰ηγ, dominated by the a₀(980), is also in agreement with recent Novosibirsk results.     

Normal state DC electrical resistivity in RE-123 superconductors

We have examined the DC electrical resistivity of RE-123 superconductors (where RE is a rare earth element) on the basis of a model of correlated electron transfer arising from the electron-phonon interaction. The small polaron stabilization energy (ε_p) has been obtained from the fitting of the normal state resistivity data. A correlation between the lattice parameters of the unit cell and ε_p has been observed which is explained on the basis of fermiology.     

The large N limit of the O(N) Heisenberg spin system in two dimensions

The large N limit of the O(N) Heisenberg spin system in two dimensions (a lattice version of the non-linear σ-model) is analysed by the collective field technique. A compact expression for the mass gap and the β-function is obtained for arbitrary but fixed gN. The strong and weak coupling limits of this expression correctly reproduce the known results.     

The NN → NNη reaction close to threshold

The pp → ppη and np → npη reactions at energies near the η production threshold are studied in a non-relativistic one boson exchange model, where the N^* (1535 MeV) S11 resonance is excited through the exchange of π, η, and ω mesons and subsequently decays into an ηN pair. Energy integrated cross sections and energy spectra of the out going η's are reported. Providing NN and ηN final state interactions are taken into account coherently, the model reproduces both the scale and energy dependence of the cross section for the pp → ppη reactions up to 100 MeV. Final state interaction corrections due to the nucleon-nucleon and meson-nucleon forces influence strongly the scale and shape of the cross sections. The shape of the energy spectra of the outgoing η's provides a clear signature of the ηN force.     

Critical behavior of diluted Heisenberg ferromagnets with competing interactions

The pure and the site-diluted classical Heisenberg model on the face centered cubic (fcc) lattice with ferromagnetic exchange J_nn between nearest neighbors and antiferromagnetic exchange J_nn = −J_nn/2 between next nearest neighbors is studied by Monte Carlo simulation. Data are generated by the heat bath algorithm for lattice sizes L = 4, 8, 12, 16, 20 and 24, using histogram reweighting techniques and sampling up to several hundred configurations of the random site disorder. From a finite size scaling analysis both the critical temperature and the critical exponents are estimated. For the pure system, the data are in very good agreement with the critical exponent estimates 1/v ≈ 1.42, β/v ≈ 0.51 obtained from other methods (as a check of the accuracy of our approach, we also study the nearest neighbor model — where J_nn ≡ 0− and again obtain very good agreement with the known behavior). However, for the diluted systems evidence for a new universality class is found. While for concentration c = 0.875 of occupied sites strong crossover phenomena preclude us from giving exponent estimates, for c = 0.75 we find 1/v ≈ 1.2 and β/v ≈ 0.45. Possible reasons why the Harris criterion may not apply for this system are discussed. The application of this study to experiments on Eu_xSr_1−xS is briefly mentioned.     

Lattice sternheimer factor of Eu(OH)3 from Mössbauer spectral study

The Mössbauer spectra of powdered Eu(OH)₃ were recorded between 300 and 4.2 K, with ¹⁵¹SmF₃ as source. The spectra showed one single absorption line having definite width but without any fine structure. The lattice Sternheimer factor γ_∞ of ¹⁵¹Eu in Eu(OH)₃ was estimated with the help of these spectra as well as by using the values of some physical parameters obtained from spectral and magnetic susceptibility measurements on pure single crystal of Eu(OH)₃, The value of lattice Sternheimer factor (= -87) thus evaluated is slightly larger than that value ( = - 80) for ¹⁵⁴Eu in ethylsulphate host. This result is consistent with the γ_∞ values of other rare-earth ions and also with the results obtained from magnetic and optical experiments on Eu(OH)₃.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.