Diffusion of particles over triangular inhomogeneous lattice with two non-equivalent sites

We investigate the diffusion of particles adsorbed on a triangular lattice with deep and shallow sites. It is shown that the character of the particle migration depends substantially on the relative jump rates from the deep and shallow sites. The site inhomogeneity imposes specific correlation betweeen successive jumps: particles perform pairs of slow and fast jumps. General analytical expressions have been derived for the chemical and jump diffusion coefficients. We have calculated coverage dependencies of the diffusion coefficients and some thermodynamic quantities for different lateral interactions between the particles. The analytical data have been compared with the numerical data obtained by kinetic Monte Carlo simulations. The agreement between the results obtained by these quite different approaches is found to be very satisfactory.     

Surface diffusion of particles adsorbed on an inhomogeneous lattice with two non-equivalent sites

We investigate surface diffusion in a system of particles with the nearest neighbor pairwise lateral interaction adsorbed on a two-dimensional inhomogeneous lattice of square symmetry with deep and shallow sites. General analytical expressions for the chemical and jump diffusion coefficients have been derived in case of strong inhomogeneity. The expressions are valid for the inhomogeneous lattices with different geometries and dimensionalities. We have calculated the coverage dependencies of the tracer, jump and chemical diffusion coefficients for different temperatures using the real-space renormalization group (RSRG) method and compared the data with the numerical results obtained by the MC simulations. The coincidence between the data obtained by these quite different methods is rather good.     

Mössbauer spectra of EuPd3B0.5, an experimental test of a microscopic theory

The Mössbauer spectra of EuPd₃B_0.5 have been interpreted on the basis of the general microscopic theory previously developed by the authors. The theory connects the valence state of the europium ion with the distribution of the neighbouring boron atoms. The general characteristics of the present spectra are fairly well accounted for by theory, and an evolution with temperature of the line intensity ratio, in good agreement with the experimental data, is predicted.     

Lineshapes in quasi-elastic scattering from species hopping between non-equivalent surface sites

We present an analytic model applied to quasi-elastic scattering from an adsorbed surface species undergoing jump diffusion between adsorption sites described by a Bravais lattice combined with a basis of multiple points. The model allows for hops between adsorption sites which are both symmetrically and energetically inequivalent. We give results for 1-D hopping, which are applicable to a species jumping between the top and bridge sites along the [11?0] direction on an fcc-(110) surface or for jumps along a step edge. In 2-D, results for hopping between fcc and hcp hollow sites and between the bridge sites of an fcc-(111) surface are presented. These examples give characteristic signatures which will allow these forms of motion to be recognized in experimental data and will enable the underlying physical parameters to be extracted by comparison with the analytical forms derived here.     

Nuclear resonance scattering of synchrotron radiation by two non-equivalent crystal sublattices in57Fe3BO6

Pure nuclear Bragg reflections off a single crystal of⁵⁷Fe₃BO₆ consisting of two antiferromagnetic sublattices were studied using Mössbauer and synchrotron radiation. Energy and time spectra were measured of different reflections (h00), when either only one sublattice was reflecting, (300), or both sublattices at comparable strength either in antiphase, (500), or in phase, (700). Characteristic line shapes and quantum beat modulations revealed the interference of the scattering by the two sublattices.     

Quadrupole interactions at europium sites in Eu3V2O7 and Eu2VO4

The electric-quadrupole interactions at the Eu sites in Eu₃V₂O₇ and Eu₂VO₄ oxides have been studied at room temperature with¹⁵¹Eu Mössbauer spectroscopy. Both divalent and trivalent Eu ions were found in the oxides. The fraction of Eu²⁺ is 17.1(8)% in Eu₃V₂O₇ and 39.0(1.6) % in Eu₂VO₄. The values of the quadrupole coupling constant, eV_zzQ_g, obtained from the fits using a full Hamiltonian method are ?6.594(50) and ?8.043(65) mm/s for Eu³⁺, and ?13.168(402) and ?18.032(134) mm/s for Eu²⁺, respectively in Eu₃V₂O₇ and Eu₂VO₄. The magnitude of eV_zzQ_g in Eu₂VO₄ is the largest ever reported for Eu²⁺ in any Eu oxide system.     

Anomalous europium luminescence in LaF3

Optical spectra (absorption, emission, excitation, decay) and dielectric relaxation were measured for divalent europium and partially for ytterbium in lanthanum fluoride crystals. Optical absorption of LaF₃–Eu²⁺ contains not only asymmetric weakly structured band at 245 nm but also less intense bands at 330, 380 nm. Broadband Eu²⁺ emission at 600 nm appeared below 80 K with decay time 2.2 μs at 7.5 K. Emission at 600 nm is attributed to so-called anomalous luminescence. Bulk conductivity is directly proportional to absorption coefficient of Eu²⁺ bands. Dielectric relaxation peak of LaF₃–EuF₃ is attributed to rotation of dipoles Eu²⁺-anion vacancy. The long-wavelength absorption at 300–400 nm region are assigned to transitions from 4f⁷Eu²⁺ ground state to states of neighbouring fluorine vacancy.     

Luminescence and crystal-field fitting of two optical isomeric complexes of europium

FT-IR and Raman vibrational spectra and electronic emission spectra have been recorded for enantiomers of europium complexes with DBM: dibenzoylmethanate 1,2, and TTFA: 2-thenoyltrifluoroacetonate 3,4, employing the chiral ligands L_SS(+)- and L_RR(-)-4,5-pinene bipyridine. Contrary to the previously published X-ray data, where geometrical differences were stated to occur for particular enantiomers, the vibrational (and the emission) spectra of the individual optical isomers of a complex are not distinguishable. Using excitation into the Eu³⁺⁵D₂ multiplet term, the emission intensity is weak from ⁵D₁, whereas a complex structure is observed for the ⁵D₀→⁷F_J transitions. Features in the vibronic sidebands exhibit similar derived vibrational energies to those observed in the Raman spectra. Fittings of 25 4f⁶ crystal-field energy levels of 2 and 4 have been attempted with some approximations concerning the local Eu³⁺ environments. The ⁵D₀ emission lifetimes are monoexponential and are 0.5 (1,2) and 0.9 ms (3,4) at room temperature.     

Valence transition and magnetic ordering in Sn doped EuPd2Si2

The sharp, temperature induced, continuous valence transition in EuPd₂Si₂ is drastically changed by doping with Sn at the Si site upto 5 at. %. Only a first order valence transition occurs for a 3% Sn doped sample and the 2⁺ component which survives the valence transition orders magnetically at 4.2 K. No valence transition at all occurs for a 5% Sn doped sample right up to 1.9 K and magnetic ordering sets in around 30 K.     

Gaussian-type functions for the 3d Rydberg and 3d correlation orbitals in B to Ne and Al to Ar

Gaussian-type basis sets for the 3d Rydberg orbitals and 3d correlation orbitals are developed for the first- and second-row main group elements. The numbers of the Gaussian-type functions (GTFs) used for the 3d orbitals are 1–3 for the former elements and 1–4 for the latter elements. The 3d Rydberg orbitals for the first-row main group elements are close to the hydrogen (H) 3d orbitals, but those of the second-row main group elements are very different from H 3d except for Al. A two d or three d GTF set suffices to model the first-row main group elements, but at least four d GTFs are necessary for the second-row main group elements. The Rydberg GTF orbitals, consisting of three GTFs, are converted into correlating orbitals by introducing a single scaling factor. The correlation energies (CEs) calculated using these correlating orbitals cover 99.4–100% of those calculated using Dunning's three primitive GTFs for the first-row main group elements, and 94.9–99.7% of the CEs of Woon and Dunning's d's for the second-row main group elements. The resulting correlating 3d orbitals were tested by picking out F₂ and Cl₂, yielding spectroscopic constants close to or more accurate than those calculated by Dunning's 3d orbitals and Woon and Dunning's 3d orbitals.     

Mössbauer studies of 151Eu in europium oxalate, europium bissalen ammonium and europium benzoate

Although a number of europium water insoluble chelates have been prepared for several decades, the covalent nature of these compounds has never been established in any quantitative fashion. Shifts in the I.R. bands and conductivity measurements of these salts were hitherto used to qualitatively compare their molecular nature. In this communique we have used temperature coefficients of ¹⁵¹Eu Mössbauer spectra to determine the Debye temperatures (θ _D) of three europium chelates: namely europium oxalate, europium bissalen ammonium (recently reported) and europium benzoate and compared their θ _D with the measured θ _D of the known ionic EuF₃. Additionally, the mean square amplitude (<x ²) of these four compounds was computed and plotted as a function of temperature giving in each case a reasonable linear plot. It was interesting to note that the θ _D of the most ionic EuF₃ was greatest (283 ± 10 K) followed by the oxalate (166 ± 15 K), then the europium bissalen ammonium (133 ± 5 K) and lastly the europium benzoate with a θ _D of (105 ± 5 K).     

Hot electron faraday-effect and non-equivalent inter-valley transfer in n-type Ge

I.R. Faraday rotation measurements have been performed on n-type Ge at 200K and 300K up to electric field strength of $\text{5.5kV}\text{cm}$. The decrease in rotation is interpreted in terms of an increase in average effective mass of the electrons. Both, nonparabolicity of the conduction band and transfer of electrons to satellite valleys have to be considered for the interpretation of the results.     

On two correlation inequalities for Potts models

The Fortuin-Kasteleyn random cluster representation ofq-state Potts models is used to extend to everyq two correlation inequalities proven previously only for even values ofq.     

The relaxation rate of europium in europium iron garnet

The relaxation rate of europium ions in europium iron garnet (EulG) was estimated by assuming that the crystal field is modulated by optical phonons. Mixing J = 2 states into the J = 0 and J = 1 states by the static rhombic fields breaks the “triangle selection rule” and makes possible this mechanism.It is concluded that the mechanism proposed here can be responsible of the order of magnitude of the line width in the ferrimagnetic resonance of EuIG, but to obtain the observed isotropy of the line width, it is necessary to make rather arbitrary assumptions concerning the dependence of phonon spectral density on frequency.     

The luminescence of bismuth and europium in Ca4YO(BO3)3

A series of Bi³⁺/Eu³⁺ singly and doubly activated Ca₄YO(BO₃)₃ phosphors were synthesized by solid-state reaction method. The structures and photoluminescent properties of the phosphors were investigated at room temperature. Under UV excitation Bi³⁺ and Eu³⁺ show a high light output. Ca₄YO(BO₃)₃:Eu³⁺ has potential application as a phosphor for fluorescent lamps. The luminescence of Bi³⁺ and Eu³⁺ in Ca₄YO(BO₃)₃ resembles more that in the rare earth oxides than that in borates. The free oxygen ion in the host lattice, which is not bonded to any boron ions seems to be responsible for that. In this host lattice energy migration between linear Eu³⁺ chains occurs. The emission of Bi³⁺ is completely quenched when Eu³⁺ is co-doped. A model was proposed to explain it.     

Microstructural analysis for europium in CdTe films

CdTe:Eu films were grown by the pulsed laser deposition method on glass substrates. The targets were prepared with three different concentrations of Cd, Te and Eu employing CdTe and EuTe powders, homogenized by ball milling. X-ray diffraction measurements showed that the samples grown with a mixture of phases related with the structure of CdTe and EuTe, with a little increase of the lattice parameter. Scanning electron micrographs revealed that CdTe:Eu films presented a texture similar to solidified drops. Optical transmission spectroscopy was used for determinate the band gap of samples. Raman spectroscopy results shown broadening of Raman features associated with the structure of CdTe and EuTe with nominal Eu content.     

Interference and correlation of two independent photons

Two independent photons, produced through the spontaneous emission of two separate emitters subject to uncorrelated dephasing processes, can display two-photon interference (i.e. coalescence into a two-photon state) when they are incident simultaneously on a beamsplitter, in a manner analogous to that of twin photons produced through degenerate parametric fluorescence. The presence of dephasing processes, however, reduces the interference contrast (i.e. the probability of coalescence), by the ratio of the coherence time to the lifetime of the emitter. Received 9 September 2002 Published online 17 December 2002 RID="a" ID="a"e-mail: izo.abram@lpn.cnrs.fr     

Spectroscopic characteristic of Ce3+ at two different sites in Ba3Lu(BO3)3 under ultraviolet excitation

A series of Ba₃Lu_{1 ? x} Ce _x (BO₃)₃ phosphors with different x value were prepared by a conventional high-temperature solid-state reaction method, and their spectroscopic characteristic were systematically investigated. The excitation and emission spectra indicate Ce³⁺ can occupy two different Lu lattice sites, and shows different luminescence properties. The spectroscopic characteristic of Ce(1) and Ce(2) which include Stokes shift and crystal field depression were investigated. The investigation indicates that Ba₃Lu(BO₃)₃: Ce³ phosphor can have potential application in white-light emitting diodes due to the near-ultraviolet excitation band and blue light emission of Ce(2) in Ba₃Lu(BO₃)₃.     

Invasion percolation between two sites in two, three, and four dimensions

The mass distribution of invaded clusters in non-trapping invasion percolation between an injection site and an extraction site has been studied, in two, three, and four dimensions. This study is an extension of the recent study focused on two dimensions by Araújo et al. [A.D. Araújo, T.F. Vasconcelos, A.A. Moreira, L.S. Lucena, J.S. Andrade Jr., Phys. Rev. E 72 (2005) 041404] with respect to higher dimensions. The mass distribution exhibits a power-law behavior, P(m)∝m^−α. It has been found that the index α for p_e<p_c, p_c being the percolation threshold of a regular percolation, appears to be independent of the value of p_e and is also independent of the lattice dimensionality. When p_e=p_c, α appears to depend marginally on the lattice dimensionality, and the relation α=τ−1, τ being the exponent associated with cluster size distribution of a regular percolation via n_s∝s^−τ, appears to be valid.