LEED crystallographic studies of the (311) surface of nickel: I. Structural analysis

An intensity analysis with low-energy electron diffraction is reported for the stepped (311) surface of nickel. Intensity versus energy curves were measured for 14 diffracted beams at normal incidence, and comparisons made with I(E) curves calculated with the layer-doubling method. The latter showed some numerical instabilities at particular energies and topmost spacings. Evidence is presented for detecting such problems by plotting, at fixed energy, emergent beam intensities as a function of topmost spacing. Calculated and experimental intensity curves were assessed with reliability indices proposed both by Zanazzi and Jona and by Pendry. Good correspondences were obtained with an unreconstructed surface model in which the topmost spacing is contracted by about 14% from the bulk value.     

Structure determination of the (100) surface of rhodium by low energy electron diffraction

Elastic low-energy electron diffraction intensity data have been measured as a function of energy for two directions of incidence for the (100) surface of rhodium. The dynamical perturbation programs of Van Hove and Tong have been used for analysing these new experimental data, and it is concluded that the normal face-centered cubic registry is maintained to the surface layer. A preliminary comparison between measured and calculated I(E) curves indicates the topmost interlayer spacing to be 1.96 ± 0.10 Å, and therefore possibly slightly expanded from the bulk interlayer spacing of 1.90 Å.     

Structural determinations of the Rh(100) and Cu(111) surfaces using the reliability factor proposed for LEED by Zanazzi and Jona

The reliability factor (R) proposed for LEED by Zanazzi and Jona has been applied to experimental and calculated intensities for the (100) surface of rhodium and for the (111) surface of copper. The calculations used the dynamical perturbation programs of Van Hove and Tong. For each metal, phase shifts were calculated both from a band structure potential and from a potential calculated with a 13 atom cluster. For Cu(111) the I(E) curves from the two potentials were indistinguishable visually and gave similar minimum R values (0.132 and 0.136); the two potentials used for rhodium showed somewhat greater differences. The approach described by Zanazzi and Jona has been supplemented by a simple statistical analysis of the errors involved in the predicted geometries. This study indicates that the topmost interlayer spacing in Cu(111) is contracted by 4.1 ± 0.6% from the bulk value; in Rh(100) the top spacing equals the bulk value to within 3%.     

Calculation of the surface energy of bcc transition metals by using the second nearest-neighbor modified embedded atom method

The surface energies for 24 surfaces of all bcc transition metals Fe, Cr, Mo, W, V, Nb and Ta have been calculated by using the second nearest-neighbor modified embedded atom method. The results show that, for all bcc transition metals, the order among three low-index surface energies E_(1 1 0) < E_(1 0 0) < E_(1 1 1) is in agreement with experimental results and E_(1 1 0) is also the lowest surface energy for various surfaces. So that from surface energy minimization, the (1 1 0) texture should be favorable in the bcc films. This is also consistent with experimental results. The surface energy for the other surfaces increases linearly with increasing angle between the surfaces (h k l) and (1 1 0). Therefore, a deviation of a surface orientation from (1 1 0) can be used to estimate the relative values of the surface energy.     

Theoretical study of spin-dependent bremsstrahlung from ferromagnetic Fe(110)

By means of a nonrelativistic one-step model Green function formalism of photoemission, spin-up- and spin-down-induced ultraviolet bremsstrahlung spectra have been calculated for ferromagnetic Fe(110) for several angles of incidence. Comparison with recent experimental data shows good agreement with regard to the existence and E(k_∥) despersion of both minority and majority spin features. Calculation of the corresponding bulk band structures and k_∥-resolved layer-by-layer quasi-particle densities of states permits a physical interpretation: most bremsstrahlung features can be understood in terms of bulk interband transitions, while one is due to surface resonance.     

Luminescence decays with underlying distributions: General properties and analysis with mathematical functions

In this work, an analysis of the general properties of the luminescence decay law is carried out. The conditions that a luminescence decay law must satisfy in order to correspond to a probability density function of rate constants are established. From an analysis of the general form of the luminescence decay law, it is concluded that the decay must be either exponential or sub-exponential for all times, in order to be represented by a distribution of rate constants H(k). Sub-exponentiality is nevertheless not a sufficient condition. Only decays that are completely monotonic have a probability density function of rate constants. The construction of the decay function from cumulant and moment expansions is studied, as well as the corresponding calculation of H(k) from a cumulant expansion. The asymptotic behavior of the decay laws is considered in detail, and the relation between this behavior and the form of H(k) for small k is explored. Several generalizations of the exponential decay function, namely the Kohlrausch, Becquerel, Mittag-Leffler and Heaviside decay functions, as well as the Weibull and truncated Gaussian rate constant distributions are discussed.     

Spin-injection efficiency and magnetoresistance in a hybrid ferromagnetic-semiconductor trilayer with interfacial barriers

We present a self-consistent model of spin transport in a ferromagnetic (FM)-semiconductor (SC)-FM trilayer structure with interfacial barriers at the FM-SC boundaries. The SC layer consists of a highly doped n²⁺ AlGaAs-GaAs 2DEG while the interfacial resistance is modeled as delta potential (δ) barriers. The self-consistent scheme combines a ballistic model of spin-dependent transmission across the δ-barriers, and a drift-diffusion model within the bulk of the trilayer. The interfacial resistance (R_I) values of the two junctions were found to be asymmetric despite the symmetry of the trilayer structure. Transport characteristics such as the asymmetry in R_I, spin-injection efficiency and magnetoresistance (MR) are calculated as a function of bulk conductivity σ_s and spin-diffusion length (SDL) within the SC layer. In general a large σ_s tends to improve all three characteristics, while a long SDL improves the MR ratio but reduces the spin-injection efficiency. These trends may be explained in terms of conductivity mismatch and spin accumulation either at the interfacial zones or within the bulk of the SC layer.     

Ground-state properties of the one dimensional electron liquid

We present a theory of the pair distribution function g(z) and many-body effective electron-electron interaction for the one dimensional (1D) electron liquid. Our approach involves the solution of a zero-energy scattering Schrödinger equation for where we implemented the Fermi hypernetted-chain approximation including the elementary diagram corrections. We present numerical results for g(z) and the static structure factor S(k) and obtain good agreement with data from diffusion Monte Carlo studies of the 1D system. We calculate the correlation energy and charge excitation spectrum over an extensive range of electron density. Furthermore, we obtain the static correlations in good qualitative agreement with those calculated for the Luttinger liquid model with long-range interactions.     

Manifestation of vortex depinning transition in nonlinear current-voltage characteristics of polycrystalline superconductor Y1−xPrxBa2Cu3O7−δ

We present our recent results on the temperature dependence of current-voltage characteristics for polycrystalline Y_1−xPr_xBa₂Cu₃O_7−δ superconductors with x=0.0, 0.1 and 0.3. The experimental results are found to be reasonably well fitted for all samples by a power like law of the form V=R(I−Ic)^a(T). Here, we assume that a(T)=1+Φ₀IC(T)/2πkBT and IC(T)=IC(0)(1−T/TC)^3/2 for the temperature dependences of the power exponent and critical current, respectively. According to the theoretical interpretation of the obtained results, nonlinear deviation of our current-voltage characteristics curves from Ohmic behavior (with a(TC)=1) below TC is attributed to the manifestation of dissipation processes. They have a characteristic temperature Tp defined via the power exponent as a(Tp)=2 and are related to the current induced depinning of Abrikosov vortices. Both TC(x) and Tp(x) are found to decrease with an increase of Pr concentration x reflecting deterioration of the superconducting properties of the doped samples.     

Diffusion in very chaotic hamiltonian systems

We study nonintegrable hamiltonian dynamics: H(I,θ) = H₀(I) + kH₁(I,θ), for large k, that is, far from integrability. An integral representation is given for the conditional probability P(I,θ, t¦I₀, θ₀, t₀) that the system is at I, θ at t, given it was at I₀, θ₀ at t₀. By discretizing time into steps of size ?, we show how to evaluate physical observables for large k, fixed ?. An explicit calculation of a diffusion coefficient in a two degrees of freedom problem is reported. Passage to ? = 0, the original hamiltonian flow, is discussed.     

Infrared spectroscopic studies of the rare gas atom perturbed S1(0) rovibron band of solid parahydrogen

We report high resolution infrared absorption studies of rare gas (Rg) atom doped solid parahydrogen in the hydrogen S₁(0) region around 4486 cm⁻¹. At low Rg atom concentrations (∼0.1%), satellite transitions appear in the S₁(0) region due to rovibrational excitation of parahydrogen molecules with one nearest-neighbor Rg atom. The Ne satellite feature differs qualitatively from the Ar, Kr, and Xe satellite features for reasons described within. The frequency of the S₁(0) satellite features linearly shift to lower energy as the polarizability of the Rg atom increases while the absorption coefficients increase with the square of the Rg atom polarizability. Rotational calculations are performed for H₂ with a nearest-neighbor Rg atom assuming a rigid hexagonal close-packed lattice structure. The calculated fine structure of the S₁(0) satellite features agree qualitatively with lifting of the 2J+1 degeneracy of the v = 1, J = 2 upper state caused by the anisotropy in the Rg-H₂ intermolecular potential. The discrepancy between the calculated and measured Rg atom S₁(0) satellite features may signal partial delocalization of the J = 2 roton onto neighboring parahydrogen molecules.     

Dynamic structure function of liquid 3He

The dynamic structure function S(Q, ω) of liquid ³He is evaluated in the impulse approximation, based on a theoretically calculated momentum distribution n(k). The gap of n(k) at the Fermi surface gives rise to discontinuities in the slope of S(Q = const., ω) which may be detectable in neutron- or γ-ray scattering experiments.     

Electronic structure and magnetic properties of the compound CeCuIn

Magnetization, magnetic susceptibility, electrical resistivity, thermoelectric power and X-ray photoemission measurements were performed on a polycrystalline sample of CeCuIn. This compound crystallizes in a hexagonal structure of the ZrNiAl type. The magnetic data indicate that CeCuIn remains paramagnetic down to 1.9 K with a paramagnetic Curie temperature of −13 K and an effective magnetic moment equal to 2.5 μ_B. The electrical resistivity has metallic character, yet in the entire temperature range studied here, it is a strongly nonlinear function of temperature. The temperature dependence of the thermoelectric power is dominated by a small positive maximum near 76 K and a deep negative minimum at about 16 K. Above 150 K the thermopower exhibits a Mott's type behavior. The positive sign of the Seebeck coefficient in this temperature region indicates that the holes are dominant charge and heat carriers. The structure of Ce 3d_5/2 and Ce 3d_3/2 XPS spectra has been interpreted in terms of the Gunnarsson-Schönhammer theory. Three final-state contributions f⁰, f¹ and f² are clearly observed, which exhibit a spin-orbit splitting Δ_SO≈18.7 eV. The appearance of the 3d⁹f⁰ component is a clear evidence of the intermediate valence behavior of Ce. From the intensity ratio I(f⁰)/[I(f⁰)+I(f¹)+I(f²)] the 4f-occupation number is estimated to be 0.95. In turn, the ratio I(f²)/[I(f¹)+I(f²)]=0.08 yields a measure of the hybridization energy that is equal to 45 meV.     

Scale-free networks by super-linear preferential attachment rule

A network growth model with geographic limitation of accessible information about the status of existing nodes is investigated. In this model, the probability Π(k) of an existing node of degree k is found to be super-linear with Π(k)∼k^α and α>1 when there are links from new nodes. The numerical results show that the constructed networks have typical power-law degree distributions P(k)∼k^−γ and the exponent γ depends on the constraint level. An analysis of local structural features shows the robust emergence of scale-free network structure in spite of the super-linear preferential attachment rule. This local structural feature is directly associated with the geographical connection constraints which are widely observed in many real networks.     

Effects of cross-correlation between the real and imaginary parts of the quantum noise in a two-mode saturation laser system

A two-mode saturation laser model with cross-correlation between the real and imaginary parts of the quantum noise is considered. The laser intensity Langevin equation and corresponding Fokker-Planck equation are derived by the phase-locking method. The effects of the cross-correlation strength λ between the real and imaginary parts of quantum noise and the cavity decay constant K on the steady-state intensity distribution Q(I₁,I₂), the mean light intensity 〈I〉, the normalization autocorrelation λ₁₁(0) and cross correlation λ₁₂(0) are studied by numerical calculation. The results show that as λ increases the Qs(I₁,I₂) show two extrema, and λ almost does not affect the 〈I〉, λ₁₁(0) and λ₁₂(0) when the laser system is operated far above threshold. Nevertheless, when the laser system is operated at and below threshold, λ makes the curves of Qs(I₁,I₂) have the higher peak and drop faster. Furthermore, it enhances the deviation of λ₁₁(0) and λ₁₂(0) and lessens the mean light intensity 〈I〉 when the laser system is operated at and below threshold.     

On the phase superlattice in a vanadium dioxide film

A statistical theory has been proposed for the formation of a heterostructure of alternating metallic and semiconducting phases in a single-crystal VO₂ film on a substrate. The size distribution function of metallic domains is found as a function of temperature. The temperature dependence of the intensity of light transmitted through the film I(T) is obtained. The depth and width of the dip in curve I(T) near a phase transition, calculated in the framework of the developed theory, agree with the experimental data.     

Doublet coulomb-nuclear scattering length and other parameters of the effective-range function for proton-deuteron scattering from an analysis of present-day data

Parameters of the effective-range function K(k ²) having a pole are found from the results that were obtained by calculating the S-wave phase shifts for doublet proton-deuteron scattering and the binding energies of three-nucleon nuclei on the basis of Faddeev equations and within the N/D method and which are known from the literature. The convergence of the expansion of K(k ²) in powers of the momentum is studied. The energy of the proton-deuteron resonance corresponding to the virtual triton state is calculated.     

Effective-range function for doublet nd scattering from an analysis of modern data

The parameters of the generalized effective-range function K(k ²) having a pole are found by using the results that were obtained by calculating the S-wave phase shift δ(E) for doublet nd scattering and the triton binding energy on the basis of Faddeev equations and within the N/D method and which were presented in the literature. The convergence of the expansion of K(k ²) in powers of momentum is studied. The binding energy of the virtual triton and the residues of the partial-wave scattering amplitudes at the poles corresponding to the bound and virtual states are calculated. Correlations between the binding energies of the bound and virtual states of the triton, on one hand, and the doublet scattering length for nd interaction, on the other hand, are considered. The function K(k ²) is also calculated within a two-body model featuring various potentials.