Effect of zone boundaries on the pressure dependence of the superconducting transition temperature of aluminum

An attempt is made to explain the pressure dependence of the superconducting transition temperatureT _c of aluminum. The model calculation is essentially the same as that of Ott and Sorbello, with the difference that here the effect of zone boundaries on the density of states is included. The model calculations given here can explain both the low pressure and the high pressure data and suggest that one should include the effect of zone boundaries to explain theT _c data for the complete pressure range.     

Charge Density Variations or Stripes in YBa2Cu3O6+y

For many years it was believed that NMR on the YBa₂Cu₃O_6+y family of superconductors does not support charge density variations or stripes. We discuss the NMR data of YBa₂Cu₃O_6+y (y > 0.63) and show that large charge density variations are actually necessary in order to explain the data.     

Effect of pre-strain on the dislocation relaxation in zinc single crystal

The ultrasonic absorption,, in a zinc single crystal was measured as a function of temperature along three different orientations. The effect of different amounts of pre-strain on the absorption is investigated. It has been found that the energy,W, activating the relaxation process is independent of the amount of pre-strain, while the attempt frequency,v ₀, and the temperature of the absorption peak,T _m, are pre-strain dependent. The relaxation strength indicates a slight maximum in the considered range of pre-strain. Seeger theory has been used to explain some features of the experimental data, however, all the existing theories fail to explain the dependence of the parametersW andv ₀ on the orientation.     

Local Electric Field Effects in Microwave and dc Electrodynamics of High-T c Metal Oxides

The model of the paraelectric, which is close to the point of the Mott–Hubbard instability, is shown to explain the dc resistivity and Hall effect temperature behavior for high-T _c superconductor metal oxides (HTSC). In the ground state the current is carried by a liquid of boson-like pairs of carriers in upper and lower Hubbard bands. The Mott–Hubbard instability corresponds to the order-of-lattice-constant length of the mean free path and results in the temperature insensitivity of Drude conductivity. Nearly linear on T resistivity results from the Curie law via the local (acting) electric field. Fermion-like carriers, temperature excited over the energy of boson-like pair dissociation (pseudo gap), explain the temperature behavior of Hall effect. Available data are compared with the model.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7?δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7– crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2–x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

FATIGUE THERESHOLD IN A 2618A ALUMINIUM ALLOY

Abstract— This work is concerned with the determination of the fatigue threshold ΔK_th for an aluminium alloy. No dependency of ΔK_th with grain size was observed. Crack closure concepts were applied in order to explain the effect of mean stress and environment. The opening load P_op was determined by the compliance method. Results show no significant variation of P_op with environment (air or vacuum) but partially explain the influence of mean stress on crack growth rates. Fractographic analysis (SEM) of fracture surfaces and evolution of compliance diagrams suggest that a change of mechanism in crack propagation occurs when stage I changes to stage II growth.     

Application of the three-valence model of photorefraction to rhodium-doped barium titanate

Abstract

Small signal and light induced absorption data taken at 633 nm and 1.06 μm for different samples of rhodium-doped barium titanate (Rh : BaTiO₃) have been analysed. These data have been used in conjunction with the three-valence model theory of photorefraction to determine N _D, the total amount of rhodium in each sample. It is shown that the values of N _D calculated at the two wavelengths for individual samples are inconsistent with each other, although no evidence for other impurities in the samples has been observed. This indicates that a more complex theory than the three-valence model is necessary to explain the photorefractive processes in Rh : BaTiO₃, and that an additional photorefractive centre may not be sufficient to account for the observed discrepancies in N _D.     

Vibrational entropy of polyatomic solids: Metal carbides,metal borides,and alkali halides

It is shown that in the high temperature vibrational entropy of polyatomic solids, the influence of the atomic masses is described by an effective mass , where c _i is the concentration of mass M _i . This mass effect explains almost all of the differences between TiC, ZrC, and HfC in the recommended entropy data of Hultgren et al. (1973). Analogous results are found for VC, NbC, and TaC and for TiB₂, ZrB₂, and HfB₂. Mass differences also explain a major part of the measured entropy differences between alkali halides AB, with A=Li, Na, or K and B=F, Cl, Br, or I. Finally, we discuss different methods, including one used in the JANAF Thermochemical Tables, to estimate standard entropies from data on chemically related compounds.     

Dielectric relaxation and ionic conductivity studies of Na<Subscript>2</Subscript>ZnP<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Na₂ZnP₂O₇ compound was obtained by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction, infrared analysis and electrical impedance spectroscopy. The impedance plots show semicircle arcs at different temperatures and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. Dielectric data were analyzed using complex electrical modulus M* for the sample at various temperatures. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity σ _d.c. follows the Arrhenius relation. The near value of activation energies obtained from the analysis of M″ and conductivity data confirms that the transport is through ion hopping mechanism, dominated by the motion of the Na⁺ ions in the structure of the investigated materials.     

Charge bag model of superconductivity in intercalated layered compounds

In certain layered compounds the superconducting transition temperature (T _c ) is enhanced on intercalation. The superconductivity in these materials arises over a charge density wave background. To explain this enhancement inT _c we propose the charge bag model analogous to the spin bag model proposed earlier by Schrieffer as a mechanism for highT _c superconductivity.     

Study of doping effects on transition temperature of La2 −x (Ca,Na, K) x CuO4 superconductors

We have investigated doping effects on the transition temperature (T _c ) of La_{2 −x} M _x CuO₄ (M=Ca, Na and K) by incorporating the effects of two-dimensional (2D) acoustic plasmons in the framework of the strong coupling theory. The contributions from 2D acoustic plasmon mechanism toT _c have been obtained from a Fourier-transformed effective potential, which has been earlier found to be successful in predicting the composition dependence ofT _c in La_{2 −x} (Ba, Sr) _x CuO₄. The results obtained by us on the variation of transition temperature with composition (x) in La_2−x M _x CuO₄ superconductors are in reasonably good agreement with the available experimental data. This success has led to the conclusion that 2D acoustic plasmons are adequate to explain the pairing mechanism and the variation ofT _c with composition (x) in cuprate superconductors.     

Critical magnetic field in layered superconductors

The nature of the dependence H _c2(T) is greatly affected by the presence of magnetic impurities and their ordering at low temperatures. The scattering leads to a change in the sign of the curvature, and consequently to an increase in the value H _c2. The theory allows one to explain recent experimental data with the overdoped cuprates which exhibits behavior that is drastically different from the conventional picture. The calculations are in excellent agreement with the data.The authors are grateful to A. Mackenzie, J. Cooper, and S. Wolf for fruitful discussions. One of us (YNO) wishes to acknowledge the support of the Humboldt Foundation. The research of VZK is supported by the U.S. Office of Naval Research under contract no. N00014-94-F0006.     

Subdominant Interactions and Hc2 in UBe13

We discuss a model based on a field-induced mixture of two odd-parity irreducible representations to explain the unusual features of H _c2(T) in the heavy fermion compound UBe₁₃. We compare its predictions with recent pressure measurements as well as with the most prominent theoretical models which have been proposed up to now.     

Ion migration in high-T c superconductors and the nature of the conducting state ind-wave superconductors

The available experimental data on the ion migration in superconducting Y-123 and Bi-2223 films under the effect of high transport currents are summarized. Irrespective of structural perfection, electrostimulated diffusion of heavy ions and oxygen gives rise to the formation of new, stable phases and partial degradation of superconductivity. A model taking into account the proximity of the superconducting, structural, and magnetic transitions in high-T _c materials is used to explain ion migration as the response of the system of ions and coherentd electrons to the external electric field. Thed-wave symmetry superconducting state is shown to possess combined, rotational + translational invariance, rotational symmetry being due to the motion of electrons in the curl field of the ions, playing the role of topological defects.     

Magnetic ordering in Cu-Zn ferrite

The variation of magnetization with temperature of the Zn _x Cu_1–x Fe₂O₄ system has been obtained between 300 K and the Néel temperature at a constant magnetic field of 5.57×10⁵ A m^–1 for x=0 to 0.8. The observations indicate the existence of a Yafet-Kittel (Y-K) type of magnetic ordering in the mixed ferrites. A molecular field analysis of the Y-K spin-ordering using a three-sublattice model is shown to explain the experimental data satisfactorily. For the sake of verification, Néel temperatures of Cu-Zn ferrites were also determined from Mössbauer studies.     

Thermal expansion of bismuth telluride

The results of X-ray and dilatometric measurements of the thermal expansion of bismuth telluride in the temperature range of 4.2–850 K have been critically analyzed. The joint statistical processing of the experimental data has been performed by the least squares method and the most reliable temperature dependences of the linear thermal expansion coefficients along the principal crystallographic axes α _a and α _c and the average linear coefficient [`(a)] _L\bar \alpha _L , as well as the density of Bi₂Te₃, have been recommended. The results indicate that the linear thermal expansion coefficients along the directions parallel and perpendicular to the cleavage planes decrease with an increase in the temperature in a narrow temperature range. At temperatures below 298 K, the character of the temperature dependences of the linear thermal expansion coefficients of Bi₂Te₃ has been analyzed in terms of the anharmonicity of the chemical bonding forces in the layer structure and anisotropy of the elastic constants. In terms of the deviations of the composition of the Bi₂Te₃ compound from the stoichiometric one and the real (defect) structure of the compound, a model has been proposed to explain the minimum in the (α _a T) dependence of Bi₂Te₃ near the melting temperature of bismuth. A method for calculating the temperature dependence of the linear thermal expansion coefficients of the anisotropic layer crystals using the data on the specific heat has been discussed, which provides good agreement of the calculated linear thermal expansion coefficients with the experimental data within the accuracy of the measurements of the linear thermal expansion coefficients.     

Test of the Anderson–Stuart model and correlation between free volume and the ‘universal’ conductivity in sodium silicate glasses

Experimental ionic conductivity σ and activation energy E _A data in the binary sodium silicate system are reviewed. Analysis and brief discussion based on 48 glasses in a wide compositional range (between 4 and 45 Na₂O mol%) are presented. Emphasis is placed on the application of the Anderson–Stuart model to describe the variation of activation energy E _A with sodium concentration. In this analysis were considered experimental parameters such as shear modulus G and relative dielectric permittivity ε, also in wide compositional range. A ‘universal’ finding is obtained using log₁₀σ vs. E _A /k _B T in 47 of the 48 glasses investigated, where E _A is the activation energy for conduction, k _B is the Boltzmann constant and T is the absolute temperature. Using conductivity and molar volume from density data, both measured at 20 °C in the same glasses, it was found a remarkable common cubic scaling relation between conductivity enhancement of the free volume due to increase in alkali content. The drastic drop in conductivity by 16 orders of magnitude for so many ion-conducting binary sodium silicate glasses is then caused by structure and ion content. The effects of shear modulus, relative dielectric permittivity and free volume are taken into account, as also the problem of phase separation. In particular, it is suggested that the glass network expansion, which is related to the available free volume, is a parameter that could partially explain the increase in ionic conductivity for this binary system.     

Magnetic Studies of Spin Wave in Fe/Ag Multilayer Films

Magnetic properties of Fe/Ag multilayer films are investigated and examined versus Fe layer thickness t _Fe. As a result, spontaneous magnetization M(T) temperature dependence has been revealed to be well described by a T ^3/2 law in all multilayer films (7 ?≤t _Fe≤60 ?). Spin-wave theory based on anisotropic ferromagnetic system has been also used to explain magnetization temperature dependence. For Fe layer thickness, approximate values for J ₀ bulk exchange interaction and J _s surface exchange interaction have been estimated. First principle calculations based on density functional theory (DFT) and Korringa–Kohn–Rostoker (KKR)—coherent potential approximation (CPA) method—combined with Local Spin Density Approximation (LSDA), are performed as well. Magnetic moment, in fcc Ag_1−x Fe _x and bcc Fe_1−x Ag _x systems, versus x is presented and discussed in terms of Fe content on magnetic coupling. Reasonable agreement between experimental data and theoretical calculations is highlighted.     

Microwave Properties of In Situ Grown MgB2 Superconducting Thin Films

The electrodynamic response at 20 GHz of c-axis oriented MgB₂ superconducting thin films is reported. Mg-rich Mg-B precursor samples were grown on MgO and Al₂O₃ single crystal substrates by a d.c. planar magnetron sputtering technique, and subsequently annealed in situ at 800°C for 10 min in a In-sealed Nb box in the presence of saturated Mg vapor. The films were characterized by a variety of structural and electronic techniques including XRD, EDS, STM-AFM analyses, and transport measurements. The dependence of the surface impedance from temperature and radiofrequency (r.f.) field amplitude was measured via a dielectric resonator technique. Temperature data clearly confirm the s-wave nature of the newly discovered superconductor, even if the value of the energy gap is smaller than BCS prediction. An effective two-band model can be applied to quantitatively explain the experimental results. In spite of previous reports claiming the absence of weak link behavior in MgB₂, the power dependence show that granularity governs the performance of these films in the microwave region.     

An introduction to chaotic and random time series analysis

Chaos refers to the paradoxical evolution of a deterministic system in a way that is disordered—to the point that the time dependence of the physical variables appears stochastic. A need for data analysis procedures to detect, model, and separate chaotic and random processes has arisen from this recently understood paradigm. Many special techniques have been designed for chaotic data; the unification of these with conventional time series analysis is a developing field. This tutorial uses examples to explain the origin of chaotic behavior and the relation of chaos to randomness. Two powerful mathematical results are described: (1) a representation theorem guarantees the existence of a specific time-domain model for chaos and addresses the relation between chaotic, random, and strictly deterministic processes, and (2) a theorem assures that information on the behavior of a physical system in its complete state space can be extracted from time-series data on a single observable. These theorems form the basis of a practical data analysis scheme, as follows: given N observations of a variable Y, i.e., {Y_n, n = 1,2,3, …, N}, define X = A * Y and maximize, with respect to the parameters of A, a function H(X) that measures degree of chaos. This maximization is carried out by minimizing the dimension covered by the data in the M-dimensional space (X_n, X_n+1, X_n+2, …, X_n+M?1). The resulting dimension D either (1) increases continuously with M or (2) levels off and remains constant (= D_max) beyond a certain point. In case (1) or if D_max is quite large X is random; if case (2) holds and D_max is small, we have chaos. The inverse of A found in this procedure is an estimate of the filter in the moving average model for Y.