Preparation and electrical properties of sintered oxides composed of Mn1.5Co(0.25+X)Ni(1.25-X)O4 ( $$0 \leqq X \leqq 0.75$$) with a cubic spinel structure

Monophase cubic spinel-type oxides of Mn_1.5Co_(0.25+X)Ni_(1.25-X)O₄ ( ) were prepared through the oxidation of specimens sintered at 1400 °C. The oxides with composition are focused on in this study, as the oxides with composition did not convert into a monophase cubic spinel structure. The electrical conductivity of the sintered bodies was confirmed to increase exponentially with increasing temperature. In the composition range of , both the electrical conductivity and the Seebeck coefficient increased with increasing X. The oxides with composition between were n-type semiconductors, whereas those with were p-type. It was concluded that electrical conduction in the specimens is controlled by small polaron hopping.     

Frictional Effects in Thin 3He Films

The recent high-precision torsional oscillator experiments of Casey et al. involving thin films of normal liquid ³He showed that the film decouples from the substrate with a time constant which is proportional to T ⁻¹ where T is the absolute temperature. We interpret this experiment by adapting a theory due to Meyerovich which was developed for dilute ³He-⁴He mixtures flowing between two relatively smooth plates. The analysis of the experiment confirms the central idea that varies as T ⁻¹. The variation of with film thickness, d, is affected by the change in the shape of the free surface of the film, due to van der Waals forces, as the film becomes thinner.     

Finite-size Effects on the Thermal Resistivity of 4He Near the Superfluid Transition

We present measurements of the thermal resistivity () near the superfluid transition temperature of ⁴He at saturated vapor pressure and confined in glass capillary arrays with rectangular capillary cross-sections of spacing L = 1 μm, width m, and length mm. We expect the finite-size effect in this rectangular geometry to provide a good approximation to that in the ideal parallel-plate geometry. The data coincide within our resolution with previous measurements for cylindrical capillaries of 1 m radius, indicating that the finite-size scaling-functions for these two geometries are indistinguishable. This stands in contrast to the scaling functions for static properties which, near the transition temperature, depend on the dimensionality of the confinement. The results are consistent with recent Monte Carlo and spin-dynamics simulations, and with renormalization-group calculations for capillaries with square cross section and .     

On Larkin-Imry-Ma State of 3He-A in Aerogel

Superfluid ³He-A shares the properties of spin nematic and chiral orbital ferromagnet. Its order parameter is characterized by two vectors and . This doubly anisotropic superfluid, when it is confined in aerogel, represents the most interesting example of a system with continuous symmetry in the presence of random anisotropy disorder. We discuss the Larkin-Imry-Ma state, which is characterized by the short-range orientational order of the vector , while the long-range orientational order is destroyed by the collective action of the randomly oriented aerogel strings. On the other hand, sufficiently large regular anisotropy produced either by the deformation of the aerogel or by applied superflow suppresses the Larkin-Imry-Ma effect leading to the uniform orientation of   . The interplay of regular and random anisotropy allows us to study many different effects.      

EPR study of V2O5–P2O5–Li2O glass system

glass system, with 0 < x 50 mol%, was prepared and investigated by EPR method. For low content of V₂O₅ all the spectra present a hyperfine structure typical for isolated V⁴⁺ ions. With the increasing of V₂O₅ content, the EPR absorption signal showing hyperfine structure is superposed by a broad line without hyperfine structure characteristic for clustered ions. At high V₂O₅ content, the vanadium hyperfine structure disappears and only the broad line can be observed in the spectra. Spin Hamiltonian parameters g , g , A , A , dipolar hyperfine coupling parameters, P, and Fermi contact interaction parameters, K, have been calculated.The composition dependence of line widths of the first two absorptions from the parallel band and of the broad line characteristic to the cluster formations was also discussed.     

The Two-dimensional Vibrating Reed: A Study of Anisotropic Pinning in High-temperature Superconductors

The pinning of flux lines on point-like defects in superconductors depends on both the magnitude and the symmetry of the order parameter. We study the anisotropy of pinning in untwinned single crystals of YBa₂Cu₃O and crystals of Bi₂Sr₂CaCu₂O employing a modified vibrating reed technique with two degrees of freedom. The pinning potential is calculated for an anisotropic d-wave superconductor in the approximation of a single pancake vortex interacting with a single point defect. The response of the reed to pinning potentials with two- and fourfold symmetry is modeled. The appearance of a twofold symmetry is demonstrated for YBa₂ Cu₃O crystals. The effect of the fourfold symmetric component is masked by flux creep effects in our experiments. Dedicated to Frank Pobell, former editor of JLTP.     

Fermi surface and extended van Hove singularity in the non-cuprate layered perovskite superconductor Sr2RuO4

With high-resolution (22 meV) angle-resolved photoemission spectroscopy, the Fermi surface of the first copper free layered-perovskite superconductor, Sr₂RuO₄, was determined. We observed three bands to cross the Fermi energy in qualitative agreement with LDA band structure calculations; one electron-like surface encircling the point in the projected Brillouin zone, and two hole-like surfaces around the point. The most striking aspect of the measurements is the observation of an extended van Hove singularity. It is located 17 meV below the Fermi energy and extends around the point for about 0.2 Å^–1 along both the — — and the — — directions.These observations demonstrate that van Hove singularities near the Fermi surface are a more generic feature of layered oxides, and call for a clarification of their exact role in oxide superconductivity.We are grateful to D. Singh for making available his band structure calculations. D. H. Lu thanks the VW Foundation for financial support.     

Tunneling with very long relaxation times in glasses,organic materials,and Nb-Ti-H (D)

The long-time (t=10–200 h) heat release from glasses, from organic materials, and from Nb-Ti-H (D) was measured at 30T70 mK. For Suprasil W glass, Dimethyl-Siloxan, Stycast 1266, Stycast 2850 FT, Vespel, and for Nb-Ti-H (D) with various Ti and D concentrations, we found . Typical values are = 0.05 nW/g for the organic materials and for Nb-Ti-H (D) and = 0.005 nW/g for the glass att=100 h after cooldown from room temperature. For charging temperaturesT _i <5 K, we find the predicted dependence (investigated for Suprasil W glass and for Nb-Ti-D). The observed time and temperature dependences agree with predictions of the conventional two-level tunneling model for amorphous materials even at these very long times. No heat release was observed for Teflon, graphite, and Al₂O₃.     

Polarization of Dielectrics by Acceleration

We argue that acceleration induces electric polarization in usual dielectrics. Both accelerations in superfluid ( and ) participate in the medium polarization. Excitations contribution to the polarization is calculated at low temperatures. Estimates of the effect show order of magnitude agreement with recent experimental results on electric effect of superflow.     

Construction of binary minimal product parity-check matrices

A parity-check matrix H of a given code is called minimal if it has minimum number of nonzero entries among all parity-check matrices representing . Let and be two binary linear block codes with minimal parity-check matrices H ₁ and H ₂, respectively. It is shown that, using H ₁ and H ₂, one can efficiently generate a minimal parity-check matrix for the product code .     

Correlation between magnetic hysteresis and magnetic relaxation in YBaCuO single crystals

The dependence of the magnetic momentm obtained from the hysteresis loops on the speed of the magnetic field sweep =dH _ext/dt is explained on the basis of Anderson's interpretation of the magnetic flux creep. In addition, a phenomenological model is suggested which predicts a linear dependence ofm on ln with the slope m/ ln , numerically equal to the relaxation rate m/ ln(t) from the usual magnetic relaxation. Such linear relations betweenm and ln were observed experimentally in single crystals of YBaCuO. Preliminary experiments on the complementary time dependent relaxation ofm after a simulated step change ofH _ext gave mostly relaxation rates close to the predicted values. The model here presented also enables one to compare the critical state in the superconductor at a field sweep rate with the critical state at some timet _eff after a step change ofH _ext. The values of analyzed in our experiments actually correspond to the critical state at timest _eff between0.04 and4 sec after an imaginary large step change ofH _ext.     

Methods for the reduction of the micropipe density in SiC single crystals

Micropipes are very harmful for SiC devices. Even one micropipe in the active area can destroy a high-voltage SiC device. Therefore, it is necessary to reduce the density of micropipes in SiC single crystals. In the present paper, we proposed methods for reducing micropipes. Restriction of screw dislocations and decrease of inclusions are the key factors to reduce the number of micropipes. (0 0 0 1) Si-face, and crystal faces acted as growth surface in different experiments. Active carbon was appended to act as carbon source. The crucible and active carbon were subjected to X-ray diffraction investigation before and after growth. The experimental results indicate that the activity of the graphite crucible was low, and it decreased with the progressing crystal growth, which increased the probability of micropipe formation. Appending active carbon can act as ample carbon source for crystal growth. The reduction of micropipes was achieved by the restrained formation of Si liquid phase. Using and crystal faces as the growth surfaces the generation of micropipes was restricted, as no new micropipe generated on the and crystal faces. At the same time, the density of edge dislocations is reduced considerably.     

Optimal-control methods for two new classes of smart obstacles in time-dependent acoustic scattering

Time-dependent acoustic scattering problems involving “smart” obstacles are considered. When hit by an incident acoustic field, smart obstacles react in an attempt to pursue a preassigned goal. Let be the three-dimensional real Euclidean space, and let be a bounded simply connected open set with a Lipschitz boundary characterized by a constant acoustic boundary impedance χ, immersed in an isotropic and homogeneous medium that fills . The closure of Ω will be denoted as . When hit by an incident field, the obstacle Ω pursues the preassigned goal through the action of a control input acting on its boundary (i.e., a quantity with dimensions of a pressure divided by a time). The obstacles considered in this paper monitor the control input acting on their boundaries in order to achieve one of the following goals: (i) be furtive in a given set of the frequency space, and (ii) appear in a given set of the frequency space and outside a given set of containing Ω and Ω _G as similar as possible to a “ghost” obstacle Ω _G having boundary acoustic impedance χ _G . It is assumed that and . The problem corresponding to the first goal will be called the definite-band furtivity problem, and the problem corresponding to the second goal will be called the definite-band ghost-obstacle problem. These two goals define two classes of smart obstacles. In this paper, these problems are modeled as optimal-control problems for the wave equation introducing a control input acting on the boundary of Ω for time . The cost functionals proposed depend on the value of the control input on the boundary of the obstacle and on the value of the scattered acoustic field generated by the obstacle on the boundary in the “furtivity case”, and on the boundary of a suitable set containing Ω and Ω _G in the “ghost-obstacle case”. Under some assumptions, the use of the Pontryagin maximum principle allows us to formulate the first-order optimality conditions for the definite-band furtivity problem and for the definite-band ghost-obstacle problem as exterior problems outside the obstacle for a system of two coupled wave equations. Numerical methods to solve these exterior problems are developed by extending previous work. These methods belong to the class of the operator-expansion methods that are highly parallelizable. Numerical experiments proving the validity of the control problems proposed as mathematical models of the definite-band furtivity problem and definite-band ghost obstacle problem are presented. The numerical results obtained with a parallel implementation of the numerical methods developed are discussed and their properties are established. The speed-up factors obtained using parallel computing are really impressive. The website: http://www.econ.univpm.it/recchioni/w11 contains animations and virtual reality applications relative to the numerical experiments.     

Representation Attacks on the Braid Diffie-Hellman Public Key Encryption

The Braid Diffie–Hellman public key cryptosystem is based on the Diffie–Hellman version of a decomposition problem (DP) in the braid group B _n . We propose a linear algebra attack on DP via the faithful Lawrence–Krammer representation . For generic and sufficiently long instance braids we recover the -image of the private key using just one matrix inversion.     

Stability analysis of a low energy vortex configuration

Recent work using the London theory of anisotropic superconductivity has shown that, for small values ofB and large values of the anisotropy, a lattice of vortex lines parallel to is not necessarily the minimum energy configuration. When is not parallel to a symmetry axis, the “double lattice”—a lattice of lines parallel to plus a lattice of lines parallel to —can have a lower energy. The question arises, however, of whether some other configuration can have a yet lower energy. To investigate this question, we perform a stability analysis of the double lattice. We find that its energy can indeed be lowered by certain deformations of the vortex lines.     

Effects of phase fluctuation on a mixture of rotating superfluids

It is shown that if the root-mean-square of the gradient of the phase fluctuation of either of the components exceeds the corresponding inverse of the coherence length or if the chemical potential exceeds ₁ ⁰ or ₂ ⁰ , where is the volume integrals of the interaction function between the components, and ₁ ⁰ , ₂ ⁰ are the densities of the two components, the mixture of two rotating superfluids has an instability.     

Counterflow-Driven Textural Phase Transitions in Superfluid 3He-A1

We have studied second sound propagation through superftuid ³ He-A ₁ filling a rectangular resonator equipped with 3 pairs of transducers. The -texture was manipulated using one transducer to drive an oscillatory counterflow while measuring the resonant response of an orthogonal transducer pair. We observed abrupt signal changes and hysteresis effects depending on drive frequencies and amplitudes. We analyzed our experiments by examining planar -textures strongly coupled to the linear second sound wave equation. Evidence of first-order phase transitions was obtained numerically. The results are qualitatively consistent with the experimental findings. Our physical intuition did not anticipate these striking discontinuous phenomena.     

Nodes of the Gap Function and Anomalies in Thermodynamic Properties of the B-Phase of Superfluid 3He

Departures of thermodynamic properties of the B-phase of three-dimensional superfluid ³He from the predictions of BCS theory are analyzed. Attention is focused on deviations of the ratios and from their BCS values, where is the pairing gap at zero temperature, T_c is the critical temperature, and C_s and C_n are the superfluid and normal specific heats. We attribute these deviations to the momentum dependence of the gap function , which becomes well pronounced when this function has a pair of nodes lying on either side of the Fermi surface. We demonstrate that such a situation arises if the P-wave pairing interaction, evaluated at the Fermi surface, has a sign opposite to that anticipated in BCS theory. Taking account of the momentum structure of the gap function, we derive a closed relation between the two ratios that contains no adjustable parameters and agrees with the experimental data. Some important features of the effective pairing interaction are inferred from the analysis.      

AC long-range phase-coherent effects of S-N-S junctions in the dirty limit

The conductance of a normal metal film (N) in contact with two superconductors (S) is calculated in the presence of a constant and oscillating V_ωcosωt bias voltage between N and S. It is shown that the conductance as a function of has a Shapiro-like step at = ħω/2e. This exists in the case when the spacing between two superconductors largely exceeds the coherence length. It is also shown that the Shapiro step height decreases rapidly when the junction size exceeds the phase breaking length.     

Coefficient of restitution and linear–dashpot model revisited

With the assumption of a linear–dashpot interaction force, the coefficient of restitution, , can be computed as a function of the elastic and dissipative material constants, k and γ by integrating Newton’s equation of motion for an isolated pair of colliding particles. If we require further that the particles interact exclusively repulsive, which is a common assumption in granular systems, we obtain an expression which differs even qualitatively from the known result . The expression allows to relate Molecular Dynamics simulations to event-driven Molecular Dynamics for a widely used collision model. This research was supported by German Science Foundation and by the G.I.F., the German-Israeli Foundation for Scientific Research and Development.