Phase diagram of the 1D Kondo lattice model

We determine the bounday of the fully polarized ferromagnetic states in the one dimensional Kondo lattice model at partial conduction electron band filling by using a newly developed infinite size DMRG method which conserves the total spin quantum numbers. The obtained paramagnetic to ferromagnetic phase bounday is bellow J 3.5 for the whole range of band filling. By this we solve the controversy in the phase diagram over the extent of the ferromagnetic region close to half filling.     

Crack analysis near vacuum brazing interface of Cu/Al dissimilar materials using Al–Si brazing alloy

Abstract

The present paper concentrates on the factors inducing cracking near the Cu/Al vacuum brazing interface using Al–Si brazing alloy. Various analysis and test methods were adopted to analyse the cause of the cracking. Experiment results indicated that two intermetallic compounds layers – δ phase and θ phase – were formed near the interface of Cu and brazing seam region. The microhardness of both the two intermetallic compounds phases reached 720 and 510 HM respectively. The brazing seam region consisted mainly of θ phase and α-Al (Cu) solid solution. Si presence had led to obvious zone liquation in the brazing seam region. Stress concentration appeared on the margin and edge region of the joint. The crack initiated on the θ phase layer, and then extended towards the brazing seam region and δ phase layer. The separation in the θ phase had an obviously brittle fracture surface.     

Observation of e2/h Conductance Steps in a Side-Gate Point Contact on In0.75Ga0.25As/In0.75Al0.25As Heterostructure

In 1996, it was suggested by K. J. Thomas et al. (Phys. Rev. Lett. 77, 135 (1996)) that 0.7(2e²/h) conductance structure in Quasi-1DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In_0.75Ga_0.25As/In_0.75Al_0.25As heterojunctions grown on GaAs substrates and obtained the value of 10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e²/h conductance steps in low electron concentration side-gate point contact.     

Superconductivity and Phase Diagram in a CuO2 Monolayer

We recall the van Hove scenario (J. Labbe and J. Bok, Europhys. Lett. 3, 1225 (1987); J. Bouvier and J. Bok, in The Gap Symmetry and Fluctuations in HTSC, J. Bok et al., eds. (Plenum, New York, 1998)) developed since 1987. It explains high T _c, anomalous isotope effect, gap anisotropy etc. We apply this scenario to the superconductive surface layer, obtained by field effect on CaCuO₂ by J. H. Schön et al. Preprint (private communication), to be published). We show that the variation of resistivity and Hall effect with temperature in the normal state can be understood by the presence of a van Hove singularity (v.H.s.) in the band structure. The doping by field effect changes the distance between the Fermi level and the v.H.s.     

Kinetics of transient liquid phase diffusion bonding process for joining aluminium metal matrix composite

Abstract

The mechanism and kinetics of the transient liquid phase diffusion bonding process in a 6061–15 wt-%SiCp composite at 570°C, 0·2 MPa, with 200 μm thick copper foil interlayer, has been investigated by microstructural characterisation of the bond region using optical microscopy, scanning electron microscopy and electron probe microanalysis. The kinetics of isothermal solidification, representing the displacement of the solid/liquid interface y (in micrometres) as a function of time t (in seconds), followed a power law relationship y?=?157t^0·07. According to this kinetic equation, the effective diffusivity of copper in the composite system was found to be ～10⁶ times higher than the lattice diffusivity, indicating the dominance of short circuit diffusion through the defect rich particle/matrix interface.     

Entropy-Driven Reentrant Phase Transitions in Even-J/Odd-J Mixtures of Linear Rotors

Depending on the parity of the rotational quantum number J, solid hydrogens exhibit either pressure-driven broken symmetry phase (BSP) transitions (even-J species – para-hydrogen (p-H₂), and ortho-deuterium (o-D₂)) or usual order-disorder phase transitions (odd-J species – o-H₂ and p-D₂) with the phase transition temperature increasing monotonically with pressure from the phase transition point at zero pressure. At the same time, for solid HD the BSP phase transition line displays a minimum, indicating that the disordered phase is reentrant. In this work a model of quantum linear rotors is used to study characteristic features of the P–T phase diagrams for ortho–para mixtures in solid H₂ and D₂. We developed a mean-field theory of even-J – odd-J mixtures of quantum linear rotors on a 3D lattice. Two limiting cases are considered: mixtures at thermodynamic equilibrium, where the conversion time is small or comparable with the thermalization time, and the opposite case, frozen mixtures, when the conversion time is large compared with other relevant times. We found that for all equilibrium linear rotor systems — the even-J – odd-J mixtures — the reentrant behavior of the phase transition lines is an entropy-driven phenomenon, as was previously found for the all-J system. Experimentally, conditions to find the reentrant BSP transition line are most favorable for H₂ mixtures, whereas the frozen monotonic phase lines should be the case for D₂ even-J – odd-J mixtures. These results may therefore be particularly useful for understanding the anomalous transition region of the BSP transition documented for D₂ mixtures.PACS numbers: 64.60 Cn, 67.80 −s, 67.90 +z, 81.40 Vw     

Lattice and spin bipolarons in metal oxides and doped fullerenes

We extrapolate the BCS theory to the strong electron-phonon and (or) electron-spin fluctuation interaction and show that in the strong-coupling limit the ground state is a charged Bose liquid of lattice and (or) spin bipolarons. Kinetic and thermodynamic properties of charged bosons on a lattice in the normal and superconducting states are discussed, and some evidence for the model is given from NMR, neutron scattering, near-infrared absorption, Hall effect, resistivity, thermal conductivity, isotope effect, heat capacity, and critical magnetic fields of high-T _c oxides. The maximum attainableT _c is estimated to be in the region of the transition from the Fermi liquid to a charged Bose liquid (polaronic superconductivity). The proposed theory is not restricted by low dimensionality and might be applied to cubic oxides such as the old BaPbBiO and to alkali-doped C₆₀ as well.We thank D. Khmelnitskii, W. Liang, J. Loram, M. Pepper, E. Salje, and J. Wheattey for helpful stimulating discussions, and J. Cooper, A. Carrington, and A. Mackenzie for extensive experimental data and discussion. A. Bratkovsky has been instrumental in elaborating the temperature dependence of the infrared absorption and the electrical conductivity. One of us (A.S.A.) appreciates the financial support from the Leverhulme Trust.     

Magnetic transitions and magnetocaloric effects in Fe0.75Mn1.35As

In Fe_0.75Mn_1.35As compound, a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase can be induced above its phase transition temperature T _s = 165 K by an external magnetic field, which leads to large magnetocaloric effects around T _s. The sign of the magnetic entropy change ΔS _M in the Fe_0.75Mn_1.35As compound is negative, not as expected as an inverse magnetocaloric effect, and the maximum value of ΔS _M is 4.2 J/kg K at 167.5 K for a magnetic field change of 5 T. Although it induces an irreversible lattice expansion, the cycling of a magnetic field does not induce an irreversible change in the magnetic transitions and magnetocaloric behaviors. The antiferromagnetism-related metamagnetic transitions with a large magnetic entropy change may provide with an opportunity in searching novel materials for magnetic refrigeration.     

Dipolar anisotropy effects in the spin relaxation of adsorbed3He monolayers

Nuclear magnetic relaxation times and their anisotropy with respect to field direction are computed for adsorbed monolayer and submonolayer ³ He films. We considerT ₁ andT ₂ for fluid and solid systems with motion via diffusion and tunneling, respectively, andT ₂ for the rigid lattice system. Calculations for the fluid system are carried out via a modification of Torrey theory and for the solid system by the method of moments. The rigid lattice fourth moment is computed to check the Gaussian approximation for the line shape. The results provide explanations of and predictions for experiments on Vycor and Grafoil substrates. Possible complications implied by our results for experiments on liquid ³ He in the millidegree region are discussed.Work supported in part by the Science Research Council (D.J.C. and B.C.) and the National Science Foundation (W.J.M.) under Grant GH 34504A1.     

The Gap-to- Tc Ratio of a van Hove Superconductor

We give an analytical expression for the gap-to-T _c ratio (R) of a superconductor with a van Hove singularity in the density of states. Our calculation yields R in very good agreement with the results obtained numerically by S. Ratanaburi et al. [J. Supercond. 9, 485 (1996)].     

Inelastic neutron scattering from the vortex lattice in type-II superconductors. I

The inelastic neutron scattering from the vortex lattice in dirty type-II superconductors is considered. In the high-field region, the correlation function for the local magnetization is given in terms of the fluctuation propagator of the superconducting order parameter. In particular it is shown that the Fourier transform of the correlation function for the local magnetization diverges as /q –G _i /^–2, when the transverse transfered momentumq approaches the reciprocal lattice vectorG _i of the Abrikosov lattice. This term gives rise to tails around each Bragg peak in the inelastic neutron-scattering cross section.     

Organic superconductors with an incommensurate anion structure

Abstract

Superconducting incommensurate organic composite crystals based on the methylenedithio-tetraselenafulvalene (MDT-TSF) series donors, where the energy band filling deviates from the usual 3/4-filled, are reviewed. The incommensurate anion potential reconstructs the Fermi surface for both (MDT-TSF)(AuI₂)_0.436 and (MDT-ST)(I₃)_0.417 neither by the fundamental anion periodicity q nor by 2q, but by 3q, where MDT-ST is 5H-2-(1,3-dithiol-2-ylidene)-1,3-diselena-4,6-dithiapentalene, and q is the reciprocal lattice vector of the anion lattice. The selection rule of the reconstructing vectors is associated with the magnitude of the incommensurate potential. The considerably large interlayer transfer integral and three-dimensional superconducting properties are due to the direct donor–donor interactions coming from the characteristic corrugated conducting sheet structure. The materials with high superconducting transition temperature, T_c, have large ratios of the observed cyclotron masses to the bare ones, which indicates that the strength of the many-body effect is the major determinant of T_c. (MDT-TS)(AuI₂)_0.441 shows a metal–insulator transition at T_MI=50 K, where MDT-TS is 5H-2-(1,3-diselenol-2-ylidene)-1,3,4,6-tetrathiapentalene, and the insulating phase is an antiferromagnet with a high Néel temperature (T_N=50 K) and a high spin–flop field (B_sf=6.9 T). There is a possibility that this material is an incommensurate Mott insulator. Hydrostatic pressure suppresses the insulating state and induces superconductivity at T_c=3.2 K above 1.05 GPa, where T_c rises to the maximum, T_c^max=4.9 K at 1.27 GPa. This compound shows a usual temperature–pressure phase diagram, in which the superconducting phase borders on the antiferromagnetic insulating phase, despite the unusual band filling.     

Symmetries of Vortex Lattice Solutions of the Bogoliubov–de Gennes Equation in a Two-Dimensional Square Lattice

This report describes symmetry properties of tetragonal vortex lattice solutions of the Bogoliubov–de Gennes equation in a two-dimensional square lattice in a uniform magnetic field. The invariance group of a tetragonal vortex lattice solution is expressed in a form of G _(l) = (e + tC _2x ) (l = 0, 2, ± 1), where tC _2x is a space rotation around the x-axis accompanied with time reversal, is a kind of fourfold rotation group, and L is the magnetic translational group of the vortex lattice state. We give a new, refined definition of local symmetric order parameters (OPs) (s-wave, d-wave, and p-wave), which have a well-defined nature such that the OP (e.g., s-wave OP) at the translated site by a lattice vector (of the vortex lattice) from a site (m, n) is expressed by the OP (e.g., s-wave) at the site (m, n) times a phase factor. Winding numbers around the origin of s-wave and d-wave OPs are obtained for four types of solutions G _(l) (l = 0, 2, ± 1). It is shown that all energy bands of quasiparticles of a vortex lattice state are doubly degenerate.     

Thermal expansion of Bi2.2Sr1.8CaCu2Ox superconductor single crystals

Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi₂Sr_1.8CaCu₂O_x. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.     

Influence of thermal cycling on fatigue strength of Cu/Sn/Cu solder joints

Abstract

The influence of thermal cycling on the fatigue life of Cu/Sn/Cu solder joints has been examined. Copper plates were bonded with tin foil (with a solder thickness of 60 µm) and suffered thermal cycling in a temperature range of 55 or 125 K. Then they were subjected to fatigue testing at a shear stress amplitude of 2 MPa and a frequency of 3.6 Hz. With the increasing number of the thermal cycles, the fatigue life decreased from 3.0×10⁵ to 5.0×10⁴ at thermal cycle 6000. However, the fatigue life did not change so much during thermal cycling in different temperature ranges. When the solder joints suffered the thermal cycling, the η phase at the bonding interface coarsened and elongated, and its arrangement became irregular. After larger numbers of thermal cycles, fine cracks appeared in the η phase parallel to the interface. After fatigue testing, circular patterns were observed inside the bonded region on a fracture surface, and their shape and size became irregular and larger with the increasing number of thermal cycles, respectively. These showed that the reduction in fatigue life was caused by improved propagation of the fatigue crack following changes in the morphology and arrangement of the η phase during thermal cycling.     

Configuration and local elastic interaction of ferroelectric domains and misfit dislocation in PbTiO3/SrTiO3 epitaxial thin films

Abstract

We have studied the strain field around the 90° domains and misfit dislocations in PbTiO₃/SrTiO₃ (001) epitaxial thin films, at the nanoscale, using the geometric phase analysis (GPA) combined with high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field––scanning transmission electron microscopy (HAADF-STEM). The films typically contain a combination of a/c-mixed domains and misfit dislocations. The PbTiO₃ layer was composed from the two types of the a-domain (90° domain): a typical a/c-mixed domain configuration where a-domains are 20–30 nm wide and nano sized domains with a width of about 3 nm. In the latter case, the nano sized a-domain does not contact the film/substrate interface; it remains far from the interface and stems from the misfit dislocation. Strain maps obtained from the GPA of HRTEM images show the elastic interaction between the a-domain and the dislocations. The normal strain field and lattice rotation match each other between them. Strain maps reveal that the a-domain nucleation takes place at the misfit dislocation. The lattice rotation around the misfit dislocation triggers the nucleation of the a-domain; the normal strains around the misfit dislocation relax the residual strain in a-domain; then, the a-domain growth takes place, accompanying the introduction of the additional dislocation perpendicular to the misfit dislocation and the dissociation of the dislocations into two pairs of partial dislocations with an APB, which is the bottom boundary of the a-domain. The novel mechanism of the nucleation and growth of 90° domain in PbTiO₃/SrTiO₃ epitaxial system has been proposed based on above the results.     

Thermodynamic properties of LaF<Subscript>3</Subscript> superionic crystals near their phase transition

We examine the behavior of the entropy (S _d), enthalpy (H), and concentration of disordered fluoride ions (n) in LaF₃ in a broad temperature range around its critical temperature using a model for structural lattice disordering. We analyze the relationship between the free energy of a crystal and LaF₃ lattice disordering parameters: entropy, S _d; anion sublattice disordering energy, E _a; and the concentration of vacancy-interstitial defect complexes, n. H(T), n(T), and S _d(T) data are obtained for the phase transition region.     

High strain rate superplasticity in powder metallurgy aluminium alloy 6061 + 20 vol.-%SiCpcomposite with relatively large particle size

Abstract

The possibility of high strain rate superplasticity (HSRS) was examined over a wide range of temperatures in a powder metallurgy aluminium alloy 6061/SiC_p composite with a relatively large SiC particle size of ~8 μm. A maximum tensile elongation of 350% was obtained at 600°C and 10^-2 s^-1. Tensile elongations over 200% were obtained in a narrow temperature range between 590 and 610°C at high strain rates of 10^-2 and 10^-1 s^-1. The current testing temperature range could be divided into two regions depending on the rate-controlling deformation mechanism. Region I is in the lower temperature range from 430 to 490°C, where lattice diffusion controlled dislocation climb creep (n = 5) is the rate-controlling deformation process, and region II is in the higher temperature range from 520 to 610°C, where lattice diffusion controlled grain boundary sliding controls the plastic flow. An abnormally large increase in activation energy was noted at temperatures above 590°C, where large tensile elonga tions over 200% were obtained at high strain rates. This increase in activation energy and high tensile ductility may be explained in terms of presence of a liquid phase created by partial melting, but such evidence could not be provided by the current differential scanning calorimetry (DSC) test. This may be because the DSC is not sensitive enough to detect the small amount of liquid phase.     

Phase transformations of Pb0.995La0.005[Zr0.95 ? ySn0.05(Mg1/3Nb2/3)y]0.99875O3 solid solutions

We have constructed phase diagrams of Pb_0.995La_0.005[Zr_{0.95 − y} Sn_0.05 (Mg_1/3Nb_2/3) _y ]_0.99875O₃ (y = 0–0.02) solid solutions in different electric fields. Our results demonstrate that, by cooling in a constant electric field, the low-temperature rhombohedral ferroelectric phase can be stabilized in a narrow temperature range. It is shown that the phase state below the Curie temperature strongly depends on the temperature-field history of the material. In particular, the low-temperature rhombohedral ferroelectric phase only appears during cooling. At the same time, the temperature-field stability region of the tetragonal antiferroelectric phase is much broader during heating. Original Russian Text ? E.A. Bikyashev, E.A. Reshetnikova, I.V. Lisnevskaya, T.G. Lupeiko, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 6, pp. 706–713.     

A Quantum Mechanical Treatment of the Vortex–Vortex Interaction in Anisotropic Superconductors

Using a new Hamiltonian of interaction [Int. J. Mod. Phys. B16, 4809 (1002); B17, 4763 (2003)], we calculate the vortex–vortex interaction energy in anisotropic superconductors. We present here the analytical formulae. The interaction energy has a minimum at a certain vortex–vortex distance. This distance decreases with the increase of the angle θ between the vortex line and the crystalline axis. Also, we calculate the elastic force and the nonharmonicity coefficients of the vortex lattice. Both coefficients decrease with increasing angle. Generally, the interaction energy decreases with the angle increase. Finally, we present the self-energy of a vortex in a lattice, the energy of a single isolated vortex and the lower critical field. Also, the surface of the vortex in the lattice has a form of a rosette with six petals, which are all equals for θ = 0 and become nonequals for θ ≠ 0. The surface of the first isolated vortex has a form of an oval for θ ≠ 0, and becomes a circle for θ = 0. PACS numbers: PACS 74.25.Qt.