Charge dynamics and spin order in doped Hubbard models

Hole motion in an antiferromagnetic (AF) environment is accompanied by the emission of spin wave excitations. Spin-wave shakeoffs are responsible for incoherent contributions to the dynamics of propagating holes. Using a spin-density-wave polaron scheme we calculate the optical conductivity () and show that the incoherent part of the hole spectrum contributes to the low-frequency part of ().Separately, we discuss the possible formation of spiral spin patterns upon doping of the half-filled one-band Hubbard model. In particular, we consider the influence of band structure effects arising from nearest- and next-nearest-neighbor hopping processes on a square lattice. Differences in the ground state spin patterns for hole and electron doping are obtained offering a possible explanation for the persistence of AF order in low electron-doped cuprate superconductors.     

Waves of general type propagating in a liquid layer sandwiched between two thermo-elastic half-spaces

Waves of general type propagating in a compressible non-viscous liquid bounded on both sides by two different generalised thermoelastic half-spaces are studied. It is found that, unlike in the case of classical (Stoneley-type) waves, the motion is not necessarily two dimensional, and that the particles of the solids and the liquid vibrate in three different planes, in general. The orientation of each of these planes varies from point to point, in general, and only under a particular condition do these planes coincide and become perpendicular to the interfaces. Further, a single scalar function determines all the physical quantities associated with the waves, and the phase velocity equation holds irrespective of the form of wave front.     

Magnetism and transport in the t-t'-J model

The normal state properties of high-T_c cuprates are investigated in terms of an extended t-J model using a spin-rotation-invariant slave-boson (SB) technique. A second-neighbour hopping t' of different sign is included to account for band structure effects of both hole- and electron-doped systems. Using the renormahzed SB quasiparticle band, the doping and temperature dependence of the Hall resistivity are explored, the results being in accord with experiments on La_2–SrCuO₄ (LSCO), YBa₂Cu₃O_6+x (YBCO), and Nd_2–CeCuO₄ (NCCO). The calculated -dependence of the dynamical magnetic structure factor S( ) shows reasonable agreement with the qualitative features of the neutron scattering cross sections in metallic LSCO- and YBCO-type systems.     

Surface wave propagation in sandy layer overlying a liquid saturated porous half-space and lying under a uniform liquid layer

Dispersion of Rayleigh-type surface waves is studied in a sandy layer under a uniform layer of homogeneous liquid lying over liquid-saturated porous solid half-space. The frequency equation in the form of a ninth-order determinant is obtained and evaluated. Special cases have been deduced and dispersion curves for the phase velocity and wave number have been plotted for a particular model.     

Thermoviscoelastic model of a layer between two bodies and conditions of their conjugation

On the basis of the Kelvin-Voigt-type thermomechanical theory of viscoelasticity, by the method of averaging over the thickness, we develop a thermoviscoelastic model of thin intermediate layer and establish generalized Winkler-type thermomechanical conjugation conditions for solid bodies in the dynamical mode for the case of imperfect thermal contact. It is shown that these conditions can be regarded as a generalization of the classical model. Relations convenient for practical applications are deduced. Various classical conditions of thermomechanical contact and their generalizations are obtained as a result of the limiting transition performed under additional assumptions concerning the moduli of elasticity of the intermediate layer. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 35–43, May–June, 2007.     

Hydrothermal wave in a shallow liquid layer

The oscillatory thermocapillary convection and hydrothermal wave in a shallow liquid layer, where a temperature difference is applied between two parallel sidewalls, have been numerically investigated in a two-dimensional model. The oscillatory thermocapillary convection and hydrothermal wave appear if the Marangoni number is larger than a critical value. The critical phase speed and critical wave number of the hydrothermal wave agree with the ones given analytically by Smith and Davis in the microgravity environment, and it travels in the direction opposed to the surface flow. Another wave traveled downstream in addition to the hydrothermal wave traveled upstream was observed in the case of earth gravity condition.     

Quantum spin dynamics of the transverse Ising model in two dimensions

We use the method of recurrence relations to obtain the time-dependent spin correlation function of the Ising model in a transverse field in 2D. We find that the correlation function decays algebraically at long times as t_– where 2.2. This is to be contrasted with the 1D case where the decay is Gaussian. We expect that in 3D the dynamical correlation will also exhibit a power law decay. Our results can be used to understand the experimental shape functions for the induced moment in LiTb_pY _1–pF ₄ .     

Surface tension effects on the nonlinear behavior of long waves in a two layer flow

 The propagation of long waves of finite amplitude at the interface of two viscous fluids in the presence of interfacial tension is examined. The effect of capillarity on the shape of the waves at the interface of two superposed fluids is investigated for a wide range of density differences, viscosity ratios and imposed pressure gradients. It is found that in planar geometry surface tension stabilizes the interfacial disturbances. Attention is given to the case in which the upper fluid is more dense and comprises a thin film above the lower fluid. With the heavier fluid on the top the flow pattern is always unstable when surface tension effects are neglected. In this case the interfacial waves do not grow forever and reach a finite amplitude only when the interfacial tension is greater than a critical value.     

Anisotropic superconductivity in the two-dimensional Hubbard model

We address the problem of anisotropic superconductivity in the two-dimensional Hubbard model. The Eliashberg equations have been generalized to the case which accounts for the anisotropy of the order parameter. Strong local correlations are treated within the mean field slave boson approximation. The superconducting transition temperatureT _c is evaluated as a function of the occupation number. Our results indicate that thed-wave state is the most likely channel for superconductivity for small concentration of holes. We have also derived an approximate analytical formula forT _c valid for any value of the occupation number. In addition, the influence of strong correlations on the electron-phonon coupling function is also discussed.     

An alternative model for elastic thermal stresses in two materials joined by a graded layer

The elastic stress distribution resulting from thermal mismatch in a film/graded layer/substrate system is a critical issue. Recently by a three-variable method Hsueh and Lee formulated a closed-form analytical solution [Hsueh CH, Lee S. Modeling of elastic thermal stresses in two materials joined by a graded layer. Composites: Part B 2003;34:747–52] in order to overcome the complexity of the traditional analytical models. The paper is devoted to developing an alternative analytical model for graded beams by Zhang’s two-variable method for multilayered beams. To illustrate applications of the present solution, specific results are calculated for the GaAs/graded GaAs–Si/Si systems. The transition phenomenon between tensile stress and compressive stress in the graded beam is investigated in detail for different graded parameters or different thickness. The present results agree well with existing analytical results. The differences between the present two-variable model and Hsueh’s three-variable model are also discussed.     

Natural convection of a conducting liquid in a spherical layer

A system of differential equations describing the natural convection of the conducting liquid in the earth’s core is solved numerically by the methods of a reference volume and finite differences.     

Thermocapillary instability of a viscoelastic liquid layer

Summary The stability of a layer of viscoelastic fluid heated from below is studied in the case that the instability is driven by surface-tension gradients at the upper surface. The operative parameter for instability is the Marangoni number and the critical value of this parameter is calculated as a function of Prandtl number, heat-transfer coefficient and elasticity parameters. It is shown that when the elasticity is very small instability sets in as steady convection, as for a Newtonian fluid, but at larger elasticities oscillatory convection is the first mode of instability to appear.With 5 Figures     

Impact of two circular plates one of which is floating on a thin layer of liquid

The paper deals with the axisymmetric unsteady problem of the collision of two circular plates, one of which is located initially on the surface of a shallow liquid layer and another is falling down on it. The presence of air between the colliding plates is taken into account. Both the air and the liquid are assumed ideal and incompressible and their flows potential. The flows in the liquid layer and between the plates are assumed one-dimensional with corrections for three-dimensional effects close to the plate edges. The present study is focused on the stage of strong interaction between the plates, during which the floating plate is accelerated and the hydrodynamic pressure in the liquid layer takes its maximum value. A simplified model of this interaction is suggested. Velocities of the plates and the hydrodynamic pressure on the bottom of the liquid layer are analytically estimated and compared with experimental results. The model provides the maximum of the hydrodynamic pressure, which can be used at the design stage. It is shown that the air flow between the moving plates is of major importance to explain the low amplitude of the measured hydrodynamic pressures.     

Hydrodynamics and mass transfer in a liquid layer at a rotating surface

The hydrodynamics and the mass transfer in a liquid layer at a rotating surface are analyzed in the boundary-layer approximation without undulation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 25, No. 4, pp. 648–655, October, 1973.     

Gas-bubble formation in liquid layer

The dynamics of bubble formation and breakaway when a gas issues into a liquid is considered. The different modes of gas-bubble formation in a liquid layer are indicated. The results of analytic investigation are compared with experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 6, pp. 1066–1071, December, 1978.     

Hole pockets in the doped 2D Hubbard model

The electronic momentum distributionn(k) of the two dimensional Hubbard model is studied for different values of the couplingU/t, electronic density n, and temperature, using quantum Monte Carlo techniques. A detailed analysis of the data on 8 × 8 clusters shows that features consistent with hole pockets at momentak = (±/2,±/2) appear as the system is doped away from half-filling. Our results are consistent with recent experimental data for the cuprates discussed by Aebi et al. (Phys. Rev. Lett. 72, 5757 (1994)). In the range of couplings studied, the depth of the pockets is maximum at n 0.9,and it increases with decreasing temperature. The apparent absence of hole pockets in previous numerical studies of this model is explained.     

Stretching of a liquid layer underneath a semi-infinite fluid

Exact similarity solutions of the Navier–Stokes equation are derived describing the flow of a liquid layer coated on a stretching surface underneath another semi-infinite fluid. In the absence of hydrodynamic instability, the interface remains flat as the layer thickness decreases in time. When the physical properties of the fluids are matched, we obtain Crane’s analytical solution for two-dimensional (2D) flow and a corresponding numerical solution for axisymmetric flow. When the rate of stretching of the surface is constant in time, the temporal evolution of the interface between the layer and the overlying fluid is computed by integrating in time a system of coupled partial differential equations for the velocity in each fluid together with an ordinary differential equation expressing kinematic compatibility at the interface, subject to appropriate boundary, interfacial, and far-field conditions. Multiple solutions are found in certain ranges of the density and viscosity ratios. Additional similarity solutions are presented for accelerated 2D and axisymmetric stretching. The numerical prefactors that appear in the analytical expressions for the interface location and wall shear stress are presented for different ratios of the densities and viscosities of the two fluids.     

High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.