Determination of In-Plane Anisotropy from Magnetoresistivity Measurements on YBa2Cu3O7−δ

The magnetoresistance of six untwinned YBa ₂ Cu ₃ O _7– crystals has been measured above T _c for IBc. Fluctuation theories allowing for an in-plane coherence length anisotropy ( _{a b} ) were used in the analyses. The results suggest that the anisotropy, = _b / _a , is considerably lower than that expected from the resistivity ratio, i.e., . Some magnetoresistive measurements with the field in the ab-plane are also presented.     

μ+SR Study on Bi2Sr2Ca1−x Y x (Cu1−y Zn y )2O8+δ around the Hole Concentration of \frac{1} {8} per Cu

In order to study the effect in different high-T _c oxides from the La-system, muon spin relaxation measurements were applied to the Zn-substituted Bi-2212 system, Bi ₂ Sr ₂ Ca _1–x Y _x (Cu _1–y Zn _y ) ₂ O ₈₊ , around the hole concentration p= per Cu. It has been revealed that the magnetic correlation between Cu spins is anomalously enhanced in the Zn-substituted samples at per Cu, proving the existence of the effect" in the Bi-2212 system as well.     

A Monte Carlo Study of Disorder in the High Pressure Mixed Crystal N2-Ar

In order to obtain a better notion of the experimental results in our laboratory, Monte Carlo calculations have been performed of the N ₂-Ar crystal on the N ₂-rich side, in the p-T region where the and phases exist in pure N ₂. Considering the enthalpy, the system prefers the Ar atoms to be located on the sphere positions. The * phase is present for mixtures down to but is most likely metastable. The *-* transition shifts to lower temperatures with decreasing . The 2 ^nd order transition within the phase continues to exist to even smaller . In contrast to the * -* transition, the transition temperature for the 2 ^nd order transition does not shift to lower temperatures. For a mixture of it is within 5 K from the pure 2 ^nd order transition at a pressure of 7.0 GPa.     

Order parameters and energies of analytic and singular vortex lines in rotating3He-A

We present the expressions of the generalized Ginzburg-Landau (GL) theory for the free energy and the supercurrent in terms of thed vector, the magnetic fieldH, and operators containing the spatial gradient and the rotation. These expressions are then specialized to the Anderson-Brinkman-Morel (ABM) state. We consider eight single-vortex lines of cylindrical symmetry and radiusR=[2m/]^–1/2: the Mermin-Ho vortex, a second analytic vortex, and six singular vortices, i.e., the orbital and radial disgyrations, the orbital and radial phase vortices, and two axial phase vortices. These eight vortex states are determined by solving the Euler-Lagrange equations whose solutions minimize the GL free energy functional. For increasing field, the core radius of the texture of the Mermin-Ho vortex tends to a limiting value, while the core radius of the texture goes to zero. The gap of the singular vortices behaves liker forr 0, where ranges between and . The energy of the radial disgyration becomes lower than that of the Mermin-Ho vortex for fieldsH6.5H*=6.5×25 G (atT=0.99T _c and forR=10L*=60 µm, or=2.9 rad/sec). ForR 2 _T ( _T is the GL coherence length) or _c2 (upper critical rotation speed), the energies of the singular vortices become lower than the energies of the analytic vortices. This is in agreement with the exact result of Schopohl for a vortex lattice at _c 2. Finally, we calculate the correction of order (1 -T/T _c ) to the GL gap for the axial phase vortex.     

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.     

Parafluidity in helium near the λ-transition and the generalized time-dependent Landau theory

A phenomenological theory of parafluidity, i.e., an enhancement of fluidity due to order-parameter fluctuations, is presented for helium near the transition. The generalized time-dependent Landau theory of second-order phase transitions is reviewed in general and is applied to the superfluid transition in helium as a particular example. In helium, it is found that parafluidity is manifested in the divergences of the mass diffusivity , the thermal conductivity , the first-sound amplitude attenuation ||^–1, and the second-sound dampling , which are all consistent with the dynamic scaling hypothesis. Here a characteristic relaxation time ₀ ||^–1 is used, where =(T–T _c )/T _c andT _c is the transition temperature. Although there are not enough experimental data to confirm our formulas, the present approach is seen to agree in order of magnitude with available experiments. Finally, the sound absorption above a ferromagnetic transition is calculated by adding a diffusion term to the generalized time-dependent Landau equation. The result thus obtained agrees in order of magnitude with experiments in nickel.Supported in part by the National Science Foundation and the Horace H. Rackham School of Graduate Studies.     

Natural convection in a rectangular enclosure in the presence of a magnetic field with uniform heat flux from the side walls

Summary A numerical study is presented for magnetohydrodynamic free convection of an electrically conducting fluid in a two-dimensional rectangular enclosure in which two side walls are maintained at uniform heat flux condition. The horizontal top and bottom walls are thermally insulated. A finite difference scheme comprising of modified ADI (Alternating Direction Implicit) method and SOR (Successive-Over-Relaxation) method is used to solve the governing equations. Computations are carried out over a wide range of Grashof number, Gr and Hartmann number, Ha for an enclosure of aspect ratio 1 and 2. The influences of these parameters on the flow pattern and the associated heat transfer characteristics are discussed. Numerical results show that with the application of an external magnetic field, the temperature and velocity fields are significantly modified. When the Grashof number is low and Hartmann number is high, the central streamlines are elongated and the isotherms are almost parallel representing a conduction state. For sufficiently large magnetic field strength the convection is suppressed for all values of Gr. The average Nusselt number decreases with an increase of Hartmann number and hence a magnetic field can be used as an effective mechanism to control the convection in an enclosure.List of symbols Ar aspect ratio,H/L - B ₀ induction magnetic field - H ₀ magnetic field,H ₀=B ₀/ _m - g gravitational acceleration - Gr Grashof number,gq(L/k)L ³/v ² - H height of the enclosure - Ha Hartmann number, - k thermal conductivity - Nu local Nusselt number - average Nusselt number - p pressure - Pr Prandtl number, / - q heat flux - t time - T dimensionless temperature, (–₀)/q(L/k) - u vertical velocity - U dimensionless vertical velocity,uL/ - v horizontal velocity - V dimensionless horizontal velocity,vL/ - x vertical coordinate - X dimensionless vertical coordinate,x/L - y horizontal coordinate - Y dimensionless horizontal coordinate,y/L - thermal diffusivity - thermal expansion coefficient - temperature - ₀ reference temperature - density - kinematic viscosity - viscosity - _m magnetic permeability - electrical conductivity - stream function - dimensionless stream function, / - dimensionless time,t/L ² - vorticity - dimensionless vorticity, L ²/ - X grid spacing inX-direction - Y grid spacing inY-direction - time increment - ² Laplacian operator     

On internal stress and activation volume in polymers

The two-site model is developed for the analysis of stress relaxation data. It is shown that the product of d In (– )/d and (- _i) is constant where is the applied stress, _i is the (deformation-induced) internal stress and = d/dt. The quantity d In ( )/d is often presented in the literature as the (experimental) activation volume, and there are many examples in which the above relationship with (- _i) holds true. This is in apparent contradiction to the arguments that lead to the association of the quantity d In (– )/d with the activation volume, since these normally start with the premise that the activation volume is independent of stress. In the modified theory presented here the source of this anomaly is apparent. Similar anomalies arise in the estimation of activation volume from creep or constant strain rate tests and these are also examined from the standpoint of the site model theory. In the derivation presented here full account is taken of the site population distribution and this is the major difference compared to most other analyses. The predicted behaviour is identical to that obtained with the standard linear solid. Consideration is also given to the orientation-dependence of stress-aided activation.     

Nuclear relaxation in bcc3He for large molar volumes

We report the results of measurements of longitudinal and transverse nuclear relaxation timesT ₁ andT ₂ in bcc³He at 300 mK and at a Larmor frequency of 3 MHz for molar volumes 22.6V24.9 cm³. It is found, even in this extended range of molar volumes, that the relaxation times increase withV as , with ₁=19±1 and ₂=18±1.5. This molar volume dependence has also been found in various thermodynamic data, including recent results of pressure measurements through the nuclear ordering temperature. This universality in molar volume dependence remains unexplained within the context of the present models of multiple exchange.     

Hole Density in Charge Stripes and Effect of Next-Nearest-Neighbor Transfer

By applying a variational Monte Carlo method to the two-dimensional t-J model, a couple of properties of striped states are studied. In the charge domain walls vertical (or horizontal) to the lattice (VDW), the hole density is favorable in energy similarly for 0.5 , whereas in the diagonal domain walls (DDW) the stable hole density is limited to = 1. Negative next-nearest-neighbor transfer (t/t < 0) further stabilizes VDW with = 0.5 and DDW with = 1 against the -wave superconducting state.     

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.     

Transport properties of helium near the liquid-vapor critical point. V. The shear viscosity of3He-4He mixtures

A torsional oscillator operating at 158 Hz has been used to measure the shear viscosity for two mixtures,³He_0.65-⁴He_0.35 and³He_0.80–⁴He_0.20. Data for each mixture are reported along near-critical isochores 0.85</ _c <1.18, where _c is the critical density. Just as in a previous paper on pure³He and⁴He, the observed viscosity includes the sum of background and critical contributions and the effect from the earth's gravity nearT _c . The analysis of the data provides (1) the background viscosity versus density and temperatureT, (2) the viscosity ratio / along the various isochores and also along selected isotherms obtained by interpolation, showing the critical contribution, and (3) the shape of the dew-bubble curve for each mixture, as obtained from the maxima in or from the discontinuities in the sloped/dT at the transition from the two-phase to the single-phase regime. A comparison between the data of these mixtures and those for pure fluids is presented. The background viscosity data along isotherms and along isochores are found to vary smoothly from³He to⁴He. The critical viscosity ratio [ / ]( _c ) for mixtures along the critical isochore is comparable with that for the pure fluids, i.e., it diverges weakly asT _c is approached. Hence, just as for previously investigated critical transport properties in³He-⁴He mixtures, there is no significant difference in behavior from that in the pure fluids.     

Thermal expansion and bulk modulus of cobalt

The ten third-order elastic (TOE) constants of hexagonal cobalt, which has nearly the idealc/a value, have been evaluated using the nearest-neighbor central interaction model for hexagonal materials proposed by Ramji Rao and Srinivasan. These TOE constants have been employed to evaluate the low-temperature limit of the thermal expansion, the Anderson-Grüneisen (AG) parameter , and the second Grüneisen constantq of cobalt. has the value 2.08 for cobalt. The temperature variation of the volume Grüneisen gamma of cobalt has been calculated on this model. The high-temperature limit has the value 2.13, which agrees well with the experimental value obtained from thermal expansion and specific heat data of cobalt. Anderson's theory has been used to explain the temperature dependence of the bulk modulus of cobalt and compared with the experimental values obtained from the elastic constant data of Fisher and Dever. The calculatedB _s values agree with the experimental values to within 1%.     

Nonlinear asymptotic theory of hypersonic flow past a circular cone

Summary The hypersonic small-disturbance theory is reexamined in this study. A systematic and rigorous approach is proposed to obtain the nonlinear asymptotic equation from the Taylor-Maccoll equation for hypersonic flow past a circular cone. Using this approach, consideration is made of a general asymptotic expansion of the unified supersonic-hypersonic similarity parameter together with the stretched coordinate. Moreover, the successive approximate solutions of the nonlinear hypersonic smalldisturbance equation are solved by iteration. Both of these approximations provide a closed-form solution, which is suitable for the analysis of various related flow problems. Besides the velocity components, the shock location and other thermodynamic properties are presented. Comparisons are also made of the zeroth-order with first-order approximations for shock location and pressure coefficient on the cone surface, respectively. The latter (including the nonlinear effects) demonstrates better correlation with exact solution than the zeroth-order approximation. This approach offers further insight into the fundamental features of hypersonic small-disturbance theory.Notation a speed of sound - H unified supersonic-hypersonic similarity parameter, - K hypersonic similarity parameter, M - M freestream Mach number - P pressure - T temperature - S entropy - u, v radial, polar velocities - V freestream velocity - shock angle - cone angle - density - density ratio, /() - ratio of specific heats - polar angle - stretched polar angle, / - (), (), () gage functions     

Nonlinear axisymmetric response of orthotropic thin truncated conical and spherical caps

Summary An approximate analytical solution of the large deflection axisymmetric response of polar orthotropic thin truncated conical and spherical shallow caps is presented. Donnell type equations are employed. The deflection is approximated by a one term mode shape satisfying the boundary conditions. The Galerkin's method is used to get the governing equation for the deflection at the hole. Nonlinear free vibration response and the response under uniformly distributed static and step function loads are obtained. The effect of various parameters is investigated.Notations A, A ^* Inward and outward amplitudes - a, b, h Base radius, inner radius and thickness of the cap - D M h ³/[12(–v ² )] - E ,E Young's moduli - H ^*,H Apex height, dimensionless apex heght:H ^*/h - N _, Stress resultants - p ^1/2 - q Uniformly distributed load - Q,Q₀ Dimensionless load: , dimensionless step load - Q, Q ₀ Dimensionless load: , step load - t, Time, dimensionless time: t - T _A Ratio of nonlinear periodT for inward amplitudeA and the linear periodT _L - w ^* Normal displacement at middle surface - w Dimensionless displacement:w ^*/h - ₁ Linear parameter of static response - Orthotropic Parameter:E /E - Mass density - ₂,₃ Quadratic and cubic nonlinearity parameters - b/a - v ,v Poisson's ratios - Dimensionless radius:r/a - ^*, Stress function, dimensionless stress function: - ₀ ^* ,₀ Linnear frequency, dimensionless frequency: With 7 Figures     

Evidence of a new vicinal state on the4He crystal interface

Equilibrium shape of hcp⁴He crystals has been studied at low temperatures by means of a high resolution optical interferometer. The profile of the crystalline interface next to a horizontal c-facet was found to have a well defined border line separating two regions characterized by different angular behavior of the surface stiffness . For surfaces tilted by a small angle 10^–4 rad with respect to the facet, we find 1/, contrary to the dependence predicted by current theories. The tremendous increase of surface stiffness in the close vicinity to the facet orientation may be considered as strong evidence of a new surface state.     

Low Temperature Heat Capacities of Submonolayer Solid 3He

Heat capacities (C) of ³ He submonolayer solids adsorbed on a graphite surface are measured down to 100 K, a factor of twenty lower temperatures than previous work. At a real densities near the commensurate solid (6.4 nm ^–2 ), an anomalous temperature dependence, C 1/T, is observed in a wide temperature range over two orders of magnitude (0.1 T 20mK). Similar behavior was observed for the commensurate solid in the second-layer with the same density, suggesting a common microscopic mechanism. It is, however, a puzzle that the solid is ferromagnetic according to recent magnetization measurements by ISSP group, while the solid is known to be antiferromagnetic. Possible explanations for this conflict, such as vacancy effects or multiple-spin exchanges modulated by a substrate potential corrugation, are discussed.     

Flux line density gradient in the mixed state of type II superconductors

The value of the critical current for a type II (1) superconducting slab placed in a magnetic fieldH _e (H _c1H _e H _c2) parallel to the sample surface and with a currentI flowing perpendicular to the magnetic field is obtained. This critical current depends on the characteristic length of the mixed state . The critical state equation for a simple system with pinning is given. The equation relating the length with the Campbell pinning length is obtained in the special case . A formula for the surface impedance of the mixed state is given.     

Collective-mode spectroscopy of textures in superfluid3He-B

A phenomenological theory for the propagation of the real squashing modes in superfluid³He-B is presented. This allows one to calculate the splitting of the real squashing (rsq) mode spectrum caused by the combined effects of magnetic field, dispersion, and texture in the experimentally important range of magnetic fields from 0 to 10³ Gauss. This serves to provide a tool for the rsq-mode spectroscopy of the -textures in³He-B. In particular, a new gyrosonic effect is suggested: the intensity of the rsq modes generated in textures depends on the sense of rotation—even when the axis of rotation coincides with the direction of the ultrasound propagation .     

Spectroellipsometric studies of 0.9PbMg1/3Nb2/3O3-0.1PbTiO3 thin films

0.9PbMg_1/3Nb_2/3O₃-0.1PbTiO₃ (PMN-PT) films of different thickness, ranging from 75 to 450 nm, were prepared on La_0.5Sr_0.5MnO₃ (LSMO)-buffered LaAlO₃ (LAO) substrates by pulsed laser deposition (PLD). The structural properties of these films were characterized by X-ray diffraction. The –2 scans indicated that all the films have a pure perovskite phase containing no impurity. The - and -scans confirmed that all the films possess a heteroepitaxial relation of PMN-PT(1 0 0) LSMO(1 0 0) LAO(1 0 0) structure. The surface morphology and cross section of the films were examined by scanning electron microscopy (SEM). Their dielectric constants and the leakage currents were measured by an impedance analyzer and a leakage digital electrometer, respectively. Furthermore, spectroellipsometry (SE) was used to characterize the depth profile, refractive index, and microstructural inhomogeneities, including voids, microroughness of surface, and the electrode/film interface of these films. Based on these studies, the correlation between the electrode/film interface and the electrical properties of the films was discussed. Our results show that the ratio of the electrode/film interface thickness to the film thickness increases as the film thickness decreases. The increase in this ratio results in deterioration of the leakage current and dielectric constant of the films.