Shear viscosity of liquid4He and3He-4He mixtures,especially near the superfluid transition

High-resolution measurements of are reported for liquid⁴He and³He-⁴He mixtures at saturated vapor pressures between 1.2 and 4.2 K with particular emphasis on the superfluid transition. Here is the mass density, the shear viscosity, and in the superfluid phase both and are the contributions from the normal component of the fluid ( _n and _n ). The experiments were performed with a torsional oscillator operating at 151 Hz. The mole fraction X of³He in the mixtures ranged from 0.03 to 0.65. New data for the total density and data for _n by various authors led to the calculation of . For⁴He, the results for are compared with published ones, both in the normal and superfluid phases, and also with predictions in the normal phase both over a broad range and close to T. The behavior of and of in mixtures if presented. The sloped/dT near T and its change at the superfluid transition are found to decrease with increasing³He concentration. Measurements at one temperature of versus pressure indicate a decreasing dependence of on molar volume asX(³He) increases. Comparison of at T, the minimum of _n in the superfluid phase and the temperature of this minimum is made with previous measurements. Thermal conductivity measurements in the mixtures, carried out simultaneously with those of , revealed no difference in the recorded superfluid transition, contrary to earlier work. In the appendices, we present data from new measurements of the total density for the same mixtures used in viscosity experiments. Furthermore, we discuss the data for _n determined for⁴He and for³He-⁴He mixtures, and which are used in the analysis of the data.     

Radiation of internal waves during vertical motion of a body through a nonuniform liquid

Energy losses to radiation of internal waves during the vertical motion of a point dipole in two-dimensional and three-dimensional cases are computed.Notation _o(z), p_o(z) density and pressure of the ground state - z vertical coordinate - v, p, perturbed velocity, pressure, and density - H(d 1n _o/dz)^–1 characteristic length scale for stratification - N=(gH^–1–g²c _o ^–2 )^1/2 Weisel-Brent frequency - g acceleration of gravity - c_o speed of sound - vertical component of the perturbed velocity - V vector operator - k wave vector - frequency - d vector surface element - W magnitude of the energy losses - (t), (r) (x)(y)(z) Dirac functions - v_o velocity of motion of the source of perturbations - d dipole moment of the doublet - _o,l length dimension parameters - _o intensity of the source Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 619–623, October, 1980.     

Velocity of second sound and superfluid density near the superfluid transition in3He-4He mixtures

Using superleak condenser transducers, the velocity of second soundU ₂ has been measured near the superfluid transition temperature T in³He-⁴He mixtures with molar concentrationsX of³He of 0.0, 0.038, 0.122, 0.297, and 0.440. We have obtained the superfluid density _s/ fromU ₂ on the basis of linearized two-fluid hydrodynamics. The results for _s/ are consistent with those obtained from the oscillating disk method, as expected from two-fluid hydrodynamics. The value of _s/ at eachX could be expressed by a single power law, _s/=k, where =1-T/R, with the experimental uncertainty. It is found that the exponent is independent of concentration forX0.44 within the experimental uncertainty. This concentration independence of is in agreement with the universality concept. From the conclusion that the values of are universal forX0.44, the concentration dependence of the superfluid component _s is expressed by an empirical equation _s(X, )=²_s(0, ). It is found that corresponds to the volume fraction of⁴He in the superfluid³He-⁴He mixture. The value of is in agreement with that obtained from the measurement of the molar volume by others.This paper is based on a thesis submitted to Tokyo University of Education in partial fulfillment of the requirements for the Ph.D. degree.     

The importance of seasonal differences in the cellulose microfibril angle in softwoods in determining acoustic properties

The influence of the micro- and mesoscopic structure of wood cell walls on the acoustic properties of softwood was investigated in a synchrotron X-ray microbeam diffraction experiment with particular attention to the seasonal differences in crystallographic features. A multiple regression analysis was performed for data from 12 different softwood species in order to determine the dependence of longitudinal relative Young's modulus (E/) and loss tangent (tan) on seasonal cellulose microfibril angles (MFAs), crystal width of cellulose microfibrils etc. We conclude that a low MFA in both latewood and earlywood yields high E/ and low tan, which is an attribute of wood used as violin or piano soundboards. Among the softwood species we characterized Sitka spruce best fits this criterion.     

Universality and the scaling laws in the superfluid phase transition of3He-4He mixtures

From the second-sound velocityU ₂ near the superfluid transition point, the superfluid densities in³He-⁴He mixtures, _s (X) and _s (), were deduced along the paths of constant³He concentrationX and of constant chemical potential difference of³He and⁴He. The following critical exponents of _s are determined: (a) =XX for _s (X) in the(X, T) plane,(b) X for _s (X) in the(, T) plane, and(c) for _s () in the(, T) plane. It is found that and _X change by about 4–6% relative to with increasing³He concentration up toX=0.4 and by 8–10% up toX=0.53. It seems that, belowX=0.53, universality hold for . Values of have been found to be in good agreement with the critical exponent of _s in pure⁴He under constant pressure. The values of and _X forX0.53 are also found to be consistent with the scaling relations in the (,T) plane of³He-⁴He mixture.Work performed in part while at the Electrotechnical Laboratory.     

Shear viscosity measurements of liquid 3He-4He mixtures above 0.5 K

Simultaneous measurements of () and of the molar volume are reported for liquid mixtures of ³He in ⁴He over the temperature range between 0.5 and 2.5 K. Here is the shear viscosity and is the mass density. In the superfluid phase, the product of the normal components, _n and _n , is measured. The mixtures with ³He molefractions 0.30 < X < 0.80 are studied with emphasis on the region near the superfluid transition T and near the phase-separation curve. Along the latter, they are compared with data by Lai and Kitchens. For X > 0.5, the viscosity singularity near T becomes a faint peak, which however fades into the temperature-dependent background viscosity as X tends to the tricritical concentration X _t. Likewise, no singularity in is apparent when T _t is approached along the phase separation branches _– and ₊. Furthermore, viscosity data are reported for ³He and compared with previous work. Finally, for dilute mixtures with 0.01 X 0.05, the results for are compared with previous data and with predictions.     

Fast adiabatic heating and temperature relaxation in near-critical fluids under zero gravity

Heat transport in supercritical CO₂ is studied under microgravity conditions. A large temperature and density region around the critical point is explored (CO₂ cells were filled at critical density= _c and off-critical densities= _c±0.18 _c). Local heating is obtained by using a small thermistor located in the bulk fluid. Through interferometric observations, a new mechanism of thermalization has been evidenced. Thermal expansion of a warm diffusing boundary layer around the heating thermistor is responsible for rapid adiabatic heating of the bulk fluid through the emission of pressure waves at the border. The scaled thickness of the thermal boundary layer follows a power law. When the heat flow stops, the bulk adiabatic heating instantaneously vanishes and the temperature relaxation inside the thermal boundary layer follows locally a diffusive process.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Fatigue crack growth behaviour of tungsten carbide-cobalt hardmetals

Fatigue crack propagation studies have been carried out on a range of WC-Co hardmetals of varying cobalt content and grain size using a constant-stress intensity factor double torsion test specimen geometry. Results have confirmed the marked influence of mean stress (throughK _max), which is interpreted in terms of static modes of fracture occurring in conjunction with a true fatigue process, the existence of which can be rationalized through the absence of any frequency effect. Dramatic increases in fatigue crack growth rate are found asK _max approaches that value of stress intensity factor ( 0.9K_IC) for which static crack growth under monotonic load (or static fatigue) occurs in these materials. Lower crack growth rates, however, produce fractographic features indistinguishable from those resulting from fast fracture. These observations, and the important effect of increasing mean free path of the cobalt binder in reducing fatigue crack growth rate, can reasonably be explained through a consideration of the mechanism of fatigue crack advance through ligament rupture of the cobalt binder at the tip of a propagating crack.     

Temperature dependence of electrical resistivity of deformed and undeformed V-rich V3Si single crystals

The electrical resistivity (T) of V-rich V₃Si single crystals (T _c-11.4 K) was measured from 4.2 to 300 K along the directions of [1 0 0] and [1 1 1] before and after plastic deformation at 1573 K. Anisotropy of (T) was observed although V₃Si has the cubic A15 structure. Plastic deformation does not affect the normal-state (T) behaviour but changes the normal-superconducting transition width T_c. At low temperatures (T _c<T 40 K), (T) varies approximately as T ⁿ where n-2.5 and this behaviour does not contradict the (0)- phase-diagram plot proposed by Gurvitch, where is the electron-phonon coupling constant and (0) is the residual resistivity.     

Flux flow resistivity and upper critical field in ideal type II amorphous superconductors

Flux flow resistivity _f and upper critical fieldH _c2 of ideal type II amorphous bulk supercbnductors Zr₃Ni and Zr₃Rh on both as-quenched and thermally relaxed states have been studied. It is found that thermal annealing does not change the temperature dependence ofH _c2 in homogeneous superconductors. The temperature and field dependence of _f in all samples studied exhibits a universal scaling relation of the form _f / _n =f(h, t), where _n is the normal state resistivity, andh andt are the reduced field and reduced temperature, respectively. The results are compared with predictions of the time-dependent microscopic theories for bulk superconductors in the dirty limit. In the low-field region (HH _c2 ) the viscosity coefficient contains both the ordinary (Bardeen-Stephen, Tinkham) and anomalous (Gor'kov-Kopnin) terms. ForHH _c2 the results agree qualitatively with the theory of Imai with pair-breaking in the anomalous term. Implications of the present results are discussed.This work is sponsored by the National Science Foundation under Grant DMR-82-02624.     

Elastodynamic parameters for dynamic interface fracture mechanics

Summary Dynamic extension of cracks running along curvilinear interfaces of brittle bimaterials subjected to mechanical crack surface loads and superimposed thermal strains acting along the ligament is considered. This paper especially addresses the provision and discussion of elastodynamic interface parameters in order to assess quantitatively the bimaterial fracture in view of the governing physical features: applied mechanical and thermal strain loading, existence of an interface, crack-tip velocity and curvature of the interface contour. By utilizing the linear theory of thermoelasticity and adopting Stroh's method of generalized complex potentials, from the corresponding boundary and continuity conditions vectorial Hilbert problems are derived. It is shown that the parameters of the eigenvalues and of the eigenvectors of the Hilbert problems can be interpreted as elastodynamic interface mechanics parameters reading (,v _p, _Hf , _Hf ). Generalized Dundurs parameters of dynamics (, ) and consequently an associated generalized Dundurs diagram of dynamics are proposed. While the aforementioned elastodynamic interface parameters (, ,v _p, _Hf , _Hf ) do not assume the interface to be damaged, interfaces with running interface cracks generally cause two additional interface parameters, denoted as bimaterial constants (, _Hf ), where the latter is specific to the curvature of the interface in conjunction with the velocity of the interface crack. However, the bimaterial constants (, _Hf ) can be traced back to interface parameters for an uncracked bimaterial, namely to (, _Hf ).     

The variation of the index of refraction in a vortex tube

Three models of a vortex tube (isentropic, isothermal, and isochoric) are considered as optical inhomogeneities. Expressions relating the index of refraction to the coordinate are obtained. The possibility of obtaining Schlieren pictures of such flows is discussed.Notation a speed of sound - n index of refraction - p pressure - r radius - T absolute temperature - v linear velocity - angular deflection of the beam - x adiabatic exponent - density - angular velocity - ¯v, ¯p etc. dimensionless parameters - v₀, ₀ values at the core bounboundary - p, etc. stagnation values - n_c index of refraction at center of vortex     

A modified Weibull treatment for the analysis of strength-test data from non-identical brittle specimens

Powder compacts (e.g., pharmaceutical tablets) manufactured on commerically available machines are not strictly identical but show inevitable variability in their weights, thicknesses and compaction pressures. Consequently, the variability in fracture-stress data obtained from such brittle specimens is greater than that due to the inherent strength variability of the material itself. A modified Weibull analysis has been developed so that a more accurate estimate of the inherent variability of the mechanical strength of the material can be derived from test data obtained from commercially produced compacts; its application is illustrated.Nomenclature D diameter - f() relative frequency of occurrence of specimens with density and volume - F minimization function - i ascending rank number of a fracture stress - m Weibull modulus - N _tot number of specimens in a batch - N() number of specimens with densities in the range to + d and volumes in the range to + d - P _f failure probability - p _u upper punch compaction pressure - t thickness - volume - w weight - W _f fracture load - density - _f fracture stress - ¯ _f mean fracture stress of a batch - ¯ _f() mean fracture stress of specimens with density and volume - ₀ scale parameter or normalizing factor - _u location parameter or threshold stress     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

In-Plane Fourfold Symmetry of the Upper Critical Field Observed in La(Sr)214 Single Crystals

In order to establish the existence of in-plane anisotropy of the upper critical field H _c2 (), the out-of-plane resistivity _c measurements were performed on La(Sr)214 single crystals with rectangular and cylindrical shape under rotating magnetic field applied within the ab-plane. Although observed _c shows non-sinusoidal twofold symmetry, clear fourfold symmetry was obtained after subtracting twofold sinusoidal component in _c which is due to the unavoidable misalignment of the magnetic field with respect to ab-plane. H _c2 () is estimated from the extracted fourfold component of _c with the flux flow theory. Angular dependence of H _c2 () was well fitted by cos(4). Since the fourfold component of H _c2 () was largest at = n/2, which corresponds to the a-axis direction, the present results strongly suggest the type symmetry in La(Sr)214. In addition, the difference in fourfold component of H _c2 at = n/2 and n/2+/4 was found to increase with decreasing temperature.     

Experimental status of the ac Josephson effect in liquid helium

The presence of an ac sound field causes several distinct effects in a system of two baths of superfluid liquid helium which are coupled by a small orifice. The primary effects of the sound field can be classified as a Z-T structure, steps, and anomalies. Each of these three effects has been subjected to various criteria which are necessary for an interpretation in terms of the ac Josephson effect. These three effects failed the criteria which were applied, and therefore they are not due to the Josephson effect. However, it is shown that the properties of these three effects correlate quite well with the expected properties of first-sound resonances within one of the two liquid baths.Financial support was provided in part by the National Science Foundation.     

AC electrical conductivity of electron beam evaporated Cu-GeO2 thin cermet films

AC electrical properties of 410 nm think 30 at.wt% Cu-70 at.wt% GeO₂ thin films are reported for the frequency range 10⁴ to 10⁶ Hz and temperature range 150 to 425 K. The loss tangent (tan ) and the dielectric loss (/₀) are found to show striking minima around a cut-off frequency 10⁵ Hz. In the lower frequency range (10⁵ Hz), ₁() ^s T ⁿ is obeyed with s (0 to 0.51) increasing as a function of temperature and n (0.10 to 0.14) showing a very weak temperature dependence. In the higher frequency region (10⁵ Hz), ₁() and /₀ increase sharply leading to the quadratic behavior of ₁() with s equal to 2. These processes are discussed by analyzing an equivalent circuit which shows that at lower frequencies, the effects of series resistance in leads and contacts can be neglected, while at higher frequencies such effect give rise to spurious ² dependance for the conductance. A weakly activated AC conductivity and a frequency exponent s that increases with increasing temperature suggest that the low frequency behavior originates from carrier migration by tunneling process.     

Theory of periodically driven,current-carrying,superconducting filaments. I. Spatially homogeneous states

Starting from the nonequilibrium theory of dirty superconductors in the Ginzburg-Landau regime, spatially homogeneous states with an applied currentI=I ₀+I ₁ cos (t) are considered. Expressions for the linear response (I₁ small) valid up to high frequencies (k _BT_c) are derived and evaluated analytically for the experimentally important case of smallI ₀ and ₀(T). Then the nonlinear response is treated for frequencies with _E1. Interesting new behavior is found for frequencies ₀ 1, where ₀ is essentially the GL relaxation time.     

Electronic specific heat of La2−xSrxCu1−yMyO4(M=Ga,Zn): Impurity effects on a D-wave superconductor

The electronic specific heat C_el was studied in Ga- and Zn-doped La_2–xSr_xCuO₄ (0.16x0.22) at T10K. Partial substitution of Ga or Zn for Cu suppresses T_c and revives the T-linear electronic specific heat, T, markedly. The (y)/n vs T_c/T_c0 relation for Zn-doped samples with x0.2 is in good agreement with the theoretical one for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x 0.2 deviate slightly from the theoretical curve. The deviation will be discussed in relation to changes in the magnetic properties of 3d electrons.