Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

Thermal transport properties of helium near the superfluid transition. II. Dilute3He-4He mixtures in the superfluid phase

Measurements of the average thermal conductivity _exp hQ/T and of the thermal relaxation time to reach steady-state equilibrium conditions are reported in the superfluid phase for dilute mixtures of³He in⁴He. Hereh is the cell height,Q is the heat flux, andT is the temperature difference across the fluid layer. The measurements were made over the impurity range 2×10^–9<X(³He)<3×10^–2 and with heat fluxes 0.3<Q<160 µW/cm². Assuming the boundary resistanceR _b , measured forX<10^–5, to be independent ofX over the whole range ofX, a calculation is given for _exp. ForQ smaller than a well-defined critical heat fluxQ _c (X) X ^0.9, _exp is independent of Q and can be identified with the local conductivity _eff, which is found to be independent of the reduced temperature = (T–T)/T for –10^–2. Its extrapolated value at T is found to depart forX10^–3 from the prediction X ^–1 , tending instead to a weaker divergence X ^–a witha0.08. A finite conductivity asX tends to zero is not excluded by the data, however. ForQ >Q _c (X), a nonlinear regime is entered. ForX10^–6, the measurements with the available temperature resolution are limited to the nonlinear conditions, but can be extrapolated into the linear regime forX2×10^–7. The results for _exp(Q),Q _c (X), and _eff(XX) are found to be internally consistent, as shown by comparison with a theory by Behringer based on Khalatnikov's transport equations. Furthermore, the observed relaxation times (X) in the linear regime are found to be consistent forX>10^–5 with the hydrodynamic calculations using the measured _eff(X). ForX<10^–5, a faster relaxation mechanism than predicted seems to dominate. The transport properties in the nonlinear regimes are presented and unexplained observations are discussed.     

Thermal conductivity and quantum diffusion in solid H2

The thermal conductivity of solid H₂ and the NMR absorption signal of isolated o-H₂ were measured simultaneously along isotherms 0.07<T<1.5 K as a function of time after a rapid cooldown from 2 K. The o-H₂ concentration ranged from 3.4% to 0.4%, and the pressure was 90 atm. During the measurements, clustering of o-H₂ particles occurred as seen from the changes both of the NMR signal amplitude and of with time t. The difference ^–1 = ^–1 ()– ^–1(0) between the thermal resistivity ^–1 (t=0) just after cool down and in equilibrium, ^–1 (), was found to change sign near 0.23 K, and this result is discussed with respect to previous experiments. The equilibrium resistivity attributed to the o-H₂ impurities, , is derived and is compared with previous determinations and with predictions. An analysis of the equilibration process for ^–1 and for the NMR signal amplitude is presented. It shows that the characteristic times are of comparable but not equal magnitude. Comparison of the derived from NMR data atP=90 and 0 atm favors resonant ortho-para conversion over quantum tunneling as the leading mechanism for quantum diffusion.     

Thermal conductivity of superconducting UPt3

The thermal conductivity of the heavy-fermion superconductor UPt₃ has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonalc axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, (H) depends on the relative orientation of field and current. On the other hand, at high fields nearH _c2 (in both samples), (H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.     

γ ? α2 Phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%)

The ₂ phase transformation in fractured high temperature stress rupture Ti-48Al-2Nb(at.%) alloy has been studied by analytical electron microscopy. ₂ and phases were found at grain boundaries. ₂ layers that suspended in layers and interfacial ledge higher than 2d ₍₁₁₁₎ at /₂ interfaces were observed in the lamellar grains. These facts indicated that ₂ phase transformation and dynamic recrystallization have occurred during high temperature stress rupture deformation. It can be concluded that deformation induced ₂ phase transformation and dynamic recrystallization resulted in the presence of particles at grain boundaries. A structural and compositional transition area between deformation-induced ₂(or ) and its adjacently original (or ₂) phases was found by HREM and EDS and is suggested as a way to transform between and ₂ phase during high temperature stress rupture deformation. The transition area was formed by slide of partial dislocations on close-packed planes and diffusion of atoms.     

Dependence of the flux flow viscosity coefficient on the Ginzburg-Landau parameter κ in superconducting indium films

The flux flow viscosity coefficient b ^–1 (in units of ₀ H _{c n} ^–1 c ^–2) of vacuum-deposited indium thin films with low values (1.0–2.8) is measured at temperatures between 0.5T _cand 0.98T _cin the weak magnetic field region. At each temperature, b ^–1 decreases as increases for larger than 1.7. The decrease of b ^–1 with increasing is more rapid as the temperature increases. From an analysis of the present results as well as the existing data on intermediate- and high- superconducting alloys, the relation between b ^–1 and is established over a wide range of . The dependence of b ^–1 on is qualitatively explained by taking into account the contribution of the normal electron dissipation to the viscosity coefficient in the Bardeen-Stephen model.Financial support provided by the Fonds National Suisse de la Recherche Scientifique.On leave from the Department of Electronic Engineering, Faculty of Engineering, Kyushu Industrial University, Fukuoka, Japan.     

Thermal conductivity and phonon scattering mechanisms in La#x2212;x#x2212;x#x2212;xMxCuO4

The thermal conductivity of high density La_2–xM_xCuO₄ (M=Ba, Sr) sintered materials was measured between 15K and 150K for the various concentrations of Ba and the phonon thermal conductivity was analyzed comparing with that of Nd_2–xCe_xCuO₄. The pretty large value for pure La₂CuO₄ was drastically diminished by substituting La by a small amount of Ba atoms especially at low temperatures. It was found that a new type of the phonon scattering center such as a two-level tunneling must be taken account of in order to explain the observed reduction.     

Microstructural characterization of CO2 laser welds in the Al-Li based alloy 8090

The microstructural development of the Al-Li-Cu-Mg-Zr alloy 8090 has been studied after autogenous CO₂ laser welding. Sheets ranging in thickness from 1–4 mm were welded at speeds of between 20–120 mm s^–1 and powers from 1.5–3.8 kW. Optical microscopy, scanning and transmission electron microscopy were used to study the as-received base metal, the heat-affected zone and the solidified fusion zone. The base metal was supplied in a superplastically formable condition and thus had an unrecrystallized grain structure containing 1–2 m sized sub-grains with sub-micrometre and precipitates in the matrix. In the fusion zone, the as-solidified grain structure was columnar at the interface with the base metal but became equiaxed in the central region of the weld pool. The weld depth and top bead width both increased with decreasing welding speed and increasing beam power within the limits investigated. The fusion zone microstructure was cellular-dendritic. Intermetallic precipitates, which are rich in copper, magnesium, silicon (and presumably lithium), formed in the cell/dendrite boundaries. Very fine-scale precipitates were present in the as-solidified -Al matrix but there was no evidence for the , S and T₁ phases. The heat-affected zone was only 100 m wide and was characterized by regions of partial melting. Radiographs of welds reveal that porosity occurred predominantly along the weld centre-line. In partial penetration welds, two types of pores were observed: near spherical and irregular. However, in fully penetrating welds, only the spherical type of porosity was present. Overall volume fractions of porosity were measured from metallographic sections and were found to vary with welding speed and weld type, i.e. partial or full penetration.     

Experimental study of the boundary between single-phase convection and boiling in underheated cryogenic liquids

The effect of pressure and underheating on the position of the boundary between heat-transfer regimes in liquid helium and hydrogen is investigated.Notation q heat flux - p pressure - =T_s–T underheating - T_s saturation temperature - T temperature of liquid - T=T_wa – T T_s=T_wa – T_s - T_wa temperature of heat-emitting surface - A,a, B, b, C constants - m, n indices - Nu Nusselt number - Ra Rayleigh number - thermal conductivity - coefficient of cubical expansion - kinematic viscosity - g acceleration - standard deviation Indices 01 conditions of convection-boiling transition - 02 conditions of boiling-convection transition Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 5–11, January, 1982.     

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.     

Grain growth in the two-phase Al-Cu alloys

Grain and phase growth in the two-phase Al-Cu alloys containing 6, 11, 1 7, 24 and 33 wt % Cu were investigated by annealing at 535 °C for 0.5–100 h. The grain and phase sizes of the phase are seen to be larger than that of the phase. The size of phase decreases whereas the size of phase increases with increasing copper content in the alloy. As such, the phase- and grain-size distributions are broader than the phase- and grain-size distributions, but the size range depends on annealing time and alloy composition. The grain sizes of the ,d , and ,d , phases can be related to the volume fraction of the phase,f , according to the equationd = 0.497d /f .     

Ultrasonic dispersion and attenuation near the liquid-gas critical point of 3He

We report ultrasonic dispersion and attenuation measurements near the liquid-gas critical point of ³He at frequencies from 0.5 to 5.0 MHz and densities from 0.89 _c to 1.15 _c . The singular part of the sound attenuation and the dispersion on the critical isochore _c = 0.0414 g/cm³ are analyzed in terms of the Kawasaki-Mistura theory. If the Ornstein-Zernike order parameter correlation function is assumed in the analysis, good agreement with our data is achieved, except close to the critical temperature T _cin the high-frequency region, where ^* = /_D 1. Here _D is the characteristic relaxation rate of the critical fluctuations. From a fit of the theory to our data, and assuming the inverse correlation length is expressed by = ₀, where = (T–T_c)/T_c with = 0.63, we obtain ₀ = (3.9 ± 0.4) × 10⁹ m^–1. It is found that a more realistic form of the correlation function, as proposed by Fisher and Langer and calculated by Bray, yields even poorer agreement with out data than does the classical Ornstein-Zernike form for ^* > 10. The same difficulties appear in the analysis of the available data for xenon. Thus, the present mode coupling theory is unable to satisfactorily describe the acoustic experiments on fluids near the liquid-vapor critical point over a large range of reduced frequencies ^*. In the appendix, we reanalyze previously reported ultrasonic data in ⁴He, taking into account the nonsingular term of the thermal conductivity. Using = 0.63, we obtain a good fit of the experiment to the theory in the hydrodynamic region with ₀ = (5.5 ± 1) × 10⁹ m^–1.Supported by a grant from 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.     

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.     

Influence of pore volume on laser performance of Nd : YAG ceramics

For present study, 1.1 at% Nd-doped YAG ceramics with a controlled pore volume (150–930 vol ppm) were fabricated by a solid-state reaction method using high-purity powders. The scattering coefficients of Nd:YAG ceramics, obtained from Fresnel' equation, increased simply with increases in the pore volume. The cw laser output power of Nd:YAG ceramics was clearly related to the scattering coefficients of the specimens examined in the present works, which in turn were affected on the pore volume. The effective scattering coefficients of Nd:YAG ceramics with a pore volume of 150 vol ppm were nearly equivalent to those of a 0.9 at%Nd:YAG single crystal by Czochralski method. As the exciting power was increased under excitation by an 808-nm diode laser, however, the laser output power of the Nd:YAG ceramics exceeded that of the Nd:YAG single crystal because of the fairly large amount of Nd additives. The lasing performance of the Nd:YAG ceramics changed drastically with change in pore volume. On the other hand, lasing performance was not affected by the existence of grain boundaries in the polycrystalline Nd:YAG ceramics.     

Thermal Conductivity of Sm3S4 System with Mixed Valence Sm Ions

The thermal conductivity () and electrical resistivity () of mixed-valence compound Sm₃S₄ have been measured in the temperature range 5 to 300 K. The present results and those presented previously [1] for the thermal conductivity between 80 to 850 K are interpreted in terms of the temperature-dependent fluctuating valence of Sm ions. Sm₃S₄ crystallizes in the cubic Th₃P₄ structure, and the cations with different valences occupy equivalent lattice sites. Divalent and trivalent Sm ions are randomly distributed in the ratio of 1:2 over all possible crystallographic cation positions (Sm²⁺ ₂Sm³⁺ ₂S^2– ₄). The behavior of the Sm₃S₄ lattice thermal conductivity _ph is extraordinary since valences of Sm ions are fluctuating (Sm³⁺Sm²⁺) with a temperature dependent frequency. In the interval 20 to 50 K (low hopping frequencies), _ph of Sm₃S₄ varies as _ph T ^–1 (it is similar to materials with static distribution of cations with different valences): at 95 to 300 K (average hopping frequencies 10⁷ to 10¹¹ Hz), _ph changes as _ph T ^–0.3 (it is similar to materials with defects). Defects in Sm₃S₄ appear because of local strains in the lattice by the electrons hopping from Sm²⁺ ions (with big ionic radii) to Sm³⁺ ions (with small ionic radii) and back (Sm²⁺Sm³⁺), at T>300 K (high hopping frequencies), _ph becomes similar to materials with homogenous mixed valence states [1].     

Coefficients of mass transfer between a fluidized bed and the surface of a capillary-porous body

Results are presented from a theoretical determination of coefficients of mass transfer between a fluidized bed of porous particles and a capillary-porous body.Notation a particle radius - F area of contact of particles with the surface of the body - f percentage of area of surface of product in contact with the bubble phase - g acceleration due to gravity - i flow of liquid mass from a unit area of the surface - N number of fluidizations - n number of particles coming into contact with a surface of unit area per unit of time - p_p, p_b capillary potentials of particles and product - R₂, R₁ radii of narrow and broad pores inside the product - r radius of capillaries in the particles - S area of the surface being treated - T temperature of the bed - t time of treatment - u percentage content of liquid in the specimen - V volume of the product being treated - v mean square component of the fluctuation velocity of the particles in the direction normal to the surface - , * standard and corrected mass-transfer coefficients determined from (5) and (9) - _b, _b, _p porosities of product determined for all and for only the small pores and the porosity of the material of the particles - _d, _m porosity of the dense phase and the porosity of the bed in the state of minimum fluidization - _b, _p angles of wetting of the materials of the product and particles, respectively, by the liquid binder - , viscosity and density of the liquid - ₀ density of the dry product - surface tension coefficient of the liquid - characteristic time of contact of particles with the surface - Re_m Reynolds number corresponding to particle radius and minimum-bed-fluidization velocity [6] Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 3, pp. 460–465, March, 1981.     

A note on the similarity solutions for free convection on a vertical plate

Summary The similarity solutions for free convection on a vertical plate when the (non-dimensional) plate temperature is x and when the (non-dimensional) surface heat flux is –x are considered. Solutions valid for 1 and 1 are obtained. Further, for the first problem it is shown that there is a value ₀, dependent on the Prandtl number, such that solutions of the similarity equations are possible only for >₀, and for the second problem that solutions are possible only for >–1 (for all Prandtl numbers). In both cases the solutions becomes singular as ₀ and as –1, and the natures of these singularities are discussed.     

Chemical Control of Underdoped and Overdoped States in Y(Ba2 ? ySry)Cu3O6 + δ

The chemical control of underdoped and overdoped states in the Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1 and 0.9) compounds has been observed by high-resolution O K-edge X-ray-absorption near-edge-structure spectra. The chemical substitution of Sr for Ba in the fully-oxygenated Y(Ba_2–y Sr _y )Cu₃O_{6 +} (0.9) compounds gives rise to high hole concentrations within both the CuO₂ planes and the out-of-plane sites, leading to the overdoped state and the decrease in the superconducting transition temperature from 92 K for y=0 to 84 K for y=0.8. In contrast, an increase in the Sr content in the oxygen-deficient Y(Ba_{2 – y} Sr _y )Cu₃O_{6 +} (0.1) compounds did not indicate superconductivity. The oxygen-deficient compounds exhibit the underdoped state due to the low hole concentration.     

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mössbauer spectra of epitaxial -Fe₁₆N₂ and -Fe₈N films have been studied and their differences are discussed in detail. The Mössbauer spectrum of -Fe₁₆N₂ can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mössbauer spectrum of -Fe₈N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for -Fe₁₆N₂ than for -Fe₈N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for -Fe₁₆N₂. The iron moments tend to locate in the film plane for -Fe₁₆N₂ and to arrange perpendicularly to the film plane for -Fe₈N.