Anisotropic spin diffusion and multiple spin echoes in3He-4He solutions

We propose a ₁-t₁-₂ pulse-NMR experiment to detect the spin-diffusion anisotropy, =D-D, in degenerate spin-polarized³He-⁴He mixtures, where D and dare the transverse and longitudinal spin diffusion coefficients. In such an experiment the nonlinearity of the dynamics produces multiple spin echoes (MSE). At the ³He concentration x₃ 4% the spinrotation parameter vanishes (M 0), so that the nonlinearity of the equations of motion is entirely due to the anisotropy. In this situation, detection of MSE amounts to observation of D. For slight anisotropy, i.e. D/D 0.25, we use a perturbation scheme similiar to that developed by Einzel et al. (in that case, for small M and small demagnetizing field) to calculate the second and third echo heights. For larger anisotropy we numerically calculate the echo heights. We find that for D/D = 0.5 the heights are 2 % of the first echo, and should be detectable. The (₁, ₂) tip-angle dependence of the D echoes is different from that of the M and demagnetization echoes, and furthermore, they occur at right angles to these echoes (in spin space). Thus, even when small spin-rotation and demagnetization effects are present, the ₁-t₁-₂ experiment provides a sensitive means of detecting the anisotropy.     

Temperature and randomness dependence of the pair-breaking parameter in superconducting aluminum films

Magnetoconductance and excess conductance due to superconducting fluctuations in aluminum films are measured in order to study the temperature dependence of the pair-breaking parameter at temperatures nearT _c . The parameter _M is estimated from the relation =/8k _B T_in, where _in is the inelastic scattering time deduced from the analysis of the magnetoconductance. The parameter _F is determined by fitting theories to data on the excess conductance at zero magnetic field. It is shown that: (1) For films with a wide range of the sheet resistanceR , 12R 200 /, the temperature dependence of _M nearT _c agrees well with the theory of Brenig et al. (2) For clean films withR 100 /, the value of _F analyzed with theories including the correction term to the Maki-Thompson contribution shows almost the same temperature dependence as _M . In a film withR 200 /, however, a discrepancy between _M and _F remains.On leave from College of General Education, Kyushu University, Ropponmatsu, Fukuoka, Japan.     

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.     

Energy Scales in the Local Magnetic Excitation Spectrum of YBa2Cu3O6+y

The wave-vector integrated dynamical spin susceptibility _2D() of YBa ₂ Cu ₃ O _6+y cuprates is considered. _2D is calculated in the superconducting state from a renormalized mean-field theory of the t–t–J-model, based on the slave-boson formulation. Besides the well-known 41 meV resonance a second, much broader peak ('hump') appears in Im _2D. It is caused by particle-hole excitations across the maximum gap ⁰ . In contrast to the resonance, which moves to lower energies when the hole filling is reduced from optimal doping, the position of this 'hump' at 2 ⁰ stays almost unchanged. The results are in reasonable agreement with inelastic neutronscattering experiments.     

Nonlinear Response of a Continuous Wave NMR Spectrometer for Samples with a Large Magnetization

We present a new insight in the response of a CW NMR spectrometer for highly magnetized samples above 100 MHz. The spectrometer is a bridge made of a magic T. The output of the bridge is proportional to the reflection coefficient, , of a resonant circuit, which is built with a coil containing the sample. The sensitivity of the reflection coefficient to the complex susceptibility of the sample, () = () – j(), depends on the quality factor, Q, of the circuit and filling factor, . When the condition Q 1 is not fulfilled, we show indeed that the use of a simple crystal detector, which is only sensitive to || gives rise to a strong nonlinear response of the spectrometer. Measurements of the complex value of by means of phase sensitive detection allow to recover a linear behavior. We discuss and illustrate those issues with a few circuits we designed for our measurements on liquid ³ He with spin polarizations up to 15 %. A method is described to build in a reproducible and predictable way resonant circuits matched to 50 in the frequency range 100 – 400 MHz with a quality factor as high as 1000 at 4 K.     

Interplay of Nuclear Magnetism and Superconductivity

The ratio of nuclear saturation magnetization and superconducting critical field, ₀ M _sat / B _S0*, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases 1, 1, and 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn ₂ , and of the metal hydride TiH _2.07 at ultralow temperatures, 17 K T 1 K, as function of static field 0 B 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn ₂ , we get a steep decrease of B _S (T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn ₂. The metal hydride TiH _2.07 , which is under present investigation, is a good candidate to show reentrant superconductivity.     

Microstructural study of ε-iron-carbonitrides formed by a glow discharge technique

The microstructure and phase transformations occurring in-iron-carbonitrides have been studied by means of X-ray and electron diffraction, electron microscopy and Mössbauer spectrometry. Ordering of the interstitial atoms, N or C, results in a hexagonal unit cell for Fe₃(C, N) with parametersa=a3 andc=c wherea andc are the lattice parameters of the hexagonal close-packed (h c p) iron unit cell. Stacking faults on (0001) planes and partial dislocations with Burgers vector b=1/31 0 ¯1 0 are observed in quenched-Fe₄ (C, N). After quench-aging, the carbonitrides show a structural hardening due to the precipitation of a metastable phase. Slow cooling of-carbonitrides with less than 25 at% interstitials leads to the precipitation of-carbonitride and ferrite in-phase grains which allows the orientation relationships between the-,- and-phases to be defined and a model of the-phase--phase transformation to be proposed.     

The morphology of α′ martensite in a two-phase (α+γ) Fe-Cr-Ni stainless steel

Both lath-shaped and martensites are induced by tensile deformation within of a two-phase ( + ) Fe-Cr-Ni stainless steel, forms from the through the at an intersection of two crystals. These are observed both when is surrounded by and when borders . The amount of strain at which both and nucleate, increases with test temperatures in the range –196 to 50° C. Adjacent laths are either twin-related, or 5, 9, 15 or 19° off the twin relationship, as found by analysing electron diffraction patterns.     

Diffraction studies of β″-magnesium sialon

The compositions of high magnesium content -magnesium sialon crystals within the 3M/4X plane of the Mg-Si-Al-O-N system indicate some form of metal atom ordering wihtin the structure. Although it is not possible using X-ray diffraction to detect weak additional reflections arising from the ordering between magnesium, aluminium, and silicon atoms, such weak reflections are revealed on electron diffraction photographs for a phase, resulting in a tripling of the hexagonal latticec-dimension. These show to have a structure very similar to rhombohedral willemite (Zn₂SiO₄), with the (Mg, Al) metal atoms ordered with respect to the silicon atoms in a 21 ratio. However, there are some additional weak diffraction spots with indices not obeying the rhombohedral condition of –h+k+l=3n. It is proposed that the structure of this is identical to that of willemite and the extra spots are a result of some form of twinning, which implies the existence of ordered microdomains.     

Paramagnetic resonance in a “dirty” spin-glass

For a spin-glass with nonmagnetic defects (n _m ^1/3l 1, where n _m is the magnetic impurity concentration and l is the mean free path) an absorption function () is derived. Three ranges of temperature and external magnetic field are considered. In the vicinity of the transition the value of () d is estimated as a function of temperature and field.     

The structure of the γ′ extended solid solution in a splat-cooled Ag-50 at.% Cu alloy

The structure of the extended solid solution in electron transparent areas of a splatcooled Ag-50 at.% Cu alloys was examined by transmission electron microscopy. This phase was usually found to be spinodally decomposed at large grain sizes (–1m in diameter), in contrast to X-ray diffraction data indicating that the solid solution was undecomposed. A solidification model for rapidly quenched eutectic alloys is proposed to account for the observed structure of the splat-cooled alloy. A transformation curve for the spinodal decomposition of is also calculated and related to predictions derived from the solidification model.     

Spectral shape dependence in the very strong coupling regime of Eliashberg theory

The effect of ² F() shape dependence on several physical properties of superconductors is studied at various values of the strong coupling indexT _c/ _ln . Our results indicate that the degree of shape dependence of each property is sensitive to the value ofT _c/ _ln . Generally, for the region we examine, 0.25T _c/ _ln 1.3, the dependence on shape is found to be higher than in the conventional strong coupling regimeT _c/ _ln 0.2. However, with the exception of the mass enhancement parameter , the amount of shape dependence does not increase steadily withT _c/ _ln and there appears to be regions of maximum shape sensitivity.     

The elastic torsion problem for a nonhomogeneous and transversely isotropic half-space

Summary The statical Reissner-Sagoci problem for nonhomogeneous transversely isotropic elastic half-space is investigated. The modulus of rigidity is assumed asC _ii (r,z)= _ii r f(z),i=4, 6 and 0, where _ii are constants. The expressions for stresses, displacement and torque are given. The analysis is based on the assumption that the tangential displacement is prescribed within the areaz=0,r1 and shearing stress is zero in the outside arear>1,z=0. The integral equation which is obtained is solved in the case whenf(z)=cosh² kz and 0, wherek is a constant.With 7 Figures     

The martensitic transformation in cold-worked Fe-Mn alloys studied by Mössbauer spectroscopy

The effect of cold-working on 16, 23 and 30 at% Mn iron-manganese alloys (C<0.05 at%) has been examined using Mössbauer spectroscopy and X-ray diffractometry. The induced martensitic transformation , depends on the composition and on the initial structure. It is found that the transformation in presence of the phase occurs only at high deformations.     

Modeling of Natural Thermogravitational Convection in Horizontal Channels with an Irregularly Shaped Cross Section

Results of a numerical investigation of the natural thermogravitational convection of air in horizontal channels with a shaped cross section are presented. Twodimensional unsteady equations of convection in the Boussinesq approximation, which are written in the velocity vortex–current function–excess temperature variables, are employed as a mathematical model. In investigating, the range of Rayleigh numbers was bounded above by the value 10⁶, which corresponds to the range of applicability of the Boussinesq approximation to air in the problem posed. The boundaryvalue problem was solved numerically by the finiteelement method of Galerkin (weak formulation). The distinctive features of the temperature and flow fields were revealed on the basis of calculations and the intensity of heat exchange was evaluated.     

Microstructure and mechanical properties of rapidly quenched L20 and L20+L12 alloys in Ni-Al-Fe and Ni-Al-Co systems

Ductile L2₀-type wires and+L1₂-type duplex wires with high strengths and large elongation in the Ni-Al-Fe and Ni-Al-Co ternary systems have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter was in the range 80 to 180m and the average grain size was 2 to 4m for the wires and 0.2 to 1.0m for the+ wires. _y, _f and _p of the wires were found to be about 360 to 760 MPa, 560 to 960 MPa, and 0.2 to 5.5%, respectively, for the Ni-Al-Fe system, those of the+ wires were about 395 to 660 MPa, 670 to 1285 MPa, and 3.5 to 17%, respectively, for the Ni-Al-Fe system, and about 260 to 365 MPa, 600 to 870 MPa, and 4.0 to 7.0%, respectively, for the Ni-Al-Co system. Cold-drawing caused a significant increase in _y and _f and the values attained were about 1850 and 2500 MPa, respectively, for Ni-20Al-30Fe and Ni-25Al-30Co wires drawn to about 90% reduction in area. The high strengths, large elongation and good cold-workability of the melt-quenched and+ compound wires have been inferred to be due to the structural change into a low-degree ordered state containing a high density of phase boundaries, suppression of grain-boundary segregation and refinement of grain size.     

Streamline curvature effects in side-channel spillway flow

Summary Steady flow in side channel spillways is investigated, thereby accounting for the effects of the streamline slope and curvature on the free surface profile. The governing system of equations is linearized in order to discuss the main features of the solutions. Subsequently, the non-linear version is solved by an implicit numerical procedure; particular attention is paid to the stability of the chosen scheme. Finally, the solutions are compared to observations and an almost perfect agreement is found.Notation a reference flow depth - b channel width - F Froude number - g gravitational acceleration - h flow depth - J average energy gradient - K Strickler's roughness coefficient - L lateral inflow length - m root of characteristic polynomial - n Manning's roughness coefficient - p lateral inflow intensity per unit channel width - q lateral inflow intensity - Q discharge - R hydraulic radius - S specific momentum - S _f frictional slope - S ₀ bottom slope - V average cross-sectional velocity - x longitudinal coordinate - X normalized longitudinal coordinate - y normalized flow depth - factor, 01 - factor, 01 - perturbative change in flow depth - relative discharge intensity - relative specific momentum With 7 FiguresSeveral symbols may appear with indices u, d, N, c and then refer to the up-and downstream end of the lateral inflow zone, and to uniform and critical flow conditions, respectively. Primes indicate derivatives with respect to the longitudinal coordinate.     

Superconducting state in M3C60: Strong vs. weak coupling

The superconducting state in the doped fullerenes is due to strong coupling (e.g.,2.1 for Rb₃C₆₀) to low-frequency intramolecular modes _L 250 cm^–1 (^21/2). The analysis is based on an equation describingT _c for any strength of the coupling and on recent isotope effect and NMR data.     

Phase transformation and impact properties of type 17-8-2 austenitic weld metals

The influence of stress relieving and ageing treatments in the range 600 to 900° C on the phase transformations and change in room temperature impact properties has been studied for two manual metal arc 17-8-2 weld metals. The transformation of the-ferrite in the range 600 to 800° C was found to conform to a classical Johnson-Mehl equation; the initial precipitates were M₂₃C₆ carbides followed by the intermetallic-phase. At higher temperatures a slower transformation rate was found suggesting a C curve type of behaviour and the dominant intermetallic phase changed to. Room temperature impact toughness values were found to change with ageing time. Below 800° C there was a consistent fall in these values which became very marked when the-phase developed at the/-boundaries, Above 800° C, spherodization of the carbides and intermetallic phases delayed the fall in the impact values, and led to significant increases in the early stages of ageing. Scanning electron microscopy confirmed that the change from ductile to brittle fracture mode was normally associated with the development of the and phases, but at 600° C the fall in impact properties could be atrributed to carbide development.     

Spin hydrodynamic equations with external disturbances and suitable Green's functions for superfluid3He-B. New onsager relations

The spin hydrodynamic equations for superfluid³He-B have been obtained for the case of external, time-dependent fields. On the basis of a microscopic approach, expressions are found for additional terms in equations containing these fields. Considering the linear response of the system to the switching on of external fields, formulas are found for suitable Green's functions (magnetization-magnetization, rotation-rotation, magnetization-rotation, rotation-magnetization). The rotation-rotation Green's function has the 1/q ² singularity characteristic of superfluid systems. Connections between Green's functions lead to relations among kinetic coefficients v, ₁, and ₂. It is also shown that there is a conserved quantityQ ^(B) = div v _s ^(B) that describes sources or magnetic type charges (monopoles) of the superfluid velocity v _s ^(B) . Comparison with the phenomenological approach suggests thatQ ^(B) is proportional to a pseudoscalar giving the projection of the spin density onto the vector describing the axis of rotation.