Effect of Ba and Zn doping in Bi2Pb0.6Sr2Ca2Cu3Oδ superconductors using ac susceptibility measurements

Ac complex susceptibility, = – i, measurements were done on the samples doped with barium and zinc (Bi₂Pb_0.6Sr₂Ca_{2 – x}M_xCu₃O, M = Ba, Zn and x = 0.02 and 0.10). The data of shows that coupling of the grains in Zn-doped samples are weaker than that of Ba-doped samples and hence it could be concluded that Zn-doped samples are dominated by the S-I-S type of weak links, whereas the Ba doped samples are dominated by the S-N-S weak links. Calculated values of I₀ is three times higher in the Ba doped samples such that the values of Josephson coupling energy, E_j is four times that of Zn doped samples. Analysis based on the sensitivity of the data of d(T)/dT versus temperature furnished further information on the two-step transitions related to the coupling of the grains in both systems.     

Thermal Expansion and Isothermal Compressibility of TlIn1 – xNdxSe2 (0 ≤ x≤ 0.08) Crystals

The thermal expansion coefficient () and isothermal compressibility ( _T ) of TlIn_{1 – x} Nd _x Se₂(0 x 0.08) crystals were measured between 77 and 400 K. In the range 77–160 K, both and _T increase with temperature, the increase in being much steeper. At higher temperatures, and _T change very little. The observed composition dependences of and _T are interpreted in terms of energy-band structure.     

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.     

Intergranular Critical Current Density in Polycrystalline (Bi-Pb)-2223 High Temperature Superconductor by ac Susceptibility Measurements

The field dependence of real () and imaginary () component of ac susceptibility of superconductors within the critical state model can conveniently be used for evaluating the critical current when field amplitude is larger than the penetration field. A method to analyze the real () and imaginary () component of fundamental ac susceptibility with the objective of extracting the temperature dependence of critical current density J _c(T) is reported. The procedure makes use of the ac susceptibility data of two polycrystalline (Bi-Pb)-2223 samples measured with different excitation amplitudes below and close to the critical temperature. The temperature dependence of J _c is extracted using the isothermal scan over and data. Results obtained from this procedure are found to be in fair agreement with J _c(T) calculated from traditional loss-maximum data.     

Magnetic properties of highly dilutedPdFex andPtFex-alloys. Part II. Susceptibility at micro- and milli-kelvin temperatures

We have measured the 16 Hz susceptibility of the giant magnetic moments induced by Fe impurities in highly dilutedPdFe_x andPtFe_x samples with 2.5 ppm x 75 ppm in a wide temperature range, 30 K T 300 mK, and at static magnetic fields 0,01 mT B 25 mT. We find spin glass freezing at Tf(X)/X0,19mK/ppm Fe forPdFe_x and the larger value 0.26 mK/ppm Fe forPtFe_x. This is the first observation of spin glass freezing inPtFe_x. In the low-temperature range T 0.5T_f(x), the susceptibilities follow — ₀ T with small zero-temperature ₀ values forPdFe_X and vanishing ₀ values forPtFe_x. In the paramagnetic high-temperature range, we find (T — )^it-1 at T 10 mK independent of x forPdFe_x, and at T 2Tf(x) dependent of x forPtFe_x with vanishing values for both systems. The data compare well to the predictions of the Thouless-Anderson-Palmer TAP approach of the Sherrington-Kirkpatrick SK model for spin glasses.     

Molar volume and magnetic susceptibility of borate glasses containing iron

A glass system was prepared from reagent grade chemicals having the formula 58 mol % B₂O₃ - 22 mol % CaO - (20 - ) mol % Al₂O₃ - mol % Fe₂O₃, where has different values between 2 and 20 mol %.The molar volume () and magnetic susceptibility () of these glasses has been measured as a function of Fe₂O₃ content. The values of and were found to increase as the Fe₂O₃ content in the glass increases in the region between 2 and 8 mol % and in the region where the content is greater than 10 to 16 mol %, while a decrease was found for samples having an Fe₂O₃ content of 10 mol % and more than 16 mol %. These results could be explained by the change in coordination of iron from FeO₆ to FeO₄, and also the change of Fe³⁺ to Fe²⁺ and change of Fe²⁺ from six coordination to four coordination.     

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.     

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.     

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.     

Study of History Effect of Vortex Matter by AC Susceptibility

We have observed numerically the history effect, based on bulk flux pinning framework, in the susceptibility (T) (acs), experimentally measured in different samples. The dependencies of history effect in (T) on frequency, ac and dc fields, and bulk pinning strength are calculated. Our calculation matches well the behavior of in 2H–NbSe₂, showing the bulk model works well. It is also shown that bulk pinning and surface pinning will result in a history effect of very different characteristics. In addition, the numerical result shows that a peak in critical current density, which is usually accompanied by the vortex melting transition, may not be observed as a dip in acs in some cases, which is a dynamic response of a sample to applied as field and is dependent on ac field frequency and amplitude.     

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.     

Nuclear susceptibility of liquid He3—II

Pulsed NMR measurements of the nuclear susceptibility of liquid He³ at various molar volumes are presented over the temperature range between 0.035 and 1°K. The scatter in the data is about 0.2%, while the absolute accuracy is estimated to be about ±1%. At temperaturesT such that ₀ T/C0.35 the susceptibility data can empirically be represented by the expression =₀[1–(x₀ T/C)²]. Here ₀ is the extrapolated molar susceptibility atT=0 for a given volume, assuming the expression to be valid below 35 m°K.C is the molar Curie constant and is a factor that appears to be somewhat dependent on molar volume and whose average value is found experimentally to be about 0.53. This result compares with the theoretical prediction by Béal-Monadet al. who used the paramagnon model for liquid He³ and obtained the expression given above with a value of =0.58. However, the observed apparentT ²dependence of the susceptibility is over a greater temperature range than the theory would lead one to expect, and this point is discussed. The experimental results are also compared with a molecular-field approach by Goldstein, who proposed a representation of the susceptibility by a universal curve T/C=f(₀ T/C), wheref is the function for the ideal gas. In spite of some systematic discrepancies, a scaled presentation of the data is in good overall agreement with the calculated curve. The data disagree with the Stoner model of a Fermi gas with interactions. Finally, the values of ₀ are compared with those of previous work.Research supported by grants from the National Science Foundation and from the Army Research Office (Durham).     

Correlation of burst pressure of GRP cylinders with the stress wave factor

Glass fibre-reinforced plastic (GRP) cylinders are increasingly used for highly stressed structural elements. The higher the demands on the materials, the higher are the fault detection requirements to be met by non-destructive materials testing methods. Acousto-ultrasonics is a valuable aid for the non-destructive evaluation of GRP composite materials, because it may be the answer to evaluating effects of subtle defects in composites. The aim of the research is to evaluate the burst pressure of GRP cylinders by acousto-ultrasonics techniques. The theoretical results have been found to be in good agreement with the experimental values. Hence the results strongly suggest that stress wave factor measurements can be exploited successfully to predict burst pressure of GRP cylinders.Nomenclature P Internal pressure, kgf cm^–2 - d Internal diameter, cm - t Thickness of cylinder, cm - (N ,N ,N ) Resultant forces, kgf - (M ,M ,M ) Moments, kg cm - [A] Extensional stiffness matrix - [B] Bending stretching coupling matrix - [D] Flexural stiffness matrix - ( ⁰ , ⁰ , ⁰ ) Midplane strains - (k ,k ,k ) Curvatures - n Number of laminae - Z Distance from midplane, cm - _u Ultimate tensile strength of GRP composite, kg cm^–2 - S _W Stress wave factor - m Material parameter - Filament winding angle     

Environmental stress cracking of glassy polymers and solubility parameters

The use of solubility parameters to predict critical stress ( _c ^* ) or strains (_c) for environmental cracking/crazing in several glassy polymers (e.g. PMMA, PPO, PS, PVC, PSF and PC) is re-examined. It is shown that the enthalpic component ( _H ) of the Flory-Huggins semi-interaction parameter () does not always give a good correlation between _c and _H even though solvent molar volume and polymer-solvent molecular interactions have already been considered. Re-analysis of available experimental data using Gent's theory shows that there is a general trend for _c ^* (or _c) to increase with . These results, therefore, support Gent's proposed mechanism of environmental stress crazing/cracking. It is finally concluded that unless a definite relationship can be established between _c or _c ^* with _H it is not possible to predicta priori _c or _c ^* , given the empirical solubility parameters of a solvent. Unfortunately, there are not many such relationships as discovered in this paper.     

Density of States in Superconductor-Normal Metal-Superconductor Junctions

We consider the ₀ -dependence of the density of states inside the normal metal of a superconductor-normal metal-superconductor (SNS) junction. Here ₀ is the phase difference of two superconductors of the junction. It is shown that in the absence of electron-electron interaction the energy dependence of the density of states has a gap which decreases as ₀ increases and closes at ₀ = . Both the analytical expressions for the ₀ -dependence of the density of states and the results of numerical simulations are presented.     

Phase transformation behavior of nanocrystalline χ-alumina powder obtained by thermal decomposition of AIP in inert organic solvent

Thermal decomposition of aluminum isopropoxide (AIP) in inert organic solvents (toluene and mineral oil) resulted in the formation of -alumina. Phase evolution by calcination at various temperatures for this alumina was studied via X-ray diffraction. The results suggest a direct transformation from -alumina to -alumina at approximately 1180°C, without the formation of -alumina phase, while still maintaining the small particle size (<100 nm). The transformation behavior was observed by TEM and the crytallite size was calculated by the Scherrer equation. The results indicate one -alumina crystal transforms into one -alumina crystal at its critical size in a nucleation step. This crystal exhibits a rapid grain growth following the transformation.     

Magnetic Properties of High-Resistivity CdTe〈In〉 and CdTe〈Cl〉 Crystals

The magnetic susceptibility of high-resistivity CdTeIn and CdTeCl crystals was measured between 4.2 and 300 K. The susceptibility was found to vary anomalously with temperature. Below 50 K, all the samples were paramagnetic. The observed anomalies are interpreted in terms of donor–acceptor pairs formed by native defects and dopant or uncontrolled impurity atoms. The effect of doping on the 300-K is related to the Van Vleck paramagnetic contribution resulting from the local electric fields of X_i–V _Cdand In_i–V _Cddefect complexes. In CdTeCl, this contribution is insignificant.     

Brittleness and thermal stability of graphites

The correlation between calculated values of thermal-stability criteria and experimentally measured values of the failing temperature gradient for 10 graphites is investigated. It is demonstrated that use of the brittleness measure makes it possible to divide the materials into two groups: relatively brittle (<0.85) and transitional (>0.85). A linear correlation between the criterion R and the failing temperature gradient T is established for materials of the transitional group, whereas a correlation between the criterion T_c and T is indicated for the relatively brittle materials.Translated from Problemy Prochnosti, No. 9, pp. 61–63, September, 1990.     

Magnetic properties of single-crystal uranium mononitride

The susceptibility of a single crystal of antiferromagnetic uranium mononitride (T _N 53 K) is reported between 4 and 1000 K. The paramagnetic susceptibility follows a modified Curie-Weiss law with a temperature-independent susceptibility ₀=8.0×10^–6 emu cm^–3 added, =–247 K, and 2.66 Bohr magnetons/U atom. Nearest and next nearest neighbor exchange constants are evaluated. The anisotropy in the ordered region is discussed on the basis of an equal distribution of antiferromagnetic domains with spin direction along any of the three 100 axes. Such an assumption is corroborated by thermal expansion measurements along the three 100 axes.Supported financially by the South African Atomic Energy Board and the Council for Scientific and Industrial Research. Experimental measurements performed at the Rand Afrikaans University.     

Thermal Conductivity of the Four-Leg Spin-Ladder System La2Cu2O5 Single Crystal

We have investigated the magnetic susceptibility, , and the thermal conductivity, , in magnetic fields for the four-leg spin-ladder system La₂Cu₂O₅ single crystal. The in a magnetic field parallel to the ladder exhibits a kink at 130 K in correspondence to the magnetic ordering. The along the ladder exhibits a peak at 25 K and a shoulder at 14 K, which are probably related to the thermal conductivity due to magnons, _magnon, and that due to phonons, _phonon, respectively. The perpendicular to the ladder, on the other hand, exhibits only one broad peak related to _phonon. The observed large anisotropy of has been explained based upon the anisotropy of _magnon.