Pulsed Nuclear Magnetic Resonance on 3He Adsorbed on Bare and 4He Preplated MCM-41 Using DC SQUID Detection

We report low field DC SQUID NMR measurements down to 1.5 K of ³He adsorbed in the pores of the mesoporous molecular sieve MCM-41. In the first experiment measurements were made on ³He adsorbed onto the bare pore walls of MCM-41 with coverages ranging from $n_{^{3}\mathrm{He}}=0.86n_{1}$ to full pores at $n_{^{3}\mathrm{He}}=1.79n_{1}$ , where n ₁ is the coverage for monolayer completion. A second experiment was performed with low ³He coverages ( $n_{^{3}\mathrm{He}}\sim0.01n_{1}$ ) on ⁴He preplated pores, where a crossover to a quasi-1D state is expected to occur at temperatures sufficiently below 700 mK. In both experiments relaxation times T ₁ and T ₂ ^* were measured as a function of temperature and coverage at frequencies from 80 to 240 kHz. The frequency dependence of the linewidth in the pure ³He experiment is extremely weak therefore T ₂ ^* ≈T ₂. The 1.5 K isotherm shows a small minimum in T ₂ ^* at a coverage corresponding to monolayer completion. In the experiment with ⁴He preplating there was no significant change in T ₁ or T ₂ ^* when the ³He coverage was doubled from $n_{^{3}\mathrm{He}}=0.01n_{1}$ to 0.02n ₁ at a ⁴He preplating of $n_{^{4}\mathrm{He}}=1.05n_{1}$ . This suggests that the relaxation times are dominated by single particle effects in the low density regime.     

Effect of nonideality on the collisional recombination coefficient in an ultracold plasma

The effect of nonideality on the dependence of the collisional recombination coefficient α in an ultracold plasma on the temperature T _e and density n _e of electrons is discussed. It has been shown that the dependence of the recombination coefficient on the density and temperature α ? n _e ² T _e ^?9/2 characteristic of a weakly nonideal plasma is transformed to α ? n _e ^1.2 T _e ^?2 when the ratio of the potential energy to the kinetic energy of an electron (nonideality parameter) reaches unity. As the nonideality parameter increases, the temperature dependence disappears and the density dependence becomes n _e ^1/2 . The existing theoretical results for various physical models have been analyzed and compared to the experimental data.     

Effective electromechanical properties of ferroelectric piezoactive composites of the crystal-ceramic type based on (Pb1−x Cax)TiO3

Based on the previously proposed models, effective electromechanical properties of the ferroelectric piezoactive composites of the “single-domain crystal-ceramic” and “polydomain crystal-ceramic” types are determined for the first time. Features of the concentration dependence of the piezoelectric coefficients of four types (e _3j ^* , d _3j ^* , g _3j ^* , and h _3j ^* ) are interpreted. Advantages of the new composites in comparison with the known ferroelectric piezoceramic materials of the PbTiO₃ type are considered and explained. It is shown that the coefficients e _3j ^* are most sensitive to a change in the system parameters; some reasons for this sensitivity are established.     

A model for paramagnetic Fermi systems

We develop a model for paramagnetic Fermi liquids. This model has both direct and induced interactions, the latter including both density-density and current-current response. The direct interactions are chosen to reproduce the Fermi liquid parametersF ₀ ^s ,F ₀ ^a ,F ₁ ^s and to satisfy the forward scattering sum rule. TheF ₁ ^a andF ₁ ^s,a for1>1 are determined self-consistently by the induced interactions; they are checked against experimental determinations. The model is applied in detail to liquid³He, using data from spin-echo experiments, sound attenuation, and the velocities of first and zero sound. Consistency with experiments gives definite preferences for values ofm*. The model is also applied to paramagnetic metals. Arguments are given that this model should provide a basis for calculating effects of magnetic fields.     

Self-diffusion of14C in polycrystalline β-SiC

The¹⁴C self-diffusion coefficients for both lattice (D _lc ^* ) and grain boundary (D _bc ^* ) transport in high purity CVDβ-SiC are reported for the range 2128 to 2374 K. The Suzuoka analysis technique revealed thatD _bc ^* is 10⁵ to 10⁶ faster thanD _bc ^* ; the respective equations are given by $$\begin{gathered} D_{I c}^* = (2.62 \pm 1.83) \times 10^8 exp\left\{ { - \frac{{(8.72 \pm 0.14)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ D_{b c}^* = (4.44 \pm 2.03) \times 10^7 exp\left\{ { - \frac{{(5.84 \pm 0.09)eV/atom}}{{kT}}} \right\}cm^2 sec^{ - 1} \hfill \\ \end{gathered} $$ A vacancy mechanism is assumed to be operative for lattice transport. From the standpoint of crystallography and energetics, reasons are given in support of a path of transport which involves an initial jump to a vacant tetrahedral site succeeded by a jump to a normally occupied C vacancy.     

Sublimation study of BiBr3

Steady-state sublimation vapour pressures of anhydrous bismuth tribromide have been measured by the continuous gravimetric Knudsen-effusion method from 369.3 to 478.8 K. Additional effusion measurements have also been made from 435.4 to 478.6 K by the torsion—effusion method. Based on a correlation of Δ_sub H ₂₉₈ ⁰ and Δ_sub S ₂₉₈ ⁰ , a recommended p(T) equation has been obtained for BiBr₃(s) $$\alpha - {\rm B}i{\rm B}r_3 :log{\text{ }}p = - C\alpha /T - 12.294log{\text{ }}T + 5.79112 \times 10^{ - 3} {\text{ }}T + 47.173$$ with Cα=(Δ _subH ₂₉₈ ⁰ +20.6168)/1.9146×10^-2 $$\beta - {\rm B}i{\rm B}r_3 :log{\text{ }}p = - C\beta /T - 23.251log{\text{ }}T + 1.0492 \times 10^{ - 2} {\text{ }}T + 77.116$$ with Cβ=(Δ _subH ₂₉₈ ⁰ +46.2642)/1.9146×10^-2 where p is in Pa, T in Kelvin, Δ _sub H ₂₉₈ ⁰ in kJ mol^?1. Condensation coefficients and their temperature dependence have been derived from the effusion measurements.     

Superconducting transition temperature of aluminum,indium, and lead fine particles

Aluminum, indium, and lead fine particles with free surfaces were made by evaporation in helium gas to study the size dependence of the superconducting transition temperatureT _c in very small particles. The transition temperature was determined from the change of diamagnetic susceptibility with temperature. The present experiment shows thatT _c rises with decrease of the average diameter \(\bar d\) for aluminum and indium, while it remains nearly constant for lead. The enhancement factorsT _c /T _c ^b (T _c ^b isT _c for the bulk material) in the case of \(\bar d = 100 {\AA}\) are determined as 1.45, 1.15, and 1.0 for aluminum, indium, and lead, respectively. The effect of the soft phonon mode arising from the increasing fraction of surface satisfactorily explains the experiment with respect to both the size dependence and the difference among elements by a rather simple model.     

Optical study of charge dynamics along thec-axis in high-T c superconductors

Comparing the optical spectra withE∥c for typical high-T _c superconducting cuprates, we discuss the charge dynamics along thec-axis. The plasma energy or the mass anisotropic factorm _* ^c //m _* ^b is one of the key parameters determining the spectrum forE∥c. In Bi₂Sr₂CaCu₂O₈ and La_2?x Sr _x CuO₄ with a largem _* ^c /m _* ^b , the plasma energy is smaller than the superconducting gap energy, and thus the supercurrent along thec-axis is a kind of Josephson current flowing through insulating layers such as the BiO layers. On the other hand, in YBa₂Cu₃O₇ with a smallm _* ^c /m _* ^a , it seems that the coherent supercurrent flows along thec-axis. Although the spectrum forE∥c strongly depends on samples, presumably due to the difference in the hole concentration, a clear anisotropy between theab- and thec-directions is observed in the characteristics energy scales such as the plasma energy as well as the reflectivity knee energy.     

Effect of a magnetic field on the specific heat jump of a superconducting alloy containing paramagnetic impurities with local states within the gap

The effect of a uniform magnetic field on the specific heat jump \({\Delta C}\) of a superconductor containing paramagnetic impurities has been investigated. The impurities are described by the Shiba-Rusinov model. Taking the normalized position of the bound state within the BCS gap ε₀=0.8 and 0.0 and the parameter α(=〈S _z ² 〉/S²)=1.0 and 1/3 as typical values, we find the detailed dependence of \({\Delta C}\) on the impurity concentration \(\bar c\) and the transition temperature T_c. In some cases \({\Delta C}\) diverges at certain values of \(\bar c\) or T_c, signaling a change of the normal-superconductor phase transition from second order to first order.     

The heat capacity of cadmium telluride at medium and high temperatures

The modified Schomeit method for processing enthalpies (H _T ⁰ -H _298.15 ⁰ ) is suggested, which provides for agreement between high-temperature data on C _p both with a reliably measured value of C _{p, 298.15} ⁰ and with derivative $\left( {\frac{{\partial C_p^0 }}{{\partial T}}} \right)_{298.15} $ . The latter gives a “smoother” joining of dependences C _p (T) obtained experimentally with different accuracy. The suggested procedure is applied to analysis of experimentally obtained distributions of heat capacity of cadmium telluride as a function of temperature, and a new approximating equation is obtained for the most reliable data in the range from 298 K to the melting point of CdTe. This procedure is compared to other known methods of simultaneous processing of C _p (T) data. It is demonstrated that the values of heat capacity of CdTe recommended by us agree with reliably determined coefficients of thermal expansion (CTE) of this compound. The characteristic Debye temperature of cadmium telluride is estimated in a wide temperature range, and the pattern of its variation with temperature is analyzed.     

Raman and infrared studies of cupric oxide

Polarized Raman and fourier-transform infrared (FTIR) measurements have been made on a single crystal of CuO. Group theory predicts nine vibrations of which three (A _g , 2B _g ) are Raman-active and six (3A _u , 3B _u ) are infrared-active. We have observed three Raman modes at 296 (A _g ), 346 (B _g ¹ ) and 636 (B _g ² ) cm^?1. We have also observed six infrared modes at 146 (B _u ³ ), 164 (A _u ² ), 355 (A _u ³ ), 480 (B _u ¹ ), 542 (?) and 603 (B _u ² ) cm^?1. The normal frequencies and eigenvectors have been calculated using Wilson’s FG method; a good fit between theory and experiment has been obtained.     

Sorption of 131I− and 131IO 3 − from aqueous solution on nickel hydroxide at 20°C

Sorption of ¹³¹I^? and ¹³¹IO ₃ ^? from aqueous solution on solid Ni(OH)₂ at 20°C was studied. It was found that in sorption from aqueous solution, after 120-min contact of the liquid and solid phases at V/m = 500 ml g^?1, with an increase in the concentration of I^? and IO ₃ ^? from 10^?5 to 10^?3 M the distribution coefficients K _d decrease from 16.4 to 5.3 ml g^?1 for ¹³¹I^? and from 113.6 to 30.8 ml g^?1 for ¹³¹IO ₃ ^? . The influence of various anions (Cl^?, NO ₃ ^? , SO ₄ ^2? , and BO ₃ ^3? ) on sorption of ¹³¹I^? and ¹³¹IO ₃ ^? from aqueous solution on solid Ni(OH)₂ was studied. It was found that, with an increase in the concentration of these ions from 10^?5 to 10^?1 M, the distribution coefficients K _d of ¹³¹I^? and ¹³¹IO ₃ ^? decrease by a factor of 3–10.     

Mechanism of Np(VI) oxidation with ozone in alkaline solutions

Bubbling of an ozone-oxygen mixture containing 0.1?C0.5 vol % O₃ at a rate of 15?C20 l h^?1 through 13 ml of a 2 × 10^?5?1 × 10^?4 M solution of Np(VI) in 0.1 and 1 M LiOH leads to the formation of Np(VII). The initial rate increases approximately in proportion to [Np(VI)] and [O ₃ ^gas ]^0.5. Up to 80% of Np(VI) is oxidized at maximum. At the O₃ concentration in the gas phase increased to 1?C4 vol %, Np(VI) is oxidized completely. Under the same conditions, Np(VI) in a concentration of (1?C5) × 10^?3 M is oxidized to almost 100%. Analysis of published data and additional experiments on the reaction of O₃ with Np(VI) ions in LiOH solutions allow a conclusion that the ozonation involves the reactions O₃ + OH^? = HO ₂ ^? + O₂, O₃ + HO ₂ ^? + OH^? = O ₃ ^? + O ₂ ^? + H₂O, and O₃ + O ₂ ^? = O ₃ ^? + O₂, followed by O ₃ ^? + NpO₂(OH) ₄ ^2? = O₂ + NpO₄(OH) ₂ ^3? + H₂O. In addition, HO ₂ ^? reduces Np(VII) and Np(VI) and reacts with O ₃ ^? . Certain contribution is made by the reaction Np(VI) + O₃ = Np(VII) + O ₃ ^? . The dependence of the Np(VII) accumulation rate on [O ₃ ^gas ]^0.5 was interpreted in terms of the concept of a heterogeneous-catalytic process.     

The composition of the gas phase over liquid lithium at 1600–6000 K in view of existence of atoms and metastable self-associates

Methods of thermodynamic simulation in view of existence of lithium atoms (Li₁), condensed and volatile Li₂–Li₅ self-associates, Li ₁ ⁺ , Li ₂ ⁺ , and Li ₃ ⁺ ions, and e-gas are used to calculate partial pressures of components of the gas phase over liquid lithium in the 1600–6000 K temperature range and the thermal characteristics of the system at 1600 and 6000 K.     

Construction of kernels G l, 0 l of the nonlinear collision integral in the Boltzmann equation for arbitrary l

It was shown in [1] that kernels L _l (v, v ₁) of linear collision integral and kernels G _{l, 0} ^l (v, v ₁, v ₂) of nonlinear collision integral are related by the Laplace transform. Here, analytical expressions are derived for nonlinear kernels G _{l, 0} ^+l (v, v ₁, v ₂) with arbitrary l for models of hard spheres and pseudo-Maxwellian molecules using the Laplace transform method.     

The Unique Properties of Superconductivity in Cuprates

Copper oxides are the only materials that have transition temperatures, T _c, well above the boiling point of liquid nitrogen, with a maximum \(T_{\mathrm {c}}^{\mathrm {m}}\) of 162 K under pressure. Their structure is layered, with one to several CuO₂ planes, and upon hole doping, their transition temperature follows a dome-shaped curve with a maximum of \(T_{\mathrm {c}}^{\mathrm {m}}\) . In the underdoped regime, i.e., below \(T_{\mathrm {c}}^{\mathrm {m}}\) , a pseudogap Δ* ∝ T* is found, with T* always being larger than T _c, a property unique to the copper oxides. In the superconducting state, Cooper pairs (two holes with antiparallel spins) are formed that exhibit coherence lengths on the order of a lattice distance in the CuO₂ plane and one order of magnitude less perpendicular to it. Their macroscopic wave function is parallel to the CuO₂ plane near 100 % d at their surface, but only 75 % d and 25 % s in the bulk, and near 100 % s perpendicular to the plane in yttrium barium copper oxide (YBCO) [1]. There are two gaps with the same T _c [2]. As function of doping, the oxygen isotope effect is novel and can be quantitatively accounted for by a vibronic theory or by the presence of bipolarons [2, 3]. These cuprates are intrinsically heterogeneous in a dynamic way. In terms of quasiparticles, bipolarons are present at low doping and aggregate upon cooling [2] so that probably ramified clusters and/or stripes are formed, leading over to a more Fermi liquid-type behavior at large carrier concentrations.     

Synthesis and adsorption properties of nanosized Mg-Al layered double hydroxides with Cl−, NO 3 − or SO 4 2− as interlayer anion

Nanosized Mg-Al layered double hydroxides (LDHs) with Cl^?, NO ₃ ^? or SO ₄ ^2? as the interlayer anion have been synthesized by a modified coprecipitation method. The obtained LDHs were confirmed to be composed of a single phase and to be highly substituted by Al (Mg/Al ratio ～1.9). The abilities of the LDHs to adsorb several harmful anions (F^?, CrO ₄ ^2? , HAsO ₄ ^2? and HSeO^3?) in aqueous solution were studied. The LDHs exhibit high adsorption abilities. The amount of adsorption onto the LDHs differed between the starting interlayer anions, and decreased in the following order of the interlayer anions: NO ₃ ^? > Cl^? > SO ₄ ^2? . The NO₃-formed Mg-Al LDH reached a CrO ₄ ^2? adsorption equilibrium state within only 30 min, much faster than those in previous reports. Thus, the nanocrystallized, highly Al substituted phase of the NO₃-formed Mg-Al LDH is found to markedly enhance the anion adsorption ability.     

Effect of diffusion-annealing time (0.5 h ≤ t ≤ 2 h) on the mechanical and superconducting properties of Cu-diffused bulk MgB2 superconductors by use of experimental and different theoretical models

This study reports the effect of different annealing time (0.5 h ≤ t ≤ 2 h) on the electrical, physical, microstructural, mechanical and superconducting properties of Cu-diffused bulk magnesium diboride (MgB₂) system by means of dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and microhardness measurements (H _v ). The room temperature resistivity (at 300 K), critical transition (T _c ^offset and T _c ^onset ) temperature, variation of transition temperature, grain size, phase purity, lattice parameter, texturing, surface morphology, crystallinity and Vickers microhardness values of the samples are evaluated and compared with each other. The resistivity results obtained reveal that the (T _c ^offset and T _c ^onset ) values of the samples produced ascend with the enhancement in the annealing time up to 1 h beyond which these values start to reduce systematically and in fact the smallest T _c ^onset of 38.1 K and T _c ^offset of 36.2 K are observed for the sample annealed for 2 h. Similarly, the SEM micrographs display that the surface morphology, crystallinity and grain connectivity improve until a certain diffusion-annealing time (1 h), and after this point, all the properties obtained start to degrade with the increase of the annealing time. Furthermore, the peak intensities, grain sizes and lattice parameters deduced from the XRD measurements illustrate that a systematic elongation in the a and c axis lengths is detected with the annealing time until 1 h beyond which a regular contraction in the lattice parameters is observed for the samples. Likewise, the peak intensities belonging to MgB₂ phase enhance with the increment of the annealing time up to 1 h after which they reduce slowly; however, a new peak belonging to Mg₂Cu phase appears in the sample annealed for the duration of 2 h, confirming both the reduction of the grain size and degradation of the critical temperature. Additionally, we have focused on the microhardness measurements for the examination of the mechanical properties of the samples studied. Experimental results of microhardness measurements are estimated using the various models such as Meyer’s law, proportional sample resistance model, modified proportional sample resistance model and Hays-Kendall (HK) approach. Based on the simulation results obtained, the Hays-Kendall (HK) approach is determined as the most suitable model describing the mechanical properties of samples prepared.