Long term stability of H atoms in HD-D2-He solids

No Heading We employed en electron spin resonance (ESR) technique for investigating the long term behavior of hydrogen and deuterium atoms in the HD-D₂ impurity helium solids that were created by sending a gas mixture [H₂]:[D₂]:[He]= 1:4:100 through a radio-frequency electrical discharge into a volume of superfluid ⁴He at T = 1.5 K. H and D atoms were stabilized inside nanoclusters of impurity molecules. The exchange tunneling reactions D + H₂ HD + H and D + HDD₂ + H proceeded to eliminate D atoms and increase the concentration of H atoms in the HD-D₂ impurity-helium solids. Local concentrations of H atoms inside the molecular nanoclusters of order 10²⁰ cm^–3 were achieved. The high concentration of H atoms was stable during 40 hours storage of the sample at T = 1.35 K. These solids are possible candidates for collective quantum phenomena of atomic hydrogen if the Bose-Einstein degeneracy regime can be attained.PACS numbers: 61.46. +w, 67.40.Yv, 76.30.–v.     

Magnetic Resonance Studies of Impurity-Helium Solids Containing Hydrogen and Deuterium Impurities

Impurity-Helium Solids (Im-He) produced by injecting a mixed beam of helium and impurity gases into superfluid ⁴He have been investigated by electron spin resonance (ESR) and nuclear magnetic resonance (NMR) techniques. NMR signals from deuterium molecules in a D₂-He solid have been studied. Only samples prepared from gaseous mixtures containing high concentrations of D₂ molecules gave observable signals. The ESR experiments were performed on H and/or D atomic impurities in Im-He solids containing H, D, H₂, and D₂ in various combinations. The exchange tunneling reactions D+H₂HD+H and D+HDD₂+H were used to generate high concentrations of H atoms (10¹⁷/cm³) in Im-He samples.     

Pressure anomaly near the triple point on the magnetic phase diagram of bcc solid3He

The nuclear magnetic phase diagram of bcc solid ³ He has been determined by pressure measurements. We have observed a pressure jump at the transition from the high field phase to the paramagnetic phase (HP) near the triple point. This jump indicates that HP is first order.     

Magnetic field and temperature dependence of the Knight shift in superconductors of small dimensions

The magnetic field dependence of the paramagnetic Knight shift in weak-coupling superconductors is evaluated within the framework of the BCS-Gor'kov theory for samples of small dimensions. The behavior of the monotonic and oscillatory parts of the Knight shift are examined in detail in the regimesT0 andTT _c ^– .     

The hP1-type phases in alloys of cadmium,mercury, and indium with tin

The hP1-type phases in alloys of Cd, Hg, and In with Sn are stable in the valence electron concentration interval 3.80–3.95. Following a rule of Raynor, the axial ratioc/a decreases with increasing valence electron concentration,N _b ^/at . Using lattice constant measurements the rule is confirmed, and by using density measurements the absence of constitutional vacancies is verified. Thec/aN _b ^/at relation of the hP1 phases is similar toc/aN _b ^/at of InSn _m (03Sn. A bonding-type proposal, explaining the stability of these structures and the dependence of their axial ratio on valence electron concentration, is derived.     

Quantum Tunneling of D(H) Atoms and 2 – in Solid Hydrogen

This paper contains three results. First, the rate constants for the tunneling reaction HD + D H + D ₂ in solid HD increase steeply with increasing temperature above 5 K, while they are almost independent of temperature below 5 K. A mechanism of a vacancy–assisted tunneling reaction is proposed to account for this temperature dependence. Second, a hydrogen atom and a hydrogen molecule form a van der Waals complex in the Ar matrix at 20 K, where the tunneling reaction HD + D H + D ₂ takes place in this complex. The analysis of well–resolved ESR spectra of the complex determined the distance between a hydrogen atom and a hydrogen molecule as 2.3 – 2.5 Å. Third, the decay rate constants of anions in solid parahydrogen decrease with decreasing temperature below 6.6 K, attain the minimum value at 5 K, and then increase with decreasing temperature in the range of 5 2.7 K. The abnormal temperature dependence of the decay rate constants below 5 K is ascribed to a phonon–scattering process of quantum diffusion.     

Quantum transport of surface electrons over liquid helium in magnetic field

The magnetoconductivity _xx of surface electrons over liquid helium at electron densities (1–9) × 10⁸ cm^–2 is measured at temperatures down to 0.5 K and magnetic fields H up to 25 kOe. _xx is shown to decrease with increasing H. For rather high temperatures and low magnetic fields dependence _xx ^–1 on H displays a classical behavior, but as the temperature is reduced or H is increased, a transition to the quantum magnetotransport regime occurs. It is shown that _xx decreases at lowering temperature, passes through a minimum and then slightly increases in the region of electron-ripplon scattering. The measured data are in good agreement with the theoretical results taking into account the preferable contribution of electron scattering on ripplons with wavelength near magnetic length. At the region of electron crystal _xx is weakly dependent on T. At low magnetic field _xx ^–1 increases with H linearly and, at H>17kOe, _xx ^–1 ~ H ^1.5.     

Half-solitons in superfluid3He-A: Novel π/2-quanta of phase slippage

Core structures of nontopological solitons between inequivalent vacua in superfluid³He-A are considered. We analyze the symmetries of these A-A interfaces, and compute their hard-core structures in the Ginzburg-Landau regime. We discuss both domain walls where the orbital anisotropy l-vector is flipped (l–l), and those with the same l(x=–) and l(x=+) asymptotics. In particular, we find new classes of A-A boundaries: these novel /2-solitons, which can occur in the absence of a change in the asymptotic l-vector field, constitute the elementary quanta of phase slippage in superfluid³He-A. We ascribe these half-solitons to a new topological scenario for the flaring-out of vorticity in the extended (k, r)-space. Edges of such walls serve to provide vortices with 1/4 quantum of circulation in³He-A. In analogy with the B-B domain walls in superfluid³He-B, solitons of pure phase slippage by —with a normal core—prove unstable in³He-A; they either fission into a pair of ordinary l-solitons—domain walls with superfluid cores both flipping the orbital anisotropy axis (i.e., l–ll), or form a bound pair of walls, each of which constitutes an l-soliton with a phase shift of /2. Our investigation of the superfluid A-A vacuum interfaces may prove useful in a broader context since the A-A boundaries exemplify the possible domain walls relevant for the Higgs-field solitons (cosmic domain walls) within the Weinberg-Salam model.     

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.     

Photoreaction of a ZnO gel film chemically modified with β-diketones

Chelate formation is confirmed by a red shift of the n * absorption peak of benzoylacetone (BzAcH) from 309 to 336 nm with its addition to a sol containing Zn²⁺ ions. The chelate bonds between Zn²⁺ and BzAc^– are mostly maintained in the gel film prepared from the sol. Irradiation of the gel film by a Xe lamp with a cut filter (_ex > 300 nm) in the presence of H₂O leads to decomposition of the chelate ring. As a result of the photolysis, ZnO–H groups, CH₃CHO and other carbonyl compounds are generated with the lost of CH₃ groups of the BzAc^– ligand and H₂O involved in the film. INDO/S calculations on a model complex (Zn(BzAc)(OEt)) assign the n * absorption to the electronic transition from a non-bonding molecular orbital (MO) distributed mainly at the phenyl group to an anti-bonding MO localized at the CO bonds. On the basis of these results, a photo-induced hydrolysis mechanism was presented to explain the formation of positive-type patterned ZnO films.     

The cubic autocatalator: The influence of the quadratic autocatalytic and the uncatalysed reactions

The proptotype chemical reaction scheme, the cubic autocatalator, A + 2B 3B; B C is taken in a closed system, with A formed from the precursor P by the simple step P A. The pooled chemical approximation is invoked whereby the concentration of P can be assumed to remain constant throughout. The effects of allowing the quadratic autocatalytic reaction A + B 2B and the uncatalysed reaction A B in the scheme are considered in detail. The full scheme is described by the non-dimensional parameters µ (measuring the reaction rate of the initiation step) ands andr (measuring the reaction rates of the quadratic autocatalytic and the uncatalysed steps respectively). It is shown, provided only thatr ors (or both) are non-zero, no matter how small, the solution remains bounded for all (positive) values of µ, whereas withr =s = 0 the solution is bounded only for µ > µ₀ (µ₀ = 0.900 32). It is shown that withr = 0 ands 0 the governing equations have a Hopf bifurcation at µ = 1 –s producing a stable limit cycle which exists for all µ in 0 < µ <1 –s. The behaviour of these limit cycles as µ 0 is also discussed.     

Stagnation flow and heat transfer in a zonal magnetic field

Summary The problem stated in the title is studied for small values of the diffusivity ratio and the magnetic force coefficient , the magnetic field being of internal origin. Uniformly valid expansions are derived for the velocity and magnetic fields in the fluid. It is found that as 1, the viscous layer is brought to rest and the current in the layer is uniform and normal to the wall.The heat transfer is next calculated at a uniformly heated wall on the assumption of small temperature variations. It is deduced that when log(^–1) approaches a certain value depending on the wall temperature etc., the thermal boundary layer separates at the stagnation point and, if dissipation is neglected, along the whole wall.     

Resonance Effect on the Tunneling Reaction: H+H2→H2+H in Solid Hydrogen

The tunneling abstraction reaction: H+H₂H₂+H in -irradiated solid hydrogen has been studied using electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) spectroscopy and gas-chromatography. The rate constant for the tunneling reaction in solid hydrogen was found to decrease with the increase in the concentration of ortho-H₂ molecules in solid hydrogen. We concluded that the decrease in the rate constant is due to the energy level mismatching between reactant species of H+H₂ trapped and product species of H₂+H trapped in the reverse induced by inhomogeneous intermolecular interactions between ortho-H² molecules in solid hydrogen. This result indicates that resonance effects play an important role in tunneling reactions in solids.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.     

Hydrothermal Synthesis of Nanocrystalline SnO2 for Gas Sensors

Nanocrystalline SnO₂ is prepared by hydrothermal synthesis (130–250°C, 2–5 h) using three different precursors and is characterized by x-ray diffraction, transmission electron microscopy, and nitrogen BET measurements. The crystallite size of SnO₂ powders (d = 4–5 nm) prepared from amorphous stannic acid gels is found to vary very little with process temperature and duration. Air anneals at 500°C for 1–20 h demonstrate that the highest stability toward crystallite growth is offered by the samples prepared by oxidizing SnSO₄ with H₂O₂ (the crystallite size increases only slightly, from 4–5 to 5–7 nm), whereas the crystallite size of the samples prepared by high-temperature hydrolysis of SnCl₄ increases markedly, from 4–5 to 16–17 nm. Nanocrystalline NiO-doped SnO₂ is prepared by hydrothermal treatment, and its physicochemical properties are investigated. Both SnO₂ and SnO₂NiO exhibit gas sensitivity, as demonstrated by consecutively exposing the samples to different gaseous atmospheres: O₂ N₂ O₂ and O₂ N₂ + C₂H₅OH O₂.     

X-ray diffraction and Ir-absorption characteristics of lanthanide orthophosphates obtained by crystallisation from phosphoric acid solution

Powders of LnPO₄ · H₂O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) prepared by crystallisation from boiling phosphoric acid (2 M H₃PO₄/1) solution were characterised by X-ray diffraction and FTIR-spectroscopy. Hexagonal LnPO₄ · H₂O (La Tb), tetragonal (Ho Lu and Y) and orthorombic DyPO₄ · H₂O crystalline modifications were identified. Ir-spectra of the hydrated hexagonal, anhydrous tetragonal LnPO₄ · H₂O (Dy, Ho, Er, Tm, Yb, Lu and Y) and anhydrous monoclinic (La Tb) are consistent with those reported in the literature. However, the hydrated tetragonal LnPO₄ · H₂O (Ho, Er, Tm, Yb, Lu and Y) display a surplus band (625 cm^–1) in the region of ₄, which was not reported in the literature. The band disappears after ignition at 950°C, while the tetragonal structure is still maintained, which may imply that it is attributed to hydrogen bonding of H₂O molecules to the phosphate oxygen in hydrated salts. Some of the phosphates, after ignition at 950°C, display additional P₂O₇ ^4– band at 1265–1267 cm^–1. That may be resulted from HPO₄ ^2– for PO₄ ^3– substitution in the phosphates crystallised in acidic (2 M H₃PO₄/1) solution.     

Amplifier based on a field-effect triode with negative current feedback

Conclusions A field-effect triode amplifier with series negative current feedback allows a voltage gain of the order of 200–300 to be obtained for a load resistance R_s1 M. The coefficient K_u begins to decrease noticeably only for a feedback resistance above 500 .The current gain reaches (8–10)·10³. Increasing the resistances R_s and R_L to hundreds of ohms has practically no effect on K_i. For a further increase of R_s and R_L the coefficient K_i decreases.The power gain reaches its maximum value (of the order of 10⁴ or more) for R_s100 and R_L=10–100 k. An increase in R_s leads to a reduction of K_pmax and to a shift of the extremum of the function K_p=f(R_L) into the range of higher values of R_L.A large input resistance of the amplifier (tens of megohms and higher) is obtained when R_s increases to 10–100 M. The maximum input resistance is obtained for R_L and R_s and may exceed values of from hundreds of megohms to several gigaohms. The minimum input resistance is hundreds of kilohms for R_L and R_s0.The minimum input resistance (5–10 k or less) is ensured for R_g and R_L0. An increase of the output resistance to hundreds of megohms or higher occurs for R_g and R_s.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 67–70, September, 1971.     

Effects Induced by 16O–18O Oxygen Isotope Exchange in Manganite Ceramics and Films

Unique sensitivity of electrical resistivity to the ¹⁶O¹⁸O isotope exchange was observed for (La_1–y Pr _y )_0.7Ca_0.3MnO₃ ceramics and epitaxial thin films. At y=0.7, substitution of ¹⁶O by ¹⁸O in ceramic samples results in the reversible transition from a metal-like to insulating state. In thin films, this metal–insulator transition occurs at y=0.5 due to the strains induced by substrate. The applied magnetic field (H2 T) transforms the sample with ¹⁸O back to the metallic state. In such magnetic fields the temperature dependence of resistivity exhibits a peak corresponding to the onset of metallic state. The shift of the peak toward low temperatures after the ¹⁶O¹⁸O exchange exceeds 50 K at 3 T for both ceramic and thin film samples. The results are interpreted in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the charge ordered insulating and the ferromagnetic metal-like states.     

The Second Order Recombination and Spin-Exchange Relaxation of Atomic Deuterium at Low Temperatures in a Low Magnetic Field

We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a ⁴ He coated sample cell, using the hyperfine resonance of (F = 1/2, m_F = –1/2) – (F = 3/2, m_F = –1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1_DD = (5.5 ± 1.3) × 10 ^–9 cm and the adsorption energy of D on ⁴ He surface was _a = 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1_DD at low field was orders of magnitude smaller than what was expected when the scaling of 1/B² dependence of the direct recombination mechanism was used, and in addition, _a was significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, G_DD = (1.4 ± 0.6) × 10 ^–10 cm ³ sec ^–1 . It was smaller than theoretical calculations.     

Preparation of lanthanide orthophosphates by crystallisation from phosphoric acid solution

Highly crystalline solids of the composition of LnPO₄ · nH₂O (Ln = La, Ce, Pr, Nd. Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) were prepared by crystallisation from boiling phosphoric acid solution with the initial concentration 2 M/1 of H₃PO₄ and 0.02 M/1 of Ln. The number of molecules of water (n) was close to 1. The crystallisation process was completed within two hours. Scanning electron micrographs showed that the diameter of the LnPO₄ · H₂O crystalline particles was increasing within the group of elements from below 1 m (LaPO₄ · H₂O) to above 5 m (TmPO₄ · H₂O). Such large crystals of lanthanide phosphates cannot be obtained by usually applied precipitation methods. The light lanthanides (La Sm) formed clusters of shapeless particles with the tendency to form rod-like crystals. The heavy lanthanides (Ho Lu, and YPO₄ · H₂O) consisted of spherical globules. The TbPO₄ · H₂O and DyPO₄ · H₂O, formed a mixture of the spherical and rod-like crystals.