Spin-polarization effect on electron attachment to atomic hydrogen on liquid helium surface

No Heading Reaction rates of electron attachment to atomic hydrogen are measured as a function of magnetic field. The reaction takes place in a two-dimensional mixture of hydrogen atoms and electrons on liquid helium surface. Surface electron density, measured by using vibrating capacitor electrometer technique, decreases when H atoms are introduced. Applied high magnetic field suppresses electron attachment, H + e^– H^–, as well as hydrogen recombination, H + H H₂. Since the electronic state of negative hydrogen, H^–, is spin singlet, electron attachment is suppressed by spin-polarization. Possible microscopic mechanisms to explain the measured magnetic field dependence of the reaction kinetics are discussed.PACS numbers: 67.65.+z, 68.     

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.     

High-Temperature Limit for the O2-Catalyzed Ortho-Para Conversion Rate in Solid Hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of molecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the temperature range 4.2–10.9 K. Conversion rate was mainly determined by the paramagnetic oxygen impurities. The catalyzed conversion rate constant increases with temperature in the range 4.2–8 K and becomes independent of temperature above 8 K. The high-temperature limit is 1.15(8) s ^–1 . The catalyzed conversion rate was shown to become temperature independent when the ortho-H ₂ diffusion time _d falls below the conversion time _cc in the first coordination sphere of O ₂ impurity. The obtained value of _cc =10.5(8) s was used to identify the position of O ₂ in H ₂ lattice as substitutional. The temperature dependence of the ortho-H ₂ diffusion rate near the high temperature limit was determined.     

Hyperfine resonance of atomic deuterium at 1 K

Atomic-deuterium gas has been studied at temperatures just above 1 K. A short rf discharge was used to dissociate solid D ₂ on the surface of a sealed pyrex bulb lined with a saturated helium film. Pulsed magnetic resonance on the - transition at its minimum frequency (309 MHz in a magnetic field of 3.9 mT) was used to observe the resulting D. The free induction decays were shortened by spin-exchange collisions with impurity H atoms present in the sample. The H density, inferred using calculated spin-exchange cross sections, was typically higher than the D density. In addition, the samples of D were short-lived, decaying exponentially in time with a strongly temperature-dependent lifetime. We were able to show that recombination with the H impurity was not the cause of the sample decay, and we propose that the decay is due to the thermally activated process wherein D atoms penetrate the liquid helium film that coats the cell walls. Analysis of the lifetime data yields the value 13.6 (6) K for the rest energy of a D atom dissolved in liquid helium, the first measurement of this quantity for any hydrogen isotope.     

Stress-induced hydrogen movement and the partial molal volume of hydrogen in AISI 4340 steel

Stress-induced movement of hydrogen to the high tensile region in AISI 4340 steel was measured at room temperature and 202 °C. A neodymium hydrogen detection method was used to determine the location and concentration of hydrogen. The AISI 4340 stress rings with local hydrogen content up to 0.69 ppm and loaded to 16.87, × 103 kg cm-2 did not fail, whereas a stress ring with 0.73 ppm did fail for the test condition cited. The predicted hydrogen concentration versus fracture time curves at constant tensile stress indicated a good agreement with the stress ring test data.The partial molal volume of hydrogen, ¯H, was calculated to be 2.47 cm3 per g-atom for AISI 4340 steel, which was greater than the reported value for alpha iron at 2.0 cm3 per g-atom. Knowing the value ¯H and the hydrogen content for equation RT In C/Co = ¯H/3 the increase of hydrogen concentration by a given applied stress can be calculated.     

An experiment to investigate the superfluid transition in a confined planar geometry

The vibrational characteristics of the FCC–cryocrystals with substitutional impurities have been analyzed. A non–central impurity atom in environment of the central host atoms is considered. The difference between equilibrium interatomic distances and r ₀ of the impurity and host atoms, respectively, causes their non–central interaction. A very strong dependence of the impurity frequency spectra on the ratio is shown. The range of the values of this ratio which corresponds to dynamical stabitity of the system under consideration is determined.     

Study on the interaction between a dislocation and impurities in KCl : Sr2+ single crystals by the Blaha effect Part I Interaction between a dislocation and an impurity for the Fleischer's model taking account of the Friedel relation

It has been previously reported for KCl : Sr²⁺ (0.035, 0.050, 0.065 mol% in the melt) single crystals that the interaction between a dislocation and the impurity can be approximated to the Fleischer's model. From the values of ₀ for the specimens, however, it was confirmed that the Friedel relation can be taken into the Fleischer's model. The ₀ is the bending angle at which the dislocation embraces the impurities under the effective shear stress at 0 K. Furthermore, the interaction between a dislocation and the impurity could be approximated to the Fleischer's model taking account of the Friedel relation. This was examined on the basis of the dependence of strain-rate sensitivity due to the impurities on temperature at about 100–200 K. Then, the critical temperature, at which the effective shear stress is zero, was determined to be 282 K. In addition, the values of the enthalpy and the Gibbs free energy of activation for the breakaway of the dislocation from the impurity were obtained for the specimen.     

Solid hydrogen and deuterium. I. Ground-state energy calculated by a lowest order constrained-variation method

The ground-state energy of solid hydrogen and deuterium is calculated by means of a modified variational lowest order constrained-variation (LOCV) method. Both fcc and hcp H₂ and D₂ are considered, and the calculations are done for five different two-body potentials. For solid H₂ we obtain theoretical results for the ground-state binding energy per particle from –74.9 K at an equilibrium particle density of 0.700 ^–3 or a molar volume of 22.3 cm³/mole to –91.3 K at a particle density of 0.725 ^–3 or a molar volume of 21.5 cm³/mole, where =2.958 Å. The corresponding experimental result is –92.3 K at a particle density of 0.688 ^–3 or a molar volume of 22.7 cm³/mole. For solid D₂ we obtain theoretical results for the ground-state binding energy per particle from –125.7 K at an equilibrium particle density of 0.830 ^–3 or a molar volume of 18.8 cm³/mole to –140.1 K at a particle density of 0.843 ^–3 or a molar volume of 18.5 cm³/mole. The corresponding experimental result is –137.9 K at a particle density of 0.797 ^–3 or a molar volume of 19.6 cm³/mole.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Investigation of the thermal and electrical transport properties of niobium and niobium films at temperatures between 9 and 50 K

The thermal conductivity and the electrical conductivity of niobium crystals and niobium films have been investigated in the normal state from 9 to 50 K. The deviations from the Matthiessen rule, w=/T+T², have been studied in detail for the thermal case. The investigation shows a slight dependence of the electron-phonon scattering coefficient upon the impurity content of the sample. With the specific electrical residual resistivity ₀ as the measure for the impurity content, the following correlation can be formulated: =1.2×10^-2[₀/-cm)]^0.04, being obtained in cm/W K. Above 20 K an additional scattering mechanism occurs. The temperature dependence of the additional resistance W between 20 and 50 K is proportional to T ^5.5 ··· T ⁴. Possible causes of this phenomenon are discussed. For the discussion, all the data available in the literature on the thermal conductivity of niobium in this temperature region are used.     

Electro-transport of hydrogen in α-iron

The migration of hydrogen in -iron under the influence of an electric field was studied in a wide temperature interval. In every case hydrogen migrated toward the cathode. The transport charges are positive and decrease exponentially with rising temperature. The effective charge is practically independent of the concentration of hydrogen in iron (in the concentration range studied). To explain the results obtained, a vacancy mechanism of electro-transport was postulated; it envisages the movement of vacancies as a result of their interaction with the current carrier flux in the metal and the diffusion of impurity atoms carried by the migrating vacancies.     

Low-density properties of gaseous,spin-aligned atomic hydrogen

The ground-state properties of bulk atomic hydrogen are calculated for the low-density gas, assuming that the atoms interact via the ³ _u ⁺ pair potential. The Monte Carlo technique is applied to a system consisting of 32 hydrogen atoms in a box, with periodic boundary conditions. These results are compared with those derived from the hard-sphere Bose-Einstein gas model.Work supported by NASA Grant No. NGR 06-002-159.     

Investigation of the gapless state induced by pressure in Hg1−x Cd x Te alloys

We report the results of an experimental study of the gapless state (GS) induced inp-type semimetallic alloys Hg _1–x Cd _x Te (0<x<0.15) by pressure. Galvanomagnetic effects in weak magnetic fields (2T<300 K) and the Shubnikov-de Haas effect have been investigated in the pressure interval 1p<15 kbar. Direct evidence for the existence inp-type semimetallic alloys of an impurity hole band overlapping with the conduction band by 3–4 me V is obtained. At liquid helium temperatures the Fermi level is located in the impurity band, so that two groups of carriers take part in transport effects: light electrons in the conduction band and heavy holes in the impurity band. The electron Fermi energyE _F is proved to be essentially constant during the transition to the GS. A linear dependence of the electron effective mass at the band edgem* (0) upon the gapE _g is obtained. A significant role of scattering of electrons into the impurity band at liquid helium temperatures is revealed.     

Model of hydrogen behaviour in enamelling grade steels

A model has been developed of the behaviour of hydrogen in enamelling-grade steels in relation to the delayed defect of blow-off of enamelled surface (fishscaling). The model is based on current theories concerning reversible and irreversible trapping of hydrogen in metallic materials. It leads to the establishment of a free hydrogen parameterC _L which can be used to assess the susceptibility of a steel to fishscaling following the usual enamelling processess. The model can also be used to study the effect of both thermomechanical steelmaking cycles and enamelling processes on resistance to the defect.Nomenclature E _aD Activation energy of hydrogen diffusion through normal lattice - E _s Saddle-point energy - E _B Trap binding energy - E _aT Trap activation energy=E_s+E_B - A Trapping site - B Normal lattice site - v ₀ Vibration frequency of hydrogen at a normal lattice site - v ₁ Vibration frequency of hydrogen at a trapping site - N _L Density of normal lattice sites for hydrogen - N _T Density of trapping sites for hydrogen - C _L Concentration of hydrogen on lattice sites - C _T Concentration of hydrogen captured on traps - k Probability of trapping=v₀ exp(–E _s/R T) - p Probability of detrapping =v ₁ N _L exp (–E _aT/R T) - n Fraction of trapping sites occupied with hydrogen atoms among the total trapping sites=C _T/N _T - t Time - T Temperature - R Gas constant     

Thermal detection of ESR on spin-polarized hydrogen: Study of surface recombination

We introduce a new method for the detection of electron-spin resonance in spin-polarized atomic hydrogen gas (H). Instead of observing the microwave power absorbed in the ESR transition, we monitor the recombination heat deposited by ESR-induced spin-up atoms (H) onto a liquid-helium coated carbon bolometer. The signal from this sensor reproduces well the ESR absorption lineshape registered by our 128 GHz homodyne spectrometer. Using the recombination detection we have achieved a density detection limit of n=3·10¹⁰ atoms/cm³ for 0.2 nW microwave power incident on the resonant cavity at the temperature T=150 mK. We have studied the decay rate of recombination heat absorbed by the bolometer after an ESR excitation pulse and the dependence of this rate on T, n and nuclear polarization of the H sample. The bolometer signal is found to be related mainly to second-order H + H recombination to ortho-H₂ on the surfaces of the sample cell. From the signals we have determined the rate constant K _bc ^c =3.2(5)·10^–5T cm²/s·K^–1/2 in the interval from T=250 to 425 mK in a field of 4.5 T.     

A study on the hydriding-dehydriding kinetics of Mg1.9Al0.1Ni

The hydrogen absorption/desorption (A/D) kinetics of hydrogen storage alloys Mg_1.9Al_0.1Ni prepared by hydriding combustion synthesis in two-phase (-) region in the temperature range from 523 to 573 K have been investigated. The hydriding/dehydriding (H/D) reaction rate constants were extracted from the time-dependent A/D curves. The obtained hydrogen A/D kinetic curves were fitted using various rate equations to reveal the mechanism of the H/D processes. The relationships of rate constant with temperature were established. It was found that the three-dimensional diffusion process dominated the hydrogen A/D. The activation energies of Mg_1.9Al_0.1Ni are 52 ± 2 and 48 ± 1 kJ/mol H₂ for the H/D processes smaller than that of Mg₂Ni, which can be explained that the improvement of H/D reaction kinetics in Mg₂Ni by using additive Al.     

Local Environment and Migration in Nickel Metal of 57Co Impurity Atoms Formed by a Nuclear Reaction

The local environment of ^{5 7}Co impurity atoms formed in nickel metal by the ^{5 8}Ni(,p) reaction was studied by emission Müossbauer spectroscopy. The positions of ^{5 7}Co atoms after irradiation and short isochronous (1800 s) heat treatments of the cyclotron targets at 500-1100 K were evaluated. The processes of impurity transfer during treatment of irradiated targets were analyzed. The rate of impurity transfer is determined by the damage of the irradiated metal lattice and stabilization forms of impurity atoms (i.e., their location in the substitution sites or at the crystallite grain boundary).     

Thermal conductivity and electrical resistivity of pure polycrystalline cobalt in the temperature range 2.5–30 K

Thermal conductivity and electrical resistivity of 99.99% pure Co sample were measured in the temperature range 2.5–30 K. The annealing, procedure of the sample (either above or below Curie temperature), followed by cooling it down to room temperature at a slow cooling rate, caused an unexpected increase in its thermal resistivity and residual electrical resistivity, contrary to the results obtained for most pure metals. Co samples either not thermally treated or annealed consist only of a HI phase as proved by X-ray and electron diffraction analyses. The result, led to the conclusion that changes of grain structure and physical defects appearing in the Co at Curie temperature and at 690 K, when phase transitions take place, should be taken into account. The electron-magnon scattering, is significant in electrical conductivity but the electron-physical defect and impurity scattering plays a dominant role in thermal conductivity. The electron-physical defect and impurity scattering is elastic (validity of the Wiedemann Franz law)) as demonstrated by the value of _{th el} = 1.0, obtained in this work.     

Ortho-para conversion in hydrogen bubbles in copper

The low-temperature heat release in copper due to ortho-para conversion in hydrogen bubbles is investigated. Deviations from a model of free solid hydrogen are observed: a more intense heat release atT10 K and a rapid (10 h) heat release after cooling to 1.3 K. The experimental results can be explained by the assumption of autocatalytic conversion in the bulk and the catalytic influence of the bubble surface.     

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.