Dynamic properties of a symmetric spin nanocontact

This paper describes a theoretical study, at a microscopic scale, of the properties of a symmetric magnetic nanocontact. In particular, we study a symmetric nanocontact separating two waveguide groups of semi-infinite spin ordered ferromagnetic monatomic chains. The individual and total conductance of bulk magnons of the chains, scattering coherently at the nanocontact, and the localised density of spin states in the nanocontact domain, are calculated and analysed. The inter-atomic magnetic exchange is varied on the nanocontact to investigate the consequences of magnetic softening and hardening for the calculated properties. Transmission and reflection scattering cross sections are calculated from elements of a Landauer type scattering matrix. The results highlight the localized spin states on the nanocontact domain and their interactions with incident magnons. The results demonstrate also the magnetic and symmetry properties of the nanocontact domain.     

Fractional quantum conductance values in Au nanoelectrodes due to hydrogen adsorption

Hydrogen adsorption in gold nanocontact electrodes in electrochemical solution is experimentally discerned. This is performed with gold nanocontact conductance histograms in an electrochemical environment in which both the electrochemical potential and the electrolyte type are varied. Different salts, acids, and hydrogen peroxide electrolytes are studied. Salts and acids exhibit at negative electrochemical potentials different fractional quantum conductance histograms peaks associated to extra stable structures due to H adsorption while these peaks do not appear for H₂O₂ where electron transfer between solution and electrodes occurs without hydrogen formation or hydrogen adsorption on the gold electrode.     

Hydrogen migration in palladium: First-principles calculations

The supercell Pd₁₆H has been investigated in order to describe the hydrogen migration in the palladium lattice, where the hydrogen atom moves between symmetric interstitial sites. Ab initio calculations of the barriers for hydrogen diffusion in relaxed and unrelaxed metal lattices have been performed in the local density approximation, and the charge transfer during motion of hydrogen has been calculated. The conclusion has been drawn that the hydrogen migration in palladium occurs predominantly along the octapore-tetrapore-octapore trajectory.     

Nanostructure of laser-deposited palladium thin films on different substrates

The morphological features of palladium thin films deposited on different substrates are described. Film deposition has been performed by means of the pulsed laser evaporation method. It is shown that the grain structure of palladium films is formed independently of the substrate roughness. Particular emphasis is placed on the correlation between gas-sensitive metal-insulator-semiconductor (MIS) sensor properties and the nanostructure of palladium films used as metal electrodes in these sensors. It is concluded that a change in the morphology of palladium films has no direct influence on the degradation of the hydrogen sulphide sensitivity of MIS sensors that arises after sensor annealing in air enriched with hydrogen.     

On the interaction of a hydrogen atom with a lithium metal surface

In this paper we adopt a molecular cluster approach to study the interaction of a hydrogen atom with a (100) surface of solid lithium metal. In this study, the spin-unrestricted Hartree-Fock equation is solved for the molecular cluster. We also solve for the potential energy surface of the LiH molecule and the Li₂ molecule. We find that the essential features of the hydrogen surface interaction can be understood in terms of the molecule two body interactions. We also find that the hydrogen atom sits below the surface of the Li metal rather than above the surface.     

Antivortex gyrotropic motion in a nanocontact

A vortex-antivortex pair can form in the free layer of a nanocontact device as a result of the Oersted field produced by the current. In a large-scale free layer having uniform magnetization boundary conditions an in-plane external magnetic field will tend to confine the vortex-antivortex pair, which undergoes gyrotropic motion about the nanocontact after an initial displacement from the static equilibrium position. With the vortex pinned to a defect at the nanocontact the antivortex dynamics can be isolated and gyrotropic precession of the antivortex will be the dominant mode. The frequency of antivortex precession increases as the external magnetic field increases, and the frequency decreases as the nanocontact current increases.     

Orbital Kondo effect in cobalt-benzene sandwich molecules

We study a Co-benzene sandwich molecule bridging the tips of a Cu nanocontact as a realistic model of correlated molecular transport. To this end we employ a recently developed method for calculating the correlated electronic structure and transport properties of nanoscopic conductors. When the molecule is slightly compressed by the tips of the nanocontact the dynamic correlations originating from the strongly interacting Co 3d shell give rise to an orbital Kondo effect while the usual spin Kondo effect is suppressed due to Hund's rule coupling. This nontrivial Kondo effect produces a sharp and temperature-dependent Abrikosov-Suhl resonance in the spectral function at the Fermi level and a corresponding Fano line shape in the low bias conductance.     

Spontaneous transformations of the magnetic structure of a film nanocontact

The magnetization distributions in a symmetric magnetic film nanocontact for oppositely magnetized ferromagnetic electrodes are analyzed based on numerically solving the Landau-Lifshitz and magnetostatic equations as a function of magnetic and geometrical factors. It is found that a symmetric magnetic configuration is unstable when the head-to-head domain wall dividing the regions with opposite orientations of magnetization is located at the center of the nanocontact. The instability arises when the uniaxial magnetic anisotropy constant reaches a certain critical value K _c below which it spontaneously leaves the center of the nanocontact. The transition from the symmetric state (wall at the center) to an asymmetric one can be continuous (second order) or discrete (first order), depending on the geometrical and physical parameters of the nanocontact (length to width ratio, anisotropy constant, and saturation magnetization). The phase diagram is constructed in terms of the variable’s nanocontact length vs. anisotropy constant. This diagram divides the symmetric and asymmetric magnetic configurations of the system. The occurrence of a tricritical point in the phase diagram is its characteristic feature.     

Simulation of the self-organization of nanocontacts in thin gold films

An efficient method has been developed for simulation of the self-organization of metal nanocontacts obtained by transmission electron microscopy. In the framework of this approach, the basic steps of nanocontact formation at room temperature are determined and the main diffusion events determining the contact formation time and the shape of the contacts are revealed. The self-organization of nanocontacts having different orientations with respect to the original fcc lattice is considered. The results obtained are in good agreement with the experimental data on nanocontact formation.     

A novel source of atomic hydrogen for passivation of defects in silicon

Palladium membranes have been used for decades for the separation of hydrogen from other gasses. In this letter the use of thin palladium leaves to act as sources of atomic hydrogen for silicon samples is explored. It has been assumed in the past that although hydrogen diffuses through palladium in atomic form, the atoms recombine to form molecular hydrogen at the surface. In this letter it is shown that hydrogen supplied to one surface of a palladium leaf can result in atomic hydrogen being released from the opposite surface at low pressure. This is demonstrated through the use of a palladium leaf in a direct plasma system which allows for atomic hydrogen to be supplied to a sample while avoiding exposure to UV radiation from the plasma and high energy charged particles. This method is shown to provide significant atomic hydrogen to silicon samples and improve passivation of silicon surfaces. Method of Shielded Hydrogen Passivation: Schematic of direct plasma chamber with a shield inserted between the plasma and the silicon sample.     

Helical ordering of hydrogen bonds between pairs of DNA bases

The interaction between hydrogen bonds and conformational elastic degrees of freedom has been investigated using the simplest model of a double-strand DNA molecule. The hydrogen bonds are described in terms of two-level quantum systems. After excluding conformational degrees of freedom, one has effective interaction among two-level systems. In the ground state of an ideal double helix, hydrogen bonds in a DNA molecule also have a helical order induced by conformational degrees of freedom. The pitch of the hydrogen-bond helix (and even its sign under certain conditions) is different from that of the basic helix pitch and, generally speaking, is incommensurate with the latter. This effect can, possibly, lead to an inversion of the sign of the circular dichroism in spectral bands, which was detected in some experiments. Zh. éksp. Teor. Fiz. 115, 940–950 (March 1999)     

The effect of hydrogenation/dehydrogenation cycles on palladium physical properties

A series of hydrogenation/dehydrogenation cycles have been performed on palladium wire samples, stressed by a constant mechanical tension, in order to investigate the changes in electrical and mechanical properties. A large increase of palladium electrical resistivity has been reported due to the combined effects of the production of defects linked to hydrogen insertion into the host lattice and the stress applied to the sample. An increase of the palladium sample strain due to hydrogenation/dehydrogenation cycles in α→β→α phase transitions is observed compared to the sample subjected to mechanical tension only. The loss of initial metallurgical properties of the sample occurs already after the first hydrogen cycle, i.e. a displacement from the initial metallic behavior (increase of the resistivity and decrease of thermal coefficient of resistivity) to a worse one occurs already after the first hydrogen cycle. A linear correlation between palladium resistivity and strain, according to Matthiessen's rule, has been found.     

Hydrogen adsorption on a clean palladium ribbon; Thermal desorption and work function measurements

The adsorption of hydrogen on a palladium ribbon has been studied by thermal desorption and work function measurements. It has been established that several heating cycles of the sample covered with hydrogen, up to 700 K lead to the repeated appearance of hydrogen thermal desorption peaks. Analogous experiments of adsorption and repeated heating cycles up to 700 K have shown work function changes decrease to zero as a result of heating and an increase again almost up to the initial value following cooling, in a much shorter time than that required for adsorption. The experimental results show that only a small part of the adsorbed hydrogen is desorbed in the temperature range of the thermal desorption peak. The major part of adsorbed hydrogen penetrates below the surface which leads to a nonequilibium increase of the bulk concentration.     

Hydrogen-induced Ostwald ripening at room temperature in a Pd nanocluster film

The structural and morphological changes occurring in an ensemble of vapor deposited palladium nanoclusters have been studied after several hydrogenation cycles with x-ray diffraction, extended x-ray-absorption fine structure spectroscopy, Rutherford backscattering spectrometry, and STM. Initial hydrogenation increased the cluster size, a result that is attributed to hydrogen-induced Ostwald ripening. This phenomenon originates from the higher mobility of palladium atoms resulting from the low sublimation energy of the palladium hydride as compared to that of the palladium metal. The universality of this phenomenon makes it important for the application of future nanostructured hydrogen storage materials.     

Interaction of hydrogen with palladium surface: FIM and FEM studies

In the present paper, we focus on the geometrical and electronic changes in palladium surface structure which appeared during its interaction with hydrogen in the presence of an external electric field. The interaction process was examined by using the field ion microscopy (FIM) as well as the field emission microscopy (FEM) techniques. In order to study the geometrical changes in substrate surface structure, the distance distribution function (DDF) was constructed on the basis of FIM patterns of both a clean and hydrogen-covered palladium surface. The electronic changes were examined by the measurement of the total energy distribution (TED) of electrons emitted from the palladium tip surface. The most pronounce examples of such changes are an expansion of the equilibrium interatomic distance in palladium surface and a shift of the Fermi level of the metal. These changes may be explained among others by palladium hydrides formation. This process is the most efficient if the field strength exceeds 23 V/nm.     

Comparison of reactivity on step and terrace sites of Pd (3 3 2) surface for the dissociative adsorption of hydrogen: A quantum chemical molecular dynamics study

The notion of “active sites” is fundamental to heterogeneous catalysis. However, the exact nature of the active sites, and hence the mechanism by which they act, are still largely a matter of speculation. In this study, we have presented a systematic quantum chemical molecular dynamics (QCMD) calculations for the interaction of hydrogen on different step and terrace sites of the Pd (3 3 2) surface. Finally the dissociative adsorption of hydrogen on step and terrace as well as the influence of surface hydrogen vacancy for the dissociative adsorption of hydrogen has been investigated through QCMD. This is a state-of-the-art method for calculating the interaction of atoms and molecules with metal surfaces. It is found that fully hydrogen covered (saturated) step sites can dissociate hydrogen moderately and that a monovacancy surface is suitable for significant dissociative adsorption of hydrogen. However in terrace site of the surface we have found that dissociation of hydrogen takes place only on Pd sites where the metal atom is not bound to any pre-adsorbed hydrogen atoms. Furthermore, from the molecular dynamics and electronic structure calculations, we identify a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage and directional dependence of atomic hydrogen diffusion on stepped palladium surface.     

高真空多层绝热结构中一氧化钯吸氢特性研究

对高真空下的一氧化钯吸附氢气的特性进行了分析和研究,实验测定了一氧化钯吸附氢气的机理,不同条件下一氧化钯对氢气的吸附特性,以及设置一氧化钯对夹层漏放气的影响。实验证明,在夹层中使用恰当的混合吸附剂,可以有效减小夹层的漏放气量,提高低温储罐的寿命。     

Effective proton-proton interaction and metallization of hydrogen

The effective proton-proton interaction in metallic hydrogen has been calculated for various densities. In the potential of this interaction, which can be represented in the form of a series of perturbation theory in the electron-proton interaction, the second- and third-order terms are taken into account. The second-order term gives a comparatively shallow potential well with one minimum. The position of the minimum corresponds to the equilibrium mutual position of protons in metallic hydrogen. When the third-order term is additionally taken into account, the potential has two characteristic minima; the position of one of them corresponds to the distance between the protons in a hydrogen molecule. The depths of these potential wells depend on the density of the system. At fairly high densities, only the potential well corresponding to the equilibrium position of protons in the metallic phase holds in the proton-proton potential. Owing to the presence of two minima in the effective proton-proton potential, the density of hydrogen at the point of transition to the metallic state can be estimated. Analysis of the effective proton-proton interaction makes it also possible to estimate the possibility of the existence of metallic hydrogen in a stable state at quite low temperatures.     

Interaction of a piezoelectric ceramic based on lead-zirconium-titanium solid solution with different solders

Thermodynamic estimation of the interaction of a piezoelectric ceramic based on the lead-zirconium-titanium (PZT) solid solution with lead, tin, copper, silver, palladium, and their alloys has been performed. The results of thermodynamic calculations have been experimentally verified by estimating the adhesion and capillary properties of solders on a PZT piezoelectric ceramic.     

The hardening and loss of strength of thin surface layers of LiF monocrystals and zirconium ceramics under conditions of cyclic nanoindenting

Low-cycle surface fatigue is still relatively poorly investigated, especially in the case of accumulation of surface fractures during nanocontact interactions between solids. A new technique of modeling fatigue during a short nanocontact interaction based on repeated dynamic nanoindenting is proposed. The results of investigation of model LiF crystals and industrial ZrO₂ ceramics show that, at a small number of cycles N of about 20, the material is hardened under the indenter. As N increases, the hardening is replaced by a loss of strength reflected in the formation and growth of microcracks.