Metastable Sites Research Articles

A First Principles Study on the Dissociation and Rotation Processes of a Single O2 Molecule on the Pt(111) Surface

Through density functional theory calculations, a detailed description on the dissociation and rotation processes of a single oxygen molecule adsorbed on the Pt(111) surface has been provided. Both dissociation and rotation reaction pathways have been identified, and some interesting phenomena related to the scanning tunneling microscopy experiments are finally explained. It is found that the prior occupation of oxygen atom on the metastable hcp-hollow site after O2 dissociation is originated from the particular structure of the intermediate state, and the low energy barrier of the O2 rotation can be attributed to an effective pathway. The experimentally observed noninteger power-law dependence of the rotation rate as a function of the current has been accurately determined by a newly developed statistical model for the inelastic electron tunneling. By considering the randomness of multielectron inelastic tunneling processes, it is found that the noninteger exponent comes from a statistical contribution of various n-electron events.

Dynamics of Molecular Adsorption and Rotation on Nonequilibrium Sites

It is generally accepted that important events on surfaces such as diffusion and reactions can be adsorption site dependent. However, due to their short lifetime and low concentration in most systems, adsorbates on nonequilibrium adsorption sites remain largely understudied. Using low-temperature scanning tunneling microscopy, site-dependent adsorption is shown for the molecule butyl methyl sulfide, which is trapped in multiple metastable adsorption sites upon deposition onto a Au(111) surface at 5 K. As this molecule does not have enough energy to diffuse to its preferred adsorption site on the surface, it is possible to study the behavior of individual molecules in a variety of nonequilibrium sites. Here we present atomic-scale data of the same chemical species in three independent, metastable adsorption sites and equilibration to a single equilibrium site as a function of either electrical or thermal excitation. Butyl methyl sulfide exhibits distinctly different physical properties at all four adsorption sites, including rotational dynamics and appearance in scanning tunneling microscopy (STM) images. An energy profile is proposed for the adsorption and equilibration of these species, and a correlation is drawn between rotational barrier and adsorption energy.

Structural Transition and Thermal Stability of a Coronene Molecular Monolayer on Cu(110)

A series of alternating structural transitions of coronene monolayers on Cu(110) surfaces are observed using in situ molecular beam epitaxy with low-energy electron diffraction when varying substrate temperature after deposition. The first transition is observed around 293 K, and the second one happens at 360 K. By employing molecular mechanics and density functional theory calculation, it is found that the first transition is due to the transition from physical adsorption to chemical adsorption, while the second transition can be explained by the adsorbed molecules diffusing from a metastable site to a thermodynamically more stable site. Moreover, the unchanged diffraction pattern in the subsequent cooling and reheating process indicates that the final monolayer structure has a high thermal stability. These transitions were compared with the reversible transitions of coronene on Ag(110).

Spin states and hyperfine interactions of iron in (Mg,Fe)SiO 3 perovskite under pressure

With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U). Our calculation supports the notion that the transition observed in recent Mössbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase.

Cloning of the sixth complement component and, spatial and temporal expression profile of MAC structural and regulatory genes in chicken

Humoral cytotoxicity results from the assembly of terminal components of complement, called membrane attack complex (MAC), which lead to the formation of pores on pathogen membranes. The complement components involved in MAC formation are C5b, C6, C7, C8alpha, C8beta, C8gamma and C9. Among them, C6 protein interacts with C5b through a metastable binding site to form a soluble C5b-6 dimer in the vicinity of the activating cell. Formation of the MAC is controlled by complement regulatory molecules, such as CD59, vitronectin and clusterin. Here, we report the molecular characterization of the C6 complement component, as well as the spatial and temporal expression profile of MAC structural (C6, C7, C8alpha, C8beta, C8gamma) and regulatory (CD59, vitronectin and clusterin) genes in chicken (Gallus gallus). The deduced polypeptide sequence of chicken C6 consists of 935 amino acid residues and exhibits 81%, 58%, 56% and 44% identity with zebra finch, human, frog and trout orthologs, respectively. The 'domain' architecture of chicken C6 resembles that of mammalian counterparts and the cysteine backbone is also conserved. MAC structural and regulatory genes are expressed in a wide range of adult chicken tissues, with the liver being the major source of their produced transcripts. The developmental expression profile of chicken MAC structural genes shows that their transcripts initially appear in the 12th embryonic day in the liver, exhibiting a pick in the 17th, while no expression was detected in the early whole embryo (day 4 and 6), as well as in the 2-day old neonate chicken liver. On the other hand, MAC regulatory genes are expressed in all the developmental stages investigated.

O-vacancy-mediated spin-spin interaction in Co-doped ZnO: First-principles total-energy calculations

The effects of O vacancy (VO) on the electronic and magnetic properties of Co-doped ZnO are examined through first-principles total-energy calculations. Our results suggest that due to the presence of VO, O 2p states are coupled with the Co 3d states. Co 3d empty minority states are broadened and move up towards the Fermi level. VO at metastable sites can induce the ferromagnetic (FM) coupling between nearest-neighbor (NN) Co ions, and the atoms around the FM-coupled NN Co ions are spin-polarized which leads to the long-ranged FM coupling between Co ions. Our results based on first-principles total-energy calculations give a possible explanation for the controversial magnetic properties of Co-doped ZnO reported in the literature.

Boron and Phosphorus Implantation Induced Electrically Active Defects in p-Type Silicon

Electrically active defects induced by ion implantation of boron and phosphorus into silicon and their recovery under isothermal annealing at 450 °C were investigated using Deep Level Transient Spectroscopy (DLTS) and Energy Resolved Tunneling Photoconductivity (ERTP) spectroscopy at cryogenic temperatures. DLTS results show electrically active deep traps located at Ev+0.35 eV and Ev+0.53 eV in boron implanted Si and at Ev+0.34 eV, Ev+0.43 eV, and Ev+0.38 eV in phosphorus implanted Si. These meta-stable defect sites were found to be either eliminated or significantly reduced in thermally annealed samples. We assigned these defect sites to hydrogen and carbon incorporated complexes formed during ion implantation. Corroborating the DLTS results, the photocurrent measurement also revealed a strong reduction of electrically active defects states, extended from EC – 0.3 eV up to the conduction band edge of Si, upon isothermal annealing.

First-principles study of oxygen incorporation and migration mechanisms in Ti2AlC

We performed density-functional calculations of oxygen incorporation and diffusion in layered Ti2AlC for a range of intrinsic- and impurity-element chemical potentials. In view of the thermal equilibrium coexistence between oxygen-dissolved Ti2AlC and the oxide scale, a thermodynamic scheme is presented that allows the comparison of the relative stability of oxygen defects in different exterior environments. The calculations show that the oxygen atom favors substitution on carbon lattice sites (OC) under oxygen-lean conditions and high temperatures, whereas the occurrence of an oxygen interstitial in the aluminum atomic layer (IO-tri) becomes more preferential in an oxygen-rich atmosphere and low temperatures. Interstitial oxygen (IO-tri) diffusion via a metastable interstitial site (IO-oct) has a comparatively low migration energy. The substitutional oxygen defect (OC) diffuses by exchanging with neighboring carbon vacancy, which needs a relatively high diffusion barrier.

THE COSMOLOGICAL CONSTANT PROBLEM AND INFLATION IN THE STRING LANDSCAPE

An earlier paper points out that a quantum treatment of the string landscape is necessary. It suggests that the wave function of the universe is mobile in the landscape until the universe reaches a meta-stable site with its cosmological constant Λ0 smaller than the critical value Λc, where Λc is estimated to be exponentially small compared to the Planck scale. Since this site has an exponentially long lifetime, it may well be today's universe. We investigate specific scenarios based on this quantum diffusion property of the cosmic landscape and find a plausible scenario for the early universe. In the last fast tunneling to the Λ0(<Λc) site in this scenario, all energies are stored in the nucleation bubble walls, which are released to radiation only after bubble collisions and thermalization. So the Λ0 site is chosen even if Λ0 plus radiation is larger than Λc, as long as the radiation does not destabilize the Λ0 vacuum. A consequence is that inflation must happen before this last fast tunneling, so the inflationary scenario that emerges naturally is extended brane inflation, where the brane motion includes a combination of rolling, fast tunnelings, slow-roll, hopping and percolation in the landscape. We point out that, in the brane world, radiation during nucleosynthesis are mostly on the standard model branes (brane radiation, as opposed to radiation in the bulk). This distinction may lead to interesting dynamics. We consider this paper as a road map for future investigations.

Comparative study of single Cu, Ag, Au, and K atoms adsorbed onSi(111)−(7×7)

Using first-principles calculations, we have studied the geometrical, electronic, and dynamic properties in detail for a single K, Cu, Ag, and Au atoms adsorbed on $\text{Si}(111)\text{\ensuremath{-}}7\ifmmode\times\else\texttimes\fi{}7$ surface. We have found that single noble metal Cu, Ag, and Au atoms favor the multicoordinate adsorption sites to saturate the maximum number of dangling bonds on $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface. For Cu, Ag, and Au atoms adsorbed on the stable or metastable adsorption sites, the bright spots in the simulated negative bias scanning tunnel microscopy (STM) images are found right on the nearby Si adatoms which are induced by a combination of the charge redistribution and the height increase in Si adatoms induced by the single metal-atom adsorption. The finite increase in charge density between Cu, Ag, and Au atoms and the nearby bonding Si atoms indicate the bond is covalent in nature. In comparison, an alkali metal K atom loses its $s$ electron to the Si surface, and the charge transfer results in the bright spot at the Si adatom in simulated negative bias STM image.

Scaling properties of a rice-pile model: Inertia and friction effects

We present a rice-pile cellular automaton model that includes inertial and friction effects. This model is studied in one dimension, where the updating of metastable sites is done according to a stochastic dynamics governed by a probabilistic toppling parameter p that depends on the accumulated energy of moving grains. We investigate the scaling properties of the model using finite-size scaling analysis. The avalanche size, the lifetime, and the residence time distributions exhibit a power-law behavior. Their corresponding critical exponents, respectively, tau, y, and yr, are not universal. They present continuous variation versus the parameters of the system. The maximal value of the critical exponent tau that our model gives is very close to the experimental one, tau=2.02 [Frette, Nature (London) 379, 49 (1996)], and the probability distribution of the residence time is in good agreement with the experimental results. We note that the critical behavior is observed only in a certain range of parameter values of the system which correspond to low inertia and high friction.

Temperature-dependent diffusion coefficients fromab initiocomputations: Hydrogen, deuterium, and tritium in nickel

The temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium, and tritium in nickel are computed using transition state theory. The coefficient of thermal expansion, the enthalpy and entropy of activation, and the pre-exponential factor of the diffusion coefficient are obtained from ab initio total energy and phonon calculations including the vibrations of all atoms. Numerical results reveal that diffusion between octahedral interstitial sites occurs along an indirect path via the metastable tetrahedral site and that both the migration enthalpy and entropy are strongly temperature dependent. However, the migration enthalpy and entropy are coupled so that the diffusion coefficient is well described by a constant activation energy, i.e., $D={D}_{0}\phantom{\rule{0.2em}{0ex}}\mathrm{exp}[\ensuremath{-}Q∕(RT)]$, with $Q=45.72$, 44.09, and $43.04\phantom{\rule{0.3em}{0ex}}\mathrm{kJ}∕\mathrm{mol}$ and ${D}_{0}=3.84\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $2.40\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$, $1.77\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}{\mathrm{m}}^{2}\phantom{\rule{0.2em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ for H, D, and T, respectively. The diffusion of deuterium and tritium is computed to be slower than that of hydrogen only at temperatures above $400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At lower temperatures, the order is reversed in excellent agreement with experiment. The present approach is applicable to atoms of any mass as it includes the full coupling between the vibrational modes of the diffusing atom with the host lattice.

Theoretical Study of NO Dissociation on Stepped Rh(221) and RhCu(221) Surfaces

Periodic density functional calculations are reported for NO dissociation on the Rh(221) and RhCu(221) surfaces. The analysis of the reaction pathways indicates that the presence of steps results not only in new adsorption sites for NO adsorption but also on the appearance of a completely new reaction pathway with activation energies which are smaller than those found previously for the regular Rh(111) surfaces. At low NO surface coverage and a small Cu density, the presence of Cu on Rh(221) does not change the reactivity of rhodium stepped surfaces toward NO dissociation, since the effective activation energy barrier is higher (1.32 eV for RhCu(221) versus 1.17 eV for Rh(221)). The essential reason is the increased metastability of the step sites. However, for stronger NO coverage or a higher Cu density, the stepped RhCu(221) surface is expected to exhibit an enhanced activity: either by increasing the occupancy of these metastable sites along the RhCu steps or by removing the possibility of highly stable pure Rh bridge step sites.

First-principles study of alloying effect of transition metals on He in titanium ditritide

Due to its inert reactivity with almost elements, 3He produced from tritium decay has extremely detrimental effects on the tritide. To refrain from this 3He-induced damage, an efficient way is to increase the stability of 3He in metal tritide by alloying. Using a first-principles discrete variational method in two cluster models, one for a low 3He concentration and the other for a high 3He concentration, the authors study the alloying effect of 3d and 4d transition metals on the stability of 3He in TiT 2 system. It is found that the preferring and metastable sites of 3He are affected by 3He concentration: 3He prefers to stay at original tetrahedral interstitial site when 3He concentration is low but moves to octahedral site when 3He concentration is high enough. A criterion of alloying effect is proposed, according to which Nb, Y, Zr, Pd, Ru, Tc, Rh, Cr, Mo and Ag are suggested to be the beneficial alloying elements for increasing the stability of 3He in the alloyed TiT 2 with a low 3He concentration and Y, Nb, Mo, Zr, Cr, Tc, Ru, Rh and Cu for that with a high 3He concentration. Our results of alloying effect are supported by the positron annihilation spectroscopy (PAS) measurements for He-implanted Ti, TiMoYAl and TiZrYAl films.

Atomic and Electronic Structures of Carbon Nanotubes on Si(001) Stepped Surfaces

We report first-principles total-energy calculations that provide energetics and electronic structures of adsorbed carbon nanotubes (CNTs) on stepped Si(001) surfaces. We find that adsorption energies strongly depend on the directions of CNTs, and that there are several metastable adsorption sites both on terraces and near step edges. We also find that the electronic structure of adsorbed metallic CNTs becomes semiconducting or remains metallic, depending on the adsorption site. Charge redistribution upon adsorption is prominent mainly at the CNT-surface interface.

Adsorption, Vibration, and Diffusion of O Atoms on Rh Low-Index and (711) Stepped Defective Surfaces

The adsorption, vibration, and diffusion of O atoms on Rh(100), Rh(111), Rh(110), and Rh(711) surfaces were studied using the 5-parameter Morse potential (5-MP) of interaction between an adatom and a metal surface cluster. Our theoretical calculations provide information about adsorption sites, adsorption geometry, binding energy, and eigenvibration. Our results agreed very well with experimental results. Four major results follow. First, the theoretical calculation showed that on the Rh(100) surface the 4-fold hollow site is the only adsorption site. Second, on the O-Rh(111) system, the 3-fold hollow site is the stable adsorption site. Third, on the Rh(110) surface at low coverage, the O atom is adsorbed preferably on the pseudo-3-fold site, while with increasing coverage, the O atom is adsorbed not only on the pseudo-3-fold site but also on the long bridge site. Last, as for the Rh(711) stepped surface, the 3-fold site on the (111) step is metastable, whereas the 4-fold sites on the (100) terrace are stable, which enables the O atoms to diffuse easily from the 3-fold to the 4-fold site at low coverage. Therefore, the O atoms are adsorbed preferrably on the stable 4-fold sites of the (100) terrace and then later as coverage increases on the metastable 3-fold site of the (110) step.

Room-Temperature Adsorption of Si Atoms on H-terminated Si(001)-2 ×1 Surface

Room-temperature adsorption of Si atoms on a hydrogen-terminated Si(001)-2 ×1 surface is studied using scanning tunneling microscopy/spectroscopy and first-principles total-energy calculations. We find that a Si atom adsorbs at the metastable bridge site of a Si dimer forming a SiH 2 cluster at room temperature. Although Si adatom has a kinetic energy sufficient to overcome the activation energy at the moment of adsorption, the reaction path to the most stable off-centered inter-bridge site is narrow and the energy dissipation takes place before the transition. The ground-state adsorption is achieved by an annealing of the sample at 520 K.

Observing sub-microsecond telegraph noise with the radio frequency single electron transistor

Telegraph noise, which originates from the switching of charge between metastable trapping sites, becomes increasingly important as device sizes approach the nanoscale. For charge-based quantum computing, this noise may lead to decoherence and loss of readout fidelity. Here we use a radio frequency single electron transistor (rf-SET) to probe the telegraph noise present in a typical semiconductor-based quantum computer architecture. We frequently observe microsecond telegraph noise, which is a strong function of the local electrostatic potential defined by surface gate biases. We present a method for studying telegraph noise using the rf-SET and show results for a charge trap in which the capture and emission of a single electron is controlled by the bias applied to a surface gate.

Mechanisms of VUV Damage in BaMgAl10O17:Eu2+

We have studied vacuum ultraviolet (VUV)-induced damage processes in BaMgAl10O17:Eu2+ using a unique accelerated life-testing device. Emission, excitation, and reflectance spectra of doped and undoped samples before and after VUV damage reveal two independent mechanisms of degradation. The observed color shift in damaged materials is due to the migration of Eu2+ ions to metastable sites in the lattice. The observed loss of brightness is due in part to the formation of color centers in the spinel layer that act as energy traps and also absorb the exciton emission of the host lattice, thus interrupting the transfer of energy from the host to the activator. Several modes of energy transfer to Eu2+ are available depending on the excitation energy.

Pattern formation in a metastable, gradient-driven sandpile.

With a toppling rule which generates metastable sites, we explore the properties of a gradient-driven sandpile that is minimally perturbed at one boundary. In two dimensions we find that the transport of grains takes place along deep valleys, generating a set of patterns as the system approaches the stationary state. We use two versions of the toppling rule to analyze the time behavior and the geometric properties of clusters of valleys, also discussing the relation between this model and the general properties of models displaying self-organized criticality.