Intermediate Minima Research Articles

Magnetic domain structure of epitaxial Gd films grown on W(110)

We present a detailed real-space spin-polarized scanning tunneling microscopy (SP-STM) study of the magnetic domain structure of Gd(0001) films epitaxially grown on W(110). To find optimal preparation conditions, the influence of the substrate temperature during deposition and of the postgrowth annealing temperature was investigated. Our results show that the lowest density of surface defects, such as step edges as well as screw and edge dislocations, is obtained for room-temperature deposition and subsequent annealing at 900 K. SP-STM data reveal small-size magnetic domains at lower annealing temperatures, evidently caused by pinning at grain boundaries and other crystalline defects. The coverage-dependent magnetic domain structure of optimally prepared Gd films was systematically investigated. For low coverage up to about 80 atomic layers (AL), we observe $\ensuremath{\mu}\mathrm{m}$-size domains separated by domain walls which are oriented approximately along the $[1\overline{1}0]$ direction of the underlying W substrate. Above a critical film thickness ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}\ensuremath{\approx}(100\ifmmode\pm\else\textpm\fi{}20)$ AL, we identify stripe domains, indicative of a spin reorientation transition from in plane to out of plane. In agreement with existing models, the periodicity of the stripe domains increases the further the coverage exceeds ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}$. While the orientation of the stripe domains is homogeneous over large distances just above ${\mathrm{\ensuremath{\Theta}}}_{\text{crit}}$, we find a characteristic zigzag pattern at $\mathrm{\ensuremath{\Theta}}\ensuremath{\gtrsim}200$ AL and irregular stripe domains beyond 500 AL. Intermediate minima and maxima of the magnetic signal indicate the nucleation of branching domains. The results are discussed in terms of various contributions to the total magnetic energy, such as the magnetocrystalline, magnetostatic, and magnetoelastic energy density.

Activation-Tunneling Dynamics of Extended Systems in a Periodic Potential Relief

The dynamics of objects of various physical origins in barrier structures of superconductors, magnetic materials, quantum crystals and other solids is determined by the rate of fluctuation decay of metastable states in intermediate minima of a potential barrier. In a low-temperature region important for many nanotechnological applications, thermal fluctuations are frozen and are replaced by quantum fluctuations, which lead to a specific phase transition in the dynamics. The character of this transition in various systems is an analog of second-order or first-order transitions. The transition temperature is dependent on the degree of metastability and can be controlled by external load. This dependence is calculated for an extended nanosystem in an oblique periodic relief like a “washboard” in a wide range of varying the load, and the results generalize the results known before.

Size- and temperature-dependent epitaxy for a strong film-substrate mismatch: The case ofPt∕MgO(001)

The growth of platinum film on MgO(001) was analyzed in a combined experimental and theoretical study. Experiments were performed by grazing-incidence x-ray diffraction at small [grazing-incidence small-angle x-ray scattering (GISAXS)] and wide [grazing-incidence x-ray scattering (GIXS)] angles, and the theory coupled ab initio calculations on model $\mathrm{Pt}∕\mathrm{Mg}\mathrm{O}(001)$ systems and large-scale simulations of supported Pt clusters using a coordination-dependent interaction potential. GISAXS data showed that the cluster aspect ratio is constant at all studied temperatures with a tendency to faceting at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The cluster spacing at the early stage of the growth is characteristic of a growth on defects, while the particle size obeys a $d\ensuremath{\sim}{t}^{0.5}$ power law assigned to dynamic coalescence. All Pt films deposited at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ show the $[100]{(001)}_{\mathrm{Pt}}\ensuremath{\Vert}[100]{(001)}_{\mathrm{Mg}\mathrm{O}}$ epitaxy. As the cluster size increases, the in-plane Pt-Pt distance increases above the bulk value $(0.393\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, passes through a maximum $(0.398\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ at a thickness of $t=0.6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and then relaxes back to the bulk value through interfacial dislocations, a behavior predicted by calculation and which appears typical of (001) epitaxy. Finally, the formation of dislocations arising from the $\mathrm{Pt}∕\mathrm{Mg}\mathrm{O}(001)$ mismatch of $\ensuremath{-}6.83%$ is evidenced. Below $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, due to a minimization of the surface energy of the film, the ${(111)}_{\mathrm{Pt}}\ensuremath{\Vert}{(001)}_{\mathrm{Mg}\mathrm{O}}$ epitaxy dominates. Beside the expected $[1\overline{1}0]{(111)}_{\mathrm{Pt}}\ensuremath{\Vert}[110]{(001)}_{\mathrm{Mg}\mathrm{O}}$ orientation, another epitaxy is revealed, i.e., $[1\overline{1}0]{(111)}_{\mathrm{Pt}}\ensuremath{\Vert}[100]{(001)}_{\mathrm{Mg}\mathrm{O}}$. Moreover, families of orientations slightly rotated relative to the former epitaxy are evidenced. Calculations reveal that intermediate minima in energy appear at angles that depend on the size of the particles. Comparison between Ni/, Pd/, Pt/, and $\mathrm{Ag}∕\mathrm{Mg}\mathrm{O}(001)$ shows that the progressive weakening of the metal-MgO bonding in this order explains the decrease in adhesion energy as well as in metal-MgO distance, which are experimentally and/or theoretically observed through that series.

Multicoordinate Driven Method for Approximating Enzymatic Reaction Paths: Automatic Definition of the Reaction Coordinate Using a Subset of Chemical Coordinates

We present a generalization of the reaction coordinate driven method to find reaction paths and transition states for complicated chemical processes, especially enzymatic reactions. The method is based on the definition of a subset of chemical coordinates; it is simple, robust, and suitable to calculate one or more alternative pathways, intermediate minima, and transition-state geometries. Though the results are approximate and the computational cost is relatively high, the method works for large systems, where others often fail. It also works when a certain reaction path competes with others having a lower energy barrier. Accordingly, the procedure is appropriate to test hypothetical reaction mechanisms for complicated systems and provides good initial guesses for more accurate methods. We present tests on a number of simple reactions and on several complicated chemical transformations and compare the results with those obtained by other methods. Calculation of the reaction path for the enzymatic hydrolysis of the substrate by dUTPase for an active-site model with 85 atoms, including several loosely bound water molecules, indicates that the method is feasible for the study of enzyme mechanisms.

Oceanic primary productivity and dissolved oxygen levels at the Cretaceous/Tertiary Boundary: Their decrease, subsequent warming, and recovery

Thirty‐six different geochemical and foraminiferal analyses were conducted on samples collected at closely spaced intervals across the Cretaceous/Tertiary (K/T) boundary exposed at Caravaca, Spain. A rapid reduction in the gradient between δ13C values in fine fraction carbonate and benthic foraminiferal calcite and a decrease in the abundance of phosphorus (a proxy for organic carbon) and calcium were recorded in sediments 0–0.5 cm above the K/T boundary. These trends imply that an abrupt mass mortality occurred among pelagic organisms, leading to a significant reduction in the flux of organic carbon to the seafloor. In addition, variations in sulfur isotope ratios, the hydrocarbon‐generating potential of kerogen (measured as the hydrogen index), and foraminiferal indices of dissolved oxygen level all imply that a rapid decrease in dissolved oxygen was coincident with the δ13C event. Evidence of the low oxygen event has also been recognized in Japan and New Zealand, suggesting that intermediate water oxygen minima were widely developed during earliest Danian time. A threefold increase in the kaolinite/illite ratio and a 1.2‰ decrease in δ18O (carbonate fine fraction) were recorded in the basal 0.1–2 cm of Danian age sediments. These trends suggest that atmospheric warming and an increase in surface water temperature occurred 0–3 kyr after the δ13C event. Recovery in the difference between δ13C values in the carbonate fine fraction and in benthic foraminiferal calcite as well as increases in phosphorus and calcium contents occur at the base of planktonic foraminiferal Zone Pla, implying that an increase in primary productivity commenced some 13 kyr after the K/T boundary.Tables A1‐A3 are available on diskette or via Anonymous FTP from kosmos.agu.org directory APENO (Username = anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.

Effect of methyl substitution on the intramolecular exchange of hydrogen-bonded protons

Of the two possible monomethyl naphthazarins only the 2-methyl derivative has been encountered. Efforts to obtain the 7-methyl derivative always lead to the 2-methyl tautomer. Previous studies suggested a near symmetric double minimum potential for the intramolecular proton exchange with a high barrier and 0.9 kcal mol −1 difference between the 2- and 7-tautomers. If there were fast exchange, the two tautomers would have been detected. Now, with a much larger and better basis set and better algorithms for finding stationary points, a new study finds a barrier of 29 kcal mol −1. Two more (intermediate) minima were found, corresponding to structures in which the protons are on different rings. However, the energy difference between the absolute minima and the intermediates is so large that the latter do not contribute to the exchange process. In monomethylnaphthazarin, the difference between the two lowest minima was found to be 1.09 to 1.79 kcal mol −1. The rotation of the methyl group changes the energy difference between the minima, but the minimum of the 2-methyl isomer is always below that of the 7-methyl isomer, thus maintaining the asymmetry of the double minimum potential, without accidental degeneracy, in contrast to dimethylnaphthazarins and 6-methyl-9-hydroxyphenalen-1-one, as already reported. The exchange process is described by two quantities: the fraction of the proton emerging into the other well (2.7 × 10 −11) and the frequency with which it leaks (6 × 10 12). This will make the 7-methyl derivative undetectable by NMR. The product of the two factors suggests that the equivalent of a full proton is exchanging every six milliseconds.

Potential Energy Surfaces for Proton Abstractions from Acetic Acid

The abstractions of hydrogen from both carbon and oxygen in acetic acid by hydride, fluoride, and hydroxide anions have been studied using ab initio electronic structure calculations. Molecular structures were optimized at the Hartree-Fock level of theory using the 6-31++G(d,p) basis set. For energetics, the 6-311++G(d,p) basis set was used, with second- and fourth-order perturbation theory corrections, for both minima and transition states. For the hydride and fluoride ion abstractions of hydrogen from carbon, a small activation energy exists at the Hartree-Fock level, but vanishes when correlation energy corrections are introduced. No other barriers are found for the abstraction reactions, but intermediate minima are found on the F{sup -} + CH{sub 3}COOH {yields} FH + {sup -}CH{sub 2}COOH surface and on the analogous OH{sup -} + CH{sub 3}COOH surface. The calculated heats of formation for both acetic acid anions are in good agreement with the experimental value. The fourth-order perturbation theory calculation of the activation energy for the isomerization of acetate to enolate ion is 50.4 kcal/mol. The G2 values for the gas phase acidities of acetic acid at the OH and CH ends of the molecule are 339.3 and 365.8 kcal/mol, respectively. The former result is in good agreementmore » with the experiment. 26 refs., 5 figs., 5 tabs.« less

Electron spin resonance spectrum of trapped in a single crystal matrix

The radical , labelled with 13C in the position shown, has been prepared by X-irradiation of single crystals of . The 13C hyperfine coupling tensor of this radical confirms previous conclusions from a study of the corresponding unlabelled species that the electron is essentially localized on the carbon atom. At room temperature the methylene group undergoes rapid reorientation about the P-C bond while comparison of the 13C tensors at 300 K and 77 K suggests that the barrier to rotation has a strong two-fold component but contains intermediate minima.

Water Vapor Distribution in the Lower Stratosphere over North and South America

Abstract : Water vapor concentrations in the upper troposphere and lower stratosphere from 70N to 40S over North and South America were deduced from solar spectra recorded on 56 flights of a high flying jet aircraft. Ambient concentrations between 10.7 and 18.3 km declined with altitude at all latitudes to a minimum at about 2 km above the local tropopause, and remained nearly constant thereafter to 18.3 km. Although seasonal variance was pronounced in the troposphere over New Mexico, changes of only a few per cent were observed in the lower stratosphere, assuming uniform mixing from 17.7 km to the top of the atmosphere. The mean latitudinal distribution of water vapor in the lower stratosphere measured during the Northern Hemispheric winter of 1967-68 showed a maximum concentration of 1.75 mg/kg at 65N and a progressive decline to a minimum of 1.25 mg/kg near 30S. Intermediate minima and maxima were also observed at 25N and 10N, respectively. (Author)