High-energy Configurations Research Articles

CHIANTI—An Atomic Database for Emission Lines. VII. New Data for X‐Rays and Other Improvements

The CHIANTI atomic database contains atomic energy levels, wavelengths, radiative transition probabilities, and collisional excitation data for a large number of ions of astrophysical interest. CHIANTI also includes a suite of IDL routines to calculate synthetic spectra and carry out plasma diagnostics. Version 5 has been released, which includes several new features, as well as new data for many ions. The new features in CHIANTI are as follows: the inclusion of ionization and recombination rates to individual excited levels as a means to populate atomic levels; data for Kα and Kβ emission from Fe II to Fe XXIV; new data for high-energy configurations in Fe XVII to Fe XXIII; and a complete reassessment of level energies and line identifications in the X-ray range, multitemperature particle distributions, and photoexcitation from any user-defined radiation field. New data for ions already in the database, as well as data for ions not present in earlier versions of the database, are also included. Version 5 of CHIANTI represents a major improvement in the calculation of line emissivities and synthetic spectra in the X-ray range and expands and improves theoretical spectra calculations in all other wavelength ranges.

Synthesis of Single Crystalline Tellurium Nanotubes with Triangular and Hexagonal Cross Sections

Single crystalline tellurium (Te) nanotubes with triangular cross sections were successfully synthesized for the first time by a simple approach of vaporizing tellurium metal and condensing the vapor in an inert atmosphere onto a suitable substrate. Tellurium gas was evaporated by heating at 350 degrees C and was condensed on the Si (100) substrate at 150-200 degrees C, in the downstream of argon (Ar) gas at a flow rate of 25 sccm for 10 min. This led to the production of nanotubes of triangular cross section along with some hexagonal ones. The formation of the nanotubes was highly dependent upon the structure of the substrate surface, Ar gas flow rate, and the deposition temperature. When the substrate is Si (111) or sapphire (0001) or when the argon flow rate is increased to 500 sccm, nanowires and nanorods were exclusively formed. Irrespective of the morphologies, all the observed Te nanostructures grew in a regular [0001] direction. The facile approach to nanotubes with a triangular cross section may facilitate some new applications as well as stimulate theoretical studies pertaining to the stability of this high-energy configuration.

Negative creep of metallic glasses as an externally-catalyzed dissipative structure within Ginzburg–Landau kinetics

Negative creep concerns annealing temperatures of metallic glasses approaching ambient where normal densification is negligible but negative dimensional change is dramatically catalyzed by a LeChatelier-like load perturbation and initially accelerated against the load by temperature and load increase. This obtains on the isotherm up to the load point and time increment where normal creep in the opposite direction begins to counter the anomalous effects. The proposed dissipative structure is based upon strain as a global average order parameter satisfying the postulated time-dependent Ginzburg–Landau (TDGL) equation and applied to Cu 60Zr 40 at 473 K. This serves as the Euler–Lagrange equation corresponding to the experimentally verified Ostwald's Step Rule which subsumes autonomous selection for near reversibility and minimum dissipation or equivalent high free energy configurations en route to equilibrium thus sustaining the G–L postulate. This quench-induced self-organizing phenomenon can be conceived as a continuous stroke potential energy-increasing heat engine driven by the quenched-in thermal energy.

An ab initio study of manganese atoms and wires interacting with carbon nanotubes

A systematic study of Mn monomers, dimers, trimers, and wires interacting with an (8, 0)semiconductor single-wall carbon nanotube (SWCN) is presented. Spin-polarizedtotal-energy ab initio calculations based on the density functional theory are used todescribe the structural, electronic and magnetic properties of all studied systems. For Mnmonomers, either outside or inside the nanotube, the most stable configuration is found tobe over the centre of the hexagonal site. The most stable geometry for outsidedimers presents the Mn atoms adsorbed directly on top of C atoms, when in ahigh-spin configuration, whereas for a low-spin configuration the Mn atoms areadsorbed on bond-centred sites at opposite sides of a hexagonal ring, with theMn–Mn bond aligned in a diagonal direction relative to the tube axis in bothcases. There are many low energy configurations (at above the lowest energy ones) at distinct orientations, for both the low and high energyconfigurations. For trimers, two kinds of Mn structures are investigated: compact or open.The compact trimers are found to be more stable than the open systems by approximately1 eV/Mn atom. A monoatomic wire in a zig-zag configuration has a binding energy that isintermediate between the open and the compact trimers, independently of the spinconfiguration. For all the investigated Mn structures adsorbed on the SWCN, eitherhigh-spin (HS) or low-spin (LS), the interactions between Mn atoms and between Mn andC atoms become stronger as the Mn coordination number increases. The resulting magneticmoments for all adsorbed systems are found to be close to their original values for thecorresponding free Mn structures.

Study of Pair Contact Formation among Hydrophobic Residues in a Model HP-36 Protein: Relationship between Contact Order Parameter and Rate of Folding and Collapse

Recent analysis of the structures of a large number of proteins in their native state has demonstrated a close relation between relative contact order parameter (COP) (which gives the average contact distance among hydrophobic residues) and the rate of protein folding (Grantcharova et al. Curr. Opin. Struct. Biol. 2001, 11, 70). We have explored the existence of such a relationship by carrying out Brownian dynamics simulations of a model protein. The model consists of 36 amino acid residues and mimics a thermostable single domain headpiece of chicken villin (HP-36) protein. Long range interactions across the protein are obtained by using a simplified hydropathy scale. The heteropolymer exhibits qualitative features of folding and correlates with the COP. We have defined and calculated the distance dependent pair correlation function among the hydrophobic residues. Nativelike states are characterized by distinct pair contacts which are mostly absent in other collapsed non-native states. Contact pair formation shows a strong dependence on contour distance separation between the pairs. Study of the dynamics of specific contact pair formation during folding shows different characteristics depending on whether the final state is (near) native or far from the native configuration. Approach to the final collapsed state is almost always faster when the final state is nativelike. Fluctuations in pair contacts are found to be smaller in nativelike conformations compared to those of higher energy configurations. This is in agreement with a recent theoretical prediction.

Core polarization effects on C4 form factors of sd-shell nuclei

Coulomb form factors of C4 transitions in even-even N = Z sd-shell nuclei ( 20Ne, 24Mg, 28Si and 32S) are discussed taking into account higher-energy configurations outside the sd-shell model space which are called core polarization effects. Higher configurations are taken into account through a microscopic theory, which allows particle-hole excitations from the 1s and 1p shells core orbits and also from the 2s1d-shell orbits to the higher allowed orbits with excitations up to 4 ω. The effect of core polarization is found essential in both the transition strengths and momentum transfer dependence of form factors, and gives a remarkably good agreement with the measured data with no adjustable parameters. The calculations are based on the Wildenthal interaction for the sd-shell model space and on the modified surface delta interaction (MSDI) for the core polarization effects.

Grain boundary structure of nanocrystalline Cu processed by cryomilling

The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction is a dislocation-controlled continuous process which consists of the formation of small-angle grain boundaries (GBs), a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle GBs. The GBs were generally curved, wavy or faceted, and heavily strained, which are typical characteristics of nanostructured materials. In addition, extrinsic dislocations were found in many GBs, indicating that most are in a high-energy non-equilibrium configuration, which is consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that GBs in nanostructured metals processed by severe plastic deformation are mostly in non-equilibrium states.

Interleukin-1 beta folding between pH 5 and 7: experimental evidence for three-state folding behavior and robust transition state positions late in folding.

The thermodynamic stability and folding kinetics of the all beta-sheet protein interleukin-1beta were measured between 0 and 4 M GdmCl concentrations and pH 5-7. Native interleukin-1beta undergoes a 3.5 kcal/mol decrease in thermodynamic stability, Delta, as pH is increased from 5 to 7. The native state parameter m(NU), measuring protein destabilization/[GdmCl], remains constant between pH 5 and 7, indicating that the solvent-exposed surface area difference between the native state and unfolded ensemble is unchanged across this pH range. Similarly, pH changes between 5 and 7 decrease only the thermodynamic stability, DeltaG(H)2(O), and not the m-values, of the kinetic intermediate and transition states. This finding is shown to be consistent with transition state configurations which continue to be the high-energy configurations of the transition state in the face of changing stability conditions. A three-state folding mechanism U right arrow over left arrow I right arrow over left arrow N is shown to be sufficient in characterizing IL-1beta folding under all conditions studied. The m-values of refolding transitions are much larger than the m-values of unfolding transitions, indicating that that the fast, T(2) (U right arrow over left arrow I), and slow, T(1) (I right arrow over left arrow N), transition states are highly similar to the intermediate I and native state N, respectively. Many of the folding properties of interleukin-1beta are shared among other members of the beta-trefoil protein family, although clear differences can exist.

Microscopic calculations of C2 and C4 form factors in sd-shell nuclei

Inelastic electron scattering to 2 + and 4 + states for the even–even N= Z sd-shell nuclei ( 20 Ne, 24 Mg, 28 Si and 32 S) are discussed taking into account higher energy configurations outside the sd-shell. Higher energy configurations, which are called core polarization effects, are included through a microscopic theory that includes excitations from the core 1 s and 1 p orbits and also from 2 s–1 d shell to the higher allowed orbits with 2ℏ ω excitations. The predictions of C2 and C4 Coulomb form factors are compared with the available experimental data. The calculations based on the Wildenthal interaction for the sd-shell model space and on the modified surface delta interaction for the core-polarization effects. The effect of higher excited configurations is found essential in both the transition strength and momentum transfer dependence of form factors, and gives a remarkably good agreement with the measured data.

High-resolution transmission electron microscope (HRTEM) study of the transformation interface and substructure in NiTiHf 40 melt–spun ribbons

The fine structures of self-accommodation spear-like martensite variant groups and the interface between the martensite region and the amorphous phase region in NiTi–Hf 40 melt–spun ribbons were studied by using high-resolution transmission electron microscopy (HRTEM). In the self-accommodation lath plate martensite group, the variants prefer to form orthogonal morphology of spear-like martensite on both sides of melt–spun ribbons. Some variant pairs were of the (011) B19′ Type I twin related with straight and coherent interface while the interface between other variants is curved and incoherent. The main substructure in variants of the group was not the (001) B19′ compound twin but the (011) B19′ Type I microtwin and (011) stacking faults. The interface between the amorphous phase and the martensite region showed high energy and nonequilibrium configuration. Some area in the interface is straight but not smooth and exhibited an irregular configuration and some area in the interface has an obvious zigzag configuration. Energy-dispersive spectroscopy (EDX) analysis of the composition in NiTi–Hf 40 ribbon showed that the Ni and Ti content were different in the martensite region and in the amorphous phase. The amorphous phase and the martensite coexisted in NiTi–Hf ribbons when the cooling rate was high enough. The higher hardness and brittleness of Hf 40 compared to those of lower Hf content NiTi–Hf alloys were due to the not retained austenite, the very small variant pair and the coexisting amorphous phase.

Internal field sources, their screening and the flow stress

The experimental investigation of the internal elastic stress fields in polycrystals of deformed Cu–Al and Cu–Mn alloys was carried out. The basic sources of the internal stress field were identified and the decrease of the field with increasing distance from the sources was determined. The average values of the internal stresses as a function of strain were measured. On this basis, the separation of sequences of dislocation substructure transformations into low-energy and the high-energy configurations was carried out.

Molecular dimension and interaction parameters of polyacrylamide in water–dimethylsulphoxide mixtures: effect of temperature

The intrinsic viscosities [ η] of polyacrylamides having different molecular sizes are measured at 30–50°C temperature in various mixtures of water (good solvent) and dimethylsulphoxide (DMSO, poor solvent). The observed result and the Huggins constant values show a significant variation of cosolvency as a function of solvent composition ( ϕ) and temperature. The nature of plots of [ η] vs. ϕ DMSO indicates relative importance of entropy factors over energetic factors in determining molecular configuration at high temperature. The unperturbed dimensions ( K θ ) of the polymer are determined by various methods, which agree well with each other and exhibit a minimum at ϕ=0.5. The temperature coefficients of the unperturbed dimension ( K ′) gives both positive and negative values depending on the solvent composition, which indicates the variation of compactness and the presence of low and high energy configurations as a function of solvent composition. Molecular extension factor ( α n) and the chain rigidity ( σ) values are evaluated and the influence of temperature is discussed.

Magnetic behavior of small magnetic particles

The magnetism of nanosized three-dimensional clusters of ferromagnetic atoms is studied as a function of temperature. Numerical simulations are used to investigate the evolution of the magnetic structure within the clusters along different thermalization processes. The occurrence of vortexlike excitations is observed when the system is cooled from a high-energy configuration. For a wide range of temperature T, the relaxation time of the cluster is found to scale with ${1/T}^{n},$ with $n=1.55\ifmmode\pm\else\textpm\fi{}0.05.$

Reconstruction of in-plane magnetization distributions from magnetic force microscope images

A three-step method is proposed for reconstructing the in-plane magnetization distribution from a magnetic force microscope (MFM) image. The magnetization distribution is initialized using some assumptions based on our knowledge about domain structures and MFM contrast mechanisms. Then the high-energy moment configurations, which are initialized based on linear variation assumption, are relaxed by performing a micromagnetic calculation using experimentally measured magnetic parameters. A section of a 360° wall in a NiFe/NiO bilayer has been selected to illustrate the reconstruction procedures. It is shown that the method is effective in revealing the wall structure related to the MFM contrast. This method avoids problems caused by noise and nonunique solutions of the micromagnetic equations.

Vitrification in a 2D Ising model with mobile bonds

A bond-disordered two-dimensional Ising model is used to simulate Kauzmann's mechanism of vitrification in liquids, by a Glauber Monte Carlo simulation. The rearrangement of configurations is achieved by allowing impurity bonds to hop to nearest neighbors at the same rate as the spins flip. For slow cooling, the theoretical minimum energy configuration is approached, characterized by an amorphous distribution of locally optimally arranged impurity bonds. Rapid cooling to low temperatures regularly finds bond configurations of higher energy, which are both a priori rare and severely restrictive to spin movement, providing a simple realization of kinetic vitrification. A supercooled liquid regime is also found, and characterized by a change in sign of the field derivative of the spin-glass susceptibility at a finite temperature.

Grain boundary structure in Al–Mg and Al–Mg–Sc alloys after equal-channel angular pressing

Samples of an Al–3% Mg alloy and an Al–3% Mg–0.2% Sc alloy were subjected to equal-channel angular pressing (ECAP) to reduce the grain size to approximately 0.2–0.3 μm. Some samples of each alloy were also annealed for 1 h at temperatures of either 423 or 673 K, respectively. High-resolution electron microscopy was used to examine the microstructure both before and after annealing. The grain boundaries after ECAP were wavy and faceted and in high-energy nonequilibrium configurations. These results were consistent with earlier observations of materials subjected to severe plastic deformation using high-pressure torsion. In addition, some grain boundaries in the Al–Mg–Sc alloy had a zigzag appearance after annealing at 673 K, where the straight portions of the boundary were identified as low-energy {111} planes. It is suggested these are mobile boundaries lying in a lowest energy configuration where mobility may be restricted by the presence of incoherent Al3Sc particles.

Banded domain structures and polarization arrangements in PbTiO 3 single crystals

Banded ferroelectric domain structures and the corresponding polarization arrangements of flux-grown PbTiO 3 single crystals have been studied using optical and electron microscopy. Most of the specimens show banded domain arrangements, which correspond to surface relief characteristics of as-grown crystals. Using crystals with different etching conditions, the three-dimensional arrangements of the domains can be constructed. It is found that banded domain boundaries show relatively high energy, which enhances the etching of these boundaries. Drawing schematic polarization configurations, one finds that head-to-head, tail-to-tail, and other unconventional polarization configurations exist constantly in PbTiO 3 crystals, indicating that the high electrical energy configurations are quite common in lead titanate crystals.

Metastability in amorphous silicon from hydrogen flips

A new metastability of hydrogen atoms in a-Si:H was found, from molecular dynamics simulations. In the higher energy configuration H is flipped to the backside of the SiH bond, and is bonded to the same silicon atom since the SiH bond is rotated by nearly 180°. This H-flip defect explains structural changes found from infrared absorption, volume dilation, and nuclear magnetic reasonance measurements.

Application of an Adaptive Algorithm to the Control of the Plasma Vertical Position

The plasma vertical position in a tokamak can be open-loop unstable with time-varying dynamics. The limitation in the output power of the control amplifier makes the time-varying unstable system particularly difficult to control. Fixed-coefficient linear controllers usually fail to maintain control in the presence of large disturbances, like edge-localized modes (ELMs), which saturate the amplifier output. During the saturation period, the vertical position of the plasma will grow exponentially with the unstable eigenvalue and may reach values that cannot be controlled by the energy provided by the control amplifier, which is limited by practical constraints. The primary sources of disturbances and measurement noise that effect the vertical position are the ELMs and the 600-Hz noise from the thyristor power supplies. The former are present in the form of pulses and appear during high-energy confinement plasma configurations. A novel nonlinear controller for the vertical position based on a discrete adaptive near-time optimum control algorithm (DANTOC) is used to stabilize the system, to maximize the stability region, and to provide robustness with respect to the aforementioned sources of disturbances and measurement noise. The controller is tested in simulation for the Joint European Torus tokamak, and the results demonstrate its feasibility in controlling the vertical position for different plasma configurations. The controller is also tested on the COMPASS-D tokamak, and the results demonstrate the improvement with respect to a simple linear proportional and derivative controller in the presence of disturbances and measurement noise.

Domain structures and polarization arrangements in PbTio3 single crystals

Ferroelectric domain structures and the corresponding polarization arrangements of flux-grown PbTiO3 single crystals have been studied using optical and electron microscopy. Crystals with different etching conditions reveal three dimensional arrangements of the domains. Most of the specimens show banded structures which exhibit relatively high energy and therefore enhances the etching of these boundaries. Drawing schematic polarization configurations, one finds that head-to-head, tail-to-tail and other unconventional polarization configurations exist constantly in PbTiO3 crystals, indicating that high electric energy configurations are quite common in lead titanate crystals.