Neighbor Pairs Research Articles

Structural Transitions and Magnetic Structure in NH4CuCl3via14N-NMR

We report results of 14N-NMR experiments on NH4CuCl3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole coupling parameter νz splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both νz and the magnetic hyperfine shift Kz are resolved in the NMR spectrum for general field directions. We then conclude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Rüegg et al. [Phys. Rev. Lett. 93 (2004), 037207], we propose that triplet dimers in the 1/4-plateau are formed not between the nearest neighbor pairs but over different chains.

Reduct and attribute order

Based on the principle of discernibility matrix, a kind of reduction algorithm with attribute order has been developed and its solution has been proved to be complete for reduct and unique for a given attribute order. Being called the reduct problem, this algorithm can be regarded as a mapping R = Reduct(S) from the attribute order space Θ to the reduct space R for an information system 〈U, C ∪ D〉, where U is the universe and C and D are two sets of condition and decision attributes respectively. This paper focuses on the reverse problem of reduct problem S = Order(R), i.e., for a given reduct R of an information system, we determine the solution of S = Order(R) in the space Θ. First, we need to prove that there is at least one attribute order S such that S = Order(R). Then, some decision rules are proposed, which can be used directly to decide whether the pair of attribute orders has the same reduct. The main method is based on the fact that an attribute order can be transformed into another one by moving the attribute for limited times. Thus, the decision of the pair of attribute orders can be altered to the decision of the sequence of neighboring pairs of attribute orders. Therefore, the basic theorem of neighboring pair of attribute orders is first proved, then, the decision theorem of attribute order is proved accordingly by the second attribute.

Carbon–carbon interactions in iron

Carbon atoms occupy interstitial sites in iron so that the configurations of any solid-solution at constant composition depend solely on the distribution of carbon atoms on the interstitial sub-lattice, this in turn being influenced by interactions between carbon atoms in close proximity. The carbon–carbon interaction energy, which influences the distribution of carbon atoms, is reviewed with a view to understanding the nature of the interaction and to highlight some recent developments in the subject. It appears that the C–C interaction energy for ferrite cannot be deduced from the thermodynamic data currently available, primarily because of the very low solubility of carbon in ferrite. On the other hand, there is ample evidence to support the view that the corresponding energy for austenite is consistent with a strong repulsion between near neighbour pairs of carbon atoms.

First-principles investigation of L10-disorder phase equilibria of Fe–Ni, –Pd, and –Pt binary alloy systems

FLAPW total energy electronic structure calculations are combined with cluster variation method in order to perform first-principles investigation of phase equilibria for three kinds of Fe-based binary alloy systems, Fe–Ni, Fe–Pd, and Fe–Pt. A particular focus of the present investigation is placed on L10-disorder phase equilibria. The lattice vibration effects are incorporated within the quasi-harmonic approximation via Debye–Gruneisen model. The ground state analysis revealed that magnetism plays a crucial role in the phase stability of each system. The calculated transition temperatures for Fe–Pd and Fe–Pt systems are in close agreement with experimental ones. The lattice vibration effects further improves the accuracy, and it is found that magnetic fine structure also affects the resultant transition temperature in Fe–Pt system. Although L10-ordered phase does not appear as a stable ordered phase in a conventional phase diagram of Fe–Ni system, the present first-principles calculation suggests the possibility of the stabilization of this phase. The effect of the second nearest neighbor pair interactions as well as multibody interactions are investigated.

Acetone hydration in supercritical water: (13)C-NMR spectroscopy and Monte Carlo simulation.

The (13)C-NMR chemical shift of acetone delta((13)C[Double Bond]O) was measured in aqueous solution at high temperatures up to 400 degrees C and water densities of 0.10-0.60 g/cm(3) for the study of hydration structure in the supercritical conditions. The average number N(HB) of hydrogen bonds (HBs) between an acetone and solvent waters and the energy change DeltaE upon the HB formation were evaluated from the delta and its temperature dependence, respectively. At 400 degrees C, N(HB) is an increasing function of the water density, the increase being slower at higher water densities. The acetone-water HB formation is exothermic in supercritical water with larger negative DeltaE at lower water densities (-3.3 kcal/mol at 0.10 g/cm(3) and -0.3 kcal/mol at 0.60 g/cm(3)), in contrast to the positive DeltaE in ambient water (+0.078 kcal/mol at 4 degrees C). The corresponding Monte Carlo simulations were performed to calculate the radial and orientational distribution functions of waters around the acetone molecule. The density dependence of N(HB) calculated at 400 degrees C is in a qualitative agreement with the experimental results. In the supercritical conditions, the HB angle in a neighboring acetone-water pair is weakly influenced by the water density, because of the absence of collective HB structure. This is in sharp contrast to the hydration structure in ambient water, where the acetone-water HB formation is orientationally disturbed by the tetrahedral HB network formation among the surrounding waters.

Long- and short-range magnetic order, charge transfer, and superconductivity inYBa2Cu3−xCoxO7

We have performed powder-neutron-diffraction measurements in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{7}$ for a large range of Co concentrations $(x=0.04--0.72)$ and determined the magnetic phase diagram. As previously observed, superconductivity disappears for $x>~0.42$ and antiferromagnetic long-range order emerges above this value. The antiferromagnetic order shows noticeable differences compared to that in unsubstituted ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+\ensuremath{\delta}}$ and surprisingly survives to a much higher hole doping. In contrast to recent results in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{7}$ single crystals, there is no evidence of any long-range antiferromagnetic order in the superconducting phase. The Co valence evolves from 4+ to 3+ with increasing Co concentration. We studied the short-range chemical and magnetic correlations between the Co ions in superconducting $(x<~0.36)$ and antiferromagnetic $(x>~0.42)$ samples. The Co ions predominantly occupy the Cu1 sites and their moments remain mostly disordered, forming random antiferromagnetic next nearest neighbor pairs, which compete with the long-range order. This can explain the low value for the ordered Cu1 site moment.

Exchange interaction between distant neighbors in mercury‐based DMS

The magnetic properties of zinc-blende, HgS-based mixed crystals containing up to a few percent of Co are reported. The magnetic susceptibility was measured in a wide temperature range starting from 1.8 K, the magnetization studies were performed in the magnetic fields up to 50 kOe at two temperatures: 1.8 K and 3 K. From the analysis of the experimental data in extended nearest neighbor pair approximation the Co–Co nearest neighbor exchange constant JNN = −33 K +1 = −2 K was determined. As in the case of Mn–Mn interaction (investigated by us previously) its radial decay for further neighbors is proportional to: ∝ R−5. The chemical trend obtained within the whole family of mercury chalcogenides is presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

DIFFICULTY FOR CONSENSUS IN SIMULTANEOUS OPINION FORMATION OF SZNAJD MODEL

In the Sznajd model of 2000, a pair of neighboring agents on a square lattice convinces its six neighbors of the pair opinion if and only if the two agents of the pair share the same opinion. Now we replace the usual random sequential updating rule (corresponding to informal encounters) by simultaneous updating (like in formal votes during meetings) and find that this change makes a complete consensus much more difficult. The reason is that for simultaneous updating some agents simultaneously receive conflicting messages from different neighbor pairs (called frustration) and thus refuse to change their opinion.

計算状態図と材料設計への展開 クラスター展開法の代数的構造に関する一考察と第一原理相平衡計算への応用

Cluster Expansion Method has been extensively employed for the first principles calculation of phase equilibria. The essential mathematical procedure is to calculate total energies for K ordered phases including pure elements, which forms K × 1 energy vector, and to operate K × K inversed matrix of correlation functions on the energy vector. The method is based on the orthogonality of the correlation functions in the thermodynamic configuration space and is versatile to various alloy systems. However, when one would like to obtain (K + 1)-st cluster interaction energy, it is not sufficient to add an arbitrary phase in the energy vector for which the total energy is to be calculated. This is because the elements of the inversed matrix of correlation functions are generally not preserved and, therefore, the net effect of the (K + 1)-st cluster interaction energy is obscured. In the present paper, algebraic aspects of the correlation functions are examined in detail and we propose some criterion to extract the net contribution of the (K + 1)-st cluster interaction energy. The criterion is applied to Fe-Pt system and the effects of 2nd nearest neighbor pair interaction and three body interaction energies on the transition temperature are discussed.

Chiral Discrimination Effects in Langmuir Monolayers of 1-O-Hexadecyl Glycerol

Chiral discrimination effects in the monolayer of 1-O-hexadecyl glycerol (3-hexadecyl-oxy-propane-1,2-diol) amphiphile are theoretically investigated on the basis of a detailed coarse-grained molecular model. The theory uses experimentally obtained information for the average tilt and azimuthal orientation of a reference molecule and minimizes the energy of neighboring molecules using a theoretical framework developed previously. On the basis of the effective pair potential (EPP) of a neighboring pair of molecules, the mutual orientation at the minimum of EPP is obtained. The predicted curvature of the domains composed of enantiomeric amphiphiles based on the mutual orientation obtained from the EPP theory agrees with the experimentally observed handedness based on results from optical studies such as Brewster angle microscopy and fluorescence microscopy. The study revealed, for the first time, interesting crossover from heterochiral preference to homochiral preference of hexadecyl glycerol amphiphile. Th...

Time-resolved photo-phonon spectroscopy of exchange coupled Cr3+ pairs in ruby

A rather simple method is used to simultaneously detect both optical absorption spectra and excited-state nonradiative transitions in 0.03 and 0.16 at. % ruby at temperature 2 K. The technique utilizes a time-resolved bolometer detection of phonons generated by the excited-state nonradiative relaxation following optical excitation with a pulsed tunable dye laser. The observed excitation spectra of phonons coincide well with currently known absorption spectra of both chromium single ions and pairs of the nearest neighbors. For the first time, the fast (≤ µs) resonant energy transfer from single chromium ions to the fourth nearest neighbors is observed directly. New strongly perturbed Cr-single-ion sites are observed.

Mutual passivation of group IV donors and nitrogen in diluted GaNxAs1−x alloys

We demonstrate the mutual passivation phenomenon of Ge donors and isovalent N in highly mismatched alloy GaNxAs1−x doped with Ge. Layers of this alloy were formed by the sequential implantation of Ge and N ions followed by pulsed laser melting and rapid thermal annealing. The mutual passivation effect results in the electrical deactivation of GeGa donors (Ge on Ga sites) and suppression of the NAs (N on As sites) induced band gap narrowing through the formation of GeGa–NAs nearest neighbor pairs. These results in combination with the analogous effect observed in Si-doped GaNxAs1−x provide clear evidence of the general nature of the mutual passivation phenomenon in highly mismatched semiconductor alloys.

CVM calculation of the b.c.c. Co–Cr–Al phase diagram

The cluster variation method (CVM) has been used as a tool for modelling the thermodynamics of the b.c.c. Co–Cr–Al system within the irregular tetrahedron approximation. The interaction parameters (nearest and next-nearest neighbour pairs, as well as tetrahedron interactions) for the three binary sub-systems have been derived following the so-called phenomenological approach, i.e. the interaction parameters have been fitted to experimental phase diagram and/or thermochemical data. As a result, the three binary phase diagrams of the system and four isothermal sections of the ternary phase diagram have been obtained. The results show that the CVM is thermodynamically self-consistent.

Pair dynamics in a glass-forming binary mixture: simulations and theory.

We have carried out molecular dynamics simulations to understand the dynamics of a tagged pair of atoms in a strongly nonideal glass-forming binary Lennard-Jones mixture. Here atom B is smaller than atom A (sigma(BB)=0.88sigma(AA), where sigma(AA) is the molecular diameter of the A particles) and the AB interaction is stronger than that given by Lorentz-Berthelot mixing rule (epsilon(AB)=1.5epsilon(AA), where epsilon(AA) is the interaction energy strength between the A particles). The generalized time-dependent pair distribution function is calculated separately for the three pairs (AA, BB, and AB). The three pairs are found to behave differently. The relative diffusion constants are found to vary in the order D(BB)(R)>D(AB)(R)>D(AA)(R), with D(BB)(R) approximately 2D(AA)(R), showing the importance of the hopping process (B hops much more than A). We introduce a non-Gaussian parameter [alpha(P)(2)(t)] to monitor the relative motion of a pair of atoms and evaluate it for all the three pairs with initial separations chosen to be at the first peak of the corresponding partial radial distribution functions. At intermediate times, significant deviation from the Gaussian behavior of the pair distribution functions is observed with different degrees for the three pairs. A simple mean-field (MF) model, proposed originally by Haan [Phys. Rev. A 20, 2516 (1979)] for one-component liquid, is applied to the case of a binary mixture and compared with the simulation results. While the MF model successfully describes the dynamics of the AA and AB pairs, the agreement for the BB pair is less satisfactory. This is attributed to the large scale anharmonic motions of the B particles in a weak effective potential. Dynamics of the next nearest neighbor pairs is also investigated.

On neighbors in geometric permutations

We introduce a new notion of ‘neighbors’ in geometric permutations. We conjecture that the maximum number of neighbors in a set S of n pairwise disjoint convex bodies in R d is O( n), and we prove this conjecture for d=2. We show that if the set of pairs of neighbors in a set S is of size N, then S admits at most O( N d−1 ) geometric permutations. Hence, we obtain an alternative proof of a linear upper bound on the number of geometric permutations for any finite family of pairwise disjoint convex bodies in the plane.

Exceptional pairs of amino acid neighbors in α-helices

Amino acids seem to have specific preferences for various locations in α-helices. These specific preferences, called singlet local propensity (SLP), have been determined by calculating the preference of occurrence of each amino acid in different positions of the α-helix. We have studied the occurrence of amino acids, single or pairs, in different positions, singlet or doublet, of α-helices in a database of 343 non-homologous proteins representing a unique superfamily from the SCOP database with a resolution better than 2.5 Å from the Protein Data Bank. The preference of single amino acids for various locations of the helix was shown by the relative entropy of each amino acid with respect to the background. Based on the total relative entropy of all amino acids occurring in a single position, the Ncap position was found to be the most selective position in the α-helix. A rigorous statistical analysis of amino acid pair occurrences showed that there are exceptional pairs for which, the observed frequency of occurrence in various doublet positions of the α-helix is significantly different from the expected frequency of occurrence in that position. The doublet local propensity (DLP) was defined as the preference of occurrences of amino acid pairs in different doublet positions of the α-helix. For most amino acid pairs, the observed DLP (DLPO) was nearly equal to the expected DLP (DLPE), which is the product of the related SLPs. However, for exceptional pairs of amino acids identified above, the DLPO and DLPE values were significantly different. Based on the relative values of DLPO and DLPE, exceptional amino acid pairs were divided into two categories. Those, for which the DLPO values are higher than DLPE, should have a strong tendency to pair together in the specified position. For those pairs which the DLPO values are less than DLPE, there exists a hindrance in neighboring of the two amino acids in that specific position of the α-helix. These cases have been identified and listed in various tables in this paper. The amount of mutual information carried by the exceptional pairs of amino acids was significantly higher than the average mutual information carried by other amino acid pairs. The average mutual information conveyed by amino acid pairs in each doublet position was found to be very small but non-zero.

Neighborhood behavior of in silico structural spaces with respect to in vitro activity spaces-a novel understanding of the molecular similarity principle in the context of multiple receptor binding profiles.

As a consequence of recent advances in the field of High Throughput Screening, the systematic testing ("in vitro profiling") of compounds against a panel of targets covering different therapeutic areas is nowadays used to generate relevant information with respect to the in vivo behavior of drug candidates. However, the development of chemoinformatics tools required for the exploitation of such data is yet in an incipient phase. In this paper, a formalism for the analysis of activity profile vectors (describing the experimental responses of compounds in each of the considered activity tests) is introduced and applied at the study of Neighborhood Behavior (NB; the hypothesis that structurally similar compounds display similar biological properties) of molecular similarity metrics. The experimental activity profiles define an Activity Space in which more than 500 drugs and reference compounds are positioned, their coordinates being inhibitory propensities in the included tests and unambiguously characterizing a molecule in terms of its receptor binding properties. While previous studies of Neighborhood Behavior had to rely on a loose classification of compounds in terms of the therapeutic areas they were designed for, here the NB of a calculated "in silico" similarity metric has been redefined as a relationships between intermolecular dissimilarity scores in the "structural" and "activity" spaces, respectively, and expressed in terms of two quantitative criteria: "consistency" (the propensity of the metric to selectively rank activity-related compound pairs among the structurally most similar pairs) and "completeness" (monitoring the retrieval rate of activity-related compound pairs among the best ranked pairs of structural neighbors). These criteria were used to calibrate and validate a similarity metric based on Fuzzy Bipolar Pharmacophore Fingerprints.

Melting of Cross-Linked DNA V. Cross-Linking Effect Caused by Local Stabilization of the Double Helix

DNA interstrand cross-links are usually formed due to bidentate covalent or coordination binding of a cross-linking agent to nucleotides of different strands. However interstrand linkages can be also caused by any type of chemical modification that gives rise to a strong local stabilization of the double helix. These stabilized sites conserve their helical structure and prevent local and total strand separation at temperatures above the melting of ordinary AT and GC base pairs. This local stabilization makes DNA melting fully reversible and independent of strand concentration like ordinary covalent interstrand cross-links. The stabilization can be caused by all the types of chemical modifications (interstrand cross-links, intrastrand cross-links or monofunctional adducts) if they give rise to a strong enough local stabilization of the double helix. Our calculation demonstrates that an increase in stability by 25 to 30 kcal in the free energy of a single base pair of the double helix is sufficient for this “cross-linking effect” (i.e. conserving the helicity of this base pair and preventing strand separation after melting of ordinary base pairs). For the situation where there is more then one stabilized site in a DNA duplex (e.g., 1 stabilized site per 1000 bp), a lower stabilization per site is sufficient for the “cross-linking effect” (18–20 kcal). A substantial increase in DNA stability was found in various experimental studies for some metal-based anti-tumor compounds. These compounds may give rise to the effect described above. If ligand induced stabilization is distributed among several neighboring base pairs, a much lower minimum increase per stabilized base pair is sufficient to produce the cross-linking effect (1 bp- 24.4 kcal; 5 bp- 5.3 kcal; 10 bp- 2.9 kcal, 25 bp- 1.4 kcal; 50 bp- 1.0 kcal). The relatively weak non-covalent binding of histones or protamines that cover long regions of DNA (20–40 bp) can also cause this effect if the salt concentration of the solution is sufficiently low to cause strong local stabilization of the double helix. Stretches of GC pairs more than 25 bp in length inserted into poly(AT) DNA also exhibit properties of stabilizing interstrand cross-links.

Stability of martensite in noble metal alloys

The stability of the martensite is controlled by the electron concentration and by the electronegativity, which is described in terms of pair interchange energies. Although the contribution of the conduction electrons determines the difference in stability between the disordered equilibrium face centred cubic and body centred cubic phases at elevated temperatures, they are only partly responsible for the enthalpy difference between the β phase matrix and the martensite. For a quantitative evaluation of the martensitic transformation it is therefore necessary to have also information on the differences in electronegativity, which control a large fraction of the enthalpy of mixing of the disordered phases, and contribute to the changes from long-range ordering. Long-range ordering takes place generally at lower temperatures in the β phase matrix, and is inherited in the martensite after the diffusionless transformation. The aim of this paper is to derive pair interchange energies that permit to describe quantitatively the martensitic transformation in noble metal alloys. As their prototypes the binary Cu–Zn, Cu–Al and ternary Cu–Zn–Al alloys are analysed in detail, because most information is available for them. It will be shown that for the quantitative description of the enthalpy of formation of the martensite and of the equilibrium phases, pair interchange energies are required which include important contributions also from more distant than first and second nearest neighbour pairs. However, the difference in enthalpy of formation between the martensite and the high temperature β phase can be well described by composition independent nearest and next nearest neighbour pair interchange energies alone.

Structure analysis of amorphous thin films of GeSbTe compounds by grazing incidence X-ray scattering

ABSTRACTWe have investigated the structures of amorphous thin films of GeSbTe compounds – the materials used for the recording layer of phase-change optical memory – through grazing incidence X-ray scattering (GIXS) measurements. Thin films with compositions of GeSb2Te4 and Ge2Sb2Te5 were deposited on Si substrates. Electron radial distribution functions (RDFs) derived from the data clearly showed evidence of the medium-range order with three peaks of the atomic pair distribution. The RDFs of both samples were very similar to those of amorphous bulk GeTe compounds reported from previous X-ray scattering experiments. The positions of the first peaks of these RDFs were consistent with the distances of the first nearest neighbor pairs reported from previous EXAFS experiments. A comparison of the RDF of the amorphous phase with that of the crystalline phase suggested that the phase change caused no significant change in the number of atoms included inside or the radius of the second nearest neighbor atomic shell, although it changed the coordination number and the distance of the first nearest neighbor atomic pairs. This may shorten the moving distance of atoms in the phase change and enable high-speed phase change.