Intrachain Correlations Research Articles

Structure of AuCN Determined from Total Neutron Diffraction.

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Structure of AuCN determined from total neutron diffraction.

The structure of gold cyanide, AuCN, has been determined at 10 and 300 K using total neutron diffraction. The structure consists of infinite [bond]Au[bond](CN)[bond]Au[bond](CN)[bond] linear chains, hexagonally packed, with the gold atoms in sheets. The Au-C and Au-N bond lengths are found to be identical, with d(Au(-C/N) = 1.9703(5) A at 300 K. This work supersedes a previous study, by others, which used Rietveld analysis of neutron Bragg diffraction in isolation, and found these bonds to have significantly different lengths (Delta d = 0.24 A) at 300 K. The total correlation function, T(r), at 10 and 300 K, has been modeled using information derived from total diffraction. The broadening of inter- and intrachain correlations differs markedly due to random displacements of the chains in the direction of the chain axes. This is a consequence of the relatively weak bonding between the chains. An explanation for the negative thermal expansion in the c-direction, which occurs between 10 and 300 K, is presented.

NMR of 89Y in the copper-oxide spin-chain compound Ca2+xY2-xCu5O10

We report NMR lineshape, spin-lattice relaxation time T 1 , and spin-spin relaxation time T 2 data at 17 MHz (8.07 T) for 89 Y in the copper-oxide spin-chain compound Ca 2+x Y 2-x Cu 5 O 10 . For x = 0, a broad, asymmetric line with width Δν 90 kHz is observed for T = 250-300 K. The spectra exhibit an appreciable average shift (ΔH/H +0.7%) and sharpen at lower temperature, possibly due to increasing intrachain ferromagnetic correlations. T 1 and T 2 decrease with decreasing temperature. The T 1 data imply a short correlation-time limit, with τ e = 3-5 x 10 -11 s. The T 2 data apparently include a contribution from dipolar interactions with copper nuclei. Relaxation time data for a doped (x=0.5) compound surprisingly show more rapid relaxation.

Correlation-induced dimensional crossovers of charge-transfer excitations in quasi-one-dimensional organic conductors

Applying the finite-temperature density-matrix renormalization-group method to the spinless fermion model on a two-leg ladder, we have calculated the dynamical structure factors for the local charge transfer processes along and across the chains. The intra-chain excitation spectra are sensitive to the inter-chain transfer integral and the chemical potential, while the inter-chain excitation spectra are sensitive to the intra-chain electron correlation. These dynamical properties are due to the collective motions of fermions along the chains and the energy-dependent confinement of fermions in the chains. At low but finite energies, the effects of the increasing inter-chain transfer integral on the local charge-transfer spectra are similar to those of the chemical potential deviating from zero.

Magnetic Properties of the Fluorite-Related La 3MO 7 Phases, M=Ru and Os : Local Moment Magnetism, Short- and Long-Range Order in 4 d and 5 d Transition Metal Oxides

The oxide materials La 3 MO 7, M=Ru and Os have been prepared, the Os phase for the first time. Both crystallize in a fluorite-related supercell with Cmcm symmetry with very similar lattice constants. A singular feature of this structure type is the presence of well- separated zig-zag chains of corner-sharing MO 6 octahedra parallel to one of the orthorhombic axes. Local moment magnetic behavior is observed for both on very different energy scales. In each case the materials exhibit Curie–Weiss paramagnetism at high temperatures with effective moments consistent with S= 3 2 , as expected for the 4 d 3 and 5 d 3 electronic configurations associated with oxidation state +5, thus providing evidence for local moment magnetism. At lower temperatures broad maxima occur near 20 K (Ru) and 110 K(Os) which can be attributed to intrachain magnetic correlations. Fits to an appropriate model yield J intra/ k values of −2.9 K for Ru and −23.2(1) K for Os. This ∼eight-fold increase is attributed to the expanded radial extent of the d-orbitals in proceeding from the 4 d to the 5 d series. Evidence for long-range antiferromagnetic order is present in the form of sharp susceptibility maxima at 10 and 45 K for M=Ru and Os, respectively.

Intrachain correlation functions and shapes of homopolymers with different architectures in dilute solution

We present results of Monte Carlo study of the monomer–monomer correlation functions, static structure factor, and asphericity characteristics of a single homopolymer in the coil and globular states for three distinct architectures of the chain: ring, open, and star. To rationalize the results we introduce the dimensionless correlation functions rescaled via the corresponding mean-squared distances between monomers. For flexible chains with some architectures these functions exhibit a large degree of universality by falling onto a single or several distinct master curves. In the repulsive regime, where a stretched exponential times a power law form (de Cloizeaux scaling) can be applied, the corresponding exponents δ and θ have been obtained. The exponent δ=1/ν is found to be universal for flexible strongly repulsive coils and in agreement with the theoretical prediction from improved higher-order Borel-resummed renormalization group calculations. The short-distance exponents θv of an open flexible chain are in a good agreement with the theoretical predictions in the strongly repulsive regime also. However, increasing the Kuhn length in relation to the monomer size leads to their fast crossover towards the Gaussian behavior. Likewise, a strong sensitivity of various exponents θij on the stiffness of the chain, or on the number of arms in star polymers, is observed. The correlation functions in the globular state are found to have a more complicated oscillating behavior and their degree of universality has been reviewed. Average shapes of the polymers in terms of the asphericity characteristics, as well as the universal behavior in the static structure factors, have been also investigated.

Quasielastic Neutron Scattering of Liquid Te50Se50in the Semiconductor-to-Metal Transition Range

We have carried out neutron scattering experiments for liquid Te 50 Se 50 at 700 K (semiconducting regime), 900 K (transition regime) and 1100 K (metallic regime). The quasielastic scattering peak is comparatively narrow at the peak positions of S ( Q ). A remarkable increase in the peak width is observed near the first minimum of S ( Q ), when the semiconductor-to-metal transition occurs. Since the first peak of S ( Q ) in this system proves to mainly reflect the inter-chain correlation and the second peak to the intra-chain correlation, the observed broadening may suggest frequent bond-switching between the chains in the metallic regime.

Long-scale conformational properties of peptide chains in β-sheets

We propose a sufficiently simple multichain model of ordered β-sheets, composed of extended macromolecules with rigid elements. The effective constants of intra- and interchain interactions describe primary and secondary structures of proteins, respectively. It is found that the long-range correlation of orientations of chain elements decreases with the separation along the same chain or between different chains according to the same asymptotic power law. The exponent in this law is determined by the ratio of the energy of thermal motion and the geometric mean of the energies of intra- and interchain interactions. The characteristic scale parameters are obtained, which define the crossover of the intra- and interchain correlation functions from the exponential law of decrease to the power one. The power law for intrachain correlations leads to a non-Gaussian behavior of the mean-square dimensions of chains. Several types of asymptotic dependences of mean-square dimensions of a chain in the β-sheet on the number of chain elements are found. Peptide chains may exist in different conformations: from extended ones to random Gaussian coils. Long-scale statistical properties of polymer systems with interchain interactions and those for polymer chains with excluded volume effects are compared.

Thermodynamic and structural properties of Yukawa hard chains

The thermodynamic and structural properties of Yukawa hard chains are considered in this work. We specifically examined the influence of the Yukawa potential range parameter λ on the thermodynamic properties, inter- and intrachain correlation functions of the chain fluid. The compressibility factor or pressure of Yukawa chain fluids was calculated via a first-order perturbation theory using hard-sphere chain fluid as a reference. Monte Carlo simulations were performed to calculate the thermodynamic properties and inter- and intrachain correlation functions of the fluid for λ=1.8 and 3.0. Simulation results for the compressibility were compared with calculations from the first-order perturbation theory and the Statistical Associating Fluid Theory-Variable Range (SAFT-VR) model. We found that both theories represent the simulated data very well. In particular, the SAFT-VR equation provides an excellent estimate of the properties of Yukawa hard chains over wide ranges of temperature and density. Simulated data for interchain and intrachain correlation functions reveal that the local structure of the Yukawa hard chains is strongly effected by the range parameter of the potential.

Synthesis, Structure and Magnetic Properties of a Chiral One-Dimensional Molecular-Based Magnet

A novel chiral organic radical 2-{4′-((S)-2″-methylbutoxy)phcnyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (1) and its metal complex, [1·Mn(II)(hfac)2] n were synthesized and characterized. The radical 1 crystallizes in the orthorhombic space group P212121 (No. 19) with a = 11.494(3) Å, b = 25.328(3) Å, c = 6.1281(5) Å, V = 1748.0(4) Å3. The complex [1·Mn(II)(hfac)2] n crystallizes in the same space group with a = 14.081(1) Å, b = 15.940(1) Å, c = 16.075(1) Å, V = 3608.1(4) Å3. X-ray crystal structure analysis of the complex revealed the formation of a helical-chain structure. The oxygen atoms of the radical 1 are ligated to two different manganese ions in cis position and exhibit the δ configuration in [1·Mn(II)(hfac)2] n . The magnetic measurements revealed that this chiral complex behaves as a ferrimagnet below T C = 4.5 K. It is characterized by a strong intrachain exchange correlation, the ratio between the intrachain and interchain exchange interactions being about 10−5.

Torsional Vibrations of Ester Groups as a Mechanism of Dielectric Relaxation of Poly(p-hydroxybenzoic Acid)

Dielectric relaxation of poly(p-hydroxybenzoic acid) (PHBA) due to torsional vibrations of the dipoles of ester groups in the main chain is considered. A dynamic model of elastically coupled rotators interacting with the crystal lattice is applied. The model is characterized by two sets of force constants which determine intrachain correlations between rotators along a given chain and interchain interactions in the crystal lattice. Correlation functions for the cosines of the rotation angles of different rotators in a chain are calculated. Fluctuations of the components of dipole moments normal to the chain axis (transverse components) and fluctuations of the components along the chain axis (longitudinal components) are considered. The transverse component is connected with the previously considered crank-shaft type of internal rotational motions of ester groups. The origin of the longitudinal component is ascribed to a complicated superposition of internal rotations and bending vibrations. The fluctuatio...

Antiferromagnetism in doped anisotropic two-dimensional spin-Peierls systems

We study the formation of antiferromagnetic correlations induced by impurity doping in anisotropic two-dimensional spin-Peierls systems. Using a mean-field approximation to deal with the interchain magnetic coupling, the intrachain correlations are treated exactly by numerical techniques. The magnetic coupling between impurities is computed for both adiabatic and dynamical lattices and is shown to have an alternating sign as a function of the impurity-impurity distance, hence suppressing magnetic frustration. An effective model based on our numerical results supports the coexistence of antiferromagnetism and dimerization in this system.

Interchain pairing in quasi-one-dimensional conductors

In this work, we tackle the problem of interchain pairing in correlated quasi-one-dimensional conductors. Using the renormalization group method, we show how the combination of interchain hopping t tT and intrachain correlations generates the opening of anisotropic superconducting and density-wave channels of fluctuations as the energy scale or the temperature is reduced. We also discuss under what conditions long-range order is possible.

Excitation spectrum and ground-state properties of theS=12Heisenberg ladder with staggered dimerization

We have studied the excitation spectrum of the $S=1/2$ quantum spin ladder with staggered dimerization by dimer series expansions, diagrammatic analysis of an effective interacting Bose gas of local triplets, and exact diagonalization of small clusters. We find that the model has two massive phases, with predominant inter-chain (rung) or intra-chain correlations. The transition from the rung dimer into the intra-chain dimer phase is characterized by softening of the triplet spectrum at $k=\pi$. The excitation spectrum as well as the spin correlations away from and close to the critical line are calculated. The location of the phase boundary is also determined.

A Twist Grain Boundary-like Twisted Smectic Phase in Monodisperse Poly(γ-benzyl α,l-glutamate) Produced by Recombinant DNA Techniques

Smectic ordering has been observed in perfectly monodisperse poly(γ-benzyl α,L-glutamate) (PBLG) which was synthesized using recombinant DNA technology. These PBLG molecules form rigid α-helical rods 11.45 nm in length. In the present study, transmission electron microscopy (TEM) and electron diffraction reveals a banded morphology with an approximately 120 nm period which provides strong evidence for helical rotation of the director field as in a cholesteric or twisted smectic. Detailed examination of the relative orientation of the banding in the morphology images and the reflections in the electron diffraction patterns, indicating interchain and intrachain correlations, leads to the conclusion that the structure observed is a twisted smectic phase. The relationship between the twist and the layering is found to be that of the twist grain boundary (TGB) phase. However, our data does not allow us to determine whether the monodisperse PBLG structure is blocky with discrete twist boundaries as in a true TGB or is a more continuously twisting structure. Thus, we will refer to the phase as TGB-like. Conventional, polydisperse PBLG is well-known to form cholesteric phases as a result of the chirality of the helical rod. The formation of a TGB-like phase in monodisperse PBLG is consistent with the superposition of a smectic-A layering resulting from the uniform rod length on the twisted texture present in the cholesteric.

Integral equation theory of flexible polyelectrolytes. I. Debye–Hückel approach

The polymer reference interaction site model (PRISM) with standard Percus–Yevick (PY) or hypernetted chain (HNC) closure equations is solved numerically for solutions of flexible polyelectrolytes. The monomers interact via a screened Coulomb potential (Debye–Hückel approach). The wormlike form factor characterized by a persistence length is used for the intrachain correlations. The equilibrium and structural results obtained for a large variety of parameters are interpreted in terms of the monomer–monomer interchain pair distribution function and of the structure factor. Issues like the scattered intensity peak position and the osmotic pressure are addressed. The domain within the phase diagram without HNC solutions is also presented.

A Study of the Structure and Electronic and Thermal Properties of Quasi-One-Dimensional La3MoO7

The structure of La3MoO7was solved inP212121withZ=4 anda=7.597(1) Å,b=7.7192(4) Å andc=11.0953(8) Å and refined to the reliability factors ofR(F)=0.0366 andwR(F2)=0.0782 for 102 variables and 5352 reflections. A structural feature of interest is the presence of zigzag chains oftrans-corner-sharing octahedra of composition MoO5−5parallel to theb-axis. Resistivity data taken along theb-axis direction show semiconducting behavior in the range 140 to 298 K with an activation energy of 0.16 eV. The magnetic susceptibility is quite complex. The main feature is a broad maximum at 655 K which is interpreted as due to intrachain spin correlations of the Mo(5 +) ions. Assuming theS=1/2 Heisenberg model this implies aJ/k=−511 K. Several other anomalies are observed at 483, 140, and 100 K. Differential scanning calorimetry data also show the 483 K feature and disclose another at 373 K not seen in the susceptibility. There is no evidence from powder neutron diffraction data of any structural changes from 298 to 10 K but no data are available above room temperature. There is an indication from neutron diffraction data for the onset of long-range antiferromagnetic order below 100 K.

Structure and physical properties of liquid chalcogens

Static and dynamic structures of supercooled liquid Te and liquid Te-Se mixtures have been investigated by neutron scattering. The structure factor S(Q) of liquid Te (l-Te) was analyzed on the assumptions that l-Te is composed of short chains with long and short covalent bonds and that the intrachain and interchain correlations could be separated. Both S(Q) and the vibrational density of states G(E) change drastically with temperature. The correlation between the structural and electronic properties is discussed.

Spin-Peierls lattice fluctuations of pure and Si- and Zn-substituted CuGeO3.

The pretransitional fluctuations of the spin-Peierls ~SP! transition of pure and Siand Zn-substituted CuGeO3 have been investigated by x-ray diffuse scattering. In pure CuGeO3 the critical fluctuations have been measured from TSP ~14.2 K! to 35 K. Although they exhibit a modest anisotropy, a crossover from a threeto a two-dimensional regime of structural fluctuations has been observed at about 1.3 K above TSP . A quantitative comparison of the thermal dependence of the x-ray intensity with that measured in the recent quasielastic neutron scattering investigation of Hirota et al., at the same reciprocal wave vector, shows that the SP critical lattice fluctuations of CuGeO3 are mostly of inelastic origin. Substitution by a fraction of a percent of Ge by Si or of Cu by Zn has a very important effect on the SP transition. In the Si compounds the intrachain and interchain correlations are severely reduced and no superlattice reflections are observed at the temperature where a drop of spin susceptibility occurs in the 0.25% Si compound. Surprisingly, the substitution has a weaker effect in the 0.6% Zn compound where a pseudotransition towards a short-range SP order is observed at about 13 K. We discuss these differences with respect to the nature of the substitution and we emphasize the key role played by the impurity-induced structural deformations in the case of the Si compounds to limit the SP order. @S0163-1829~96!05422-7#

Rouse Dynamics of Block Copolymers

We present rigorous analysis for the dynamic intrachain correlations of block copolymers within the Rouse model, which uncovers current misunderstandings of the dynamics of unentangled block copolymers in solutions and melts. Analytic predictions are made for the dynamic correlations at large and small distances, respectively, providing a critical test for the theory of Benmouna et al. which ignores memory effects and is based on a “dumbbell” model. At large distances, differences exist with regard to the k and composition dependence of the internal mode relaxation time. At small distances, on the other hand, we find non-Markovian stretched exponential decay, as opposed to their Markovian simple exponential decay, and for the correlation between two dynamically asymmetric blocks, a single mode with an intermediate decay constant, in contrast to the linear combination of the two intrablock correlation decay modes obtained by Benmouna et al. Furthermore, for structurally and dynamically symmetric but composition asymmetric diblock copolymers, they do not formally recover their predictions for homopolymers.