Short-range Order Correlations Research Articles

Magnetic short-range order and Griffiths-like phase in the S-1/2 spin-dimer ferromagnet BaCu(SeO3)2

The polycrystals of S = 1/2 spin-dimer compound BaCu(SeO3)2 have been synthesized by using solid-state reaction. Without long-range order, the magnetic susceptibility χ(T) curve presents two anomalies at T1 = 5 K and TG = 58 K due to antiferromagnetic (AFM) short-range order and short-range ferromagnetic (FM) correlations, respectively, which is further supported by the specific heat and electron spin resonance data. The high-temperature χ(T) curve follows the Curie-Weiss law with θCW = 11.3 K, showing the dominant intradimer FM interaction. The magnetization M(H) curve at 2 K is characteristic of an AFM-FM transition at 0.4 T, suggesting the weak interdimer AFM couplings. The density function theory calculations and quantum Monte Carlo simulations allow an estimation of the intradimer and interdimer exchanges with J1/kB = 39.09 K, J2/kB = −1.51 K, and J3/kB = −1.68 K. Interestingly, the χ−1(T) curve within θCW <T <TG deviates downward from paramagnetic Curie-Weiss behavior, mimicking the Griffiths-like phase. This is further supported by the ac magnetic susceptibility and magnetic relaxation measurements. The competing intradimer FM and interdimer AFM exchanges with a subtle ratio between them are proposed to be responsible for the formation of Griffiths-like phase.

Routes of Theophylline Monohydrate Dehydration Process Proposed by Mid-Frequency Raman Difference Spectra

Two routes of the dehydration process of theophylline monohydrate have been proposed in this work from mid-frequency Raman difference spectra (MFRDS) results and experiments. MFRDS can establish short-range order correlations among various theophylline crystal forms. MFRDS results indicate that the short-range order of metastable Form III is most similar to that of monohydrate, which explains that Form III is the main dehydration products in the mild dehydration process. The phenomenon that unstable amorphous theophylline intermediate phase would appear during the dehydration process of theophylline monohydrate was confirmed indirectly by Powder X-ray Diffraction (PXRD) and optical microscope and reported in the previous reports, which could cause the nucleation of Form II, as MFRDS results indicate short-range order of amorphous solid dispersion of theophylline is most similar to that of Form II. MFRDS analysis shows the advantages in studying the phase transformation of small organic molecule crystals.

Self-Consistency of the Theory of Elementary Stage Rates of Reversible Processes and the Equilibrium Distribution of Reaction Mixture Components

An analysis is presented of one of the key concepts of physical chemistry of condensed phases: the theory self-consistency in describing the rates of elementary stages of reversible processes and the equilibrium distribution of components in a reaction mixture. It posits that by equating the rates of forward and backward reactions, we must obtain the same equation for the equilibrium distribution of reaction mixture components, which follows directly from deducing the equation in equilibrium theory. Ideal reaction systems always have this property, since the theory is of a one-particle character. Problems arise in considering interparticle interactions responsible for the nonideal behavior of real systems. The Eyring and Temkin approaches to describing nonideal reaction systems are compared. Conditions for the self-consistency of the theory for mono- and bimolecular processes in different types of interparticle potentials, the degree of deviation from the equilibrium state, allowing for the internal motions of molecules in condensed phases, and the electronic polarization of the reagent environment are considered within the lattice gas model. The inapplicability of the concept of an activated complex coefficient for reaching self-consistency is demonstrated. It is also shown that one-particle approximations for considering intermolecular interactions do not provide a theory of self-consistency for condensed phases. We must at a minimum consider short-range order correlations.

Local strain heterogeneity and elastic relaxation dynamics associated with relaxor behavior in the single-crystal perovskite Pb(In1/2Nb1/2)O3−PbZrO3−Pb(Mg1/3Nb2/3)O3−PbTiO3

Recently, $\mathrm{Pb}(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbZr}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PIN-PZ-PMN-PT) relaxor single crystals were demonstrated to possess improved temperature-insensitive properties, which would be desirable for high-power device applications. The relaxor character associated with the development of local random fields (RFs) and a high rhombohedral-tetragonal (R-T) ferroelectric transition temperature $({T}_{\mathrm{R}\ensuremath{-}\mathrm{T}}g120{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ would be critical for the excellent properties. A significant effect of the chemical substitution of $\mathrm{I}{\mathrm{n}}^{3+}$ and $\mathrm{Z}{\mathrm{r}}^{4+}$ in PMN-PT to give PIN-PZ-PMN-PT is the development of local strain heterogeneity, which acts to suppress the development of macroscopic shear strains without suppressing the development of local ferroelectric moments and contribute substantially to the RFs in PIN-PZ-PMN-PT. Measurements of elastic and anelastic properties by resonant ultrasound spectroscopy show that PIN-PZ-PMN-PT crystal has a quite different form of elastic anomaly due to Vogel-Fulcher freezing, rather than the a discrete cubic-$T$ transition seen in a single crystal of PMN-28PT. It also has high acoustic loss of the relaxor phase down to ${T}_{\mathrm{R}\ensuremath{-}\mathrm{T}}$. Analysis of piezoresponse force microscopy phase images at different temperatures provides a quantitative insight into the extent to which the RFs influence the microdomain structure and the short-range order correlation length $\ensuremath{\langle}\ensuremath{\xi}\ensuremath{\rangle}$.

Paramagnetic susceptibility and correlation functions of a [formula omitted]-dimensional classical Heisenberg ferromagnet via the two-time Green function method

In the present paper we investigate the paramagnetic susceptibility and the short-range order correlation functions of a d-dimensional classical isotropic ferromagnetic Heisenberg model with short-range exchange interactions by employing the two-time Green function method in classical statistical mechanics. Here we use Tyablikov–Callen-like decouplings for higher order Green functions and a formula for magnetization recently obtained by extension of the well known Callen method developed many years ago for the quantum isotropic Heisenberg model. Although our analysis is true for any temperature and dimensionality, we focus on one-, and some two- and three-dimensional lattices of experimental interest and derive asymptotic expressions for susceptibility and correlation functions within the paramagnetic phase close to the phase boundary and in the high-temperature regime. Besides, we present a Fourier series expansion method for deriving the high-temperature behaviors of the correlation functions. Our predictions, as obtained from a genuine classical many-body framework, may constitute a good reference point for the quantum counterparts emerging in the classical limit at the same level of approximation. Of course, although the classical spin models have unrealistic properties at sufficiently low temperatures, our classical analysis provides, in a relatively simple way as compared to a quantum treatment, an experimentally interesting scenario at finite temperature and dimensionalities d≥1.

3D-ΔPDF: a routine method for analyzing diffuse scattering from single crystals.

With the recent advances in x-ray area detectors, high quality diffuse scattering became readily available. This allows to investigate local order in single crystals and provides invaluable information about real structures of disordered crystals, e.g. stacking probabilities, or structural dynamics. Recently, a new method called Three Dimensional Difference Pair Distribution Function (3D-ΔPDF) analysis was introduced [1]. It provides direct access to the short range order correlations and allows to investigate the diffuse scattering of both static and dynamic origin in a unified fashion . The method is similar to the powder PDF, but availability of three dimensional diffraction data provides several key differences. Firstly, it allows elimination of Bragg peaks and refinement of diffuse scattering alone which conveniently separates the investigation of average structure from investigation of ordering. Secondly, due to reduced overlap of PDF signals, the correlation coefficients at very long interatomic vectors become accessible. The 3D-ΔPDF analysis can be performed in the newly developed program Yell [2]. The program supports all types of correlations and contains a fast FFT-based method for diffuse scattering calculation, and the constraints of arbitrary arithmetic expressions.

Structural Evolution of Supercritical CO2 across the Frenkel Line.

Here, we study structural properties of the supercritical carbon dioxide and discover the existence of persistent medium-range order correlations, which make supercritical carbon dioxide nonuniform and heterogeneous on an intermediate length scale. We report on the CO2 heterogeneity shell structure where, in the first shell, both carbon and oxygen atoms experience gas-like-type interactions with short-range order correlations while within the second shell, oxygen atoms essentially exhibit a liquid-like type of interactions due to localization of transverse-like phonon packets. Importantly, we highlight a catalytic role of atoms inside of the nearest-neighbor heterogeneity shell in providing a mechanism for diffusion and proving the existence of an additional thermodynamic boundary in the supercritical carbon dioxide on an intermediate length scale. Finally, we discuss important implications for answering the intriguing question whether Venus may have had CO2 oceans and urge for an experimental detection of this persistent local-order heterogeneity.

A new layered triangular antiferromagnet Li4FeSbO6: spin order, field-induced transitions and anomalous critical behavior

Structure, electrochemical, magnetic and resonance properties of new layered antimonate Li(4)FeSbO(6) were comprehensively studied using powder X-ray diffraction, cyclic voltammetry, magnetic susceptibility, heat capacity, electron spin resonance and Mössbauer spectroscopy. In the crystal structure the iron ions form the triangular network within (LiFeSbO(6))(3-) layers alternating with nonmagnetic lithium layers. The electrochemical activity studied implies an Fe(3+)/Fe(4+) redox couple at 4.3 V (ox.) and 3.9 V (red.) thereby revealing that Li can be reversibly extracted. The long-range antiferromagnetic order was found to occur at the Néel temperature, T(N) ≈ 3.6 K, confirmed both by the magnetic susceptibility data and specific heat ones. The effective magnetic moment is estimated to be 5.93 μ(B)/f.u. and satisfactorily agrees with theoretical estimations assuming high-spin configuration of Fe(3+) (S = 5/2). In the magnetically ordered state, though, the magnetization demonstrates rather peculiar behavior. An additional anomaly on the M(T) curves appears at T(2) < T(N) in moderate magnetic field. The positions of transitions at T(N) and T(2) separate increasingly with increasing external field. Multiple measurements consistently demonstrated field-sensitive moving of magnetic phase boundaries constituting a unique phase diagram for the compound under study. The complex low-dimensional (2D) nature of magnetic coupling was confirmed by the dynamic magnetic properties study. Electron spin resonance from Fe(3+) ions in paramagnetic phase is characterized by a temperature independent effective g-factor of 1.99 ± 0.01. However, the distortion and broadening of the ESR line were found to take place upon approaching the magnetically ordered state from above. The divergence of the temperature-dependent linewidth is analyzed in terms of both critical behavior close to long-range magnetic order and the Berezinskii-Kosterlitz-Thouless (BKT)-type transition. Heat capacity measurements even at zero field manifested an appearance of the additional anomaly at temperatures below the Néel temperature. The temperature dependence of ESR intensity, linewidth and shift of the resonant field imply an extended region of short-range order correlations in the compound studied. The rich variety of the anomalies in magnetic and resonance properties makes this new antimonate a very interesting system to investigate the multiple phase transitions and competing exchange interaction due to the critical role of the layered structure organization accompanied by the frustration effects in triangular antiferromagnets.

Observation of a Griffiths-like phase in the paramagnetic regime ofErCo2

A systematic x-ray magnetic circular dichroism study of the paramagnetic phase ofErCo2 has recently allowed us to identify the inversion of the net magnetization of the Co net momentwith respect to the applied field well above the ferrimagnetic ordering temperature,Tc. The study of small-angle neutron scattering measurements has also shown the presence of shortrange order correlations in the same temperature region. This phenomenon, which we havedenoted parimagnetism, may be related to the onset of a Griffiths-like phase in paramagneticErCo2. We have measuredac susceptibility on ErCo2 as a function of temperature, applied field and excitation frequency. Severalcharacteristics shared by systems showing a Griffiths phase are present inErCo2, namely the formation of ferromagnetic clusters in the disordered phase, the loss ofanalyticity of the magnetic susceptibility and its extreme sensitivity to an applied magneticfield. The paramagnetic susceptibility allows us to establish that the magnetic clusters areonly formed by Co moments as well as the intrinsic nature of those Co moments.

Coulomb correlations and the Wigner–Mott transition

Evidence for metal–insulator transitions in dilute 2D electron gases has sparked controversy and debate. A new model suggests such behaviour could arise from strong correlations driven by non-local Coulomb interactions, providing an alternative view to that which considers disorder to be the over-riding influence. Strong correlation effects, such as a marked increase in the effective mass of the carriers of electricity, recently observed in the low-density electron gas1 have provided spectacular support for the existence of a sharp metal–insulator transition in dilute two-dimensional electron gases2. Here, we show that strong correlations, normally expected only for narrow integer-filled bands, can be effectively enhanced even far away from integer-filling, owing to incipient charge ordering driven by non-local Coulomb interactions. This general mechanism is illustrated by solving an extended Hubbard model using dynamical mean-field theory3. Our findings account for the key aspects of the experimental phase diagram, and reconcile the early viewpoints of Wigner and Mott. The interplay of short-range charge order and local correlations should result in a three-peak structure in the electron spectral function, which can be observed in tunnelling and optical spectroscopy. These experiments will discriminate between the Wigner–Mott scenario and the alternative perspective that views disorder as the main driving force for the two-dimensional metal–insulator transition4.

Orientationally correlated colloidal polycrystals without long-range positional order

We probe the local and global structure of spin-coated colloidal crystals via laser diffraction measurements and scanning electron and atomic force microscopies, and find that they are unique three-dimensional orientationally correlated polycrystals, exhibiting short-range positional order but long-range radial orientational correlations, reminiscent of-but distinct from-two-dimensional colloidal hexatic phases. Thickness and symmetries are controllable by solvent choice and spin speed. While the polycrystallinity of these colloidal films limits their applicability to photonics, we demonstrate their feasibility as templates to make crack-free magnetic patterns.

Transport of light in amorphous photonic materials

Amorphous photonic materials based on dense assemblies of high refractive index spherical particles are presented. Light transmission through these photonic glasses shows a nontrivial wavelength dependence. The transmission spectra can be quantitatively reproduced by modeling the optical properties starting from their building blocks. Our results emphasize the relevance of including short range order correlations and an appropriate effective refractive index in the analysis of light transmission through amorphous photonic materials.

Comparison of short-range-order in liquid- and rotator-phase states of a simple molecular liquid: A reverse Monte Carlo and molecular dynamics analysis of neutron diffraction data

The short-range order (SRO) correlations in liquid- and rotator-phase states of carbon tetrachloride are revisited here. The correlation of some angular magnitudes is used to evaluate the positional and orientational correlations in the liquid as well as in the rotator phase. The results show significant similitudes in the relative position of the molecules surrounding a central one but striking differences in their relative orientations, which could explain the changes in SRO between the two phases and the puzzling behavior of the local density in the liquid and rotator phases.

Short-range order and pair correlations in a binary solid solution with a square lattice

Using the example of a disordered AyB1 − y solid solution whose atoms are distributed over squarelattice sites, it is shown that correlations in the first coordination shell inevitably cause correlations in the second and more distant coordination shells. These induced correlations decay with distance extending to the ninth coordination shell in a square lattice. The induced correlations are calculated as a function of the correlation in the first coordination shell for binary solid solutions with various compositions. This dependence is described by a third-degree polynomial in the first-shell correlation to within the computational accuracy. The coefficients of the polynomial calculated in this paper using computer simulation can be useful in developing short-range order theory and in calculating the phase diagrams of binary solid solutions.

Modeling of short-range order in a defect square lattice

The short-range order and pair correlations in a square lattice in which some part of sites are occupied by A atoms and the other are vacant have been investigated by computer simulation. It is shown that, in the presence of interactions in the first coordination sphere of an A y □1 − y solid solution with a square lattice, induced pair correlations damp only in the ninth coordination sphere. A functional dependence between the correlation in the first coordination sphere and the induced correlations in the next coordination spheres has been established for different compositions of the solid solution.

Magnetism and electron correlations

Electron correlation effects on magnetism are discussed from a local point of view on the basis of the dynamical coherent potential approximation (CPA) which completely takes account of the dynamical spin and charge fluctuations within the single-site approximation. The dynamical CPA is equivalent to other single-site theories of strongly correlated electrons: the many-body CPA, the dynamical mean-field theory, and the projection operator method CPA. The method is shown to describe both the dynamical effects on magnetic properties and the many-body band structure on the same footing. The effects of orbital degeneracy and the realistic calculation scheme are discussed with emphasis on Hund's rule coupling. Two types of theories for non-local correlations, the molecular dynamics approach and the self-consistent projection operator approach, are presented. The effects of magnetic short-range order and non-local correlations are discussed with some numerical results.

Interatomic voids in analysis of computer model structures of liquids and glasses

To analyze the structure of noncrystalline systems (liquids, glasses), it is suggested that the functions to be calculated should include not only the conventional atom-atom radial distribution function and structural factor, but also the same functions for a system of points representing the centers of interatomic voids. The points are located at the nodes of Voronoi polyhedra or, which is equivalent, at the centers of the spheres circumscribed around the Delaunay simplices. The system of points (system {D}) is unambiguously defined by the atomic coordinates and conveys additional information on the structure of the system because each interatomic void is determined by the relative positions of several atoms. Using a homogeneous noncrystalline packing model of atoms and inhomogeneous models derived from it as an example, we demonstrate that system {D} is sensitive to the structural features on medium scales and may be used along with the conventional atom-atom functions to study structural correlations other than short-range order correlations.

Spectral density and density of states in a superconductor in an exactly solvable model of a pseudogap state

A simple, exactly solvable model of a pseudogap state induced by fluctuations of dielectric short-range order is used to study the peculiarities of the electronic spectral density and density of states of a superconductor in the model of the Fermi surface with hot patches. The problem is considered for arbitrary values of the short-range order correlation length ξcorr. It is shown that the absence of self-averaging of the superconducting order parameter over the random field of dielectric fluctuations causes the spectral density and density of states to change significantly. The superconducting character of these quantities persists in a wide temperature range above the temperature T c of the superconducting transition, which is uniform over the whole sample.

Absence of 2kF splitting in the diffuse scattering from Cu3Au at the (001) surface.

We report a new type of short-range order correlations at the (001) surface of Cu3Au which no longer produces the 2k(F)-splitting characteristic for the bulk short-range order scattering. We present the temperature dependence of this phenomenon and a theoretical interpretation of its origin. We argue that this new surface effect is caused by a drastic change of the strain-induced interactions at the surface.

Gas and Liquid Transfer in Narrow Pores

A microhydrodynamic approach is formulated to describe the molecular flows in micro- and mesoporous systems, including the region of capillary phenomena, where one needs a single set of equations characterizing the flows of both dense gases and liquids. The set of equations of the transfer of dense fluids in narrow pores is closed by using the simplest molecular model, specifically, the lattice gas model, which describes the behavior of fluids over wide ranges of concentrations (from the gas to the liquid state) and temperatures, including the critical region. This model takes into account the volume of molecules and the intermolecular interactions in a quasi-chemical approximation, which retains short-range order correlations. The model reflects the strong anisotropy of the molecular distributions along a normal to the pore wall (because of the adsorption forces) and along the pore axis if there is capillary condensation. The derived set of equations is a set of finite-difference equations in increments of coordinates (instead of derivatives). The dissipative coefficients have a nonlocal anisotropic character. The equations of the transfer in near-wall cells and the prospects of the microhydrodynamic approach are discussed.