Ising model with a new class of four-spin interactions

The Ising model on a Union-Jack lattice, described by a Hamiltonian with second-neighbor pair-pair, four-spin, infinite-range interactions is considered. The model is solved exactly and the results are compared with MFA predictions. Within the exact treatment two new classes of phase transitions are obtained. The first one includes transitions from a disordered to a metastable, ordered and then to a stable and ordered phase with decreasing temperature. The metastable phase does not appear if the temperature is increased. The second one contains transitions between ordered and partialy ordered, partialy frustrated phases.     

Formation of the metastable phase upon crystallization of light amorphous Mg-Ni-Y alloys

The decomposition of the amorphous phase upon heating of the Mg₈₇Ni₁₃ and Mg₈₃Ni₁₃Y₄ amorphous alloys prepared by rapid quenching of the melt is investigated. The nanocrystalline structure is formed at the first crystallization stage and contains crystals of magnesium and the metastable phase. The structure of the metastable phase (the lattice parameters and space group) is determined. The possible chemical compositions of the metastable phase are considered. No differences are revealed in the structures of the metastable phases in the yttrium-containing and yttrium-free alloys. Heating of the alloys results in decomposition of the metastable phase. After completion of the transformations, the alloy structure consists of Mg and Mg₂Ni in accordance with the equilibrium phase diagram.     

Simulations of spinodal nucleation in systems with elastic interactions

Systems with long-range interactions quenched into a metastable state near the pseudospinodal exhibit nucleation that is qualitatively different from classical nucleation near the coexistence curve. We observe nucleation droplets in Langevin simulations of a two-dimensional model of martensitic transformations and determine that the structure of the nucleating droplet differs from the stable martensite structure. Our results, together with experimental measurements of the phonon dispersion curve, allow us to predict the nature of the droplet. The results have implications for nucleation in many solid-solid transitions and the structure of the final state.     

Strongly frustrated random antiferromagnets

A high dimensionality calculation (Weiss like) has been carried out for antiferromagnetism (AFM) in structures with many sublattices. By allowing quenched disorder in the exchange interactions our results clearly exhibit the interplay between the effects of lattice frustration and disorder on the system's properties. For given number of sublattices present, there are several possible phases (ordering of the spins) and as many metastable states in the ergodic phases. It is found that the glassy behavior, and metastability, for multi-sublattices systems is substantially enhanced as compared with simple structures, exhibiting structure dependent de Almeida-Thouless lines. Strongly disordered systems have the long-range AFM ordering, ergodic metastable states and glassy phases intermingled in a non-trivial way. Also, even small fluctuations in the exchange parameters do induce sizeable glassy behavior in structures with many sublattices. Spin glass behavior in apparently non-disordered systems as certain pyrochlores may be accounted for within the present context. Received 9 April 1999 and Received in final form 8 June 1999     

一维钻石链反铁磁Ising模型磁化的模拟

采用Monte Carlo模拟方法对自旋为1/2的一维钻石链反铁磁Ising系统的磁化行为进行研究.在这个系统中,反铁磁的交换作用和三角结构导致存在自旋阻挫.重点研究不同的反铁磁自旋交换作用对系统磁行为和自旋构型的影响.模拟磁化曲线中M=M_S/3磁化平台,得到平台宽度随不同的自旋交换相互作用强弱的变化关系,以及出现磁化平台时格点的自旋构型;给出亚稳态存在的条件及亚稳态时的微观构型.研究磁滞回线随温度的变化关系.结果表明,随着温度的升高,磁滞回线逐渐减小,最终消失.     

Nonlinear dynamics for proper ferroelastic transitions for which the landau condition is violated

On the basis of a lattice model the domain structure for ferroelastic transformations is examined. The model accounts for both strongly nonlinear and competing interactions which allow for, in some situations, the propagation of nonlinear excitations. The model can be mostly applied to proper ferroelastic transformations of which In-Tl, Ti-Ni, etc.… are good prototypes. The phonon dispersion of the transverse acoustic waves obtained in the linearized case is discussed and the results show first a phonon softening at nonzero wave-number and next an upwards convexity of the phonon branch near the long-wavelength limit. This can be seen as pre-transitional effects. In a fully nonlinear case we consider vanishing dilatation transformations and the continuum approximation is applied to the one-dimensional version. Then, we investigate nonlinear excitations; three main classes of solution are found: i) quasi-harmonic solutions corresponding to periodically modulated structures in space which is a precursor effect of the elastic transformation, ii) an array of solitons made of periodic arrangements of parent-elastic domains and iii) a moving strain soliton. All the significant results are numerically illustrated by means of the microscopic model. Finally, the similarity to martensitic transformations and some extensions of the model are outlined.     

The temporal evolution of population densities of excited states in atomic oxygen thin plasmas

Population densities of transient oxygen plasmas have been calculated solving a system of differential equations, which describes the temporal evolution of the considered quantum levels. The results, which refer to a temperature of 1 eV and to electron number densities ranging from 10⁸ to 10¹⁴ cm^-3, show the strong importance of the metastable 3s⁵S state in determining the evolution of these plasmas. The present results are then applied to the calculation of relaxation times of selected quantum levels and to the definition of quasistationary conditions of plasmas having low lying excited states (i.e. states belonging to the same principal quantum number as the ground state).     

Structure of hydrophobically and hydrogen-bonded complexes between amphiphilic copolymer and polyacid in water

We observed by SANS and NMR the structure of intermolecular complexes formed through hydrogen bonding and hydrophobic interactions between a polyacid and a neutral copolymer surfactant (PEO-PPO-PEO). The polyacid is perdeuterated and the contrast variation method enables us to measure separately the structure factor of each component in the complex and the cross structure factor as a function of the pH and the temperature. The evolution of NMR spectra and relaxation times with pH and temperature give a complementary insight into the inner structure of the aggregates. The interaction between the aggregates and the aggregation number of polyacid chains are governed by electrostatic effects; they depend on the polyacid charge and are insensitive to temperature. The number of copolymer chains which results from the cooperative action of hydrogen bonding and hydrophobic interactions is sensitive to both pH and temperature. The complexation preserves the micellar core-corona structure of the copolymer and shrinks the polyacid chains which adopt a compact structure. The non-dissociated polyacid sequences are bound to the PPO part of the copolymer forming the hydrophobic core of the complex, whereas the ionized polyacid sequences form with the PEO a stabilizing hydrated corona.Received: 27 April 2004, Published online: 3 August 2004PACS: 61.12.-q Neutron diffraction and scattering - 82.56.-b Nuclear magnetic resonance - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 82.70.-y Disperse systems; complex fluids     

Lamellar structure and nanomechanical properties of quasicrystalline Al-Cu-Fe alloys

The kinetics of structural phase transformations in quasicrystal-forming Al-Cu-Fe alloys with compositions in the region of stability of the icosahedral (i) phase has been investigated. It has been shown that, depending on the development of metastable transformations i → pentagonal phases P1 and P2, a homogeneous lamellar structure (i + P1 + P2) or a polygrain i-phase is formed in the alloys. The P-h diagrams obtained upon nanoindentation, atomic force microscopy, and scanning electron microscopy of indentations have demonstrated signs of elasto-plastic deformation of the alloys with lamellar and polygrain icosahedral structures. It has been found that, in contrast to the polygrain icosahedral alloys with a normal size effect of nanoindentation, the alloys with a lamellar structure are characterized by a nonmonotonic dependence of the hardness (H) on the maximum load (P _max) and exhibit the effect of strain hardening in the range of loads 50 mN ≤ P _max < 500 mN. The strain hardening is considered as the result of resistance exerted by boundaries of the lamellar structure to the development of plastic deformation.     

Crystallographic variant selection of martensite at high stress/strain

The phenomenological theory of martensitic transformation is well understood that the displacive phase transformations are mainly influenced by the externally applied stress. Martensitic transformation occurs with 24 possible Kurdjomov-Sachs (K-S) variants, where each variant shows a distinct lattice orientation. The elegant transformation texture model of Kundu and Bhadeshia for crystallographic variant selection of martensite in metastable austenite at various stress/strain levels has been assessed in this present research. The corresponding interaction energies have also been evaluated. Encouraging correlation between model prediction and experimental data generation for martensite pole figures at many deformed austenite grains has been observed at different stress/strain levels. It has been investigated that the mechanical driving force alone is able to explain the observed martensite microtextures at all stress/strain levels under uniaxial tensile deformation of metastable austenite under low temperature at a slow strain rate. The present investigation also proves that the Patel and Cohen’s classical theory can be utilized to predict the crystallographic variant selection, if it is correctly used along with the phenomenological theory of martensite crystallography.     

On the virial coefficients of the equation of state of a system of non-axial molecules

A thermodynamic perturbation theory in which all angle-dependent interactions are considered as a perturbation of the central potential is applied to calculate the second and third virial coefficients of a fluid composed of non-axial molecules. The influence of a large number of anisotropic pair- and three-body non-ādditive interactions has been considered. Experimental values have been used for the dipole moment, quadrupole tensor and for anisotropic polarizability. The parameters for the central Lennard-Jones (12-6) potential have been determined from the viscosity data. The relative contribution of each branch of pair and triplet interactions has been evaluated as a function of temperature for molecules possessing both the axial- and non-axial symmetries. It has been shown that the non-axial approximation is an improvement over the axial one. Theoretical results have been compared with the experimental data of CH₃OH.     

Transformations of C-type defects on Si(100)-2 × 1 surface at room temperature – STM/STS study

We present a scanning tunneling microscopy study of the C-type defects on the Si(100)-2 × 1 surface and their transformations into other defect forms at room temperature. A model of the C defect as a dissociated water molecule was adopted for interpretation of the observed transformations. We explained the transformations by hopping the H or OH between bonding sites on Si dimers. Newly, the most stable defect form, corresponding to the H and hydroxyl group adsorbed on the same dimer, is reported. Real time observations provided an explanation for the defect C2-C2 described earlier. A reversible transition of this defect into another form, not revealed yet, is presented. Electronic structure of the observed defects is studied by means of scanning tunneling spectroscopy. Measured spectra show semiconducting character of the C defect. Spectra of the other defect forms are discussed.     

热处理引起3C-ZnS结构转变的X-射线衍射分析

为了论证层错是硫化锌晶体中交流电致发光的一个必要条件,我们把3C-ZnS单晶体在1100℃热处理后引入了层错.同时,我们对热处理前后的结构,用劳埃背反射法和文献[8、13]推导的公式进行了分析.结果表明,沿3C-结构<111>四个等价方向都可以产生无序、孪晶和2H-结构,而且当热处理时间足够长时,结构转变趋向于一个<111>方向.这些实验结果,在有关的文献中尚未见到.     

Influence of temperature and anisotropic pressure on the phase transitions in alpha-cristobalite

The role of temperature and anisotropy of the applied load in the pressure-induced transformations of alpha-cristobalite is investigated by means of first principles molecular dynamics combined with the metadynamics algorithm for the study of solid-solid phase transitions. We reproduce the transition to alpha-PbO2 as found in experiments and we observe that the transition paths are qualitatively different and yield different products when a nonhydrostatic load is applied, giving rise to a new class of metastable structures with mixed tetrahedral and octahedral coordination.     

Propagation and ignition of fast gasless detonation waves of phase or chemical transformation in condensed matter

Fast self sustained waves of chemical or phase transformations, observed in several contexts in condensed matter effectively result in “gasless detonation". The phenomenon is modelled by coupling the reaction diffusion equation, describing chemical or phase transformations, and the wave equation, describing elastic perturbations. The coupling considered in this work involves (i) a dependence of the sound velocity on the chemical (phase) field, and (ii) the destruction of the initial chemical equilibrium when the strain exceeds a critical value (strain induced phase transition). Both the case of an initially unstable state (first order kinetics) and metastable state (second order kinetics) are considered. An exhaustive analytic and numerical study of travelling waves reveals the existence of supersonic modes of transformations. The practically important problem of ignition of fast waves by mechanical perturbation is investigated. With the present model, the critical strain necessary to ignite gasless detonation by local perturbations is determined. Received 18 November 1999     

Non-uniformities in polymer/liquid crystal mixtures

We theoretically model the nucleation of nematic droplets during phase ordering in mixtures of a flexible polymer and a low-molecular-weight liquid crystal. By appealing to classical nucleation theory (CNT), we calculate the energy barrier to nucleation and the size of a critical nucleus. We study the influence of a metastable intermediate phase on the nucleation of the nematic. Below a triple point in the phase diagram, there are two distinct mechanisms for the formation of a nematic nucleus: 1) direct nucleation from the isotropic phase and 2) nucleation via a precursor metastable isotropic phase. We calculate the crossover concentration as a function of temperature, delineating the regions of the phase diagram in which each mechanism prevails. In the latter case, the presence of a hidden metastable isotropic-isotropic binodal may either promote or delay the nucleation of a nematic phase. Received 9 August 2002 RID="a" ID="a"e-mail: matuyama@chem.mie-u.ac.jp     

Metastable states of spin glasses on random thin graphs

In this paper we calculate the mean number of metastable states for spin glasses on so called random thin graphs with couplings taken from a symmetric binary distribution . Thin graphs are graphs where the local connectivity of each site is fixed to some value c. As in totally connected mean field models we find that the number of metastable states increases exponentially with the system size. Furthermore we find that the average number of metastable states decreases as c in agreement with previous studies showing that finite connectivity corrections of order 1/c increase the number of metastable states with respect to the totally connected mean field limit. We also prove that the average number of metastable states in the limit is finite and converges to the average number of metastable states in the Sherrington-Kirkpatrick model. An annealed calculation for the number of metastable states of energy E is also carried out giving a lower bound on the ground state energy of these spin glasses. For small c one may obtain analytic expressions for . Received 14 October 1999 and Received in final form 14 December 1999     

Structural relaxation in metastable alloys

A mechanism of stress relaxation in metastable alloys is considered. It is demonstrated that structural transformations during a relaxation test lead to anomalously high stress relaxation. Two stages of structural relaxation are revealed: formation of defect-free channels and decomposition of a solid solution. A new method of program hardening of metastable alloys based on the structural relaxation processes is suggested. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 56–61, November, 2007.     

Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling

ABSTRACT

Neutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3?mol fraction of methanol (x_M) under the supercritical (618?K, 100?MPa) and ambient (298?K, 0.1?MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.