Solid state phase transitions characterized by ESR and XAS

Measurements of the relaxation time, τ of electron systems to a disturbance, by two different spectroscopic methods are examined in detail, with the purpose to establish how the presence of fluctuations near a solid state phase transition are made evident in insulators, conductors and superconductors. The absolute temperature and the relaxation time determine the thermodynamic stability of the electronic system near a phase transition by the Uncertainty Principle. At a given temperature T, Landau and Lifshitz obtain the stability from the lower limit of the uncertainty in entropy in units of the Boltzmann constant, ΔS/k_B<<1 when T τ>>3.82 K ps. Magnetic resonance can measure τ>>10⁻¹⁰ s, when v=9 GHz. X-ray spectroscopy can measure τ<10⁻¹⁶s for hv>5 keV. The results extract information about phenomena that occur at the phase transition by following the evolution of spectral features versus T and crystal orientation. Electron spin resonance identifies the phase transition by the evolution of doublet, triplet and antiferromagnetic resonance, and energy loss. Analysis of the X-ray absorption near an element edge determines one, the relative valence: V(Cu in chains)−V(Cu in planes) ≈1 in YBa₂Cu₃O_7−δ, two, the appearance of allowed Cu K pre-edge quadrupole transitions at T_c, three, the enhancement of Ba L3,2 edge transitions by an order of magnitude, just above T_c, at a crystal orientation of the c-axis to the X-ray polarization vector of 8 π/18, and four, difference X-ray absorption spectra, relative to the transition temperature, identify the bonds as well as the atoms involved in the transition. The figure abstract shows the changes in electron density obtained by temperature difference X-ray absorption near the Y K-edge in YBa₂Cu₃O_7−δ below T_c.     

Stochastic models of firstorder nonequilibrium phase transitions in chemical reactions

The steady states of a simple nonlinear chemical system kept far from equilibrium are analyzed. A standard macroscopic analysis shows that the nonlinearity introduces an instability causing a transition analogous to a thermodynamic first-order phase transition. Near this transition the system exhibits hysteresis between two alternative steady states. Fluctuations are introduced into this model using a stochastic master equation. The solution of this master equation is unique, preventing two alternative exactly stable states. However, a quasi-hysteresis occurs involving transitions between alternative metastable steady states on a time scale that is longer than that of the fluctuations around the mean steady state values by a factor of the forme , where ø is the height of a generalized thermodynamic potential barrier between the two states. In the thermodynamic limit this time scale tends to infinity and we have essentially two alternative stable steady states.     

Composition and polyamorphism in supercooled yttria-alumina melts

By extending recent work on liquid-liquid transitions in supercooled yttria-alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.     

Tests of time-reversal invariance in complex systems

The random-matrix theory for the effects of time-reversal non-invariance (TRNI) on energy level, strength and cross-section fluctuations in complex systems is reviewed. Applied to the compound-nuclear data this gives bounds on rms TRNI matrix elements. Using a fluctuation-free form of statistical spectroscopy bounds on α, the relative magnitude of the TRNI nucleon-nucleon interaction, is deduced. In all three cases we find α ≲ (2–3) × 10⁻³ at high (∼ 99%) statistical confidence. Suggestions are made about experiments which should improve the bounds.     

Active membranes studied by X-ray scattering

In view of recent theories of “active” membranes, we have studied multilamellar phospholipid membrane stacks with reconstituted transmembrane protein bacteriorhodopsin (BR) under different illumination conditions by X-ray scattering. The light-active protein is considered as an active constituent which drives the system out of equilibrium and is predicted to change the collective fluctuation properties of the membranes. Using X-ray reflectivity, X-ray non-specular (diffuse) scattering, and grazing incidence scattering, we find no detectable change in the scattering curves when changing the illumination condition. In particular the intermembrane spacing d remains constant, after eliminating hydration-related artifacts by design of a suitable sample environment. The absence of any observable non-equilibrium effects in the experimental window is discussed in view of the relevant parameters and recent theories.     

Proper Orthogonal Decomposition of the mixing layer flow into coherent structures and turbulent Gaussian fluctuations

The Proper Orthogonal Decomposition is used to decompose fluctuating turbulent flows into a coherent non-Gaussian component and background fluctuations. An application is performed from 2D experimental data of a turbulent plane mixing layer flow. The analyses of the energy spectra and the Probability Density Function of the velocity field show that POD extracts an incoherent part approaching the quasi-Gaussian distribution properties. The background fluctuations are homogeneous with small amplitude. New future applications are then conceivable like the modeling of the incoherent part for particular inflow condition generation methodology and the analysis of the cyclic velocity field variabilities in Internal Combustion engine flow. To cite this article: Ph. Druault et al., C. R. Mecanique 333 (2005).     

Dissipation and large thermodynamic fluctuations

The results of recent work of Kipnis, Olla, and Varadhan on the dynamic large deviations from a hydrodynamic limit for some interacting particle models are formally extended to a general hydrodynamic situation, including non-equilibrium steady states, as a fluctuation-dissipation hypothesis. The basic conjecture is that the exponent of decay in the probability of a large thermodynamic fluctuation is given by the dissipation of the force required to produce the fluctuation. It is shown that this hypothesis leads to a nonlinear version of Onsager-Machlup fluctuation theory that had previously been proposed by Graham. A direct consequence of the theory is a dynamic variational principle for the most probable thermodynamic history subject to imposed constraints (Onsager's principle of least dissipation). Following Graham, the theory leads also to a generalized potential, analogous to an equilibrium free energy, for the nonequilibrium steady state and an associated static variational principle. Finally, a formulation of nonlinear fluctuating hydrodynamics is proposed in which the noise enters multiplicatively so as to reproduce the conjectured large-deviations theory on a formal analogy with the results of Freidlin and Wentzell for finite-dimensional systems.     

Models of structure and properties of H-bonded molecular mixtures

Results on the modeling of molecular interactions, supramolecular organization and physicochemical properties of binary mixtures of organic solvents are presented. The mixtures, both with negative deviation from ideality, such as propanone–trichloromethane, dimethylsulfoxide–trichloromethane, and with positive deviation, such as 1,4-dioxane–methanol, 1,4-dioxane–ethanol, trichloromethane–methanol, trichloromethane–ethanol, have been studied. Models for the aggregation in mixtures due to specific intermolecular interactions are proposed. Integral and differential parameters of the supramolecular organization were obtained. Thermodynamic characteristics of aggregation (equilibrium constants, Gibbs energy, enthalpy and entropy) and structural parameters of the aggregates have been determined. Long-range molecular correlations and cooperativity of H-bonds have been established. The manifestations of the supramolecular ordering in the physicochemical properties of the mixtures are discussed.     

α-glucosidase inhibitory,antioxidant activity,and GC/MS analysis of Descurainia sophia methanolic extract: In vitro,in vivo,and in silico studies

Due to the presence of various phenolic compounds in D.sophia, this plant may have an inhibitory effect on α-Glc and ultimately diabetes control. Therefore, this work aims to scrutinize total phenolic, flavonoid contents, antioxidant capacity, and α-Glc inhibitory activity in aerial parts of methanolic D.sophia extract. The methanolic flower extracts were selected from among aerial parts for the experimental study of anti-diabetic effects by α-Glc inhibitory assays. The flower extracts were also studied by GC/MS to detect the compounds. The total phenolic and flavonoid contents were 21.38 ± 0.93 GAE/g and 96.2 ± 0.20 QE/g, respectively. The IC₅₀ value of flower extract for α-Glc inhibition with mixed (Competitive/non-competitive) mode was found to be 20.34 ± 0.11 mg/ml. Furthermore, in-vivo studies showed that the blood glucose level reduced after consumption of flower extract compared to the control group. Twenty-one compounds were identified by GC/MS technique. These compounds were assessed for high docking scores against α-Glc in silico. Docking score calculations exhibited that the DES-α-Glc complex had a significantly higher binding energy (-6.13 Kcal/mol) than other compounds. The DES-α-Glc complex which displayed a higher docking energy value than the ACR was subjected to MDs studies. The findings of this study suggest that the flower extract of D.sophia can be used as a suitable additive in syrups or foods with anti-diabetic capacity.