Self-consistent Mean-field Model Research Articles

Exploring the nematic to twist-bend nematic phase transition: calorimetric and optical measurements

ABSTRACT We investigate the nematic (N) to twist-bend nematic (NTB) phase transition in binary mixtures of a symmetric liquid crystal dimer (CB7CB) and a calamitic bi-phenyl mesogen (7OCB). As the molar concentration of 7OCB increases, both the Iso–N and N–NTB transition temperatures decrease gradually, consequently, the width of the nematic phase above the NTB phase can suitably be varied from ~12 K to 46 K. Notably, as the nematic range expands, there is a remarkable reduction in transitional entropy, eventually approaching negligible or zero values. Additionally, the heat capacity peak and the phase shift disappear when transitional entropy reaches zero. This behaviour diverges from typical N–SmA and N–SmC phase transitions, where transitional entropy decreases near the tricritical point but the heat capacity peak persists. Our findings indicate a finite positive δ(∆n) across all concentrations at the N–NTB phase transition, suggesting a gradually weakening first-order nature. This contrasts with theoretical predictions of a second-order N–NTB phase transition, as our experimental data consistently supports a first-order nature. The self-consistent mean-field Landau Model successfully validates our experimental data, establishing the universal first-order nature of the N–NTB phase transition, with experimental entropy values in good agreement with theoretical prediction.

Theoretical studies on structural properties and decay modes of $$^{284-375}$$119 isotopes

Within the axially deformed relativistic mean field with $$\hbox {NL3}^*$$ parametrisation, the different bulk properties like binding energy, quadrupole deformation parameter, separation energies, density profile and shape co-existence for Z = 119 isotopic chain within the mass range 284 $$\le$$ A $$\le$$ 375 are computed. Further, a competition between possible decay modes such as α-decay, $$\beta$$ -decay and spontaneous fission (SF) of the isotopic chain of Z = 119 superheavy nuclei under study is systematically analysed within self-consistent relativistic mean field model and a close agreement is noticed among the calculations performed using various semi-empirical formulae and also with the estimations made by finite range droplet model (FRDM) wherever available. Our analysis confirmed that α–decay is restricted within the mass range 284 $$\le$$ A $$\le$$ 375 and thus being the dominant decay channel in this mass range and there is no possibility of $$\beta$$ –decay for the considered isotopic chain. In addition, we forecasted the α–decay chain of fission survival nuclides, i.e. $$^{284-296}119$$ and found as one α chain from $$^{284} 119$$ and $$^{296} 119$$ , $$2 \alpha$$ chains from $$^{285}119$$ and $$^{295} 119$$ consistently, $$3\alpha$$ chains from $$^{286} 119$$ and $$^{294} 119$$ consistently, $$4 \alpha$$ chains from $$^{287} 119$$ consistently, six consistent α chains from $$^{288-293} 119$$ . A comparative study of SF and α-decay half-live computed using the formula of Santhosh et al and Coulomb proximity potential model (CPPM), respectively, for the fission surviving isotopic chain reveals that isotopes $$^{288-293} 119$$ exhibit $$6\alpha$$ chains followed by SF, isotopes $$^{295,296} 119$$ exhibit $$4\alpha$$ chains followed by SF, and the rest of the nuclei show continuous α chains. This study has also established that the alpha half-life values computed using $$\hbox {Q}_\alpha$$ (RMF) agree with half-life values computed using experimental $$\hbox {Q}_\alpha$$ values within 1 order difference. Thus, such studies can be of great significance to the experimentalists in very near future for synthesizing the isotopes of Z = 119 superheavy nuclei.

Crystal Plasticity Modeling of Anisotropic Hardening and Texture Due to Dislocation Transmutation in Twinning.

In crystalline materials, dislocations are three-dimensional lattice distortions that systematically distort twin interfaces that they encounter. This results in dislocation dissociation events and changes in the atomic structure of the interface. The manner in which the interface distorts drive the product of the dissociation event, and consequently, the incident dislocation core and the magnitude and relative direction of the Burgers vector govern these slip-twin interaction phenomena. Recent characterization studies using transmission electron microscopy as well as advanced molecular dynamic simulations have shown that slip dislocations, whether striking or struck by a {102} twin boundary, dissociate into a combination of twinning disconnections, interfacial disclinations (facets), jogs, and other types of dislocations engulfed inside the twin domains, called transmuted dislocations. While twinning disconnections were found to promote twin propagation, the dislocations incorporated inside the twin are of considerable importance to hardening and damage initiation as they more significantly obstruct slip dislocations accommodating plasticity of the twins. In this work, the dislocation transmutation event and its effect on hardening is captured using a dislocation density based hardening model contained in a visco-plastic self-consistent mean-field model. This is done by allowing the twins to increase their dislocation densities, not only by virtue of slip inside the twin, but also through dislocations that transmute from the parents as the twin volume fraction increases. A correspondence matrix rule is used to determine the type of converted dislocations while tracking and parameterizing their evolution. This hypothesis provides a modeling framework for capturing slip-twin interactions. The model is used to simulate the mechanical response of pure Mg and provides a more physically based approach for modeling stress-strain behavior.

Phase-space methods for the spin dynamics in condensed matter systems.

Using the phase-space formulation of quantum mechanics, we derive a four-component Wigner equation for a system composed of spin-[Formula: see text] fermions (typically, electrons) including the Zeeman effect and the spin-orbit coupling. This Wigner equation is coupled to the appropriate Maxwell equations to form a self-consistent mean-field model. A set of semiclassical Vlasov equations with spin effects is obtained by expanding the full quantum model to first order in the Planck constant. The corresponding hydrodynamic equations are derived by taking velocity moments of the phase-space distribution function. A simple closure relation is proposed to obtain a closed set of hydrodynamic equations.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

A self-consistent mean-field model for polyelectrolyte gels.

We present a novel approach to modeling polyelectrolyte gels, exploiting the analogy between star-branched polymers and polymer networks as a computationally inexpensive yet reliable alternative to full-scale simulations. In the numerical mean-field model of a star-like polymer we modify the boundary conditions to represent an infinite network. We validate the predictions of our new model against a coarse-grained simulation model. We also validate it against a phenomenological analytical model which has been previously shown to agree with simulations in a limited range of parameters. The mean-field model explicitly considers local density gradients and agrees with the simulation results in a broad range of parameters, beyond that of the analytical model. Finally, we use the mean-field model for predictions of the swelling behaviour of weak polyelectrolyte gels under different pH conditions. We demonstrate that the local density gradients are important and that the ionization of the weak polyelectrolyte gel is significantly suppressed. Under the studied conditions the effective pKA is about one unit higher than that of the free monomer. This shift in the effective pKA stems from the different pH values inside and outside the gel.

Structural and decay properties of Z = 132, 138 superheavy nuclei

In this paper, we analyze the structural properties of Z = 132 and Z = 138 superheavy nuclei within the ambit of axially deformed relativistic mean-field framework with NL3 * parametrization and calculate the total binding energies, radii, quadrupole deformation parameter, separation energies, density distributions. We also investigate the phenomenon of shape coexistence by performing the calculations for prolate, oblate and spherical configurations. For clear presentation of nucleon distributions, the two-dimensional contour representation of individual nucleon density and total matter density has been made. Further, a competition between possible decay modes such as $\alpha$ -decay, $\beta$ -decay and spontaneous fission of the isotopic chain of superheavy nuclei with Z = 132 within the range $ 312 \le A \le 392$ and $318 \le A \le 398$ for Z = 138 is systematically analyzed within self-consistent relativistic mean-field model. From our analysis, we inferred that the $\alpha$ -decay and spontaneous fission are the principal modes of decay in majority of the isotopes of superheavy nuclei under investigation apart from $\beta$ -decay as dominant mode of decay in 318-322138 isotopes.

Investigation of the Mg isotopes using the shell-model-like approach in relativistic mean field theory**Supported by NSFC (11465001,11275098, 11275248, 11505058,11165001) and Natural Science Foundation of Inner Mongolia of China (2016BS0102)

Ground state properties for Mg isotopes, including binding energies, one- and two-neutron separation energies, pairing energies, nuclear matter radii and quadrupole deformation parameters, are obtained from the self-consistent relativistic mean field (RMF) model with the pairing correlations treated by a shell-mode-like approach (SLAP), in which the particle-number is conserved and the blocking effects are treated exactly. The experimental data, including the binding energies and the one- and two-neutron separation energies, which are sensitive to the treatment of pairing correlations and block effects, are well reproduced by the RMF+SLAP calculations.

Functional Interfaces for Transparent Organic Electronic Devices: Consistent Description of Charge Injection by Combining In Situ XPS and Current Voltage Measurements with Self-Consistent Modeling

The interface properties between Sn-doped In2O3 (ITO) and the organic semiconductor α-NPD are studied using in situ X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) as well as with in situ current–voltage analysis in combination transport simulations using a self-consistent mean field model. In particular, ITO is sputtered onto α-NPD as required for transparent or inverted organic light-emitting diodes. We identify deposition conditions, which leave the organic molecules intact. The barrier heights determined by XPS/UPS for the inverted interfaces between undoped and doped α-NPD and ITO are 1.0 and 1.1 eV, respectively. These are in good agreement with barrier heights extracted from current–voltage simulations if the band width of the highest occupied molecular orbital (HOMO) is taken into account. The HOMO bandwidth determined by UPS is σUPS = 0.22 eV and that derived from simulations is σsim = 0.23 eV.

Coherent spin-light-induced mechanisms in the semirelativistic limit of the self-consistent Dirac-Maxwell equations

We present a self-consistent mean-field model based on a two-component Pauli-like equation that incorporates quantum and relativistic effects (up to second-order in 1/c) for both external and internal electromagnetic fields. By taking the semi-relativistic limit of the Dirac-Maxwell equations in the presence of an external electromagnetic field we obtain an analytical expression of a coherent light-induced mean-field Hamiltonian. The latter exhibits several mechanisms that involve the internal mean fields created by all the electrons and the external electromagnetic field (laser). The role played by the light-induced current density and the light-induced second-order charge density acting as sources in Maxwell's equations are clarified. In particular, we identify clearly four different mechanisms involving the spins that may play an important role in coherent ultrafast spin dynamics.

Interplay between proton-neutron pairing and deformation in self-conjugated medium mass nuclei

We employ a model combining self-consistent mean-field and shell model techniques to study the competition between particle-like and proton-neutron pairing correlations in fp-shell even-even self-conjugate nuclei. Deformation effects are realistically and microscopically described. The resulting approach can give a precise description of pairing correlations and eventually treat the coexistence of different condensate formed of pairs with different total spin/ isospin. The standard BCS calculations are systematically compared with approaches including correlation effects beyond the independent quasi-particle picture. The competition between proton-neutron correlations in the isoscalar and isovector channels is also analyzed, as well as their dependence on the deformation properties.

Theoretical study of triaxial shapes of neutron-rich Mo and Ru nuclei

Background: Recently, transition quadrupole moments in rotational bands of even-mass neutron-rich isotopes of molybdenum and ruthenium nuclei have been measured. The new data have provided a challenge for theoretical descriptions invoking stable triaxial deformations. Purpose: To understand experimental data on rotational bands in the neutron-rich Mo-Ru region, we carried out theoretical analysis of moments of inertia, shapes, and transition quadrupole moments of neutron-rich even-even nuclei around $^{110}$Ru using self-consistent mean-field and shell model techniques. Methods: To describe yrast structures in Mo and Ru isotopes, we use nuclear Density Functional Theory (DFT) with the optimized energy density functional UNEDF0. We also apply Triaxial Projected Shell Model (TPSM) to describe yrast and positive-parity, near-yrast band structures. Results: Our self-consistent DFT calculations predict triaxial ground-state deformations in $^{106,108}$Mo and $^{108.110,112}$Ru and reproduce the observed low-frequency behavior of moments of inertia. As the rotational frequency increases, a negative-$\gamma$ structure, associated with the aligned $\nu(h_{11/2})^2$ pair, becomes energetically favored. The computed transition quadrupole moments vary with angular momentum, which reflects deformation changes with rotation; those variations are consistent with experiment. The TPSM calculations explain the observed band structures assuming stable triaxial shapes. Conclusions: The structure of neutron-rich even-even nuclei around $^{110}$Ru is consistent with triaxial shape deformations. Our DFT and TPSM frameworks provide a consistent and complementary description of experimental data.

Predictions of inter-granular cracking and dimensional changes of irradiated polycrystalline graphite under plane strain

An original model is proposed that predicts the response of a polycrystalline graphite agglomerate to temperature change and irradiation. The model explicitly considers the graphite crystal anisotropy and the development of cracks along the interfaces between differently oriented grains. Using only graphite single crystal data, the model yields predictions of the macroscopic elastic stiffness, thermal expansion coefficient, irradiation-induced creep and irradiation-induced dimensional change (initial shrinkage followed by swelling at higher fast neutron fluence). The behaviour of the virgin polycrystal is properly predicted using a self-consistent mean-field model. However, predicting the pattern of inter-granular cracks requires a finite element model with cohesive zones. The latter simulations are performed on a periodic model polycrystal in 2D, leading to a qualitative agreement with experimental data.

Interacting boson model with energy density functionals

A brief overview of the recent advancement in the microscopic study of the interacting boson model (IBM) is given. A new nucleon-boson mapping method has been proposed recently, that derives the IBM Hamiltonian based on the self-consistent mean-field model with the microscopic energy density functional (EDF). The mean-field total energy surface computed with a given EDF is mapped onto the analogous energy expectation value in the boson condensate, thereby the energy spectrum and the transition rates are generated. Since the EDF framework allows an accurate global description of nuclear intrinsic properties, the IBM is derived in a unified way, basically for any situation of low-energy quadrupole collective states of nuclei. The basic notion of the new mapping technique is sketched.

Self-consistent mean-field model for palmitoyloleoylphosphatidylcholine-palmitoyl sphingomyelin-cholesterol lipid bilayers.

The connection between membrane inhomogeneity and the structural basis of lipid rafts has sparked interest in the lateral organization of model lipid bilayers of two and three components. In an effort to investigate anisotropic lipid distribution in mixed bilayers, a self-consistent mean-field theoretical model is applied to palmitoyloleoylphosphatidylcholine (POPC)--palmitoyl sphingomyelin (PSM)--cholesterol mixtures. The compositional dependence of lateral organization in these mixtures is mapped onto a ternary plot. The model utilizes molecular dynamics simulations to estimate interaction parameters and to construct chain conformation libraries. We find that at some concentration ratios the bilayers separate spatially into regions of higher and lower chain order coinciding with areas enriched with PSM and POPC, respectively. To examine the effect of the asymmetric chain structure of POPC on bilayer lateral inhomogeneity, we consider POPC-lipid interactions with and without angular dependence. Results are compared with experimental data and with results from a similar model for mixtures of dioleoylphosphatidylcholine, steroyl sphingomyelin, and cholesterol.

Lateral organization of complex lipid mixtures from multiscale modeling.

The organizational properties of complex lipid mixtures can give rise to functionally important structures in cell membranes. In model membranes, ternary lipid-cholesterol (CHOL) mixtures are often used as representative systems to investigate the formation and stabilization of localized structural domains ("rafts"). In this work, we describe a self-consistent mean-field model that builds on molecular dynamics simulations to incorporate multiple lipid components and to investigate the lateral organization of such mixtures. The model predictions reveal regions of bimodal order on ternary plots that are in good agreement with experiment. Specifically, we have applied the model to ternary mixtures composed of dioleoylphosphatidylcholine:18:0 sphingomyelin:CHOL. This work provides insight into the specific intermolecular interactions that drive the formation of localized domains in these mixtures. The model makes use of molecular dynamics simulations to extract interaction parameters and to provide chain configuration order parameter libraries.

SELF-CONSISTENT PHENOMENOLOGICAL THEORY OF CHARGE INJECTION AT THE CONDUCTOR/INSULATOR INTERFACE

ABSTRACT Self-consistent mean-field model of charge injection based on the matching of electric displacement and electrochemical potential in the electrode and the insulator is extended so as to account for discreteness of charge carriers. The improved scheme includes both the Schottky barrier lowering due to the individual image charge and the barrier change due to the field penetration into the injecting electrode. Applications of the theory to the cases of high-permittivity dielectric capacitors and organic light-emitting diodes are exemplarily presented.

Shell-model Hamiltonian from self-consistent mean-field model: [formula omitted] nuclei

We propose a procedure to determine the effective nuclear shell-model Hamiltonian in a truncated space from a self-consistent mean-field model, e.g., the Skyrme model. The parameters of pairing plus quadrupole–quadrupole interaction with monopole force are obtained so that the potential energy surface of the Skyrme Hartree–Fock+BCS calculation is reproduced. We test our method for N=Z nuclei in the fpg- and sd-shell regions. It is shown that the calculated energy spectra with these parameters are in a good agreement with experimental data, in which the importance of the monopole interaction is discussed. This method may represent a practical way of defining the Hamiltonian for general shell-model calculations.

Adsorption Of Bar-domain Proteins To Charged Lipid Membranes Causes Deformations And Lipid Demixing

Many proteins participating in cellular processes contain BAR domains that have been implicated in membrane shaping and deformation. These BAR domains can either induce significant membrane curvatures or sense high-curvature regions on cell membranes, but the mechanism for this action is still not well understood. One suggestion is that BAR domains work collectively and achieve significant bilayer deformations only through a suitable organization at membrane interfaces. In contrast, evidence from some atomistic simulations suggests that a single BAR can substantially deform a lipid membrane locally. Here we present results from a self-consistent mean-field model of BAR domain association on membranes, suggesting that a single Amphyphisin BAR is capable of producing a steady state, where the initially near-planar membrane curves significantly. However, using our approach we predict that such deformation will occur only for membrane patches that have the propensity to attain high spontaneous curvature, and that such favorable preconditioning may be the result of either local lipid demixing, or of a preceding insertion of the BAR domain's amphyphatic N-helix. Both events have been predicted to bring about asymmetry in the two membrane monolayers. To contrast, our simulations also show that local segregation of charged lipids under the influence of the adsorbing BAR domain alone cannot produce high enough asymmetry between bilayer leaflets, and that in the absence of additional energetic sources that favor membrane asymmetry, the membrane will remain near-flat within fluctuations upon BAR adsorption. Thus, we conclude that N-helix insertions may have a critical mechanistic role in the function of the BAR domain, and that the electrostatic interactions between BAR and membrane are essential for sensing and stabilization of bilayer curvature.

Systematics of collective correlation energies from self-consistent mean-field calculations

The collective ground-state correlations stemming from low-lying quadrupole excitations are computed microscopically. To that end, the self-consistent mean-field model is employed on the basis of the Skyrme-Hartre-Fock (SHF) functional augmented by BCS pairing. The microscopic-macroscopic mapping is achieved by quadrupole-constrained mean-field calculations which are processed further in the generator-coordinate method (GCM) at the level of the Gaussian overlap approximation (GOA). We study the correlation effects on energy, charge radii, and surface thickness for a great variety of semi-magic nuclei. A key issue is to work out the influence of variations of the SHF functional. We find that collective ground-state correlations (GSC) are robust under change of nuclear bulk properties (e.g., effective mass, symmetry energy) or of spin-orbit coupling. Some dependence on the pairing strength is observed. This, however, does not change the general conclusion that collective GSC obey a general pattern and that their magnitudes are rather independent of the actual SHF parameters.

Correlation patterns of NIKKEI index constituents: Towards a mean-field model

An analysis of minute-tick data from the Japanese stock index market is reported for a three-year period of 2000/7/4–2003/6/30. Correlation patterns and principal component distributions were determined for 180 constituents of the NIKKEI 225 index, excluding the effects of after-hours trading and constituent revisions. The first principal component describes about 30% of the total variance in constituent log returns (subject to slow decrease with the size of the correlation window), suggesting that a small number of physical parameters may describe the internal dynamics of the index, allowing for an adiabatic representation of index dynamics, and a self-consistent mean-field model of its constituents. Finally, it is shown that the introduction of a time gap into minute-tick data significantly improves the correlations of the price-weighed index with its constituents, even when such gap inserts are strictly penalized. This phenomenon corresponds to a heterogenous response time of index constituents to the adiabatic collective motion and also demonstrates the inhomogeneous nature of equidistant time ticks in financial trading.