Minimum free energy for an electromagnetic conductor with memory

A closed expression is determined in the frequency domain for the minimum free energy associated with a state of a linear electromagnetic conductor with memory effects, using the fact that this quantity is equal to the maximum recoverable work obtainable from the given state of the material. Another equivalent expression is also derived and applied to evaluate explicit formulae for a discrete spectrum model. In particular, for such a model we present the results corresponding to only one inverse time decay for each of the three kernels of the constitutive equations. These results clearly show the effects of various parameters on the expression for the minimum free energy. Copyright © 2008 John Wiley & Sons, Ltd.     

The minimum free energy for incompressible viscoelastic fluids

A general closed expression is given in the frequency domain for the isothermal minimum free energy of an incompressible viscoelastic fluid, whose constitutive equation is expressed by a linear functional of the history of strain. Another equivalent form of the minimum free energy is also derived and used to study the particular case of a discrete model material response. Copyright © 2006 John Wiley & Sons, Ltd.     

Practical estimation of a splitting parameter for a spectral method for the ternary Cahn–Hilliard system with a logarithmic free energy

We propose a practical estimation of a splitting parameter for a spectral method for the ternary Cahn–Hilliard system with a logarithmic free energy. We use Eyre's convex splitting scheme for the time discretization and a Fourier spectral method for the space variables. Given an absolute temperature, we find composition values that make the total free energy be minimum. Then, we find the splitting parameter value that makes the two split homogeneous free energies be convex on the neighborhood of the local minimum concentrations. For general use, we also propose a sixth‐order polynomial approximation of the minimum concentration and derive a useful formula for the practical estimation of the splitting parameter in terms of the absolute temperature. The numerical tests are phase separation and total energy decrease with different temperature values. The linear stability analysis shows a good agreement between the exact and numerical solutions with an optimal value s. Various computational experiments confirm that the proposed splitting parameter estimation gives stable numerical results. Copyright © 2016 John Wiley & Sons, Ltd.     

Random quantum channels II: Entanglement of random subspaces, Rényi entropy estimates and additivity problems

In this paper we obtain new bounds for the minimum output entropies of random quantum channels. These bounds rely on random matrix techniques arising from free probability theory. We then revisit the counterexamples developed by Hayden and Winter to get violations of the additivity equalities for minimum output Rényi entropies. We show that random channels obtained by randomly coupling the input to a qubit violate the additivity of the p-Rényi entropy, for all p>1. For some sequences of random quantum channels, we compute almost surely the limit of their Schatten S₁→Sp norms.     

Scaling Laws in Microphase Separation of Diblock Copolymers

Summary. In this note, we study a nonlocal variational problem modeling microphase separation of diblock copolymers ([22], [3], [21]). We apply certain new tools developed in [5] to determine the principal part of the asymptotic expansion of the minimum free energy. That is, we prove a scaling law for the minimum energy and confirm that it is attained by a simple periodic lamellar structure. A previous result of Ohnishi et al. [23] was for one space dimension. Here, we obtain a similar result for the full three-dimensional problem. Received February 20, 2001; accepted June 2, 2001 Online publication August 13, 2001     

The stress driven instability in elastic crystals: Mathematical models and physical manifestations

Summary Equilibrium equations and stability conditions for the simple deformable elastic body are derived by means of considering a minimum of the static energy principle. The energy is supposed to be sum of the volume (elastic) and the surface terms. The ability to change relative positions of different material particles is taken into account, and appropriate natural definitions of the first and second variations of the energy are introduced and calculated explicitly. Considering the case of negligible magnitude of the surface tension, we establish that an equilibrium state of a nonhydrostatically stressed simple elastic body (of any physically reasonable elastic energy potential and of any symmetry) possessing any small smooth part of free surface is always unstable with respect to relative transfer of the material particles along the surface. Surface tension suppresses the mentioned instability with respect to sufficiently short disturbances of the boundary surface and thus can probably provide local smoothness of the equilibrium shape of the crystal. We derive explicit formulas for critical wavelength for the simplest models of the internal and surface energies and for the simplest equilibrium configurations. We also formulate the simplest problem of mathematical physics, revealing peculiarities and difficulties of the problem of equilibrium shape of elastic crystals, and discuss possible manifestations of the above-mentioned instability in the problems of crystal growth, materials science, fracture, physical chemistry, and low-temperature physics.     

Monotone strain-stress models for shape memory alloys hysteresis loop and pseudoelastic behaviorx

To describe the behavior of Shape Memory Alloy we use a thermomechanical model, founded on a free energy which is a convex function with respect to the strain and to the martensitic volume fraction, and a concave one with respect to the temperature. The material parameters of the model are experimentally determined.Received: November 26, 2001; revised: March 20, 2002     

A tensor model for liquid crystals on a spherical surface

Rod-like molecules confined on a spherical surface can organize themselves into nematic liquid crystal phases. This can give rise to novel textures displayed on the surface, which has been observed in experiments. An important theoretical question is how to find and predict these textures. Mathematically, a stable configuration of the nematic fluid corresponds to a local minimum in the free energy landscape. By applying Taylor expansion and Bingham approximation to a general molecular model, we obtain a closed-form tensor model, which gives a free energy form that is different from the classic Landau-de Gennes model. Based on the tensor model, we implement an efficient numerical algorithm to locate the local minimum of the free energy. Our model successfully predicts the splay, tennis-ball and rectangle textures. Among them, the tennis-ball configuration has the lowest free energy.     

Minimizing the condition number of a positive definite matrix by completion

Summary. We consider the problem of minimizing the spectral condition number of a positive definite matrix by completion: \noindent where is an Hermitian positive definite matrix, a matrix and is a free Hermitian matrix. We reduce this problem to an optimization problem for a convex function in one variable. Using the minimal solution of this problem we characterize the complete set of matrices that give the minimum condition number. Received October 15, 1993     

The mean-field approximation in quantum electrodynamics: The no-photon case

We study the mean-field approximation of quantum electrodynamics (QED) by means of a thermodynamic limit. The QED Hamiltonian is written in Coulomb gauge and does not contain any normal ordering or choice of bare electron/positron subspaces. Neglecting photons, we properly define this Hamiltonian in a finite box [−L/2; L/2)³, with periodic boundary conditions and an ultraviolet cutoff λ. We then study the limit of the ground state (i.e., the vacuum) energy and of the minimizers as L goes to infinity, in the Hartree-Fock approximation. In the case with no external field, we prove that the energy per volume converges and obtain in the limit a translation-invariant projector describing the free Hartree-Fock vacuum. We also define the energy per unit volume of translation-invariant states and prove that the free vacuum is the unique minimizer of this energy. In the presence of an external field, we prove that the difference between the minimum energy and the energy of the free vacuum converges as L goes to infinity. We obtain in the limit the so-called Bogoliubov-Dirac-Fock functional. The Hartree-Fock (polarized) vacuum is a Hilbert-Schmidt perturbation of the free vacuum and it minimizes the Bogoliubov-Dirac-Fock energy. © 2006 Wiley Periodicals, Inc.     

弹性地基上自由边矩形薄板几个问题的注记

对于弹性地基上自由边矩形薄板的弯曲、稳定和振动问题,本文选择了一个挠曲函数,它能精确满足自由边全部边界条件以及自由角点的条件.应用能量变分原理,给出了确定挠曲函数中待定参数的方程,以及稳定性方程和频率方程,给出了求最小临界力和最小固有频率的一般公式.     

Variational problems with free interfaces

Summary A large class of problems arise in the material sciences involving free interfaces. To establish the existence and regularity (including the regularity of free interfaces) of solutions has been an important and interesting issue. Here we were able to do so in a model case accounted in optimal designs.The method developed in this paper is rather general and may be useful for many other related problems.     

The minimum rank problem: A counterexample

We provide a counterexample to a recent conjecture that the minimum rank over the reals of every sign pattern matrix can be realized by a rational matrix. We use one of the equivalences of the conjecture and some results from projective geometry. As a consequence of the counterexample we show that there is a graph for which the minimum rank of the graph over the reals is strictly smaller than the minimum rank of the graph over the rationals. We also make some comments on the minimum rank of sign pattern matrices over different subfields of R.     

Optimally Adjusted Mixture Sampling and Locally Weighted Histogram Analysis

Consider the two problems of simulating observations and estimating expectations and normalizing constants for multiple distributions. First, we present a self-adjusted mixture sampling method, which accommodates both adaptive serial tempering and a generalized Wang–Landau algorithm. The set of distributions are combined into a labeled mixture, with the mixture weights depending on the initial estimates of log normalizing constants (or free energies). Then, observations are generated by Markov transitions, and free energy estimates are adjusted online by stochastic approximation. We propose two stochastic approximation schemes by Rao–Blackwellization of the scheme commonly used, and derive the optimal choice of a gain matrix, resulting in the minimum asymptotic variance for free energy estimation, in a simple and feasible form. Second, we develop an offline method, locally weighted histogram analysis, for estimating free energies and expectations, using all the simulated data from multiple distributions by either self-adjusted mixture sampling or other sampling algorithms. This method can be computationally much faster, with little sacrifice of statistical efficiency, than a global method currently used, especially when a large number of distributions are involved. We provide both theoretical results and numerical studies to demonstrate the advantages of the proposed methods.     

Singularly perturbed elliptic systems

We study the existence and concentration behavior of positive solutions for a class of Hamiltonian systems (two coupled nonlinear stationary Schrödinger equations). Combining the Legendre–Fenchel transformation with mountain pass theorem, we prove the existence of a family of positive solutions concentrating at a point in the limit, where related functionals realize their minimum energy. In some cases, the location of the concentration point is given explicitly in terms of the potential functions of the stationary Schrödinger equations.     

Variational formulation of a modified couple stress theory and its application to a simple shear problem

A variational formulation is provided for the modified couple stress theory of Yang et al. by using the principle of minimum total potential energy. This leads to the simultaneous determination of the equilibrium equations and the boundary conditions, thereby complementing the original work of Yang et al. where the boundary conditions were not derived. Also, the displacement form of the modified couple stress theory, which is desired for solving many problems, is obtained to supplement the existing stress-based formulation. All equations/expressions are presented in tensorial forms that are coordinate-invariant. As a direct application of the newly obtained displacement form of the theory, a simple shear problem is analytically solved. The solution contains a material length scale parameter and can capture the boundary layer effect, which differs from that based on classical elasticity. The numerical results reveal that the length scale parameter (related to material microstructures) can have a significant effect on material responses.      

Triblock Copolymer Theory: Ordered ABC Lamellar Phase

Summary. The ABC lamellar phase of a triblock copolymer in the strong segregation region is studied on periodic and bounded intervals. In the periodic case we find a family of local minimizers of the free energy functional all with a fine lamellar structure. Among these local minimizers we identify the one most favored by the free energy, and hence determine the thickness of lamellar microdomains. In the bounded interval case we show that perfect lamellar structure does not exist due to the boundary effect. We view the strong segregation limit as a Γ -limit of the free energy by a proper choice of the material sample size. The key step is the spectral analysis of a large matrix resulting from the second derivative of the Γ -limit.     

Maximum Recoverable Work for a Rigid Heat Conductor with Memory

In this paper we study the problem of giving an explicit formula for the minimum free energy for a rigid heat conductor, with memory both in the expression of the heat flux and in the one of the internal energy. Two equivalent expressions are derived in terms of Fourier-transformed quantities; one of them is used to obtain the results related to the particular case of a discrete spectrum model.     

Rheology of polarizable non-piezoelectromagnetic material in relativity

Following the work of Carter on nonlinear perfectly elastic solid and perfect nonlinearly polarizable nonconducting solid, we have constructed models whose free gravitational field is of Petrov typeD: (i) in inertial reference frame (IRF), (ii) with pure expansion and (iii) with pure rotation with the assumption that the flow field is expressible in terms of two real null vectors of the Newman-Penrose (N-P) tetrad. By using the strain variation equation, the necessary and sufficient conditions on the dynamical variables are obtained in Newman-Penrose version. We observe that the initial pressure tensor depends on the polarizable and electromagnetic properties of the material. Further, we conclude that there does not exist such a material with pure expansion but there exists such a material moving rigidly with or without rotation. We obtain the Hawking energy conditions and invariants for this material in IRF.     

Interaction of phase-transformations and plasticity – a multi-phase micro-sphere approach

We present an efficient model for the simulation of solid to solid phase-transformations in polycrystalline materials. As a basis, we implement a scalar-valued Gibbs-energy-barrier-based phase-transformation model making use of statistical physics. In this work, we particularly adopt the model for the simulation of phase-transformations between an austenitic parent phase and a martensitic tension and compression phase. The incorporation of plasticity phenomena is established by enhancing the Helmholtz free energy functions of the material phases considered, where the plastic driving forces acting in each phase are derived from the overall free energy potential. The coupled model is embedded into a micro-sphere formulation in order to simulate three-dimensional boundary value problems—a technique well-established in the context of computational inelasticity at small strains. It is shown that the model is capable of reflecting experimentally observed behaviour. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)