A kinetic theory and benchmark predictions for polymer-dispersed, semi-flexible macromolecular rods or platelets

The goals of this paper are: to present a mean-field kinetic theory for the hydrodynamics of macromolecular high aspect ratio rods or platelets dispersed in a polymeric solvent; and, to apply the formalism to predict the impact due to a polymeric versus viscous solvent on the classical Onsager isotropic-nematic equilibrium phase diagram and on the monodomain response to imposed steady shear. The kinetic theory coupling between the nanoscale rods or platelets and the polymeric solvent is incorporated through a mean-field potential that reflects the enormous particle-polymer surface area and the particle-polymer interactions across this interfacial area. To determine predictions of this theory on the equilibrium and sheared monodomain phase diagrams, we present a reduction procedure which approximates the coupled Smoluchowski equations for the polymer chain probability distribution function (PDF) and the nano-particle orientational PDF in favor of a coupled system of equations for the rank 2 second-moment tensors for each PDF. The reduced model consists of an 11-dimensional dynamical system, which we solve using continuation software (AUTO) to predict the modified Onsager equilibrium phase diagram and the modified Doi-Hess shear phase diagram due to the physics of polymer-particle surface interactions.     

Magnetic superlattices with variable interlayer exchange coupling: a new approach for the investigation of low-dimensional magnetism

We have investigated the magnetic order in an [Fe(2)/(VHx)(13)] x 200 superlattice as a function of temperature and hydrogen content in the vanadium layers. A J(radially)-T magnetic phase diagram was established where J(radially) denotes the interlayer exchange coupling between adjacent Fe planes. We propose that Fe/V superlattices, in which the ratio of interlayer to intralayer coupling can be tuned continuously and reversibly via hydrogen in the nonmagnetic vanadium, offer a new approach for the study of low-dimensional magnetism and crossover effects near the transition from ferromagnetic to antiferromagnetic order.     

Compensation point (or points) of nanoscaled transverse Ising thin films: effects of surface dilution

The phase diagram (critical temperature and compensation temperature) and the total magnetizations in a nanoscaled transverse Ising thin film with thickness L and a negative interlayer coupling at the surfaces are investigated by the use of both the effective-field theory with correlations and the mean-field theory. Particular emphasis is given to the effects of the surface and its dilution on them. We have found that, for appropriate values of the system parameters, compensation point (or points) may be obtained in the present system with L?=?5 (or L?=?10).     

New mean-field theory of the tt't'J model applied to high-T(c) superconductors

We introduce a new mean-field approach to the tt't'J model that incorporates both electron-like quasiparticle and spinon excitations as suggested by some experiments and numerical studies. It leads to a mean-field phase diagram which is consistent with that of hole and electron doped cuprates. Moreover, it provides a framework to describe the observed evolution of the electron spectral function from the undoped insulator to the overdoped Fermi metal for both hole and electron doping. The theory also provides a new non-BCS mechanism leading to superconductivity.     

Frustration in a transverse Ising nanoisland: effects of interlayer coupling

The phase diagram and magnetizations in a transverse Ising nanoisland are examined by using the effective field theory with correlations. The nanoisland is constructed from two layers with nine atoms in each which are coupled by the interlayer coupling. We present some characteristic phenomena in them which come from the frustration induced by an interlayer coupling and two transverse fields.     

参数修改对铁电薄膜相变性质的影响

在关联有效场理论的框架内, 利用微分算子技术, 详细地计算了基于横场伊辛模型描述的对称铁电薄膜系统的相变性质. 根据薄膜各层自旋平均值构成的一系列耦合方程, 推导出可以用来计算任意层的具有不同表面层的薄膜相图的解析通式方程, 讨论了参数修改对薄膜相互作用参数从FPD (铁电相占主导地位的相图)到PPD (顺电相占主导地位的相图)过渡值和参数空间中各相变区域的影响. 在与平均场近似进行比较的结果显示, 关联有效场理论所得到的铁电薄膜的铁电性在某种程度上比平均场近似下的结果减弱. 关键词： 铁电薄膜 横场伊辛模型 相图 居里温度     

Dynamical generation of a Coulomb phase in the mean-field approach to Z(N) lattice gauge theories

The phase diagram of Z(N) lattice gauge theories is examined in the mean-field approach. It is shown how large non-perturbative fluctuations around the mean-field may naturally occur in these theories. When this happens, a massless phase which is absent in the mean-field approximation is dynamically generated. An order parameter which characterizes the Coulomb to Higgs phase transition is introduced. The predictions for the values of the critical coupling for this transition are in excellent agreement with the Monte Carlo data in four dimensions.     

Phase diagram for a t-J bilayer: role of interlayer couplings

The phase diagram for a t-J bilayer as a function of interplanar hopping, t_⊥ and hole concentration, x is presented for a few different values of interplanar exchange, J_⊥ using variational Monte Carlo calculations. The phase diagram shows rich features, such as a coexistence of antiferromagnetism and superconductivity at underdoping, planar (d-wave) and interplanar (d_z-wave) superconducting correlations for small and large J_⊥, respectively at optimal and overdoping. Another unusual feature appears in the form of a dome shaped structure in the phase diagram where the superconducting correlations are initially assisted as interplanar hopping is enhanced for small t_⊥, while larger t_⊥ is found to be detrimental to superconductivity.     

Mean-field theory of the three-state chiral clock model

The phase diagram of the three-state chiral clock model, which is known to exhibit commensurate and incommensurate ordered modulated structures, is investigated in the mean-field approximation. First a numerical analysis of the mean-field equations is presented. It is based in the main on the observation that these equations define a non-linear mapping in a four dimensional space. This method of analyzing the mean-field theory proves particularly useful in the determination of the pinning transition of the incommensurate structures. Next the phase diagram is investigated analytically by means of a Landau expansion modified such as to include domain walls. It is found that in the vicinity of the order-disorder transition most features of the phase diagram can be explained quantitatively by this expansion. Finally we present a systematic lowtemperature expansion of the mean-field theory, showing that the low-temperature phase diagram obtained in the mean-field approximation is different from that of the full model.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

On the Interplay of Magnetic and Molecular Forces in Curie–Weiss Ferrofluid Models

We consider a mean-field continuum model of classical particles in R ^d with Ising or Heisenberg spins. The interaction has two ingredients, a ferromagnetic spin coupling and a spin-independent molecular force. We show that a feedback between these forces gives rise to a first-order phase transition with simultaneous jumps of particle density and magnetization per particle, either at the threshold of ferromagnetic order or within the ferromagnetic region. If the direct particle interaction alone already implies a phase transition, then the additional spin coupling leads to an even richer phase diagram containing triple (or higher order) points.     

Purely quadrupolar spin-1 Ising model in a transverse field

A quantum Ising-like spin-1 model characterized by quadrupolar interaction, coupled to an external anisotropic field with both dipole and quadrupole momenta is analyzed. The general phase diagram (including temperature), as well as order parameter, specific heat, and susceptibility are evaluated in the mean-field approximation and exibit a rich structure with transitions of 2° and 1° order and tricritical points. ForT=0 the phase diagram is examined also by a recently formulated improved version of mean-field theory which has the usual mean-field theory as its zero-th order approximation.     

Bilayer quantum hall systems at filling factor nu = 2: An exact diagonalization study

We present an exact diagonalization study of bilayer quantum Hall systems at filling factor nu = 2 in the spherical geometry. We find the high-Zeeman-coupling phase boundary of the broken symmetry canted antiferromagnet is given exactly by previous Hartree-Fock mean-field theories, but that the state's stability at weak Zeeman coupling has been qualitatively overestimated. In the absence of interlayer tunneling, degeneracies occur between total spin multiplets due to the Hamiltonian's invariance under independent spin rotations in top and bottom two-dimensional electron layers.