Jahn-Teller, charge and magnetic ordering in half-doped manganese oxides

The phase diagram of half-doped manganite systems of formula A _0.5 A ^′ _0.5MnO₃ is investigated within a single-orbital model incorporating magnetic double-exchange and superexchange, together with intersite Coulomb and electron-lattice interactions. Strong Jahn-Teller and breathing mode deformations compete together and result in shear lattice deformations. The latter stabilize the charge-ordered CE-type phase, which undergo first-order transitions with temperature or magnetic field to either Ferromagnetic metallic or Paramagnetic insulating phases. An essential feature is the self-consistent screening of Coulomb and electron-phonon interactions in the ferromagnetic phase. Received 28 November 2000     

The role of local environment in charge ordering of perovskite-like oxides

The properties of magnetically diluted perovskite-like complex oxides La_{2 ? x} A _x MO₄ (M is Fe or Mn and A is Sr or Ba) have been studied. Charge and spin ordering is suggested to be interrelated with the local structural characteristics and the electronic structure of nanoclusters of transition elements found in the ceramics studied.     

Electron charge ordering in the extended Hubbard model

The Hubbard model on 3-dimensionalsc lattice with an attractive or repulsive on site interaction, augmented by a renulsive nearest neighbour interaction and its atomic limit are studied using the method of infrared bounds. Conditions upon interactions for the existence of a temperature driven phase transition to so called charge ordered phase are derived. A lower bound in terms of interactions for the corresponding critical temperature is calculated.     

Charge fluctuation,charge ordering,and zero-gap state in organic conductors

We have carried out a series of measurements of angular dependence of solid-state NMR spectrum using single crystal samples on various organic molecular conductors, in order to investigate the natures of the electronic states at low temperatures. We confirmed a charge ordered insulating state in α-(BEDT-TTF)₂I₃ and large charge disproportionation in the metallic state of this salt. In another charge ordered system, θ-(BEDT-TTF)₂RbZn(SCN)₄, we observed unusual NMR line broadening, proportional to resonance shift, in the metallic state above the transition. We found that this broadening is due to charge disproportionation, or more correctly, due to the inhomogeneity of local susceptibility at nuclear sites and analyzed its dynamics. We observed similar broadening in various organic molecular conductors as well, such as θ-(BEDT-TTF)₂CsZn(SCN)₄, an exotic Bechgaad salt, (TMTSF)₂FSO₃, and λ-type BETS salts, λ-(BETS)₂(Fe,Ga)Cl₄. We found the mechanism of CD in each system is different, respectively.     

Fermi liquid theory of the impurity-lattice relaxation for a simple mixed-valence impurity model

Ward identities are used to relate the charge relaxation of a simple spinless mixed-valence impurity with its charge susceptibility at low temperatures. The result is compared with the Korringa relation for the Anderson model and the Kondo problem.     

Low dimensional ordering on a lattice model

A simple d-dimensional lattice model is proposed, incorporating some degree of frustration and thus capable of describing some aspects of molecular orientation in covalently bound molecular solids. For d = 2 the model is shown to be equivalent to the standard two-dimensional Ising model, while for d > 2 it describes a peculiar transition from an isotropic high temperature phase to a low-dimensional anisotropic low temperature state. A general mean field analysis is presented and compared to some exact limit properties.     

Spin, charge, and orbital ordering in La0.5Sr1.5Mno4

We have analyzed the experimental evidence of charge and orbital ordering in La0.5Sr1.5MnO4 using first principles band structure calculations. Our results suggest the presence of two types of Mn sites in the system. One of the Mn sites behaves as an Mn3+ ion, favoring a Jahn-Teller distortion of the surrounding oxygen atoms, while the distortion around the other is not a simple breathing mode kind. Band structure effects are found to dominate the experimental spectrum for orbital and charge ordering, providing an alternate explanation for the experimentally observed results.     

Growth model for atomic ordering: the case for quadruple-period ordering in GaAsSb alloys

Quadruple-period ordering in GaAsSb alloys is studied both theoretically and experimentally. A growth model is proposed to account for the observed three-dimensional (3D) ordered structure. The model is qualitatively different from the widely accepted surface reconstruction and dimerization-induced ordering models that strictly speaking explain only the in-plane 2D patterns. Here, we show that the already ordered substrate will affect the reconstruction of the growth front with respect to the substrate to ensure a correct stacking of the individual 2D ordered layers into the observed 3D lattice.     

Uniformly frustrated XY model without a vortex-pattern ordering

The uniformly frustrated XY model with f=1/3 on a dice lattice is shown to possess an accidental degeneracy of its ground states so well developed that the difference between the free energies of fluctuations does not lead to the stabilization of a particular vortex pattern down to zero temperature. Nonetheless, at low temperatures the system is characterized by a finite helicity modulus whose vanishing (at a finite temperature) is related to the dissociation of half-vortex pairs.     

A simple model for ordering in adsorbed layers

Order-disorder phase transitions in adsorbed and confined fluids with directional interactions are studied using lattice density functional theory. A new model is developed that is capable of predicting both order-disorder and condensation phase transitions. For systems with weak interactions, the results of this model are compared with both lattice Monte Carlo simulation data and simple isotherm theories that are commonly used to fit experimental data. The results show that these simple isotherms are incapable of duplicating complex behaviour exhibited by anisotropic molecules. When sufficiently strong interactions are present, confined fluids with directional interactions may spontaneously form ordered structures. It is shown that the ordering predicted by this model can result in the assembly of long chains. Such ordering has been observed experimentally in magnetorheological fluids in a magnetic field. It is shown that the orientation of the chains predicted by this model can be controlled by adjusting the molecule-surface interaction. It may be possible to create nanoscale devices that exploit this type of molecular switching.     

Two band model for superconducting copper oxides

Assuming that predominant charge carriers in superconducting cooper oxides are holes on O sites, we derive a microscopic model representing a coupled system of spin degrees of freedom on Cu sites and a low concentration of mobile holes. The analysis of a single magnetic polaron shows a large enhancement of the hole effective mass and a strong sensitivity of the antiferromagnetic ground state on doping. Possible mechanisms for the hole pairing are also discussed.     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

K左右出现典型再入型自旋玻璃行为,同时观察到了负的磁化率异常. 结果表明,Nd0.5Ca0.5MnO3体系在低温下存在着典型的相分离和多种复杂的磁相互作用竞争机制,这为强关联锰氧化物体系物理机制的理解提供了重要的实验资料.     

Hawking-like radiation does not require a trapped region

We discuss the issue of quasiparticle production by "analogue black holes" with particular attention paid to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that, in order to obtain a stationary and Planckian emission of quasiparticles, it is not necessary to create a trapped region in the acoustic spacetime (corresponding to a supersonic regime in the fluid flow). It is sufficient to set up a dynamically changing flow asymptotically approaching a sonic regime with sufficient rapidity in laboratory time. This result is generic to curved-space quantum field theory, the "analogue spacetimes" we consider providing a guide to physical intuition, and a possible route to laboratory experiments.     

Directional ordering of fluctuations in a two-dimensional compass model

In the Mott insulating phase of the transition metal oxides, the effective orbital-orbital interaction is directional both in orbital space and in real space. We discuss a classical realization of directional coupling in two dimensions. Despite extensive degeneracy of the ground state, the model exhibits partial orbital ordering in the form of directional ordering of fluctuations at low temperatures stabilized by an entropy gap. Transition to the disordered phase is shown to be in the Ising universality class through exact mapping and multicanonical Monte Carlo simulations.     

Effect of size and charge on ordering of a highly charged concentrated macroions in suspension

An efficient fast Fourier transform method has been employed to determine correlation function [g(r*)] using the structure factor [S(Q*)] calculated with the rescaled mean spherical approximation (RMSA) and the DLVO potential. Based on this function a parametric (size and charge) study of the ordering in a highly charged and concentrated macroions (an ideal colloid) has been made. The strength of the correlation increases with the increase in the charge on macroions and it saturates after acertain value. Similarly, a critical diameter of the particle depending on the charge on it has been found at which normal feature of the ordering disappears.     

The polarizability model for ferroelectricity in perovskite oxides

This article reviews the polarizability model and its applications to ferroelectric perovskite oxides. The motivation for the introduction of the model is discussed and nonlinear oxygen ion polarizability effects and their lattice dynamical implementation outlined. While a large part of this work is dedicated to results obtained within the self-consistent-phonon approximation, nonlinear solutions of the model are also handled, which are of interest to the physics of relaxor ferroelectrics, domain wall motions, and incommensurate phase transitions. The main emphasis is to compare the results of the model with experimental data and to predict novel phenomena.