Microscopic theory of longitudinal sound velocity in CDW and SDW ordered cuprate systems

We address here the self-consistent calculation of the spin density wave and the charge density wave gap parameters for high-T_c cuprates on the basis of the Hubbard model. In order to describe the experimental observations for the velocity of sound, we consider the phonon coupling to the conduction band in the harmonic approximation and then the expression for the temperature dependent velocity of sound is calculated from the real part of the phonon Green’s function. The effects of the electron–phonon coupling, the frequency of the sound wave, the hole doping concentration, the CDW coupling and the SDW coupling parameters on the sound velocity are investigated in the pure CDW phase as well as in the co-existence phase of the CDW and SDW states. The results are discussed to explain the experimental observations.     

Macroscopic spin and charge transport theory

According to the general principle of non-equilibrium thermodynamics, we propose a set of macroscopic transport equations for the spin transport and the charge transport. In particular, the spin torque is introduced as a generalized `current density' to describe the phenomena associated with the spin non-conservation in a unified framework. The Einstein relations and the Onsager relations between different transport phenomena are established. Specifically, the spin transport properties of the isotropic non-magnetic and the isotropic magnetic two-dimensional electron gases are fully described by using this theory, in which only the macroscopic-spin-related transport phenomena allowed by the symmetry of the system are taken into account.     

Microscopic theory of dipole-exchange spin waves in magnetic multilayers

A microscopic model is employed to calculate the spectrum of dipole-exchange spin waves in multilayers in which thin ferromagnetic films are separated by non-magnetic spacers. Two configurations are considered: in one the films have magnetizations parallel to each other, in the other the magnetizations are antiparallel. The calculations extend a previous microscopic formalism that allows the calculation of the dipole-exchange spin wave spectrum in thin films. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness.Received: 26 May 2004, Published online: 12 August 2004PACS: 75.30.Ds Spin waves - 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces     

Pseudopotential theory of spin susceptibility of metals

We derive an expression for the exchange enhanced spin susceptibility (χ_s) of metals by using the pseudopotential formalism and degenerate perturbation theory. The advantage of our theory over the recent spin-density-functional formalism is that we have included spin-orbit effects. Our results for χ_s of alkali metals agree with the experimental results.     

Microscopic theory of charge-density wave systems

An effective action (in imaginary time) for the phase of the order parameter is derived using the path integral formulation of the microscopic theory. After analytic continuation, the classical equation of motion is derived. Dissipation is found to arise from impurity scattering and from the screened Coulomb interaction with normal electrons. Similarities to a recent theory of Josephson junctions are discussed.     

Interplay of spin and charge channels in zero-dimensional systems

We study the interplay of charge and spin (zero-mode) channels in quantum dots. The latter affects the former in the form of a distinct signature on the differential conductance. We also obtain both longitudinal and transverse spin susceptibilities. All these observables, underlain by spin fluctuations, become accentuated as one approaches the Stoner instability. The nonperturbative effects of zero-mode interaction are described in terms of the propagation of gauge bosons associated with charge [U(1)] and spin [SU(2)] fluctuations in the dot, while transverse spin fluctuations are analyzed perturbatively.     

Landau theory of charge and spin ordering in the nickelates

Guided by experiment and band structure, we introduce and study a phenomenological Landau theory for the unusual charge and spin ordering associated with the Mott transition in the perovskite nickelates, with chemical formula RNiO3, where R=Pr, Nd,Sm, Eu, Ho, Y, and Lu. While the Landau theory has general applicability, we show that for the most conducting materials, R=Pr, Nd, both types of order can be understood in terms of a nearly nested spin-density wave. Furthermore, we argue that in this regime, the charge ordering is reliant upon the orthorhombic symmetry of the sample, and therefore proportional to the magnitude of the orthorhombic distortion. The first order nature of the phase transitions is also explained. We briefly show by example how the theory is readily adapted to modified geometries such as nickelate films.     

Random-walk theory and ordered phases in lattice-gas systems

The random-walk formalism that describes correlation functions in a homogenous system is here extended to cover correlations in ordered phases of a lattice gas. The general method is illustrated by application to certain lattice gases on linear, square and honeycomb lattices, treated under the Percus-Yevick approximation.     

Fluctuation susceptibility relations for classical spin systems

We prove identities between integrated Ursell functions and derivatives of the pressure in the thermodynamic limit, for multicomponent classical spin systems which obey the Lee-Yang theorem and some form of Gaussian domination, when the susceptibility is finite (T>T _c). Following Refs. 3 and 4, we view the moment generating function of the magnetization as the inverse of an infinitely divisible characteristic function. Fluctuation susceptibility relations of all orders then follow by bounding the corresponding cumulants, taken in zero external field. High-order cumulants are bounded in terms of the susceptibility using Gaussian and Simon's inequalities for short-range interactions.     

Diagonalization-free implementation of spin relaxation theory for large spin systems

The Liouville space spin relaxation theory equations are reformulated in such a way as to avoid the computationally expensive Hamiltonian diagonalization step, replacing it by numerical evaluation of the integrals in the generalized cumulant expansion. The resulting algorithm is particularly useful in the cases where the static part of the Hamiltonian is dominated by interactions other than Zeeman (e.g. in quadrupolar resonance, low-field EPR and Spin Chemistry). When used together with state space restriction tools, the algorithm reported is capable of computing full relaxation superoperators for NMR systems with more than 15 spins.     

On the theory of the magnetic,electric, and mixed susceptibility in magnetically and electrically ordered systems

On the basis of the unified macroscopic approach taking into account the initial structural state of the system and the parameters of external effects (frequency and orientation of probe and displacing fields), relations have been obtained for the frequency and orientation dependences of the magnetic, electric, and mixed susceptibility and magnetic capacity for magnets, ferroelectrics, and ferroelectromagnets, respectively.     

Differential susceptibility of spin glass - ferromagnetic systems

The differential susceptibility of double transition spin glass - ferromagnets is calculated as a function of temperature and external field based upon the theory by Sherrington and Kirkpatrick. Good agreement with experiments on the $\text{Pd}$FeMn system has been obtained.     

Order parameter and static susceptibility of spin glasses in mean-field theory

Summary The spin glass phase of a system of Ising spins interacting via the RKKY interaction is investigated at a mean-field level. The results around the transition temperature are at sharp variance with those previously found by other authors and suggest a first-order transition. The static susceptibility is found to deviate from Fischer's predictions. A test of the validity of scaling predictions is also performed.

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Riassunto La fase di ≪spin glass≫ di un sistema di spin di Ising interagenti tramite un'interazione del tipo RKKY è studiata in campo medio. I risultati nell'intorno della temperatura di transizione sono in netto contrasto con quelli in precedenza trovati da altri autori e suggeriscono una transizione del prim'ordine. Si trova che la suscettività statica devia dalle predizioni di Fischer. Si effettua anche una verifica della validità delle leggi di scala.

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Резюме На уровне среднего поля исследуется фаза спинового стекла для системы спинов Изинга, взаимодействующих через RKKY взаумодействие. Полученные результаты для температуры перехода сильно отличаются от результатов, полученных ранее другими авторами, и предполагают переход первого порядка. Обнаружено, что статическая восприимчивость отличается от предсказаний Фишера. Также проводится проверка справедливости предсказаний подобия.

     

Microscopic theory of substrate-induced gap effect on real AFM susceptibility in graphene

In this article, the novel (G ^′/G)-expansion method is successfully applied to construct the abundant travelling wave solutions to the KdV–mKdV equation with the aid of symbolic computation. This equation is one of the most popular equation in soliton physics and appear in many practical scenarios like thermal pulse, wave propagation of bound particle, etc. The method is reliable and useful, and gives more general exact travelling wave solutions than the existing methods. The solutions obtained are in the form of hyperbolic, trigonometric and rational functions including solitary, singular and periodic solutions which have many potential applications in physical science and engineering. Many of these solutions are new and some have already been constructed. Additionally, the constraint conditions, for the existence of the solutions are also listed.     

Microscopic theory of fluctuations in itinerant electron systems — Ferromagnetism

A many body theoretical method for calculating the effect of low energy fluctuations on the dynamic susceptibility χ(q, ω⁺) is presented. For the case of ferromagnetism, the contribution of one internal spin fluctuation is obtained and is seen to be much larger than the temperature dependent part of the Stoner or RPA term. It leads to the observed Curie-Weiss law for χ(0, 0) (e.g. in Ni where comparison with experiment is made).     

On the dynamic mean field theory of spin glasses

We derive in detail Sompolinsky's mean field theory of spin glasses using a diagram expansion of the effective local Langevin equation of Sompolinsky and Zippelius. We use a simpler generating functional than in the literature, on which the quenched average is very easily done. We pay special attention to the existence of an external field. We show that there are two different types of singularities for ω=0 in the equations. The first type, which leads to Parisi'sq(0), is connected with the local magnetisation. The second type, which leads toq′(x), is connected with the nonergodic behaviour. We show that the continuous limit of discrete Sompolinsky solutions has to be taken in order to be in accordance with the fluctuation dissipation theorem on infinite time scales. We discuss carefully the question of dynamical stability. We show that Sommers' solution is unstable only on an infinite time scale and thus remains an acceptable equilibrium theory with a broken symmetry. We argue that for ω=0 a formal violation of the fluctuation dissipation theorem is physically expected if the relaxation times are of the order of the switching time of the external field. From this point of view the spin-glass state is a steady state but not a real equilibrium state.     

Growth of ordered domains beyond a dynamic instability in dissipative systems

We furnish evidence supporting our conjecture that the growth of structure which results at the onset of a center manifold in driven systems operating far from equilibrium can be described by a power law with a single exponent. More specifically, if the dissipative structure is contained in a center manifold and thus its stability is warranted, the linear dimensions for domains of organized spatial cells increase ast ^1/2.