Transport properties in Simplified Double Exchange model

Transport properties of the manganites by the double-exchange mechanism are considered. The system is modeled by a simplified double-exchange model, i.e. the Hund coupling of the itinerant electron spins and local spins is simplified to the Ising-type one. The transport properties such as the electronic resistivity, the thermal conductivity, and the thermal power are calculated by using Dynamical mean-field theory. The transport quantities obtained qualitatively reproduce the ones observed in the manganites. The results suggest that the Simplified double exchange model underlies the key properties of the manganites.Comment: 5 pages, 5 eps figure

Charge-ordered ferromagnetic phase in manganites

A mechanism for charge-ordered ferromagnetic phase in manganites is proposed. The mechanism is based on the double exchange in the presence of diagonal disorder. It is modeled by a combination of the Ising double-exchange and the Falicov-Kimball model. Within the dynamical mean-field theory the charge and spin correlation function are explicitely calculated. It is shown that the system exhibits two successive phase transitions. The first one is the ferromagnetic phase transition, and the second one is a charge ordering. As a result a charge-ordered ferromagnetic phase is stabilized at low temperature.Comment: To appear in Phys. Rev.

Anomaly in Spin Excitation Spectrum of Double-Exchange Systems with Randomness

Spin excitation spectrum of the double-exchange model is studied in the presence of randomness. Spin wave approximation in the ground state shows that the randomness significantly modifies the spectrum from the cosine-like one in the pure system to that with anomalies such as broadening, anti-crossing and gap opening. The origin of anomalies is speculated to be modulation of effective ferromagnetic coupling by the Friedel oscillation. These anomalies qualitatively reproduce the spin excitation spectrum in colossal magnetoresistance manganites whose Curie temperatures are relatively low. Our results suggest that randomness control is an important notion to understand effects of the A-site substitution which has previously been understood as the bandwidth control.Comment: 4 pages including 3 figure

Phase separation and the segregation principle in the infinite-U spinless Falicov-Kimball model

The simplest statistical-mechanical model of crystalline formation (or alloy formation) that includes electronic degrees of freedom is solved exactly in the limit of large spatial dimensions and infinite interaction strength. The solutions contain both second-order phase transitions and first-order phase transitions (that involve phase-separation or segregation) which are likely to illustrate the basic physics behind the static charge-stripe ordering in cuprate systems. In addition, we find the spinodal-decomposition temperature satisfies an approximate scaling law.Comment: 19 pages and 10 figure

Colossal magnetoresistance and quenched disorder in manganese oxides

We give an overview on several recent topics of colossal magnetoresistive manganites in both experiments and theories, focusing on the effect of quenched disorder. The disorder is intrinsically involved since the compounds are solid solutions, and its importance has been pointed out in several experiments of transport and magnetic properties. Recent progress in the experimental control of the strength of disorder is also reviewed. Theoretically, the effect of the disorder has been explored within the framework of the double-exchange mechanism. Several efforts to understand the phase diagram and the electronic properties are reviewed. We also briefly discuss a recent topic on the effect of disorder on competing phases and the origin of colossal magnetoresistance.Comment: 5 pages, 4 figures, proceedings submitted to SPQS200

Exact solution of the Falicov-Kimball model with dynamical mean-field theory

The Falicov-Kimball model was introduced in 1969 as a statistical model for metal-insulator transitions; it includes itinerant and localized electrons that mutually interact with a local Coulomb interaction and is the simplest model of electron correlations. It can be solved exactly with dynamical mean-field theory in the limit of large spatial dimensions which provides an interesting benchmark for the physics of locally correlated systems. In this review, we develop the formalism for solving the Falicov-Kimball model from a path-integral perspective, and provide a number of expressions for single and two-particle properties. We examine many important theoretical results that show the absence of fermi-liquid features and provide a detailed description of the static and dynamic correlation functions and of transport properties. The parameter space is rich and one finds a variety of many-body features like metal-insulator transitions, classical valence fluctuating transitions, metamagnetic transitions, charge density wave order-disorder transitions, and phase separation. At the same time, a number of experimental systems have been discovered that show anomalies related to Falicov-Kimball physics [including YbInCu4, EuNi2(Si[1-x]Gex)2, NiI2 and TaxN].Comment: 51 pages, 40 figures, submitted to Reviews of Modern Physic

A theory of double exchange in infinite dimensions

This paper gives a simplified model of the double exchange which is a kind of indirect exchange interaction between localized magnetic moments. The presented model is solved exactly in the case of infinite -dimensional space. Equations for single-particle Green's function and magnetization of the localized spins subsystem are obtained. It is shown that our simple double exchange model reveals an instability to the ferromagnetic ordering of localized moments. Magnetic and electric properties of this system on Bethe lattice with $z=\infty$ are investigated in detail

Strongly correlated Falicov-Kimball model in infinite dimensions

In this paper we have examined the strongly correlated Falicov-Kimball model in infinite dimensions with the help of a diagrammatic technique for the Hubbard X-operators. This model is represented by the simplified $t{-}J$ model with introduced intra-atomic level energy $\varepsilon^0$ for localized particles. For the Bethe lattice with $z\to\infty$, we have found that the obtained equations for the band Green's function and self-energy coincide with the corresponding Brandt-Mielsch equations taken at $U\to\infty$, and are resolved in analytical form both in the homogeneous phase and in the chessboard phase. In the latter case we have obtained the equation for the order parameter defining the chessboard-like distribution of localized particles. Instability of the homogeneous phase and properties of the chessboard phase are investigated in detail. In particular, it is found that the temperature dependence of the chessboard order parameter has reentrant behaviour for some range of values of $\varepsilon^0$