Spin Dynamics of Double-Exchange Manganites with Magnetic Frustration

This work examines the effects of magnetic frustration due to competing ferromagnetic and antiferromagnetic Heisenberg interactions on the spin dynamics of the double-exchange model. When the local moments are non-colinear, a charge-density wave forms because the electrons prefer to sit on lines of sites that are coupled ferromagnetically. With increasing hopping energy, the local spins become aligned and the average spin-wave stiffness increases. Phase separation is found only within a narrow range of hopping energies. Results of this work are applied to the field-induced jump in the spin-wave stiffness observed in the manganite Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \le x \le 0.4$.Comment: 10 pages, 3 figure

A Monte Carlo Method for Fermion Systems Coupled with Classical Degrees of Freedom

A new Monte Carlo method is proposed for fermion systems interacting with classical degrees of freedom. To obtain a weight for each Monte Carlo sample with a fixed configuration of classical variables, the moment expansion of the density of states by Chebyshev polynomials is applied instead of the direct diagonalization of the fermion Hamiltonian. This reduces a cpu time to scale as $O(N_{\rm dim}^{2} \log N_{\rm dim})$ compared to $O(N_{\rm dim}^{3})$ for the diagonalization in the conventional technique; $N_{\rm dim}$ is the dimension of the Hamiltonian. Another advantage of this method is that parallel computation with high efficiency is possible. These significantly save total cpu times of Monte Carlo calculations because the calculation of a Monte Carlo weight is the bottleneck part. The method is applied to the double-exchange model as an example. The benchmark results show that it is possible to make a systematic investigation using a system-size scaling even in three dimensions within a realistic cpu timescale.Comment: 6 pages including 4 figure

Control of the finite size corrections in exact diagonalization studies

We study the possibility of controlling the finite size corrections in exact diagonalization studies quantitatively. We consider the one- and two dimensional Hubbard model. We show that the finite-size corrections can be be reduced systematically by a grand-canonical integration over boundary conditions. We find, in general, an improvement of one order of magnitude with respect to studies with periodic boundary conditions only. We present results for ground-state properties of the 2D Hubbard model and an evaluation of the specific heat for the 1D and 2D Hubbard model.Comment: Phys. Rev. B (Brief Report), in pres

Frequency-dependent spin susceptibility in the two-dimensional Hubbard model

A Quantum Monte Carlo calculation of dynamical spin susceptibility in the half-filled 2D Hubbard model is presented for temperature $T=0.2t$ and an intermediate on-site repulsion $U=4t$. Using the singular value decomposition technique we succeed in analytically continuing the Matsubara Green's function into the real frequency domain and in deriving the spectral representation for the longitudinal and transverse spin susceptibility. The simulation results, while contradicting the random-phase approximation prediction of antiferromagnetic long-range order at this temperature, are in agreement with an extension of a self-consistent renormalization approach of Moriya. The static susceptibility calculated using this technique is qualitatively consistent with the $\omega \rightarrow 0$ simulation results.Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclose

Spin Excitation Spectrum of La1−xAx_{1-x}A_xMnO3_3

As an effective model to describe perovskite-type manganates (La,$A$)MnO$_3$, the double-exchange model on a cubic lattice is investigated. Spin excitation spectrum of the model in the ground state is studied using the spin wave approximation. Spin wave dispersion relation observed in the inelastic neutron scattering experiment of La$_{0.7}$Pb$_{0.3}$MnO$_3$ is reproduced. Effective values for the electron bandwidth as well as Hund's coupling is estimated from the data.Comment: 10 pages LaTeX including 4 PS figure

Magnetic Order in the Double Exchange Model in Infinite Dimensions

We studied magnetic properties of the double exchange (DE) model with S=1/2 localized spins at T=0, using exact diagonalization in the framework of the dynamical mean field theory. Obtained phase diagram contains ferromagnetic, antiferromagnetic and paramagnetic phases. Comparing the phase diagram with that of the DE model with classical localized spins, we found that the quantum fluctuations of localized spins partly destabilize the ferromagnetism and expand the paramagnetic phase region. We found that phase separations occur between the antiferromagnetic and paramagnetic phases as well as the paramagnetic and ferromagnetic ones.Comment: 11 pages, LaTeX, 9 eps-figure

Density-Matrix functional theory of strongly-correlated lattice fermions

A density functional theory (DFT) of lattice fermion models is presented, which uses the single-particle density matrix gamma_{ij} as basic variable. A simple, explicit approximation to the interaction-energy functional W[gamma] of the Hubbard model is derived from exact dimer results, scaling properties of W[gamma] and known limits. Systematic tests on the one-dimensional chain show a remarkable agreement with theBethe-Ansatz exact solution for all interaction regimes and band fillings. New results are obtained for the ground-state energyand charge-excitation gap in two dimensions. A successful description of strong electron correlations within DFT is achieved.Comment: 15 pages, 6 figures Submitted to PR

Spin Diffusion in Double-Exchange Manganites

The theoretical study of spin diffusion in double-exchange magnets by means of dynamical mean-field theory is presented. We demonstrate that the spin-diffusion coefficient becomes independent of the Hund's coupling JH in the range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal magnetoresistive manganites in the metallic part of their phase diagram. Our study reveals a close correspondence as well as some counterintuitive differences between the results on Bethe and hypercubic lattices. Our results are in accord with neutron scattering data and with previous theoretical work for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio

Dynamical Mean-Field Solution for a Model of Metal-Insulator Transitions in Moderately Doped Manganites

We propose that a specific spatial configuration of lattice sites that energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites constitutes approximately a spatially random two-energy-level system. Such an effect results in a mechanism of metal-insulator transitions that appears to be different from both the Anderson transition and the Mott-Hubbard transition. Correspondingly, a disordered Kondo lattice model is put forward, whose dynamical mean-field solution agrees reasonably with experiments.Comment: 4 pages, 2 eps figures, Revtex. First submitted to PRL on May 16, 199

Theory of Insulator Metal Transition and Colossal Magnetoresistance in Doped Manganites

The persistent proximity of insulating and metallic phases, a puzzling characterestic of manganites, is argued to arise from the self organization of the twofold degenerate e_g orbitals of Mn into localized Jahn-Teller(JT) polaronic levels and broad band states due to the large electron - JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence