H. Röder

Calculation of densities of states and spectral functions by Chebyshev recursion and maximum entropy

We present an efficient algorithm for calculating spectral properties of large sparse Hamiltonian matrices such as densities of states and spectral functions. The combination of Chebyshev recursion and maximum entropy achieves high-energy resolution without significant roundoff error, machine precision, or numerical instability limitations. If controlled statistical or systematic errors are acceptable, CPU and memory requirements scale linearly in the number of states. The inference of spectral properties from moments is much better conditioned for Chebyshev moments than for power moments. We adapt concepts from the kernel polynomial method, a linear Chebyshev approximation with optimized Gibbs damping, to control the accuracy of Fourier integrals of positive nonanalytic functions. We compare the performance of kernel polynomial and maximum entropy algorithms for an electronic structure example. {copyright} {ital 1997} {ital The American Physical Society}

Double degeneracy and Jahn-Teller effects in colossal-magnetoresistance perovskites

Jahn-Teller (JT) electron-phonon-coupling effects in the colossal-magnetoresistance perovskite compounds La{sub 1{minus}{ital x}}{ital A}{sub {ital x}}MnO{sub 3} are investigated. Electron-electron correlations between two degenerate Mn {ital e}{sub {ital g}} orbitals are studied in the Gutzwiller approximation. The static JT distortion and nonadiabatic polaron effects are studied in a modified Lang-Firsov approximation. We find that (i) the electron or hole character of the charge carrier depends on the static JT distortion and (ii) due to the two-component nature of the JT coupling, fluctuations in the JT distortion direction contribute to the charge transport in similar fashion as the local spins. {copyright} {ital 1996 The American Physical Society.}

PHONONS FROM NEUTRON POWDER DIFFRACTION

The spherically averaged structure function S([vert bar][bold q][vert bar]) obtained from pulsed neutron powder diffraction contains both elastic and inelastic scattering via an integral over energy. The Fourier transformation of S([vert bar][bold q][vert bar]) to real space, as is done in the pair density function (PDF) analysis, regularizes the data, i.e., it accentuates the diffuse scattering. We present a technique which enables the extraction of off-center ([vert bar][bold q][vert bar][ne]0) phonon information from powder diffraction experiments by comparing the experimental PDF with theoretical calculations based on standard interatomic potentials and the crystal symmetry. This procedure [dynamics from powder diffraction] has been [ital successfully] implemented as demonstrated here for two systems, a simple metal fcc Ni and an ionic crystal CaF[sub 2]. Although computationally intensive, this data analysis allows for a phonon based modeling of the PDF, and additionally provides off-center phonon information from neutron powder diffraction. [copyright] [ital 1999] [ital The American Physical Society]

Theory of colossal magnetoresistance

Abstract The history and recent developments in studying (colossal) magnetoresistance in perovskite manganese oxides show that there is growing evidence for strongly coupled spin, charge and lattice degrees of freedom. Together with disorder, these provide the microscopic driving forces for local and inhomogeneous textures. The modeling and experimental probes for localized charge—spin—lattice (polaron) structures, and their multiscale ordering, can be considered in terms of a growing synergy of solid state physics and materials science perspectives.

HIGH-TEMPERATURE THERMODYNAMICS OF THE FERROMAGNETIC KONDO-LATTICE MODEL

We present a high-temperature series expansion for the ferromagnetic Kondo-lattice model in the large coupling limit, which is used to model colossal magnetoresistance (CMR) perovskites. Our results show the expected crossover to Curie-Weiss behavior at a temperature related to the bandwidth. Estimates for the magnetic transition temperatures are in the experimentally observed range. The compressibility shows that the high-temperature charge excitations can be modeled by spinless fermions. The CMR effect itself, however, warrants the inclusion of dynamic effects and cannot be explained by a static calculation. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic properties of the double-exchange model

We study the ferromagnetic (FM) Kondo lattice model in the strong-coupling limit (the double-exchange (DE) model). The DE mechanism proposed by Zener to explain ferromagnetism is found to have unexpected properties when there is more than one itinerant electron. We find that, in general, the many-body ground state of the DE model is not globally FM ordered (except for in special filled-shell cases). Furthermore, the low-energy excitations of this model are distinct from those in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.

Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals

The effect of the powder average on the peak shapes and positions in neutron pair distribution functions of polycrystalline materials is examined. It is shown that for highly anisotropic crystals, the powder average leads to shifts in peak positions and to non-Gaussian peak shapes. The peak shifts can be as large as several percent of the lattice spacing.

Spin-Peierls ground states in an electron-lattice periodic Anderson model

Novel dimerized antiferromagnetic (homogeneous spin-Peierls) and inhomogeneous-lattice antiferromagnetic (inhomogeneous spin-Peierls) ground states are found in a one-dimensional (1-D) and a two-dimensional (2-D) electron-lattice periodic Anderson model, respectively. Coexistence and mutual enhancement of the Peierls lattice distortion and the antiferromagnetic long-range-order are emphasized. The stoichiometric (half-filling) phase diagrams for the 1-D and 2-D cases are strongly dependent on the hybridization and electron-lattice coupling. For non-stoichiometric fillings, local lattice distortion (coupled spin-charge-lattice small-polaron) states are found: these are discussed in the context of, for example, Ce-based, heavy-fermion systems. Relations to volume collapse phenomena and inorganic spin-Peierls systems, and the similarity of the phase diagrams to those of organic superconductors are also described.

Persistent current in the ferromagnetic Kondo lattice model

In this paper, we study the zero-temperature persistent current in a ferromagnetic Kondo lattice model in the strong coupling limit. In this model, there are spontaneous spin textures at some values of the external magnetic flux. These spin textures contribute a geometric flux, which can induce an additional spontaneous persistent current. Since this spin texture changes with the external magnetic flux, we find that there is an anomalous persistent current in some region of magnetic flux: near {Phi}/{Phi}{sub 0}=0 for an even number of electrons and {Phi}/{Phi}{sub 0}=1/2 for an odd number of electrons. {copyright} {ital 1997} {ital The American Physical Society}

The local atomic structure and phonons in Ba0.5Sr0.5TiO3

The study of the local atomic structure of the solid solution of Ba0.5Sr0.5TiO3 (BST) has shown that its microscopic properties are quite distinct from those of the end members of the series, BaTiO3 (BTO) and SrTiO3 (STO). In contrast to BTO and STO, the BST structure changes as a function of temperature at short length scales, but not on average. However, from the extraction of phonon dynamics from the diffraction measurement, the presence of a soft mode behavior similar to the soft mode observed in BTO is suggested. This indicates that lattice instabilities are driven by mechanisms that are similar to those observed in the ferroelectric transition of BTO and the structural transition of STO.