Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in La1−xAxMnO3La_{1-x}A_xMnO_3

A classical model for the lattice distortions of \lax is derived and, in a mean field approximation, solved. The model is based on previous work by Kanamori and involves localized Mn d-electrons (which induce tetragonal distortions of the oxygen octahedra surrounding the Mn) and localized holes (which induce breathing distortions). Parameters are determined by fitting to the room temperature structure of $LaMnO_3$. The energy gained by formation of a local lattice distortion is found to be large, most likely $\approx 0.6$ eV per site, implying a strong electorn-phonon coupling and supporting polaronic models of transport in the doped materials. The structural transition is shown to be of the order-disorder type; the rapid x-dependence of the transition temperature is argued to occur because added holes produce a "random" field which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late

Dynamic Jahn-Teller Effect and Colossal Magnetoresistance in La1−xAxMnO3La_{1-x}A_xMnO_3

A model for $La_{1-x}Sr_xMnO_3$ which incorporates the physics of dynamic Jahn-Teller and double-exchange effects is presented and solved via a dynamical mean field approximation. In an intermediate coupling regime the interplay of these two effects is found to reproduce the behavior of the resistivity and magnetic transition temperature observed in $La_{1-x} Sr_x MnO_3$.Comment: 11 pages. Latex. Minor revisions, including improvement of discussion of state with frozen-in lattice distortion. Figures (available from [email protected]) unchange

Interplay of charge and orbital ordering in manganese perovskites

A model of localized classical electrons coupled to lattice degrees of freedom and, via the Coulomb interaction, to each other, has been studied to gain insight into the charge and orbital ordering observed in lightly doped manganese perovskites. Expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. The expressions are analyzed for several hole configurations, including that experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find that, although the preferred charge and orbital ordering depend sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are on the order of the observed charge ordering transition temperature. The effects of additional strains are also studied. Some results for La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately describe this material because the high temperature phase is metallic.Comment: 12 pages in RevTex, 5 figures in PS files, to appear in Phys. Rev. B (New paragraphs and references added, typos corrected

On the Fermi Liquid to Polaron Crossover I: General Results

We use analytic techniques and the dynamical mean field method to study the crossover from fermi liquid to polaron behavior in models of electrons interacting with dispersionless classical phonons.Comment: 42 pages, 13 figure

On the Fermi Liquid to Polaron Crossover II: Double Exchange and the Physics of "Colossal" Magnetoresistance

We use the dynamical mean field method to study a model of electrons Jahn-Teller coupled to localized classical oscillators and ferromagnetically coupled to ``core spins'', which, we argue, contains the essential physics of the ``colossal magnetoresistance'' manganites $Re_{1-x} A_x MnO_3$. We determine the different regimes of the model and present results for the temperature and frequency dependence of the conductivity, the electron spectral function and the root mean square lattice parameter fluctuations. We compare our results to data, and give a qualitative discussion of important physics not included in the calculation. Extensive use is made of results from a companion paper titled: ``On the Fermi Liquid to Polaron Crossover I: General Results''.Comment: 34 pages, 10 figures. Depends on previous paper titled "On the Fermi Liquid to Poalron Crossover I: General Result

Effects of uniaxial strain in LaMnO_3

The effects of uniaxial strain on the structural, orbital, optical, and magnetic properties of LaMnO_3 are calculated using a general elastic energy expression, along with a tight-binding parameterization of the band theory. Tensile uniaxial strain of the order of 2 % (i.e., of the order of magnitude of those induced in thin films by lattice mismatch with substrates) is found to lead to changes in the magnetic ground state, leading to dramatic changes in the band structure and optical conductivity spectrum. The magnetostriction effect associated with the Neel transition of bulk(unstrained) LaMnO_3 is also determined. Due to the Jahn-Teller coupling, the uniform tetragonal distortion mode is softer in LaMnO_3 than in doped cubic manganates. Reasons why the observed (\pi \pi 0) orbital ordering is favored over a (\pi \pi \pi) periodicity are discussed.Comment: 9 figures, submitted in Phys. Rev.

Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller Coupling in Perovskite Manganites

Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for Mn perovskite oxides. We study the undoped Mott insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as in the experimental indications, and investigate their ground-state properties by quantum Monte Carlo method. At half filling where the number of $e_{g}$ electron is one per site on average, the Coulomb interaction opens a Mott gap and induces a staggered orbital ordering. The opening of the Mott gap is, however, substantially slower than the mean-field results if the Jahn-Teller coupling is absent. The synergy between the strong correlation and the Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces realistic amplitudes and stabilization energy of the Jahn-Teller distortion. Upon doping, the orbital ordering stabilized by the Coulomb interaction is destroyed immediately. Toward the metal-insulator transition, the short-ranged orbital correlation is critically enhanced in metals, which should be related to strong incoherence of charge dynamics observed in experiments. Our model, moreover, exhibits a uniform ordering of $d_{x^{2}-y^{2}}$ orbital in a wide region of doping in agreement with experimental indications.Comment: 4 pages LaTeX including 3 PS figures, submitted to J.Phys.Soc.Jp

Unified theory of phase separation and charge ordering in doped manganite perovskites

A unified theory is developed to explain various types of electronic collective behaviors in doped manganites R$_{1-x}$X$_x$MnO$_3$ (R = La, Pr,Nd etc. and X = Ca, Sr, Ba etc.). Starting from a realistic electronic model, we derive an effective Hamiltonianis by ultilizing the projection perturbation techniques and develop a spin-charge-orbital coherent state theory, in which the Jahn-Teller effect and the orbital degeneracy of e$_g$ electrons in Mn ions are taken into account. Physically, the experimentally observed charge ordering state and electronic phase separation are two macroscopic quantum phenomena with opposite physical mechanisms, and their physical origins are elucidated in this theory. Interplay of the Jahn-Teller effect, the lattice distortion as well as the double exchange mechanism leads to different magnetic structures and to different charge ordering patterns and phase separation.Comment: 10 ReVTEX pages with 4 figures attache

Evidence for charge localization in the ferromagnetic phase of La_(1-x)Ca_(x)MnO_3 from High real-space-resolution x-ray diffraction

High real-space-resolution atomic pair distribution functions of La_(1-x)Ca_(x)MnO_3 (x=0.12, 0.25 and 0.33) have been measured using high-energy x-ray powder diffraction to study the size and shape of the MnO_6 octahedron as a function of temperature and doping. In the paramagnetic insulating phase we find evidence for three distinct bond-lengths (~ 1.88, 1.95 and 2.15A) which we ascribe to Mn^{4+}-O, Mn^{3+}-O short and Mn^{3+}-O long bonds respectively. In the ferromagnetic metallic (FM) phase, for x=0.33 and T=20K, we find a single Mn-O bond-length; however, as the metal-insulator transition is approached either by increasing T or decreasing x, intensity progressively appears around r=2.15 and in the region 1.8 - 1.9A suggesting the appearance of Mn^{3+}-O long bonds and short Mn^{4+}-O bonds. This is strong evidence that charge localized and delocalized phases coexist close to the metal-insulator transition in the FM phase.Comment: 8 pages, 8 postscript figures, submitted to Phys. Rev.

Ordering and Fluctuation of Orbital and Lattice Distortion in Perovskite Manganese Oxides

Roles of orbital and lattice degrees of freedom in strongly correlated systems are investigated to understand electronic properties of perovskite Mn oxides such as La_{1-x}Sr_{x}MnO_{3}. An extended double-exchange model containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller coupling is derived under full polarization of spins with two-dimensional anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital degrees of freedom are investigated by using the quantum Monte Carlo method. In undoped states, it is crucial to consider both the Coulomb interaction and the Jahn-Teller coupling in reproducing characteristic hierarchy of energy scales among charge, orbital-lattice and spin degrees of freedom in experiments. Our numerical results quantitatively reproduce the charge gap amplitude as well as the stabilization energy and the amplitude of the cooperative Jahn-Teller distortion in undoped compounds. Upon doping of carriers, in the absence of the Jahn-Teller distortion, critical enhancement of both charge compressibility and orbital correlation length is found with decreasing doping concentration. These are discussed as origins of strong incoherence in charge dynamics. With the Jahn-Teller coupling in the doped region, collapse of the Jahn-Teller distortion and instability to phase separation are obtained and favorably compared with experiments. These provide a possible way to understand the complicated properties of lightly doped manganites.Comment: 22 pages RevTeX including 25 PS figures, submitted to Phys.Rev.B, replaced version; two figures are replaced by Fig.17 with minor changes in the tex