Roles of orbital in magnetoelectronic properties of colossal magnetoresistive manganites (original) (raw)
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Theory of Manganites Exhibiting Colossal Magnetoresistance
2004
The electronic properties of many transition metal oxide systems require new ideas concerning the behaviour of electrons in solids for their explanation. A recent example, subsequent to that of cuprate superconductors, is of rare earth manganites doped with alkaline earths, namely Re 1−x A x M nO 3 , which exhibit colossal magnetoresistance, metal insulator transition and many other poorly understood phenomena. Here we show that the strong Jahn Teller coupling between the twofold degenerate (d x 2 −y 2 and d 3z 2 −r 2) e g orbitals of M n and lattice modes of vibration (of the oxygen octahedra surrounding the M n ions) dynamically reorganizes the former into a set of states (which we label ℓ) which are localized with large local lattice distortion and exponentially small intersite overlap, and another set (labelled b) which form a broad band. This hitherto unsuspected but microscopically inevitable coexistence of radically different ℓ and b states, and their relative energies and occupation as influenced by doping x, temperature T , local Coulomb repulsion U etc., underlies the unique effects seen in manganites. We present results from strong correlation calculations using the dynamical mean-field theory which accord with a variety of observations in the orbital liquid regime (say, for 0.2
Colossal magnetoresistance manganites: A new approach
2003
Manganites of the LA 1-x Ca x MnO 3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn-Teller (JT) coupling with phonons the twofold degenerate e g states at the Mn sites dynamically reorganize themselves into localised, JT polarons l with exponentially small inter-site hopping, and band-like, nonpolaronic states b, leading to a new 2-band model for manganites which includes strong Coulomb and Hund's couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulatormetal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of the l and b states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.
Structural effect on colossal magnetoresistivity in manganites: Bond versus band
The atomic structure is an important component in the mechanism of the colossal magnetoresistance ͑CMR͒ phenomenon mainly through the ionic size effect. In the current understanding, this factor controls the CMR phenomenon by changing the electron bandwidth, while the estimated changes in the bandwidth appear to be too small to be of significance. By way of the atomic pair distribution function analysis of pulsed neutron diffraction data of model compounds, we suggest that the principal effect of the ionic size is to change the polaron formation energy through the change in the local elastic response.
Direct Observation of High-Temperature Polaronic Behavior in Colossal Magnetoresistive Manganites
Physical Review Letters, 2004
The temperature dependence of the electronic and atomic structure of the colossal magnetoresistive oxides La1−xSrxMnO3La_{1-x}Sr_{x}MnO_{3}La1−xSrxMnO3 (x = 0.3, 0.4) has been studied using core and valence level photoemission, x-ray absorption and emission, and extended x-ray absorption fine structure spectroscopy. A dramatic and reversible change of the electronic structure is observed on crossing the Curie temperature, including charge localization and spin moment increase of Mn, together with Jahn-Teller distortions, both signatures of polaron formation. Our data are also consistent with a phase-separation scenario.
Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites
Journal of Physics-condensed Matter, 2010
Correlation-induced spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites are calculated with realistic parameters in order to provide a quantitative comparison with experimental results for spin stiffness, magnon dispersion, magnon damping, anomalous zone-boundary magnon softening, and Curie temperature. The role of orbital degeneracy, orbital ordering, and orbital correlations on spin dynamics in different doping regimes is highlighted.
Limited local electron-lattice coupling in manganites: An electron diffraction study
Physical Review B, 2008
Pr,Ca)MnO 3 is the archetypal charge-ordered manganite, but in Pr 0.48 Ca 0.52 MnO 3 we find (using convergent-beam electron diffraction and dark-field images) that the superlattice period is locally incommensurate with respect to the parent lattice, and that the superlattice orientation possesses significant local variations. This suggests that local electron-lattice coupling never overwhelmingly dominates the rich physics of manganites, even in the most extreme scenarios that produce the largest colossal magnetoresistance effects.
Lattice and superexchange effects in doped CMR manganites
Journal of Magnetism and Magnetic Materials, 2004
We report on the influence of the lattice degrees of freedom on charge, orbital and spin correlations in colossal magnetoresistance (CMR) manganites. For the weakly doped compounds we demonstrate that the electron-phonon coupling promotes the trapping of charge carriers, the disappearance of the orbital polaron pattern and the breakdown of ferromagnetism at the CMR transition. The role of different superexchange interactions is explored.
Microscopic spin interactions in colossal magnetoresistance manganites
Physical Review B, 2002
Using inelastic neutron scattering we measured the microscopic magnetic coupling associated with the ferromagnetic clusters of the ''colossal magnetoresistance'' compound Pr 0.70 Ca 0.30 MnO 3. When the insulatingto-metal ͑I-M͒ transition is induced by an external magnetic field there is a discontinuous change in the spin-wave stiffness constant. This result implies that the I-M transition is not achieved by the simple percolation of micron-sized metallic clusters as currently believed, but involves a first-order transformation.
Physical Review B, 2006
The one-and two-orbital double-exchange models for manganites are studied using Monte Carlo computational techniques in the presence of a robust electron-phonon coupling ͑but neglecting the antiferromagnetic exchange J AF between the localized spins͒. The focus in this effort is on the analysis of charge transport. Our results for the one-orbital case confirm and extend previous recent investigations that showed the presence of robust peaks in the resistivity versus temperature curves for this model. Quenched disorder substantially enhances the magnitude of the effect, while magnetic fields drastically reduce the resistivity. A simple picture for the origin of these results is presented. It is also shown that even for the case of just one electron, the resistance curves present metallic and insulating regions by varying the temperature, as it occurs at finite electronic density. Moreover, in the present study these investigations are extended to the more realistic two-orbital model for manganites. The transport results for this model show large peaks in the resistivity versus temperature curves, located at approximately the Curie temperature, and with associated large magnetoresistance factors. Overall, the magnitude and shape of the effects discussed here resemble experiments for materials such as La 0.70 Ca 0.30 MnO 3 , and they are in agreement with the current predominant theoretical view that competition between a metal and an insulator, enhanced by quenched disorder, is crucial to understanding the colossal magnetoresistance ͑CMR͒ phenomenon. However, it is argued that further work is still needed to fully grasp the experimentally observed CMR effect, since in several other Mn oxides an antiferromagnetic chargeordered orbital-ordered state is the actual competitor of the ferromagnetic metal.
Physical Review B, 2003
We report neutron scattering results on the spin dynamics in the paramagnetic ͑PM͒ and ferromagnetic ͑FM͒ states of the 50% hole-doped manganites Pr 1/2 Sr 1/2 MnO 3 and Nd 1/2 Sr 1/2 MnO 3. In the PM phase, these systems exhibit two kinds of diffuse scatterings: an isotropic quasielastic diffuse scattering and a dynamical ridge-type, i.e., two-dimensional FM diffuse scattering. With decreasing temperature, both systems enter the pure FM metallic state, but the spin dynamics in the FM state possess a strong A-type antiferromagnetic feature. In fact, Nd 1/2 Sr 1/2 MnO 3 enters a canted antiferromagnetic phase at lower temperature through a second order phase transition. These behaviors in spin dynamics demonstrate the existence of the static d x 2 Ϫy 2-type orbital ordering in the PM and FM states of 50% hole-doped manganites which have relatively larger one-electron bandwidth.