X-ray and neutron scattering measurements directly demonstrate the existence of polarons in the paramagnetic phase of optimally-doped colossal magnetoresistive oxides. The polarons exhibit short-range correlations that grow with decreasing temperature, but disappear abruptly at the ferromagnetic transition because of the sudden charge delocalization. The "melting" of the charge ordering as we cool through TC occurs with the collapse of the quasi-static polaron scattering, and provides important new insights into the relation of polarons to colossal magnetoresistance.PACS numbers: 75.30. Vn, 75.30.Et, 71.30.+h, 71.38.+i Manganese oxides have attracted tremendous interest because they exhibit colossal magnetoresistance (CMR) -a dramatic increase in the electrical conductivity when they order ferromagnetically. The basic relationship between ferromagnetism and conductivity in doped manganese oxides has been understood in terms of the doubleexchange mechanism [1,2], where an itinerant e g electron hops between Mn 4+ ions, providing both the ferromagnetic exchange and electrical conduction. In addition, an important aspect of the physics of manganese oxides is the unusually strong coupling among spin, charge, and lattice degrees of freedom [2,3]. These couplings can be tuned by varying the electronic doping, electronic bandwidth, and disorder, giving rise to a complex phase diagram in which structural, magnetic, and transport properties are intimately intertwined. The charge-ordered phases represent one of the most intriguing results of balancing these couplings, and have been observed at low temperature in insulating, antiferromagnetically ordered manganites, but are incompatible with double exchangemediated ferromagnetism seen in optimally-doped CMR systems.In comparison to the cubic manganites such as La 1−x A x MnO 3 (A=Sr, Ca, Ba), the two-layer Ruddlesden-Popper compounds La 2−2x Sr 1+2x Mn 2 O 7 [4], where x is the nominal hole concentration, are advantageous to study because the reduced dimensionality strongly enhances the spin and charge fluctuations. The crystal structure is body-centered tetragonal (space group I4/mmm) [5] with a ≃ 3.87Å and c ≃ 20.15 A, and consists of MnO 2 bilayers separated by (La,Sr)O sheets. In the intermediate doping regime (0.32 ≤ x < 0.42), the ground state is a ferromagnetic metal, and the magnetoresistance is found to be strongly enhanced near the combined metal-insulator and Curie transition at T C (112 K for the x=0.4 system of present interest [6]). The present results reveal diffuse scattering associated with lattice distortions around localized charges, i.e. polarons, in the paramagnetic phase. The formation of lattice polarons above the ferromagnetic transition temperature T C has been inferred from a variety of measurements [7], but detailed observation via diffuse x-ray or neutron scattering in single crystals has been lacking until now [8]. Through such measurements, we have observed the collapse of quasi-static polaron scattering when the metallic, ferromagnetic sta...

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The mechanism behind fast intersystem crossing in transition-metal complexes is shown to be a result of the dephasing of the photoexcited state to the phonon continuum of a different state with a significantly different transition metal-ligand distance. The coupling is a result of the spin-orbit interaction causing a change in the local moment. A recurrence to the initial state is prevented by the damping of the phonon oscillation. The decay time is faster than the oscillation frequency of the transition metal-ligand stretch mode, in agreement with experiment. For energies above the region where the strongest coupling occurs, a slower "leakage-type" decay is observed. If the photoexcited state is lower in energy than the state it couples to, then there is no decay.

We investigate the cascade decay mechanism for ultrafast intersystem crossing mediated by the spin-orbit coupling in transition-metal complexes. A quantum-mechanical description of the cascading process that occurs after photoexcitation is presented. The conditions for ultrafast cascading are given, which relate the energy difference between the levels in the cascading process to the electron-phonon self energy. These limitations aid in the determination of the cascade path. For Fe 2+ spin-crossover complexes, this leads to the conclusion that the ultrafast decay primarily occurs in the manifold of antibonding metal-to-ligand charge-transfer states. We also give an interpretation for why some intermediate states are bypassed.

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Using a model Hamiltonian, we discuss how we could interpret data obtained from transition metal K-edge resonant inelastic x-ray scattering (RIXS) experiments. By analyzing the creation of valence excitations from the screening of the core hole and calculating corresponding RIXS spectra for metals and insulators, we find that the probability for excitations depends not only on the total energy but also on the asymmetric screening dynamics between electrons and holes.

We have studied a model of bipolaron (negative-U) centers and magnetic impurities which includes a repulsive on-site Coulomb interaction as well as a coupling of the electronic on-site energy to the displacement amplitude of a local phonon mode. By means of a generalized Schrieffer-Wolff transformation, we derive an effective strong-coupling (or weak-hybridization)Hamiltonian for the low-lying states of the system. In the case of a bipolaron center (where the phonon-mediated on-site attraction dominates over the Coulomb repulsion), the effective Hamiltonian describes conductionelectron scattering and electron-pair tunneling processes between the localized impurity orbital and the conduction band of the metallic host. In the limit of very strong electron-phonon coupling, the pair tunneling matrix elements become exponentially small, whereas conduction electron scattering is suppressed only algebraically. The impurity orbital occupation number is then almost "frozen in" and the bipolaron center becomes inefficient in enhancing the superconducting pairing correlations of the host. In the case of a magnetic impurity (with a predominant repulsive Coulomb interaction), the effective Hamiltonian reduces to the form of an ordinary Kondo spin-exchange model. The spin-exchange coupling constants are not affected by the presence of the electron-phonon interaction.

In this paper, we examine the mean field electronic structure of the f 1 − f 2 Anderson lattice model in a slave boson approximation, which should be useful in understanding the physics of correlated metals with more than one f electron per site such as uranium-based heavy fermion superconductors. We find that the multiplet structure of the f 2 ion acts to quench the crystal field splitting in the quasiparticle electronic structure. This is consistent with experimental observations in such metals as U P t 3