Tsezar F. Seman

Effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites

We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation between NdMnO3 and LaMnO3, and obtain the relations between distortions. We find good agreement between theoretical results and experimental data.

Role of Complex Energy Landscapes and Strains in Multiscale Inhomogeneities in Perovskite Manganites

Perovskite manganites have attracted considerable attention recently due to inhomogeneities in multi-functional properties, observed by various high resolution probes. We present an analysis of the essential role played by complex energy landscapes in the nanometer to micron-scale inhomogeneities observed in perovskite manganites using a model expressed in terms of symmetrized atomic-scale lattice distortion modes. We also discuss the origin for the stability of large metal and insulator domains in the absence of defects. We demonstrate that an intrinsic mechanism, which specifically involves longrange interactions between strain fields, the Peierls-Nabarro energy barrier, and complex energy landscapes with multiple metastable states is responsible for the inhomogeneities in perovskite manganites. This is in contrast to an extrinsic mechanism such as chemical randomness. We highlight experimental results which support our intrinsic, rather than extrinsic, mechanism.

Strongly momentum-dependent screening dynamics in La0.5Sr1.5MnO4 observed with resonant inelastic x-ray scattering

We report strongly momentum-dependent short-ranged charge screening dynamics in CE-type charge, orbital, and spin ordered La0.5Sr1.5MnO4, based on Mn K-edge resonant inelastic x-ray scattering data. Through a comparison with theoretical calculations, we show that the observed momentum dependence reflects highly localized, nearest-neighbor screening of the transient local charge perturbation in this compound with an excitonlike screening cloud, rather than delocalized screening. The size of the screening cloud is estimated to be about 0.4-0.5 interatomic distances.

Photoinduced nonequilibrium dynamics in charge ordered materials

We study the nonequilibrium dynamics of photoinduced phase transitions in charge ordered (CO) systems with a strong electron-lattice interaction and analyze the interplay between electrons, periodic lattice distortions, and a phonon thermal reservoir. Simulations based on a tight-binding Hamiltonian and Boltzmann equations reveal partially decoupled oscillations of the electronic order parameter and the periodic lattice distortion during CO melting, which becomes more energy efficient with lower photon energy. The cooling rate of the electron system correlates with the CO gap dynamics, responsible for an order of magnitude decrease in the cooling rate upon the gap reopening. We also find that the time-dependent frequency of coherent oscillation reflects the dynamics of the energy landscape, such as transition between single-well and double-well, which sensitively depends on the photon energy and the pump fluence. The results demonstrate the intricate nonequilibrium dynamics in CO materials.

Momentum dependence in K-edge resonant inelastic x-ray scattering and its application to screening dynamics in CE-phase La0.5Sr1.5MnO4

We present a formula for the calculation of K-edge resonant inelastic x-ray scattering on transition-metal compounds, based on a local interaction between the valence shell electrons and the 1s core hole. Extending a previous result, we include explicit momentum dependence and a basis with multiple core-hole sites. We apply this formula to a single-layered charge-, orbital-, and spin-ordered manganite, La0.5Sr1.5MnO4, and obtain good agreement with experimental data, in particular with regards to the large variation of the intensity with momentum. We find that the screening in La0.5Sr1.5MnO4 is highly localized around the core-hole site and demonstrate the potential of K-edge resonant inelastic x-ray scattering as a probe of screening dynamics in materials.