arXiv subjects

Sumio Ishihara

Publications and source records attributed to Sumio Ishihara.

64 records · Page 4Linked to original sources

arXiv1999

Theory of Orbital Ordering, Fluctuation and Resonant X-ray Scattering in Manganites

A theory of resonant x-ray scattering in perovskite manganites is developed by applying the group theory to the correlation functions of the pseudospin operators for the orbital degree of freedom. It is shown that static and dynamical informations of the orbital state are directly obtained from the elastic, diffuse and inelastic scatterings due to the tensor character of the scattering factor. We propose that the interaction and its anisotropy between orbitals are directly identified by the intensity contour of the diffuse scattering in the momentum space.

cond-mat.str-el↗

arXiv1999

Orbital Ordering and Resonant X-ray Scattering in Layered Manganites

In layered manganites with orbital and charge orderings, the degeneracy of the Mn $4p$ orbitals as well as the $3d$ ones is lifted by the effects of the $4p$ bands and the local Coulomb interactions. We formulate the atomic scattering factor for the resonant x-ray scattering in the memory function method by taking into account these effects on an equal footing. It is shown that the polarization dependences of the scattering intensities at the orbital and charge superlattice reflections observed in LaSr$_{2}$Mn$_2$O$_7$ are caused by the local and itinerant characters of $4p$ electrons, respectively. We examine the type of the orbital ordered state.

cond-mat.str-el↗

arXiv1999

Temperature dependence of the resistivity in the double-exchange model

The resistivity around the ferromagnetic transition temperature in the double exchange model is studied by the Schwinger boson approach. The spatial spin correlation responsible for scattering of conduction electrons are taken into account by adopting the memory function formalism. Although the correlation shows a peak lower than the transition temperature, the resistivity in the ferromagnetic state monotonically increases with increasing temperature due to a variation of the electronic state of the conduction electron. In the paramagnetic state, the resistivity is dominated by the short range correlation of scattering and is almost independent of the temperature. It is attributed to a cancellation between the nearest-neighbor spin correlation, the fermion bandwidth, and the fermion kinetic energy. This result implies the importance of the temperature dependence of the electronic states of the conduction electron as well as the localized spin states in both ferromagnetic and paramagnetic phases.

cond-mat.str-el↗

arXiv1998

Theory of Orbital Excitation and Resonant Inelastic X-ray Scattering in Manganites

We study theoretically the collective orbital excitation named orbital wave in the orbital ordered manganites.The dispersion relation of the orbital wave is affected by the static spin structure through the coupling between spin and orbital degrees of freedom. As a probe to detect the dispersion relation, we propose two possible methods by utilizing resonant inelastic x-ray scattering. The transition probability of the orbital wave scattering is formulated, and the momentum and polarization dependences of the structure factor are calculated in several types of the orbital and spin structures. The elastic x-ray scattering in the L-edge case to observe the orbital ordering is also discussed.

cond-mat.str-el↗

arXiv1998

Polarization Dependence of Anomalous X-ray Scattering in Orbital Ordered Manganites

In order to determine types of the orbital ordering in manganites, we study theoretically the polarization dependence of the anomalous X-ray scattering which is caused by the anisotropy of the scattering factor. The general formulae of the scattering intensity in the experimental optical system is derived and the atomic scattering factor is calculated in the microscopic electronic model. By using the results, the X-ray scattering intensity in several types of the orbital ordering is numerically calculated as a function of azimuthal and analyzer angles.

cond-mat.str-el↗

arXiv1998

Phase Diagram in Manganese Oxides

We study theoretically the phase diagram of perovskite manganites taking into account the double degeneracy of the $e_g$ orbitals in a $Mn^{3+}$ ion. A rich phase diagram is obtained in the mean field theory at zero temperature as functions of $x$ (hole concentration) and $J_S$ (antiferromagnetic interaction between $t_{2g}$ spins). The global features of the phase diagram is understood in terms of the superexchange and double exchange interactions, which are strongly depends on types of the occupied $e_g$ orbitals. The strong electron correlation induces the orbital polarization, which controls the dimension of the conduction band. A sequential change of the spin and orbital structures with doping holes is consistent with the recent experiments. In particular, metallic A-type (layered) antiferromagnetic state is found for $x\sim0.5$ with the uniform $d_{x^2-y^2}$ orbital ordering. Effects of the Jahn-Teller distortion are also studied. A short version of this paper has been already published (to appear in PRB-rapid June), but this paper contains additional and more detailed results.

cond-mat.str-el↗

arXiv1997

Theory of Anomalous X-ray Scattering in Orbital Ordered Manganites

We study theoretically the anomalous X-ray scattering as a new probe to observe the orbital orderings and excitations in perovskite manganites. The scattering matrix is given by the virtual electronic excitations from Mn $1s$ level to unoccupied Mn $4p$ level. We find that orbital dependence of the Coulomb interaction between Mn $3d$ and Mn $4p$ electrons is essential to bring about the anisotropy of the scattering factor near the K edge. The calculated results in $MnO_6$ clusters explain the forbidden reflections observed in $La_{0.5}Sr_{1.5}MnO_4$ and $LaMnO_3$. A possibility of the observation of the orbital waves by the X-ray scattering is discussed.

cond-mat.str-el↗

arXiv1997

Pressure Effects in Manganites with Layered Perovskite Structure

Pressure effects on the charge and spin dynamics in the bilayer manganite compounds $La_{2-2x}Sr_{1+2x}Mn_2O_7$ are studied theoretically by taking into account the orbital degrees of freedom. The orbital degrees are active in the layered crystal structure, and applied hydrostatic pressure stabilizes the $3d_{x^2-y^2}$ orbital in comparison with $3d_{3z^2-r^2}$. The change of the orbital states weakens the interlayer charge and spin couplings, and suppresses the three dimensional ferromagnetic transition. Numerical results, based on an effective Hamiltonian which includes the energy level difference of the orbitals, show that the applied pressure controls the dimensionality of the spin and charge dynamics through changes of the orbital states.

cond-mat.str-el↗

arXiv1997

Phase Diagram in R_{1-x}A_xMnO_3

We study the phase diagram of R_{1-x}A_xMnO_3 (R=La, Pr, Nd, Sm ; A=Ca, Sr, Ba) by taking into account the degeneracy of $e_g$ orbitals and the anisotropy of the transfer integral. The electron-electron interaction is treated in the mean field approximation with the optimization of the spin and orbital structures. The global phase diagram is understood in terms of the two interactions, i.e., the super exchange interaction for small $x$ and the double exchange interaction for larger $x$ modified by the orbital degeneracy. The dimensionality of the electronic energy band resulting from the orbital structure is essential to determine the phase diagram. The effects of the Jahn-Teller distortion are also studied.

cond-mat.str-el↗

arXiv1996

Orbital Liquid in Perovskite Transition-Metal Oxides

We study the effects of the degeneracy of the $e_g$ orbitals as well as the double exchange interaction with $t_{2g}$ spins in perovskite transition-metal oxides. In addition to the spin field ${\vec S}_i$, the isospin field ${\vec T}_i$ is introduced to describe the orbital degrees of freedom. The isospin is the quantum dynamical variable, and is represented by the boson with a constraint. The dispersion of this boson is flat along $(π/a,π/a,k_z)$ ($a$: lattice constant) and the other two equivalent directions. This enables the orbital disordered phase down to low temperatures. We interpret some of the anomalous experiments, i.e., optical absorption and d.c. resistivity, in the low temperature ferromagnetic phase of La$_{1-x}$Sr$_x$MnO$_3$ with $x > 0.2$ in terms of this orbital liquid picture.

cond-mat↗