Tetsuya Mutou

CHARGE-ORBITAL STRIPE STRUCTURE IN LA1-XCAXMNO3 (X = 1/2, 2/3)

We propose the origin of the charge-ordered stripe structure with the orbital ordering observed experimentally in La_(1-x)Ca_(x)MnO_(3) (x=1/2, 2/3), in which the long-range Coulomb interaction plays an essential role. We study a Hubbard model with doubly-degenerate e_g orbitals, and treat the on-site Coulomb interaction (U) and the nearest-neighbor one (V) with the Hartree-Fock approximation. Both the charge and orbital ordering structures observed in experiments are reproduced in a wide region of the U-V phase diagram determined by the present study. The stability of the orbital ordering is also confirmed by the perturbation theory.

Temperature-Induced Gap Formation in Dynamic Correlation Functions of the One-Dimensional Kondo Insulator: Finite-Temperature Density-Matrix Renormalization-Group Study

Combination of the finite-temperature density-matrix renormalization-group and the maximum entropy presents a new method to calculate dynamic quantities of one-dimensional many-body systems. In the present paper, density of states, local dynamic spin and charge correlation functions of the one-dimensional Kondo insulator are discussed. Excitation gaps open with decreasing temperature and the gaps take different values depending on channels. The excitation spectra change qualitatively around the characteristic temperature corresponding to the spin gap which is the lowest energy scale of the Kondo insulator.

Origin of the charge-ordered stripe with the orbital ordering in manganese oxides

Abstract The origin of the charge-ordered stripe with the orbital ordering observed experimentally in some manganese oxides is proposed. It is emphasized that the long-range Coulomb interaction plays an essential role. We study a Hubbard model with doubly degenerate orbitals, and treat the on-site Coulomb interaction ( U ) and the nearest-neighbor one ( V ) with the Hartree–Fock approximation. Charge-orbital stripe structures observed in experiments are reproduced in a wide region of the U – V phase diagram. The stability of the stripe structure is confirmed by perturbation theory.

Melting of an antiferromagnetic Heisenberg spin chain by external magnetic fields

We study the magnetic-field-induced insulator - metal transition in the chain model at quarter filling. In this model the nearest-neighbour hopping energy t competes with the next-nearest-neighbour exchange energy , and for a small the system is an insulator accompanied by a charge ordering. Response to external magnetic fields is investigated numerically by the density-matrix renormalization-group method. We have found that a transition from the charge ordered insulating phase to a metallic phase occurs at a finite strength of the magnetic field for small . The transition is an example of magnetic-field-induced melting transition of a quantum antiferromagnetic Heisenberg spin chain system.