Photoinduced coherent oscillations in the one-dimensional two-orbital Hubbard model

We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schr\"odinger equation for the numerically exact many-electron wave function, we obtain time-dependent optical response functions. The calculated spectra show characteristic coherent oscillations that vary with the frequency of probe light. A simple analysis for the dominant oscillating components clarifies that these photoinduced oscillations are caused by the quantum interference between photogenerated states. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appears around the charge-transfer peak.Comment: 5 pages, 5 figure

Frustration-induced eta inversion in the S=1/2 bond-alternating spin chain

We study the frustration-induced enhancement of the incommensurate correlation for a bond-alternating quantum spin chain in a magnetic field, which is associated with a quasi-one-dimensional organic compound F5PNN. We investigate the temperature dependence of the staggered susceptibilities by using the density matrix renormalization group, and then find that the incommensurate correlation becomes dominant in a certain range of the magnetic field. We also discuss the mechanism of this enhancement on the basis of the mapping to the effective S=1/2 XXZ chain and a possibility of the field-induced incommensurate long range order.Comment: 4 pages, 5 figures, replaced with revised version accepted to PR

Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators

We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp

Fractional S^z excitation and its bound state around the 1/3 plateau of the S=1/2 Ising-like zigzag XXZ chain

We present the microscopic view for the excitations around the 1/3 plateau state of the Ising-like zigzag XXZ chain. We analyze the low-energy excitations around the plateau with the degenerating perturbation theory from the Ising limit, combined with the Bethe-form wave function. We then find that the domain-wall particles carrying $S^z=\pm 1/3$ and its bound state of $S^z=\pm 2/3$ describe well the low-energy excitations around the 1/3 plateau state. The formation of the bound state of the domain-walls clearly provides the microscopic mechanism of the cusp singularities and the even-odd behavior in the magnetization curve.Comment: 13 pages, 15 figure

Photoinduced charge and spin dynamics in strongly correlated electron systems

Motivated by photoinduced phase transition in manganese oxides, charge and spin dynamics induced by photoirradiation are examined. We calculate the transient optical absorption spectra of the extended double-exchange model by the density matrix renormalization group (DMRG) method. A charge-ordered insulating (COI) state becomes metallic just after photoirradiation, and the system tends to recover the initial COI state. The recovery is accompanied with remarkable suppression of an antiferromagnetic correlation in the COI state. The DMRG results are consistent with recent pump-probe spectroscopy data.Comment: 5 pages, 4 figure

Production of Kaluza-Klein States at Future Colliders

Perturbative breaking of supersymmetry in four-dimensional string theories predict in general the existence of new large dimensions at the TeV scale. Such large dimensions lie in a domain of energies accessible to particle accelerators. Their main signature is the production of Kaluza-Klein excitations which can be detected at future colliders. We study this possibility for hadron colliders (TEVATRON, LHC) and $e^+ e^-$ colliders (LEP-200, NLC-500).Comment: 13 pages, LATEX, 4 postscript figures appended at the end, CPTH-A293.0294 and IEM-FT-84/9

Magnetic phase diagram of the S=1/2 antiferromagnetic zigzag spin chain in the strongly frustrated region: cusp and plateau

We determine the magnetic phase diagram of the antiferromagnetic(AF) zigzag spin chain in the strongly frustrated region, using the density matrix renormalization group method. We find the magnetization plateau at 1/3 of the full moment accompanying the spontaneous symmetry breaking of the translation, the cusp singularities above and/or below the plateau, and the even-odd effect in the magnetization curve. We also discuss the formation mechanisms of the plateau and cusps briefly.Comment: 4 pages, 8 figures, revised version, to appear in J.Phys.Soc.Jp

Self-Consistent Tensor Product Variational Approximation for 3D Classical Models

We propose a numerical variational method for three-dimensional (3D) classical lattice models. We construct the variational state as a product of local tensors, and improve it by use of the corner transfer matrix renormalization group (CTMRG), which is a variant of the density matrix renormalization group (DMRG) applied to 2D classical systems. Numerical efficiency of this approximation is investigated through trial applications to the 3D Ising model and the 3D 3-state Potts model.Comment: 12 pages, 6 figure