Nobuya Maeshima

Photoinduced Metallic Properties of One-Dimensional Strongly Correlated Electron Systems

We study photoinduced optical responses of one-dimensional strongly correlated electron systems. Optical conductivity spectra are calculated for the ground and photoexcited states in a one-dimensional Hubbard model at half filling by the exact diagonalization method. It is found that, in the Mott insulator phase, the photoexcited state has large spectral weights including the Drude weight below the optical gap. As a consequence, the spectral weight above the optical gap is markedly reduced. These results imply that a metallic state is induced by photoexcitation. A comparison between the photoexcited and hole-doped states shows that photoexcitation is similar to chemical doping.

Dynamics of photoexcited states in one-dimensional dimerized Mott insulators

Dynamical properties of photoexcited states are theoretically studied in a one-dimensional Mott insulator dimerized by the spin-Peierls instability. Numerical calculations combined with a perturbative analysis have revealed that the lowest photoexcited state without nearest-neighbor interaction corresponds to an interdimer charge transfer excitation that belongs to dispersive excitations. This excited state destabilizes the dimerized phase, leading to a photoinduced inverse spin-Peierls transition. We discuss the purely electronic origin of midgap states that are observed in a latest photoexcitation experiment of an organic spin-Peierls compound, K-TCNQ (potassium-tetracyanoquinodimethane).

Photoexcitation-Energy-Dependent Transition Pathways from a Dimer Mott Insulator to a Metal

We theoretically study pump-photon-energy-dependent pathways of a photoinduced dimer-Mott-insulator-to-metal transition, on the basis of numerical solutions to the time-dependent Schrodinger equation for the exact many-body wave function of a two-dimensional three-quarter-filled extended Peierls–Hubbard model. When molecular degrees of freedom inside a dimer are utilized, photoexcitation can weaken the effective interaction or increase the density of photocarriers. In the organic dimer Mott insulator, κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br, the intradimer and the interdimer charge-transfer excitations have broad bands that overlap with each other. Even in this disadvantageous situation, the photoinduced conductivity change depends largely on the pump photon energy, confirming the two pathways recently observed experimentally. The characteristic of each pathway is clarified by calculating the modulation of the effective interaction and the number of carriers involved in low-energy optical excitations. The pump-phot...

Polaronic quasiparticle picture for generation dynamics of coherent phonons in semiconductors: Transient and nonlinear Fano resonance

We examine generation dynamics of coherent phonons (CPs) in both of polar and non-polar semiconductors -- such as GaAs and Si -- based on a polaronic-quasiparticle (PQ) model. In the model concerned, the PQ operator is composed of two kinds of operators. One is a quasiboson operator -- defined as a linear combination of a set of pairs of electron operators -- and the other is a longitudinal optical (LO) phonon operator. The problem of transient and non-linear Fano resonance (FR) is tackled in particular; the vestige of this quantum interference effect was observed exclusively in lightly

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

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Magnetic Field versus Temperature Phase Diagram of the Spin-1/2 Alternating-Bond Chain Compound F5PNN

-doped Si immediately after carriers were excited by an ultrashort pulse-laser [M. Hase et. al., Nature 426, 51 (2003)], though not observed yet in GaAs. It is shown that the phonon energy state is embedded in a continuum state formed by a set of adiabatic eigenstates of the quasiboson. This result implies the possibility of manifestation of the transient FR in the present optically-non-linear system.

Photoinduced melting and charge order in quarter-filled organic conductors: Itinerant electron systems with competing interactions

CREST, JST, CREST, Tokyo 102-0075, Japan(Dated: November 29, 2010)We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedomin the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schr¨odingerequation for the numerically exact many-electron wave function, we obtain time-dependent opticalresponse functions. The calculated spectra show characteristic coherent oscillations that vary withthe frequency of probe light. A simple analysis for the dominant oscillating components clarifiesthat these photoinduced oscillations are caused by the quantum interference between photogeneratedstates. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appearsaround the charge-transfer peak.

Laser-controlled exciton Fano resonance in semiconductor superlattices

We have measured the specific heat of the S = 1/2 alternating-bond Heisenberg antiferromagnetic chain compound pentafluorophenyl nitronyl nitroxide in magnetic fields using a single crystal and powder. A sharp peak due to field-induced magnetic ordering (FIMO) is observed in both samples. The H – T phase boundary of the FIMO of the single crystal is symmetric with respect to the central field of the gapless field region H C1 ≤ H ≤ H C2 , whereas it is distorted for the powder whose ordering temperatures are lower. We discuss possibility that an effective pressure caused by mixing the powder with grease, which is reported for various organic compounds, plays an important role for the distorted phase boundary. An analysis employing calculations based on the finite temperature density matrix renormalization group suggests that the pressure-induced frustration enhances incommensurate spin correlation leading to the distorted phase boundary for the powder.

Parallelization of the R-matrix propagation method for the study of intense-laser-driven semiconductor superlattices

Photoinduced charge dynamics in one- and two-dimensional organic conductors are studied theoretically in extended Peierls-Hubbard models. For quasi-one-dimensional (EDO-TTF)2PF6, photoinduced change in the charge order pattern from (0110) to (1010) is accompanied by probe-energy-dependent oscillations of conductivity. This is caused by coexistence of charge order and delocalized electrons. For quasi-two-dimensional α-(BEDT-TTF)2I3 and θ-(BEDT-TTF)2RbZn(SCN)4, photoinduced melting of the horizontal-stripe charge order proceeds easier in the α-type salt than in the θ-type salt. This is because the charge order in the θ-type salt is more strongly stabilized by electron-phonon interactions.

Quantum Generation Dynamics of Coherent Phonons: Analysis of Transient Fano Resonance

Quantum control of excitonic Floquet states in semiconductor superlattices driven by an intense monochromatic laser is examined from a theoretical point of view. High-resolution optical absorption spectra, calculated using multichannel scattering theory with the R-matrix propagation method, clarified that the excitonic Fano resonance structure is induced by the laser field. Each of the physical quantities related to this resonance-such as the spectral intensity, an asymmetry parameter (q-parameter), a resonance width, and so on-shows a characteristic extremum as a function of laser strength (F(ac)) in the vicinity of a critical value of F(ac) where dynamic localization is realized. It has also been shown that this F(ac)-dependence is caused by an ac-Zener coupling between two photon sidebands. Further, we have shown that these quantities are also controlled by changing the laser frequency (ω), as well as F(ac), and the underlying physics is explained on the basis of anticrossing behavior of the two photon sidebands.