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arXiv:1511.00365 · Physical review. B./Physical review. B

Photoinduced correlated electron dynamics in a two-leg ladder Hubbard system

Hiroshi Hashimoto, Sumio Ishihara

Published Apr 22, 2016• 17 citations
arXivHTMLPDF

Abstract

Photoinduced carrier dynamics in a correlated electron system on a coupled two-leg ladder lattice are studied. The two-leg ladder Hubbard model is analyzed utilizing the exact diagonalization method based on the Lanczos algorithm in finite-size clusters. In order to reveal the transient carrier dynamics after photoirradiation, we calculate the low-energy components of the hole kinetic energy, the pair-field correlation function, the optical conductivity spectra, and other characteristics. It is shown that the photoinduced metallic-like state appears in a half-filled Mott insulating state, while the low-energy carrier motion is suppressed by photoirradiation in the case of hole-doped metallic states. These photoinduced changes in the electron dynamics are associated with changes in the carrier-pair coherence; they are not attributed to a naive thermalization, and are associated with a ladder-lattice effect instead. Based on the numerical results, optical control of hole pairs using double-pulse pumping is demonstrated. Implications for recent optical pump-probe experiments are presented.

DOI
10.1103/physrevb.93.165133

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Photoinduced carrier dynamics in a correlated electron system on a coupled two-leg ladder lattice are studied. The two-leg ladder Hubbard model is analyzed utilizing the exact diagonalization method based on the Lanczos algorithm in finite-size clusters. In order to reveal the transient carrier dynamics after photoirradiation, we calculate the low-energy components of the hole kinetic energy, the pair-field correlation function, the optical conductivity spectra, and other characteristics. It is shown that the photoinduced metallic-like state appears in a half-filled Mott insulating state, while the low-energy carrier motion is suppressed by photoirradiation in the case of hole-doped metallic states. These photoinduced changes in the electron dynamics are associated with changes in the carrier-pair coherence; they are not attributed to a naive thermalization, and are associated with a ladder-lattice effect instead. Based on the numerical results, optical control of hole pairs using double-pulse pumping is demonstrated. Implications for recent optical pump-probe experiments are presented.
The charge carrier dynamics of doped electronic correlated systems on ladders and chains, subject to ultrafast photoirradiation, is investigated using the time-dependent Lanczos method. The time-resolved optical conductivity and the temporal profiles of other relevant quantities, including the doublon number, the kinetic energy, and the interaction energy, are calculated. Two competitive factors that can influence the transient charge carrier dynamics are identified as the thermal effect and the charge effect. We demonstrate that the analysis of their interplay can provide an intuitive way to understand the numerical results and the recent optical pump-probe experiment on a two-leg ladder cuprate.
We study photoinduced optical responses of one-dimensional strongly correlated electron systems. The optical conductivity spectra are calculated for the ground state and a photoexcited state in the one-dimensional Hubbard model at half filling by using 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 largely reduced. These results imply that a metallic state is induced by photoexcitation. Comparison between the photoexcited and hole-doped states shows that the photoexcitation is similar to chemical doping.
Photoinduced nonequilibrium states in the Mott insulators reflect the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. The spin degrees of freedom will also contribute to the competition in a different manner depending on lattice geometry. We investigate pulse-excited optical responses of a half-filled two-leg Hubbard ladder and compare them with those of a one-dimensional extended Hubbard chain. Calculating the time-dependent optical conductivity, we find that strong monocycle pulse inducing quantum tunneling gives rise to anomalous suppression of photo-induced in-gap weight, leading to negative weight. This is in contrast to finite positive weight in the Hubbard chain. The origin of this anomalous behavior in the two-leg ladder is attributed to photoinduced localized exciton that reflects strong spin-singlet dimer correlation in the ground state.
Photoinduced spin and charge dynamics in double-exchange model are numerically studied. The Lanczos method and the density-matrix renormalization-group method are applied to one-dimensional finite-size clusters. By photon irradiation in a charge-ordered (CO) insulator associated with antiferromagnetic (AFM) correlation, both the CO and AFM correlations collapse rapidly, and appearances of new peaks inside of an insulating gap are observed in the optical spectra and the one-particle excitation spectra. Time evolutions of the spin correlation and the in-gap state are correlated with each other, and are governed by the transfer integral of conduction electrons. Results are interpreted by the charge kink/antikink picture and their effective motions which depend on the localized spin correlation. Pump-photon density dependence of spin and charge dynamics are also studied. Roles of spin degree of freedom are remarkable in a case of weak photon density. Implications of the numerical results for the pump-probe experiments in perovskite manganites are discussed.
We study the optical properties of an electron–hole two-band Hubbard model with interactions of electron–electron (hole–hole) repulsion U and electron–hole attraction U '. In the case of two types of correlated insulators at half filling, viz. , the Mott–Hubbard type and the biexciton type, interband linear absorption/gain spectra including excitonic effects are calculated analytically using two-site dynamical mean-field theory and ladder approximation. We show that an exciton-like peak structure emerges because of an “exciton” in the sea of correlated fermions when U ' is larger than a certain critical value. The binding energy of the photoinduced excitonic bound state and the spectral intensity are evaluated as a function of U '. The emergence and absence of the exciton-like peak suggests that there are two different insulating mechanisms in the Mott–Hubbard insulating state of the electron–hole system: one is attributable to only U and the other is attributable to both U and U '; the latter is called the excitonic Mott–Hubbard insulating state. The intrinsic difference of the mechanisms cannot be captured from only single-particle properties such as the density of states, but it appears clearly in optical properties reflecting two-particle correlations.
The photoexcited state in a correlated electron system with a spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where the energy difference between the two orbitals is introduced. The photoexcited metastable state is examined based on the effective model Hamiltonian derived using the two-orbital Hubbard model. Spin-state change is induced by photoirradiation in the low-spin band insulator near the phase boundary, and it is found that a high-spin state is stabilized by creating a ferromagnetic bound state with photodoped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside the bound state. The time evolution of photoexcited states is simulated in the time-dependent mean-field scheme. It is found that pair annihilations of photodoped electrons and holes generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.