Relaxation Dynamics of Photocarriers in One-Dimensional Mott Insulators Coupled to Phonons

Abstract

It is a fundamental task in strongly correlated electron systems to examine interdependence among spin, charge, orbital, and lattice degrees of freedom. Recently, the examinations of the interdependence under nonequilibrium conditions become quite urgent issues in various on-going subjects. Ultrafast transient optics is an approach exploring new functionalities of materials and observing properties hidden in equilibrium conditions. The most important subject is photoinduced phase transition of low-dimensional transition metal oxides and organic materials with strong electron correlation.One-dimensional (1D) Mott insulators, Sr 2 CuO 3 and halogen-bridged Ni compounds, show photoinduced insulator-to-metal transition accompanied with its picosecond recovery to the insulating state. 1,2 The time scale of the recovery is three order of magnitude faster than that for semiconductors. 3 The 1D Mott insulators also exhibit gigantic third-order optical nonlinearity, and because of these two properties, they are promising future opto-electronics materials. 4 The complete description of optically excited Mott insulators is thus desired, but key theoretical concepts in nonequilibrium are still under construction.The photocarriers of the 1D Mott insulators are called holon and doublon representing empty and doubly occupied sites, respectively. A holon and a doublon recombine by emitting energy to other elementary excitations. A problem is to clarify a pass way of energy dissipation due to the recombination. Two possible candidates are spin and phonon excitations, since antiferromagnetic (AF) exchange energy and highest phonon frequencies are of the same order (∼0.1 eV). 5 Since high-energy states created by optical excitation may violate the separation of spin and charge degrees of freedom inherent in 1D electron systems, 6 a pass way for energy dissipation through a spin channel can be expected. However, recent numerical studies have shown robustness of the spin-charge separation for nonequilibrium steady states. 7, 8 It is thus necessary to make clear a coupling of spin and charge degrees of freedom under photoirradiation. As for phonon relaxation, pump-probe experiments have been done for various TTF-TCNQ salts with different magnitudes of electron-phonon (EP) coupling. 9 K-and Rb-TCNQ show spin-Peierls (SP) transition at T c =395 K and 220 K, respectively, and their photocarriers are once localized as polarons at around 70 fs, and then recombine with a few ps. On the other hand, ET-F 2 TCNQ does not show SP transition, and metallic photocarriers decay within 200 fs. Therefore, a fundamental question to be answered is about what is driving force of ultrafast relaxation of the 1D Mott insulators when the charge carriers couple weakly with spin and lattice. Since EP coupling is also present in semiconductors, we need to answer another question why phonon relaxation in the Mott insulators is much faster than that in the semiconductors.In this Letter, we incorporate time dependent vector potential of laser pulse into density-m...

Photoexcitation of electronic instabilities in one-dimensional charge-transfer systems

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Exciton recombination in one-dimensional organic Mott insulators

“…In the following we will show that a multiphonon emission can be a viable recombination mechanism in 1D organic Mott insulators, somewhat specific to organic materials with energetic intramolecular vibrations and strong electron-phonon coupling [17,18]. The mechanism bears similarity with recently proposed multiphonon exciton decay in semiconducting carbon nanotubes [19], while in standard semiconductors such a scenario seems to be inefficient [20].…”

Interaction quench in the Holstein model: Thermalization crossover from electron- to phonon-dominated relaxation

“…27-29, the non-equilibrium dynamics of one or two polarons in the Holstein model was investigated with a time-dependent exact diagonalization method. As for many-electron problems, previous works have investigated systems with classical phonons 31 or quantum phonons 32 in one-dimension. Many-electron problems in two-dimensional systems have been studied with an exact diagonalization method for the Holstein-Hubbard model 33 or with a weak-coupling perturbation theory for the Holstein model.…”