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Physical Review Letters

Dynamics of Photoexcited States in Strongly Correlated Electron Systems

Akira Takahashi, Hiroki Gomi, Masaki Aihara

Published Oct 29, 2002• 33 citations
Publisher

Abstract

We theoretically investigate the dynamics of the photoexcited state in the strongly correlated low-dimensional electron systems. In the two-dimensional case, the ultrafast relaxation originating from the transfer of photogenerated charges in the antiferromagnetic background is followed by the slower one originating from the spin structure rearrangement with the new charge distributions. This clearly shows the difference in the relaxation between charge and spin degrees of freedom. In the one-dimensional case, spin-charge separation holds and the mechanical coherence is preserved.

DOI
10.1103/physrevlett.89.206402

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We theoretically investigate the dynamics of the photoexcited state in the strongly correlated low-dimensional electron systems. In the two-dimensional case, the ultrafast relaxation originating from the transfer of photogenerated charges in the antiferromagnetic background is followed by the slower one originating from the spin structure rearrangement with the new charge distributions. This clearly shows the difference in the relaxation between charge and spin degrees of freedom. In the one-dimensional case, spin-charge separation holds and the mechanical coherence is preserved.
We theoretically investigate the dynamics of photoexcited states in strongly correlated low-dimensional electron systems by analyzing the transient four-wave mixing (TFWM). We adopt the effective Hamiltonian of the Hubbard model for the photoexcited states in the strong correlation case, and TFWM intensity is obtained by numerically calculating the time development of the photoexcited state. We find that TFWM intensity sensitively reflects the relaxation due to the diffusion of the photogenerated charges and the relaxation due to the spin structure rearrangement to the diffused charge distribution. This enables us to clarify the interplay of charge and spin degrees of freedom. In the two-dimensional case, a well-separated two-step relaxation is observed; the slower spin relaxation follows the faster charge relaxation as a result of the coupling between the spin and charge degrees of freedom. This shows that the spin-charge separation approximately holds and the coupling between the spin and charge degrees of freedom is weak. In the one-dimensional case, only the charge relaxation is observed. Furthermore, there is no correlation between the relaxations before and after the second pulse. These are characteristic behaviors inherent in the spin-charge separation in strongly correlated electron systems.
The effective Hamiltonian of the Hubbard model for multiphoton excited states is derived assuming a strong Coulomb repulsion. Diagonalizing the effective Hamiltonian exactly, the lowest energy one-photon excited state is calculated in the two-dimensional Hubbard model at half-filling, and its physical properties are investigated. For t/U< 0.0165,t/U<~0.0165,t/U< 0.0165, where t is the magnitude of the transfer integral between the nearest-neighbor sites and U is an on-site Coulomb repulsion energy, ferromagnetic spin order is photogenerated from the antiferromagnetic ground state, and, for t/U> 0.0166,t/U>~0.0166,t/U> 0.0166, antiferromagnetic spin order survives one-photon excitation. Photoinjected opposite charges are unbound because of strong electron correlation for t/U< 0.08,t/U<~0.08,t/U< 0.08, and bound for t/U> 0.08.t/U>~0.08.t/U> 0.08. The probability that the photoinjected opposite charges are on nearest-neighbor sites is strongly suppressed through all the correlation regions including this charge-bound region. We see a tendency toward spin-charge separation, but the coupling is not small in the one-photon excited state. The local spin structure around the bound positive and negative charges injected by photoexcitation is quite different from that induced by two-hole doping; this result suggests that multiphoton excitation will generate an electronic order which differs from that generated by doping.
We theoretically investigate the dynamics of insulator-metal transition induced by light-pulse excitation in one-dimensional (1D) and two-dimensional (2D) Mott insulators. We adopt the Pariser-Parr-Pople model, and pump-probe signal is obtained by numerically calculating the time development of the state excited by a light pulse. In the 1D case, when photoexcitation density is below about 10%, an extremely small portion of the energy eigenstates dominates the optical process, and the spin-charge separation holds in these dominant energy eigenstates. As a result, the Mott gap and the short-range antiferromagnetic (AF) spin order are preserved, and the spin relaxation does not occur. When the density is above the value, a metallic state without the Mott gap is photogenerated, and the magnitude of the short-range AF spin order is significantly reduced by photoexcitation. This is consistent with the experimentally observed photoinduced Mott transition. Furthermore, the spin relaxation occurs in the metallic state. This photoinduced Mott transition is a manifestation of the spin-charge coupling in the intensely photoexcited states. In the 2D case, the spin and the charge degrees of freedom are coupled even in the weakly photoexcited states, and the characteristic crossover associated with the excitation density is not observed.
We study the properties of photogenerated states of strongly correlated electron systems (SCES) with using the 'excited-state' t-J Hamiltonian. The lowest energy state in the two-photon excited manifold is calculated for the 20 site two-dimensional square lattice at half filling. The analysis of the off-diagonal order suggests that the d-wave superconducting state is photogenerated. We propose an optical method using the virtual transition to avoid the temperature growth arising from the optical excitation. We also investigate the transient behavior of the photoexcited state in SCES, which distinctively reflects the spin-charge separation in SCES.
We theoretically investigate the properties of multiphoton excited states in strongly correlated one-dimensional (1D) and two-dimensional (2D) electron systems at half filling using the extended Hubbard model. In the 2D case, we find the photoexcited states where two or more than two holons and doublons are strongly bound, and the cluster of bound holons and doublons coexists with the surrounding antiferromagnetic (AF) spin order, for the realistic Coulomb interaction parameters. The holon-doublon cluster (HDC) states have a unique charge aggregation mechanism originating from the spin-spin interaction in the AF spin background as well as from the direct Coulomb interaction. In the 1D case, the HDC states are destabilized by the spin-spin interaction in contrast to the 2D case. These charge aggregation mechanisms are the outcomes of exotic natures of holons and doublons, and the difference in the mechanism between the 1D and 2D cases is due to the difference in the coupling between spin and charge degrees of freedom. We also calculate the light-induced absorption that enables us to experimentally observe the predicted HDC states.
Using a nonequilibrium implementation of the extended dynamical mean-field theory (EDMFT) we simulate the relaxation after photoexcitation in a strongly correlated electron system with antiferromagnetic spin interactions. We consider the $t\ensuremath{-}J$ model and focus on the interplay between the charge and spin dynamics in different excitation and doping regimes. The appearance of string states after a weak photoexcitation manifests itself in a nontrivial scaling of the relaxation time with the exchange coupling and leads to a correlated oscillatory evolution of the kinetic energy and spin-spin correlation function. A strong excitation of the system, on the other hand, suppresses the spin correlations and results in a relaxation that is controlled by hole scattering. We discuss the possibility of detecting string states in optical and cold-atom experiments.