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International Journal of Modern Physics B

Photogenerated States in a Highly Correlated Electron System

M. Aihara, Akira Takahashi, H. Gomi

Published Aug 10, 2003• 0 citations
Publisher

Abstract

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.

DOI
10.1142/s0217979203021010

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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 two-dimensional electron systems. The effective Hamiltonian of the Hubbard model for the multiphoton excited states is derived assuming strong Coulomb repulsion. Diagonalizing the effective Hamiltonian exactly, the lowest energy state in two-photon excited manifold is calculated in the 20 site two-dimensional square lattice at half filling. Analysis of correlation functions suggests that the antiferromagnetic spin order characteristic of the half filled ground state is destroyed by two-photon excitation, and two different d-wave superconducting states and a spiral spin state are photogenerated.
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 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.
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.
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 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.