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Physical review. B, Condensed matter

Photoinduced superconducting states in strongly correlated electron systems

Akira Takahashi, Hiroki Gomi, Masaki Aihara

Published Sep 3, 2002• 15 citations
Publisher

Abstract

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.

DOI
10.1103/physrevb.66.115103

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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 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.
We investigate the Hubbard model on a two-dimensional cluster of 18 sites using Lanczos algorithm for different strengths of Coulomb repulsion in the low doping region. Energies and correlation functions are given for half-filling and two holes. In the strong coupling regime we compare our results with the $t\ensuremath{-}J$ model. We confirm the tendency of holes to repel, although a significant value of the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave superconductive order parameter is measured. These data represent a benchmark for future quantum simulations.