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 theoretically investigate the dynamics of insulator-metal transition induced by light-pulse excitation in one-dimensional Mott insulators. We adopt the Pariser--Parr--Pople model and the pump-probe signal is obtained by numerically calculating the time development of the state excited by light pulse. When the 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 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, spin relaxation occurs in the metallic state. This photoinduced Mott transition is a manifestation of the spin-charge coupling in the intensely photoexcited states.
We theoretically investigate the effect of static disorder on the dynamics of photoexcited states in one-dimensional Mott insulators. We numerically calculate the time development of the state excited by a light pulse in the Pariser-Parr-Pople model, where disorder is introduced in the site energy and the transfer integral. In the weaker excitation case, the spin-charge coupling is induced by the disorder in the transfer integral. The effects of the disorder in the site energy on the spin-charge coupling are much weaker than those in the transfer integral. In the stronger excitation case, the spin and the charge degrees of freedom are coupled in the photoexcited states even in regular systems. The spin relaxations are enhanced by the disorders, and there are no significant differences in the effects of disorders on the spin relaxations between these two kinds of disorders.
We theoretically investigate the dynamics of photoexcited states in one-dimensional Mott insulators. We adopt the Pariser–Parr–Pople model, and numerically calculate the time development of the nonequilibrium state excited by a light pulse. In the weaker light excitation case where the maximum photoexcitation density is below about 18%, the charge carriers in the photoexcited states are holons and doublons, and the geminate recombination of bound doublon–holon pairs, namely, the Auger recombination process dominates the decay of the photogenerated holons and doublons with the realistic Coulomb parameters. In the stronger light excitation case where the density is above the value, the charge carriers of the photoexcited states cannot be described by holons and doublons. As a result, the Auger recombination process of doublon–holon pairs is not the main decay route of the photogenerated charges. Furthermore, the spin and the charge degrees of freedom are coupled, and spin relaxation occurs in the photoexcited states. As the density of the photogenerated charge carriers becomes larger, the decay time decreases. This tendency is consistent with experiments.
We theoretically investigate the light absorption spectrum of two-dimensional (2D) Mott insulators. We employ the numerical diagonalization method to calculate the light absorption spectrum with using the effective Hamiltonian for the extended Hubbard model, which is valid when the on-site Coulomb interaction energy
U is much larger than the nearest-neighbor transfer integral
t and the nearest-neighbor Coulomb interaction energy
V. For
V=0, the absorption spectrum consists of a broad band with a width of about
16t and a sharp central peak, and the energy eigenstates contributing to the absorption spectrum do not have the antiferromagnetic (AF) spin order, when
U is sufficiently larger than
t. These features result from the spin-charge interplay inherent in the very strong correlation region where charge transfer term is dominant. With decreasing
U∕t, the peak structure in absorption spectrum becomes unclear and some low energy eigenstates have the AF spin order, as a result of the spin-spin interaction term. For large
V∕t, the dominant peak arises in the lower energy region of the spectrum. A large number of holon-doublon bound states, which have nearly the same charge but different spin structures, are responsible for this peak, in contrast to the conventional exciton state. This is also in contrast to the charge bound states in one-dimensional (1D) Mott insulators, where a single energy eigenstate dominates the optical transition moment as a result of the spin-charge separation. The essentially different absorption spectra between the 1D and 2D Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.
Abstract We theoretically investigate the effect of static disorder on the dynamics of photoexcited states in onedimensional Mott insulators. We adopt the Pariser‐Parr‐Pople model, where disorder is introduced in the site energy and the transfer integral. We numerically calculate the time development of the state excited by a light pulse, and investigate the time dependence of the nearest‐neighbor spin‐spin interaction energy, the on‐site Coulomb interaction energy, and the kinetic energy of photogenerated charges. In the weaker excitation case, the energy transfer from the charge degrees of freedom to the spin degrees of freedom does not occur in the regular systems as a result of the spin‐charge separation. When the disorder in the transfer integral is introduced, the spin‐spin interaction energy changes with time after photoexcitation, showing that the spin‐charge coupling is induced in this case. The effects of the disorder in the site energy on the spincharge coupling are much weaker than those in the transfer integral. On the contrary, the energy transfer from the on‐site Coulomb interaction energy to the kinetic energy is strongly suppressed by these two kinds of disorders. In the stronger excitation case, the spin and the charge degrees of freedom are coupled in the photoexcited states, and the two kinds of energy transfers occur even in regular systems. Furthermore, these energy transfers occur in irreversible way, suggesting that these transfers are relaxation phenomena. These relaxations are enhanced by the disorders. There are no significant differences in the effects of disorders on these relaxations between the disorder in the site energy and that in the transfer integral. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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.