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Physica status solidi. C, Conferences and critical reviews/Physica status solidi. C, Current topics in solid state physics

Effect of disorder on the relaxation of photoexcited states in one‐dimensional Mott insulators

Mami Segawa, Akira Takahashi, Masaki Aihara

Published Oct 16, 2008• 2 citations
Publisher

Abstract

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)

DOI
10.1002/pssc.200879889

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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)
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 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.
Abstract In order to study relaxation rates of photoexcited states in correlated electron systems, we calculate the time evolution of phonons' kinetic energies after photoexcitation of a one‐dimensional half‐filled Hubbard model coupled with different types of phonons, which modulate transfer integrals, site energies, and on‐site repulsion strengths. Without or with weak dimerization, the energy transfer to phonons that modulate on‐site repulsion strengths is the largest. It increases rapidly as a function of the corresponding displacement. This suggests that the rapid relaxation of photoexcited states in one‐dimensional correlated electron systems is allowed without change in symmetry by phonons that modulate Coulomb repulsion strengths. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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 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.