Theoretical study for ultrafast dynamics of photoexcited states in strongly correlated low-dimensional electron systems by analyzing nonlinear optical responses

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It has been heatedly discussed whether spin-charge separation is the root cause of various novel physical properties of strongly correlated low-dimensional electron systems, but it is still controversial. Since light couples only to the charge degrees of freedom, the spin dynamics is driven by the photoinduced charge motion through the coupling between spin and charge degrees of freedom. Therefore, the ultrafast optical excitation provides an ideal stage to investigate the problem of spin-charge separation. Considering these points, we have investigated the dynamics of photoexcited states in strongly correlated low-dimensional electron systems, and we have obtained the following results.We show that transient four wave mixing intensity sensitively reflects the relaxations due to the charge and spin motions. 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. This shows that the coupling between the spin and charge degrees of freedom is weak in this case. In the one-dimensional case, spin-charge separation holds almost rigorously. 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 spin order, for the realistic Coulomb interaction parameters. The holon-doublon cluster states have a unique charge aggregation mechanism originating from spin-spin interaction in the antiferromagnetic spin background as well as from the direct Coulomb interaction in the two-dimensional case. The charge aggregation mechanism is the outcomes of exotic natures of holons and doublons. We also find that the essentially different absorption spectra between the one-dimensional and the two-dimensional Mott insulators originate from the difference in the coupling between spin and charge degrees of freedom.