Linear and nonlinear optical properties of one-dimensional Mott insulators consistingof Ni-halogen chain and CuO-chain compounds

We studied the linear and nonlinear optical responses in the one-dimensional (1D) Mott insulators of the halogen (X)-bridged nickel compounds (the NiX-chain compounds), [Ni(chxn)₂X]Y₂ [X,Y=Cl; X,Y=Br; X=Cl, Y=NO₃: (chxn)=cyclohexanediamine] and the copper oxide (CuO) chain compounds, A₂CuO₃ (A=Sr and Ca). The excitation profiles of the photoconductivity as well as the photoluminescence efficiency measurements show that charge-transfer (CT) excited states in the NiX-chain compounds form excitonic bound states, while the excitonic effect is relatively small in Sr₂CuO₃ and negligible in Ca₂CuO₃. The relatively large excitonic effect in the NiX-chain compounds is attributable to the strong 1D confinement of the electronic states. The temperature dependence of the ε₂ spectra reveals that the spectral widths Γ_CT of the CT bands are dominated mainly by the electron–lattice interaction, which is smaller in the NiX-chain compounds than in the CuO-chain ones. The χ⁽³⁾(−ω;0,0,ω) spectra of the 1D Mott insulators were obtained by the electroreflectance spectroscopy. The maximum values of ∣Imχ⁽³⁾(−ω;0,0,ω)∣ in the 1D Mott insulators(∼10⁻⁵–10⁻⁸ esu) were considerably larger than those in other 1D semiconductors such as 1D band insulators of silicon polymers, and 1D Peierls insulators of π-conjugated polymers and halogen-bridged Pt compounds (∼10⁻⁸–10⁻¹⁰ esu). To elucidate the enhancement of ∣Imχ⁽³⁾(−ω;0,0,ω)∣ in the 1D Mott insulators, we have compared the nature of the photoexcited states of the 1D Mott insulators with those of the 1D band and Peierls insulators. In the 1D Mott insulators, the odd and even CT excited states are nearly degenerate. This degeneracy induces the large transition dipole moment between these two states and then leads to the enhancement of χ⁽³⁾. Such a feature in the 1D Mott insulators is independent of the magnitude of the excitonic effect, although the excitonic effect sharpens the χ⁽³⁾ spectrum and enhances the maximum value of ∣χ⁽³⁾∣. In the 1D band and Peierls insulators, on the other hand, the splitting between the lowest excited state with odd parity and the second-lowest one with even parity is as large as the exciton binding energy. It leads to the diminution of the transition dipole moment between these two excited states and hence of χ⁽³⁾. These differences of the photoexcited states between the 1D Mott insulators and others have been explained in terms of the 1D extended Peierls–Hubbard model.