We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ for all-optical switching.

We present large-scale correlated calculations of the linear optical absorption spectrum of oligo-acenes containing up to seven benzene rings. For the calculations we used the Pariser-Parr-Pople (PPP) Hamiltonian, along with the configuration interaction (CI) technique at various levels such as the full CI, the quadruple CI, and multireference singles-doubles CI. The role of Coulomb parameters used in the PPP Hamiltonian was examined by considering standard Ohno parameters, as well as a screened set of parameters. A detailed analysis of the many-body character of the important excited states contributing to the linear absorption has also been performed. The results of our calculations have been compared extensively with the theoretical work of other authors, as well as with experiments.

In this paper we report first-principles calculations on the ground-state electronic structure of two infinite one-dimensional systems: (a) a chain of carbon atoms and (b) a chain of alternating boron and nitrogen atoms. Meanfield results were obtained using the restricted Hartree-Fock approach, while the many-body effects were taken into account by second-order Møller-Plesset perturbation theory and the coupled-cluster approach. The calculations were performed using $6-{31G}^{**}$ basis sets, including the d-type polarization functions. Both at the Hartree-Fock (HF) and the correlated levels, we find that the infinite carbon chain exhibits bond alternation with alternating single and triple bonds, while the boron-nitrogen chain exhibits equidistant bonds. In addition, we also performed density-functional-theory-based local-density-approximation (LDA) calculations on the infinite carbon chain using the same basis set. Our LDA results, in contradiction to our HF and correlated results, predict a very small bond alternation. Based upon our LDA results for the carbon chain, which are in agreement with an earlier LDA calculation [E.J. Bylaska, J.H. Weare, and R. Kawai, Phys. Rev. B 58, R7488 (1998)], we conclude that the LDA significantly underestimates Peierls distortion. This emphasizes that the inclusion of many-particle effects is very important for the correct description of Peierls distortion in one-dimensional systems.