Kaoru Iwano

Theory for Spectral Shape of Optical Absorption in Halogen-Bridged Mixed-Valent Metal Complexes

Spectral shapes of optical absorptions in the charge density wave states of halogen-bridged mixed-valent metal complexes are theoretically determined for the first time, using a one-dimensional extended Peierls-Hubbard model. Two important effects beyond the mean field theory are taken into account; the electron-hole (e-h) correlation on the metal atoms, and the classical fluctuation of the halogen sublattice coordinates. It is shown that the exciton strongly affects the spectral shape of the charge transfer (CT) band. When the e-h binding is weak, the CT band is the mixture of the exciton absorption and the interband ones. When it is strong, on the other hand, the exciton dominates over all the CT band and, consequently, its higher energy side becomes very close to a Lorentzian. From the comparison with experimental data, it is concluded that platinum complexes correspond to strong binding cases. The intragap absorptions associated with nonlinear excitations such as solitons and polarons are also studied...

Relaxation processes yielding nonlinear excitations in the extended Peierls-Hubbard model for photoexcited MX chains

The relaxations occurring after photoexcitation in the MX chains are investigated, using a one-dimensional extended Peierls-Hubbard model. The special emphasis is placed on the processes in which the nonlinear excitations such as solitons and polarons are created in the background of the charge-density wave (CDW). The way of relaxations changes its nature, depending mainly on the relative strengths of the on-site and nearest-neighbor Coulombic repulsions U and V , respectively, to the on-site electron-lattice interaction S . When U or both U and V dominate over S , soliton-pair states are unstable compared with the self-trapped exciton (STE) in the localized limit of the model. However, the neutral-soliton pair can be the lowest structural excitation in intermediate-transfer cases, stabilized by the effect of itineracy. As for the adiabatic potential surfaces, the uniform CDW is connected to the neutral-soliton pair, while the STE to the charged-soliton pair. It is expected that the STE will make a transi...

Effective conversion of CDW sites to SDW states in Pd-Ni mixed-metal MX chains

We theoretically study the interplay between a charge-density-wave (CDW) and spin-density-wave (SDW) in mixed-metal MX chains. We assume that two kinds of metals, namely, Pd and Ni, are randomly distributed in a one-dimensional chain. Since all-Pd and all-Ni compounds show CDW and SDW, respectively, the gradual mixing of such a type gives a very good viewpoint for understanding both the states more deeply. Using a Peierls-Hubbard model and a mean-field approximation, we find that the CDW state is drastically suppressed by the replacement of Pd sites with Ni ones. As a result, the Ni concentration of 50% is enough to destroy the CDW completely if the Hubbard- U at Ni sites is large enough as expected in real materials. This means that all the Pd sites become Pd 3+ with no halogen displacements and, as a whole, a modified SDW state is formed. Such a peculiar state, which does not exist in pure MX chains, can explain the results of ESR and Raman experiments.

Nonlinear Optical Spectra in Halogen-Bridged Mixed-Valent Platinum Complexes

Nonlinear optical properties in halogen-bridged mixed-valent platinum complexes are studied theoretically for the first time. Two types of nonlinearities are treated here. One is the electric-field modulation of the absorption spectrum, and the other is the third-harmonic generation. The spectral shapes of these nonlinearities are determined by a method in which both effects of the electron-hole correlation and the lattice fluctuations are taken into account. As for the electric-field modulation, the difference spectrum observed in the visible region for [Pt(en) 2 ][Pt(en) 2 Cl 2 ](ClO 4 ) 4 is well reproduced with features such as the rigid shift of the main charge-transfer band and the newly appearing peak due to the parity-even exciton. On the other hand, in the spectrum of the frequency-dependent intensity of the third-harmonic generation, several enhancements are found: the three-photon resonance structure due to the parity-odd exciton and the free elctron-hole continuum, and the two-photon resonance...

Large second-order optical nonlinearity in a ferroelectric molecular crystal of croconic acid with strong intermolecular hydrogen bonds

Linear and nonlinear optical responses in a molecular crystal, croconic acid, showing electronic-type ferroelectricity were studied by reflection and second harmonic generation spectroscopy. The second-order nonlinear susceptibility χ(2) was very large, exceeding 10−6 esu in the near-infrared region. The enhancement of χ(2) was attributed to the large dipole moment of the lowest π–π* transition and the large difference between the molecular dipole moments for the ground state and the photoexcited state. We deduced the molecular orbitals (MOs) and dipole moments responsible for the large χ(2) by comparing the experimental optical parameters and MO calculation results based upon density functional theory.

Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy

In electronic-type ferroelectrics, where dipole moments produced by the variations of electron configurations are aligned, the polarization is expected to be rapidly controlled by electric fields. Such a feature can be used for high-speed electric-switching and memory devices. Electronic-type ferroelectrics include charge degrees of freedom, so that they are sometimes conductive, complicating dielectric measurements. This makes difficult the exploration of electronic-type ferroelectrics and the understanding of their ferroelectric nature. Here, we show unambiguous evidence for electronic ferroelectricity in the charge-order (CO) phase of a prototypical ET-based molecular compound, α-(ET)2I3 (ET:bis(ethylenedithio)tetrathiafulvalene), using a terahertz pulse as an external electric field. Terahertz-pump second-harmonic-generation(SHG)-probe and optical-reflectivity-probe spectroscopy reveal that the ferroelectric polarization originates from intermolecular charge transfers and is inclined 27° from the horizontal CO stripe. These features are qualitatively reproduced by the density-functional-theory calculation. After sub-picosecond polarization modulation by terahertz fields, prominent oscillations appear in the reflectivity but not in the SHG-probe results, suggesting that the CO is coupled with molecular displacements, while the ferroelectricity is electronic in nature. The results presented here demonstrate that terahertz-pump optical-probe spectroscopy is a powerful tool not only for rapidly controlling polarizations, but also for clarifying the mechanisms of ferroelectricity.

Propagating Librations in Ice XI: Model Analysis and Coherent Inelastic Neutron Scattering Experiment

We report on propagating librational (rotational) modes intrinsic to ice XI. Contrary to the theoretical picture presumed so far, namely, that all librational modes are localized in each water molecule, our calculations identify large dispersions for some of the modes, which basically originate from intermolecular dipole–dipole interactions. On the basis of this finding, the known incoherent inelastic neutron scattering spectrum and infrared absorption spectrum are newly analyzed to give the interpretation that the spectra are mixtures of contributions of localized and propagating modes. Furthermore, we perform an experiment on coherent inelastic neutron scattering using the MAPS spectrometer in order to search ω– Q space more directly than in a previous work and find that the observation also strongly suggests the existence of large dispersions.

DFT calculations for the high-temperature structure of (EDO-TTF)2PF6: Identification of an electronic molecular dimer

Density-functional theory (DFT) calculations are performed based on the high-temperature structure of (EDO-TTF)2PF6, a quasi-one-dimensional molecular compound that shows both thermal and photoinduced phase transitions. In this structure, the EDO-TTF molecules are one-dimensionally aligned, accompanied with weak dimerization. Contrary to a common sense, our DFT calculations reveal that the pair having a shorter mutual distance has a weaker intermolecular coupling than the pair with a longer one; the latter is appropriate to be called an electronic dimer. We also estimate the corresponding transfer energies and discuss their relevance to spin correlations and optical excitations.

Eilenberger equations in trans-polyacetylene with random disorder

Eilenberger equations are derived for trans-polyacetylene with random impurities. The Takayama-Lin-Liu-Maki model is generalized to include the impurity effect. Two types of disorders, namely, site-type and bond-type are studied. Since the original derivation, for dirty superconductors, does not work for the Peierls system, new derivation is given with the help of self-consistent argument. The equations are solved, using the assumption that dimerization pattern is uniform. It turns out there are no differences between the two types of disorders and we get exactly the same results as for the gapless superconductors with paramagnetic impurities. Electronic density of states and order parameter are calculated as functions of disorder parameter. They are compared with the results by Xu and Trullinger, obtained by supersymmetry arguments. Present method gives an overall agreement as far as the density of states is concerned. There is a difference in the order parameter.

Ultrafast Photoinduced Electric-Polarization Switching in a Hydrogen-Bonded Ferroelectric Crystal

Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching 20  μC/cm^{2}, which originates from the strong coupling of proton and π-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.