R.T. Fu

Anab initioapproach to phonon spectrum oftrans-polyacetylene

By an ab initio calculation based on the local density approximation of density-functional theory, the force constants up to the third nearest-neighbour carbon-hydrogen unit are determined to obtain the phonon spectrum of trans-polyacetylene for the first time. The calculated phonon frequencies at the point coincide well with those of the observed infrared and Raman spectra of trans-polyacetylene. The motion of hydrogen atoms is shown to be important in determining the phonon spectrum of polyacetylene due to the contribution of the bending between the CH and CC bonds.

Third nonlinear optical spectrum of Pt-Cl complex

Abstract The recent experimental data of THG (third harmonic generation) spectrum χ (3) ( ω ) of Pt-Cl complex is well interpreted by the present theory based on CDW model, which not only shows that the observed peak around 1.8 eV is produced by the two-photon resonance between the bottom of valence band and the top of conduction band, but also predicts that there exists one more peak at 0.9 eV outside the observed region.

The localized modes of soliton and infrared absorption in polyacetylene

Abstract Starting from SSH model of polyacetylene, anew infrared active localized mode of the soliton is found, it is the staggered mode g s , in which all the odd atoms are fixed whereas the even atoms oscillate in alternate directions. It is shown that there only exist three infrared active localized modes around the soliton, i.e. Goldstone mode, third mode and staggered mode. The effect of the electron-electron interaction on the localized modes is also studied, and the results show that all the localized modes survive e-e interaction. These three infrared active modes are well corresponding to the observed three infrared absorption lines in 900, 1288 and 1395 cm −1 .

Torsional structure in poly(para-phenylene vinylene)*

Abstract A direct approach to calculate the phonon dispersion curves in poly( para -phenylene vinylene, PPV) by using a parameterized tight-binding force constant method is presented. The calculated phonon dispersion curves for the first time show a pronounced soft mode in responding to a specific twisted structure in PPV, in very good agreement with experiments. A torsion angle of 22 ° is predicted in PPV chain, consistent with experimental data in oligomers. I

Dynamics of charge transfer and bond distortion in C60

Abstract The static theory shows that the bond structure of C 60 is distorted by charge transfer and the distortion of C 60 − is localized in the equatorial area to form a string polaron with symmetry D 5 d . This paper further studies the dynamical process of forming the polaron. The time-dependence of the bond distortion demonstrates the evolution of the lattice relaxation from the symmetry I h to D 5 d , and the calculated relaxation time agrees with the experiment.

Pairing energy of hole-doped C60

Abstract The pair-binding energy of hole-doped C 60 is calculated by the Gutzwiller scheme. It is found that the pairing energy of hole-doped C 50 is higher than that of electron-doped C 60 . The implication of this difference for superconductivity is discussed.

Two-photon resonance in THG of polymer

Abstract The THG spectrum φ 3 (ω) of trans -polyacetylene has two peaks at 0.6 and 0.9 e V. While the first peak is unanimously understood to originate from the three-photon resonance across the energy gap, how to interpret the second peak is a controversial issue. Both the single-particle and correlated electron theories show that the two-photon resonance can give the second peak, but its position is higher than 0.9 eV . A phenomenological consideration indicates that the excitation-dependent damping shifts the second peak to the lower energy. This paper presents a microscopic theory of the excitation-dependent damping, from which the THG spectrum φ 3 (ω) can be calculated and agrees quantitatively with the experimental data.

Laminar structure of bond distortion and electronic states in charged C60

Abstract For doped C 60 , the charge transfer distorts the bond structure of C 60 and forms some self-trapping electronic states. The present theory shows that the charged C 60 possesses following remarkable features: 1). Both the bond distortion and the self-trapping states have laminar structure and are localized in the equatorial area. 2). The symmetry is reduced from I h of the pristine C 60 to D 5 d of the charged C 60 . 3). The carbon atoms in charged C 60 are divided into eight layers with an inversion center, and there appear four groups of carbon atoms. The atoms in different groups are nonequivalent, then the spectral line in the NMR gets a fine structure consisting of four sub-lines with the strength ratio 1:1:2:2. 4). The charged C 60 has two self-trapping electronic states, one is 0.06eV above HOMO with odd parity, another is 0.05eV below the LUMO with even parity.

An ab initio approach to phonon spectrum of trans-polyacetylene

By an ab initio calculation based on the local density approximation of densityfunctional theory, the force constants up to the third nearest-neighbour carbon-hydrogen unit are determined to obtain the phonon spectrum of trans-polyacetylene for the first time. The calculated phonon frequencies at the 0 point coincide well with those of the observed infrared and Raman spectra of trans-polyacetylene. The motion of hydrogen atoms is shown to be important in determining the phonon spectrum of polyacetylene due to the contribution of the bending between the CH and CC bonds. Trans-polyacetylene [1], as a prototype of conducting polymers, has been studied extensively due to its promising electric, magnetic and optical properties [2]. In particular, Su, Schrieffer and Heeger (SSH) [3] proposed a soliton model to explain these properties of conducting polymers. The resulting physics turned out to be quite rich and the model has been very successful in interpreting the experimental phenomena. However, in the SSH model some approximations have been made to describe trans-(CH)x , the main one being that only a translation of the CH group as a whole in the chain direction is considered among the six degrees of freedom for the group. To study the physical properties quantitatively, especially the vibrational spectra in detail, one has to consider all these coordinates. Vibrational spectra of trans-(CH)x have been studied in many ways. Within the discrete one-dimensional SSH model, all vibrational modes around the soliton excitation [4] as well as in the dimerized lattice [5] are obtained. Three infrared localized modes are found to fit the observed three infrared absorption peaks in doped polyacetylene in total. Furthermore, by use of a two-dimensional model, including the contribution of the bending between the CC bonds, a few more localized modes are found [6], depending on the adjustable model parameters, which are not completely clear for trans-(CH)x . Based onab initio second-order Møller–Plesset perturbation calculations of trans-oligoenes, Hirataet al [7] extrapolated the vibrational force field of trans-(CH)x using scale factors. Although their result seems close to the experimental data, it is still believed that further parameter-free calculations are needed for the phonon spectra. Ab initio methods based on density-functional theory (DFT) are now common and there are well established tools for studying structual and vibrational properties of materials. The plane-wave pseuodopotential method and local-density approximation (LDA) to DFT have provided a simple framework and the accuracy and predictive power of this method have been demonstrated convincingly in a large variety of systems. The calculation of reliable phonon spectra in semiconductors is well within reach of DFT. In this letter, we report the result of anab initio calculation on the dimerized lattice of trans-polyacetylene based on the 0953-8984/97/240351+04

CHARGE-TRANSFER AND BOND RELAXATION IN FULLERENE

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