arXiv subjects
Publications and source records attributed to Kikuo Harigaya.
The third-order nonlinear optical susceptiblity chi(3) of a C60 molecule is calculated by using a free-electron model as well as by taking Coulomb interactions into account. The free-electron model yields chi(3) magnitudes which are in agreement with experiment. Althugh Coulomb interactions reduce chi(3) by one order of magnitude, local-field corrections can be invoked to recover the agreement.
An elastic anomaly, observed in the heavy fermi liquid state of Ce alloys (for example, CeCu$_6$ and CeTe), is analyzed by using the infinite-$U$ Anderson lattice model. The four atomic energy levels are assumed for f-electrons. Two of them are mutually degenerate. A small crystalline splitting $2Δ$ is assumed between two energy levels. The fourfold degenerate conduction bands are also considered in the model. We solve the model using the mean field approximation to slave bosons, changing the Fermi energy in order to keep the total electron number constant. The nonzero value of the mean field of the slave bosons persists over the temperatures much higher than the Kondo temperature. This is the effect of the constant electron number. Next, the linear susceptibility with respect to $Δ$ is calculated in order to obtain the renomalized elastic constant. The resulting temperature dependence of the constant shows the downward dip. We point out the relation of our finding with the experimental data.
Effects on C60 by thermal fluctuations of phonons, misalignment of molecules in a crystal, and other intercalated impurities are simulated by disorder potentials. The SSH model for doped C60 is solved with gaussian bond and site disorders. Polarons and dimerizations in lightly doped cases (C60^{-1,-2}) are relatively stable against disorders, indicated by peak structures in distribution functions of bond length and charge density.
Possible dimerization patterns and electronic structures in fullerene tubules as the one-dimensional pi-conjugated systems are studied with the extended Su-Schrieffer-Heeger model. We assume various lattice geometries, including helical and nonhelical tubules. The model is solved for the half-filling case of $π$-electrons. (1) When the undimerized systems do not have a gap, the Kekule structures prone to occur. The energy gap is of the order of the room temperatures at most and metallic properties would be expected. (2) If the undimerized systems have a large gap (about 1eV), the most stable structures are the chain-like distortions where the direction of the arranged trans-polyacetylene chains is along almost the tubular axis. The electronic structures are ofsemiconductors due to the large gap.
We review the recent theoretical treatment of fullerenes as pi-conjugated systems. Polaronic properties due to the Jahn-Teller type effects are mainly discussed. (1) A Su-Schrieffer-Heeger type electron-phonon model is applied to fullerenes: C_60 and C_70, and is solved with the adiabatic approximation to phonons. When the system (C_60 or C_70) is doped with one or two electrons (or holes), the additional charges accumulate along almost an equatorial line of the molecule. The dimerization becomes the weakest along the same line. Two energy levels, the occupied state and the empty state, intrude largely in the gap. The intrusion is larger in C_70 than in C_60. These are ``polarons'' in doped fullerenes. It is also found that C_60 and C_70 are related mutually with respect to electronical structures as well as lattice geometries. (2) We apply the model to the fullerene epoxide C_60O. It has the polaron-type lattice distortion around the oxygen, and also shows the energy level intrusion in the gap. (3) Optical properties of C_60 are calculated and discussed. In the absorption of the doped molecule, a new peak structure is present owing to the polaronic distortion. In the luminescence of the neutral C_60, the spacing between H_g(8)-phonon side-band peaks and the relative intensities agree well with experiments. In the dispersion of the third harmonic generation, the magnitudes of |chi^(3)| agree with those of experiments at the resonance of the lowest allowed transition as well as in the region away from the resonance.
We consider luminescence in photo-excited neutral C_60 using the Su-Schrieffer-Heeger model applied to a single C_60 molecule. To calculate the luminescence we use a collective coordinate method where our collective coordinate resembles the displacement of the carbon atoms of the Hg(8) phonon mode and extrapolates between the ground state "dimerisation" and the exciton polaron. There is good agreement for the existing luminescence peak spacing and fair agreement for the relative intensity. We predict the existence of further peaks not yet resolved in experiment. PACS Numbers : 78.65.Hc, 74.70.Kn, 36.90+f
Possible dimerization patterns and electronic structures in fullerene tubules as the π-conjugated systems are studied with the extended Su-Schrieffer- Heeger model. We assume various lattice geometries, including helical and nonhelical tubules, and tubules with end caps. The model is solved for the half-filling case of π-electrons. (1) When the undimerized systems do not have a gap, the Kekulé structures tend to occur. These structures are commensurate with the boundary condition in the direction perpendicular to the tubular axis. The energy gap is of the order of the room temperatures at most. Thus, the nearly metallic properties would be expected. (2) If the undimerized systems have a large gap (\sim 1eV), the most stable structures are the chain-like distortions where the direction of the arranged {\sl trans}-polyacetylene chains is along almost the tubular axis. When we try to obtain the Kekulé structures, the mismatch of the boundary condition occurs and they are energetically unfavorable. The electronic structures are of semiconductors due to the large gap.
We propose a Su-Schrieffer-Heeger type electron-phonon model for C_{60} with O defects and solve by the adiabatic approximation. Two new properties are obtained. (1) The dimerization becomes weaker around the oxygen. Two localized states appear deep in the gap. Optical transition between them is allowed. This accords with the recent optical absorption data. (2) Oxygens are predicted to cluster on the surface of $\soc$. PACS numbers: 3640, 7155, 6165, 3120P
We extend the Su-Schrieffer-Heeger model of polyacetylene to C_{60} and C_{70} molecules, and solve numerically. The calculations of the undoped systems agree well with the known results. When the system (C_{60} or C_{70}) is doped with one or two electrons (or holes), the additional charges accumulate almost along an equatorial line of the molecule. The dimerization becomes weaker almost along the same line. Two energy levels intrude largely in the gap. The intrusion is larger in C_{70} than in C_{60}. Therefore, ``polarons'' are predicted in doped buckminster- fullerenes. We calculate optical absorption coefficient for C_{60} in order to look at how ``polarons'' will be observed. It is predicted that there appears a new peak at the lower energy than the intergap transition peaks. It is also found that C_{60} and C_{70} are related mutually with respect to electronical structures as well as lattice geometries. (to be published in Phys. Rev. B 45, June 15 issue)
The frequency dependence of third harmonic generation (THG) in C_{60} is calculated, making use of a tight-binding model for pi-electrons. The magnitudes of the THG, about 10^{-12} esu, near zero frequency, agree with those in experiments for the low-energy region. We can also explain the order of the magnitude, 10^{-11} esu, around the three-photon resonance peak due to the lowest allowed excitation, recently measured by Meth et al. At higher energies, we predict a large enhancement of the THG at 3 omega \sim 6eV as a result of double resonance enhancement.