Resonant Raman scattering from amplitude modes in trans-(CH)x and -(CD)x

Journal ArticleThe three resonantly enhanced Raman vibrations in trans-(CK)x and ~(CD)X are described in terms of amplitude modes associated with the Peierls gap. The dispersion in the Raman-line profiles is explained by inhomogeneity. A narrow distribution of the effective electron-phonon coupling constant is required to account for the observed spectra in detail. The resonant Raman vibrations and the ir-active modes induced by doping or photogeneration are all described by the same form of dressed phonon propagator

Disorder-induced TA Raman lines in mixed Cu-halide crystals

Journal ArticleThe Raman spectra of CuClxBr1=x and CuBrxI1=x solid solutions are shown to contain extremely intense disorder-induced first-order TA phonons. The intensity of these lines is a function of the concentration x, and is highest in CuCl0.6oBr0.40. The TA spectrum, well resolved at low temperature, fits well the density of states (DOS) calculated from inelastic-neutron-scattering data. Assuming a disorder model in which Cu+ populates the central site or off-center sites, the high-intensity of this line and its temperature and pressure dependence are explained. The Gruneisen parameters of the TA lines were measured in various mixed crystals, and fit well other related thermodynamic parameters. The low-temperature 2TA lines of both pure and mixed crystals fit the calculated two-phonon DOS. The spectral analysis at high temperature is discussed, and it is found that ω2TA (T) follows nicely 2ωTA(T)

Amplitude and phase modes in trans-polyacetylene: resonant Raman scattering and induced infrared activity

Journal ArticleThe resonant Raman scattering (RRS) from the three Ag modes of trans-polyacetylene and the charged-induced ir modes are analyzed with use of the amplitude- and phase-mode theories. It is shown that the observed phonon frequencies and the relative intensities of all modes obtained at various laser excitation energies ħωL is accounted for by a single phonon propagator which also describes the charge-induced infrared-active modes. The dispersion of the RRS frequencies with ħωL exhibits inhomogeneity of the sample which in turn provides the functional dependence of the rr-electron gap Eg on an effective coupling parameter λ. We show that inhomogeneity in both the electron-phonon and the electron-electron interaction parameters yields inhomogeneity in λ. The experimental gapversus-λ relation is consistent with the Peierls model but allows for weak electron-electron interactions which enhance the gap. We propose a method by which the distribution in λ, P(λ), is directly derived from the experimental spectra. It appears that different samples show different breadth for the distribution function; samples with sharper RRS features have narrower P(λ). We give an experimental estimate of the σ-bond contribution to the force constant of the carbon-carbon stretching mode and the electron-phonon interaction parameter. The pinning parameter of the charged carriers and its distribution are derived directly from the infrared absorption spectra induced either by doping or by photogeneration. The pinning of the doping-induced carriers is stronger and its distribution is wider; giving thus rise to the broader lines in the doping-induced infrared spectra. The mass of the photogenerated solitons is estimated from the relative strength of the infrared spectra and is approximately equal to the band effective mass of the electrons

Phonon anomalies in Cu halides

Journal ArticleA great number of the observed phonon anomalies in the zinc-blende phase of Cu-halide crystals are explained by means of a dynamical model. It is assumed that a Cu ion populates four equivalent off-center sites or its ideal position. The off-center Cu+ population is temperature and pressure dependent. With regard to phonons, this system is described as having a two-mode behavior of the optical vibrations at k=0; one is due to Cu+ at the ideal positions and the other is due to Cu+ at the off-center tunneling states. The latter has basically also a T2 symmetry, but a shorter correlation length. This explains the appearance, width, and polarization properties of the "extra" lines observed in Raman and infrared, as well as their irregular intensity temperature and pressure dependence. Consequently, the polariton dispersion and the resonant Raman data in CuCl can readily be understood. This model also explains the anomalous intensity k and T dependence of the inelastic-neutron-scattering in CuCl. The trend in the phonon characteristics, observed in Cu halides and which is an integral part of this model, follows the same course deduced from the intensity temperature dependence of Bragg x-ray and neutron scattering data

Photogeneration of confined soliton pairs (bipolarons) in polythiophene

Journal ArticleFrom photoinduced absorption and light-induced electron-spin resonance, we demonstrate that the dominant photocarriers generated in polythiophene with excitation above the energy gap (Eg) are charged bipolarons (spin 0). The observation of bipolarons (B2± ) rather than polarons (Pf ) as the dominant photoexcitations demonstrates that the Coulomb contribution to the bipolaron energy (UB ) is sufficiently small that P± + P± B± 2 . From the analysis of the spectra, we find UB/Eg = 0.12

Phonon-polaritons in Cu halides: anomalies and their temperature dependence

Journal ArticleThe polariton Raman spectra of CuCl, CuBr, and Cul crystals are studied at various temperatures of their zinc-blende phase. Two polar modes are proven to exist in CuCl at room temperature and below; their oscillator-strength ratio changes with temperature and follows the temperature dependence of the lattice constant. In CuBr also two polar modes were observed at 300 and 80 K, but not at 2 K. In the latter case their oscillator-strength ratio shows an exponential temperature dependence. The two polar modes and their oscillator-strength temperature dependence are explained, assuming a temperature-dependent secondary mimimum in the potential energy of the copper ion. Polariton spectra of Cul at room temperature and below show just one polar mode and behave regularly

Detection of soliton shape modes in polyacetylene

Journal ArticleWe observed soliton shape modes by photoinduced absorption in partially isomerized (CH)x and (CD)x, coexisting with the soliton translational modes. The shape modes' ir activity, their dependence on the degree of isomerization, and the photoinduced spectra are explained in detail by a symmetry-breaking interaction which allows coupling of shape modes and translational modes. We also observed soliton shape-mode absorption in fully isomerized samples as phonon sidebands associated with optical transitions of neutral solitons above 1.5 eV

Raman-scattering study of pressure-induced phase transitions in CuI

Journal ArticleThe pressure-induced phase transitions: zinc-blende (ZB) --* rhombohedral -> tetragonal in Cul were studied by Raman scattering. The rhombohedral primitive cell is presented as a slightly distorted ZB unit cell that is twice as long as the primitive cell in the [111] direction. By folding back the ZB Brillouin zone, an approximation to the phonon dispersion curves of the rhombohedral structure in the [111] direction can be obtained. A comparison can then be made between the rhombohedral Raman frequencies and those obtained by neutron scattering at the ZB zone boundary at L. The agreement was found to be satisfactory. A similar analysis is made of the tetragonal phase; it can be related to the zinc-blende structure by means of a doubled primitive cell which involves a slight distortion of the iodine sublattice but a large rearrangement of the copper sublattice. The Brillouin zone must be folded along one cubic axis. The mode Griineisen parameters were calculated for the rhombohedral phase and all of them were found to be positive including that of the E mode that corresponds to TA(L) of the ZB structure

Classification of disorder and extrinsic order in polymers by resonant Raman scattering

Journal ArticleResonant Raman scattering is used to quantify the effect of disorder and extrinsic order on Peierls systems such as (CH)X . We introduce "X plots" in which the phonon and gap distribution are related and characterize the system by the functional dependence of the gap 2 A on the coupling X. For c/s-rich trans-(CH)X the disorder is extrinsic with energy ∆' P and p = 1. For all trans-(CH)X the X plot yields intrinsic disorder, i.e., variation in the e-p coupling. cis-(CH)x is ordered with a 5% extrinsic gap of the full energy gap of 1.95 eV

Photomodulation spectroscopy of iodine-doped polyacetylene; phase transition from soliton lattice to metal

Journal ArticleUsing the photomodulation technique for iodine I3-)-doped polyacetylene, we have found spectroscopic evidence for an abrupt phase transition at 5% I3-. The transition is from the lightly doped phase best described as soliton lattice to the heavily doped metallic phase. The phase transition is characterized by gradual disappearance of the single photoinduced soliton band in the semiconducting phase and a sudden appearance of two correlated photoinduced absorption bands associated with photogenerated charged defects in the metallic phase