I. Batistic

Axial oxygen-centered lattice instabilities in YBa sub 2 Cu sub 3 O sub 7 : An application of the analysis of extended x-ray-absorption fine structure in anharmonic systems

Analysis of polarized copper {ital K}-edge extended x-ray-absorption fine-structure (EXAFS) measurements on YBa{sub 2}Cu{sub 3}O{sub 7} for temperatures 10{le}{ital T}{sub nom}{le}105 K indicates that the axial oxygen moves in a double-well potential which softens within a fluctuation region associated with the onset of superconductivity in this material. This fluctuation follows from the coupling between the phonons derived from this double-well potential and the superconducting order parameter. The advantages of EXAFS compared to crystallographic measurements in discerning these aspects of the local structure are discussed. Metrical parameters and the characteristics of the potential are determined by curve fits of the EXAFS. This method is based on the calculation of radial distribution functions from selected model potentials, the forms of which are determined along with the absorber-scatterer distance, scatterer type, and number, by optimization of fits of the experimental data with the EXAFS calculated from these parameters. Unlike perturbative treatments, this approach is ideally suited for highly anharmonic systems because the putative potential can be a very close approximation to the real one and it also offers the advantage of providing dynamical information not available through perturbative treatments.

Ultragap edge states in mixed halide chain solids

Abstract Resonance Raman spectroscopy has been performed above and below the intervalence charge transfer band for mixed halide [Pt(en) 2 ][Pt(en) 2 × 2 ](ClO 4 ) 4 en=ethylenediamine, X=Cl x Br 1−x , Cl x l 1−x , and Br x l 1−x ) linear chain MX solids. Strong resonance enhancement is observed for local state vibrational modes in tuniing between the band gaps of the constituent PtX species and well to the blue of the band gaps of both species in the case of PtBr x l 1−x . These ultragap states have been attributed to edge states created by the junction between segments with different halides. In the case of PtCl x Br 1−x and PtBr x l 1−x , we observe ultragap edge state vibrational modes at ca. 210 and 181 cm −1 that can be attributed to phonons dominated by the PtBr segment motions near the PtBr-PtCl interface; similar vibrational modes are observed for PtCl and Ptl segments in the mixed halide solids. Ultragap edge states are explicitly demonstrated within theoretical calculations of a discrete, 3 4 - filled , 2-band, tight binding, extended Peierls-Hubbard model that employs mean-field and direct-space random phase approximations.

MX chains: 1-D analog of CuD planes?

We study a two-band Peierls-Hubbard model for halogen-bridged mixed-valence transition metal linear chain complexes (MX chains). We include electron-electron correlations (both Hubbard and PPP-like expressions) using several techniques including calculations in the zero-hopping limit, exact diagonalization of small systems, mean field approximation, and a Gutzwiller-like Ansatz for quantum phonons. The adiabatic optical absorption and phonon spectra for both photo-excited and doping induced defects (kinks, polarons, bipolarons, and excitons) are discussed. A long period phase which occurs even at commensurate filling for certain parameter values may be related to twinning. The effect of including the electron-phonon in addition to the electron-electron interaction on the polaron/bipolaron (pairing) competition is especially interesting when this class of compounds is viewed as a 1-D analog of high-temperature superconductors. 6 refs., 4 figs.

Incommensurate ground states of the commensurate Peierls-Hubbard Hamiltonian: Superlattice phases in the MX compounds

Abstract We study a two-band Peierls-Hubbard model for halogen-bridged mixed-valence transition metal linear chain complexes (MX chains). Electron-electron correlations (both Hubbard and PPP-like expressions) are incorporated using several techniques. A sequence of long period (non-2k F , superlattice) phases is found to occur even at exactly commensurate filling as the strength of on-site electron-phonon coupling is varied. These phases are understood as ordering of “bipolaronic” defects with respect to the commensurate 2k F or other states. Such long period phases may have been observed in MX chains via STM experiments [1]. The superlattice phases are analogous to intrinsic twinning textures. The effect of including the electron-phonon in addition to the electron-electron interaction on the polaron/bipolaron (real space pairing) competition is especially interesting when this class of compounds is viewed as a 1-D analog of high T c superconductors.

Modeling of nonlinear optic and ESR response of CDW MX Materials

Abstract We report results on the nonlinear optic and ESR response of the PtX MX chain materials calculated using a discrete, 3 4 -filled, two-band, tight-binding Peierls-Hubbard model. We calculated electroabsorption (EA) spectra for the three PtX (X=Cl, Br, I) charge-density-wave (CDW) materials and find good agreement with the experimental data. We also obtain EA spectra for localized defects in PtBr. In addition, the field orientation dependence of the electron spin resonance spectra associated with the spin carrying defects is calculated for PtX materials and compared with ESR data on photoinduced defects.

Theory of MX chain materials : a 2-band Peierls-Hubbard model

Abstract Theoretical modeling of halogen-bridged mixed valence insulators such as [Pt(en)2] [Pt(en)2Cl2](ClO4)4 (en = N2C2H8) is described in terms of an extended Peierls-Hubbard tight-binding model incorporating 3 4 - filling of 2-bands. Ground and polaronic defects are described and characterized by optical absorption and by infra-red and resonance raman spectroscopy. Striking agreement with recent experiments is pointed out, including distinct asymmetry between electron- and hole-polarons. Finally, analogies are drawn with modeling of oxide high-temperature superconductors.

Femtosecond dynamics, photoexcitation and ESR spectra of MX chain solids

Abstract Non-linear adiabatic dynamics and ESR spectra associated with non-linear excitations in MX chain (M = Pt; X = Cl, Br, I) materials are numerically studied within a discrete, 3/4-filled, two-band, tight-binding extended Peierls-Hubbard model. Both Hartree-Fock (HF) adiabatic molecular relaxation and molecular dynamics techniques are employed to investigate the time evolution of solitons, polarons, bipolarons in charge-density-wave (CDW) ground state materials. The time evolution of excitons, defect pairs and/or breathers is studied subsequent to photoexcitation. The ESR spectra associated with the resulting spin carrying non-linear excitations such as neutral solitons, triplet excitons as well as electron and hole polarons are then calculated within the above model. The superhyperfine structure in the ESR spectra is attributed to a small spin density present on the halogen sublattice.

Tuning dimensionality in low-dimensional electronic materials

Abstract We demonstrate how dimensionality can be tuned in complex low-dimensional electronic materials via small perturbations in competing molecular forces. The delicate balance between molecular level forces on observed dimensionality in materials is illustrated by the 1-d to 3-d structural reorganization following deuteration of the ancillary ligands in the halogen-bridged transition-metal charge-transfer complex [Pt(en) 2 I 2 ][Pt(en) 2 ]I 2 (PtI) where en denotes ethylenediamine. Specifically, the impact of competing forces on dimensionality is clearly demonstrated by the temperature-dependent phase transitions in perdeuterated PtI (D-PtI), where small changes in inter- and intra-sheet interactions drive mesoscopic structural changes.

ESR spectra of photoinduced defects in MX chain solids

Abstract We calculate the electron spin resonance (ESR) spectra associated with the spin carrying nonlinear excitations such as neutral solitons, triplet excitons as well as electron and hole polarons for halogen-bridged mixed-valence transition metal (MX: M = Pt, X = Cl,Br,I) linear chain materials within a discrete, 3/4-filled, two-band, tight-binding extended Peierls-Hubbard model 1 . The strongly distorted and localized CDW material, PtCl, is contrasted with the weak CDW, delocalized material, PtBr. The hyperfine structure in the ESR spectra arises from the spin density on the Pt atoms while the superhyperfine structure is attributed to a small spin density present on the halogen sublattice. These theoretical predictions are compared with the recent experimental ESR data on photoinduced defects in PtX materials.

Optical properties of MX chain materials: an extended Peierls-Hubbard model☆

Abstract We describe theoretical modeling of both pure (MX) and mixed-halide (MX x X′ 1− x ) halogen (X)-bridged transition metal (M) linear chain complexes in terms of an extended Peierls-Hubbard, tight-binding Hamiltonian with 3/4-filling of two bands. Both inter- and intra-site electron-phonon coupling are included. Electronic (optical absorption), lattice dynamic (IR, Raman) and spin (ESR) signatures are obtained for the ground states, localized excited states produced by impurities, doping or photoexcitation (excitons, polarons, bipolarons, solitons) and the edge states (which occur in mixed-halide crystals, e.g. PtCl x Br 1− x ). Adiabatic molecular dynamics is used to explore photodecay channels in pure and impure systems for ground states as well as in the presence of pre-existing polaronic states: 71.38+ i , 78.30− j , 71.10+ x .