Quantum lattice fluctuations in a model electron-phonon system

An analytical approach, based on the unitary transformation method, has been developed to study the effect of quantum lattice fluctuations on the ground state of a model electron-phonon system. To study nonadiabatic case, the Green's function method is used to implement the perturbation treatment. The phase diagram and the density of states of fermions are obtained. We show that when electron-phonon coupling constant $\alpha^{2}/K$ decreases or phonon frequency $\omega_{\pi}$ increases the lattice dimerization and the gap in the fermion spectrum decrease gradually. At some critical value the system becomes gapless and the lattice dimerization disappears. The inverse-square-root singularity of the density of states at the gap edge in the adiabatic case disappears because of the nonadiabatic effect, which is consistent with the measurement of optical conductivity in quasi-one-dimensional systems.Comment: 9 pages, 4 ps figures include

Effect of quantum lattice fluctuations on the optical-absorption spectra of halogen-bridged mixed-valence transition-metal complexes

The effect of quantum lattice fluctuations on the optical-absorption spectra in the ground state of halogen-bridged mixed-valence transition-metal linear-chain complexes is studied by using a one-dimensional extended Peierls-Hubbard model. The nonadiabatic effects due to finite phonon frequency $\omega_{\pi}>0$ are treated through an energy-dependent electron-phonon scattering function $\delta(k^{\prime},k)$ introduced by means of an unitary transformation. The calculated optical-absorption spectra do not have the inverse-square-root singularity, but they have a peak above the gap edge and there exists a significant tail below the peak, which are consistent with the optical-absorption coefficient or the optical conductivity measurements of this material.Comment: 10 pages, 6 figure

Pressure-induced phase transitions of halogen-bridged binuclear metal complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O

Recent contrasting observations for halogen (X)-bridged binuclear platinum complexes R_4[Pt_2(P_2O_5H_2)_4X]nH_2O, that is, pressure-induced Peierls and reverse Peierls instabilities, are explained by finite-temperature Hartree-Fock calculations. It is demonstrated that increasing pressure transforms the initial charge-polarization state into a charge-density-wave state at high temperatures, whereas the charge-density-wave state oppositely declines with increasing pressure at low temperatures. We further predict that higher-pressure experiments should reveal successive phase transitions around room temperature.Comment: 5 pages, 4 figures embedded, to be published in Phys. Rev. B 64, September 1 (2001) Rapid Commu

Photoinduced charge separation in Q1D heterojunction materials: Evidence for electron-hole pair separation in mixed-halide MXMX solids

Resonance Raman experiments on doped and photoexcited single crystals of mixed-halide $MX$ complexes ($M$=Pt; $X$=Cl,Br) clearly indicate charge separation: electron polarons preferentially locate on PtBr segments while hole polarons are trapped within PtCl segments. This polaron selectivity, potentially very useful for device applications, is demonstrated theoretically using a discrete, 3/4-filled, two-band, tight-binding, extended Peierls-Hubbard model. Strong hybridization of the PtCl and PtBr electronic bands is the driving force for separation.Comment: n LaTeX, figures available by mail from JTG ([email protected]

Nonadiabatic effects in a generalized Jahn-Teller lattice model: heavy and light polarons, pairing and metal-insulator transition

The ground state polaron potential of 1D lattice of two-level molecules with spinless electrons and two Einstein phonon modes with quantum phonon-assisted transitions between the levels is found anharmonic in phonon displacements. The potential shows a crossover from two nonequivalent broad minima to a single narrow minimum corresponding to the level positions in the ground state. Generalized variational approach implies prominent nonadiabatic effects:(i) In the limit of the symmetric E-e Jahn- Teller situation they cause transition between the regime of the predominantly one-level "heavy" polaron and a "light" polaron oscillating between the levels due to phonon assistance with almost vanishing polaron displacement. It implies enhancement of the electron transfer due to decrease of the "heavy" polaron mass (undressing) at the point of the transition. Pairing of "light" polarons due to exchange of virtual phonons occurs. Continuous transition to new energy ground state close to the transition from "heavy" polaron phase to "light" (bi)polaron phase occurs. In the "heavy" phase, there occurs anomalous (anharmonic) enhancements of quantum fluctuations of the phonon coordinate, momentum and their product as functions of the effective coupling. (ii) Dependence of the polaron mass on the optical phonon frequency appears.(iii) Rabi oscillations significantly enhance quantum shift of the insulator-metal transition line to higher values of the critical effective e-ph coupling supporting so the metallic phase. In the E-e JT case, insulator-metal transition coincide with the transition between the "heavy" and the "light" (bi)polaron phase at certain (strong) effective e-ph interaction.Comment: Paper in LaTex format (file jtseptx.tex) and 9 GIF-figures (ppic_1.gif,...ppic_9.gif

Soliton excitations in halogen-bridged mixed-valence binuclear metal complexes

Motivated by recent stimulative observations in halogen (X)-bridged binuclear transition-metal (M) complexes, which are referred to as MMX chains, we study solitons in a one-dimensional three-quarter-filled charge-density-wave system with both intrasite and intersite electron-lattice couplings. Two distinct ground states of MMX chains are reproduced and the soliton excitations on them are compared. In the weak-coupling region, all the solitons are degenerate to each other and are uniquely scaled by the band gap, whereas in the strong-coupling region, they behave differently deviating from the scenario in the continuum limit. The soliton masses are calculated and compared with those for conventional mononuclear MX chains.Comment: 9 pages, 10 figures embedded, to be published in J. Phys. Soc. Jpn. 71, No. 1 (2002

Characterization of halogen-bridged binuclear metal complexes as hybridized two-band materials

We study the electronic structure of halogen-bridged binuclear metal (MMX) complexes with a two-band Peierls-Hubbard model. Based on a symmetry argument, various density-wave states are derived and characterized. The ground-state phase diagram is drawn within the Hartree-Fock approximation, while the thermal behavior is investigated using a quantum Monte Carlo method. All the calculations conclude that a typical MMX compound Pt_2(CH_3CS_2)_4I should indeed be regarded as a d-p-hybridized two-band material, where the oxidation of the halogen ions must be observed even in the ground state, whereas another MMX family (NH_4)_4[Pt_2(P_2O_5H_2)_4X] may be treated as single-band materials.Comment: 16 pages, 11 figures embedded, to be published in Phys. Rev.

Reference Force Field and CDW Amplitude of Mixed-Valence Halogen-Bridged Pt Complexes

The spectroscopic effects of electron-phonon coupling in mixed-valence chlorine-bridged Pt chains complexes are investigated through a parallel infrared and Raman study of three compounds with decreasing Pt-Pt distance along the chain. The e-ph interaction is analyzed in terms of the Herzberg-Teller coupling scheme. We take into account the quadratic term and define a precise reference state. The force field relevant to this state is constructed, whereas the electronic structure is analyzed in terms of a simple phenomenological model, singling out a trimeric unit along the chain. In this way we are able to account for all the available optical data of the three compounds, and to estimate the relevant microscopic parameters, such as the e-ph coupling constants and the CDW amplitude.Comment: 10 pages, compressed postscript, 6 Tables and 5 Figures also in a compressed ps.Z file. Revision is in the submission format only (postscript instead of tex

Asymptotic normality of families of probability measures

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