Two-band Peierls-Hubbard Model Research Articles

Competing Ground States of Metal–Halide Ladders

Based on a symmetry argument, we investigate the ground-state properties of newly synthesized metal-halide ladder compounds (C_8_H_6_N_4_)[Pt(C_2_H_8_N_2_)X]_2_(ClO_4_)_4_ 2H_2_O (X=Cl, Br, I). Employing a fully dressed two-band Peierls-Hubbard model, we systematically reveal possible charge- or spin-ordered states. Numerical phase diagrams demonstrate a variety of competing Peierls and Mott insulators with particular emphasis on the transition between two types of mixed-valent state of Pt^II^ and Pt^IV^ driven by varying interchain hopping integrals and Coulomb interactions.

Self-doping effect on the Mott transition accompanied with threefold charge ordering in (DCNQI) 2Cu

The commensurate state with threefold lattice distortion in the insulating phase of (DCNQI) 2Cu is studied based on a two-band Peierls–Hubbard model by using the density-matrix renormalization-group method. With strong electron correlation among the d electrons, self-consistent lattice modulation strongly blocks the charge transfer between the π and d orbitals in order to keep the commensurability condition even when the π–d level difference is widely varied. A transition to an incommensurate phase requires a large deviation of the π–d level difference from the optimal case.

Strong commensurability effect on metal-insulator transition in (DCNQI) 2 Cu

Stability of three-fold lattice distortion in the insulating state of (DCNQI) 2 Cu isstudied by exactly diagonalizing a two-band Peierls Hubbard model on the 6×2 lattice. Self-doping is essentially important and caused by strong commensurability pinning, which are a consequence of moderate coupling of DCNQI π electrons with lattice and strong correlation of Cu d electrons.

Quantum and Thermal Phase Transitions of Halogen-Bridged Binuclear Transition-Metal Complexes

Aiming to settle the controversial observations for halogen-bridged binuclear transition-metal (MMX) complexes, finite-temperature Hartree-Fock calculations are performed for a relevant two-band Peierls-Hubbard model. Thermal, as well as quantum, phase transitions are investigated with particular emphasis on the competition between electron itinerancy, electron-phonon interaction and electron-electron correlation. Recently observed distinct thermal behaviors of two typical MMX compounds Pt_2(CH_3CS_2)_4I and (NH_4)_4[Pt_2(P_2O_5H_2)_4I]2H_2O are supported and further tuning of their electronic states is predicted.

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.

Competing Ground States of the New Class of Halogen-Bridged Metal Complexes

Based on a symmetry argument, we study the ground-state properties of halogen-bridged binuclear metal chain complexes. We systematically derive commensurate density-wave solutions from a relevant two-band Peierls-Hubbard model and numerically draw the the ground-state phase diagram as a function of electron-electron correlations, electron-phonon interactions, and doping concentration within the Hartree-Fock approximation. The competition between two types of charge-density-wave states, which has recently been reported experimentally, is indeed demonstrated.

Optical excitations along CuO chains in copper oxides.

Optical responses in a one-dimensional two-band Peierls-Hubbard model near 5/8 filling (i.e., near 1/4 filling in the effective one-band model) are studied within inhomogeneous Hartree-Fock and random-phase approximations. Low-energy collective excitations are produced by a sliding motion of a spin-density wave. In the commensurate case (i.e., at 5/8 filling) or near this filling with moderate strength of intersite electron-lattice coupling, the spin-density wave (with polaronic defects in the latter case) is pinned by commensurability with the lattice, and the lowest-frequency optical response lies in the infrared regime. Otherwise, the spin-density wave is incommensurate and moves freely unless it is pinned by extrinsic impurities. Implications for midinfrared absorptions observed in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ and ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ compounds are discussed. With sufficiently weak hopping between the Cu and O sites, a ferromagnetic phase is found.

Excitations of the one-dimensional, two-band Peierls-Hubbard model at three-quarters filling in the large-U limit.

We investigate the ground state and the excitation spectrum associated with the localized defect states of the three-quarter-filled, one-dimensional Peierls-Hubbard model. In the continuum limit with an infinite, on-site electron-electron repulsion energy U, the Hamiltonian is mapped onto the N=1 Gross-Neveu model, in which irrationally charged soliton-antisoliton pairs are formed upon doping, while polarons are unstable. Corrections due to finite-U effects are discussed. However, discreteness effects are found to support excitations absent from the continuum field theory. In particular, for large band separation a novel type of polaron can be formed with a highly localized electronic wave function surrounded by an extended pattern of local lattice distortion.

Photoinduced charge separation, intrinsic self-doping and device applications of heterojunction materials: Mixed-halide MX solids

Halogen-bridged mixed-valence transition metal linear chain complexes (or MX chains, M: Pt, Pd, Ni; X: Cl, Br, I) are highly anisotropic, quasi-one-dimensional (Q1D) materials that are related to conducting polymers, mixed stack charge-transfer salts, and oxide superconductors in terms of their low dimensionality and competing electron-phonon and electron-electron interactions. Each set of counterion and equatorial ligand that holds the MX chains in a three-dimensional array acts as a different “template” for the apparently soft PtX chain geometry by determining the M-M distance. Recently we prepared single crystals of mixed-halide MX x X′ 1− x materials which consist of segments of pure MX and MX′ producing junctions between the distinct halide segments. We have extensively studied the following three systems: (1) PtCl x Br 1− x , (2) PtCl x I 1− x and (3) PtBr x I 1− x for a variety of concentrations x . For the PtCl x Br 1− x system we obtained direct spectroscopic evidence for charge separation from resonance Raman (RR) experiments on doped and photoexcited single crystals: electron polarons prefer to locate on the PtBr segment while the hole polarons are trapped within PtCl segments. Theoretical calculations based on a two-band Peierls-Hubbard model demonstrate strong hybridization of PtCl and PtBr electronic bands as the driving force for separation. In contrast to PtCl x Br 1− x , both the PtCl x Br 1− x and the PtCl x Br 1− x systems exhibit qualitatively different behavior, such as intrinsic “self doping”. Important photovoltaic device applications of these novel electronic materials are discussed.

Vibronic, Optical, and Magnetic Excitations around Solitons in a One-Dimensional Two-Band Peierls-Hubbard Model

Nonlinear doping states in a charge-density-wave background and those in a spin-density- wave background are investigated through their response functions in vibronic, optical and magnetic channels in a one-dimensional two-band Peierls-Hubbard model. A random phase approximation is used for their analysis on the basis of totally unrestricted Hartree-Fock solutions. Doping-induced changes of excitation spectra in these different channels are distinctive to the nature of individual doping states

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

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.

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.