21 research outputs found
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 decreases or phonon
frequency 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
are treated through an energy-dependent electron-phonon
scattering function 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
Photoinduced infrared absorption of quasi-one-dimensional halogen-bridged binuclear transition-metal complexes
We investigate the optical conductivity of photogenerated solitons in
quasi-one-dimensional halogen-bridged binuclear transition-metal MMX complexes
with particular emphasis on a comparison among the three distinct groups:
A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O (X=Cl,Br,I; A=Na,K,NH_4_,...),
Pt_2_(RCS_2_)_4_I (R=C_n_H_2n+1_) and Ni_2_(CH_3_CS_2_)_4_I, which exhibit a
mixed-valent ground state with the X sublattice dimerized, that with the M_2_
sublattice dimerized and a Mott-Hubbard magnetic ground state, respectively.
Soliton-induced absorption spectra for A_4_[Pt_2_(P_2_O_5_H_2_)_4_X]nH_2_O
should split into two bands, while that for Pt_2_(RCS_2_)_4_I and
Ni_2_(CH_3_CS_2_)_4_I should consist of a single band. The excitonic effect is
significant in Ni_2_(CH_3_CS_2_)_4_I.Comment: 4 pages, 3 figure
Photoinduced charge separation in Q1D heterojunction materials: Evidence for electron-hole pair separation in mixed-halide solids
Resonance Raman experiments on doped and photoexcited single crystals of
mixed-halide complexes (=Pt; =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]
Novel Density-Wave States of Two-Band Peierls-Hubbard Chains
Based on a symmetry argument we systematically reveal Hartree-Fock
broken-symmetry solutions of the one-dimensional two-band extended
Peierls-Hubbard model, which covers various materials of interest such as
halogen-bridged metal complexes and mixed-stack charge-transfer salts. We find
out all the regular-density-wave solutions with an ordering vector or
. Changing band filling as well as electron-electron and electron-phonon
interactions, we numerically inquire further into the ground-state phase
diagram and the physical property of each state. The possibility of novel
density-wave states appearing is argued.Comment: 10 pages, 6 PS figures, to appear in Phys. Lett.
Ground-state properties of a Peierls-Hubbard triangular prism
Motivated by recent chemical attempts at assembling halogen-bridged
transition-metal complexes within a nanotube, we model and characterize a
platinum-halide triangular prism in terms of a Peierls-Hubbard Hamiltonian.
Based on a group-theoretical argument, we reveal a variety of valence
arrangements, including heterogeneous or partially metallic charge-density-wave
states. Quantum and thermal phase competitions are numerically demonstrated
with particular emphasis on novel insulator-to-metal and insulator-to-insulator
transitions under doping, the former of which is of the first order, while the
latter of which is of the second order.Comment: 9 pages, 7 figures. to be published in J. Phys. Soc. Jpn. Vol. 79,
No.
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
Quantum Breathers in a Nonlinear Lattice
We study nonlinear phonon excitations in a one-dimensional quantum nonlinear
lattice model using numerical exact diagonalization. We find that multi-phonon
bound states exist as eigenstates which are natural counterparts of breather
solutions of classical nonlinear systems. In a translationally invariant
system, these quantum breather states form particle-like bands and are
characterized by a finite correlation length. The dynamic structure factor has
significant intensity for the breather states, with a corresponding quenching
of the neighboring bands of multi-phonon extended states.Comment: 4 pages, RevTex, 4 postscript figures, Physical Relview Letters (in
press
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