221 research outputs found
Nonequilibrium Green's-Function Approach to the Suppression of Rectification at Metal--Mott-Insulator Interfaces
Suppression of rectification at metal--Mott-insulator interfaces, which is
previously shown by numerical solutions to the time-dependent Schr\"odinger
equation and experiments on real devices, is reinvestigated theoretically by
nonequilibrium Green's functions. The one-dimensional Hubbard model is used for
a Mott insulator. The effects of attached metallic electrodes are incorporated
into the self-energy. A scalar potential originating from work-function
differences and satisfying the Poisson equation is added to the model. For the
electron density, we decompose it into three parts. One is obtained by
integrating the local density of states over energy to the midpoint of the
electrodes' chemical potentials. The others, obtained by integrating lesser
Green's functions, are due to the couplings with the electrodes and correspond
to an inflow and an outflow of electrons. In Mott insulators, incoming
electrons and holes are extended over the whole system, avoiding further
accumulation of charge relative to the case without bias. This induces
collective charge transport and results in the suppression of rectification.Comment: 18 pages, Figs. 1(b), 2, and 8 replaced. Corrected typo
Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene--Chloranil
For electronic states and photoinduced charge dynamics near the neutral-ionic
transition in the mixed-stack charge-transfer complex
tetrathiafulvalene--chloranil (TTF-CA), we review the effects of Peierls
coupling to lattice phonons modulating transfer integrals and Holstein
couplings to molecular vibrations modulating site energies. The former
stabilizes the ionic phase and reduces discontinuities in the phase transition,
while the latter stabilizes the neutral phase and enhances the discontinuities.
To reproduce the experimentally observed ionicity, optical conductivity and
photoinduced charge dynamics, both couplings are quantitatively important. In
particular, strong Holstein couplings to form the highly-stabilized neutral
phase are necessary for the ionic phase to be a Mott insulator with large
ionicity. A comparison with the observed photoinduced charge dynamics indicates
the presence of strings of lattice dimerization in the neutral phase above the
transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: I. Threshold Behavior in Ionic-to-Neutral Transition
Photoinduced dynamics of charge density and lattice displacements is
calculated by solving the time-dependent Schr\"odinger equation for a
one-dimensional extended Peierls-Hubbard model with alternating potentials for
the mixed-stack organic charge-transfer complex, TTF-CA. A pulse of oscillating
electric field is incorporated into the Peierls phase of the transfer integral.
The frequency, the amplitude, and the duration of the pulse are varied to study
the nonlinear and cooperative character of the photoinduced transition. When
the dimerized ionic phase is photoexcited, the threshold behavior is clearly
observed by plotting the final ionicity as a function of the increment of the
total energy. Above the threshold photoexcitation, the electronic state reaches
the neutral one with equidistant molecules after the electric field is turned
off. The transition is initiated by nucleation of a metastable neutral domain,
for which an electric field with frequency below the linear absorption peak is
more effective than that at the peak. When the pulse is strong and short, the
charge transfer takes place on the same time scale with the disappearance of
dimerization. As the pulse becomes weak and long, the dimerization-induced
polarization is disordered to restore the inversion symmetry on average before
the charge transfer takes place to bring the system neutral. Thus, a
paraelectric ionic phase is transiently realized by a weak electric field. It
is shown that infrared light also induces the ionic-to-neutral transition,
which is characterized by the threshold behavior.Comment: 24 pages, 11 figure
Polaron Formation in the Three-Band Peierls-Hubbard Model for Cuprate Superconductors
Exact diagonalization calculations show a continuous transition from
delocalized to small polaron behavior as a function of intersite
electron-lattice coupling. A transition, found previously at Hartree-Fock level
[Yonemitsu et al., Phys. Rev. Lett. {\bf 69}, 965 (1992)], between a magnetic
and a non magnetic state does not subsist when fluctuations are included. Local
phonon modes become softer close to the polaron and by comparison with optical
measurements of doped cuprates we conclude that they are close to the
transition region between polaronic and non-polaronic behavior. The barrier to
adiabatically move a hole vanishes in that region suggesting large mobilities.Comment: 7 pages + 3 poscript figures, Revtex 3.0, MSC-199
Phonon Assisted Multimagnon Optical Absorption and Long Lived Two-Magnon States in Undoped Lamellar Copper Oxides
We calculate the effective charge for multimagnon infrared (IR) absorption
assisted by phonons in the parent insulating compounds of cuprate
superconductors and the spectra for two-magnon absorption using interacting
spin-wave theory. Recent measured bands in the mid IR [Perkins et al. Phys.
Rev. Lett. {\bf 71} 1621 (1993)] are interpreted as involving one phonon plus a
two-magnon virtual bound state, and one phonon plus higher multimagnon
absorption processes. The virtual bound state consists of a narrow resonance
occurring when the magnon pair has total momentum close to .Comment: 4 page
Charge-Transfer Excitations in One-Dimensional Dimerized Mott Insulators
We investigate the optical properties of one-dimensional (1D) dimerized Mott
insulators using the 1D dimerized extended Hubbard model. Numerical
calculations and a perturbative analysis from the decoupled-dimer limit clarify
that there are three relevant classes of charge-transfer (CT) states generated
by photoexcitation: interdimer CT unbound states, interdimer CT exciton states,
and intradimer CT exciton states. This classification is applied to
understanding the optical properties of an organic molecular material,
1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its
photoinduced transition from the dimerized spin-singlet phase to the regular
paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is
the interdimer CT exciton state and the second lowest state is the intradimer
CT exciton state.Comment: 6 pages, 6 figures, to be published in J. Phys. Soc. Jp
Nonlinear Conduction by Melting of Stripe-Type Charge Order in Organic Conductors with Triangular Lattices
We theoretically discuss the mechanism for the peculiar nonlinear conduction
in quasi-two-dimensional organic conductors \theta-(BEDT-TTF)2X
[BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene] through the melting of
stripe-type charge order. An extended Peierls-Hubbard model attached to
metallic electrodes is investigated by a nonequilibrium Green's function
technique. A novel current-voltage characteristic appears in a coexistent state
of stripe-type and nonstripe 3-fold charge orders, where the applied bias melts
mainly the stripe-type charge order through the reduction of lattice
distortion, whereas the 3-fold charge order survives. These contrastive
responses of the two different charge orders are consistent with the
experimental observations.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jp
Dynamic changes in cerebral oxygenation by two methods during cardiac surgery and postoperative cognitive decline
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