249 research outputs found
Enhanced coherent dynamics near a transition between neutral quantum-paraelectric and ionic ferroelectric phases in the quantum Blume-Emery-Griffiths model
Nonequilibrium dynamics are studied near the quantum phase transition point
in the one-dimensional quantum Blume-Emery-Griffiths model. Its pseudo-spin
component represents an electric polarization, and
corresponds to ionicity, in mixed-stack charge-transfer complexes that exhibit
a transition between neutral quantum-paraelectric and ionic ferroelectric (or
antiferroelectric) phases. The time-dependent Schr\"odinger equation is solved
for the exact many-body wave function in the quantum paraelectric phase. After
impact force is introduced on a polarization locally in space and time,
polarizations and ionicity coherently oscillate. The oscillation amplitudes are
large near the quantum phase transition point. The energy supplied by the
impact flows linearly into these oscillations, so that the nonequilibrium
behavior is uncooperative.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: III. Interference Caused by a Double Pulse
In order to study consequences of the differences between the
ionic-to-neutral and neutral-to-ionic transitions in the one-dimensional
extended Peierls-Hubbard model with alternating potentials for the TTF-CA
complex, we introduce a double pulse of oscillating electric field in the
time-dependent Schr\"odinger equation and vary the interval between the two
pulses as well as their strengths. When the dimerized ionic phase is
photoexcited, the interference effect is clearly observed owing to the
coherence of charge density and lattice displacements. Namely, the two pulses
constructively interfere with each other if the interval is a multiple of the
period of the optical lattice vibration, while they destructively interfere if
the interval is a half-odd integer times the period, in the processes toward
the neutral phase. The interference is strong especially when the pulse is
strong and short because the coherence is also strong. Meanwhile, when the
neutral phase is photoexcited, the interference effect is almost invisible or
weakly observed when the pulse is weak. The photoinduced lattice oscillations
are incoherent due to random phases. The strength of the interference caused by
a double pulse is a key quantity to distinguish the two transitions and to
evaluate the coherence of charge density and lattice displacements.Comment: 16 pages, 8 figure
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: II. Linear Behavior in Neutral-to-Ionic Transition
Dynamics of charge density and lattice displacements after the neutral phase
is photoexcited is studied by solving the time-dependent Schr\"odinger equation
for a one-dimensional extended Peierls-Hubbard model with alternating
potentials. In contrast to the ionic-to-neutral transition studied previously,
the neutral-to-ionic transition proceeds in an uncooperative manner as far as
the one-dimensional system is concerned. The final ionicity is a linear
function of the increment of the total energy. After the electric field is
turned off, the electronic state does not significantly change, roughly keeping
the ionicity, even if the transition is not completed, because the ionic
domains never proliferate. As a consequence, an electric field with frequency
just at the linear absorption peak causes the neutral-to-ionic transition the
most efficiently. These findings are consistent with the recent experiments on
the mixed-stack organic charge-transfer complex, TTF-CA. We artificially modify
or remove the electron-lattice coupling to discuss the origin of such
differences between the two transitions.Comment: 17 pages, 9 figure
Charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4: Cooperative effects of electron correlations and lattice distortions
Combined effects of electron correlations and lattice distortions are
investigated on the charge ordering in \theta-(BEDT-TTF)2RbZn(SCN)4
theoretically in a two-dimensional 3/4-filled extended Hubbard model with
electron-lattice couplings. It is known that this material undergoes a phase
transition from a high-symmetry metallic state to a low-symmetry insulating
state with a horizontal-stripe charge order (CO) by lowering temperature. By
means of the exact-diagonalization method, we show that electron-phonon
interactions are crucial to stabilize the horizontal-stripe CO and to realize
the low-symmetry crystal structure.Comment: 7 peges, 7 figures, accepted for publication in Phys. Rev.
Photoinduced coherent oscillations in the one-dimensional two-orbital Hubbard model
We study photoinduced ultrafast coherent oscillations originating from
orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By
solving the time-dependent Schr\"odinger equation for the numerically exact
many-electron wave function, we obtain time-dependent optical response
functions. The calculated spectra show characteristic coherent oscillations
that vary with the frequency of probe light. A simple analysis for the dominant
oscillating components clarifies that these photoinduced oscillations are
caused by the quantum interference between photogenerated states. The
oscillation attributed to the Raman-active orbital excitations (orbitons)
clearly appears around the charge-transfer peak.Comment: 5 pages, 5 figure
Electron-Phonon Driven Spin Frustration in Multi-Band Hubbard Models: MX Chains and Oxide Superconductors
We discuss the consequences of both electron-phonon and electron-electron
couplings in 1D and 2D multi-band (Peierls-Hubbard) models. After briefly
discussing various analytic limits, we focus on (Hartree-Fock and exact)
numerical studies in the intermediate regime for both couplings, where unusual
spin-Peierls as well as long-period, frustrated ground states are found. Doping
into such phases or near the phase boundaries can lead to further interesting
phenomena such as separation of spin and charge, a dopant-induced phase
transition of the global (parent) phase, or real-space (``bipolaronic'')
pairing. We discuss possible experimentally observable consequences of this
rich phase diagram for halogen-bridged, transition metal, linear chain
complexes (MX chains) in 1D and the oxide superconductors in 2D.Comment: 6 pages, four postscript figures (appended), in regular Te
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
Mott insulating state in a quarter-filled two-orbital Hubbard chain with different bandwidths
We investigate the ground-state properties of the one-dimensional two-band
Hubbard model with different bandwidths. The density-matrix renormalization
group method is applied to calculate the averaged electron occupancies as a
function of the chemical potential . Both at quarter and half fillings,
"charge plateaux" appear in the - plot, where diverges and
the Mott insulating states are realized. To see how the orbital polarization in
the one-quarter charge plateau develops, we apply the second-order perturbation
theory from the strong-coupling limit at quarter filling. The resultant
Kugel-Khomskii spin-orbital model includes a field coupled to
orbital pseudo-spins. This field originates from the discrepancy between the
two bandwidths and leads to a finite orbital pseudo-spin magnetization.Comment: 4 pages, 2 figures, Proceedings of LT2
Nonequilibrium transport and optical properties of model metal--Mott-insulator--metal heterostructures
Electronic properties of heterostructures in which a finite number of
Mott-insulator layers are sandwiched by semi-infinite metallic leads are
investigated by using the dynamical-mean-field method combined with the Keldysh
Green's function technique to account for the finite bias voltage between the
leads. Current across the junction is computed as a function of bias voltage.
Electron spectral functions in the interacting region are shown to evolve by an
applied bias voltage. This effect is measurable by photoemission spectroscopy
and scanning tunneling microscopy. Further predictions are made for the optical
conductivity under a bias voltage as a possible tool to detect a deformed
density of states. A general discussion of correlated-electron based
heterostructures and future prospect is given.Comment: 11 pages, 11 figures, published versio
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
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