91 research outputs found
Electronic Ferroelectricity and Frustration
Beyond a conventional classification of ferroelectricity, there is a class of
materials where electronic degrees of freedom and electronic interactions are
directly responsible for electric polarization and ferroelectric transition.
This is termed electronic ferroelectricity. In this article, we review
electronic ferroelectricity from a view point of frustration. Experimental and
theoretical examinations in spin driven ferroelectric materials, recently
termed multiferroics, are introduced. Spin frustration caused by competing
magnetic interactions is of prime importance for this type of ferroelectricity.
Charge driven ferroelectricity where electronic charge order induces electric
polarization is reviewed. In particular, exotic dielectric and
magneto-dielectric properties in layered iron oxides are focused on. Through a
number of recent experimental and theoretical researches, charge fluctuation
due to frustrated geometry plays essential roles on electronic ferroelectricity
in this compound.Comment: 13 pages, 13 figures, to be published in J. Phys. Sco. Jpn. Special
Topics "Novel States of Matter Induced by Frustration
Hole dynamics in spin and orbital ordered vanadium perovskites
Hole dynamics in spin and orbital ordered vanadates with perovskite structure
is investigated. A mobile hole coupled to the spin excitation (magnon) in the
spin G-type and orbital C-type (SG/OC) ordered phase, and that to the orbital
excitation (orbiton) in the spin C-type and orbital G-type (SC/OG) one are
formulated on an equal footing. The observed fragile character of the (SG/OC)
order is attributed to the orbiton softening caused by a reduction of the
taggered magnetic order parameter. It is proposed that the qualitatively
different hole dynamics in the two spin-orbital ordered phases in vanadates can
be probed by the optical spectra.Comment: 4pages, 4figure
Transient Carrier Dynamics in a Mott Insulator with Antiferromagnetic Order
We study transient dynamics of hole carriers injected at a certain time into
a Mott insulator with antiferromagnetic long range order. This is termed
``dynamical hole doping" as contrast with chemical hole doping. Theoretical
framework for the transient carrier dynamics are presented based on the two
dimensional model. Time dependences of the optical conductivity spectra
as well as the one-particle excitation spectra are calculated based on the
Keldysh Green's function formalism at zero temperature combined with the
self-consistent Born approximation. At early stage after dynamical hole doping,
the Drude component appears, and then incoherent components originating from
hole-magnon scatterings start to grow. Fast oscillatory behavior due to
coherent magnon, and slow relaxation dynamics are confirmed in the spectra.
Time profiles are interpreted as that doped bare holes are dressed by magnon
clouds, and are relaxed into spin polaron quasi-particle states. Characteristic
relaxation times for Drude and incoherent peaks strongly depend on momentum of
a dynamically doped hole, and the exchange constant. Implications to the recent
pump-probe experiments are discussed.Comment: 13 pages, 14 figure
Photoinduced charge-order melting dynamics in a one-dimensional interacting Holstein model
Transient quantum dynamics in an interacting fermion-phonon system are
investigated. In particular, a charge order (CO) melting after a short
optical-pulse irradiation and roles of the quantum phonons on the transient
dynamics are focused on. A spinless-fermion model in a one-dimensional chain
coupled with local phonons is analyzed numerically. The infinite time-evolving
block decimation algorithm is adopted as a reliable numerical method for
one-dimensional quantum many-body systems. Numerical results for the
photoinduced CO melting dynamics without phonons are well interpreted by the
soliton picture for the CO domains. This interpretation is confirmed by the
numerical simulation for an artificial local excitation and the classical
soliton model. In the case of the large phonon frequency corresponding to the
antiadiabatic condition, the CO melting is induced by propagations of the
polaronic solitons with the renormalized soliton velocity. On the other hand,
in the case of the small phonon frequency corresponding to the adiabatic
condition, the first stage of the CO melting dynamics occurs due to the energy
transfer from the fermionic to phononic systems, and the second stage is
brought about by the soliton motions around the bottom of the soliton band.
Present analyses provide a standard reference for the photoinduced CO melting
dynamics in low-dimensional many-body quantum systems.Comment: 11pages, 13 figure
Double-Exchange Interaction in Optically Induced Nonequilibrium State: A Conversion from Ferromagnetic to Antiferromagnetic Structure
The double-exchange (DE) interaction, that is, a ferromagnetic (FM)
interaction due to a combination of electron motion and the Hund coupling, is a
well known source of a wide class of FM orders. Here, we show that the DE
interaction in highly photoexcited states is antiferromagnetic (AFM). Transient
dynamics of quantum electrons coupled with classical spins are analyzed. An ac
field applied to a metallic FM state results in an almost perfect N\'eel state.
A time characterizing the FM-to-AFM conversion is scaled by light amplitude and
frequency. This hidden AFM interaction is attributable to the electron-spin
coupling under nonequilibrium electron distribution.Comment: 6 pages, 7 figures (including supplemental material
Magnetoelectric effect in organic molecular solids
The Magnetoelectric (ME) effect in solids is a prominent cross correlation
phenomenon, in which the electric field () controls the magnetization
() and the magnetic field () controls the electric
polarization (). A rich variety of ME effects and their potential in
practical applications have been investigated so far within the
transition-metal compounds. Here, we report a possible way to realize the ME
effect in organic molecular solids, in which two molecules build a dimer unit
aligned on a lattice site. The linear ME effect is predicted in a long-range
ordered state of spins and electric dipoles, as well as in a disordered state.
One key of the ME effect is a hidden ferroic order of the spin-charge composite
object. We provide a new guiding principle of the ME effect in materials
without transition-metal elements, which may lead to flexible and lightweight
multifunctional materials.Comment: 22 pages, 5 figure
Photocontrol of magnetic structure in an itinerant magnet
We study the photoinduced magnetic transition in an itinerant magnet
described by the double-exchange model, in which conduction electrons couple
with localized spins through the ferromagnetic (FM) Hund coupling. It is shown
that intense light applied to the FM ground state induces an antiferromagnetic
(AFM) order, in contrast to the AFM-to-FM transition due to the photocarrier
injection. In particular, we focus on the mechanism for instability of the FM
structure by the light irradiation. The magnon spectrum in the Floquet state is
formulated on the basis of the pertrubative expansion of the Floquet Green
function. The magnon dispersion shows softening at momentum in the
square lattice with increasing the light amplitude, implying photoinduced AFM
instability. This result is mainly attributed to a nonequilibrium electron
distribution, which promotes low-energy Stoner excitations. The transient
optical conductivity spectra characterized by interband excitations and Floquet
sidepeaks are available to identify the photoinduced AFM state.Comment: 13 pages, 9 figure
Nonequilibrium susceptibility in photoinduced Floquet states
Nonequilibrium susceptibility in photoinduced Floquet states is studied. We
analyze an electron system coupled with a heat bath in a time-periodic
oscillating electric field. Spin/charge susceptibility is formulated on the
basis of the Floquet Green function method, and is calculated numerically in a
wide range of amplitude and frequency of light. When the frequency is larger
than the bandwidth, the susceptibility is enhanced due to the dynamical
localization effect, and their peak positions in the momentum space are shifted
by the Fermi surface deformation. In the case of the small frequency and
amplitude, multiple-peak structure emerges in the susceptibility, originating
from the multiple Floquet bands which cross the Fermi level. To confirm those
numerical results and provide the interpretation, an approximated expression of
the susceptibility is derived for small electric-field amplitude.Comment: 15 pages, 10 figure
Photoinduced correlated electron dynamics in a two-leg ladder Hubbard system
Photoinduced carrier dynamics in a correlated electron system on a coupled
two-leg ladder lattice are studied. The two-leg ladder Hubbard model is
analyzed by utilizing the exact diagonalization method based on the Lanczos
algorithm in finite size clusters. In order to reveal the transient carrier
dynamics after photoirradiation, we calculate the low-energy components of the
hole kinetic energy, the pair-field correlation function, the optical
conductivity spectra and others. It is shown that the photoinduced
metallic-like state appears in a half filled Mott insulating state, while the
low-energy carrier motion is suppressed by photoirradiation in hole doped
metallic states. These photoinduced changes in electron dynamics are associated
with changes in the carrier-pair coherence, and are not attributed to a naive
thermalization but to a ladder-lattice effect. Based on the numerical results,
optical controls of hole pairs by using the double-pulse pumping are
demonstrated. Implications to the recent optical pump-probe experiments are
presented.Comment: 9 pages, 10 figure
Orbital Ordering and Resonant X-ray Scattering in Layered Manganites
In layered manganites with orbital and charge orderings, the degeneracy of
the Mn orbitals as well as the ones is lifted by the effects of the
bands and the local Coulomb interactions. We formulate the atomic
scattering factor for the resonant x-ray scattering in the memory function
method by taking into account these effects on an equal footing. It is shown
that the polarization dependences of the scattering intensities at the orbital
and charge superlattice reflections observed in LaSrMnO are
caused by the local and itinerant characters of electrons, respectively.
We examine the type of the orbital ordered state.Comment: 4 pages, 3 figure
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