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 $t-J$ 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 (${\bm E}$) controls the magnetization (${\bm M}$) and the magnetic field (${\bm H}$) controls the electric polarization (${\bm P}$). 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 $(\pi,\pi)$ 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 $4p$ orbitals as well as the $3d$ ones is lifted by the effects of the $4p$ 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 LaSr$_{2}$Mn$_2$O$_7$ are caused by the local and itinerant characters of $4p$ electrons, respectively. We examine the type of the orbital ordered state.Comment: 4 pages, 3 figure