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This work reviews the experimental studies on photo-induced cooperative phenomena (photo-induced phase transitions, PIPT), which are related to ultrafast structural and electronic-state dynamics. The review includes a brief history of this research field. The current growth of PIPT research is facilitated by deep collaboration between quantum beam science and materials science. Meanwhile, owing to developments in ultrafast quantum beam technology, the concept of PIPT itself has largely expanded. In the initial stage of PIPT research, the dynamical PIPT process was thought to be governed by energy relaxation from the photoexcited state, which is a classical and incoherent process. Therefore, the main research target was realizing the quasi-stable state on the energy surface of the ground state. Using ultrashort pulsed light and quantum beam technologies (including X-ray and electron technologies), one can now directly probe the ultrafast and coherent electronic dynamics of PIPT materials coupled with changes in their structural properties. This new technology has realized a new ordered phase called the hidden state, which is unique to the photoexcited state. Modern laser technology has also enabled a controlled PIPT process utilizing coherent coupling between the light field and electronic states in materials. Exploiting these leaps in experimental techniques, we have expanded the way of controlling PIPT dynamics from classical (incoherent) to coherent/quantum processes. This review discusses the recent developments in PIPT tuning via quantum (coherent) control methods based on charge–lattice (orbital)–spin coupled materials.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNi1-xFexO3 (0.25 ≤ x ≤ 0.5). The low-temperature phase was found to have a polar structure (space group of R3c) with a Bi3+0.5(1+x)Bi5+0.5(1-x)Ni2+1-xFe3+xO3 charge distribution and short-range ordering of Bi3+ and Bi5+. The volume reduction upon heating that was induced by charge transfer between Bi5+ and Ni2+ decreased with increasing x because of the reduction in the amount of Ni2+. Simultaneous polar-nonpolar transition also contributed to NTE, and a composition-independent enhanced volume reduction of ∼2% was observed.

BiCoO3 is a PbTiO3 type of perovskite oxide with a giant tetragonal distortion (c/a = 1.27) that shows a pressure-induced transition from tetragonal to orthorhombic phases accompanied by a large volume shrinkage at 3 GPa. In this study, we carried out electron doping of BiCoO3 by substituting Ti4+ for Co3+ in order to destabilize the tetragonal phase and observe a giant negative thermal expansion (NTE) at ambient pressure. BiCo1-xTixO3 (x = 0, 0.1, 0.2, and 0.25) was successfully obtained by using high-pressure synthesis. However, the c/a ratio of the tetragonal phase was almost constant against x (≤0.2), and NTE was not observed at any x, suggesting that the tetragonal distortion coupled with high-spin Co3+ is robust against electron doping. In x = 0.25, a metastable orthorhombic phase was obtained by the high-pressure synthetic process, while it partially transformed into a tetragonal phase after annealing at 600 K. The stability of the giant tetragonal phase is strongly connected with the spin state of Co3+.

Perovskite-oxide Bi1–xPbxNiO3 for 0.60 ≤ x ≤ 0.80 was found to show a polar orthorhombic-to-nonpolar orthorhombic phase transition accompanied by negative thermal expansion. Bi1–xPbxNiO3 showed suc...

One of the most interesting topics regarding spin-crossover (SC) materials is optical control of the electronic state. In this chapter, recent trials on the photocontrol of some SC cobaltites are reviewed. In contrast to iron SC complexes, the time scale required for photonic change is on the sub-picosecond level for the cobaltites in general, so femtosecond laser spectroscopy is indispensable for revealing photoinduced SC phenomena. On the basis of an ultrafast linear and nonlinear spectroscopic technique using femtosecond laser pulses, we discuss the photoinduced change of the electronic structure as well as real space dynamics for some exotic SC cobaltites, Pr\(_{0.5}\)Ca\(_{0.5}\)CoO\(_3\) and BiCoO\(_3\).

To investigate photoinduced phenomena in various materials and molecules, ultrashort pulsed x-ray and electron sources with high brightness and high repetition rates are required. The x-ray and electron’s typical and de Broglie wavelengths are shorter than lattice constants of materials and molecules. Therefore, photoinduced structural dynamics on the femtosecond to picosecond timescales can be directly observed in a diffraction manner by using these pulses. This research created a tabletop ultrashort pulsed electron diffraction setup that used a femtosecond laser and electron pulse compression cavity that was directly synchronized to the microwave master oscillator (∼3 GHz). A compressed electron pulse with a 1 kHz repetition rate contained 228 000 electrons. The electron pulse duration was estimated to be less than 100 fs at the sample position by using photoinduced immediate lattice changes in an ultrathin silicon film (50 nm). The newly developed time-resolved electron diffraction setup has a pulse duration that is comparable to femtosecond laser pulse widths (35–100 fs). The pulse duration, in particular, fits within the timescale of photoinduced phenomena in quantum materials. Our developed ultrafast time-resolved electron diffraction setup with a sub-100 fs temporal resolution would be a powerful tool in material science with a combination of optical pump–probe, time-resolved photoemission spectroscopic, and pulsed x-ray measurements.

Spontaneous polarization (

Spontaneous polarization (Ps) of novel order-disorder type lead-free ferroelectric CaMnTi2O6 was successfully enhanced by partial V4+ substitution for Ti4+. A synchrotron X-ray diffraction study revealed that the polar displacement of octahedrally coordinated (Ti, V) in CaMn(Ti1-xVx)2O6 (0 ≤ x ≤ 0.4) increases with V4+ substitution having Jahn-Teller activity owing to the d1 electronic configuration. Our magnetic study suggested the presence of antisite disorder between Ca2+ and square planar coordinated Mn2+ associated with Mn-V intermetallic charge transfer for x ≥ 0.4, resulting in decreases in spontaneous polarization and the ferroelectric-paraelectric transition temperature. This is the first report on the enhanced polarization owing to the Jahn-Teller distortion of V4+ without stereochemical Pb2+ or Bi3+.

Understanding and controlling atomic motions in solid are essential in ionic materials, such as ion conductors, electrolyte and electrodes of batteries, and gas separators. Although photoinduced proton transport in solid materials has been demonstrated, photoinduced transport of heavier ions, such as the oxide ion, in solid materials remains entirely elusive. Herein, we apply ultrafast time-resolved electron diffraction to observe the large displacement of oxide ions in a strongly correlated material, EuBaCo2O5.39, under near-ultraviolet photoexcitation at room temperature. The mechanism of the photoinduced transport of oxide ions is revealed via the combination of ultrafast optical spectroscopy and density functional theory calculations. Our findings show that photoexcitation induces ionic motions in a ceramic crystal at room temperature through the charge transfer that strongly couples with large ionic displacement under the nonequilibrium condition.

BiCoO

Photoexcitation effects on ferroelectricity were investigated in a hydrogen-bonded supramolecular cocrystal by using nonlinear femtosecond spectroscopy. Irradiation of a terahertz pulse that caused...

複電荷希土類酸化鉄材料YbFe2O4は電荷秩序によって分極が発生すると期待されているが、直接的に電子分極を観測したことはなかったが第二次高調波の観測及び中性子による構造解析から電荷秩序に起因した電子分極が存在することを発見した。またこれらの結果から電荷秩序の点群が定まり、長らく議論されていた電荷秩序構造を一意的に導出することができた。

We fabricated a (111) oriented epitaxial thin film of ferroelectric iron oxide, BiFeO3 and investigated the nonlinear optical properties and its femtosecond dynamics. The angle dependence of second...

In a polar material, a laser pulse can trigger nonlinear optical phenomena, such as second-harmonic generation (SHG). The authors investigate the variation of SHG in BiCoO${}_{3}$ upon irradiation with femtosecond terahertz pulses. Applying a THz pulse, they observe enhancement of SHG by more than 50% at room temperature, the response being so fast that it simply follows the THz pulse. This huge, ultrafast enhancement, explained in terms of a photon dressed state, could be used to control nonlinear optical properties at the femtosecond timescale.

Photoirradiation effect in a ferroelectric cocrystal Hdppz-Hca is investigated in terms of time-resolved second harmonic (SH) generation measurements. By the photoexcitation with a visible pulse (530 nm) causing the intramolecular transition of Hca molecule, the SH intensity was suddenly suppressed by ≈30% on the time scale of picosecond, while the reflectivity scarcely changed. This result is discussed based on reversal of the polarized domain by the photoexcitation.

Over the past two decades (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene), which exhibits a metal&ndash, insulator (M&ndash, I) phase transition with charge&ndash, ordering (CO), has been investigated energetically because of attractive characteristics that include ultrafast and massive photoinduced spectral and structural changes. In contrast, while its crystal structure has much in common with the (EDO-TTF)2PF6 crystal, the organic alloy system of [(EDO-TTF)0.89(MeEDO-TTF)0.11]2PF6 (MeEDO-TTF = 4,5-ethylenedioxy-4&prime, methyltetrathiafulvalene) exhibits a quite different type of M&ndash, I phase transition that is attributed to Peierls instability. Here, an optical study of the static absorption spectra and the time-resolved changes in the absorption spectra of [(EDO-TTF)0.89(MeEDO-TTF)0.11]2PF6 are reported. The observed absorption spectra related to the electronic structure are highly anisotropic. With a reduction in temperature (T), the opening of a small optical gap and a small shift in the center frequency of the C=C stretching mode are observed along with the M&ndash, I phase transition. Additionally, photoinduced transient states have been assigned based on their relaxation processes and transient intramolecular vibrational spectra. Reflecting small valence and structural changes and weak donor&ndash, anion interactions, a photoinduced transient state that is similar to the thermal-equilibrium high-T metallic phase appears more rapidly in the alloy system than that in (EDO-TTF)2PF6.

To clarify the mechanism of the later process of photoinduced phase transition (PIPT) in organic charge-transfer complexes, we examined by time-resolved infrared vibrational spectroscopy two dimeri...

The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 ${\mathrm{cm}}^{\ensuremath{-}1}$. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system.

Using time-resolved near-infrared reflectance spectroscopy and time-resolved mid-infrared vibrational spectroscopy, we studied photoinduced phase transition of the charge-ordered insulating phase in a charge-transfer complex (EDO-TTF)2PF6 (EDO-TTF: ethylenedioxy-tetrathiafulvalene) in the hundred picosecond range after photoexcitation. The temporal profiles at 0.83–1.03 eV, which are a characteristic of the photoinduced charge-disproportionate phase immediately after photoexcitation, suggested the formation of a new metastable phase in the hundred picosecond range. Time-resolved vibrational spectra at 1300–1700 cm–1, where charge- and structure-sensitive C═C stretching vibrational modes are located, elucidated that the nature of the new phase is very close to that of the high-temperature metallic phase and it takes about 100 ps for the new phase to emerge accompanied with charge and structure fluctuation.

Search for the Photo-Induced Hidden Phase in Inorganic and Organic Systems S. Koshiharaa,∗, K. Onda, Y. Okimoto and T. Ishikawa JST, CREST & Department of Materials Science, Tokyo Institute of Technology Meguro-ku Tokyo 152-8551, Japan Department of Environmental Chem. and Engineering, Tokyo Institute of Technology Nagatsuta-Yokohama, 226-8503, Japan We review the dynamical behavior of strongly electron–lattice (orbital) coupled system induced by photo-excitation of fs laser pulse. By virtue of the femtosecond (fs) spectroscopy and picosecond (ps) time-resolved X-ray diffraction measurements, we demonstrate that the photo-irradiation creates new ordered state so called as “hidden phase” which can never be realized under thermo-equilibrium condition. This exotic phase commonly plays the key role for realizing the gigantic photo-induced changes in optical properties of inorganic and organic electron–lattice strongly coupled systems.

Titanium dioxide (TiO2), with its large band gap, has attracted much attention due to its excellent photocatalytic activity. TiO2 ball-shaped nano-particles were deposited on silicon substrates by a thermal oxidation approach. With an increase in the annealing temperature the surface morphology and the structure of TiO2 remained stable, exhibiting good heat stability; meanwhile, the hydrogen production rate also increased. The femtosecond pump-probe spectroscopic study showed that the lifetime of carriers of the samples as- deposited and post-annealed at different temperatures were longer than 20 ps.

The ultrafast dynamics of photoinduced phase transition (PIPT) in a crystal of Pr1–xCax MnO3 (x = 0.37) has been studied by means of femtosecond time-resolved spectroscopy. Just after photoirradiation of 1.58 eV light at 100 K, ultrafast change of reflectivity (ΔR/R) was observed within ∼1 ps utilizing the probe light of 0.5 eV due to the ferromagnetic metallic phase transition. While the fast component of ΔR/R gradually increases with increasing the excitation intensity, the slow component observed at the delay time of ∼1 ps shows unusual dependence on the excitation intensity. The signature of ΔR/R changes from negative to positive with the excitaiton power, which cannot be explained by the photo-thermal heating effect. In addition, we investigated the temperature dependence of the ΔR/R at 0.5 eV and observed the sudden change of ΔR/R around TN, which indicates that the efficiency of PIPT is closely correlated with the ordering of spin and extra electron in Mn4+. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photo-control of material properties on femto- (10−15) and pico- (10−12) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr14-xCaxCu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature., Controlling material properties on sub-picosecond scales using photons could allow for ultrafast optoelectronic devices. Here, the authors propose an ultrafast photoinduced metal-to-insulator transition in a two-leg ladder cuprate superconductor based on time-resolved reflection spectroscopy.

Time-resolved linear and nonlinear optical responses were investigated in an organic supramolecular ferroelectric material composed of protonated 2,3-di(2-pyridinyl)pyrazine (H-dppz) and deprotonated chloranilic acid (Hca). We irradiated nanosecond laser pulses (λ = 532 nm) on the crystal, pumped the intramolecular excitation of the Hca molecule, and observed a clear redshift of the molecular vibrational modes of C=O and C–O− just after the photoexcitation. Each softened mode gradually relaxed on different time scales, indicating that the electrons of the Hca molecules were redistributed after the photoexcitation. By the same excitation, a large suppression of the second-harmonic (SH) intensity was observed, driven by the macroscopic disordering of the transferred protons. The decay time of the SH intensity was longer than those of the vibrational modes, suggesting that the microscopic vibrations and macroscopic ferroelectricity have dynamics on different time scales.

In this paper, a competition between a ferromagnetic metal and a charge/orbital-ordered insulator has been investigated for RE 1− x AE x MnO 3 ( x = 0.45 ) with RE and AE being rare earth elements, ranging from La to Gd, and alkaline earth elements, Ca, Sr and Ba, respectively. A phase diagram is deduced in the plane of the effective one-electron bandwidth versus the magnitude of quenched disorder, which can be controlled by the averaged value and the variance of the RE/AE ionic radii, respectively. In the vicinity of the phase boundary, the ferromagnetic transition temperature is critically suppressed, and the colossal magnetoresistance (CMR) is enhanced.

Neutral–ionic (NI) phase transition is a reversible switching of organic charge-transfer complexes between distinct valence states by external stimuli. This phase transformation in the low-dimensional system is demonstrated to provide a variety of novel dielectric, structural, and electronic properties. Importantly, ionization of the electron donor–acceptor pairs is usually accompanied by a ferroelectric or antiferroelectric order of the molecular lattice, leading to huge dielectric response near the transition point. Although these characteristics are potentially useful for future electronic and optical applications, the thermally accessible NI transition (TINIT) is still an extremely rare case. The TINIT compounds including some new materials are overviewed in order to provide convenient guides to their design and experimental identifications. The phase transition and dielectric properties can be closely controlled in various ways depending on chemical and physical modifications of the crystals. Among them, a quantum phase transition and relaxor ferroelectricity, both of which are currently attracting subjects from both scientific and practical perspectives, are highlighted as the first achievements in organic charge-transfer complexes.

The neutral–ionic (N–I) phase transitions of the molecular valence state have been investigated by Raman spectroscopy and x-ray diffraction for a series of isostructural charge-transfer complexes, ...

We report on synthesis and electronic characterization of single-crystalline film of Sr2CoO4 with square-lattice CoO2 sheets (K2NiF4-type structure). In this compound, we have found both ferromagnetic and metallic behaviors with fairly high Curie temperat

The research on ferroelectric materials-mostly inorganic compounds or organic polymers-is increasingly motivated by both basic scientific concerns and the potential for practical applications in electronics and optics. Ferroelectricity in organic solids would be important for the development of all-organic electronic and photonic devices. The conventional approach to making organic ferroelectrics is based on the use of polar molecules. Here we report that through supramolecular assembly of nonpolar conjugated molecules, a remarkable ferroelectric response can be obtained in co-crystals of low-molecular-weight organic compounds. Co-crystals of phenazine and chloranilic acid reveal large spontaneous polarization and sizable room-temperature dielectric constants exceeding 100. The present findings provide an approach to making potentially useful organic ferroelectric materials.

Neutral–ionic (NI) phase transition has been investigated by various chemical modifications in the prototype complexes of TTF (or DMTTF)-p-chloranil. Successive lattice expansion due to the enlarged molecular size serves to lower the NI transition temperature. From the dielectric properties and the molecular ionicity, we demonstrate the novel examples of the NI transition, which is affected by the quantum fluctuation effect.

A phase transition in an organic charge-transfer complex, which originates from the neutral-ionic valence instability, can be tuned toward zero kelvin with use of external pressure or chemical modification as a control parameter. The phase diagram and observed dielectric behaviors are typical of quantum paraelectricity, yet this zero-kelvin transition point namely, the quantum critical point, accompanies large quantum fluctuation of the molecular charge, as demonstrated by the molecular vibrational mode spectra. The result indicates that the π-electron transfer between donor and acceptor molecules is coupled with the zero-point lattice dynamics around the quantum critical point.

We studied the photoinduced ionic to neutral phase transition in tetrathiafulvalene-p-chloranil (TTF-CA) using time-resolved infrared vibrational spectroscopy. By monitoring the charge sensitive b1uν10 band of CA after photoexcitation by a 1.55 eV pulse, we found that the photoinduced N-phase has the same charge of the N-phase in thermal equilibrium and there is no charge variation during the photoinduced phase transition. We also found that the photoinduced N-phase grows through at least two steps over 300 ps.

As-grown epitaxial composition-spread thin films of a colossal magnetoresistive compound, La 1− x Sr x MnO 3 (0≤ x ≤1), are fabricated by a combinatorial laser molecular beam epitaxy (laser MBE). High throughput characterization for studying x dependencies of structural, magnetic and electronic properties is demonstrated by the combination of a concurrent X-ray diffractometer, a scanning superconducting quantum interference device (SQUID) microscope and an infrared microscope spectrometer, respectively.

Successful fabrication of epitaxial thin films of La 2- x Sr x NiO 4 (0.5≤ x ≤1.4) has enabled us to investigate the change in charge dynamics upon the insulator–metal transition (IMT). The charge gap induced by (π,π) charge ordering as observed in optical spectra for x =0.5 is decreased with the increase of x , while the pseudo-gap feature subsists at least up to x =1.0 beyond IMT critical point ( x c ≈0.9). In the metallic phase near IMT, the Hall coefficient shows remarkable enhancement at low temperatures. The results indicate the predominant role of the charge ordering/correlation in the IMT.

The impurity (Cr3+)-doping effect on the stability of charge and orbital ordering has been systematically investigated for Nd1/2Ca1/2Mn1−yCryO3 crystals by measurements of magnetotransport and x-ray diffraction. The random field in terms of eg orbital deficiencies on the Cr sites drives the charge and orbital correlations to dynamical and short range, which is most relevant to the high-resistive state exhibiting colossal magnetoresistance. In the Cr-doped manganite, we can observe the coexistence of ferromagnetic–metallic and charge–orbital ordered phases, their spatial distributions, diffuse x-ray scattering, magnetic-field annealing, and the aging effect on the magnetic and electric properties, etc. These phenomena are reminiscent of those of relaxor ferroelectrics composed of ferroelectric clusters embedded in a paraelectric matrix. We propose that the mixed-valent manganite can be viewed as a “magneto- and electrorelaxor.”

Variation of the ferroelectric phase transition has been investigated for a series of isomorphous donor (D)-acceptor (A) charge-transfer complexes composed of 4,4'-dimethyltetrathiafulvalene (DMTTF) and tetrahalo-p-benzoquinones by measurements of dielectric susceptibility, X-ray diffraction, and infrared molecular vibrational spectra. The neutral-ionic phase transition of DMTTF-p-chloranil at 65 K accompanies a dielectric peak anomaly associated with the DA stack dimerization. Successive halogen replacement by bromine in the component tetrahalo-p-benzoquinone molecule expands the lattice along the DA stack of the DMTTF complex, and reduces the critical temperature steeply toward zero temperature in an analogous way to the critical behavior of quantum ferroelectrics. The 2,6-dibromo-substituted compound showing the behavior of the quantum paraelectricity is located in the immediate vicinity of the quantum critical point of this phase diagram as signified also by enhancement of the ionicity, the dielectric susceptibility, and the dynamical dimeric distortion at the lowest temperature.

We examined the ultrafast response of the metallic high-temperature phase in the conducting charge transfer complex (EDO-TTF)2PF6. A large reflectivity change of approximately 10% was observed when the intra-molecular band was excited by a weak 3.1 eV ultraviolet light pulse. The lifetimes of the photo-induced states were 0.2 and 0.7 ps in the Drude-like band and the intra-molecular band, respectively. Measurement of the photo-induced spectrum just after photo-excitation and simulation using the Drude model revealed that the electronically excited EDO-TTF molecules shortened the relaxation time of conduction electrons and reduced the reflectivity of the Drude-like band.