Interplay between electronic correlations and coherent structural dynamics during the monoclinic insulator-to-rutile metal phase transition in VO2

We report direct observations of the structural and electronic dynamics of the photoinduced insulator-metal transition in VO(2), by means of time-resolved photoemission spectroscopy. These observations provide new insights into the processes involved in this transition. Slightly above the threshold of the photoinduced phase transition, the different response times of the electrons and the lattice reveal the electronic nature of the band gap collapse. At high excitation densities, we find that the phase transition is induced nonthermally in an ultrashort time scale. Moreover, we identify different V 3p dynamics indicating the existence of different structural pathways. These results represent a clear demonstration of the potential of time-resolved core level photoelectron spectroscopy to study ultrafast dynamics in condensed matter.     

Photoinduced phase transition to a new macroscopic spin-crossover-complex phase

We clarified for the first time that the photoinduced phase is quite different in structure from the thermally induced phase by the resonant Raman spectroscopy in the spin-crossover complex, [Fe(2-pic)(3)]-Cl2EtOH. In the photoinduced phase we observed a number of additional lines assigned to infra-active vibrational modes which are strongly prohibited by selection rules in the thermally induced phase. These results indicate that a dramatic symmetry lowering should take place in the photoinduced phase. The cooperative Jahn-Teller transformation is a plausible candidate for the symmetry lowering in the photoinduced phase.     

Initial-Condition Sensitivity and Nonlinear Relaxation in Photo-Induced Ionic-to-Neutral Phase Transition in Tetrathiafulvalen- p -Chloranil Crystals

Recent experimental results of the photoinduced ionic-to-neutral (NI) phase transition in tetrathiafulvalen- p -chloranil (TTF-CA) crystals are reviewed with emphasis on the state-sensitive features and nonlinear properties. Frenkel-type and charge transfer (CT)-type excited states can induce the NI transition but with different characteristics; the transition can be induced only above threshold-excitation intensities in the case of CT excitation, whereas it is induced without any threshold for Frenkel-type excitation. The threshold that implies nonlinear processes of CT excited states is strongly state-dependent and temperature-dependent. Femtosecond time-resolved studies have resolved three distinctive sequential steps in the dynamics; formation of the precursor of N-phase domains, the local proliferation of photoinduced changes, and the process of forming N-phase orders. Origin of the nonlinear processes and the mechanism of the NI transition have been discussed based on these results.     

Domino mechanisms in photoinduced phase transitions

Photoinduced structural phase transitions via excited electronic states are discussed theoretically using a one-dimensional model composed of localized electrons and lattices under the adiabatic or diabatic approximation. We show that the global structural change by photoexcitation only at a site is possible, and we clarify conditions for the occurrence of such phenomena. Spatiotemporal dynamics of nonequilibrium first-order phase transitions is also investigated in detail in terms of photoinduced nucleations and domino processes of the domain boundaries (domain walls), which are in striking contrast to the mean-field dynamics. In the adiabatic regime, after the spontaneous emission of a photon, an initial local structural change (i) remains locally, (ii) induces cooperatively a global structural change, or (iii) disappears and returns to the initial phase. Dynamical features of the case (ii) are characterized by the deterministic (semichaotic) domino process; domain walls between the two phases move determinis-tically at a constant velocity (with changing speed) without further spontaneous emissions in the case of strong (weak) dissipation. In the diabatic regime, similar three types of structural change exist. The domain-wall dynamics is described as the stochastic domino process, which is accompanied by the successive radiative transitions. A new theoretical treatment is also proposed to study crossover between the adiabatic and diabatic regimes.     

A model of Photoinduced Regular-Dimerized Stack Transitions

We present a model of photoinduced regular-dimerized stack transitions and discuss theoretically the behavior of the organic molecular mixed-stack charge-transfer crystal, Tetrathiafulvalene-Chloranil (TTF-CA). We investigate both the photoinduced structural phase transitions and the thermal phase transition between the regular-stack neutral phase at high temperatures and the dimerized-stack ionic phase at low temperatures. Our model is based on a simple phenomenological insight that the molecules have three stable displacements in the crystal. The cooperative effect is accounted for by introducing harmonic coupling between the molecules. It is the first attempt to include not only the photo-excitation but also the temperature effect on an equal footing.     

Theory of photoinduced phase transitions in itinerant electron systems

Theoretical progress in the research of photoinduced phase transitions is reviewed with closely related experiments. After a brief introduction of stochastic evolution in statistical systems and domino effects in localized electron systems, we treat photoinduced dynamics in itinerant-electron systems. Relevant interactions are required in the models to describe the fast and ultrafast charge-lattice-coupled dynamics after photoexcitations. First, we discuss neutral-ionic transitions in the mixed-stack charge-transfer complex, TTF-CA. When induced by intrachain charge-transfer photoexcitations, the dynamics of the ionic-to-neutral transition are characterized by a threshold behavior, while those of the neutral-to-ionic transition by an almost linear behavior. The difference originates from the different electron correlations in the neutral and ionic phases. Second, we deal with halogen-bridged metal complexes, which show metal, Mott insulator, charge-density-wave, and charge–polarization phases. The latter two phases have different broken symmetries. The charge-density-wave to charge–polarization transition is much more easily achieved than the reverse transition. This is clarified by considering microscopic charge-transfer processes. The transition from the charge-density-wave to Mott insulator phases and that from the Mott insulator to metal phases proceed much faster than those between the low-symmetry phases. Next, we discuss ultrafast, inverse spin-Peierls transitions in an organic radical crystal and alkali-TCNQ from the viewpoint of intradimer and interdimer charge-transfer excitations. Then, we study photogenerated electrons in the quantum paraelectric perovskite, SrTiO₃, which are assumed to couple differently with soft-anharmonic phonons and breathing-type high-energy phonons. The different electron–phonon couplings result in two types of polarons, a “super-paraelectric large polaron” with a quasi-global parity violation, and an “off-center-type self-trapped polaron” with only a local parity violation. The former is equivalent to a charged and conductive ferroelectric domain, which greatly enhances both the quasi-static electric susceptibility and the electric conductivity. Finally, we outline the development of time-resolved X-ray diffraction experiments, which directly accesses the dynamics of electronic, atomic and molecular motions in photoexcited materials. They are extremely useful when a three-dimensional structural long-range order is established and changes the symmetry.     

Temperature behaviour of photoinduced modulated state in FeBO3:Ni

A photoinduced modulated state in FeBO₃:Ni is found to occur by means of a phase transition when the temperature changes. Dependences of the transition temperature on the value and orientation of the external magnetic field and on illumination intensity are obtained. Experimental data are discussed in terms of a model involving interactions between photoinduced Fe³⁺ ions, the impurity system and the magnetic system.     

Composition-dependent photosensitivity in As-S glasses induced by bandgap light: structural origin by Raman scattering

Massive photoinduced short- and medium-range structural changes (photopolymerization) in As-S glasses are induced by near-bandgap light and studied by Raman scattering. Structural changes involve bond restructuring in sulfur-rich nanodomains of these nanoscale-phase-separated glasses. The spectral dependence of the photopolymerization effect demonstrates that various wavelengths can be used to optically change the structure of As-S glasses. The immense structural changes are relevant to recent findings about the role of bandgap light illumination for fabricating channel waveguides in noncrystalline arsenic sulfides.     

Orbital ordering and spin-ladder formation in La2RuO5

The semiconductor-semiconductor transition of La2RuO5 is studied by means of augmented spherical wave electronic structure calculations as based on density-functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results hint towards an orbital ordering scenario which, markedly different from the previously proposed scheme, preserves the local S=1 moment at the Ru sites in the low-temperature phase. The unusual magnetic behavior is interpreted by the formation of spin ladders, which result from the structural changes occurring at the transition and are characterized by antiferromagnetic coupling along the rungs.     

Photoinduced Structural Changes in Poly(4-Vinyl Pyridine): A Luminescence Study

In the present work we show a way of controlling photoluminescence (PL) properties through photoinduced quasi-crystal formation in a system based on poly(4-vinyl pyridine) (P4VPy). Under UV irradiation at 380 nm, concentrated solutions of P4VPy in pyridine turn into gel. This phase transition results in changes in the optical properties of this polymer. The position of the PL maximum can be changed continuously from 440 to 480 nm during irradiation. After several minutes of UV irradiation a new red-shifted PL at 492 nm appears upon excitation by light of a wavelength corresponding to that of the initial PL maximum, which is also red-shifted during irradiation. Solutions of P4VPy in pyrimidine show similar behavior, but those in pyridazine do not exhibit such behavior. We have found that the reason for the observed changes in the electronic properties is a photoinduced directional ordering of polymer molecules in a special quasi-crystal formation. The process originates from a structural change in the side chain of P4VPy, namely, protonation of the polymeric pyridine after solvation. During irradiation, the polymeric pyridinium ion interacts with neutral polymeric pyridine molecules. Interchain interaction through hydrogen bonds lead to an electronic property change. We observed that the process of photoinduced sol-gel transformation is reversible. Mechanical perturbation or heating can convert the gel back to a fluid solution. The red-shifted PL is not observed, and the initial PL is blue-shifted to 450 nm and stays there.     

Dynamical aspects of the photoinduced phase transition in spin-crossover complexes

We report a dynamical study on the photoinduced cooperative changes of the spin configurations in single crystals of the organometal spin-crossover complex. In the photoswitching process between low- and high-spin states, nonlinear characteristics such as thresholdlike behavior, incubation period, and phase separation have been observed. These results demonstrate that the cooperative intersystem crossing mediated by spin-lattice interaction plays a key role in the driving process of a new class of nonequilibrium phenomena so called photoinduced phase transition.     

Local structure changes of Cu55 cluster during heating

The structural relaxation of a cluster containing 55 atoms at elevated temperatures is simulated by molecular dynamics. The interatomic interactions are given by using the embedded atom method （EAM） potential. By decomposing the peaks of the radial distribution functions （RDFs） according to the pair analysis technique, the local structural patterns are identified for this cluster. During increasing temperature, structural changes of different shells determined by atom density profiles result in an abrupt increase in internal energy. The simulations show how local structural changes can strongly cause internal energy to change accordingly.     

Measurements of photoinduced refractive index changes in bacteriorhodopsin films

We report the pump-probe measurements of nonlinear refractive index changes in photochromic bacteriorhodopsin films. The photoinduced absorption is caused by pump beam at 532 nm and the accompanying refractive index changes are studied using a probe beam at 633 nm. The proposed technique is based on a convenient and accurate determination of optical path difference using digital interferometry-based local fringe shift. The results are presented for the wild-type as well as genetically modified D96N variant of the bacteriorhodopsin.      

Ultrafast photovoltaic response in ferroelectric nanolayers

We show that light drives large-amplitude structural changes in thin films of the prototypical ferroelectric PbTiO3 via direct coupling to its intrinsic photovoltaic response. Using time-resolved x-ray scattering to visualize atomic displacements on femtosecond time scales, photoinduced changes in the unit-cell tetragonality are observed. These are driven by the motion of photogenerated free charges within the ferroelectric and can be simply explained by a model including both shift and screening currents, associated with the displacement of electrons first antiparallel to and then parallel to the ferroelectric polarization direction.     

Evidence for a cubic-to-icosahedral transition of quasi-free Pd-H-clusters controlled by the hydrogen content

An in situ synchrotron radiation study of quasi-free five nanometer-sized palladium clusters during hydrogen absorption is combined with molecular dynamics simulations to investigate the structural development. In the diffraction patterns, strong intensity changes are found that provide evidence for a structural phase transformation that is significantly different from the α--Pd-H bulk phase transition. The structural transition is reversible and driven by the hydrogen concentration. The intensity changes are consistent with a cubic-to-icosahedral structural phase transition obtained in molecular dynamical simulations using embedded-atom-method potentials. Received 15 October 2001 and Received in final form 7 February 2002     

Photoluminescence and photoinduced magnetic phase transition in an organic radical TTTA crystal studied by two-photon absorption

Photoluminescence as well as photoinduced diamagnetic to paramagnetic phase transition in a strongly correlated electron system, an organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal, was investigated under two-photon excitation with different photon densities. Below the threshold photon density to drive the photoinduced phase transition (PIPT), the diamagnetic phase shows a broad luminescence band with a large Stokes shift, whose intensity obeys almost second power law of the excitation photon density. Above the threshold photon density, on the other hand, the diamagnetic to paramagnetic phase transition effectively takes place with a large conversion yield and a steep response instead of an occurrence of the photoluminescence, indicating that the phase transition is optically induced by two-photon absorption. As far as we know, this is the first observation of the PIPT phenomena mediated by two-photon absorption.     

Theory for Difference Between Photoinduced Phase and Thermally Excited Phase

A possible difference between the photoinduced phase and the thermally excited one is studied by using a two-dimensional extended Peierls-Hubbard model, which includes a strong electron-phonon coupling and a on-site interelectron repulsion, as well as an anharmonic lattice potential. Because of this anharmonicity, the system undergoes a first order phase transition from an insulating CDW state to a metallic one at a high temperature. Although some sign of an SDW order is expected to appear due to this repulsion, it is always hidden in any equilibrium phase of the present system. In fact, it is hidden, not only in the CDW ground state, but also in this metallic one, since the high temperature itself destroys the SDW order, far before the CDW-metal transition occurs, while a photo-excitation at low enough temperature is shown to generate a local metastable SDW domain. Therefore, to observe the presence of such Coulomb interaction and the resultant broken symmetry, a nonequilibrium photoinduced phase is shown to be most straightforward. Thus, the photoinduce phase transition can make an interaction appear as a broken symmetry only in this phase, even though this interaction is almost completely hidden in all the equilibrium phases from low temperature to high ones.     

Capturing one-dimensional precursors of a photoinduced transformation in a material

Achieving control of photoinduced phase transitions requires understanding how materials work during transformation induced by a laser pulse. Here we investigate the precursors of a photoinduced phase transition in the highly cooperative charge-transfer molecular crystal tetrathiafulvalene-p-chloranil and provide key insights. The photogeneration of one-dimensional nanoscale clusters was detected by time-resolved diffuse x-ray scattering with 50-ps time resolution. Such clustering of structurally relaxed electronic excitations is expected to be a common process in many materials presenting photoinduced transformations.     

光诱导液晶中偶氮聚合物形成相光栅的研究

报道了一种新型偶氮聚合物掺杂液晶的复合体系.该体系中的偶氮聚合物在光诱导下发生顺反式异构形成与液晶分离的相光栅.通过建立多指数模型研究该相光栅在He-Ne光的辐照下折射率的变化情况,并且通过对聚合物掺杂液晶的样品加上连续变化的电压来研究其对电场的响应过程.同时研究了聚合物的单体掺杂液晶后随光场和电场的变化情况.结果表明,光诱导下聚合物掺杂液晶的样品折射率改变值经45s达到饱和,弛豫时间为10min;单体掺杂时,饱和时间与弛豫时间分别为0.17s和0.9s.外加电场在1.0V、1.4V、2.6V和4.0V时光栅的衍射效率会突然下降,而单体在变化的外电场下衍射无明显变化.