Tadahiko Ishikawa

Effects of Chemically Induced Contraction of a Coordination Polyhedron on the Dynamical Magnetism of Bis(phthalocyaninato)disprosium, a Single-4f-Ionic Single-Molecule Magnet with a Kramers Ground State

Chemically induced longitudinal contraction of the square-antiprism coordination polyhedron of a peripherically substituted bis(phthalocyaninato)dysprosiumate(III), a dysprosium-based single-4f-ionic single-molecule magnet having a J z = +/- (13)/ 2 Kramers doublet ground state, resulted in drastic changes in dynamical magnetism including a doubling of the energy barrier, a 2-order-of-magnitude decrease of the spin reversal rate, a significant rise of the blocking temperature, and the first observation of the emergence of a large remanent magnetization.

Direct observation of collective modes coupled to molecular orbital-driven charge transfer

The making of a molecular movie Phase transitions familiar from everyday life, such as boiling or melting, are caused by changing the temperature. In the laboratory, however, researchers can also change the phase of a material by shining intense light on it. During such transitions, changes occur in both the electronic and lattice structure of the material. Ishikawa et al. used ultrafast optical and electron diffraction probes to monitor both types of change simultaneously during a photo-induced phase transition in a molecular crystal. The resulting molecular movies showed expansion of the intermolecular distance, flattening of the molecules, and tilting of molecular dimers. Science, this issue p. 1501 Ultrafast spectroscopy and electron diffraction are used to create molecular movies of a phase transition in Me4P[Pt(dmit)2]2. Correlated electron systems can undergo ultrafast photoinduced phase transitions involving concerted transformations of electronic and lattice structure. Understanding these phenomena requires identifying the key structural modes that couple to the electronic states. We report the ultrafast photoresponse of the molecular crystal Me4P[Pt(dmit)2]2, which exhibits a photoinduced charge transfer similar to transitions between thermally accessible states, and demonstrate how femtosecond electron diffraction can be applied to directly observe the associated molecular motions. Even for such a complex system, the key large-amplitude modes can be identified by eye and involve a dimer expansion and a librational mode. The dynamics are consistent with the time-resolved optical study, revealing how the electronic, molecular, and lattice structures together facilitate ultrafast switching of the state.

Isotope Effect on Photoconductivity in Quantum Paraelectric SrTiO3

Photoconductivity and various photo-magnetotransport properties have been studied in quantum paraelectric SrTiO 3 crystals. SrTi 16 O 3 shows giant photoconductivity, as reported before. The photodoped carrier in the 16 O system is of electron type and has high mobility beyond 10 4 cm 2 V -1 s -1 at 5 K. Its number can reach the order of 10 15 cm -3 . In contrast, isotope-substituted SrTi 18 O 3 is not a good photoconductor. In addition, the photocarriers mobility in the 18 O sample is negligibly small. These results suggest a strong correlation between the quantum fluctuation and the giant photoconductivity in this quantum paraelectric system. In other words, the study of photocarrier dynamics is a novel approach to the investigation of quantum fluctuation in dielectric materials.

Photoinduced Phenomena in Quantum Paraelectric Oxides by Ultraviolet Laser Irradiation

A photo-induced effect in the quantum dielectric system, such as SrTiO3, KTaO3, and CaTiO3, has been investigated under rather small external DC electric field. The dielectric measurement was performed under ultraviolet (UV) laser excitation combined with DC electric field. The gigantic photo-induced effects of dielectric properties of the quantum paraelectric systems have been firstly observed in the perovskite-type materials. The photoinduced dielectric effect was obtained as a function of DC electric field under constant UV irradiation.

Structural Transitions from Triangular to Square Molecular Arrangements in the Quasi-One-Dimensional Molecular Conductors (DMEDO-TTF)2XF6 (X = P, As, and Sb)

A series of quasi-one-dimensional molecular conductors (DMEDO-TTF)(2)XF(6) (X = P, As, and Sb), where DMEDO-TTF is dimethyl(ethylenedioxy)tetrathiafulvalene, undergo characteristic structural transitions in the range of 130-195 K for the PF(6) salt and 222-242 K for the AsF(6) salt. The dramatic structural transition is induced by the order of the ethylenedioxy moiety, and the resulting anion rotation leads to the reconstruction of the H···F interaction between the methyl groups and the anions. The unique hydrogen bonds play a crucial role in the transition. As a result, the molecular packing is rearranged entirely; the high-temperature molecular stacks with an ordinary quasi-triangular molecular network transforms to a quasi-square-like network, which has never been observed among organic conductors. Nonetheless, the low-temperature phase exhibits a good metallic conductivity as well, so the transition is a metal-metal (MM) transition. The resistivity measured along the perpendicular direction to the conducting ac-plane (ρ(⊥)) and the calculation of the Fermi surface demonstrate that the high-temperature metal phase is a one-dimensional metal, whereas the low-temperature metal phase has considerable interchain interaction. In the SbF(6) salt, a similar structural transition takes place around 370 K, so that the quasi-square-like lattice is realized even at room temperature. Despite the largely different MM transition temperatures, all these salts undergo metal-insulator (MI) transitions approximately at the same temperature of 50 K. The low-temperature insulator phase is nonmagnetic, and the reflectance spectra suggest the presence of charge disproportionation with small charge difference (0.14).

Photoinduced Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-p-chloranil Studied by Time-Resolved Vibrational Spectroscopy

We studied the photoinduced neutral-to-ionic phase transition in tetrathiafulvalene- p -chloranil (TTF-CA) using time-resolved infrared (IR) vibrational spectroscopy with a broadband femtosecond IR laser pulse and a linear IR detector array. After photoexcitation, the strength of the TTF a g ν 3 band increased, indicating that dimerization is activated within ∼1 ps. In contrast, the strength of the CA b 1 u ν 10 band, whose frequency is proportional to the degree of charge transfer (ρ), decreased, and a broad weak band appeared in the lower-frequency region. This implies that large fluctuations in charge transfer continued until about 20 ps after the photoexcitation. These results indicate a large difference between the electronic structures of the ionic phases generated by thermalexcitation and photoexcitation, although a dimerized structure was observed in both cases.

Probing charge/orbital correlation in La1.2Sr1.8Mn2O7 by Raman spectroscopy

Raman spectra have been investigated for a single crystal of La 1.2 Sr 1.8 Mn 2 O 7 that shows colossal magnetoresistance around the Curie temperature T C = 126 K. Several broad phonon peaks grow w...

Photo-Induced In-Gap States in SrTiO3 Probed by Photoemission Spectroscopy under Ultraviolet Illumination

We report on an electronic structure study of SrTiO 3 under ultraviolet illumination. Photoemission measurements at 20 K under ultraviolet illumination show that the illumination induces electronic states within the band gap. The photo-induced in-gap states show the power-law energy distribution with exponent of 3.4–3.7 and weakly depends on the strength of illumination. The photo-induced in-gap states indicate that most of the photo-excited electrons are trapped by local lattice distortions with various size scale. We argue that the free carriers in the Ti 3 d band and the localized Ti 3 d electrons and O 2 p holes within the band gap contribute to the photoconductivity and the ferroelectricity under the ultraviolet illumination, respectively.

Giant Photoconductivity in Quantum Paraelectric Oxides

Photoconductivity and photo-magneto-transport properties have been measured in quantum paraelectric SrTiO3 crystals. We have found a large isotope effect in the photocurrent value in this system. This isotope effect suggests an important contribution of the quantum fluctuations for the giant photoconductivity in the quantum paraelectric system.

Ultrafast Real Space Dynamics of Photoexcited State in a Layered Perovskite-Type Spin Crossover Oxide La1.5Sr0.5CoO4

Ultrafast optical responses were investigated using a fs laser system on a layered perovskite-type cobalt oxide, La1.5Sr0.5CoO4 with a checkerboard-type charge ordering. To investigate the spatial variation of the photoexcited state, we measured the transient changes of three independent optical quantities, i.e., transmittance, reflectivity, and backside reflectivity, as well as their fluence dependence at 0.25 eV. Using the numerical calculations based on Maxwells equations, we calculated not only the dielectric constant but also the decay length of the photoexcited region (\(L\)) and analyzed the real-space dynamics of the photoexcited area in terms of the fluence and time dependence of \(L\). With the above information on the time dependence of \(L\) and Kramers–Kronig analysis, we numerically calculated the transient optical conductivity spectra, which showed instantaneous formation of a polaron-like absorption peak in the mid-infrared region. A 10 fs pump–probe reflection measurement revealed that t...