A-Site Columnar-Ordered Perovskite CaZnVO as a Pauli-Paramagnetic Metal.

In this study, we successfully synthesized a novel A-site columnar-ordered perovskite CaZnVO. This compound features a square-planar-coordinated Zn disorder, which is the same characteristic as the centrosymmetric paraelectric CaMnTiO. Unlike CaMnTiO, which shows a centrosymmetric paraelectric-noncentrosymmetric ferroelectric transition, CaZnVO retains Pauli-paramagnetic metallicity arising from itinerant V d electrons and centrosymmetry down to 5 K.

Nonlinear Optical Properties in an Epitaxial YbFeO Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFeO, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal.

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BiPbFeO with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that BiPbFeO is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal 4/ structure with √2 × √2 × 2 unit cell and has unusually Pb charge disproportionated BiPbPbFeO charge distribution. The valence of transition metal in BiPbO changes from 3.

Generation of sub-100 fs electron pulses for time-resolved electron diffraction using a direct synchronization method.

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.

Enhanced Negative Thermal Expansion Induced by Simultaneous Charge Transfer and Polar-Nonpolar Transitions.

Negative thermal expansion (NTE) induced by simultaneous mechanisms, that is, charge transfer and polar-nonpolar transitions, was observed for the first time in BiNiFeO (0.25 ≤ ≤ 0.5).

Robust Giant Tetragonal Distortion Coupled with High-Spin Co in Electron-Doped BiCoO.

BiCoO is a PbTiO type of perovskite oxide with a giant tetragonal distortion (/ = 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 BiCoO by substituting Ti for Co in order to destabilize the tetragonal phase and observe a giant negative thermal expansion (NTE) at ambient pressure.

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

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.

Photoinduced change in the charge order pattern in the quarter-filled organic conductor (EDO-TTF)2PF6 with a strong electron-phonon interaction.

The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)2PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation.

Large dielectric susceptibility associated with proton transfer in a supramolecular structure of chloranilic acid and 5,5'-dimethyl-2,2'-bipyridine.

A 1:1 adduct of chloranilic acid with 5,5'-dimethyl-2,2'-bipyridine, in which two kinds of molecules are connected by infinite hydrogen bond chains, exhibits a distinct dielectric phase transition when cooled. Below T(c)(=318 K) the hydrogen atoms participating in hydrogen bonding undergo long-range ordering and form an antiferroelectric-like state, taking a single minimum potential in the high-temperature phase (T>T(c)) due to the bifurcate hydrogen bond system. The proton-transfer phenomenon was clearly observed by electron density distribution analysis using a maximum entropy method of synchrotron x-ray diffraction data.

Ferroelectricity near room temperature in co-crystals of nonpolar organic molecules.

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.

Quantum phase transition in organic charge-transfer complexes.

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 pi-electron transfer between donor and acceptor molecules is coupled with the zero-point lattice dynamics around the quantum critical point.