Y. Okimoto

Metallic ordered double-perovskite Sr2CrReO6 with maximal Curie temperature of 635 K

Electric, thermal, optical, and magnetic properties have been investigated for ferromagnetic (ferrimagnetic) ordered double perovskites, A2CrReO6 (A=Sr and Ca). Sr2CrReO6 is found to be a metallic ferromagnet with a high Curie temperature (TC=635 K), possibly the highest-TC half metal among the perovskite family. By contrast, Ca2CrReO6 is a ferromagnetic Mott insulator with TC of 360 K.

Rapid construction of a phase diagram of doped Mott insulators with a composition-spread approach

We propose a method of rapid construction of a structural–magnetic–electronic phase diagram of doped Mott insulators. The composition-spread method is utilized for fabricating a film whose doping concentration varies from 0 to 1 continuously. The concurrent x-ray diffractometer that measures x-ray diffraction spectra of all the composition simultaneously, the scanning superconducting quantum interference device microscope, and the infrared optical spectroscopy are employed for characterizing the film. A demonstration is given for a colossal magnetoresistive material, La1−xSrxMnO3.

Control of magnetic-field effect on electroluminescence in Alq3-based organic light emitting diodes

The magnetic-field effect on electroluminescence (EL) has been investigated for the tris-(8-hydroxyquinolino) aluminum (Alq3)-based organic light emitting diode. The EL intensity sharply increases up to 8% with increasing magnetic field to 500Oe at room temperature. The magnetic field effect on EL depends on the interface structure between a hole transporting and a light emitting layers, indicating the importance of the spin-state dynamics of the electron-hole pairs at the interface.

Anomalous valence fluctuation near a ferroelectric transition in an organic charge-transfer complex

Dielectric susceptibility, X-ray diffraction and infrared molecular vibrational spectra have been investigated for a charge-transfer complex, tetrathiafulvalene (TTF)-2-bromo-3,5,6-trichloro- p -benzoquinone (QBrCl 3 ) to probe a ferroelectric valence transition at T c =66 K in comparison with the typical first-order neutral-ionic transition in the isostructural analog, TTF- p -chloranil (QCl 4 ). Around T c , the anomalously rapid fluctuation of molecular valence between the quasi-neutral and quasi-ionic state is evidenced by the temperature variation of infrared spectra, signaling the quantum motion of the neutral-ionic domain walls on the one-dimensional molecular stacks.

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.

Charge/orbital ordering in perovskite manganites

Abstract We have studied the phenomena relevant to charge/orbital ordering in perovskite manganites. In the three bandwidth controlled systems, La 0.7 (Sr 1− y Ca y ) 0.3 MnO 3 ( x =0.3), (Nd 1− y Sm y ) 0.55 Sr 0.45 MnO 3 ( x =0.45), and Pr 0.65 (Sr 1− y Ca y ) 0.35 MnO 3 ( x =0.35), the variation of the magnetic and electronic properties with y has been studied as the hole doping level is kept constant. As y increases (the effective one-electron bandwidth decreases), the colossal magnetoresistance (CMR) or charge/orbital ordering appears. That is, the CMR is caused by competition between double exchange and charge/orbital ordering. As another issue, the electronic and magnetic properties of the ordered double perovskite Sr 2 FeMoO 6 ( T C ∼420 K) have been studied by utilizing single crystals.

Anisotropy of Mott-Hubbard Gap Transitions due to Spin and Orbital Ordering in LaVO3 and YVO3

Anisotropic optical spectra coupled with antiferromagnetic spin ordering (SO) and orbital ordering (OO) have been investigated for single crystals of LaVO 3 and YVO 3 . The orbital-dependent Mott–Hubbard gap transitions are observed around 2 eV. The transitions composed of the two peaks show distinct anisotropy and selection rules, reflecting the respective SO and OO patterns. The temperature dependence of the anisotropic transitions clearly indicates the SO/OO correlation and the evolution of the order parameters.

Direct observation of photoinduced magnetization in a relaxor ferromagnet

Persistent ferromagnetic magnetization (M) as large as 0.6 μB per Mn site can be induced by irradiating visible laser pulses on a thin film of 1% Cr-doped Pr0.5Ca0.5MnO3. The enhancement of M upon the photoexcitation was also confirmed by the scanning superconducting quantum interference device microscope as an increase of magnetic field from ferromagnetic domains. The observed photoinduced transition from a metastable charge/orbital ordered state into a ferromagnetic one is likely to be assisted by the microscopic phase separation characteristic of such a relaxor ferromagnet as the present Cr-doped manganite.

Optical Study of Metal-Insulator Transition in SmBaCo 2O 6-δ Single Crystal

The optical reflectivity spectrum and its temperature dependence were measured on a cleaved (001) surface of SmBaCo 2 O 5.6 . With an increase in temperature, the reflectivity below 1 eV increases ...

Orbital-driven variation of electronic structures in tetragonal La1/2Sr1/2MnO3 as investigated by optical spectroscopy

Optical conductivity spectra (σ(ω)) were investigated for coherently strained epitaxial films of tetragonal La 1/2 Sr 1/2 MnO 3 with various c / a ratios (1.04, 1.00, and 0.98). With decreasing c / a , σ(ω) in the ground state shows a successive dramatic change from an insulating gap-like to metallic but highly incoherent spectrum, then to a pseudo-gap one, which reflects the strain-induced e g orbital ordering/disordering and the resultant different spin structures. The variation of the electronic structure induced by such a strain-orbital coupling is discussed in comparison with some theoretical models.