The electronic conduction through a Pr 0.7 Ca 0.3 MnO 3 thin film is investigated by measurements using dc and pulsed biases. Semiconducting Pr 0.7 Ca 0.3 MnO 3 films sandwiched by electrodes show both hysteretic and asymmetric behaviors in current-voltage characteristics. The observed conduction characteristics exhibit the spacecharge-limited-current effect, and the hysteretic behavior can be ascribed to a carrier trapping and detrapping of the trap sites in the manganite. The hysteresis induces a colossal electroresistance (CER) of more than 5000% at room temperature. The CER ratio is independent of the duration time of pulses from an infinite (dc) down to 150 ns, indicating that the carrier filling of all the traps can be completed within a short time.
The electroforming and the resistance-switching behaviors in magnetite, Fe3O4, by the application of an appropriate electric field are demonstrated on a lateral device with multiple electrodes. By means of this device, both the location and the nature of the change in Fe3O4 are specified from the electrical measurements and Raman spectroscopy. The switching phenomenon is caused in maghemite, γ-Fe2O3, which is formed by oxidation of Fe3O4, near an interface of an anode. The authors argue that the switching motion is originated in a redox reaction between the Fe3O4 and γ-Fe2O3.
The optical reAectivity spectra for the bc surface of a large Bi2Sr&CaCu208+, crystal were investigated over a wide energy range (30-30000 cm ') at room temperature and 6 K. In the spectrum for E~~e, only the phonon peaks were observed without any electronic contribution down to 30 cm ', whereas for E~~~b a Drude-like spectrum was observed. Even below T, the spectrum for E~~c does not show any remarkable change except for a small change in the phonon spectrum. This indicates an extremely small plasma frequency for E~~c, less than 30 cm ', and thus quite a large mass anisotropy m, */m&*) 10 . Such a small plasma frequency supports the conduction model along the c axis, that the supercurrent Bows by tunneling through insulating (semiconductor) layers such as the Bi-0 layers. Strong anisotropy in the crystal structure as well as in the electronic states is one of the characteristic properties of the high-T, superconducting cuprates (HTSC), which may be essential for the mechanism of high-T, superconductivity. Although the physical properties in the ab plane have been intensively investigated from many points of view, there are less reports on the properties in the c direction. ' This is not only due to the fact that the main interest is concentrated on the electronic states within the CuOz plane but also due to the limitation of available sample size along the c axis. Especially for the measurement of far-infrared spectra, we need sample sizes thicker than 2 mm. To understand the electronic states in the HTSC, the conduction mechanism along the c axis should be an important subject of study, being related to the problem of the conduction dimensionality.Recently large single crystals of La& "Sr"Cu04 (LSCO) have been successfully grown by a travelingsolvent-Coating-zone (TSFZ) method, which have made it possible to measure the optical spectra for E~~c over wide energy and temperature ranges. The most interesting feature is the abrupt appearance of a sharp edge below T, in the reAectivity spectrum. Tamasaku, Nakamura, and Uchida attributed this feature not to the superconducting gap but to the plasma edge for E~~c . » YBazCu307 (YBCO), the spectrum for E~~c shows a clear electronic contribution below 0.5 eV even at room temperature, which is consistent with the metallic dc conductivity along the c axis. When the temperature decreases below T"only the electronic part of the spectrum seems to change dramatically, while the superposed phonon structures change in strength and damping. ' The almost unity reflectivity (R -1.0) is observed below 100 cm '. Therefore, in contrast to the in-plane spectrum, the c axis spectrum seems to strongly depend on material, presumably due to the structural difference among the materials.This fact motivated us to investigate another typical HTSC, BizSrzCaCuzOs+, (BSCCO), which is quite different in the crystal structure from YBCO but exhibits the same T, value as YBCO. There are a few reports on the room-temperature spectra of BSCCO for E~~c. In spite of the use of several pieces of BSCCO ...
Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.
We have investigated the control of crystal orientation in misfit-type layered cobaltite Ca3Co4O9 thin films by rf-planar magnetron sputtering and succeeded in growing epitaxial films with c-axis, a-axis and b-axis orientations normal to the substrate. Anisotropic transport properties (parallel to the CoO2 layers and perpendicular to the CoO2 layers) were measured using the b-axis-oriented epitaxial films with a layered structure perpendicular to the substrate surface. The resistivity parallel to the CoO2 layers (ρ a ) is about 8 mΩ cm at room temperature while that perpendicular to the CoO2 layers (ρ c ) is about 300 mΩ cm; the anisotropy is estimated to be about 40. Thermoelectric anisotropy is not considerably pronounced; the parallel Seebeck coefficient S a is measured to be 110 µV/K and the perpendicular S c is 40 µV/K at room temperature.
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