Superconductivity in higher titanium oxides

Recent renewal of the highest transition temperature in a conventional superconductor of the sulfer hydride attracts much attention to exploring simple compounds with the lighter elements, situated in unconventional conditions. We report the discovery of superconductivity in simple oxides of Ti4O7 and g-Ti3O5 in a thin-film form having deliberately tuned epitaxial structures and off-stoichiometry. These higher titanium oxides join in a class of simple-oxide superconductors, and g-Ti3O5 now holds the highest superconducting transition temperature of 7.1 kelvin among them. The mechanism behind the superconductivity is discussed on the basis of electrical measurements and theoretical predictions. We conclude that superconductivity arises from unstabilized bipolaronic insulating states.Comment: 25 pages, 4 figures in main text, 14 pages, 11 figures in supplemental informatio

Optical Properties of ZnO-based Quantum Structures

The optical properties of ZnO quantum wells, which have potential application of short-wavelength semiconductor laser utilizing a high-density excitonic effect, were investigated. Stimulated emission of excitons was observed at temperatures well above room temperature due to the adoption of the lattice-matched substrates. The mechanism of stimulated emission from ZnO quantum wells is discussed in this paperComment: 14 pages, 10 figures, proceeding of the Euro-MRS 2005 held in Strasbourg, to appear in Superlattices and Microstructure

Origin of the Median Tectonic Line

The southern maginal region of the HT/LP Ryoke metamorphic belt is characterized by a mylonite zone and nappe structures. The tectono-metamorphic history in the southern margin of the Ryoke belt, and the initial stage of formation of the Median Tectonic Line (MTL) were studied at the middle-southern region along the MTL in the Chubu district. Four phases of deformation can be recognized in this region. The first deformation (D1) formed the S1 foliation. The structural features of D1 can not be clearly identified. The metamorphic conditions during D1 were assumed to be medium pressure in the Mikawa Plateau. Extensive growth of porphyroblasts occurred after D1 and before D2 (inter-D1-D2) under non-deformational conditions. The metamorphic conditions changed with a remarkable decrease in pressure from D1 to inter-D1-D2. The D2 deformation was penetrative and involved pressure-solution, resulting in a distinct foliation (S2), which is typically observed in the Ryoke metamorphic rocks. The metamorphic conditions changed with a distinct increase in temperature from inter-D1-D2 to D2. The oldest plutonic mass of the Older Ryoke granites (the Kamihara tonalite) was deformed during D2. The D3 deformation formed a large-scale recumbent fold during the earlier stage of D3. During the later stage of D3, the deformation was concentrated in the lower structurally portions, resulting in the formation of the large-scale mylonite zones and nappe structures. The mylonite zones are developed horizontally and have a top to the west shear sense. The high temperature portion of the Ryoke belt was emplaced onto the low temperature portion. The second older plutonic mass of the Older Ryoke granites (the Tenryukyo granite) were emplaced and deformed during D3. The D3 deformation occurred with distinct decrease in temperature and pressure. The intrusion of the Younger Ryoke granites took place after D3. The D4 deformation is characterized by the formation of the nappe complex and crush melange. The nappe structures of D4 characteristically contain the rocks formed on the surface and rocks derived from other terrane. During the later stage of D4, the Ryoke nappe complex was thrust over the Sambagawa rocks. After the coupling of the Ryoke belt and the Sambagawa belt, the high-angle MTL was formed with a sinistral strike-slip component, resulting in the formation of upright folds. D1, inter-D1-D2, and D2 are typical of the Ryoke belt, and are the result of regional metamorphic processes. Whereas D3 and D4 are characteristic of the southern margin of the Ryoke belt and represent a distinct decrease in temperature and pressure. The initial stage of the MTL is correlated with the D3 and D4 tectonism, and occurred with the uplift of the Ryoke belt from depth and the transportation of the Ryoke belt toward the south. The western portion of the Ryoke belt was uplifted at about 90 Ma and was followed by the uplift of the eastern portion at 60 Ma. The high-angle MTL and the upright folds postdate the tectono-metamorphic processes of the Ryoke belt and the formation of the initial MTL as a horizontal shear zone

Electronic properties across metal-insulator transition in \beta-pyrochlore-type CsW2O6 epitaxial films

In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic \beta-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of \beta-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-levels photoemission spectra indicated the drastic changes across the MIT. In the valence band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of W 4f core-level spectrum suggested the charge disproportionation of W5+ and W6+ in the insulating phase. The change of spectral shape in the Cs 4d core levels reflected the rattling motion of Cs+ cations. These results strongly suggest that CsW2O6 is a novel material, in which MIT is driven by the charge disproportionation associated with the rattling motion.Comment: 8 pages, 6 figure

Hole Transport in p-Type ZnO

A two-band model involving the A- and B-valence bands was adopted to analyze the temperature dependent Hall effect measured on N-doped \textit{p}-type ZnO. The hole transport characteristics (mobilities, and effective Hall factor) are calculated using the ``relaxation time approximation'' as a function of temperature. It is shown that the lattice scattering by the acoustic deformation potential is dominant. In the calculation of the scattering rate for ionized impurity mechanism, the activation energy of 100 or 170 meV is used at different compensation ratios between donor and acceptor concentrations. The theoretical Hall mobility at acceptor concentration of $7 \times 10^{18}$ cm$^3$ is about 70 cm$^2$V$^{-1}$s$^{-1}$ with the activation energy of 100 meV and the compensation ratio of 0.8 at 300 K. We also found that the compensation ratios conspicuously affected the Hall mobilities.Comment: 5page, 5 figures, accepted for publication in Jpn. J. Appl. Phy

Majority-Carrier Mobilities in Undoped and \textit{n}-type Doped ZnO Epitaxial Layers

Transparent and conductive ZnO:Ga thin films are prepared by laser molecular-beam epitaxy. Their electron properties were investigated by the temperature-dependent Hall-effect technique. The 300-K carrier concentration and mobility were about $n_s \sim 10^{16}$ cm$^{-3}$ and 440 cm$^{2}$/Vs, respectively. In the experimental `mobility vs concentration' curve, unusual phenomenon was observed, i.e., mobilities at $n_s \sim 5\times$ 10$^{18}$ cm$^{-3}$ are significantly smaller than those at higher densities above $\sim 10^{20}$ cm$^{-3}$. Several types of scattering centers including ionized donors and oxygen traps are considered to account for the observed dependence of the Hall mobility on carrier concentration. The scattering mechanism is explained in terms of inter-grain potential barriers and charged impurities. A comparison between theoretical results and experimental data is made.Comment: 5 pages, 1 figure, conference on II-VI compounds, RevTe

Why Some Interfaces Cannot be Sharp

A central goal of modern materials physics and nanoscience is control of materials and their interfaces to atomic dimensions. For interfaces between polar and non-polar layers, this goal is thwarted by a polar catastrophe that forces an interfacial reconstruction. In traditional semiconductors this reconstruction is achieved by an atomic disordering and stoichiometry change at the interface, but in multivalent oxides a new option is available: if the electrons can move, the atoms don`t have to. Using atomic-scale electron energy loss spectroscopy we find that there is a fundamental asymmetry between ionically and electronically compensated interfaces, both in interfacial sharpness and carrier density. This suggests a general strategy to design sharp interfaces, remove interfacial screening charges, control the band offset, and hence dramatically improving the performance of oxide devices.Comment: 12 pages of text, 6 figure