Go Kinoda

A transparent metal: Nb-doped anatase TiO2

This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti1−xNbxO2 films with x=0.002–0.2. The resistivity of films with x⩾0.03 is 2–3×10−4Ωcm at room temperature. The carrier density of Ti1−xNbxO2 can be controlled in a range of 1×1019to2×1021cm−3. The internal transmittance for films with x⩽0.03 (40nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In2−xSnxO3 and ZnO.

Ta-doped Anatase TiO2 Epitaxial Film as Transparent Conducting Oxide

We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5×10-4 Ω cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.

Carrier induced ferromagnetism in Nb doped Co:TiO2 and Fe:TiO2 epitaxial thin film

We have investigated the carrier induced magnetic properties of anatase Ti1−x−yNbxMyO2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95−xNbxCo0.05O2, the n-type carrier density could be controlled in a wide range (4.9×1017cm−3to2.7×1021cm−3) by Nb doping (x=0–0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95−xNbxCo0.05O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x<0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94−xNbxFe0.06O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3K, while room-temperature ferromagnetism appears at x=0.01.

Intrinsic Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ

We have investigated the Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ films grown on Al2O3 (0001). The Faraday spectra strongly depended on the film thickness, revealing the significant effect of the multiple reflections of light. The intrinsic Faraday spectra of rutile Ti1−xCoxO2−δ were corrected for this multiple reflection effect on the basis of an optical model and compared with the absorption spectra in detail. The quantitatively good correspondence between intrinsic Faraday ellipticity and optical band edge strongly suggests that rutile Ti1−xCoxO2−δ is an intrinsic ferromagnetic semiconductor. The exchange parameters, N0α and N0β, were estimated as the order of ∼0.1eV.

Tunneling Spectroscopy of Non-Oxide Perovskite Superconductor MgCNi3

We report the tunneling spectroscopy of non-oxide perovskite superconductor MgCNi3 polycrystals using a low-temperature scanning tunneling microscope. The obtained tunneling spectra clearly show a superconducting energy gap structure, which can be well fitted by the BCS function with smearing parameter Γ. The value of the superconducting gap Δ is estimated to be 1.10–1.15 meV at 3 K, resulting in a ratio of 2Δ/kBTc=3.3–3.4 for Tc of 7.8 K. This result suggests that MgCNi3 is a conventional BCS s-wave superconductor in the weak-coupling regime.

Enhancement of Magneto-Optical Properties of Anatase Co:TiO2 Co-Doped with Nb

We have fabricated Ti0.95-xNbxCo0.05O2 thin films by the pulsed laser deposition technique. Carrier density was found to be almost identical to the Nb concentration up to x=0.06, indicating that doped Nb atoms generate carriers with >80% efficiency. In this doping range, magneto-optical properties were significantly enhanced by Nb-doping. The remanent Faraday rotation at a wavelength of 405 nm was as high as 5.6 ×103 deg/cm, which is approximately one order of magnitude higher than that of a film free of Nb-doping. The present results support the hypothesis that conduction electrons play an essential role in the ferromagnetism of Co-doped TiO2.

Magnetic Properties of Rutile Ti1-xFexO2 Epitaxial Thin Films

Growth conditions for pulsed laser deposition of ferromagnetic Ti1-xFexO2 films with the rutile structure have been optimized on the basis of magnetooptical Kerr effect (MOKE) measurements. Thus, it was found that Ti0.94Fe0.06O2 films prepared under very limited growth conditions, at a substrate temperature of 650–675°C and an oxygen pressure of ~1 ×10-6 Torr, show ferromagnetism at room temperature. The optimized films revealed a saturation magnetization of 1.3 µB and a coercive field of 0.14 kOe. From XPS measurements, the valence state of Fe was determined to be 3+, ruling out the possibility that ferromagnetism may arise from Fe3O4 nanoparticles.

Electronic structures of two-phase microstructures α and β in heavily Pb-doped Bi2Sr2CaCu2Oy single crystals investigated by scanning tunneling microscopy/spectroscopy

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) of a high-Jc superconductor, heavily Pb-doped Pb0.6Bi1.4Sr2CaCu2Oy, at low temperatures below Tc. The obtained STM images showed a lamella-type phase separation into Pb-poor and Pb-rich regions. At 4.3 K, values of energy gap Δ, deduced from STS spectra, showed considerable inhomogeneity in the range of 20–80 meV, while Δ variations across the phase boundaries were less significant. At 63 K, in contrast, we found that Δ altered sharply at the boundaries particularly in the annealed samples, resulting from the suppression of electronic inhomogeneity in each phase. This suggests that the phase boundaries act as strong pinning centers at higher temperatures.

Low Temperature Scanning Tunneling Spectroscopy Studies of High Jc NdBa2Cu3O7 − δ Single Crystals

We report low temperature scanning tunneling spectroscopy (STS) studies on NdBa2Cu3O7 − δ (Nd123) single crystals. Two characteristic spectra with a nearly ohmic background spectrum and a V-shaped background are observed on the cleaved surfaces. The former spectum shows an inhomogeneous distribution of a superconducting energy gap (2Δ) for STS differential conductance map. The latter shows a homogeneous distribution. It is probable that those differences are attributed to a tunneling current from different surface layers. The temperature dependence of tunneling conductance spectrum with a V-shaped background reveals that the superconducting gap disappears around Tc, and no pseudogap behavior exists above Tc.

Inhomogeneous electronic structures in heavily Pb-doped Bi2Sr2CaCu2Oy single crystals probed by low temperature STM/STS

Abstract We have performed cryogenic scanning tunneling microscopy/spectroscopy (STM/STS) of heavily Pb-doped Bi 2 Sr 2 CaCu 2 O y single crystals to investigate local electronic structures in the overdoped regime. The obtained STM/STS results at 4.3 K clearly showed local inhomogeneity of gap structure Δ ( Δ =20–60 meV) in a scale of several nm, suggesting the coexistence of sperconducting and pseudogap-like regions, even in the overdoped regime.