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Kunihito Koumoto - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic carrier transport properties in layered cobaltate Epitaxial Films grown by reactive solid phase epitaxy
    Applied Physics Letters, 2009
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hideo Hosono, Yuichi Ikuhara, Shinichi Nakagawa, Rong Huang, Kunihito Koumoto
    Abstract:

    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 Epitaxial Film grown on the m-plane of an α-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroEpitaxially grown with the CoO2 conducting planes inclined by ∼43° against the α-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was ∼1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.

  • thermoelectric performance of Epitaxial thin Films of layered cobalt oxides grown by reactive solid phase epitaxy with topotactic ion exchange methods
    International Journal of Applied Ceramic Technology, 2007
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    Abstract:

    This article reviews high-quality Epitaxial Film growth of layered cobalt oxides by reactive solid-phase epitaxy (R-SPE) with topotactic ion-exchange methods. Epitaxial Film of Na 0.8 CoO 2 was grown on a (0001)-oriented α-Al 3 O 3 substrate by R-SPE using CoO Film as the starting material. The Na 0.8 CoO 2 Epitaxial Films were converted into high-quality Epitaxial Films of Sr 0.32 Na 0.21 CoO 2 and [Ca 2 CoO 3 ] x CoO 2 by topotactic ion-exchange methods. The Sr 0.32 Na 0.21 CoO 2 Film exhibited better stability against moisture than that of the Na 0.8 CoO 2 Film, while it retained the good thermoelectric properties of Na 0.8 CoO 2 . The [Ca 2 COO 3 ] x CoO 2 Film exhibited a high electrical conductivity of 2.95 x 10 2 S/cm and a large Seebeck coefficient of +125μV/K at 300K.

  • Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)
    Inorganic Chemistry, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    We have developed a unique multistep Film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate Epitaxial Films of superconducting sodium−cobalt oxyhydrate, Na0.3CoO2·1.3H2O. An Epitaxial Film of Na0.8CoO2 grown on an α-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na0.3CoO2·1.3H2O Epitaxial Film. The Film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the Epitaxial growth of superconducting Na0.3CoO2·1.3H2O.

  • Fabrication and thermoelectric properties of layered cobaltite, γ-Sr0.32Na0.21CoO2 Epitaxial Films
    Applied Physics Letters, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    Thermoelectric properties of γ-Sr0.32Na0.21CoO2 Epitaxial Films, which were fabricated on the (0001)-face of α-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with γ-Na0.8CoO2 Epitaxial Films, were measured to clarify the effect of Sr2+-ion exchange of γ-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF=S2σ) of the γ-Sr0.32Na0.21CoO2 Film (300K) were 8.9×102Scm−1, +120μVK−1, and 1.2×10−3Wm−1K−2, respectively. These values were relatively large compared to those of γ-Na0.8CoO2 Epitaxial Films (σ=1.2×103Scm−1, S=+95μVK−1, and PF=1.1×10−3Wm−1K−2). The γ-Sr0.32Na0.21CoO2 Epitaxial Film exhibited much improved chemical stability against moisture compared to the γ-Na0.8CoO2 Epitaxial Film. The Sr2+-ion exchange of γ-Na0.8CoO2 is vital for practical thermoelectric applications.

  • Thermoelectric Properties of Epitaxial Films of Layered Cobalt Oxides Fabricated by Topotactic Ion-Exchange Methods
    2006 25th International Conference on Thermoelectrics, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Tomohiro Saito, Yuichi Ikuhara, Kunihito Koumoto
    Abstract:

    Epitaxial Film of a layered cobalt oxide, Ca3Co4 O9, was fabricated on a (0001)-face of alpha-Al2 O3 substrate by a topotactic ion exchange method using a Na0.8CoO2 Epitaxial Film as a precursor. High-resolution X-ray diffraction and atomic force microscope measurements revealed that the Film was high-quality (001)-oriented Ca 3Co4O9 with stepped & terraced surface morphology. The Film exhibits a high electrical conductivity of 2.95 times 102 Scm-1 and a large Seebeck coefficient of ~+125 muVK-1, which leads to the thermoelectric power factor of 4.5 times 10-4 Wm-1 K-1 at 300 K

Hideo Hosono - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic carrier transport properties in layered cobaltate Epitaxial Films grown by reactive solid phase epitaxy
    Applied Physics Letters, 2009
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hideo Hosono, Yuichi Ikuhara, Shinichi Nakagawa, Rong Huang, Kunihito Koumoto
    Abstract:

    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 Epitaxial Film grown on the m-plane of an α-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroEpitaxially grown with the CoO2 conducting planes inclined by ∼43° against the α-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was ∼1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.

  • Epitaxial growth of high mobility cu2o thin Films and application to p channel thin Film transistor
    Applied Physics Letters, 2008
    Co-Authors: Kosuke Matsuzaki, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Toshio Kamiya, Hideo Hosono
    Abstract:

    Cu2O Epitaxial Films were grown for high mobility p-channel oxide thin-Film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase Epitaxial Films with hole Hall mobilities ∼90 cm2 V−1 s−1 comparable to those of single crystals (∼100 cm2 V−1 s−1). TFTs using the Epitaxial Film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory (∼0.26 cm2 V−1 s−1 and ∼6, respectively).

  • superconductivity in Epitaxial thin Films of co doped srfe2as2 with bilayered feas structures and their magnetic anisotropy
    Applied Physics Express, 2008
    Co-Authors: Hidenori Hiramatsu, Masahiro Hirano, Takayoshi Katase, Toshio Kamiya, Hideo Hosono
    Abstract:

    Superconducting Epitaxial Films of Fe-based layered arsenide, Co-doped SrFe2As2, were grown at 700 °C on mixed perovskite (La,Sr)(Al,Ta)O3(001) single-crystal substrates by pulsed-laser deposition. Both the Epitaxial Film and an (001)-oriented Film grown at 600 °C exhibited superconducting transitions at ~20 K. The zero-resistance states of the Epitaxial Film were sustained under a magnetic field (H) of 9 T at 9 K when H was parallel to the c-axis, while they were sustained at higher temperatures up to 10 K for H parallel to the a-axis. This is the first demonstration of superconducting thin Films of FeAs-based new superconductors.

  • Photoluminescence from Epitaxial Films of perovskite-type alkaline-earth stannates
    Applied Physics Express, 2008
    Co-Authors: Kazushige Ueda, Kenji Nomura, Tsuyoshi Maeda, Kensuke Nakayashiki, Katsuhiko Goto, Yutaka Nakachi, Hiroshi Takashima, Koichi Kajihara, Hideo Hosono
    Abstract:

    The Epitaxial Films of the recently developed calcium and strontium stannates phosphors with perovskite or its related structure were fabricated by the pulsed-lased deposition method. The Films were all highly transparent in the visible region, and showed intense luminescence of several colors under ultraviolet excitation. Light scattering at the substrates' surfaces varied the appearance of photoluminescence. In combination with Epitaxial-Film-growth technologies for many perovskite functional materials, the obtained results provide a promise for future developments of multifunctional optoelectronic devices.

  • Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)
    Inorganic Chemistry, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    We have developed a unique multistep Film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate Epitaxial Films of superconducting sodium−cobalt oxyhydrate, Na0.3CoO2·1.3H2O. An Epitaxial Film of Na0.8CoO2 grown on an α-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na0.3CoO2·1.3H2O Epitaxial Film. The Film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the Epitaxial growth of superconducting Na0.3CoO2·1.3H2O.

Hiromichi Ohta - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic carrier transport properties in layered cobaltate Epitaxial Films grown by reactive solid phase epitaxy
    Applied Physics Letters, 2009
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hideo Hosono, Yuichi Ikuhara, Shinichi Nakagawa, Rong Huang, Kunihito Koumoto
    Abstract:

    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 Epitaxial Film grown on the m-plane of an α-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroEpitaxially grown with the CoO2 conducting planes inclined by ∼43° against the α-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was ∼1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.

  • thermoelectric performance of Epitaxial thin Films of layered cobalt oxides grown by reactive solid phase epitaxy with topotactic ion exchange methods
    International Journal of Applied Ceramic Technology, 2007
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    Abstract:

    This article reviews high-quality Epitaxial Film growth of layered cobalt oxides by reactive solid-phase epitaxy (R-SPE) with topotactic ion-exchange methods. Epitaxial Film of Na 0.8 CoO 2 was grown on a (0001)-oriented α-Al 3 O 3 substrate by R-SPE using CoO Film as the starting material. The Na 0.8 CoO 2 Epitaxial Films were converted into high-quality Epitaxial Films of Sr 0.32 Na 0.21 CoO 2 and [Ca 2 CoO 3 ] x CoO 2 by topotactic ion-exchange methods. The Sr 0.32 Na 0.21 CoO 2 Film exhibited better stability against moisture than that of the Na 0.8 CoO 2 Film, while it retained the good thermoelectric properties of Na 0.8 CoO 2 . The [Ca 2 COO 3 ] x CoO 2 Film exhibited a high electrical conductivity of 2.95 x 10 2 S/cm and a large Seebeck coefficient of +125μV/K at 300K.

  • Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)
    Inorganic Chemistry, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    We have developed a unique multistep Film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate Epitaxial Films of superconducting sodium−cobalt oxyhydrate, Na0.3CoO2·1.3H2O. An Epitaxial Film of Na0.8CoO2 grown on an α-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na0.3CoO2·1.3H2O Epitaxial Film. The Film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the Epitaxial growth of superconducting Na0.3CoO2·1.3H2O.

  • Fabrication and thermoelectric properties of layered cobaltite, γ-Sr0.32Na0.21CoO2 Epitaxial Films
    Applied Physics Letters, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    Thermoelectric properties of γ-Sr0.32Na0.21CoO2 Epitaxial Films, which were fabricated on the (0001)-face of α-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with γ-Na0.8CoO2 Epitaxial Films, were measured to clarify the effect of Sr2+-ion exchange of γ-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF=S2σ) of the γ-Sr0.32Na0.21CoO2 Film (300K) were 8.9×102Scm−1, +120μVK−1, and 1.2×10−3Wm−1K−2, respectively. These values were relatively large compared to those of γ-Na0.8CoO2 Epitaxial Films (σ=1.2×103Scm−1, S=+95μVK−1, and PF=1.1×10−3Wm−1K−2). The γ-Sr0.32Na0.21CoO2 Epitaxial Film exhibited much improved chemical stability against moisture compared to the γ-Na0.8CoO2 Epitaxial Film. The Sr2+-ion exchange of γ-Na0.8CoO2 is vital for practical thermoelectric applications.

  • Thermoelectric Properties of Epitaxial Films of Layered Cobalt Oxides Fabricated by Topotactic Ion-Exchange Methods
    2006 25th International Conference on Thermoelectrics, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Tomohiro Saito, Yuichi Ikuhara, Kunihito Koumoto
    Abstract:

    Epitaxial Film of a layered cobalt oxide, Ca3Co4 O9, was fabricated on a (0001)-face of alpha-Al2 O3 substrate by a topotactic ion exchange method using a Na0.8CoO2 Epitaxial Film as a precursor. High-resolution X-ray diffraction and atomic force microscope measurements revealed that the Film was high-quality (001)-oriented Ca 3Co4O9 with stepped & terraced surface morphology. The Film exhibits a high electrical conductivity of 2.95 times 102 Scm-1 and a large Seebeck coefficient of ~+125 muVK-1, which leads to the thermoelectric power factor of 4.5 times 10-4 Wm-1 K-1 at 300 K

Kenji Sugiura - One of the best experts on this subject based on the ideXlab platform.

  • anisotropic carrier transport properties in layered cobaltate Epitaxial Films grown by reactive solid phase epitaxy
    Applied Physics Letters, 2009
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hideo Hosono, Yuichi Ikuhara, Shinichi Nakagawa, Rong Huang, Kunihito Koumoto
    Abstract:

    Herein we report the anisotropic carrier transport properties of a layered cobaltate, NaxCoO2 Epitaxial Film grown on the m-plane of an α-Al2O3 substrate using reactive solid-phase epitaxy. Scanning and transmission electron microscopy analyses revealed that NaxCoO2 was heteroEpitaxially grown with the CoO2 conducting planes inclined by ∼43° against the α-Al2O3 substrate surface. The electrical resistivity parallel to the CoO2 planes was ∼1/5 of the perpendicular one, but the parallel Seebeck coefficient was about twice as large as the perpendicular one. Hence, a higher thermoelectric efficiency in the cobaltates can be obtained within the CoO2 planes.

  • thermoelectric performance of Epitaxial thin Films of layered cobalt oxides grown by reactive solid phase epitaxy with topotactic ion exchange methods
    International Journal of Applied Ceramic Technology, 2007
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kunihito Koumoto
    Abstract:

    This article reviews high-quality Epitaxial Film growth of layered cobalt oxides by reactive solid-phase epitaxy (R-SPE) with topotactic ion-exchange methods. Epitaxial Film of Na 0.8 CoO 2 was grown on a (0001)-oriented α-Al 3 O 3 substrate by R-SPE using CoO Film as the starting material. The Na 0.8 CoO 2 Epitaxial Films were converted into high-quality Epitaxial Films of Sr 0.32 Na 0.21 CoO 2 and [Ca 2 CoO 3 ] x CoO 2 by topotactic ion-exchange methods. The Sr 0.32 Na 0.21 CoO 2 Film exhibited better stability against moisture than that of the Na 0.8 CoO 2 Film, while it retained the good thermoelectric properties of Na 0.8 CoO 2 . The [Ca 2 COO 3 ] x CoO 2 Film exhibited a high electrical conductivity of 2.95 x 10 2 S/cm and a large Seebeck coefficient of +125μV/K at 300K.

  • Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)
    Inorganic Chemistry, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    We have developed a unique multistep Film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate Epitaxial Films of superconducting sodium−cobalt oxyhydrate, Na0.3CoO2·1.3H2O. An Epitaxial Film of Na0.8CoO2 grown on an α-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na0.3CoO2·1.3H2O Epitaxial Film. The Film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the Epitaxial growth of superconducting Na0.3CoO2·1.3H2O.

  • Fabrication and thermoelectric properties of layered cobaltite, γ-Sr0.32Na0.21CoO2 Epitaxial Films
    Applied Physics Letters, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    Thermoelectric properties of γ-Sr0.32Na0.21CoO2 Epitaxial Films, which were fabricated on the (0001)-face of α-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with γ-Na0.8CoO2 Epitaxial Films, were measured to clarify the effect of Sr2+-ion exchange of γ-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF=S2σ) of the γ-Sr0.32Na0.21CoO2 Film (300K) were 8.9×102Scm−1, +120μVK−1, and 1.2×10−3Wm−1K−2, respectively. These values were relatively large compared to those of γ-Na0.8CoO2 Epitaxial Films (σ=1.2×103Scm−1, S=+95μVK−1, and PF=1.1×10−3Wm−1K−2). The γ-Sr0.32Na0.21CoO2 Epitaxial Film exhibited much improved chemical stability against moisture compared to the γ-Na0.8CoO2 Epitaxial Film. The Sr2+-ion exchange of γ-Na0.8CoO2 is vital for practical thermoelectric applications.

  • Thermoelectric Properties of Epitaxial Films of Layered Cobalt Oxides Fabricated by Topotactic Ion-Exchange Methods
    2006 25th International Conference on Thermoelectrics, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Tomohiro Saito, Yuichi Ikuhara, Kunihito Koumoto
    Abstract:

    Epitaxial Film of a layered cobalt oxide, Ca3Co4 O9, was fabricated on a (0001)-face of alpha-Al2 O3 substrate by a topotactic ion exchange method using a Na0.8CoO2 Epitaxial Film as a precursor. High-resolution X-ray diffraction and atomic force microscope measurements revealed that the Film was high-quality (001)-oriented Ca 3Co4O9 with stepped & terraced surface morphology. The Film exhibits a high electrical conductivity of 2.95 times 102 Scm-1 and a large Seebeck coefficient of ~+125 muVK-1, which leads to the thermoelectric power factor of 4.5 times 10-4 Wm-1 K-1 at 300 K

Masahiro Hirano - One of the best experts on this subject based on the ideXlab platform.

  • Epitaxial growth of high mobility cu2o thin Films and application to p channel thin Film transistor
    Applied Physics Letters, 2008
    Co-Authors: Kosuke Matsuzaki, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Toshio Kamiya, Hideo Hosono
    Abstract:

    Cu2O Epitaxial Films were grown for high mobility p-channel oxide thin-Film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase Epitaxial Films with hole Hall mobilities ∼90 cm2 V−1 s−1 comparable to those of single crystals (∼100 cm2 V−1 s−1). TFTs using the Epitaxial Film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory (∼0.26 cm2 V−1 s−1 and ∼6, respectively).

  • superconductivity in Epitaxial thin Films of co doped srfe2as2 with bilayered feas structures and their magnetic anisotropy
    Applied Physics Express, 2008
    Co-Authors: Hidenori Hiramatsu, Masahiro Hirano, Takayoshi Katase, Toshio Kamiya, Hideo Hosono
    Abstract:

    Superconducting Epitaxial Films of Fe-based layered arsenide, Co-doped SrFe2As2, were grown at 700 °C on mixed perovskite (La,Sr)(Al,Ta)O3(001) single-crystal substrates by pulsed-laser deposition. Both the Epitaxial Film and an (001)-oriented Film grown at 600 °C exhibited superconducting transitions at ~20 K. The zero-resistance states of the Epitaxial Film were sustained under a magnetic field (H) of 9 T at 9 K when H was parallel to the c-axis, while they were sustained at higher temperatures up to 10 K for H parallel to the a-axis. This is the first demonstration of superconducting thin Films of FeAs-based new superconductors.

  • Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)
    Inorganic Chemistry, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Hiroshi Yanagi, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    We have developed a unique multistep Film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate Epitaxial Films of superconducting sodium−cobalt oxyhydrate, Na0.3CoO2·1.3H2O. An Epitaxial Film of Na0.8CoO2 grown on an α-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na0.3CoO2·1.3H2O Epitaxial Film. The Film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the Epitaxial growth of superconducting Na0.3CoO2·1.3H2O.

  • Fabrication and thermoelectric properties of layered cobaltite, γ-Sr0.32Na0.21CoO2 Epitaxial Films
    Applied Physics Letters, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Kunihito Koumoto
    Abstract:

    Thermoelectric properties of γ-Sr0.32Na0.21CoO2 Epitaxial Films, which were fabricated on the (0001)-face of α-Al2O3 substrates by ion exchange processes between Na+ and Sr2+ ions with γ-Na0.8CoO2 Epitaxial Films, were measured to clarify the effect of Sr2+-ion exchange of γ-Na0.8CoO2 on the thermoelectric properties. Electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF=S2σ) of the γ-Sr0.32Na0.21CoO2 Film (300K) were 8.9×102Scm−1, +120μVK−1, and 1.2×10−3Wm−1K−2, respectively. These values were relatively large compared to those of γ-Na0.8CoO2 Epitaxial Films (σ=1.2×103Scm−1, S=+95μVK−1, and PF=1.1×10−3Wm−1K−2). The γ-Sr0.32Na0.21CoO2 Epitaxial Film exhibited much improved chemical stability against moisture compared to the γ-Na0.8CoO2 Epitaxial Film. The Sr2+-ion exchange of γ-Na0.8CoO2 is vital for practical thermoelectric applications.

  • Thermoelectric Properties of Epitaxial Films of Layered Cobalt Oxides Fabricated by Topotactic Ion-Exchange Methods
    2006 25th International Conference on Thermoelectrics, 2006
    Co-Authors: Kenji Sugiura, Hiromichi Ohta, Kenji Nomura, Masahiro Hirano, Hideo Hosono, Tomohiro Saito, Yuichi Ikuhara, Kunihito Koumoto
    Abstract:

    Epitaxial Film of a layered cobalt oxide, Ca3Co4 O9, was fabricated on a (0001)-face of alpha-Al2 O3 substrate by a topotactic ion exchange method using a Na0.8CoO2 Epitaxial Film as a precursor. High-resolution X-ray diffraction and atomic force microscope measurements revealed that the Film was high-quality (001)-oriented Ca 3Co4O9 with stepped & terraced surface morphology. The Film exhibits a high electrical conductivity of 2.95 times 102 Scm-1 and a large Seebeck coefficient of ~+125 muVK-1, which leads to the thermoelectric power factor of 4.5 times 10-4 Wm-1 K-1 at 300 K