M. Hirano

Epitaxial growth of transparent p-type conducting CuGaO2 thin films on sapphire (001) substrates by pulsed laser deposition

Transparent p-type conducting CuGaO2 thin films were prepared on α-Al2O3 (001) single-crystal substrates by pulsed laser deposition. The films were grown epitaxially on the substrates in an as-deposited state. X-ray pole figure analysis revealed that the films were composed of two types of epitaxial grains, both with c axes oriented perpendicular to the surface and a axes rotated 60° with respect to each other around the c axis. Observation of the CuGaO2 thin films by atomic force microscopy and high-resolution transmission electron microscopy substantiated this conclusion. The films have high optical transparency (∼80%) in the visible region, and the energy gap of CuGaO2 for direct allowed transition was estimated to be 3.6 eV. p-type conductivity was confirmed by Seebeck and Hall measurements. The electrical conductivity, carrier (positive hole) density, and Hall mobility of the films at room temperature were 6.3×10−2 S cm−1, 1.7×1018 cm−3, and 0.23 cm2 V−1 s−1, respectively.

Frontier of transparent conductive oxide thin films

The recent advancement of transparent conductive oxide thin films is reviewed using our strategy. Topics focused are the lowest resistive indium tin oxide films, deep-ultraviolet (deep-UV) transparent conductive oxides (TCO) thin film, p–n homo-junction based on bipolar CuInO2, and UV-light emitting diode based on p–n hetero-junction. A frontier of transparent oxide semiconductors is opening as a consequence of discovery of p-type TCO.

Pseudoisotropic Upper Critical Field in Cobalt-Doped SrFe2As2 Epitaxial Films

We present resistivity measurements of the complete superconducting upper critical field (H{c2}) phase diagram as a function of angle (theta) and temperature (T) for cobalt-doped SrFe2As2 epitaxial films to 0.5 K and 50 T. Although H{c2}(theta) at 10 K is indistinguishable from that derived from a single-band anisotropy model, the apparent anisotropy H{c2}{ perpendicularc}/H{c2};{ parallelc} linearly decreases to 1 at low T, with H{c2}(0)=47 T. The data are well described by a two-band model with small, opposing anisotropies for the bands. This unusual relationship is confirmed by the observation of a local maximum for H{c2};{ parallelc} at low T.

Pulsed laser deposition system for producing oxide thin films at high temperature

A pulsed laser deposition system capable of raising substrate temperatures up to 1500 °C in oxygen atmosphere has been developed to grow oxide thin films with excellent crystalline quality. The high temperature is achieved by condensation onto a substrate of infrared light emitted from a halogen lamp through a SiO2 rod serving as an optical guide. Deposition temperatures are regulated ranging from 1500 to 150 °C, under a partial oxygen pressure from 100 to 5×10−6 Pa. An appropriate thermal insulation between the substrate holder and the surrounding components suppresses the heat flow inbetween, leading to a smooth rotation of the substrate holder and preventing the mechanical components from being oxidized. A high-precision five-axis alignment mechanism equipped with a substrate manipulator allows for the development of an application in coaxial impact-collision ion scattering spectroscopy. As a demonstrative experiment, highly oriented Ga2O3 films, which are useful as deep ultraviolet transparent semicon...

Metamagnetic behavior and Kondo breakdown in heavy-fermion CeFePO

We report that nonmagnetic heavy-fermion (HF) iron oxypnictide CeFePO with two-dimensional XY-type anisotropy shows a metamagnetic behavior at the metamagnetic field H(M)≃4  T perpendicular to the c axis and that a critical behavior is observed around H(M). Although the magnetic character is entirely different from that in other Ce-based HF metamagnets, H(M) in these metamagnets is linearly proportional to the inverse of the effective mass, or to the temperature where the susceptibility shows a peak. This finding suggests that H(M) is a magnetic field breaking the local Kondo singlet, and the critical behavior around H(M) is driven by the Kondo breakdown accompanied by the Fermi-surface instability.

Doping dependence of the upper critical field and Hall resistivity of LaFeAsO1-x Fx (x=0, 0.025, 0.05, 0.07, 0.11, and 0.14)

The electrical resistivity ({rho}{sub xx}) and Hall resistivity ({rho}{sub xy}) have been measured over wide composition range using 60 T pulsed magnets. While the superconducting phase diagram (T{sub c}, x) displays the classic dome-shaped structure, we find that the upper critical field (H{sub c2}) increases monotonically with decreasing fluorine concentration (x), with the largest H{sub c2} {>=} 75 T for x = 0.05. {rho}{sub xx} and {rho}{sub xy} data provide evidence for the multiband electronic structure for all x {<=} 0.14 in LaFeAsO{sub 1-x}F{sub x}, and demonstrate the quantum phase transition near x {approx} 0.05. Both the multiband structure and quantum phase transition play crucial roles in the large H{sub c2}.

Carrier doping into MgIn2O4 epitaxial thin films by proton implantation

Epitaxial thin films of MgIn2O4 were grown on MgO(100) single-crystal substrates through the pulsed laser deposition technique. X-ray diffraction measurements revealed film orientations MgIn2O4(h00)//MgO(h00) and MgIn2O4(0k0)//MgO(0k0), respectively. Proton implantation was applied to generate carrier electrons in the films. The electrical conductivity of the as-deposited films is below ∼10−7 S cm−1 at room temperature. The maximum conductivity of ∼70 S cm−1 was obtained by the implantation. Hall voltage measurements revealed that H+ implantation causes carrier generation in proportion to H+ fluence without reduction of electron mobility. Following the post-annealing process resulted in further enhancement of the conductivity in each H+-implanted film, as conductivity and generation efficiency were found to increase up to ∼2×102 S cm−1 and ∼95% at the maximum, respectively. This differs from the behavior of polycrystalline films in which conductivity decreased by post-annealing due to a decrease in the Ha...

Pressure effect of superconducting oxypnictide LaFeAO1−xFx and related materials

Pressure dependence on superconducting transition temperature (Tc) has been investigated for iron-based superconductor LaFeAsO1−xFx, SmFeAsO1−xFx and Ca(Fe1−xCox)AsF. The Tc increases largely for LaFeAsO1−xFx with a small increase of pressure, and decreases with further compression. In SmFeAsO1−xFx the Tc decreases with increasing pressure. The increase of Tc in LaFeAsO1−xFx seems to be related to the suppression of magnetic ordering phase. Pressure-induced superconductivity was observed for these materials. The common features on 1111 type superconductors are discussed.

Pressure enhancement of superconductivity in an iron-based layered compound LaFeAsO1-xFx

Electrical resistivity measurements under high pressure were performed on iron-based superconductor LaFeAsO1-xFx. A steep increase in superconducting transition temperature (Tc) was observed for optimum-doped LaFeAsO0.89F0.11, whereas under-doped LaFeAsO0.95F0.05 shows a moderate increase in Tc with applying pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa. A ratio c/a was observed to decrease with applying pressure, which is similar to the pressure effect on cuprate high-Tc superconductors.

Electromagnetic properties of undoped LaFePnO (Pn P, As)

Undoped LaFePnO (Pn = P, As) shows metallic resistivity and a tetragonal phase at room temperature. The electrical resistivity (?) of undoped LaFeAsO exhibits a sudden decrease around Tanom ~160 K with decreasing the temperature and increases with decreasing the temperatures (T) below another threshold temperature (Tmin). It makes a contrast to undoped LaFePO, which shows neither of Tanom nor Tmin. The Tanom corresponds to a crystallographic phase transition: undoped LaFeAsO shows an orthorhombic phase at temperatures < 160 K. A positive magnetoresistance (MR = (?-?0) / ?0) also appears below Tanom. DFT calculations indicate that spin moment of LaFeAsO is 1.56 ?B / Fe in an antiferromagnetic spin configuration in a tetragonal phase, while spin moment of LaFePO is almost quenched.