Taizo Sasaki

Optical and Electrical Properties of Tin Oxide Films

The electrical and optical properties of the transparent conducting tin oxide films were studied experimentally. The observed Hall effect and Seebeck effect show that the present specimens are n -type conductor whose carrier density is 10 19 ∼10 20 cm -3 . The optical measurements made it clear that the fundamental energy gap was about 4 ev and the plasma frequency of the conducting carriers lied in the near infrared region. By comparing the observed optical transmission and reflection with the theoretical values which can be calculated with the electrically measured constants, the effective mass of the carriers was determined to be about 1/5 of the free electron mass. It will be discussed in conclusion that, to satisfy the thermoelectric power relation with this small effective mass, we must assume that the Coulomb scattering of impurity ions is the predominant scattering mechanism in the transparent conducting tin oxide films.

Unconventional superconductivity and nearly ferromagnetic spin fluctuations in NaxCoO2.yH2O

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na x CoO 2 · y H 2 O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na x CoO 2 were examined, a SC bilayer-hydrate sample with T c ∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/ T 1 in the SC sample, it was found that the coherence peak is absent just below T c and that 1/ T 1 is proportional to temperature far below T c . These results, which are in qualitative agreement with the previous result by Fujimoto et al. , suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/ T 1 T of the SC sample shows a gradual increase below 100 K down to T c , whereas 1/ T 1 T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/ T 1 T and χ bulk in the SC sample, the increase in 1/ T 1 T is at...

Fermi surface and anisotropic spin-orbit coupling of Sb(111) studied by angle-resolved photoemission spectroscopy.

High-resolution angle-resolved photoemission spectroscopy has been performed on Sb(111) to elucidate the origin of anomalous electronic properties in group-V semimetal surfaces. The surface was found to be metallic despite the semimetallic character of bulk. We clearly observed two surface-derived Fermi surfaces which are likely spin split, demonstrating that the spin-orbit interaction plays a dominant role in characterizing the surface electronic states of group-V semimetals. The universality or dissimilarity of the electronic structure in Bi and Sb is discussed in relation to the granular superconductivity, electron-phonon coupling, and surface charge or spin density wave.

Electronic properties and applications of MXenes: a theoretical review

The recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has brought a new opportunity to materials science and technology. This review highlights the computational attempts that have been made to understand the physics and chemistry of this very promising family of advanced two-dimensional materials, and to exploit their novel and exceptional properties for electronic and energy harvesting applications.

Marcasite osmium nitride with high bulk modulus: First-principles calculations

The authors propose marcasite OsN2 as a structure of the experimentally synthesized orthorhombic OsN2 compound. From the first-principles calculations, they find that the marcasite structure is more stable than the fluorite and pyrite ones and its lattice constants are in good agreement with the experiment. The calculated elastic constants for marcasite OsN2 satisfy the stability condition. The band structure shows that marcasite OsN2 is metallic. There is a pseudogap around the Fermi level of the total density of states of marcasite OsN2, which may contribute to its stability. The appearance of the pseudogap is mainly caused by the anisotropic connectivity of OsN6 octahedra.

OH terminated two-dimensional transition metal carbides and nitrides (MXenes) as ultralow work function materials

MXenes are a set of two-dimensional transition metal carbides and nitrides that offer many potential applications in energy storage and electronic devices. As an important parameter to design new electronic devices, we investigate the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principles calculations. From our calculations, it turns out that the OH terminated MXenes attain ultralow work functions between 1.6 and 2.8 eV. Moreover, depending on the type of the transition metal, the F or O functionalization affects increasing or decreasing the work functions. We show that the changes in the work functions upon functionalizations are linearly correlated with the changes in the surface dipole moments. It is shown that the work functions of the F or O terminated MXenes are controlled by two factors: the induced dipole moments by the charge transfers between F/O and the substrate, and the changes in the total surface dipole moments caused by surface relaxation upon the functionalization. However, in the cases of the OH terminated MXenes, in addition to these two factors, the intrinsic dipole moments of the OH groups play an important role in determining the total dipole moments and consequently justify their ultralow work functions.

Nanometer-thin layered hydroxide platelets of (Y0.95Eu0.05)2(OH)5NO3·xH2O: exfoliation-free synthesis, self-assembly, and the derivation of dense oriented oxide films of high transparency and greatly enhanced luminescence

Ultra-thin (Y0.95Eu0.05)2(OH)5NO3·xH2O (Y/Eu LRH) nanoplates (down to ∼4 nm thick) have been successfully synthesized in one step, without exfoliation, through hydrothermal treatment of mixed nitrate solution in the presence of tetrabutylammonium hydroxide (TBAOH). Highly [111] oriented, dense (Y0.95Eu0.05)2O3 phosphor films (∼60 nm thick) with excellent optical transparency (∼86%) and a greatly enhanced luminescence intensity (∼4 times that of the powder form) have been constructed via self-assembly of ultra-thin Y/Eu LRHs nanoplates, followed by proper annealing.

Soft X-Ray Absorption Spectra of Metals and Alloys. I. Be, Al, Sb, Bi and Al-Mg Alloys

Soft X-ray absorption spectra of Be, Al, Sb and Bi, and Al-Mg alloys have been obtained by the use of synchrotron radiation emitted from the 750 MeV electron synchrotron at the Institute for Nuclear Study, Tokyo, as a continuous light source. The K-absorption structure of beryllium was found to agree with the density of states calculated by Loucks and Cutler. Some correspondence was found between the maxima in the L 2,3 -absorption spectrum and the K-absorption spectrum of aluminum. A remarkable band was observed in the L 2,3 -absorption spectrum of aluminum at 23 eV above the edge. This band is not attributable to the singularity in the state density of the conduction band. A tentative interpretation based on the delayed onset of transitions 2 p -∞ d is proposed. The L 2,3 -absorption spectrum of aluminum in Al-Mg alloy shows a remarkable degeneration of the edge, while the L 2,3 -absorption edge of magnesium in the same alloy seems relatively unaffected. This behavior is similar to the effect reported b...

The effect of the interlayer element on the exfoliation of layered Mo2AC (A = Al, Si, P, Ga, Ge, As or In) MAX phases into two-dimensional Mo2C nanosheets

Abstract The experimental exfoliation of layered, ternary transition-metal carbide and nitride compounds, known as MAX phases, into two-dimensional (2D) nanosheets, is a great development in the synthesis of novel low-dimensional inorganic systems. Among the MAX phases, Mo-containing ones might be considered as the source for obtaining Mo2C nanosheets with potentially unique properties, if they could be exfoliated. Here, by using a set of first-principles calculations, we discuss the effect of the interlayer ‘A’ element on the exfoliation of Mo2AC (A = Al, Si, P, Ga, Ge, As or In) MAX phases into the 2D Mo2C nanosheets. Based on the calculated exfoliation energies and the elastic constants, we propose that Mo2InC with the lowest exfoliation energy and the highest elastic constant anisotropy between C11 and C33 might be a suitable compound for exfoliation into 2D Mo2C nanosheets.

A 10 m grazing incidence monochromator for high resolution soft X-ray spectroscopy at the photon factory

Abstract A description of a novel 10 m grazing incidence monochromator is given on the optical and the mechanical design features as well as on its performance achieved. The monochromator was designed and constructed at the Photon Factory for high resolution spectroscopic studies in the soft X-ray region.