Kouichi Takase

Electrical resistivity and photoluminescence spectrum of layered oxysulfide (LaO)CuS

Abstract On growth conditions the dependence of the electrical resistivity and the photoluminescence (PL) spectrum of a layered oxysulfide (LaO)CuS have been investigated. The electrical resistivity shows semiconducting behavior and its magnitude decreases with the increase in the sintering temperature and time, which is considered to introduce structural defects such as Cu or La vacancies. The PL spectrum consists of six emission bands which are assigned to a direct interband transition and the transitions originating in two kinds of donor and acceptor levels corresponding to defect centers in the band gap. The PL spectra depend on the growth conditions. The introduction of lattice imperfection increases the intensity of the wide emission bands, and the (LaO)CuS sample visually appears to be ‘white’ under UV excitation. The white luminescence is an important property for the application to display back-light.

Charge density distribution of transparent p-type semiconductor (LaO)CuS

The charge density distributions of layered oxysulfide (LaO)CuS, known as a p-type transparent semiconductor, have been investigated by analyzing the synchrotron radiation powder diffraction profile with the maximum entropy method/Rietveld method. The bonding character of the Cu–S bond is revealed to be covalent. Meanwhile, the O–La bonding has both ionic and covalent characters. The number of electrons estimated by integrating the charge density around each atom gave direct evidence that each CuS and LaO layer is electrically almost neutral.

Electron correlation in the FeSe superconductor studied by bulk-sensitive photoemission spectroscopy

We have investigated the electronic structures of recently discovered superconductor FeSe by soft-x-ray and hard-x-ray photoemission spectroscopy with high bulk sensitivity. The large Fe 3d spectral weight is located in the vicinity of the Fermi level (EF), which is demonstrated to be a coherent quasi-particle peak. Compared with the results of the band structure calculation with local-density approximation, Fe 3d band narrowing and the energy shift of the band toward EF are found, suggesting an importance of the electron correlation effect in FeSe. The self energy correction provides the larger mass enhancement value (Z^-1=3.6) than in Fe-As superconductors and enables us to separate a incoherent part from the spectrum. These features are quite consistent with the results of recent dynamical mean-field calculations, in which the incoherent part is attributed to the lower Hubbard band.

In-field Jc improvement by oxygen-free pyrene gas diffusion into highly dense MgB2 superconductor

Oxygen-free pyrene gas as a carbon (C) dopant was delinked and incorporated into highly dense MgB2 structure via a gas phase diffusion method. The technique offers the advantages that molecular C is homogeneously distributed into MgB2 and substituted at the boron sites without any severe deterioration of grain connectivity. The C substitution causes a significant shrinkage of the a-lattice parameter and an increase in the lattice strain, resulting in high disorder. The introduction of structural disorder as a result of C doping leads to a considerable enhancement of the in-field critical current density (Jc) and upper critical field.

Magnetic properties and specific heat of new spin glass Mn0.5Fe0.5PS3

We have found spin glass phenomena in a layered Mn0.5Fe0.5PS3 compound and investigated its magnetic properties and specific heat. The temperature dependence of the zero field cooled (ZFC) magnetization along the easy axis, M∥ under dc applied field Ha below 1 kOe shows a cusp at Tg=32.6 K. The FC and ZFC curves measured at Ha below 1 kOe separate at almost the same temperature as Tg. For Ha larger than 1 kOe, the temperature at which FC and ZFC curves separate deviates from Tg and shifts toward lower temperature and the peak becomes broader. Magnetic hysteresis is still observed above 70 kOe at 2 K. The remarkable feature of this spin glass phase is that the spin freezing is not suppressed up to a rather high magnetic field compared with similar spin glass (Mn,Fe)TiO3. The cusp temperature of χ′(ω) does not show magnetic field amplitude dependence from 1 to 10 Oe and increases with an increase in frequency, obeying the Vogel–Fulcher rule. The temperature dependence of χ″(ω) shows two extra peaks at Tm1 a...

Three-dimensional electronic structures and the metal-insulator transition in Ruddlesden-Popper iridates

In this study, we systematically investigate 3D momentum(

Disorder-sensitive superconductivity in the doped iron silicide superconductor (Lu1―xRx)2Fe3Si5 (R = Sc, Y, and Dy)

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Pressure dependence of the magnetic properties of MnPS3

)-resolved electronic structures of Ruddlesden-Popper-type iridium oxides Sr

Magnetic Properties of LnOZnSb (Ln = La, Ce, Pr)

_{n+1}

Structural defects effect on ferromagnetism of layered oxysulfide (La1−xCaxO)Cu1−xNixS

Ir