Published papers

Magnetic nature has been investigated for the novel ayered oxysulfide Sr_2_Cu_2_CoO_2_S_2_ which crystallizes in an unusual intergrowth structure with alternating square-planar CoO_2_ planes and anti-PbO type Cu_2_S_2_ layers. Sr_2_Cu_2_CoO_2_S_2_ is the strongly correlated antiferromagnet with T_N_ (=200K) obtained from measurements of the magnetic contribution d(χT)/dT. Below 80K(=T_1_), the difference between ZFC and FC magnetic susceptibilities are observed, indicating that Sr_2_Cu_2_CoO_2_S_2_ exhibits a spin-glass-like (spin-frustrated) transition. Above 200K(=T_2_), magnetic susceptibility shows a broad maxium indicative of 2D antiferromagnetic coupling in the square-planar CoO_2_ planes. From the NPD HERMES experiment, below 200K(=T_2_) we have confirmed the occurrences of new additional reflection peaks, which can be indexed on the K_2_NiF_4_-type magnetic cell and can be therefore assigned to the square-planar CoO_2_ planes with antiferromagnetic ordering. The magnetic reflection peaks are clearly seen in the NPD pattern at 75K in the spin-glass-like (spin-frustrated) region, indicating a possibility that Sr_2_Cu_2_CoO_2_S_2_ is the unusual spin-glass-like material.

Magnetic and electronic nature have been investigated for the novel layered oxysulfide Sr_2_Cu_2_CoO_2_S_2_ (space group I/4mmm). Sr_2_Cu_2_CoO_2_S_2_ is the antiferromagnet with T_N_ (=200 K) determined by the measurements of neutron powder diffraction and magnetic susceptibility (χ). The dχ/dT shows a broad maximum indicative of two-dimensional antiferromagnetic nature, which is not originated from the (Cu^+^S)_2_ layers but the strongly correlated Co^2+^O_2_ square-planes revealed by the X-ray photoemission spectroscopy.

Nonmetal-metal transition temperature Tt of (Ni1-xVx)0.98S increases up to x ~ 0.05, and then Tt decreases. The sign of thermoelectric power S below Tt changes from positive to negative with increasing x. These results indicate that the doped holes due to vacancies are compensated by doped electrons due to V and that the change of Tt concerns this compensation.

The zero-field-cooled magnetization at the magnetic field H = 300 Oe shows anomolous kinks at TA and TB (>TA far below the reentrant spin-glass-like transition temperature Tf (above which a long-range ferrimagnetic ordering is set up). The irreversibility between the zero-field-cooled and the field-cooled magnetizations, and the time decay of the thermoremanent magnetization (TRM) are observed below the ferrimagnetic Curie temperature TC. The decay feature can be described by Ogielski's power law TRM = At - whose exhibits peaks at TA, TB, Tf and TC. The peak of at TC can be fitted by the combination of Ising type and Ginzburg-Landau type expressions. Three stepwise changes of the spin-glass-like state below Tf and the feature of the coexistence of short-range spin-glass and long-range magnetic ordering components above Tf are discussed.

We present a k-dependent study of the single hole states in Sr_2_CuO_2_Cl_2_. We demonstrate that the controversial "1 eV peaks" in the high Tc's are quasi-particles derived from the same O 2p states as the Zhang-Rice (ZR) singlets, but of different symmetry and intensity in those regions of the Brillouin zone where the hybridization with the correlated Cu 3d states vanishes by symmetry. We use this new source of information to estimate the quasiparticle weight of the ZR singlets, discuss the quasiparticle line shape, and suggest a strong k dependence of the self-energy.

In order to study the change of the electronic states from the charge-transfer(CT) type metal to the Mott-Hubbard (MH) one, X-ray photoemission spectra are measured on the NiAs-type (Co_1-x_V_x_)_0.91_S (x ≤ 0.50) and V_0.91_S. The asymmetry parameter α for S 2s and 2p core level spectra decrease with increasing x. This result indicates that the local density of states of S atoms at the Fermi level decreases as x increases; the system changes from the CT-type metal to the MH-one with x. The change in V 2p line shape with x suggests the possibility of the considerable hybridization between V 3d and S 3p in (Co_1-x_V_x_)_0.91_S at lower x region. This feature would be related to the appearance of the extra peak at binding energy E_B_ = 2-3 eV in the valence band X-ray photoemission spectra.

Layered cobalt oxysulfide Sr_2_Cu_2_CoO_2_S_2_ crystallizes in an unusual intergrowth structure which represents a combination of SrCu_2_S_2_ and SrCoO_2_. Sr_2_Cu_2_CoO_2_S_2_ is a p-type antiferromagnetic semiconductor with T_N_(=200 K). And below T_t_(=125 K), the antiferromagnetic nature is changed from 2D to 3D. We studied Seebeck coefficient of Sr_2_Cu_2_CoO_2_S_2_ and its Cu-doped compounds. Their Seebeck coefficients near Tt tend to enhance by the Cu-doping into CoO_2_ square planes. The curves of the temperature dependence of Sr_2_Cu_2_Cu_x_Co_1-x_O_2_S_2_ is changed near T_N_, and the change points shift to high temperature by Cu-doping. The behavior resembles in TN shift of the magnetic property. The transport properties of Cu-doped Sr_2_Cu_2_CoO_2_S_2_ is affected by their magnetism.

Sr_4-x_La_x_Cu_2_Mn_3_O_7.5_S_2_ with alternating Cu_2_S_2_ and perovskite oxide layers have been synthesized and characterized. The perovskite oxide layers in Sr_2_CuMO_3_S (M=Fe and Ga) and A_4_Cu_2_Mn_3_O_7.5_S_2_ (A=Sr and La) have the stacking sequence with -MO_2_-SrO-SrO-MO_2_- and -MnO_2_-AO-Mn_1.5_-AO-MnO_2_-, respectively. Sr_2_CuFe_1-x_Ga_x_O_3_S (x=0, 0.05) is semiconducting. The electrical resistivity of the x=0.05 sample is ten times lower than that of the x=0 sample, indicating that the conduction path is in Ga-introduced iron oxide layer. Sr_4_Cu_2_Mn_3_O_7.5_S_2_ exhibits a phase transition from paramagnetic to antiferromagnetic states below 100K. In Sr_4-x_La_x_Cu_2_Mn_3_O_7.5_S_2_ (x=0, 0.1, 0.2, 0.4), the lattice parameters a and c decerease with increasing x. All samples are semiconducting. The electrical resistivity is lowered with increasing x; that of the x=0.4 sample is one-hundred times lower than that of the x=0 sample.