Satoshi Taniguchi

Spin Gap Behavior of S=1/2 Quasi-Two-Dimensional System CaV4O9

Spin susceptibility and spin-lattice relaxation rate of 51 V nuclear moments of CaV 4 O 9 with tetragonal structure have been measured. They exhibit characteristics of spin gap systems. This provides the first example of quasi-two-dimensional spin systems with spin gap.

Study of Spin-Gap Formation in Quasi-Two-Dimensional S= 1/2 System CaV 4O 9: Neutron Scattering and NMR

Neutron inelastic scattering study has been performed on aligned single crystals of quasi-two-dimensional spin-gap system CaV 4 O 9 . By analyzing the dispersion curves and the structure factor of the triplet excitation, a physical picture of the present spin system has been constructed: The plaquette singlet units of four S =1/2 V-atoms within four “corner-linked” VO 5 pyramids form the two-dimensional linkage of the spins by the interactions between the V-spins within the “edge-linked” VO 5 pyramids. We have remeasured the NMR relaxation rates and obtained the spin-gap behavior consistent with the neutron results.

Indium Content Dependence of Electron Velocity and Impact Ionization in InAlAs/InGaAs Metamorphic HEMTs

We have fabricated InxAl1-xAs/InxGa1-xAs metamorphic high electron mobility transistors (MHEMTs) with x=0.36, 0.43, and 0.53 on GaAs, and lattice-matched (LM) HEMTs with x=0.53 on InP. Using the HEMTs, we have carried out systematic studies on the electron velocity and the impact ionization in the InGaAs channels with several indium contents. The electron velocity is determined by the gate length dependence of cut-off frequency. Although high-density crystalline defects propagate into the channels of the MHEMTs, the electron velocity is not affected by the defects. Impact ionization phenomena, which dominate on-state breakdown voltage, have been investigated by gate-current analysis in the prebreakdown regime of the HEMTs. We have estimated the characteristic electric field of the impact ionization for InGaAs. It has been found that the gate current due to the impact ionization of the MHEMT with x=0.53 is smaller than that of the LMHEMT, which is explained by the enhanced electron-hole recombination by the crystalline defects in the MHEMT.

Evolution of Spin Gap in the Excitation Spectra of Quasi-Two-Dimensional S=1/2 System CaV4O9.

Neutron inelastic scattering studies have been performed on the quasi-two-dimensional spin-gap system CaV 4 O 9 . An energy gap of about 10 meV has been clearly observed in the magnetic excitation spectra at low temperatures. The Q-point of the minimum excitation energy may not correspond to the so-called (π, π), (π, 0) or Γ-points in the reciprocal cell of the two-dimensional tetragonal lattice. With decreasing temperature, the scattering intensity in the energy region around 12 meV begins to increase at about 100 K, which almost coincides with the temperature at which the susceptibility has a maximum value. This result may provide important information to understand the “spin-gap anomaly” of high- T c oxides.

Structural studies on the quasi two-dimensional Mott system BaCo1-xNixS2-δ by the X-ray Rietveld method

Structural parameters of BaCo 1- x Ni x S 2 (0 ≤ x ≤1) with quasi two-dimensional electrons have been determined by the X-ray Rietveld method. This sulfide exhibits the Mott transition with varying x . The interatomic distance between Co/Ni and S(1) at the apex site of a Co 1- x Ni x S 5 square-pyramid increases with x , which is consistent with the electronic configuration (3 d 3 z 2 - r 2 ) 1+ x of Co/Ni ions. The structural data of BaCo 0.9 Ni 0.1 S 1.85 show that the sulfur vacancies are within (Co/Ni)S planes.

Transport, Magnetic and Thermal Properties of La3Ni2O7-δ

The metal-insulator transition of La 3 Ni 2 O 7-δ with 2-dimensional electrons has been studied. Various physical properties can be understood by introducing a model of charge orderings in the NiO 2 planes induced by oxygen orderings. Arguments on the non-occurrence of the superconductivity are also given.

Two-Dimensional Observation of Grain Movements in Elongated and Aligned Grain Structure during High Temperature Deformation

It is understood that grains move by grain boundary sliding, and change their relationship to each other during superplastic deformation. Ideal two-dimensional observation of grain movements from the specimen surface is difficult even in the shear deformation because grains move three-dimensionally according to the stress distribution against the specimen surface. In this study, ODS steel with elongated grains aligned along one direction was deformed perpendicular to the aligned axis to achieve ideal two-dimensional grain movements. Surface height profiles with a laser microscope showed small amount of three-dimensional grain movements, while two-dimensional grain movements and rotations were appeared by observations before and after the deformation with SEM-EBSD.

Optimum Design of Detection Coil System for Vibrating Sample Magnetometer

The optimum design of a detection coil system for Foner-type vibrating sample magnetometers is discussed. It was found by an analysis based on numerical computations that one of these coil configurations has an extremely broad saddle point for sample displacements, which minimizes any undesirable influence due to sample mispositioning. These results can be applied to the conventional Foner-type magnetometers in order to improve the accuracy of measurements.

Structural Studies on the Phase Transition of La3Ni2O6.92 at about 550 K.

Structural studies of La 3 Ni 2 O 6.92 with a double layered perovskite structure have been performed by the X-ray Rietveld analyses between 300 K and 650 K. The interatomic distance between Ni and the apical oxygen (on the outside of NiO 2 double layer) rapidly increases around the phase-transition temperature T B ∼550 K with decreasing temperature. Then, the energy of the (3 z 2 - r 2 ) band is considered to shift downwards with respect to that of the ( x 2 - y 2 ) band with decreasing temperature and the lower Hubbard band of the former becomes to be (almost) fully occupied at the transition temperature, which induces the Mott transition (delocalized-localized transition) of the (3 z 2 - r 2 ) electrons.

Low-Temperature Magnetization and Spin-Wave Excitations in Gd

The magnetization of Gd has been measured in the temperature 4<T<310 K at constant magnetic field using a vibrating sample magnetometer. We present an exact analysis of spin-wave excitations for Gd in the presence of a magnetic field, H. The low temperature magnetization is found to obey the form : Δσ(T)/σ(0)=BZ(3/2,Tg/T)T3/2+CZ(5/2,Tg/T)T5/2, where B and C are constants, Z(3/2,Tg/T) and Z(5/2,Tg/T) are the characteristic functions of T and H, indicating magnetic field effects.