Hisashi Sasaki

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.

Neutron Scattering Study on the Quasi-One-Dimensional Spin-Gap System CuNb2O6.

Inelastic neutron measurements have been performed on a quasi-one-dimensional spin-gap system CuNb 2 O 6 . From the dispersion curve of the triplet excitation, the compound has been found to have a ferromagnetic-antiferromagnetic alternating bond chain. The temperature ( T ) dependence of the magnetic excitation spectra indicates that both the singlet correlation and the antiferromagnetic correlation begin to grow with decreasing T at around T 0 , where the spin susceptibility becomes maximum.

Spin-gap behavior of CuNb2O6

The magnetic properties of two polymorphs of CuNb 2 O 6 with quasi-one-dimensional zig-zag spin ( S =1/2) chains of Cu 2+ ions are studied. The magnetic excitation spectra of Cu spins measured by neutron scattering, and the T -dependence of the longitudinal spin relaxation rate of 93 Nb nuclei by NMR indicate the existence of the spin gap in the monoclinic phase whose magnitude is consistent with the one deduced by the detailed analyses of uniform magnetic susceptibility at low temperatures. The behavior in the monoclinic phase is in clear contrast to that found in the orthorhombic phase.

Pseudo Gap Formation Studied by B 2 u Phonon Measurements

Neutron scattering studies on a c -axis polarized B 2 u phonon have been carried out by using single crystals of YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.7 and YBa 2 Cu 2.9 Zn 0.1 O 6.75 . For YBa 2 Cu 3 O 7 , sharp decreases in the energy and width of the phonon mode have been observed with decreasing temperature T at around the superconducting transition temperature T c . For the underdoped samples, the gradual downward deviations of these quantities from the ordinary T -dependence begin far above T c with decreasing T , which is considered to be associated with the formation of the pseudo gap with the same symmetry as that of the superconducting order parameter, i.e., the ( x 2 - y 2 )-like one. These deviations become significant, with decreasing T , at temperatures higher than the so-called spin-gap temperature T SG of YBa 2 Cu 3 O 6.7 (∼150 K). Observed insensitive nature of the anomaly above T c to the Zn-doping can be explained by considering the wave vector ( q ) dependence of the pseudo gap and the coupl...

Phase Diagram and Pressure Effects on Transport Properties of BaCo1-xNixS2

Transport measurements under the hydrostatic pressure p up to 18.8 kbar have been carried out on single crystals of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits the Mott metal-insulator (M-I) transition both with varying x and with applying pressure. Based on resistivity data for x =0.00, 0.05, 0.14 and 0.18, a T - p - x phase diagram of BaCo 1- x Ni x S 2 has been constructed. The phase diagram is similar to that of (V 1- x M x ) 2 O 3 (M=Ti and Cr), although the present system does not exhibit clear discontinuity of the transition. Detailed transport behavior observed near the M-I transition induced by the external pressure are presented. For x =0.00, where a large Mott energy gap exists at ambient pressure, the intrinsic critical behavior of the Mott M-I transition seems to be observed, while for the samples with x ≠0, randomness effects caused by the Ni-substitution for Co seems to be reflected in the critical behavior.

Dynamical magnetic properties of BaCo1-xNixS2 near the Mott transitions induced by pressure and carrier-number control

Dynamical magnetic properties of BaCoS 2 which has a layered structure and exhibits Mott metal-insulator (M-I) transition with the Ni-substitution for Co and/or with increasing external pressure p , have been studied by means of neutron inelastic scattering in the pressure range of \(0{\lesssim}p{\lesssim}15\) kbar. By comparing the results with those of BaCo 0.82 Ni 0.18 S 2 , we find that characteristics of the magnetic fluctuations near the two M-I transitions induced by pressure and by the Ni-substitution for Co are rather different, even when they are compared under the conditions of similar Neel temperatures and similar antiferromagnetic staggered moments at low temperatures.

Studies of Pressure-Induced Mott Metal-Insulator Transition of BaCoS2.

Transport, thermal and elastic properties under the hydrostatic pressure p up to 21.2 kbar have been studied on the layered compound BaCoS 2 , which exhibits the Mott insulator→metal transition wit...

Thermal Expansion Coefficient of BaCo1-xNixS2

Measurements of the linear thermal expansion coefficient α have been carried out on both polycrystalline pellets and single crystals of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits the Mott transition with varying x . In the region of x ≤0.22, where antiferromagnetic state exists below T N , α exhibits the finite discontinuity Δα at T N , as is expected in ordinary cases of second order phase transition. The sign of the discontinuity Δα{≡α( T N -0)-α( T N +0)} changes from positive to negative at x ∼0.07 with increasing x , which can consistently be explained by considering the x -dependence of the electronic structure of this system.

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.

Neutron Scattering Study on Metal-Insulator Transitions of BaCoS 2 Induced by External Pressure and by Ni-Doping

Magnetic excitation spectra of BaCo 1- x Ni x S 2 , which has a layered structure and exhibits Mott metal-insulator (M-I) transition with varying x and/or with increasing external pressure, have been measured and analyzed. The random phase treatment has been found to be able to basically describe the observed dynamical magnetic behavior except in the temperature region where the intrinsic magnetic correlation length is larger than the average separation of Ni atoms. The characteristic energy of the spin fluctuation in two kinds of metallic phases induced by the Ni-doping and by the external pressure p are rather differrent. These results indicate that the pressure-induced Mott transition is mainly driven by the increase of the transfer energy, while the doping-induced transition is driven by the deviation of the electron number of each relevant band from unity.