Hiroshi Harashina

Cu-site doping effects, transport and magnetic properties of high-Tc oxides and their hole concentration dependence

Abstract Doping effects have been studied by using La 2− y Sr y Cu 1−χ M χ O 4 (M = Zn, Ni and Mg) and YBa 2 (Cu 1−χ Zn χ ) 3 O 6+ y . The rate of T c -suppression, |d T c /dχ|, is found to be inversely proportional to the carrier concentration p . The disappearance of the superconductivity with Cu-site doping seems to be primarily due to the loss of the itinerant nature of the holes, which is supported by the variation of the spectral function χ(ω) of the magnetic excitations with temperature, studied by neutron scattering on YBa 2 (Cu 0.965 Zn 0.035 ) 3 O 6.75 . The peak of Hall coefficient versus T curves and the “spin gap” effect found in the T -dependence of the resistivities ϱ become less significant with Zn-doping.

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.

Transport and Magnetic Studies of BaCo1-xNixS2

Transport and magnetic properties have been studied for BaCo 1- x Ni x S 2 with a two-dimensional electron system which exhibits the Mott transition upon varying x . Various anomalous behaviors of these properties can be considered to be similar to those of high- T c Cu oxides, indicating that Cu-oxides are not the only compounds with the well-known “anomalous normal state”.

Magnetic Structure of Nd2Mo2O7

Neutron diffraction studies have been carried out on a single crystal of the metallic pyrochlore ferromagnet Nd 2 Mo 2 O 7 . The magnetic structures proposed at 4 K is non-collinear for both the Mo- and Nd-moments, where the net moments of the two systems align antiparallel. In the temperature region between the Curie point (∼93 K) and ∼30 K, the moments primarily at Mo sites orders with the ferromagnetic magnetization, where the structure seems to be non-collinear, too. The ordering of the Nd moments gradually develops and becomes significant below ∼20 K with decreasing temperature. Discussion on the relationship between the magnetic structure and the anomalous transport properties is presented.

Nature of spin freezing transition of geometrically frustrated pyrochlore system R2Ru2O7 (R=rare earth elements and Y)

Abstract At the spin-glass-like transition of yttrium ruthenium pyrochlore system Y 2 Ru 2 O 7 , a significant jump of the specific heat has been found, suggesting the second order-like nature of the transition. It is commonly observed for other pyrochlore systems R 2 Ru 2 O 7 (R=various rare earth elements RE). For all REs studied here, the transition temperature T G is larger for the larger R-ions, which may be due to the increase of the transfer energies of the 4 d electrons between the neighboring Ru atoms with increasing RE ionic radius. Neutron Rietveld analyses on Y 2 Ru 2 O 7 and Nd 2 Ru 2 O 7 have revealed that below T G , the Ru spins are in an almost long-range ordered antiferromagnetic state, though it is magnetically glassy in a macroscopic sense, and that the lattice system does not participate in the transition. It is concluded that the specific heat jump is created by the purely magnetic origin.

Static Correlation and Dynamical Properties of Tb3+-moments in Tb2Ti2O7 –Neutron Scattering Study–

Static and dynamical properties of the magnetic moment system of pyrochlore compound Tb 2 Ti 2 O 7 with strong magnetic frustration, have been investigated down to the temperature T =0.4 K by neutr...

Neutron Diffraction Study of Pyrochlore Compound R2Ru2O7(R=Y,Nd) above and below the Spin Freezing Temperature.

Neutron Rietveld analyses have been carried out on Y 2 Ru 2 O 7 and Nd 2 Ru 2 O 7 to clarify the primary mechanism of the transition at T G to the spin-glass-like state which is accompanied by the specific heat jump. The results indicate that the lattice system does not seem to participate in the transition and changes of the neutron diffraction data with decreasing temperature through T G can consistently be understood by the ordering of magnetic moments at Ru sites to an almost long range ordered state, even though the low temperature phase is glassy in the macroscopic sense.

Electronic structure of the quasi two-dimensional Mott system BaCo1-xNixS2

By means of neutron powder diffraction on antiferromagnetic samples of BaCo 1- x Ni x S 2 , the spin structure has been determined. The magnitudes of the ordered moments are ∼2.9µ B at 10 K for x =0.0 and ∼1.7µ B at 100 K for x =0.175. The results are not compatible with the electronic configuration ( 3d 3 z 2 - r 2 ) 1+ x , which was previously assumed to explain the physical behavior of the system. Instead, the configuration seems to be ( 3d y z or 3d z x ) 1 ( 3d 3 z 2 - r 2 ) 1 ( 3d x 2 - y 2 ) 1 in BaCoS 2 . Each Ni atom substituted for Co adds one electron to this system. Based on this basic picture of the electronic structure, various physical properties of BaCo 1- x Ni x S 2 ever published are discussed.

Transport, magnetic and thermal properties of BaCo1-xNixS2

Physical properties of quasi two-dimensional electrons of BaCo 1- x Ni x S 2 which exhibits the Mott metal-insulator transition are reported. Various anomalous behaviors which are similar to those of high- T c Cu oxides have been found. It is suggested that these anomalies are induced in the metallic and magnetically correlated state without long range order and therefore with very strong low energy magnetic fluctuations. It remains unclear if the system exhibits a decreasing tendency of the electronic specific heat coefficient γ upon approaching the Mott insulating phase, which is one of the remarkable anomalous behaviors of high- T c oxides. Superconductivity has not been found down to 1.4 K.

Temperature Dependence of the Magnetic Excitation Spectra of YBa2Cu2.9Zn0.1O6.75

Neutron scattering measurements of magnetic excitations in YBa 2 Cu 2.9 Zn 0.1 O 6.75 (T c ≤10 K) have been performed. In contrast to the corresponding pure sample with the same oxygen number ( T c ≃66 K) where no peak of the antiferromagnetic correlation was detected for excitation energies below 15 meV, we have observed clear scattering contribution in the same energy range at all temperatures studied. A detailed study of χ ′′ (ω) at (π, π) in this Zn-substituted sample suggests 1) an existence of the gap-like structure at high temperatures (100 K< T <200 K) and 2) the enhancement of the spectral weight at low energies along with the loss of the itinerant nature of the electrons at low temperatures ( T <50 K).