Isamu Oguro

Magnetic Properties of Single Crystals of CexLa1-xB6

Measurements of the magnetization process at 4.2 K and 1.3 K for magnetic field up to about 150 kOe and of the temperature dependence of the susceptibility between 4.2 K and room temperature have been done in order to investigate Kondo states of dilute and dense Kondo system in single crystalline Ce x La 1- x B 6 (0.03 ≦ x ≦1.0). The magnetic phase diagram is constructed above 0.6 K for Ce 0.5 La 0.5 B 6 , Ce 0.75 La 0.25 B 6 and CeB 6 . A largely anisotropic magnetization and the temperature dependence of susceptibility in dilute alloys indicate the \(\varGamma _{8}\) quartet Kondo state contrary to the previous report. Dependence of Kondo temperature on the Ce concentration is very weak.

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

Abstract We have studied an anomalous peak effect in a Bi 2 Sr 2 CaCu 2 O 8+ y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10 -2 s t 4 s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Variable range hopping conduction in bulk samples composed of single-walled carbon nanotubes

An experimental study of the electrical transport phenomena in magnetic fields H up to 15 T in pressed samples of a porous system composed of single-walled carbon nanotubes in the temperature range 1.39 K <T<300 K is reported. Below 60 K, the exp[1/T1/4] temperature dependence of the direct current electrical resistance R(T,0) is found. Low-field transverse magnetoresistance (MR), [R(T,H)−R(T,0)]/R(T,0), below 60 K is negative and is discussed in terms of a quantum interference effect on a three-dimensional variable range hopping (3D–VRH) mechanism. Based on the results of R(T,0) and MR, we propose 3D–VRH conduction within the nanotube bundles (intrabundle hopping) or between the nanotube bundles (interbundle hopping) through their contacts.

Dense Kondo behavior in CeB6 and its alloys

Abstract CeB 6 shows a typical dense Kondo behavior and reveals three characteristics phases, named I, II and III. The phase I is characterized by the incoherent Kondo state, the mysterious phase II seems to be the quadrupolar ordering of the Γ 7 –Γ 8 mixing type, and in phase III the complicated multi-Q ordering due to the competition between the exchange and the coherent Kondo lattice appear. The substitution of nonmagnetic atoms for Ce atom causes no clear change in the Kondo temperature T K . Also, no visible change in the magnetic properties in the phase I was observed, while the decrease of ordered temperatures is observed in common for all types of substitution.

Crystal Growth and Characterization of the Kondo Semimetal CeNiSn

Several single crystals of CeNiSn and CeNi 0.95 T 0.05 Sn (T=Co and Cu) were grown and characterized by metallographic examination, electron-probe microanalysis and the measurements of resistivity, magnetoresistance, magnetic susceptibility and specific heat. Below 10 K, the semiconductor-like behavior found for dirty or substituted crystals is suppressed with decreasing impurities. For a crystal purified by the solid-state electrotransport technique, the resistivity along the orthorhombic a axis is metallic with the T 2 dependence, whereas those along the b and c axes increase below 3 K and pass through weak maxima at 0.8 K. These results suggest that the ground state of CeNiSn is not semiconducting but semimetallic with a pseudogap which closes along the a axis.

Characterization of a Non-Superconducting Cuprate, Bi2Sr2YCu2O8.5

A new Bi-containing cuprate, Bi2Sr2YCu2O8.5, was found in the course of substitution study for high Tc compounds Bi2Sr2CaCu2O8+y. Bi2Sr2YCu2O8.5 is a semiconductor and undergoes an antiferromagnetic transition at about 13 K which is considered to originate from the ordering of Cu in the CuO2 planes. An electron diffraction study revealed the existence of a superstructure. The unit cell taking account of the superstructure is 1×8×1 of the original unit cell (5.47×5.43×30.18 A3). Analogy of three kinds of oxide systems, (La1-xSrx)2CuO4, YBa2Cu3O6+x, and Bi2Sr2CaxY1-xCu2O8+y, is discussed.

Anisotropy of Bi2Sr2CaCu2O8+y revisited

Abstract The anisotropic superconductivity in Bi 2 Sr 2 CaCu 2 O 8+ y is investigated by precision measurements of resistivity and torque as a function of the magnetic field angle in the reversible regime. Scaling of the angular dependences of these quantities with an effective field, H (sin 2 θ+γ −2 cos 2 θ) 1 2 , θ being the field angle measured from the basal plane, suggests values of the anisotropy parameter γ≈200 or larger, which substantially exceeds previous estimates. In the small θ region, the effect of a layer gains importance.

Anomalous magnetic, transport and thermal properties in the half-metallic magnet UNiSn

Measurements of magnetic susceptibility, electrical resistivity, thermoelectric power and thermal expansion have been performed on the half-metallic magnet UNiSn. A second-order transition from a paramagnetic semiconductor to an antiferromagnetic metal takes place at T N =47 K without any significant lattice deformation. Unlike in the usual antiferromagnets, the curve of magnetic susceptibility sharply turns up at T N . Magnetic fields up to 13.5 T have no significant influence either on T N or the semiconducting energy gap. On the contrary, hydrostatic pressure increases T N at a rate of d T N /d p =0.50 K/kbar, but suppresses the gap energy at a rate of d E g /d p =-0.6 meV/ kbar. Thermoelectric power has an extremely sharp peak at 42 K, which is 5 K lower than T N .

Structures and Upper Critical Fields of High Tc Superconductors (RE)Ba2Cu3Ox

A systematic study of structure and superconductivity of (RE)Ba2Cu3Ox (RE: rare earch) is carried out. All superconducting (RE)Ba2Cu3Ox have oxygen deficient perovskite structure. As the ion radius of the RE ion increases, lattice parameters increase and the orthorhombic distortion becomes smaller. Superconducting transition temperature is lower for (RE)Ba2Cu3Ox with smaller RE ion radius. Upper critical fields (Hc2) are measured resistively up to 90 kOe. All superconducting (RE)Ba2Cu3Ox have similar values of dHc2/dT|Tc to that of YBa2Cu3Ox. The estimated Hc2 at absolute zero ranges from 1250 kOe to 1850 kOe.

Crystal growth of YbTX (T=Cu, Ag, Pt, Au; X=Sn, Sb) and the magnetic and transport properties

Abstract Single crystals of YbTX (T=Cu, Ag, Pt, Au: X=Sn, Sb) have been synthesized by the Bridgeman method. We found that the compounds YbCuSn, YbPtSn and YbAuSn crystallize in the TiNiSi-, ZrNiAl- and TiNiSi-type structure, respectively. The electrical resistivity ρ, magnetic susceptibility χ and magnetization M were measured. The results of χ( T ) revealed that the valence of the Yb ions is divalent in all the compounds except for YbPtSn. By contrast, YbPtSn with trivalent Yb ions orders antiferromagnetically at T m = 3.5 K and exhibits a metamagnetic transition at 4.5 kOe. As the magnetic field is increased to 150 kOe, M(H) of YbPtSn saturates at a value of 2.31 μ B f.u. 1 , whereas M(H) for other compounds gradually increases to values less than 0.02 μ B f.u 1 . All the compounds are metallic and the analysis of ρ( T ) by using the Bloch-Gruneisen law leads to the characteristic temperatures ranging from 90 to 170 K.