Toshizo Fujita

Mössbauer study of YBa2 (Cu1-xFex)3O7-δ : Superconductivity and magnetic order

Abstract The electronic and magnetic properties of the ceramic superconductor YBa2 (Cu1-xFex)3O7-δ with x =0.010, 0.017, 0.033 and 0.067 have been studied microscopically by Mossbauer technique. We have shown that Fes are substituted for both Cu 1 and Cu 2 sites. We have also shown that the samples with x >; 0.01 undergo a magnetic transition at low temperatures without destroying the superconductivity, and that the magnetic transition temperature TM increases with x. The coexistence of the superconductivity and the magnetic order is discussed, and it is argued that spin fluctuations of Cu are related to the superconductivity.

Crystallographic, Magnetic, and Superconductive Transitions in (La1-xBax)2CuO4-y

Three types of phase transitions in (La1-xBax)2CuO4-y–orthorhombic to tetragonal, spin density wave (SDW) and superconductive–were traced as a function of the Ba concentration, x. The obtained phase diagram indicates that the SDW state does not coexist with superconductivity, whereas samples both in orthorhombic and tetragonal phases can be superconducting.

Anomalous specific heat of CeB6

Abstract Specific heat measurement of CeB6 has been performed over the temperature region 1.6–77 K. Particularly the low-temperature anomaly, which consists of a sharp peak and a broad satellite, was examined in applied magnetic field up to 1.8 T. The observed field dependence and the entropy consideration reveal that both the two peaks are related to some intrinsic phase transitions which involve a complicated process of magnetic ordering. In the light of the reported data on magnetic properties, the anomalous features of this compound are discussed in terms of the unusual exchange interaction which differs remarkably in magnitude between the ground state and the excited state.

Two-Dimensional Fermi Liquid Behavior of the Superconductor Sr2RuO4

We report detailed normal state measurements of the resistivity, magnetic susceptibility and specific heat of high-quality single crystals of Sr 2 RuO 4 , the only copper-free, layered perovskite superconductor ( T c ≈1 K). It is intriguing that despite its very strong anisotropy (the ratio of the interlayer to in-plane resistivity exceeds 500) arising from the essentially two-dimensional Fermi surface, the normal state properties are consistent with a universal description for a strongly correlated Fermi liquid. Therefore, Sr 2 RuO 4 serves as an ideal reference material for the investigation of the limits of applicability of Fermi liquid theory to other highly correlated layered compounds, including high- T c cuprates.

Specific heat of the dense Kondo compound CeCu6

Abstract The specific heat of CeCu6 has been measured over a wide temperature range 150 mK–70 K. The ratio C/T was found to be as large as 1.45 J/K2 mol at low temperatures, indicating the heavy Fermi-liquid nature of a Kondo-lattice ground state.

Ce- and Yb-based Kondo semiconductors

Abstract Kondo semiconductors are a class of strongly correlated f-electron materials whose low-energy excitations exhibit a (pseudo)gap at low temperatures. The significant difference between the pseudogap in orthorhombic CeNiSn and CeRhSb and the real gap in cubic Ce 3 Bi 4 Pt 3 and YbB 12 is highlighted by a comparison of transport properties. Low-temparature measurements of the magnetoresistance, Hall coefficient and specific heat of CeNiSn have revealed a field-induced excitation of the coherent low-carrier state. A systematic study of CeNi 1− x T x Sn ( T = Co, Cu and Pt ) has shown that the residual carriers in CeNiSn are immobilized by 1% substitution irrespective of the substitutes. Further substitution with 5% Cu induces a magnetic instability at T = 0.

Anomalous Change in Crystalline Structure of (La1-xBax)2CuO4-δ

The crystal structure of (La 1- x Ba x ) 2 CuO 4-δ (0≤ x ≤0.12) was extensively investigated by powder X-ray diffraction over a wide temperature range down to 4.2 K. A structural sequence of tetragonal-orthorhombic-tetragonal phases was found on cooling for the samples with concentrations of 0.035< x <0.10. Here a new structural phase diagram is mapped in the T - x plane.

Structural phase transition in (La1−xBax)2CuO4−δ

Abstract The structural phase transition in (La 1− x Ba x ) 2 CuO 4−δ (0≤ x ≤0.12) was extensively investigated by powder X-ray diffraction over a wide temperature range down to 4.2 K. As temperature decreases, a structural sequence of tetragonal (I4/mmm)-orthorhombic (Cmca)-tetragonal (P4 2 /ncm) phases was found for the samples with Ba concentrations of 0.035 x XY model. The Cmca-P4 2 /ncm transition is of the first order and interpreted in terms of instability at the Z point of the Brillouin zone boundary.

Superconductivity in YBa2Cu3-yNiyO7-δ

Superconductivity in alloyed compounds YBa2Cu3-yNiyO7-δ has been studied in order to gain direct insights on the roles different Cu sites play in electrical conduction. Electron probe microanalysis as well as X-ray powder diffraction analysis shows that Ni is actually substituted for Cu in the oxygen-deficient perovskite compound for y at least as much as 0.27. Electrical resistivity measurements revealed that the substituted Ni does not substantially destroy the superconductivity.

Unusual low-temperature properties of Ce compounds

Abstract Low-temperature properties of Ce-based ternary compounds are reviewed. The groundstate of 4f electrons from Ce exhibits a wide variety of unusual features, including magnetic ordering, nonmagnetic or superconductive heavy-fermion state, valence fluctuation, etc. Especially we will concentrate on the equi-atomic series CeTIn and CeTSn (T: transition element), of which the unusual properties will be discussed in terms of competition between the Kondo effect and the indirect RKKY interaction among Ce spins. CeNiSn is a specially interesting example which has a small energy gap growing at low temperature, probably due to coherence effect of heavy quasiparticles in the Kondo lattice.