Kuniyuki Koyama

Antiferro-Quadrupole Ordering of CeB6 Studied by Resonant X-Ray Scattering

Under zero magnetic field, a quadrupolar order parameter at \(\mbi{q}_{Q}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})\) in a typical antiferro-quadrupole (AFQ) ordering compound CeB 6 has been observed f...

Antiferro-quadrupole Ordering of CeB

Under zero magnetic field, a quadrupolar order parameter at \(\mbi{q}_{Q}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})\) in a typical antiferro-quadrupole (AFQ) ordering compound CeB 6 has been observed f...

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The characteristic of intergrain phase ordering transition in a superconductive ceramic of YBa 2 Cu 4 O 8 is examined at zero external DC field by AC magnetic measurements. From the nonlinear response at the weak AC field limit, a negative divergence of nonlinear susceptibility χ 2 is concluded at the intergrain transition temperature T c2 but no remarkable anomaly is found at the intragrain one T c1 . This critical phenomenon may suggest the onset of chiral glass ordering at T c2 and necessarily a frustration in the intergrain Josephson type coupling network.

Studied by Resonant X-ray Scattering

Structure, resistivity and susceptibility are studied in the quenched Bi2Sr2Ca1-xNdxCu2O8+δ system. The quenched samples with 0x1 have a single phase of the Bi2Sr2CaCu2O8+δ structure, as do the furnace-cooled samples. The oxygen concentration decreases slightly by about 0.05 upon quenching from 800°C to liquid nitrogen temperature. The Tc remains a constant (~90 K) for x0.15, and decreases linearly with x for 0.15x0.45, and superconductivity disappears in the samples with x0.45. Referring to the previous results for the furnace-cooled Bi2Sr2Ca1-xNdxCu2O8+δ system, the relationship between the Tc and the hole concentration in [Cu-O] is confirmed to be universal in both the quenched and the furnace-cooled Bi2Sr2Ca1-xNdxCu2O8+δ systems.

Negative Divergence of Nonlinear Susceptibility at the Intergrain Phase Ordering Transitionin a Superconductive Ceramic of YBa2Cu4O8

The superconductive ordering of a YBa 2 Cu 4 O 8 ceramic with a single phase and of homogeneous sub-micron size grains, is studied by systematic DC magnetic measurements under weak magnetic fields. Successive phase transitions of a hierarchical nature are identified to occur at two temperatures T C1 (≃ 80 K) and T C2 (≃ 37 K). The transition at T C1 is that from the normal into the intra-grain superconductive state. The transition at T C2 is that into the inter-grain superconductive state and results in an infinite superconductive network over the whole sample. Taking a possible Josephson junction type coupling into account, the second transition at T C2 is speculated to be a long range ordering among the superconductive grains and similar to that for a three-dimensional XY ferromagnet.

Electrical, Magnetic and Superconducting Properties of the Quenched Bi2Sr2Ca1-xNdxCu2O8+δ System

Structure, resistivity and susceptibility have been studied in the Ba2Nd1-xPrx Cu3O7-δ system. The samples with 0≤x≤1 have a single phase of the orthorhombic Ba2YCu3O7-δ structure. The Tc decreases linearly with increasing x and superconductivity is not observed for the samples of x≥0.3. The valences of Nd and Pr ions in the samples were determined from the susceptibility measurement as Nd3+ and Pr4+, respectively. It is supposed naturally that the valence of Cu2 ions (Cu ions in the two-dimensional Cu–O plane) is affected strongly by the neighboring rare earth ions. The results show that the valence of Cu2 ions is significant for the superconductivity in the Ba2RECu3O7-δ system.

Successive Phase Transitions of a Hierarchical Nature in a YBa2Cu4O8 Ceramic Superconductor.

We report the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic point of view. The nuclear spin–lattice relaxation time T 1 of Sb nuclei deviates from the relation T 1 T = constant above 4.2 K, where 1/ T 1 T has a Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with θ∼30 K. The temperature dependence of the Knight shift of 139 La nuclei, which is related to the susceptibility at q = 0, is scaled to that of 1/ T 1 T above 40 K. This relation strongly suggests that ferromagnetic fluctuations are predominant in LaFe 4 Sb 12 . We also point out that LaFe 4 Sb 12 is situated close to the ferromagnetic instability due to the small Weiss temperature in the Curie–Weiss behavior of 1/ T 1 T and the Knight shift.

Electrical, Magnetic and Superconducting Properties of the High-Tc Superconductor Ba2Nd1-xPrxO7-δ

We have synthesized the polycrystalline samples of Ge-based filled skutterudites R Pt 4 Ge 12 ( R =La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt 4 Ge 12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt 4 Ge 12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt 4 Ge 12 and NdPt 4 Ge 12 show the well-localized nature of 4 f -electrons. The NMR measurements in CePt 4 Ge 12 revealed that the nuclear spin–lattice relaxation rate 1/ T 1 shows an activation-type temperature dependence 1/ T 1 ∝exp (-Δ/ k B T ) with an energy gap Δ/ k B =150 K above 100 K, whereas it is proportional to the temperature belo...

Evidence for Ferromagnetic Fluctuations in Filled Skutterudite LaFe4Sb12:Sb-NQR and La-NMR

XPS core level and valence band spectra of sintered Bi2Sr2Ca1-xNdxCu2Oy which manifests a superconducting to semiconducting phase change around x=0.5 have been measured. The fraction of Cu1+ to the total Cu ions evaluated from Cu2p spectra amounts to 7% in Bi2Sr2CaCu2Oy, but decreases to 0% with increasing Nd content. From Ca2p, Sr3d, Nd3d, Bi4f and O1s spectra, it is inferred that Ca2+ and Sr2+ are mixed and distributed over both Ca and SrO layers, whereas Nd3+ and Sr2+ are rather separate. From the valence band spectra, a finite density of states at EF is deduced for Ca-rich oxides, but it diminishes in Bi2Sr2NdCu2Oy. The analysis of ionic configurations based on the mixed valencies of copper and oxygen ions gives a consistent account of the above spectra and the superconductor to semiconductor transition.

Electrical, Magnetic and NMR Studies of Ge-Based Filled Skutterudites RPt4Ge12 (R = La, Ce, Pr, Nd)

Abstract Superconducting properties are examined in Y 1−x Pr x Ba 2 Cu 4 O 8 synthesized by the citrate pyrolysis method followed by sintering in a one atm oxygen flow. Lattice constants increase linearly, while the orthorhombicity decreases linearly with increasing x. Meissner effect measurements indicate that the onset T c decrease monotonically with the Pr substitution, and disappears at x =0.75. This is caused by the change in the hole concentration with the substitution of Pr for Y.