Hideo Ihara

New High-Tc Superconductor Family of Cu-Based Cu1-xBa2Can-1CunO2n+4-δ with Tc>116 K

A new high-temperature and nontoxic superconductor family of Cu1-xBa2Can-1CunO2n+4-δ with critical temperature (Tc)>116 K was discovered. These materials were prepared by the high-pressure technique. A superconductor sample with the highest T c contains mainly a Cu1-x Ba2Ca3Cu4O12-δ phase. The T c value of the Cu1-x Ba2Ca3Cu4O12-δ phase is 116.4 K. The phase has two kinds of tetragonal structures: body-centered tetragonal (bct) and simple tetragonal. The lattice parameters of bct are a=7.7114 A and c=35.986 A. The disordering or full occupation of Cu-vacancy sites leads to the simple tetragonal phase with the space group of P4/mmm and the half values of the lattice constants (a=3.8557 A and c=17.993 A) of the bct phase. The relationship between the lattice constant c and the number of CuO2 layers n is c/2=8.4+3.2(n-1) ( A). The small CuO2-block spacing (8.4 A) of this family suggests a lower anisotropy and higher J c than in Hg- and Tl-based superconductor families.

Sputtered nanodots: A costless method for inducing effective pinning centers in superconducting thin films

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3–5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude.

How to make superconducting-anisotropy least in high-Tc cuprate superconductors

Abstract The guiding principles to reduce superconducting-anisotropy were derived for high-T c cuprate superconductors. Based on the principles, we have attained the least superconducting-anisotropy γ=1.6 for a Cu-based CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) superconductor. Further lower superconducting-anisotropy has been proposed for Cu-1245 (γ=1.3) and Cu-1256 (γ≌1.0) Cu-based superconductors.

Transformation of C60 fullerenes into a superhard form of carbon at moderate pressure

A very hard and conducting form of carbon has been obtained from pure C60 fullerenes, as well as its mixture with aluminium at nonhydrostatic pressures of 2.6–3 GPa and a temperature of 700 °C. The hardness of the material is more than 4000 kg/mm2. Its electrical conductivity is about 100 Ω−1 cm−1 and is weakly dependent on temperature in the range 4.2–300 K. X‐ray diffraction and Raman measurements show lack of the long‐range crystalline order in the transformed material.

Evidence of Critical Oxygen Concentration at y=6.7∼6.8 for 90 K Superconductivity in Ba2YCu3Oy

A study of precise oxygen concentration dependence of the superconducting volume fraction in Ba2YCu3Oy (6.22≤y≤6.96) revealed the phase diagram of this superconducting system. Three characteristic phases are found based on the oxygen concentration: (1) a nonsuperconducting tetragonal phase for 6.2<y<6.4, (2) a low-Tc (Tc= 55~70 K) orthorhombic phase for 6.4≤y<6.8 and (3) a high-Tc (Tc=90 K) orthorhombic phase for 6.8≤y<7.0. Evidence for a distinct critical concentration at y=6.7~6.8, necessary to sustain the bulk high-Tc (90 K) superconductivity, is presented.

Coexistence of superconductivity and antiferromagnetism in multilayered high-Tc superconductor HgBa2Ca4Cu 5Oy: Cu-NMR study

We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa 2 (Ta 4 Cu 5 O y (Hg-1245) with T c = 108 K, which is composed of two types of CuO 2 planes in a unit cell; three inner planes(IPs) and two outer planes (OPs). The Cu-NMR study has revealed that the optimally doped OP undergoes a superconducting (SC) transition at T c = 108 K, whereas the three underdoped IPs do an antiferromagnetic (AF) transition below T N ∼60 K with the Cu moments of ∼(0.3-0.4)μ B . Thus bulk superconductivity with a high value of T c =108 K and a static AF ordering at T N =60 K are realized in the alternating AF and SC layers. The AF-spin polarization at the IP is found to induce the Cu moments of ∼0.02μ B at the SC OP, which is the AF proximity effect into the SC OP.

Sputter Synthesis of Ba2YCu3Oy As-Deposited Superconducting Thin Films from Stoichiometric Target–A Mechanism of Compositional Deviation and Its Control

A mechanism of compositional deviation of Ba-Y-Cu-O films during sputtering has been investigated, and a process for obtaining as-deposited superconducting films from a stoichiometric target has been developed. The main factor of the deviation is the radiation of high-energy electrons emitted from the oxide target with a low work function onto the growing surface. By eliminating the electron bombardment, films with the same composition were obtained reproducibly. As-grown films 4000 A thick deposited at 580°C showed Tc ( ρ=0) of 82.5 K and an excellent surface smoothness.

Valence Study of Orthorhombic and Tetragonal Ba2YCu3Oy: The Role of Oxygen Vacancies in High-Tc Superconductivity

The valency of Cu ions in the orthorhombic and tetragonal phases of high-Tc superconductor Ba2YCu3Oy (6.22<y<6.96) has been studied by near-edge X-ray absorption spectroscopy. The results indicate that the Cu valency at Cu1 sites on the z=1/2 plane is strongly correlated with the high-Tc (90 K) superconductivity, while the Cu valency of Cu2 sites on the basal planes affects the low-Tc (50-60 K) superconductivity of the orthorhombic phase.

Spin Correlation in High-T c Cuprate HgBa2Ca2Cu3O8+δ with T c=133 K –An Origin of T c-Enhancement Evidenced by 63Cu-NMR Study–

The Knight shift, the nuclear spin-lattice relaxation rate, 1/ T 1 , and the transverse relaxation rate, 1/ T 2G , of 63 Cu have been separately measured for the square and the pyramidal CuO 2 planes in HgBa 2 Ca 2 Cu 3 O 8+δ (Hg1223) with the highest transition temperature, T c =133 K to date. From the quantitative analysis of 1/ T 1 and 1/ T 2G , it is shown that the product of the staggered susceptibility and the characteristic energy of spin fluctuations around a zone boundary, χ Q Γ Q , for the square CuO 2 plane site is larger than that for YBa 2 Cu 3 O 7 (YBCO 7 ), although that for the pyramidal CuO 2 plane site is almost the same. This enhancement of χ Q Γ Q for the square site is considered to be responsible for the higher T c in Hg1223 than in YBCO 7 .

New High-Tc Superconductor Ag1-xCuxBa2Can-1CunO2n+3-δ Family with Tc> 117 K

A new high-temperature and nontoxic superconductor Ag1-x Cux Ba2Can -1Cun O2n +3-δ family with T c>117 K was discovered. These materials are prepared by the high-pressure technique. A superconductor sample with the highest T c contains mainly a Ag1-x Cux Ba2Ca3Cu4O11-δ (x=0.75) phase. The phase has a body-centered tetragonal structure with lattice constants of a=7.7270 A and c=36.222 A. The disordering of Ag sites leads to the space group of P4/mmm and the lattice constants of a=3.8635 A and c=18.111 A. The T c value of the Ag1-x Cux Ba2Ca3Cu4O11-δ phase is 117.1 K. The relationship between the lattice constant c and the number of CuO2 layers n is c/2=5.3+3.2n ( A). The small CuO2-block spacing (8.5 A) of this family suggests a lower anisotropy and higher J c than Hg- and Tl-based superconductor families.