Takayuki Nakane

Effects of Sr substitution on the magnetic-field irreversibility in superconductive Cu(Ba1−ySry)2YbCu2O6+z

Abstract The effects of the Sr-for-Ba substitution on the H irr characteristics of the Cu(Ba 1− y Sr y ) 2 YbCu 2 O 6+ z (Cu-1212) system is established at two different oxygen-content levels in the range of 0.0≤ y ≤0.4. In the fully oxygenated sample series, an optimum was seen at y =0.2 so that the H irr line of the present Cu(Ba 0.8 Sr 0.2 ) 2 YbCu 2 O 6.96 sample clearly exceeds that of the CuBa 2 YCu 2 O 6.92 compound. For the oxygen-deficient sample series, the H irr characteristics was depressed with increasing Sr content even though the unit cell decreased. Thus, the H irr characteristics is mostly controlled by other factors than the thickness of the nonsuperconductive blocking block.

Oxygen non-stoichiometry and hole distribution in multi-layered copper oxides: understanding of the magnetic-irreversibility characteristics

Abstract The dependence of the magnetic-irreversibility characteristics on the amount of excess oxygen and the hole distribution in superconductive multi-layered copper oxides, M m A 2 Q n −1 Cu n O m +2+2 n ± δ or M – m ( A ) 2 ( Q ) ( n −1) n :P/RS, is discussed with relevant examples of such phases, i.e. Cu−1 (Ba,Sr) 2 (Yb) 12:P, Bi-2 (Sr) 2 (Ca,Y) 12:RS, Hg-1 (Ba) 2 (Ca) 23:RS, Cu-1 (Ba) 2 (Ca) 23:P and Cu-1 (Ba) 2 (Ca) 34:P. At a fixed cation stoichiometry, the irreversibility-field line, H irr ( T ), is shifted in a continuous manner to higher magnetic fields with increasing oxygen content. This trend applies not only to the M – m ( A ) 2 ( Q ) ( n −1) n :P/RS phases with a perovskite-type (P) charge reservoir but also to those with a rock-salt-type (RS) charge reservoir, and furthermore, in both cases to both under- and overdoped regions. However, when the increase in the oxygen content is accompanied with aliovalent cation substitution, e.g. trivalent-Y-for-divalent-Ca substitution in the Bi-2 (Sr) 2 (Ca,Y) 12:RS system, the H irr characteristics are not necessarily enhanced with increasing oxygen content but may rather be depressed. This provides us with one of the experimental evidences supporting the general conclusion that it is not the oxygen content but the concentration and the distribution of holes which control the H irr characteristics.

Evaluation of irreversible property in superconducting Cu(Ba0.8Sr0.2)2YbCu2O6+z powder

Abstract The irreversibility field of superconducting Cu(Ba 0.8 Sr 0.2 ) 2 YbCu 2 O 6+ z (so-called Yb-123) powder was carefully measured as a function of temperature using a SQUID magnetometer. The DC magnetic susceptibility showed a gradual transition near the critical temperature, suggesting a wide distribution of the critical temperature. For a detailed characterization of the material, the evaluation of the mean critical temperature and its distribution width is important. These are estimated by fitting the theoretical result of the DC magnetic susceptibility in field cooled and zero field cooled processes to the experimental result. The obtained result proves that this material showed a superior irreversibility field than usual Y-123 superconductor.

Magnetic-field irreversibility in superconductive Cu-m212 (m=1, 2)

Abstract The magnetic-field irreversibility ( H irr ) characteristics of the CuBa 2 YCu 2 O 7− δ (Cu-1212 or so-called “123”) and Cu 2 Ba 2 YCu 2 O 8− δ (Cu-2212 or so-called “124”) phases are studied with respect to the hole-doping level and the isovalent-substitution effect. For both the phases the H irr characteristics are enhanced with increasing overall hole-doping level. When comparing fully oxygenated samples, the H irr line of the Cu-2212 phase is located at magnetic fields lower than that of the Cu-1212 phase. The Sr(II)-for-Ba(II) substitution is not effective in enhancing the H irr characteristics of the Cu- m 212 phases. It rather lowers the H irr line in the case of the Cu-1212 phase. On the other hand, the Yb(III)-for-Y(III) substitution enhances the H irr characteristics of the Cu-1212 phase. Both types of isovalent substitution, i.e. Sr-for-Ba and Yb-for-Y, result in contraction of the lattice dimensions, but work oppositely in terms of the effect on the H irr characteristics. Therefore it is considered that the control over the H irr characteristics is not simply achieved by the lattice-dimension change but rather by the change in the homogeneity of the hole-density distribution over the crystal.

Evaluation of the Effect of Area Factors on the Grain Connectivity of Ex-Situ

The <i>J</i> _c-<i>B</i> performance of <i>ex-situ</i> MgB₂ tape is thought to depend on the grain-boundary coupling between the grains of an <i>ex-situ</i> MgB₂ core and between particles within a grain. This work investigates the influence of these area-dependent factors on the <i>J</i> _c-<i>B</i> performance and the measured grain connectivity of <i>ex-situ</i> MgB₂ cores. While the influence of area factors on the <i>J</i> _c - B performance of <i>ex-situ</i> MgB₂ tapes is small, the effect on measured grain connectivity is larger. Therefore, the relationship between the <i>J</i> _c - B performance of <i>ex-situ</i> MgB₂ tape and grain connectivity should carefully take this phenomenon into account. Moreover, this study reveals that different ways to evaluate the grain connectivity have advantages and disadvantages for detecting the influence of the area factors. We propose an evaluation guideline for the grain connectivity of <i>ex</i>-<i>situ</i> MgB₂ cores in order to analyse the reasons for area-dependent changes. We believe that this work will improve our understanding of grain connectivity and allow it to be used as feedback in investigations of the fabrication of <i>ex-situ</i> MgB₂ tape with high <i>J</i> _c - B performance.

{\rm MgB}_{2}

For a series of CuBa2RECu2O7−δ (Cu−1(Ba)2RE12; RE= rare earth element) single-phase cation-stoichiometric samples with a fixed oxygen content, δ=0.07±0.01, a clear dependence of the Hirr characteristics on the ionic radius of RE is revealed. With decreasing ionic radius of RE, the Hirr characteristics are remarkably enhanced such that the samples containing heavy REs exhibit very high Hirr values. This result implies that through isovalent cation substitution alone carrier-density distribution and thereby the superconductivity characteristics may be controlled.

Cores

Abstract The crucial role of the overall hole-doping level in controlling the magnetic irreversibility field ( H irr ) values has been well documented for a number of copper-oxide superconductors. Here we show that not only the high overall hole-doping level but also the homogeneity of the hole distribution over the layered superconductive M m A 2 Q n −1 Cu n O m +2+2 n ± δ or M – m ( A ) 2 ( Q ) ( n −1) n crystal contributes to enhance the intrinsic H irr characteristics. To control the homogeneity of the hole distribution, i.e. the distribution of holes (i) between the superconductive CuO 2 –( Q -CuO 2 ) n −1 block and the non-superconductive A O– M m O m ± δ – A O blocking block and (ii) within the superconductive CuO 2 –( Q -CuO 2 ) n −1 block, isovalent cation substitutions are found effective. To demonstrate this, we summarize our studies on the effects of isovalent substitutions at Q and A sites on the H irr characteristics of polycrystalline Cu(Ba,Sr) 2 RECu 2 O 7− δ [Cu-1 (Ba,Sr) 2 (RE) 12] samples containing various “heavy rare earths” up to RE=Yb 0.6 Lu 0.4 and Sr at the Ba site up to 40%, and also for single-crystal (Hg,Pb)(Ba,Sr) 2 Ca 2 Cu 3 O 8+ δ [(Hg,Pb)-1 (Ba,Sr) 2 (Ca) 23] samples with a Sr-for-Ba substitution level up to 75%.

Consequence of isovalent rare earth substitution to magnetic irreversibility in cation-stoichiometric CuBa2RECu2O6.93±0.01

Abstract For CuBa 2 RECu 2 O 7− δ (Cu-1212:P), the dependence of magnetic irreversibility field ( H irr ) on the ionic radius of rare earth element (RE) has been studied. CuBa 2 RECu 2 O 7− δ samples with the RE ionic radii [ r (RE)] varying from 0.982 A (for RE=Yb 0.6 Lu 0.4 ) to 1.079 A (for RE=Sm) were studied. The oxygen content of all the samples was set at a fixed level, i.e. δ =0.06±0.03. With decreasing r (RE), the H irr characteristics, i.e. the H irr vs (1− T / T c ) plot, was clearly enhanced. The CuBa 2 (Yb 0.6 Lu 0.4 )Cu 2 O 6.92 sample showed a record-high H irr value of ∼9.0 T at 77 K. This result implies that through isovalent cation substitution the carrier-density distribution may be optimized to facilitate the superior H irr property.

Isovalent cation substitutions to control the intrinsic Hirr characteristics of superconductive copper oxides

The utilization of MgB₂ powder obtained from the core of in-situ MgB₂ tape improves the high magnetic field J_c - B performance of Fe-sheathed MgB₂ tape fabricated via an ex-situ powder-in-tube (PIT) technique. This work reveals a further improvement by means of the addition of SiC nano particle. The highest J_c at 4.2 K of the tape have a values of 4.6 x 10³ A/cm² (I_c ap26 A at 12 T) and 9.7 x 10³ A/cm² (I_c ap 54 A at 10 T) for the tape with 10 mol% SiC post-annealed at 700deg C. These values are believed to be the highest known J_c values of ex-situ MgB₂ tape. However, they were lower than the values of in-situ MgB₂ tape with 10 mol% SiC (J_c 6.3 X 103 A/cm² ap 43 A at 12 T and 1.3 x 104 A/cm² : I_c ap 88 A at 10 T). The reason for the poorer J_c -B performance of the ex-situ MgB₂ tape is thought to be due to a lower B_irr and poorer grain connectivity.

Rare-earth-ion-size control on magnetic irreversibility field in CuBa2RECu2O7−δ

We report comprehensive studies of the crystallographic,magnetic, and thermal properties of a spinel-type magnetically frustrated compound, CoAl2O4, and a magnetically diluted system,Co1-xZnxAl2O4. These studies revealed the effects of dilution and disorder when the tetrahedral magnetic Co ion was replaced by the nonmagnetic Zn ion. Low-temperature anomalies were observed in magnetic susceptibility at x<0.6. A multicritical point was apparent at T = 3.4 K and x = 0.12, where the antiferromagnetic, spin glass-like, and paramagnetic phases met. At that point, the quenched ferromagnetic component induced by a magnetic field during cooling was sharply enhanced and was observable below x = 0.6. Around x = 0.6, magnetic susceptibility and specific heat were described by temperature power laws, chi ~ C/T ~ T-d, in accord with the site percolation threshold of the diamond lattice. This behavior is reminiscent of a quantum critical singularity. We propose an x-temperature phase diagram in the range 0<x<1 for Co1-xZnxAl2O4. The transition temperature of CoAl2O4 determined from magnetic susceptibility measured under hydrostatic pressure increased with increasing pressure.