Meissner Volume Fraction Research Articles

Possible phase separation in the underdoped La 2− xSr xCuO 4 studied by the magnetic susceptibility

In order to investigate the inhomogeneity of the superconductivity, we have grown a large-sized single-crystal of La 2− x Sr x CuO 4 (LSCO), in which the Sr concentration, x, continuously changes in the underdoped regime, and measured the magnetic susceptibility on field cooling for several pieces of single crystal with different x values by slicing the single crystal. It has been found that the Meissner volume fraction, regarded as corresponding to the superconducting (SC) volume fraction in a sample, increases with increasing x. This suggests that a phase separation into SC and normal-state regions takes place in the underdoped regime as well as in the overdoped regime of LSCO.

Possible microscopic phase separation in the overdoped regime of La2−xSrxCuO4 studied by the magnetic susceptibility and electrical resistivity

Precise measurements of the magnetic susceptibility and the in-plane electrical resistivity in the overdoped regime of La2−xSrxCuO4 have been carried out using single crystals of good quality, in order to investigate the inhomogeneity of the superconductivity in the overdoped regime of high-Tc cuprates. From the magnetic-susceptibility measurements, it has been found that the Meissner volume fraction decreases with increasing x, indicating that phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime. From the in-plane resistivity measurements, it has been found that the SC transition width becomes broad with increasing x, which is possibly due to the increase of the volume fraction of the proximity region between SC and normal-state regions in the phase-separated CuO2 plane. It appears that microscopic phase separation takes place in the overdoped regime and that the proximity region becomes dominant with increase x.

Possible phase separation in the overdoped regime of La2−xSrxCuO4

In order to investigate the possible inhomogeneous superconductivity in the overdoped high-Tc cuprates, suggested from the muon-spin-relaxation measurements, we have grown a single crystal of La2−xSrxCuO4 in which the Sr concentration, x, continuously changes in the overdoped regime and measured the magnetic susceptibility of several pieces of single crystals with different x values by slicing the single crystal. It has been found that the Meissner volume fraction, regarded as corresponding to the superconducting (SC) volume fraction in a sample, as well as Tc rapidly decreases with increasing x. It has been concluded that a phase separation into SC and normal-state regions takes place in a sample of La2−xSrxCuO4 in the overdoped regime. The separated two phases in the overdoped regime may be a hole-rich normal Fermi-liquid state region and a hole-poor superconducting one. Otherwise, holes doped into the Cu 3d orbital may destroy the superconductivity around themselves, producing normal-state regions in the superconducting sea.

Synthesis, structure, and superconductivity of (La1.5Pb0.5-xSrx)CuOz

The substitution of strontium for lead in the material (La1.5Pb0.5−xSr x )CuO z , x = 0–0.15 has been carried out. A stable and reproducible single phased superconducting materials can be obtained inside an evacuated quartz tube. The X-ray diffraction pattern shows that the superconducting phase can be indexed on the basis of an orthorhombic symmetry (Fmmm) for x = 0 and on the basis of tetragonal symmetry (I4/mmm) for x > 0. The transition temperature T c increases as the strontium substitution parameter x increases. We observed the maximal T c around x = 0.15 with 38 K with fairly large Meissner volume fraction of 38% (FC).

High-pressure synthesis of CuBa 2Ca n−1 Cu nO x ( n=3, 4) superconductors without external oxidizing agents

We have successfully synthesized non-stoichiometric BaCuO 2+ δ as a precursor for the high-pressure synthesis of CuBa 2Ca n−1 Cu n O x [Cu-12( n−1) n] superconductors. The oxygen content of BaCuO 2+ δ was determined by a coulometric titration method. Using this precursor but no external oxidizing agents, samples which were superconducting with T c=116 K have been obtained by sintering at 1100°C under 5 GPa. The T c=116 K phase was identified to be the CuBa 2Ca 3Cu 4O x (Cu-1234) phase. The Meissner volume fraction of the sample was estimated at ca. 30% at 5 K. A sample which contained a superconducting phase with T c at 80 K was also obtained at a temperature as low as 950°C. The 80 K phase was identified to be the CuBa 2Ca 2Cu 3O x (Cu-1223) phase. Thus, the Cu-1234 and Cu-1223 phases were successfully synthesized without the presence of any external oxidizing agents.

Evidence for Antiferromagnetic Order in Bi2Sr2CuO6Phase with Stoichiometric Cation Composition

We have succeeded in synthesizing the insulating phase of Bi2Sr2CuO6+δ(Bi2201) with the stoichiometric cation composition by use of a heat treatment technique in an evacuated tube with titanium powders. The excess oxygen contentδand the average valence of Bi were determined electrochemically by the coulometric titration method. The value ofδin the insulating phase was found to be 0.02. The hole concentrationhwas also estimated with values ofδand the average valence of Bi in Bi2201. As a result, temperature dependence of resistivity andhdependence of Tc and Meissner volume fraction indicated behaviors similar to those of other high-Tc systems. The antiferromagnetic order of Cu2+spins in the insulating Bi2201 phase has been confirmed by nuclear magnetic resonance. Thus, we can obtain the three distinct electronic states observed in high-Tc oxides, i.e., over-doped metallic, superconducting, and antiferromagnetic phases in Bi2201 compounds by changing only the oxygen content. The static parameters of the hyperfine field and the electric field gradient at Cu sites in Bi2Sr2CuO6.02were found to be almost comparable to those of other antiferromagnetic phases of high-Tc oxides.

NQR studies in oxygen annealed La 2− xBi xCuO 4+δ

La NMR and NQR studies on polycrystalline samples of La 2CuO 4+δ and La 2− x Bi x CuO 4+δ ( x = 0.05–0.20) Bi doped samples, lower superconducting transition temperature and larger Meissner volume fraction were measured compared with those of La 2CuO 4+δ. No phase separation was observed in X-ray diffraction patterns at room temperature and 7 K. La and Cu NQR signals were observed, showing no evidence of the existence of antiferromagnetic internal field. New resonance lines observed around 57 MHz are probably NQR signals of the Cu nuclei at the vicinity of the excess oxygen atom.

Extended defects in the H-reduced YBa2Cu3O7

By exposure of YBa2Cu3O7 to hydrogen at moderate temperatures, the superconductivity properties, Tc and Meissner volume fraction, are, depending on the treatment conditions, differently affected. Although no major changes are detected by X-ray diffraction, the microstructure of the obtained products is quite complex, and a significant amount of disorder is produced. The results are also compared with those produced in the reduction performed with n-butyllithium.

High pressure synthesis of infinite-layered (Sr 1−yCa y) 0.9La 0.1CuO 2

Homogeneous solid solution (Sr 1−yCa y) 0.9La 0.1CuO 2 with infinite layered structure could be prepared in a range 0≤y<0.2 applying high pressure synthesis. Both a- and c-lattice parameters shrank with the Ca 2+ substitution because of the smaller ionic size of Ca 2+ than Sr 2+. Tc (onset) slightly reduced and Meissner volume fraction decreased with the Ca 2+ substitution as well as unit cell volume from the values for Sr 0.9La 0.1CuO 2.

Oriented Bulk Consolidation of Bi–Sr–Ca–Cu–O by Shock-Loading Method

Shock compaction experiments were carried out for Bi2Sr2CaCu2Oy particles sorted to 46–74 µ m and packing density of 50–82% at a shock pressure of 8 GPa. Bulk density reached 95% by shock compaction independent of the porosity. The most effective orientation was achieved by shock loading for the packing density of 67% and with post annealing at 870°C. The texture of the shocked plus postannealed specimen showed crystal growth not only within the c-plane but also along the c-axis. The Meissner volume fraction of the shock compacted specimen decreased with decreasing packing density but the onset temperature did not change. The magnetic torque of shock plus annealed specimen showed the alignment of particles.

Defect chemistry of Ba 1-xK xBiO y superconductors

The critical temperature of Ba 1- x K x BiO y was determined as a function of oxygen content y. With increasing oxygen content. T c and the Meissner volume fraction increased, reached a maximum and decreased. The logarithm of the conductivity σ at 250 and 100 K showed an oxygen content dependence similar to T c and it was concluded that T c has a close relation with log σ in the normal state. Pycnometry on Ba 1- x K x BiO y revealed that the ion deficiency reaches as high as 12%.

Synthesis and superconductivity of oxycarbonates of the Tl-1201 phase

TlBa x Sr 4- x Cu 2(CO 3)O y oxycarbonates with the Tl-1201 structure were synthesized over a wide range of x (1.0≤ x ≤3.2) and examined by X-ray powder diffraction, electron diffraction, high-resolution electron microscopy and magnetometry methods. New phases with the Tl-1201 structure having 6 and 7 times periodicities along the b-axis were isolated. The former was stable for a wide Ba-rich range (1.8≤ x≤3.2), and the latter was observed for a smaller Ba content range (1.2≤ x≤1.6). The phase stability was found to depend on the synthesis condition. The maximum T c of the as-synthesized specimen in this series occurred at about 70 K around x=2.0. The Meissner volume fraction of the as-synthesized specimen ( x=2.4) was 30% at 5 K.

炭酸塩銅酸化物超伝導体（Ｔｃ＝９２Ｋ）の高圧合成

A cupric oxide carbonate superconductor with a Tc of 92K has been synthesized under high pressure of 6GPa. A single phase sample with a large Meissner volume fraction of 20% is obtained when a mixture with a nominal composition (Ca0.3Sr0.7)3Cu2 C0.75 Oy encapsulated with an oxidizer of KClO4 is treated at 1473K and 6GPa for 30 minutes. The phase crystallizes basically in tetragonal structure with α=3.851Å and c=10.941Å, while an orthorhombic supercell of α×4α×2c forms, most probably as a result of ordering of carbon vacancies along the b-axis. An irreversibility line was measured under magnetic fields up to 5T.

The effect of Pb doping in HgBa 2Ca 2Cu 3O 8+δ superconductor

Polycrystalline samples with the starting compositions of (Hg 0.8Pb 0.2) x Ba 2Ca 2Cu 3O 8+δ (1.0⩽ x⩽2.5) were prepared under various synthetic conditions. Both powder X-ray-diffraction analysis and energy dispersive X-ray spectroscopy revealed that the Pb ions were successfully doped into the Hg-1223 phase. High-quality Pb-doped Hg-1223 samples for the starting composition with x=1.5 were obtained when heated at 675°C only for 1 h. The resistivity (ϱ) of such a sample was low, being ∼8×10 3 Ω cm at 300 K, and the ϱ versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high- T c cuprates. The zero resistance due to superconductivity started at 122.2 K. When the sample was reduced in flowing argon gas, it exhibited a Meissner signal below 132 K and its Meissner volume fraction exceeded 20% at 5 K. According to thermoelectric power measurements, the carrier density in the sample was reversibly dependent on the oxygen partial pressure, and the sample post-annealed in flowing oxygen gas was likely overdoped.

The bismuth oxycarbonate Bi 2Sr 6−χCu 3O 10(CO 3) 2. : A new 40 K superconductor, second member of the series (Bi 2Sr 2CuO 6) n(Sr 2CuO 2CO 3) n'

A new superconductor, Bi 2Sr 6−χCu 3(CO 3) 2O 10, has been isolated for 0⩽χ⩽1. This oxycarbonate exhibits a T c, onset of 40 K, with a Meissner volume fraction of 20%. The powder XRD and the ED study of this phase give evidence for an orthorhombic lattice with the following parameters: a=5.469 (2) A ̊ , b=5.483 (2) A ̊ and c=54.26 (1) A ̊ ; extra reflections, in incommensurate positions along b ∗, are observed corresponding to b'~9×5.5 Å. The HREM investigation shows that the structure of this phase corresponds to the intergrowth of double Sr 2CuO 2CO 3 type layers with single Bi 2Sr 2CuO 6 type layers. Thus, this oxide corresponds to the member “ n=1, n'=2” of the structural series (Bi 2Sr 2CuO 6) n (Sr 2CuO 2CO 3) n' . It is compared to the 30 K superconductor Bi 2Sr 4Cu 2O 8CO 3, first member of the series ( n= n'=1).

A 62 K superconductor with an original structure: Sr 4− xBa xTlCu 2CO 3O 7

A new superconductor with an original structure, Sr 4− x Ba xTlCu 2CO 3O 7 has been isolated for 1≤ x≤2. It crystallizes in a A-type space group with a=3.84 A ̊ ≈a p, b≈8 × a p, c≈17.0 A ̊ . The HREM study of this new curprate shows that it derives from the Tl 0.5Pb 0.5Sr 4Cu 2CO 3O 7 structure by a shearing mechanism. Indeed, it can be described as ribbons of the latter structure, four octahedra thick, shifted c/2 with respect to each other. It results in infinite single perovskite layers waving along the (001) plane and connected through flat mixed layers [(TIO) 4(CO) 4]∞. This oxycarbonate can also be described as a 1201-type structure in which the [TIO]∞ layers are replaced by mixed [(TIO) 4(CO) 4]∞ layers. The susceptibility measurements show that this cuprate exhibits a significantly higher critical temperature than the 1201-cuprates. The T c onset of the synthesized phase is 56 K and it can be increased up to 62 K by hydrogen annealing; in the same way, one observes a large Meissner volume fraction of 35% at 4.2 K.

Copper oxycarbonates with a layered structure, new promising high Tc superconductors

The ability of CO 3 groups to accomodate the perovskite framework opens the route to the possible synthesis of new copper oxycarbonates with a layered structure, which must be considered as promising high Tc superconductors. A review of the crystal chemistry of these recently synthesized new materials is given here. The best performance has been reached for the oxycarbonate Tl 0.5Pb 0.5Sr 4Cu 2CO 3O 7 which exhibits a Tc up to 70 K and a Meissner volume fraction of 70% at 4.2 K.

The bismuth oxycarbonate Bi 2Sr 4Cu 2CO 3O 8: A new superconductor with a Tc onset of 30 K

A new high- T c superconductor, Bi 2Sr 4Cu 2CO 3O 8, with a T c onset of 30 K, has been synthesized. The XRD and HREM studies of this oxycarbonate show that it exhibits an orthorhombic incommensurate structure with a=5.466 A ̊ , b=q×5.46 A ̊ , c=39.50 A ̊ and q=1/ k=8.85 to 9. A structural model is proposed which corresponds to a regular intergrowth of undulating 2201-type layers Bi 2Sr 2CuO 6, with Sr 2CuO 2CO 3-type layers, so that it represents the n= n'=1 member of the intergrowth series (Bi 2Sr 2 CuO 6) n . (Sr 2CuO 2CO 3) n' . Though it is characterized by a low Meissner volume fraction of about 5%, this new phase is of great interest for the understanding of superconductivity in these compounds, since the two components of this structure are either not superconducting, for Sr 2CuO 2CO 3, or exhibit a lower T c of 22 K for Bi 2Sr 2CuO 6; moreover, the fact that no superconductivity has been observed in the BiSrCuO system above 22 K shows that the introduction of CO 3 groups in this system enchances superconductivity. The possible synthesis of new high- T c superconductors Bi 2 n A 4 n Cu 3 n−1 CO 3O 11 n−3 ( A= Sr, La) is discussed.

Introduction of nitrate groups in the “123” structure: The 82 K superconductor YCaBa 4Cu 5(NO 3) 0.3(CO 3) 0.7O 11

The introduction of nitrate groups in the “123”-type matrix has been performed for the first time. A new superconductor, YCaBa 4Cu 5(NO 3) 0.3(CO 3) 0.7O 11 with a T c onset of 82 K and a Meissner volume fraction of 15% at 4.2 K has been synthesized. The X-ray diffraction and electron diffraction study shows that its orthorhombic cell corresponds to a superstructure of YBa 2Cu 3O 7 with “2a 123×b 123×2c 123” , the subcell parameters are a 123=3.879(1) A ̊ , b 123=3.885(1) A ̊ , c 123=11.536 (3) A ̊ . HREM investigation shows that its structure is derived from the YBa 2Cu 3O 7 structure by replacing one row of CuO 4 groups (Cu1) out of two by a mixed row of NO 3 and CO 3 groups. The latter groups point one of their apices toward the adjacent row of CuO 4 groups, so that one observes rows of CuO 5 pyramids running along b . The superconducting properties of this phase, contrary to the oxycarbonate Y 1.6Ca 0.4Ba 4Cu 5CO 3O 11, are explained in terms of hte hole carrier density.

Increase of the Meissner volume fraction in O 2-HIP processed La 1.85Ca 1.15Cu 2O 6 samples

The conditions, especially the annealing process, for synthesizing superconducting La 1.85Ca 1.15Cu 2O 6 were investigated. The samples were annealed in an Ar-20% O 2 gas mixture of 2000 atm, using an O 2-HIP (hot isostatic pressing) technique. Annealing at 1090°C made samples superconducting with T c ∼ 50 K. The longer the duration of the annealing, the larger was the Meissner signal from the sample. The Meissner volume fraction for a 100 h processed sample was estimated to be ∼ 3% at 5 K. It was found that an additional annealing at a low temperature, e.g. 600°C in a high-pressure oxygen gas or 450°C in flowing oxygen gas, was effective to increase nearly thrice the Meissner volume fraction of the sample.