Resistivity and thermoelectric power of Bi2Sr2Ca−xPrxCu2Oy system

Samples of Bi₂Sr₂Ca_1−xPr_xCu₂O_y have been characterized by resistivity and thermoelectric power measurements. All metallic samples show superconductivity with a maximum T_c = 90 K at X = 0.2. The sample of x = 0.6 shows a crossover from hopping conduction at low temperature above T_c to metallic conduction at high temperature. For the metallic samples below x = 0.6, the results of thermoelectric power are well fitted by both of a phenomenological band spectrum model and the Nagaosa and Lee model.     

Electronic structure of (Nd, Ce)2(Ba, Nd)2Cu3Oy

The dependence of T_c on oxygen content was determined for (Nd_0.67Ce_0.33)₂(Ba_0.67Nd_0.33)₂Cu_3.01O_y. It was found that superconduction appears in a range of y=9.10−9.16 and that T_c is enhanced remarkably by the increase of oxygen content. In an oxygen content range where superconduction appeared, the Ce valence was nearly +4 and the Cu valence changed from 2.31 to 2.33. The conductivity was measured in a wide temperature range between T_c and 1200 K as a function of oxygen partial pressure. The high-temperature conductivity increased with increasing oxygen content. It was found that both carrier concentration and mobility are increased with oxygen content. The conduction at low temperatures was semiconductor-like, and increased with increasing oxygen content, where the energy gap E_A-E_V between acceptor level and top of filled band approached zero.     

Effect of Hg addition on synthesis of Bi-based superconductors

We have investigated the effect of Hg addition on the superconducting properties of BiSrCaCuO system. Polycrystalline samples with nominal composition Bi₂Sr_2−xHg_xCa₁Cu₂O_y and Bi₂Sr_2−xHg_xCa₂Cu₃O_y (x=0.3) were synthesized and used to investigate the phase evolution by XRD, superconducting behaviour by R–T measurement and the structural grain boundary effects by SEM. From these measurements, it has been noticed that the phases obtained with both types of compositions are the same as Bi2212 but the T_c values are different. With additional annealing, T_{c zero} values were raised from 60 to 72 K in Bi₂Sr_2−xHg_xCa₁Cu₂O_y and 64 to 92 K in Bi₂Sr_2−xHg_xCa₂Cu₃O_y. Also, an improved grain boundary linkage has been observed by SEM for the 92 K sample.     

The effect of gallium substitution on the superconductivity of the BiSrCaCuO system

Magnetic susceptibility, X-ray diffraction and resistivity measurements of the system Bi_1.4Pb_0.6Sr₂Ca_2−xGa_xCu₃O_y are reported for 0 x 2. The high-T_c 2223 phase with a T_c of 107 K for x = 0 exists for x 0.3. The low-T_c 2212 phase with a T_c of 75 K for x = 0 exists for the full range of x. The highest values of the critical temperature and the largest volume fraction of the low-T_c phase in compounds with Ga occur for x = 0.5 ± 0.1. The identification of CaO by X-ray diffraction for x 0.6 indicates that Ga replaces Ca in the compound.     

Electrical transport and superconductivity in the Al2O3-Bi2Sr1.8Ca1.2Cu2Oy and MgO-Bi2Sr1.8Ca1.2Cu2Oy composites

We have measured the resistivities of Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites with the nominal Bi₂Sr_1.8Ca_1.2Cu₂O_y volume fraction, ₂₂₁₂, ranging from 0.15 to 1.00. For the Al₂O₃-Bi₂Sr_1.8Ca_1.2Cu ₂O_y composites, we find for the samples with ₂₂₁₂≥0.6 that the superconducting transition temperature, T_c, is not disturbed by the addition of Al₂O₃. For ₂₂₁₂<0.3, no zero-resistivity state is observed. For the MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y composites, T_c is barely disturbed for the samples with ρ₂₂₁₂≥0.7. No superconducting state is observed for the samples with ρ₂₂₁₂<0.35. The variation of (300 K) with ρ₂₂₁₂ indicates a three-dimensional percolating Bi-Sr-Ca-Cu-O matrix occurring at ρ₂₂₁₂≈0.19 and ≈0.15 in Al₂O₃-Bi₂Sr_1.8Ca_1.2 Cu₂O_y and MgO-Bi₂Sr_1.8Ca_1.2Cu₂O_y, respectively. Both resistivity and magnetization measurements suggest that the reactions of Bi₂Sr_1.8Ca_1.2Cu₂O_y with MgO are weaker than with Al₂O₃.     

Synthesis and properties of Hg0.7Pb0.3(BaSr)2Ca2Cu3Oz superconductors

We have synthesized Hg(Pb)-1223 with substitutions of Sr for Ba, i.e. Hg_0.7Pb_0.3Ba_2−xSr_xCa₂Cu₃O_z (x = 0.0–2.0) by the ampoule method. The spray-drying method and thermal decomposition under vacuum were applied for the preparation of homogeneous precursors. Samples with T_c = 110–128 K were obtained without oxygen annealing. We found that substitution of Ba by Sr remarkably relaxed the requirements on the precursors preparation and allowed their handling under ambient conditions.     

A study on the nonstoichiometry of tin oxides by coulometric titration

In this investigation, nonstoichiometries and defect structures of tin oxides were studied between 694 and 990 K by coulometric titration using solid state electrolyte (YSZ) cells. The relationship between nonstoichiometry of the oxide (x) and equilibrium oxygen partial pressure (P_o2) was expressed by the proportionality: xP_O2^−1/6. An intermediate oxide phase, Sn₃O₄ between Sn and SnO₂ was observed in the temperature range of 696–732 K. The standard Gibbs energy of formation of Sn₃O₄ via the reaction; was found to be ΔG^o_Sn3O4 = −1163960+417.36 T (J/mol). The standard Gibbs energy change for the defect formation reaction in SnO_2−x was calculated to be ΔG^o_SnO2−x = 3.05×10⁵−38.97 T (J/mol)).     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 K and 87 K for two samples of different origin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.     

Systematic thermopower measurements of the thallium cuprates Tl(Ba,Sr)2Cam−1CumO2m+3−δ and Tl2Ba2Cam−1CumO2m+4+δ

The thermoelectric power (TEP) S versus temperature has been systematically investigated for several series of the superconducting cuprates Tl(Ba,Sr)₂Ca_m−1Cu_mO_2m+3−δ (m = 2, 3) and Tl₂Ba₂Ca_m−1Cu_mO_2m+4+δ (m = 1, 2, 3). The consideration of the S(T_c) curves allows two important points to be found evidence for. The first one deals with the fact that all these superconducting thallium cuprates are systematically overdoped whatever T_c, and whatever the number of Cu or Tl layers; no underdoped superconducting cuprate could be obtained. The second point shows that there exist two classes of Tl cuprates: the weakly overdoped cuprates that exhibit a T_{c max} ≥ 100 K (all the triple copper layer cuprates and the 2212 cuprates) and those which can be heavily doped that exhibit a T_{c max} ≤ 90 K (the 2201 and the 1212 cuprates). The different behavior of thallium cuprates compared to YBa₂Cu₃O_7−δ and to bismuth cuprates is discussed.     

Magnetic properties of the Bi1.95Sr2.05−xLaxCuOy (Bi-2201) superconducting phase

We have investigated the reversible mixed-state magnetization M of three lanthanum substituted Bi_1.95Sr_2.05−xLa_xCuO_y (Bi-2201) ceramic samples having different critical temperatures T_c ranging from 20.0 to 35.5 K. As for the Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) phase, we found that anisotropy of Bi-2201 is large. A manifestation of this anisotropy is the field independent magnetization M^* observed at a temperature T^*. In the framework of the London model, and including thermal fluctuations of vortices, we found for the temperature dependence of the penetration depth λ_ab(T) = λ_ab(0)[1 − (T/T_c0)ⁿ]^−1/2, with n 1.7 and λ_ab (T = 0) 4000 Å. The estimated upper critical fields μ₀H_c2,c are of the order of 10 T. We observe a peculiar negative slope M/T at low temperature and sufficiently high external magnetic field. This feature seems to be a characteristic of the Bi-2201 phase. However, we do not know whether it is associated with the superconducting mixed-state. A small amount of magnetic impurities could also be responsible for this behavior. Finally, the behavior of the reversible magnetization of the Bi-2201 samples investigated, which are situated at the optimal and in the overdoped region, did not indicate any unusual temperature dependence for the upper critical field H_c2,c.     

Giant pressure effect in oxygen deficient YBa2Cu3Ox

The pressure effect on T_c of polycrystalline and single crystalline YBa₂Cu₃O_x investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x> 6.93 the single crystals show a negative dT_c/d p, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dT_c/d P = 0.4 K/GPa which points to pressure effects on T_c aside from charge transfer. In the underdoped region x < 6.93 the dT_c/d p values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT_c/d p increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT_c/d p drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa₂Cu₃O_x structure and can be explained by considering the anisotropic structure of YBa₂Cu₃O_x. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO₂-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dT_c/d p at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T_c to the equilibrium value T_c(p) at this pressure with a time constant depending on the oxygen content x. A decrease x results in a peak effect in dT_c/d p at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dT_c/d p first drops to 5 K/GPa at x = 6.6, but the increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible T_c increase (dT_c/d p)_O due to pressure induced oxygen ordering via oxygen motion between unit cells.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

Thermoelectric power and irreversibility field of (Hg,Tl)2Ba2Can−1CunOy (n=2–5) superconductors

We have measured the thermoelectric power, S, and the irreversibility field, H_irr, of the superconducting samples of (Hg,Tl)₂Ba₂Ca_n−1Cu_nO_y (n=2–5). S values for the (Hg,Tl)-22(n−1)n above their T_c's drastically increase with increasing n. Judging from the S values at room temperature, the n=2 and 3 samples are located in the overdoped region, the n=4 is in the almost optimally doped and the n=5 is in the underdoped region. H_irr values for the n=4 and 5 samples are higher than those for the as-synthesized and annealed n=3 samples. It is considered that the enhancement in H_irr is due not only to the increase of hole concentration but to the increase in the number of CuO₂ sheets.     

Specific heat of Sm1+xBa2−xCu3Oysolid solution of orthorhombic and tetragonal structure

The specific heats of Sm_1+xBa_2−xCu₃O_y solid solution of orthorhombic and tetragonal structure were measured in the temperature range 80–300 K. The data were analyzed in the framework of the Debye model with dilatation correction. The tendency to lower the high-temperature limit of the Debye temperature, θ^HT_D, with the oxygen deficiency was noticed. In contrast, the increase of Sm substitution causes a rise of θ^HT_D. The temperature dependence θ_D(T) was calculated for each compound from the series for the whole temperature region investigated.     

Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Influence of fluctuations on thermal conductivity of high-Tc YBa2Cu3O7−δ superconductor

To study a behavior of the thermal conductivity near T_c specific heat and thermal diffusivity of the YBa₂Cu₃O_7−δ high-T_c ceramics were simultaneously measured. Close to T_c = 92.30 K the thermal diffusivity and the thermal conductivity discovered minima and the specific heat – maximum. Quantitative analysis of the influence of thermodynamical fluctuations showed the same power laws with Gaussian exponent equal to 0.5 and existing of crossover from the 3D Gaussian to 3D XY critical behavior in the specific heat and thermal conductivity at the approach to T_c. To explain the minimum in thermal conductivity at T_c we propose a mechanism of scattering of phonons on the superconducting fluctuations.     

Preparation of melt-textured NdBa2Cu3Oy bulk with Nd4Ba2Cu2O10 addition

The NdBa₂Cu₃O_y (Nd123) bulk superconductor, to which Nd₄Ba₂Cu₂O₁₀ (Nd422) particles were intentionally added, was prepared through the so-called MMTG process in Ar (99% purity) flowing atmosphere at an ambient pressure. The quasi-single crystal thus grown was about 1 cm × 1 cm × 1 cm in dimension. It turned out that the Nd422 particles were uniformly distributed in the Nd123-phase matrix in a fashion similar to the distribution of the intentionally added Y211 particles in the Y123 phase matrix. The superconducting transition temperature T_c for the sample subjected to post-annealing in oxygen atmosphere was 94 K. The critical current density J_c was determined to be 45 000 A/cm² at 77 K and 1 T, when the field was applied parallel to the c-axis of the sample. To the best of our knowledge, the J_c value is the highest and the size of the quasi-single crystal is the largest in the melt-textured Nd123 bulk superconductors so far reported.     

Effect of sodium substitution on superconductivity in Bi-based (2212) system

Samples with the nominal composition Bi₂Sr₂Ca_1−xNa_xCu₂O_y (if with X = 0.05, 0.2, 0.4 and 0.5) were prepared by the solid state reaction method. The role of Na and the effect of various heat treatments in the 2212 composition were studied. From the X-ray diffraction data it is found that the samples annealed for longer duration have shown a single phase 2212. From the DC four-probe resistivity data, it was found that the T_c (zero) varies from 79 to 89 K. It is believed that doping with an alkali element would reduce the oxygen content due to monovalency and gives an effect similar to that during heat treatment under low oxygen pressure or quenching. The presence of sodium in the samples was confirmed by electron probe microanalysis.     

Over-doping of Tl2Ba2CuO6 due to charge transfer Tl-(Cu-O)

Superconductivity was observed in La substituted Tl₂Ba₂CuO₆. Both T_c and unit cell dimension, a, increase with increasing La concentration, indicating that an over-doping state in Tl₂Ba₂CuO₆, which was due to charge transfer Tl^3−t-(Cu-O)^p as supported from XPS measurements, was relieved by La substitution for Ba.     

Study on the superconducting composite material formation in the system Bi2Sr2CaCu2O8+x/Al-containing phases

Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr_1.5Ca_0.5Al₂O_z and (Sr_1−xCa_x)₃Al₂O₆ (x = 0.4 − 0.45) were determined to be chemically compatible with Bi_2.18Sr₂CaCu₂O_8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr_1.5Ca_0.5Al₂O_z was found to be in equilibrium only with the melt and the (Sr_1−xCa_x)₃Al₂O₆ phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr_1.5Ca_0.5Al₂O_z ↔ (Sr_1−xCa_x)₃Al₂O₆ + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr_1.5Ca_0.5Al₂O_z and Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr_1.5Ca_0.5Al₂O_z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr_1−xCa_x)₃Al₂O₆ imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr_1−xCa_x)₃Al₂O₆ materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content.