Structure and properties of Bi2Sr2CaCu2Ox/Bi2Sr2CuOx superlattices prepared by RF magnetron sputtering

We have prepared Bi₂Sr₂CaCu₂O_x/Bi₂Sr₂CuO_x (2212/2201) superlattices with various stacking periodicity by multitarget RF magnetron sputtering. Their crystal structure and electrical properties were investigated. The superlattice was found to be successfully constructed from the data of XRD patterns, AES, etc. The critical temperature increased almost linearly with increasing thickness of the 2212 phase up to a thickness of 6 unit cells. On the other hand, it did not decrease as the thickness of the 2201 layers was increased. The unit cell/unit cell superlattice showed T_{c zero} of 30 K.     

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.     

Out-of-plane conductivity in Bi2Sr2CuOy crystals

We present extensive measurements of anisotropic resistivity on Bi₂Sr₂CuO_y crystals grown from melts with different Bi/Sr ratios and doped with Pb. We find that the temperature variation of the c-axis resistivity _c(T) is correlated with the in-plane resistivity _ab. Depending on the starting compositions, the normal-state in-plane resistivity _ab can either show localized conduction at low temperature or be metallic (d_ab/dT < 0) in the whole temperature range. Correspondingly, a change of the T dependence of _c from nonmetallic (d_c/dT < 0) in the whole measured temperature range (4.2–300 K) to a mixed (d_c/dT < 0 at low T but d_c/dT> 0 at high T) conduction is observed. In accompany, the magnitude of _c at low T decreases by about two orders. We have quantified the trend of the _c(T) and examined some current proposals concerning the out-of-plane transport.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Effects of 230 MeV Au irradiation on Bi2Sr2CaCu2Ox

Bi₂Sr₂CaCu₂O_x tapes were irradiated using 230 MeV Au¹⁴⁺ ions. Columnar defects were presumably produced due to irradiation. Zero-field-cooling (ZFC) magnetization increased up to a fluence of 1.6 × 10¹¹ Au⁺/cm², but field-cooling (FC) magnetization decreased, indicating the strong pinning effects resulting from the columnar defects. The critical current density as well as the irreversibility field, obtained from the hysteresis loops, were enhanced. Irreversibility fields are fitted by H_irr = A exp(−T/T_A). An effective activation energy for flux motion was obtained from the measurements of magnetization relaxation. The features of flux pinning as a result of the columnar defects were compared with those of point defects brought about by 120 MeV O⁷⁺ irradiation.     

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.     

Temperature dependence of the resistivity and its anisotropy in n-type Nd1.85Ce0.15CuO4 single crystal

Measurements of the in-plane and c-axis normal-state resistivity in a superconducting Nd_1.85Ce_0.15CuO_4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, _ab(T) and _c(T) displays a metallic-like positive temperature coefficient of resistivity with a basic T² dependence. We discuss some possible origins of this peculiar temperature dependence.     

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.     

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.     

Normal-state resistivity of superconducting Bi1.95Sr1.65La0.4CuO6+δ single crystals

The anisotropy of the normal-state resistivity of a number of superconducting Bi_1.95Sr_1.65La_0.4CuO_6+δ single crystals has been measured by using both a six- and a four-terminal technique. We find that the ab-plane resistivity _ab in general increases almost linearly with temperature (d_ab/dT> 0). the temperature variation of the c-axis resistivity _c, however, is strongly sample dependent. Both metallic-like (d_c/dT> 0) and mixed temperature dependences of _c, including d_c/dT < 0 are observed. The mixed _c(T) data can be well described by fitting to _c(T) = A + BT + C/T, indicating that the temperature dependence of _c(T) is the result of a competition between metallic and non-metallic terms. The temperature T_min at which _c reaches a minimum increases with increasing d_ab/dT, suggesting that the metallic term of _c arises from misaligned Cu---O planes. This is confirmed by an electron-microscope (HRTEM) analysis of the samples. The anisotropy ratio _c/_ab is of the order of 10³ but increases monotonically with decreasing temperature, indicating differences in the scattering processes parallel and perpendicular to the Cu---O plane.     

Field-induced parameters of re-entrant magnets and concentrated spin glasses

We have used electron spin resonance measurements to derive the temperature and frequency dependences of the field-induced magnetization [M(T, f)] and anisotropy field [H_an (T)] in a number of amorphous alloys belonging to the series (Fe_pNi_1−p)₇₅P₁₆B₆Al₃. In re-entrant (p > p_c, the critical concentration for ferromagnetism) alloys at hi gh frequencies (f = 35 GHz, field ≈ 12 kOe) M reduces as T^3/2 at high T and as T below ≈ 40 K, the deviation from T^3/2 becoming more marked as p → p⁺_c. For p close to p_c, lowering the frequency first causes the T term to increase and ultimately ( ≈ 4 GHz) changes the variation of M with T to that discovered previously for concentrated spin glasses, namely M is constant at low T and drops linearly at high temperatures. For the re-entrants, the results are interpreted on the basis of a model which invokes an energy gap in the spin-wave spectrum, introduces a non-zero density of states of the gap energy and takes into consideration a low-q cut-off in the spin-wave integral in thelow-T (T) regime.In the concentrated spin glasses [M (0) - M (T)]/ M (0) is well represented by the function [exp (Δ / T) - 1]^-1, where Δ has values close to the corresponding Curie-Weiss temperatures θ_p but much larger than the respective spin glass transition temperatures T_SG. The temperature dependence of H_an is largely given by the function (1 - T/T^*), where T^* is equal to the zero-field freezing temperature for the re-entrants and T_SG for the spin glasses, respectively.     

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.     

Intergranular properties of YBCO and BSCCO ceramic superconductors at low fields

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J_c(T) on sintered, non-oriented YBa₂Cu₃O_7−δ and Bi₂Sr₂CaCu₂O_8+δ ceramic samples at zero field. Below the critical temperature, potential variations, J_c(T) ≈ (1−t_j)^m with t_j = T/T_j, have been found, T_j being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J_c values. Moreover, the effect of texture has been also considered on Bi₂Sr₂CaCu₂O_8+δ ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the χ′(h₀) extrapolation at zero filed has also been correlated with the evolution of the intergranular penetration depth, λ_J(T).     

Measurement of the slope parameter for the neutral pion spectrum in KL → πππ decay

The π⁰ spectrum in the K_L⁰ → 3π decay was measured using a wire chambers magnetic spectrometer. In the usual approximation, the matrix element can be expressed as: M² ≈ 1 + 2a₀(M_K/M_π²)(2T_π⁰−T_π⁰max) + a₁(M_K²/M_π⁴)(2T_π⁰−T_π⁰max)². We obtained a₀ = −0.282 ± 0.011 and a₁ consistent with zero.     

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.     

Influence of Nd substitution on the superconducting properties of ceramics in the 2212 system Bi2−wPbwSr2−xCa1−yNdx+yCu2O8+z

In the system Bi_2−wPb_wSr_2−xCa_1−yNd_x+yCu₂O_8+z different fractions of Nd are substituted on either Sr of Ca sites in order to introduce intrinsic insulating pinning centres. It is shown that a Nd concentration around x or y = 0.2 is likewise favourable with an average Nd---Nd distance in the range of the coherence length in the a, b-plane. However, clear evidence of flux pinning is only present for charge compensation with Pb²⁺. A simultaneous substitution of the Bi-based 2212 superconductor with moderate amounts of Nd³⁺ and Pb²⁺ improves the superconducting properties by strengthening the flux pinning forces.     

The incommensurate modulation in the Bi2Sr2−xCaxCuO6 superconductor, and its relation to the modulation in Bi2Sr2−xCaxCu2O8

The modulation of the (221) superconducting Bi₂Sr_2−xCa_xCuO₆ phase has been analyzed by X-ray diffraction, using four-dimensional symmetry theory. The pseudo-orthorhombic diffraction pattern is a superposition of two twin related components, such that the main reflections with hkl and hk m superimpose, but the satellite hklm and hk m do not. The latter feature allows separation of the twin intensities. The modulation analysis in superspace group P:Aa:1 shows displacements similar to those observed in the 2212 compound [Y. Gao et al., Science 241 (1988) 954] but with generally larger displacements. In particular, the c-axis displacement amplitude of the Cu atoms is increased to almost 0.5 Å. This and the obliqueness of the q vector (q=0.214a^*+0.61c^*) indicate the absence of a restraining influence exerted by the CuO₂/Sr(Ca)/CuO₂ double layers in the multi-Cu layer phases.     

Oxygen nonstoichiometry and high temperature conductivity of the 2201 phase of the Bi---Sr---Cu---O superconducting oxide

The oxygen nonstoichiometry and high temperature conductivity of Bi_2.12Sr_1.86Cu_1.02O_y (2201 phase) were determined as a function of temperature (T) and oxygen partial pressure (P_O2). The dependences of conductivity on T and P_O2 were similar to those of oxygen content, and two subphases, and β, were found in such cases as 2212 and 2223 phases. From the result a close relationship was inferred between conductivity and oxygen content, and the mobility and carrier density were calculated. The carrier density of 2201 phase was smaller than those of the 2212 and 2223 phases.     

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi₂Sr₂R_1.33Ce_0.67Cu₂O_10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi₂Sr₂Cu₂O_10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi₂Sr₂Eu_2−xCe_xCu₂O_10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples.