Vortex Excitations Above Tc in the Cuprate Superconductor Bi2Sr2Ca2Cu3O10 as Revealed by ESR

Using electron spin resonance (ESR) technique we have obtained data evidencing the existence of magnetic vortices in high-temperature superconductors at temperatures above the critical one T _c. We have studied magnetic excitations in Bi₂Sr₂Ca₂Cu₃O₁₀ single crystals above T _c with the method of surface spin decoration. The surface layer of diphenylpicrylhydrazyl was used as a sensitive probe of magnetic field distortions. The temperature dependence of the ESR signal parameters has indicated that far above T _c the magnetic flux of a sample is affected by the superconducting order parameter fluctuations while close to T _c its changes are due to vortex-type excitations.     

Increase in the diamagnetic response from low-density Bi1.8Pb0.3Sr1.9Ca2Cu3Ox high-temperature superconductors and Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag composites

Low-density polycrystalline Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors with a foamlike microstructure and composites consisting of this superconductor and silver in an amount of 20, 25, and 30 vol % are synthesized. The microstructure, as well as the temperature and field dependences of the magnetization, M(T) and M(H), are studied. It is found that, in Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x high-temperature superconductors and Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x + Ag composites, the diamagnetic response is enhanced and the screening properties are improved compared with high-temperature polycrystalline superconductors with the same composition that are prepared by the standard technology. The observed effect is explained by the features of magnetic flux penetration into a porous medium.     

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.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting, superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^–1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^–1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.     

关于MgB2/Al2O3超导薄膜电阻转变和各向异性的研究

利用电子束蒸发方法将MgB₂超导薄膜沉积到Al₂O₃(001)衬底上.采用标准的四引线法研究了磁场平行和垂直超导薄膜ab平面下的电阻转变.一个激活能模型 U(T,H)=U₀(1-T/(T_c+δ))ⁿ(1-H/H 关键词： 2/Al₂O₃')" href="#">MgB₂/Al₂O₃ 超导体 电阻转变 各向异性     

Coherence transition in granular YBa2Cu3O7-δ, YBa2Cu2.95Zn0.05O7-δ, and YBa1.75Sr0.25Cu3O7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa₂Cu₃O_7-δ, YBa₂ (Cu_2.95Zn_0.05)O_7-δ and Y(Ba_1.75Sr_0.25)Cu₃O_7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature T_c, where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature T_c0. This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of T_c, but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa₂Cu₃O_7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition.     

Thermal transport in the oxygenated magnetic superconductor, Eu1.5Ce0.5RuSr2Cu2O10+δ

The thermal conductivity and thermopower are reported for a hole doped Eu_1.5Ce_0.5RuSr₂Cu₂O_10+δ sample that has been annealed at 1100 K under an oxygen pressure of 54 atm. At T_c=45 K superconductivity and weak ferromagnetism coexist (T_m=180 K). Weak features in the thermopower, S(T), and thermal conductivity, κ(T), are observed both at T_m and at T^*=140 K. The thermopower begins to decrease sharply toward zero at T_c, and there is an extremely sharp increase of about 30% in the thermal conductivity at T_c. This “first order” transition may be related to the sudden appearance of a spontaneous vortex phase at T_c. A small shoulder is observed in κ(T) in the temperature range T=5–13 K.     

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.     

Crossover effects and finite-size scaling on the temperature dependence of paraconductivity in YBa2Cu3O6.9 and Bi2Sr2CaCu30x compounds

An investigation into the superconducting order parameter thermodynamic fluctuations and their manifestations on paraconductivity in cuprate superconductors is done using a renormalized Gaussian approach based on the Ginzburg–Landau theory. The temperature dependence of paraconductivity is affected by repulsive interactions between Cooper pairs and does not follow the universal power laws predicted by the conventional Aslamazov–Larkin theory. In addition to the well known Lawrence–Doniach crossover from three to two dimensions, we also highlight the crossover from one-dimensional to two-dimensional behavior and the crossover from weak two-dimensional to strong two-dimensional critical behavior in the vicinity of the critical temperature. These dimensional crossovers result from the resistance between Cooper pairs due to critical and thermal fluctuations which cause a transition from a metastable state to one with a smaller current. Two illustrative examples (the cases of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.9}$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_3{\mathrm{0}}_x$ compounds) are provided in support of the analysis, so as to demonstrate the usefulness of the approach.     

Mechanisms of the pancake vortex and vortex line movement in the high Tc superconductors Bi2Sr2CaCu2O8 δand La1.93Sr0.07CuO4 δ

In the strict sense, it is not very clear why with magnetic field increasing, the normal-superconductive (NS) transition becomes broad for Bi2Sr2CaCu2O8 δ(Bi2212) while the NS transitions are almost parallel for La1.93Sr0.07CuO4 δ(La214). In this paper, R-T relations are measured by the six-probe method. We propose a moving mechanism of the pancake vortex and vortex line for Bi2212. The theoretical curves fit the experiment data well.     

Origin of the metal-insulator transition in the superconducting series Bi2Sr2Ca1−xYxCu2O8+δ

The metal-insulator transition in the solid solution Bi₂Sr₂Ca_1−xY_xCu₂O_8+δ (0≤x≤1) has been investigated by TGA (oxygen content) and by X-ray absorption spectroscopy (Bi and Cu valence states). Resistivity and AC magnetic susceptibility measurements have shown that the superconducting properties and the metallic behavior vanish for x>0.55. The oxygen content δ is larger than x/2 for x≤0.3 and smaller than x/2 for x≥0.6. For x=0, the Cu K edge shows a shift towards high energy with respect to the Cu(II) oxide La₂CuO₄; this shift decreases with increasing x in agreement with the decrease of the doping hole density and the variations of the physical properties. For 0≤x≤0.3, the Bi L₃ edge shows a shift of 1 eV towards low energy with respect to the Bi(III) oxide Bi₂O₃ in agreement with the charge transfer between [CuO₂]_∞ and [BiO]_∞ planes. This shift also decreases with increasing x, but is still present for the x=0.6 composition for which δ is smaller than x/2. A model of the metal-insulator transition in this series is proposed based on the fact that the intercalation of excess oxygen raises the bottom of the Bi-O band with respect to the Fermi level and decreases the contribution of the Bi-O electron pocket to the hole density.     

Influence of flat milling on vortex matching effect in Bi2Sr2CaCu2O8+y with antidot array

Significant enlargements of antidot diameter by Ar-ion milling were observed in Bi₂Sr₂CaCu₂O_8+y single-crystal films with antidot arrays as well as the thinning of the films. In an original sample with triangular array of antidots, whose diameter is about 200 nm, a few dip structures by the matching effect were observed in the vortex-flow resistance as a function of magnetic field. With increasing the milling time, the number of the dips increases and the appearance of the flow resistance becomes periodic oscillations. These features can be explained mainly by the increase of the antidot diameter.     

Tunneling studies on single crystals of superconducting Bi2Ca1−xYxSr2Cu2O8+δ

Tunneling studies have been carried out on single crystals of Bi₂Ca_1−xSr_xCu₂O_8+δ over a wide range of compositions wherein the hole concentration varies by a factor of 2.5. The 2Δ value varies between 25 meV and 75 meV over the composition range studied, but scales with 2Δ/k_BT_c≈9.5 throughout.     

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 Kand 87 Kfortwosamplesofdifferentorigin) 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.     

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂Ca₂Cu₃O₁₀ whose T_c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency ν_Q. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T_2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.     

Effect of lead addition to Bi-based 2222 superconducting cuprates Bi2-xPbxSr2(Y, Ce)2Cu2O10+δ

Bi_2-xPb_xSr₂(Y, Ce)₂Cu₂O_10+δ, a new family of lead containing Bi-2222 compounds has been synthesized and effects of annealing in high pressure of oxygen studied. Lead-free Bi₂Sr₂(Y, Ce)₂Cu₂O_10+δ synthesized under flowing oxygen is known to be non-superconducting, but annealing under high oxygen pressure (100 bar) at 500°C, induced superconductivity with T_c=25 K. On substitution of lead (x>0.4), the superconductivity appears even in samples synthesized under flowing oxygen (1 bar). T_c increases with oxygen annealing pressure for all compositions, and it also increases with Pb content at a given oxygen pressure. These findings are discussed.     

Studies on fabrication of Ag/Tl2Ba2Ca2Cu3O10/CdSe heterostructures with electrochemical technique

The Ag/Tl₂Ba₂Ca₂Cu₃O₁₀/CdSe heterostructure was fabricated at room temperature by soft electrochemical processing technique for the first time. The formation of the heterostructure with non-diffusive interfaces was confirmed by X-ray diffraction. The crystallite sizes determined for Tl-2223 and CdSe films were 33 nm and 25 nm, respectively. The Tl₂Ba₂Ca₂Cu₃O₁₀ film electrodeposited onto Ag-substrate has shown the superconducting transition temperature T_c at 116.5 K and J_c = 2.1 × 10³ A/cm². These values were found to improve after the deposition of CdSe onto Ag/Tl-2223 films. The effect of red He-Ne laser irradiation on the superconducting properties of heterostructure are studied and discussed at length in this paper.     

Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12−y high temperature superconductors

Measurements of the a.c.susceptibility (χ=χ′+iχ″) have been made on the Mg substituted high T_C superconducting system, CuBa₂(Mg_xCa_1−x)₃Cu₄O_12−y (Cu-1234) with x=0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(J_C) made from the field dependent χ″-T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (γ=ξ_ab/ξ_c) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties.     

Crystal structure of Tl0.5Pb0.5Sr2Ca2Cu3O9 at 300 K and around Tc (118 K)

The crystal structure including the cation distribution, of a polycrystalline sample of nominal composition Tl_0.5Pb_0.5Sr₂Ca₂Cu₃O₉ with T_c = 118.2 K has been determined using resonant synchrotron X-ray diffraction data collected at the Cu K, Tl L_III and Sr K edges and time-of-flight powder neutron diffraction data. No oxygen deficiency was observed, but cation disorder at all the non copper sites according to the formula (Tl_0.60Pb_0.40)(Sr_1.60Ca_0.40)(Ca_1.93Tl_0.07) Cu₃O₉ gives a mean hole concentration of 0.18(1) per Cu atom for the three CuO₂ planes, consistent with the high T_c for this material. Analysis of five time-of-flight powder neutron diffraction data between 80 and 150 K have revealed a possible discontinuity in the variation of the c lattice parameter at T_c, due to an anomaly in the position of the apical oxygen atoms.     

Combinative energy, oxygen deficiency and superconductivity in LnBa2Cu3O7-x （Ln=Nd, Er, Sm）

The correlation among the combinative energy, superconductivity, oxygen content, the position of holes in different planes, and the position of holes in the Cu(2)-O plane in LnBa2Cu3O7-x (Ln=Nd, Er, and Sin) has been investigated on the basis of a block model. The results indicate that the combinative energy decreases with increasing Tc in all of these compounds. And also, the combinative energies are obviously different with holes at different positions in the Cu(2)-O plane when the oxygen deficiency is low. However, this difference becomes less with increasing the oxygen deficiency.The effect of the holes in different positions on the combinative energy supplies some clue to the understanding of an unresolved problem, i.e. whether the distribution of carriers in the CuO2 plane is uniform or inhomogeneous. The results not only show that the structural characters, the combinative energy between two structural blocks and the superconductivity are closely interrelated in this class of compounds, but also reveal some differences among these systems. The relationship between the combinative energy and the Tc value in NdBa2Cu3O7-x shows some features different from the systems containing Er or Sm.