Effect of Al Substitution on Structural and Electrical Properties of Bi1.6Pb0.4Sr2CaCu2−x M x O8+δ Superconducting Ceramics

In this work we study the effect on structural and electrical properties of superconducting compound Bi_1.6Pb_0.4Sr₂CaCu_2?y M _y O_8+δ were M=Al (with y=0–0.6). The samples were prepared by the solid-state reaction method. The samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), direct current (DC) resistivity versus temperature ρ(T) and alternative current (AC) susceptibility measurements. Structural analysis shows that the crystalline lattice structure of the prepared sample belongs, mainly, to the superconductive tetragonal phase Bi(Pb)2212. The SEM micrographs show that in the undoped sample the grain size has a random distribution with few grains greater than 5 μm. The grains are very dense and well connected. A quite different microstructure is obtained for the doped samples of which grains are more connected with a flat characteristic shape of Bi(Pb)2212 superconductors. All samples exhibit a superconducting character and T _c and the superconducting volume fraction decrease with increasing rate of aluminum.     

Substitution Effect of Pr for Ca in Bi2Sr2CaCu2O8+δ System

The Bi₂Sr₂CaCu₂O₈₊ system samples doped with Pr on Ca sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction, and photoemission experiments have been performed. Both X-ray diffraction and photoemission measurements show that Pr ion enters the lattice with a valence higher than 3⁺, which supports the hole-filling mechanism of the suppression of superconductivity.     

Atomic-plane-selective STM imaging and STS on the Bi2Sr2CaCu2O8+δ cleaved surface

In STM experiments on Bi₂Sr₂CaCu₂O₈₊ .     

Theory of the Phase-Sensitive Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junction Experiments

We studied theoretically the phase-sensitive c-axis Bi₂Sr₂CaCu₂O_8+δ (BSCCO) bicrystal twist Josephson junction experiments of Li et al. (Phys. Rev. Lett. 83, 4160 (1999)), calculating the critical current as a function of the twist angle in a wide variety of models. The data provide strong evidence that the c-axis tunneling in BSCCO is incoherent, and that the order parameter is s-wave for T ≤ T _c.     

Necking Effect on the Growth of Bi2Sr2CaCu2O8+δ Single Crystal by TSFZ Method

High-quality Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals have been grown by a travelling solvent floating zone method with a necking technique using a single crystal seed. The full-width at half maximum of the X-ray rocking curves was improved from 120 arcsec to 70 arcsec by the necking technique. The topographic results show that the crystals grown using the necking technique have better crystallinity. Especially, this technique is very effective on the growth of the atomically substituted single crystals. It results not only in the improvement of crystallinity but also in the enhancement of the single crystal size, especially for the crystals substituted by other elements.     

High-temperature X-ray Study of Melting and Solidification of the Bi2Sr2CaCu2O8+δ on Silver

The melting and solidification of a Bi ₂ Sr ₂ CaCu ₂ O ₈₊ compound on Ag were studied by in-situ high-temperature x-ray analysis. Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag samples were heated by the heating schedule known as partial melting and slow cooling under a 1-100%-O ₂/He gas flow. A strong c-axis alignment of the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ crystals was observed at the incongruent-melting temperature where the Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and the molten phase coexist. Similarly, a strong c-axis alignment was observed at the initial stage of solidification. However, the subsequent solidification of the residual molten phase caused less aligned Bi ₂ Sr ₂ CaCu ₂ O ₈₊ on Ag. An improved heating schedule is proposed which increases the critical current of Bi ₂ Sr ₂ CaCu ₂ O ₈₊/Ag superconducting tapes.     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

Substitution Effect of Sn for Cu in the Bi2Sr2CaCu2−xSnxO8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Sn on Cu sites were synthesized. Resistivity temperature dependence, X-ray powder diffraction and photoemission experiments have been performed. Suppression of superconductivity in the Sn-doped Bi₂Sr₂CaCu₂O_8+δ system has been observed. X-ray photoemission measurements show that Sn ion enters the lattice. Our results reveal that Sn ions are mostly in bivalent states.     

Atomic force microscopy observation of Bi2Sr2CaCu2O8+δ thin films prepared by molecular beam epitaxy

We observed the surface structures of 1200 Å-thick c-axis oriented Bi₂Sr₂CaCu₂O₈₊ thin films, which we intend to use for the base electrode of a planar type Josephson junction. The films were prepared by molecular beam epitaxy on the Nd:YAlO₃(001)substrates at a growth rate of 20 Å/min.Atomic force microscopy observation revealed that the surface of the films prepared at a substrate temperature of 780 °Cand an ozone pressure of 2.3 × 10^–3 Pahas island-like structures. Their shape is like a stack of cylindrical plates and the diameter of the island is typically 250 nm.Most of the steps have a height of 15 16 Å,which corresponds to the primitive-cell thickness of Bi₂Sr₂CaCu₂O8+.We discuss the result in comparison with that observed in Bi₂Sr₂Ca₂Cu₃O₁₀₊ films grown by metal organic chemical vapor deposition and YBa₂Cu₃O_7– films by sputtering. The occurrence of the structure with several steps is attributed to the insufficient supply of elements. The roughness less than 100 Åis favorable for the fabrication of junctions with thin barriers.     

Distribution of the Impurity Resonance Energy in Bi2Sr2CaCu2O8+δ Observed by Scanning Tunneling Spectroscopy

Using scanning tunneling microscopy and spectroscopy (STM/STS) at 4.2 K, we observed Zn impurity resonance states in single crystals of Zn substituted Bi₂Sr₂Ca(Cu_1?x Zn _x )₂O_8+δ . Zn impurity resonances were characterized by the peak structure in the local density of states (LDOS). The peaks were appeared near the E _F on each of Zn impurity sites. The peak energy showed a wide distribution from ?7 to 5 meV. The average peak energy was ?1.5 meV and the variance was 1.8 meV. We investigated the correlation between the peak energy at a Zn site and the energy gap value in the region where the Zn site is embedded. Although we observed about hundred Zn sites, no correlation between the local energy gap and the peak energy was observed.     

Study of Current-Voltage Characteristics of the Mesa-Structured Bi2Sr2CaCu2O8+δ Single Crystals under Magnetic Fields

To study on the interlayer coupling between Cu-O superconducting layers in Bi ₂ Sr ₂ CaCu ₂ O ₈₊ single crystals, we have performed the current-voltage measurements along the c-axis in the magnetic fields perpendicular and parallel to the axis at 4.2K. A pronounced effect of magnetic fields to the current-voltage characteristics appears in the multiple-branch behaviors, when the magnitude of the parallel field exceeds the 2D-3D dimensional-crossover field B _2D . This is considered as a consequence of missing a long-range order of the correlation of the vortex lines along the c-axis.     

The Peculiarities of Magnetic Flux Dynamics at Magnetothermal Instability in Textured Bi2Sr2CaCu2O8+δ

The magnetic flux dynamics was investigated in a textured Bi₂Sr₂CaCu₂O₈₊ sample using a miniature Hall probe placed on the surface of the superconductor. The process of magnetic flux penetration into the superconductor was found to have a complex structure and consist of two processes with different time scales. The first one has a length of tens of microseconds and is considered as locally adiabatic. The second one has a length comparable with the time of thermal diffusion. The importance of the above two processes in the development of magnetic instability is analyzed.     

c-axis Superfluid Response and Quasiparticle Conductivity in Bi2Sr2CaCu2O8+δ and Bi2Sr2CuO6+δ

Josephson plasma resonance for underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and Bi ₂ Sr ₂ CuO ₆₊ have been measured by sweeping the microwave, frequency continuously. The resonance enables us to determine the superfluid density and quasiparticle conductivity in the c-axis accurately. We show that the. superfluid response and the low energy excitations out of the condensate in the c-axis of these materials are very different from those in the ordinary Josephson multilayer tunnel junctions. The Josephson coupling energy in single layer Bi ₂ Sr ₂ CuO ₆₊ is more than 5000 times smaller than is predicted in interlayer tunneling model.     

Bi2Sr2CaCu2O8+δ c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry

Li et al. found that the critical current density across atomically clean c-axis twist junctions of Bi ₂ Sr ₂ CaCu ₂ O ₈₊ is the same as that of the constituent single crystal, , independent of the twist angle ₀ , even at and below T _c . We investigated theoretically if a -wave order parameter might twist by mixing in d _xy -wave components, but found that such mixing cannot possibly explain the data near to T _c . Combined with group theoretical arguments, we then conclude that the order parameter contains at least a substantial s-wave component, but does not contain any purported -wave component, except possibly below a second, unobserved phase transition. By studying tunneling models, we further conclude that the intrinsic c-axis Josephson tunneling in Bi ₂ Sr ₂ CaCu ₂ O ₈₊ is likely to be mostly incoherent.     

Synthesis of Bi2Sr2CaCu2O8+δ Single Crystals by Alumina Crucible

Journal of Superconductivity and Novel Magnetism - The contamination by alumina crucible and the effect of Al impurity on the crystal growth of Bi2Sr2CaCu2O8+δ are always a thorny problem. In...     

Effect of the Oxidation State of Pb on the Superconducting Transition Temperature of Bi2 – x Pb x Sr2CaCu2O8 + δ

A procedure is developed for determining the Pb⁴⁺ concentration in Pb-substituted Bi-based 2212 superconductors. The effect of the Pb⁴⁺ : Pb²⁺ ratio on the superconducting transition temperature of 2212 is studied. The results indicate that the highest T _c is offered by Bi_1.8Pb_0.2Sr₂CaCu₂O_{8 +} containing the smallest amounts of Pb and Cu in their higher oxidation states.     

Effect of Ce Substitution on the Magnetoresistivity and Flux Pinning Energy of the Bi2Sr2Ca1−x Ce x Cu2O8+δ Superconductors

In this study, the effect of Ce doping on the properties of Bi₂Sr₂Ca_1?x Ce _x Cu₂O_8+δ ceramic superconductors, with x=0.0, 0.01, 0.05, 0.1, and 0.25, has been investigated. Samples’ precursors were prepared using the conventional solid state method and subsequently textured using the Laser Floating Zone technique. The magnetoresistance measurements were studied under various applied magnetic fields. The activation energies, irreversibility fields (H _irr ), upper critical fields (H _c2) and coherence lengths at 0 K (ξ(0)) were calculated from the resistivity versus temperature (ρ–T) curves, under DC magnetic fields up to 5 T. The thermally activated flux flow model has been applied in order to calculate the flux pinning energies. The results indicated that H _c2(0) varied from 416.19 to 115 T and the flux pinning energies varied from 1.46 to 0.042 eV at 0 T.     

Systematic Magnetization Measurements on Single Crystalline Bi2Sr2CaCu2O8+δ with Columnar Defects

We have performed systematic magnetization measurements on single crystalline Bi ₂ Sr ₂ CaCu ₂ O ₈₊ with columnar defects of B = 0.005 to 1 T by using a SQUID magnetometer. Magnetization hysteresis curves of the pristine sample show a weak irreversible behavior in the vortex liquid state, suggesting the existence of the new vortex state in the vortex liquid state. This weak irreversible region persists systematically in the samples with columnar defects even up to B = 1 T. It is shown that the weak hysteresis of magnetization is sensitive to the disorder level of the sample and shifts systematically to higher temperature and field region with increasing the number of columnar defects. This behavior clearly indicates that effective pinning mechanism exists even in the vortex liquid state and generates a finite critical current.