Bi-Sr-Ca-Cu-O Research Articles

The true pinning force range in Bi 2Sr 2CaCu 2O 8+ δ single crystals at low temperatures

We study the effective profile of the pinning potential well in Bi 2Sr 2CaCu 2O 8+ δ single crystals using transverse ac magnetic susceptibility. The results show that the harmonic approximation for the potential well (linear regime) is valid up to vortex displacements r∼ ξ, where ξ is the coherence length. In spite of this, it is necessary to induce displacements of the vortex structure much larger than ξ to depin it from its potential. This is shown to take place even in the single vortex limit.

Evaluation of junction parameters with control of carrier concentration in Bi 2Sr 2CaCu 2O 8+ δ stacked junctions

The control of the critical current density ( J C) and the junction resistance ( R N) along the c axis of intrinsic Josephson junctions (IJJs) on a high- T C superconductor is very important for applying the IJJs to electronic devices. For controlling these junction parameters, we have clarified the relationship of J C, R N, and the carrier concentration in Bi 2Sr 2CaCu 2O 8+ δ whiskers by changing the carrier concentration with an annealing process. We determined the electrical transport characteristics of the IJJs. As a result, the J C increased, and the R N decreased systematically when the carrier concentration increased. The values of J C and R N could be controlled by a change in the carrier concentration.

Characteristics of two-stacked intrinsic Josephson junctions with a submicron loop on a Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) single crystal whisker

We report on serial dc superconducting quantum interference devices (dc SQUIDs) arrays of two-stacked intrinsic Josephson junctions (IJJs) on a Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) whisker with a submicron hole fabricated by a 3-D focused ion beam (FIB) etching method. The junction area, the number of elementary junctions, and the loop area of the SQUIDs are 3, 20, and 0.42 μm 2, respectively. The SQUIDs show typical I– V characteristics of IJJs with a critical current I C of 12 A at 4.2 K. The modulation parameter β L(=2 I C L/ Φ 0) is 6.4, and the inductance L of the SQUID is 1120 pH. That includes the inductances of the elementary junction around a SQUID loop. In a strong magnetic field, the Fraunhofer pattern is obtained and well fitted to the calculated value from the junction size; Δ B= Φ 0/ tW; t is the interlayer distance; and W is the junction width. In a low magnetic field, modulation depth of approximately 10% of the I C corresponding to the value of β L is observed. The modulation period of 80 G approximately agrees with the value calculated from the SQUID.

Characteristics of intrinsic Josephson junctions using a Bi 2Sr 2CaCu 2O x thin film estimated overall in a chip

Integrated test chips of Bi 2Sr 2CaCu 2O x thin-film-type intrinsic Josephson junctions (IJJs) are fabricated, and the yield of showing proper IJJ characteristics is examined. The thin-film type is expected to be suitable for applications to integrated circuits using THz electromagnetic waves, but its low fabrication yield is an issue to be solved. The chip size is 12 × 12 mm 2 in which 90 IJJs are included. Bi 2Sr 2CaCu 2O x thin films are prepared via two steps. First the films are deposited on a low temperature (200 °C) substrate by a precisely controlled evaporation method. Next they are annealed at high temperatures (875 and 878 °C). The annealing time is varied to find an optimum condition. The best chip has a 70% yield of showing multiple-branch structure that is the most typical current vs voltage characteristics of IJJs. The relationship between the yield and the surface smoothness and the IJJ size dependence of the yield are also discussed.

Temperature dependence of composition ratio of Bi 2Sr 2CaCu 2O 8+ δ film by PLD method

We have prepared Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) thin films by a pulse laser deposition (PLD) method followed by post-annealing. The thin films were grown on MgO substrates at 400–630 °C and 100 mTorr oxygen. They were about 200 nm thick. The crystalline structure of thin films deposited at lower temperature (∼470 °C) was amorphous. The films deposited at higher temperature (∼550 °C) began to crystallize ( c-axis oriented) but it turned out that Bi decreased with an increase temperature. After deposition, post-annealing were performed at 860–880 °C in a tubular furnace with mixed gas (Ar:O 2 = 93:7, 1 atm, 0.2 l/min flow) for 3 h. Bi did not decrease with the increase of post-annealing temperature up to 870 °C. The critical temperature of film was 78 K, and the root-mean-square (RMS) roughness in 10 μm square was less than 10 nm.

Influence of some growth parameters upon the crystal growth of Bi-2212 family superconductor by traveling solvent floating zone method

Crystals of Bi 2Sr 2CaCu 2O y , Bi 2Sr 2PrCu 2O y and Bi 2Sr 2GdCu 2O y were grown using the traveling solvent floating zone (TSFZ) method, with the dimensions of approximately 6 mm×2 mm×0.2 mm . The primary crystallization field (PCF) was determined by analyzing the composition of the melt zone after self-adjusting during growth. Relative to Bi-2212 solid solution region, the PCF of Bi 2Sr 2PrCu 2O y shifted towards the CuO-rich corner, and the PCF of Bi 2Sr 2GdCu 2O y was located approximately at the BiO 1.5-rich and CuO-rich side, in the pseudo-ternary BiO 1.5–(Sr, Pr, Gd)O–CuO phase diagram. Such variation in solvent compositions may significantly influence the segregation behavior of each constituent so as to influence the maximum stable growth rates of each compound. The minimum critical values of G/ R are 5.39×10 11 Ksm −2 of Bi 2Sr 2GdCu 2O y , 4.5×10 11 Ksm −2 of Bi 2Sr 2CaCu 2O y , and 2.94×10 11 Ksm −2 of Bi 2Sr 2PrCu 2O y . These data obtained from fundamental estimations are consistent with experimental results, indicating a decreasing behavior of growth difficulty.

The study of inter and intragranular pinning behavior of oxygen irradiated textured polycrystalline Bi 2Sr 2CaCu 2O 8+ δ and Bi 1.84Pb 0.34Sr 1.91Ca 2.03Cu 3.06O 10+ δ superconductors

Inter and intragranular pinning potentials of 116 MeV oxygen ion irradiated textured polycrystalline Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) and Bi 1.84Pb 0.34Sr 1.91Ca 2.03Cu 3.06O 10+ δ (Bi-2223) have been studied from dc magnetization at low (50 Oe) and high (10,000 Oe) fields respectively. There is an increase in intergranular pinning potential ( U 0) in Bi-2223 in contrast to anomalous decreases in Bi-2212 with increasing dislocation density. Intragranular pinning potential is governed by the concentration of vacancy type defects rather than its size as observed in proton irradiation. Pinning has been explained in the light of statistical dilute pinning model.

Magneto-optical imaging of magnetic flux distribution in high- Tc superconductors

Prototype systems of home-made magneto-optical microscopes were fabricated, and preliminary studies were carried out using Bi 2Sr 2CaCu 2O 8+ δ single crystals and an YBa 2Cu 3O 7− δ superconductor vortex flow transistor. In the study using BSCCO crystals, we succeeded in the observation of magnetic flux penetration into half-peeled thin flake region on the crystal surface, and it was found that the magnetic fluxes penetrate in characteristic one-dimensional alignment almost along the crystal a-axis. On the other hand, in the study using the YBCO device clear changes in the generated magnetic field distribution could be detected corresponding to the current direction.

Vortex crossing lattice phase transition in single crystalline Bi 2Sr 2CaCu 2O 8+ δ

The vortex state in highly anisotropic overdoped Bi 2Sr 2CaCu 2O 8+ δ superconductor with T c=84.5 K has been studied by the local ac-permeability ( μ= μ ′−i μ″) measurements in magnetic fields tilted away from the c-axis to the ab-plane. In the present measurements, we have observed the vortex lattice melting transition, the peak effect, and several anomalies of the local ac-response in the vortex solid phase that suggest the novel phase transitions in the vortex crossing lattice state.

Thermodynamic assessment of the BiO x–SrO–CaO system

The BiO x –SrO–CaO system is the most complex of the four ternary systems forming the BiO x –SrO–CaO–CuO x system where the superconducting phases Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2Ca 2Cu 3O 10+ δ are found. The BiO x –SrO–CaO system has received relatively little attention since it does not contain any superconducting phases itself. Several of the phases in this system appear during processing of superconductors, though. Using the Calphad approach, we have assessed the experimental data of this system and optimised a set of thermodynamic parameters from which the phase diagram and any thermodynamic property can be calculated. The experimental data available are quite scattered and often contradictory, but the thermodynamic description presented here is consistent with most of what is known about this system.

Thermodynamic calculation of the BiO 1.5–CaO–CuO x system

The BiO 1.5–CaO–CuO x system is by far the simplest of the four ternary systems forming the BiO x –SrO–CaO–CuO x system where the superconducting phases Bi 2Sr 2CaCu 2O 8+ δ and Bi 2Sr 2Ca 2Cu 3O 10+ δ are found. The only solution phase extending into the system is the liquid phase. Using a single interaction parameter the experimentally determined [M.-L. Carvalho et al., Physica C 336 (2000) 1] eutectic temperature at 1024 K in 1 bar O 2 could be fitted and the complete phase diagram calculated.

E– J properties of Bi 2Sr 2CaCu 2O 8 thick films

In order to evaluate the effect of the microstructure on E– J properties, the E– J properties for Bi2212 thick films prepared under three different conditions were measured. The measured E– J properties were analyzed based on the percolation model, and the local J c ( J cl) distribution was evaluated. The distribution of the local J c was much affected by its microstructures. For the well aligned sample with large plate-like grains, which is similar to Ag-sheathed Bi2212 tape, the distribution of the local J c at percolation transition field B GL was found to be asymmetric and reached the high J cl region.

Observation of vortex states in Bi 2Sr 2CaCu 2O 8+ y under tilted fields by Bitter decoration

We studied vortex states in Bi 2Sr 2CaCu 2O 8+ y under tilted fields by means of Bitter decoration. The results in large in-plane fields up to 3000 Oe show zigzag arrangement of vortices in addition to the chains. A possible origin of this novel arrangement of vortices is discussed.

Josephson vortex states in Bi 2Sr 2CaCu 2O 8+ δ obtained from vortex flow resistance measurements

We found the periodic oscillations in Josephson-vortex-flow resistance with the current perpendicular to the Cu–O layers of Bi 2Sr 2CaCu 2O 8+ δ . As the flow resistance is closely related to the configurations of the Josephson vortices, we have observed clear boundaries between the triangular-lattice phase and others. It is strongly suggested that the flow resistance measurements open the window to investigate magnetic phase diagram of Josephson vortices in strongly anisotropic high T c superconductors.

Doping dependence of crossing vortex lattice in Bi 2Sr 2CaCu 2O 8+ y

Local magnetization hysteresis is studied in a highly anisotropic superconductor Bi 2Sr 2CaCu 2O 8+ y under tilted magnetic fields using a micro Hall probe. At moderately high temperatures, we found novel multiple peak structures in the local magnetization hysteresis curves. These peaks appear only in the increasing branch of hysteresis curves when a magnetic field is applied nearly parallel to the superconducting plane. Possible origins of the novel peak structures are discussed on the basis of the evolution of the peaks with temperature.

Observation of quantized vortices by cryocooler-based scanning Hall probe microscope

We have developed a scanning Hall probe microscope (SHPM) system utilizing closed-cycle cryocooler. The Hall probe used in this system is fabricated from a GaAs/GaAlAs two-dimensional electron gas. A stepping-motor-driven XYZ translator is used with a resolution better than 0.1 μm and maximum scan range of 20 × 20 mm 2. The spatial resolution of the system is about 5 μm and magnetic resolution is about 100 mG. By using this system, we have successfully resolved the quantized vortices on the cleaved surface of Bi 2Sr 2CaCu 2O 8+ y single crystal.

Non-monotonous temperature dependence of the second peak effect in an excessively-oxygen doped Bi 2Sr 2CaCu 2O 8+ δ single crystal

A study has been carried out on the second peak effect of Bi 2Sr 2CaCu 2O 8+ δ single crystal with excessive oxygen doping based on its magnetization data. The low temperature ( T<30 K) magnetization curve reveals a full valley structure on the temperature dependent curves of both second peak field H sp( T) and its corresponding onset field H on( T), in conjunction with the appearance of a much broader temperature range of the peak effect. The high temperature ( T>30 K) data are shown to exhibit a transition from ordered to glassy flux line state. The data for H on( T) in those two temperature regimes have been nicely fitted with corresponding theoretical models for strong disorder induced effects.

Influence of the bismuth deficit on the structural and electric properties of the Bi 2Sr 2CaCu 2O y thin films synthesized by molecular beam epitaxy

BiSrCaCuO thin films were grown on (1 0 0) SrTiO3 substrates by molecular beam epitaxy (MBE) with variation of the Bi deposition time. A new 2 x212 family with x varied between 1 and 0 was grown. The X-ray study, the Rutherford back scattering (RBS), reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were used to characterize the films. It was shown that the growth method used leads to intergrowth nanostructures. The transport measurements of BiSrCaCuO thin films were performed. The results analysed using the theory of percolation show a 2D character of conductivity in the films studied.

Evaluation of crossing energy between pancake and Josephson vortices in Bi 2Sr 2CaCu 2O 8+ y

In highly anisotropic superconductors, magnetic fields tilted from c-axis give rise to a novel ground state of vortices, where pancake vortices (PVs) and Josephson vortices (JVs) coexist and attract to each other. We have evaluated the crossing energy between PVs and JVs experimentally by comparing it with the line energy of PV stacks in Bi 2Sr 2CaCu 2O 8+ y crystals with artificially introduced parallel grooves on the surface. The obtained value of the crossing energy is slightly larger than the theoretical estimate.

Lower boundary field of the triangular lattice phase in Josephson vortices

The Josephson-vortex flow resistance oscillates as a function of magnetic field when a triangular vortex lattice is formed. In order to study the lower boundary field of the triangular-lattice phase of Josephson vortex, the vortex-flow resistance as a function of magnetic field parallel to the ab-plane was measured over a wide range of temperatures on a Bi 2Sr 2CaCu 2O 8+ y (Bi-2212) single crystal whose oxygen content was controlled by annealing in vacuum. We have found that the starting field H start of the oscillations, which corresponds to the boundary field of the triangular-lattice phase, decreases systematically with increasing anisotropy parameter ( γ) of Bi-2212 at all measured temperatures. This behavior is consistent with various theoretical expectations that the boundary field is inversely proportional to γ.