Bi2Sr2CaCu2O8 Single Crystals Research Articles

Role of Density of States in the Fluctuation Conductivity of Bi2Sr2CaCu2O8 and Tl2Ba2CaCu2O8 Single Crystals

The out-of-plane resistivity ρc (T, B) of Bi2Sr2CaCu2O8 and Tl2Ba2CaCu2O8 single crystals has been measured in weak magnetic fields applied perpendicular to the ab-plane. The measurement show a well pronounced peak at a temperature Tm. The external magnetic field shifts Tm towards lower temperatures and increases the magnitude of the peak. We demonstrate that the ρc (T, O) is well described by the fluctuation theory, developed by Dorin et al., in the Gaussian approximation over a wide range of temperature, but in the presence of field, fluctuations are better described in Hartree approximation.

ANISOTROPIC RESISTIVITIES OF SINGLE CRYSTAL Bi2Sr2CaCu2O8+δ WITH DIFFERENT OXYGEN CONTENTS

The in-plane resistivity ρab(T) and the out-of-plane ρc(T) have been extensively measured for pure single crystal Bi2Sr2CaCu2O8+δ(Bi2212) annealed at different oxygen pressures.The ρc(T) and anisotropy (ρc(T)/ρab(T)) decrease rapidly with increasing carrier concentration.It is found that the out-of-plane resistivity decreases linearly with decreasing temperature down to about 120K for the overdoped sample,its resistivity anisotropy is of weak temperature dependence.The data of ρc(T) and ρc(T)/ρab(T) are well fitted by the bipolaron theory proposed by Alexandrov and Mott.

Improvement of the vortex pinning properties by increasing the size of columnar defects

The influence of the size of columnar defects on pinning properties of high-Tc superconductors has been investigated. Bi2Sr2CaCu2O8 single crystals (Bi-2212) were irradiated with heavy ions using different experimental conditions. The irradiation induced defect morphology was studied by High Resolution Electron Microscopy (HREM). Different values of columnar defect radius, R, were achieved by changing the electronic stopping power and the incident ion energy. Pinning properties of columnar defects were studied through magnetic measurements (H→//c→). It is shown that pinning properties are improved by the enlargement of defect size. A new strategy was also used: by tilting the crystal during the irradiation process, columnar defects of elliptical section in the (a→,b→) planes were introduced. The section size of defects in the (a→,b→) planes, Rab, is larger for these tilted samples than for defects parallel to the c→ axis. It is shown that such a larger size of the track projections on the (a→,b→) planes can also enhance the pinning efficiency. The enhancement of pinning properties observed for larger Rab could be directly related to the enhancement of pinning energy with Rab.

Suppressed rf dissipation in Ag17+107 ion irradiated Bi2Sr2CaCu2O8 single crystals by enhanced flux line tilt modulus

We have studied the isothermal, magnetic field (H‖c) dependent rf power P(H) dissipation (Hrf‖a) in the superconducting state of Bi2Sr2CaCu2O8 single crystals prior to and after irradiation with 250 MeV Ag17+107 ions. In the pristine state, P(H) shows an initial decrease with increase in field, reaches a minimum at HM(T) and increases monotonically for H>HM(T). This behavior arises when the electromagnetic coupling between the pancake vortices in adjacent CuO layers becomes dominant on increasing the field and minimizes the distortions of the flux lines by confining the 2D vortices. In the post irradiated state, such an initial decrease and the minimum in P(H) is not observed but only a much reduced rf dissipation that monotonically increases with field from H=0 onwards is seen. We attribute this difference to the strong enhancement of the tilt modulus C44 of the flux lines on irradiation when the pancake vortices in adjacent CuO bilayers are pinned along the track forming a well-stacked flux line in the field direction (‖c). We have also observed that the rf dissipation disappears at a certain temperature Tsf, at which the normal core of the flux line becomes commensurate with the columnar track diameter.

Role of c-axis field components in making the transport E(J) characteristics of (Bi,Pb)2Sr2Ca2Cu3Ox tapes insensitive to the direction of field within the tape plane

We report extended (over four decades) electric field versus current density characteristics of (Bi,Pb)2Sr2Ca2Cu3O10 tapes in magnetic fields applied along each of the three principal tape axes at 77 K, and along the two principal axes in the tape plane at 4.2 K. The characteristics are almost independent of field direction in the tape plane at both temperatures. Similar insensitivity of the critical current to magnetic field direction observed in other polycrystalline high temperature superconductors has been ascribed to the percolative nature of current flow, while the insensitivity in (Bi,Pb)2Sr2Ca2Cu3O10(2223) films and Bi2Sr2CaCu2O8 single crystals has been attributed to c-axis conduction across Josephson junctions. We show that the insensitivity of the critical current density to the field direction of these tapes follows naturally from the fact that the dissipation depends only on the field component lying along the c axis of individual 2223 grains. Lorenz-force driven motion of the pancake vortices that form within each 2223 grain provides a dissipation mechanism that involves only the c-axis components of the field. The angular dispersion of the grains in the tape ensures a c-axis field component in most grains, regardless of the applied field direction. This produces the remarkable insensitivity of E(J) over some four orders of magnitude of the electric field.

Electrical Resistivity of Iodine-Intercalated Bi2Sr2CaCu2O8 Subjected to a Magnetic Field

In this work we report on the intercalation of single crystals of Bi2Sr2CaCu2O8 with iodine to form the stage-1 intercalated complex IBi2Sr2CaCu2O8, and on the electrical resistivity of this final product, both as a function of temperature, T, (between 10 K and 300 K) and magnetic field, H, (going up to 15 T). Both ρ ab , the resistivity in the ab-plane, and ρ c , the resistivity along the c-axis have been studied for H // ab and H // c. We find that the metallic character of electrical conduction along the c-axis observed in the normal state in intercalated samples (as compared with the semiconducting character in the pristine host) is maintained when a magnetic field is applied, confirming that, in the normal state, a reduction in the anisotropy accompanies the intercalation process. In the superconducting state, however, we find that the magnetic field causes a broadening of the superconducting transition (as in non-intercalated Bi2Sr2CaCu2O8), especially when H // c, indicating that the superconducting state retains a highly anisotropic character after intercalation. The resistivity data in the flux-flow regime have also allowed us to estimate values of the activation energy for vortex movement.

Effect of Lower Critical Fields on Magnetization of a Single-Crystal Bi2Sr2CaCu2O8+d Bulk Superconductor.

To test the utility of the modified Kim-Anderson critical-state model for study of the magnetic response of ceramic superconductors, we measured the initial magnetization, hysteresis loops and ac complex susceptibilities Xn=X'n-iXn(n =1, 2, 3, 5) of a single-crystal Bi2Sr2CaCu2O8+d bulk sample. The results were simulated in the framework of the model. Analyses of Xn have shown that generation of the higher harmonics can be qualitatively reproduced, where the material parameters k and B0, surface barriers ΔH and lower critical fields Hcl, involved were used as fitting parameters. Comparison of the observed data with the calculated results has revealed that nonzero values of Hcl, play an essential role in the magnetic characteristics of the sample.

Evidence of d-wave pairing in the thermal conductivity of YBa2Cu3O7−d and Bi2Sr2CaCu2O8 single crystals

The electronic contribution to the thermal conductivity of a d x 2−y 2-wave superconductor is calculated within a variational method. By considering the scattering of electrons by point defects and acoustic phonons, the behavior of the thermal conductivity of YBa2Cu3O7−δ and Bi2Sr2CaCu2O8 single crystals can be well described with realistic values of the physical parameters. Besides, the values obtained for the transport electron-phonon coupling constant λ tr lie in the same range of magnitude than those derived from electrical resistivity measurements. b]References

Oxygen displacement in a Ag17+107 ion irradiated Bi2Sr2CaCu2O8 single crystal

We have studied the magnetic field (H∥c) dependent rf dissipation (Hrf∥a) in an as-grown Bi2Sr2CaCu2O8 single crystal prior to and after irradiation with 250 MeV Ag17+107 ions. In a comparison of the responses from the as-grown crystal with an air-annealed crystal, features due to oxygen deficient regions acting as weak links in the former are identified. These features disappear immediately after irradiation of the as-grown crystal. We attribute such behavior to the displacement of oxygen from columnar tracks to deficient regions thus eliminating the weak links. Losses from the same irradiated as-grown crystal stored at 300 K for 60 days show that the features similar but not identical to those observed in the pristine state have reappeared implying that the displaced oxygen is in a metastable configuration in the deficient regions and hence is mobile due to thermal effects even at 300 K.

In-Plane Paraconductivity of Bi2Sr2CaCu2O8 Single Crystals with Different Resistivity Anisotropies

Superconducting fluctuations have been studied by systematic measurements of the in-plane paraconductivity in Bi2Sr2CaCu2O8 single crystals with different resistivity anisotropies. It has been observed that the interlayer coupling strength decreases dramatically with the anisotropy of the sample, which significantly influences the dimensional behaviour of the in-plane paraconductivity. For all the samples studied, the in-plane paraconductivity is in good agreement with the Lawrence and Doniach model. For the sample of highest resistivity anisotropy, the behaviour of the in-plane paraconductivity is basically two-dimensional, but deviates significantly from two-dimensional behaviour as the resistivity anisotropy decreases.

C-axis tunneling between adjacent superconducting CuO2 bi- or trilayers

We have investigated the subgap tunneling characteristics in single crystals of Bi2Sr2CaCu2O8+δ and thin films of Tl2Ba2Ca2Cu3O10+δ with intrinsic Josephson junctions in the resistive state. The low temperature current voltage characteristics (IVC) exhibits a high subgap conductivity. The results are compared to a calculated IVC for tunnel junctions between two identical d-wave superconductors. The calculated quasiparticle branch agrees reasonable with the measured IVC. However, the presence of a small s-wave component can not be excluded. Besides the global IVC we studied fine structures which were observed in the IVC of both materials. These structures can be explained by a resonant coupling of infrared active c-axis phonons to the Josephson ac currents between adjacent pairs of superconducting layers.

Non-resonant rf absorption evidence for reentrant melting of vortex lattice in Bi2Sr2CaCu2O8 single crystals

We have studied the magnetic field dependent rf (20 MHz) losses in Bi2Sr2CaCu2O8 single crystals in the low field and high temperature regime. Above HCl the dissipation begins to decrease as the field is increased and exhibits a minimum at HMHCl. For H>HM the loss increases monotonically. We attribute the decrease in loss above HCl to the stiffening of the vortex lines due to the attractive electromagnetic interaction between the 2D vortices (that comprise the vortex line at low fields) in adjacent CuO bilayers. The minimum at HM implies that the vortex lines are stiffest and hence represents a transition into vortex solid state from the narrow vortex liquid in the vicinity of HCl. The increase in loss for H>HM marks the melting of the vortex lattice and hence a second transition into vortex liquid regime. We discuss our results in the light of recent theory of reentrant melting of the vortex lattice by G. Blatter et al. (Phys. Rev.B 54, 72 (1996)).

Enhancement of vortex fluctuation in Bi2Sr2CaCu2O8 single crystal by iodine intercalation

Vortex fluctuation effects in pristine and iodine-intercalated Bi2Sr2CaCu2O8 single crystals were studied in magnetic fields of 1 T ≤ H ≤ 5 T parallel to the c-axis. We found that iodine-intercalated specimen showed strong positional fluctuations of the vortices even far below the transition temperature due to the reduction of the interlayer coupling. Also, we obtained various thermodynamic parameters (Hc(0), Hc2(0), λab(0), and ξab(0)) using the Hao-Clem and other related models.

Supercurrents in c-axis Pb/Bi2Sr2CaCu2O8 tunnel junctions

We investigated Josephson junctions made on the surface of Bi2Sr2CaCu2O8 single crystals with Pb as the counterectrode. Junctions exhibit a clear lead gap at low temperatures as well as a nonzero supercurrent. In microwave fields Shapiro steps appear at multiples of hf/2e showing that the supercurrent is of first order. Consequences for a potential s-wave order parameter component in Bi2Sr2CaCu2O8 are discussed.

Zero-crossing steps in intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ

We have investigated the response to microwave irradiation of small mesas fabricated on single crystals of Bi2Sr2CaCu2O8+δ. We observe zero-crossing steps in the current-voltage characteristic which are consistent with the intrinsic Josephson junctions in the mesa stack phase-locking to the external microwave field.

Magnetic AC susceptibility in Bi2Sr2CaCu2O8 single crystals

We measured the temperature dependence of the ac susceptibility in Bi2Sr2CaCu2O8 single crystals. The penetration fieldH* was determined, at different temperatures, from the field dependence ofX″ in the framework of the critical state model. Using a very simple formula, we estimated the values of the critical current density near Tc, which are consistent with the usually reported data for this material.

Characteristic features of vortex depinning in a layered superconductor

The field H*(T) for the onset of dissipation is estimated self-consistently from the results of an investigation of the transverse resistance and current-voltage characteristics of a Bi2Sr2CaCu2O8 (BSCCO-2212) single crystal in a mixed state. It is established that H* is close to Hc1 in the interval Tc/2⩽T⩽Tc. Rapid growth of H*(T), accompanied by a transformation of the current-voltage characteristics, as the temperature decreases below ≈ 40 K attests to the formation of a nonvanishing region of stability of an ordered state of the vortex system, possibly as a result of a change in the effective dimensionality of the fluxoid.

Linear low-temperature dependence of the surface impedance of a Ba0.6K0.4BiO3 single crystal

The temperature dependences of the real part R s and the imaginary part X s of the surface impedance Z s =R s +iX s of the superconductor Ba0.6K0.4BiO3 (T c ≃30 K) are measured at a frequency of 9.4 GHz. Its temperature dependence Z s (T) and that of the complex conductivity σ s (T) can be described on the basis of a two-fluid model under two assumptions: The density of superconducting carriers increases linearly, and the relaxation time increases as a power law (∝1/T 5), with decreasing temperature T<T c . This model also describes well the curves Z s (T) and σs (T) recently measured for YBa2Cu3O6.95 and Bi2Sr2CaCu2O8 single crystals.

Pinning in BSCCO above the ordinary irreversibility line

Frequency-dependent observations of magnetic flux structures are used to show that pinning plays a principal role in the whole mixed state in Bi2Sr2CaCu2O8 (BSCCO) single crystals. We speculate that the random pinning force on the moving vortices may dominate over thermal fluctuations and considerably modify the position of the vortex lattice phase transition.

Morphology of growth of Bi2Sr2CaCu2O8 single crystals

Morphology of growth of Bi2Sr2CaCu2O8 single crystals