Bi2Sr2CaCu2O8 Single Crystals Research Articles

Direct observation of vortex pinning by dislocations in Bi2Sr2CaCu2O8 single crystals

A technique allowing simultaneous observation of vortex images and crystal defects in superconductors by transmission electron microscopy (TEM) has been developed. The vortices are visualized by the Bitter decoration method followed by preparation of the samples for TEM. The technique was used to study the effect of crystal dislocations on the flux line lattice (FLL) in single crystals of Bi2Sr2CaCu2O8. Individual dislocations having a local density comparable with that of the vortices are found to cause local disorder in the otherwise well-ordered FLL, acting as attractive pinning centres. We show that the main contribution to pinning in our experiments comes from strain fields around dislocations. Relevant pinning mechanisms are discussed such as those arising from local changes of superconducting parameters caused by the dislocation strain fields or from vortex-dislocation interaction of an elastic nature.

Observation of current strings in Bi2Sr2CaCu2O8 single crystals.

A flux step between regions of magnetic flux of opposite polarities in the remanent state of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single crystals is observed by means of a high resolution magneto-optical technique. This step indicates a current string inside the crystal platelet which is similar to the string appearing in the solution of the critical state problem for a cylindrical type-II superconducting wire with transport current if one takes into account the finite lower critical field ${\mathit{H}}_{\mathit{c}1}$. The conditions for the existence of the current string and its properties are discussed.

Ultrasonic study of the elastic softening in superconducting Bi2Sr2CaCu2O8 single crystals above room temperature

The elastic moduli associated with the a-b plane are obtained by the pulse echo overlap technique from room temperature to 200°C for Bi 2 Sr 2 CaCu 2 O 8 single crystals. The temperature dependence of elastic moduli, especially C', exhibits an anomalous variation. The change of C' with decreasing temperature is an overall trend of softening over a wide temperature range, and the maximum amplitude of softening reaches about 24%. This behavior is strikingly different from that of other moduli measured in the experiment. Three obvious softening minima of C' around 65, 90, and 150 °C reflect the property of lattice instability and a shear-type phase-like transition. The relation between the lattice instability and the softening of the elastic mode is discussed

Neutron depolarisation studies in Bi2Sr2CaCu2O8 + x superconductors

We have studied the time dependence of the polarisation state of a neutron beam transmitted through single crystal Bi2Sr2CaCu2O8 + x (Tc = 91 K), after removal of an applied magnetic field. We have used two dimensional polarisation analysis to separate the ordered and disordered components of the pinned flux. Using a magnetising field of 140 mT along the c-axis, we observe a critical temperature at T ∼ 32 K, above which the vortex structure appears to melt.

Path of magnetic flux lines through high-Tc copper oxide superconductors

A SERIOUS impediment to many potential applications of the high-transition-temperature (high-Tc) copper oxide superconductors is the relative ease with which magnetic flux lines move within these materials, thereby producing finite electrical resistance1,2. To devise methods for rigidly fixing flux lines in these materials, which is necessary to achieve a truly superconducting (zero resistance) state, requires an understanding of their fundamental properties. In clean, conventional type II superconductors, flux lines or vortices can be modelled well as rigid objects that pass straight through a sample. In the high-Tc materials, however, comparatively short coherence lengths, large anisotropies and large accessible thermal energies lead to more complex and fascinating behaviour, giving for example entangled flux lines and two-dimensional pancake vortices3–5. Some detail of the vortex lattice has been resolved previously6–13, although it is not clear how vortices pass through these materials. Here we address this critical issue by simultaneously decorating the positions of flux lines at opposite sides of single-crystal Bi2Sr2CaCu2O8 (BSCCO) high-Tc superconductors using the Bitter technique14,15. These new data enable us to quantify the wandering of vortices as they pass through the BSCCO high-Tc materials and address the elasticity of the vortex lattice. This information will be useful for devising effective strategies for pinning flux lines to the crystal lattice.

Fluctuation phenomena in excess conductivity and magnetization of single-crystal Bi2Sr2CaCu2O8+y.

We present an analysis of the measured excess conductivity which results from the fluctuation of the superconducting order parameter of a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ single crystal and found it was fit best by the two-dimensional Aslamazov-Larkin theory. The magnetoconductivity and magnetization studies are in good agreement with the two-dimensional (2D) scaling behavior in the fremework of Lawrence-Doniach model revealing the 2D nature of the fluctuation near the mean-field transition temperature in the presence of a field.

Gap anisotropy in Bi2Sr2CaCu2O8+ delta by ultrahigh-resolution angle-resolved photoemission.

Ultrahigh-resolution (DELTAE up to 10 meV) angle-resolved photoemission spectroscopy on single crystals of Bi2Sr2CaCu2O8+delta (Bi-2212) show a highly anisotropic superconducting energy gap: the gap is minimized, with a value close to zero, along the GAMM

Direct STM mapping of the superconducting energy gap in single crystal BI2SR2CACU2O8+x

Scanning tunneling spectroscopy applied to BiO cleavage planes of 90 KTc BSCCO 2212 at 4.2 K simultaneously provides topography and localdI/dV spectra (superconducting DOS). The spectra, which are similar to recent photoemission spectra, confirm a large gap parameter Δ(x, y) associated with an apparently gapless DOS on the uppermost layer. Transverse spatial variations of Δ on a 100 A scale are attributed to variations in BiO metallicity, presumably originating in oxygen stoichiometry variations in an unannealed crystal. We identify two characteristicdI/dV spectral shapes with regions of metallic and nonmetallic BiO layers, and can relate these by the McMillan model of the superconducting proximity effect. A sharply peaked spectral shape, similar to that observed in photoemission, is predicted for the metallic BiO layer induced superconducting by its proximity to the underlying CuO2 planes. The short mean free path and short coherence length imply that both tunneling and photoemission spectra are heavily weighted toward contributions from the BiO layer in fully metallic 2212. The present results and analysis thus suggest that the superconducting proximity effect influences the lineshapes seen by both techniques.

Anisotropy in ac losses near the irreversibility field of Bi2Sr2CaCu2O8

The ac susceptibility under a biased dc field near the irreversibility field (Hirr) of a Bi2Sr2CaCu2O8 single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of theϰ″ vs.Hdc curve forH∥a-axis have been roughly explained from a reversible (elastic) fluxoid motion, while those forH∥ c-axis have been explained from a thermally assisted flux-flow (TAFF) model. The obtained parameters are discussed in relation to anisotropic flux-pinning mechanisms in the layered structure of this compound.

Magnetostriction of HTSC single crystals

We have measured the sample length change of HTSC single crystals in the superconducting state in a magnetic field. The magnetic field was applied parallel to the crystallinec-axis direction and was swept at a constant rate of 10 mT/s up to 6 T. The sample length change was measured along theab-plane direction, i.e., perpendicular to the field. We have found for the Bi2Sr2CaCu2O8 single crystals that the relative sample length change (ΔL/L) is extraordinarily large for a superconductor, on the order of 10−4. For the case of (La1−xSr x )2CuO4, the amount of magnetostriction was not as large as for the Bi system, but the magnetostriction curves had many similarities. For both compounds, a relaxation phenomenon was observed when the field sweep was suspended at a certain field value. These observations are discussed in relation to flux line pinning.

Far-infrared spectra of Bi2Sr2CaCu2O8 single crystal

The far-infrared spectra of a Bi2Sr2CaCu2O8 single crystal at 2.7 and 298 K is reported for the spectral range of 30–350 cm−1. Several characteristic bands of Cooper pairs are found at 2.7 K in the far infrared region, and accompanied by low-energy side bands.

The residual resistance in the superconducting region of the Bi2Sr2CaCu2O8 single crystal

In the oxide superconductor Bi2Sr2CaCu2O8 the residual resistance tails down into the superconducting region below the critical temperature; this is promoted by a magnetic field. For melt-grown single crystals the behaviour is measured and analysed on the basis of the theory of Ambegaokar and Halperin. Two parameters, superconducting transition temperature and current through the spontaneously formed Josephson junction, are assigned to fit the measurements to the theory. The origin of the residual resistance is explained quantitatively by thermal fluctuation. The dependence on the magnetic field is interpreted as the external magnetic field reducing the area of the Josephson junction, introducing a normal conducting area as vortex lines.

STM characteristics of BiSrCaCuO single crystals in normal and superconducting states

The low temperature scanning tunneling microscope (LTSTM) is used to study the local electronic properties on different crystallographic planes of Bi 2Sr 2CaCu 2O y single crystals in their normal and superconducting states. In different locations selected on the topographic images the I– V characteristics are measured at 4.2 K. In particular locations various features are observed: the superconducting gap, the Coulomb blockade and resonance tunnelling. Most of the observed spectroscopic characteristics indicate the strong Coulomb blockade induced by surface or impurity states. On some a− c planes of single crystals in the superconducting state the individual Cu-O and Bi-O layers can be recognized on the STM images owing to their different contributions to superconductivity.

Comparative study of pinning and creep in Tl2Ba2CaCu2O8 and Bi2Sr2CaCu2O8 single crystals.

We have compared the pinning and creep in two identically shaped single crystals, Tl 2 Ba 2 CaCu 2 O 8 (Tl 2:2:1:2) and Bi 2 Sr 2 CaCu 2 O 8 (Bi 2:2:1:2) using magnetometry. The critical current density, J c , deduced from the M-H hysteresis loops is found to be the highest in Bi 2:2:1:2 at low temperatures (T<10 K). The field dependence of J c , in this temperature range, is similar for both crystals. At higher temperatures, J c is found to decrease more rapidly with magnetic field in Bi 2:2:1:2 than in Tl 2:2:1:2

Loss of interplane correlation in Bi2Sr2CaCu2O8 single crystals.

By means of dc magnetization and the ac response of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6}$ single crystals it is shown that at the dc irreversibility line the vortex system has no long-range order in the c direction. We find an energy dissipation peak at 7 Hz for interplane current that takes place at a temperature well below the irreversibility line. In this sense, the irreversibility line marks the temperature where quasi-two-dimensional vortices are depinned. The experimental data clearly show the different nature of two dissipation peaks in the susceptibility: one related to the interplane currents and the other associated with the intraplane ones.

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

We have studied an anomalous peak effect in a Bi2Sr2CaCu2O8+y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10-2 s<t<104s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Unconventional angular dependence of the microscopic magnetic field distribution in single crystal Bi2Sr2CaCu2O8 from muon-spin rotation studies

The angular dependence of the microscopic magnetic field distribution in single-crystal Bi2Sr2CaCu2O8 has been measured for a range of applied fields using the muon-spin rotation technique. The results show significant deviations from the predictions of the anisotropic 3D model, which may reflect the extremely anisotropic, quasi-2D nature of the flux structure. The temperature dependence of the field distribution at 400 mT has also been measured for a range of orientations, showing disagreement with the predictions of a simple two-fluid model.

Growth of Bi2Sr2CaCu2O8 single crystals from Bi-rich melts

High-quality Bi2Sr2CaCu2O8 single crystals with dimensions up to 15x4x0.05 mm3 were grown from the starting composition Bi2+xSr2CaCu2Oy, where x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7. Different amounts of Bi2O3 in excess are included in each crystal growth run to act as flux for the growth and the optimum concentrations of the flux amount have been found. Structure characterization of a high-quality 2212 free crystal of 5x1.8x0.02 mm3 was conducted by the (00l), (h00) and (0k0) X-ray diffractions, using a rotating anode diffractometer, and the back-reflection Laue photograph. It is found that melting incongruency is the main problem encountered in the growth of large and pure-phase single crystals from Bi2O3-rich melts and we have shown that, for x=0.8, crystals of 2212 phase and crystals of 2201 phase coexisting with CuO are obtained from the two segregated parts of the Bi-rich melts.

Investigation of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals at 2-MM wavelengths

Experimental investigations of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-Tc superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz.

Flux jumps in high Tc single crystals

Abstract The magnetic responce of LaSrCuO and BiSrCaCuO single crystals has been investigated under quickly varying applied magnetic field. Sharp repetitive jumps in the magnetization vs. field curve were observed and studied as a function of the magnetic field sweep rate. The effect is interpreted as an avalanche — like flux flow, caused by a run-away break of the superconductor stability and the transition from isothermal to adiabatic limit of the flux propagation.