Bi2Sr2CaCu2O8 Single Crystals Research Articles

Surface logarithmic correlations in BiSrCaCuO single crystals

Low temperature STM techniques are applied to study statistical properties of the BiO plane in 2212 high- T c single crystals. The correlation functions of height deviations in x and k spaces have been numerically obtained. Long range power law correlations have been found and attributed to half lattice cell jumps of known height in the c direction. Theoretical analysis of the correlation function resulting, from the assumption that the jumps on the surface are formed by two oppositely charged partial screw dislocations, gives very good agreement with the experimentally found behavior.

The use of a semiconductor Hall probe to study the irreversibility line in BSCCO

A miniature Hall probe using a two-dimensional electron gas in an InAlAs/InGaAs heterostructure has been used to study the magnetic properties of a small Bi2Sr2CaCu2O8 single crystal, on a lateral scale of 10-20 mu m2, by placing it close to the crystal surface. The results on the irreversibility line are compared with previously reported data obtained by other techniques. The use of such detectors for real-time study has also been considered.

Field and Temperature Dependence of Susceptibility of Single Crystal Bi2Sr2CaCu2O8

Susceptibility measurements on Bi2Sr2CaCu2O8 single crystals do reveal field-induced-suppression of sharp phase transition. No apparent anisotropy of the susceptibility was observed when magnetic field was applied along the three main directions a, b and c. The three dimensional isotropic fluctuation diamagnetism theory is proposed to explain our experimental finding of isotropic susceptibility. In a wide range of magnetic field, the temperature dependence of susceptibility does not show any obvious change in its character when the sample enters into expected vortex-lattice state from vortex-liquid state.

Scaling of the energy gap withT c in Bi2Sr2CaCu2O8+?

The results of a high-resolution photoemission study using synchrotron radiation of two single crystals of Bi2Sr2CaCu2O8+δ, with different critical temperaturesT c due to a variation in oxygen stoichiometry δ are reported. Within experimental accuracy, the energy gap 2Δ is found to scale withT c , amounting to a reduced gap parameter of 2Δ/k BTc≅7.4. Employing resonant photoemission at the O−2s and Cu−3p thresholds, two spectral peaks at binding energies of 180 meV and 320 meV were identified as predominantly O−2p-and Cu−3d 4s-derived states, respectively.

Microwave properties and characterization of co-evaporated BSCCO thin films

An extensive characterization of Bi-Sr-Ca-Cu-O (BSCCO) thin films deposited by co-evaporation on LaAlO3 and SrTiO3 substrates has been performed. The films had a Tc(R=0) of approximately 78 K, and were predominantly c-axis oriented, with critical current densities (Jc) at 4.5 K of 1.6*106 and 1.1*106 A cm-2, for the samples on SrTiO3 and LaAlO3 respectively. The microwave properties of the films were examined by three techniques. The complex conductivity ( sigma *= sigma 1-j sigma 2) and the magnetic penetration depth ( lambda ) were measured by power transmission at 30.6 GHz; the surface resistance (RS) was measured using a cavity resonator at 58.9 GHz, and the transmission line losses were determined by measuring the quality factor (Q) of a linear microstrip resonator at 10.4 and 20.2 GHz. The complex conductivity for the film on LaAlO3 was determined to be (2.0-j10)*105 S m-1 at 77 K. It was observed that in the superconducting state sigma 1 deviates from both the Bardeen-Cooper-Schrieffer (BCS) theory and the two-fluid model. Values of lambda were found to be approximately 2.0 and 1.1 mu m at 77 K and 20 K respectively, and were obtained for the film on LaAlO3. The value of lambda at 20 K was approximately three times larger than that of BSCCO single crystals. RS values of 865 and 1391 m Omega were obtained for the films on SrTiO3 and LaAlO3, respectively, at 77 K and 58.9 GHz. Unloaded Q factors at 20 K of approximately 1100 and 800 at 10.4 and 20.2 GHz respectively, were measured for the BSCCO resonator. Unloaded Q values of 290 and 405 at 20 K were obtained at 10.4 GHz and 20.2 GHz respectively, for an all gold (Au) resonator.

Optical study of domains inBi2Sr2Ca1Cu2O8-δsingle crystals

Abstract Using polarizing microscopy in reflected light mode, we were able to observe domain structure in the BiSrCaCuO single crystals (BSCCO) at room temperature. Formation of domains structure is a phenomenon ocurring in ferroic phase transitions. In this paper we present experimental results of optical studies of the domain structure observed in BSCCO single crystals. We observed {110} domains walls only. No phase transition was observed at high temperature when the crystal was heated up to 700°C.

Observation of the initial growth of semiconductor films on the Bi-based superconducting single crystal by RHEED method low temperature reaction of Ge with BSCCO single crystal

We investigated the initial growth of Ge semiconductor films on the clean surface of 80 K phase Bi Sr Ca Cu O(BSCCO) single crystal, using reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). The growth of Ge was compared with that of Au. The RHEED observation showed that, when Au was deposited up to 100Åon the BSCCO surface at the substrate temperature ( T s) = room temperature (RT= ∼ 20°C) and 300°C, Au grew in an amorphous form and in polycrystalline form, respectively, 20Åthick Ge films deposited on the BSCCO below 300°C were amorphous. The XPS results indicated that Au hardly reacted with the BSCCO, but Ge reacted with the BSCCO at the interface at RT. The formation of the amorphous phase at Ge deposition came from the interface reaction with the BSCCO.

Critical currents and magnetization of Bi2Sr2CaCu2O8 single crystals

A single crystal of Bi2Sr2CaCu2O8 was exposed to fast neutron irradiation to fluences of 5*1020 m-2 and 2.5*1021 m-2 (E>0.1 MeV). DC magnetization measurements were made with an 8T-SQUID magnetometer in order to study the influence of radiation-induced defects on flux pinning. For the lower fluence the authors find an enhancement of pinning and a shift of the irreversibility line only up to 2 T. After irradiation to a fluence of 2.5*1021 m-2, where the defect density is higher, the irreversibility line is shifted for all measured fields to higher temperatures.

ANISOTROPIC THERMOELECTRIC POWER OF Bi2Sr2CaCu2O8 SINGLE CRYSTAL

The temperature dependences of the thermoelectric power of a single crystal Bi2Sr2CaCu2O8 were measured in both the a and b directions. We discovered the anisotropy of thermoelectric power in the a-b plane of the crystal. Suggeste that the differences between Sa and Sb are caused by the modulate structure in the b direction. A satisfactory explanation is given by taking into account the contributions of the relatively mobile electrons Sd, the relatively localized electrons Sh and the phonon drag effect Sg.

Superconductivity and morphological studies on Bi2Sr2CaCu2O8 single crystals grown from stoichiometric and nonstoichiometric melts

Single crystals of Bi2Sr2CaCu2O8 (2212) have been grown by self-flux technique using stoichiometric and non-stoichiometric melts of excess CuO and Bi2O3. Single-crystal and powder X-ray diffraction studies have been made on the grown crystals to confirm their single crystallinity and structure respectively. Resistivity and susceptibility measurements provide information on the superconducting nature of the crystals. The effects of fluxing agents and starting composition on surface morphology and superconducting properties have been discussed.

Angular dependence of the field-cooled, zero-field cooled and remanent magnetization in YBA2Cu3O7 and Bi2Sr2CaCu2O8 single crystals

Abstract We have studied the angular dependence of the zero-field-cooled (M ZFC ), field-cooled (M FC ) and remanent magnetization (M rem ) of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 single crystals, by rotating the crystal relative to the applied field direction. Two compatible mechanisms, namely shape anisotropy and intrinsic anisotropy are used to describe the angular dependence of these quantities. At sufficiently low temperatures, the pinning forces are strong and the relation M FC =M ZFC +M rem holds for all orientations. The angular dependence of M ZFC is due only to variation of the demagnetization factor by rotation; whereas the angular dependence of the remanent magnetization can be interpreted by using the two mechanisms. For larger fields or higher temperature the pinning forces are weaker and |M FC | ZFC +M rem|.

Correlation of flux line pinning and irradiation damage in Bi-2212 single crystals after 0.5GeV iodine irradiation

Abstract Bi2Sr2CaCu2O8 single crystals were irradiated with 0.5GeV iodine ions which cause latent tracks along their paths with 5–10 nm in diameter. These tracks act as very effective pinning centers. After irradiation the magnetic hysteresis and pinning energy is strongly enhanced as long as the density of flux lines is smaller than the density of the tracks. The maximum increase of magnetization current, which is more than one order of magnitude at fields below one Tesla, was reached at a dose of 2.5 · 1011 ions/cm2.

A novel method to avoid demagnetization effect in magnetization studies of Bi2Sr2CaCu2O8 single crystals with the field perpendicular to the CuO2 plane

Abstract Magnetization measurement was performed on stacked samples of Bi2Sr2CaCu2O8 single crystals with magnetic field applied along the stacking direction, being parallel to the crystal c-axis. This configuration with varying sample size within the ab-plane, made possible an accurate determination of the field dependence of the critical current density Jc (∥ ab-plane) at temperatures below 30 K. However, the size-dependent magnetization hysteresis was found to vanish at higher temperatures, suggesting either a surface barrier or granularity of the crystals becomes a dominant factor for the observed irreversibility.

Influence of sample microstructures on the decoration pattern

Bitter decoration experiments were performed on melt-textured YBa 2Cu 3O y, single-crystal YBCO and BiSrCaCuO samples at 36K in an external magnetic field of 25G. A flux line density gradient was observed from the edge to the centre of the single crystal YBCO sample. In melt-textured YBCO samples ferromagnetic particles mainly concentrate on the regions of strong pinning. However, no Bitter-patterns were found on the surfaces of BiSrCaCuO single crystals.

Diagnosis of Surface Stability of 80K-Phase BiSrCaCuO Single Crystals with and without Li Doping

The surface structure of 80 K-phase BiSrCaCuO single crystals with and without Li doping has been studied using low-energy electron diffraction (LEED) at elevated temperatures. Although the superconducting critical temperature rises slightly with Li doping, the cleaved surface becomes amorphous in vacuum at temperature ∼100 degrees lower than that of the undoped sample (600°C).

Synthesis of high-Tc superconductors at Uppsala University: ceramics and crystal growth

Synthetic and process studies of the new high-Tc super-conductors are carried out using the methods of powder technology. The equipment for these studies at the Institute of Chemistry, University of Uppsala is described. The laboratory is equipped with jaw crusher, various mills, powder mixer, spray dryer and several sintering furnaces. The laboratory has excellent facilities for powder characterization, such as a semi-automatic image analyzer, a system for X-ray line broadening analysis, BET apparatus, sample divider and a sieving machine. Single-crystal growth experiments are also reported. Single crystals of YBa2Cu3O7 and Bi2Sr2CaCu2O8 were grown by the off-stoichiometric method. The largest crystals of YBa2Cu3O7 were obtained with the soaking temperature of 980°C and a cooling rate of 2°C h−1. A soaking temperature of 925°C and a cooling rate of 40°C h−1 produced the largest Bi2Sr2CaCu2O8 single crystals. A single crystal examination showed that the structure of Bi2Sr2CaCu2O8 is incommensurate along the b axis. The lattice parameters were a = 5.40 Å, b = 25.25 Å, c = 30.62 Å.

Optical constants spectra for single crystal Bi2Sr2CaCu2O8+ x with different stoichiometry

Optical constants spectra for Bi 2 Sr 2 CaCu 2 O 8+ x single crystals with different stoichiometry have been studied in 1 ÷ 4.5 eV region. Direct relation between T c and 3.8 eV and 4.3 eV bands intensities have been established. The bands are supposed to be due to BiO (triply split) - Bi6p transitions in M point of BZ.

Non-logarithmic magnetic relaxation in Bi 2.2Sr 1.7CaCu 2O 8 single crystals; evidence for collective flux pinning

High precision magnetic relaxation measurements have been performed on BiSrCaCuO single crystals with the field H applied parallel to the c-axis. At temperatures T/ T c > 0.5, the data exhibit two different field regimes. One for H< H∗ ( H∗ corresponds to the field where full flux penetration of the sample is achieved) where the ZFC and RM magnetization relaxes logarithmically with time in agreement with the conventional flux creep model; a second one for H > H ∗ where the relaxation follows a [ln( t/ t 0] -α dependence. This relaxation law is predicted in the collective flux creep theory for J ⪡ J c as well as in the vortex-glass model. In the temperature range 0.25 < T/ T c < 0.4, there is a change in character of the measured decay of the magnetization. This change in character is interpreted to arise from thermal fluctuations of the flux-line lattice giving rise to a considerable reduction in the pinning strength above the depinning temperature.

Layer-by-layer epitaxial growth of a Bi2Sr2CuO6 thin film on a Bi2Sr2CaCu2O8 single crystal

The epitaxial growth of a Bi2Sr2CuO6 (2201) thin film on a Bi2Sr2CaCu2O8 (2212) single crystal has been performed using computer-controlled laser molecular beam epitaxy. The surface of the 2212 single crystal used as the substrate is smooth and invariant under the growth condition at 640 °C in NO2 pressure of 1×10−5 mbar. The growth process of the 2201 film has been observed by in situ reflection high-energy electron diffraction (RHEED), and the layer-by-layer growth of the 2201 phase is confirmed by the oscillation of RHEED intensities. During the growth, a modulated surface structure which is characteristic of the Bi cuprate crystals is always present.

X-Ray Photoelectron Spectroscopy Studies of 7 K-Phase Bi-System Single Crystals

The Bi-system single crystals with c-axis unit cell length of ∼24 Å, i.e., Bi-Sr-Cu-O (BSCO), Bi-Pb-Sr-Cu-O (BPSCO), Bi-Sr-Ca-Cu-O (BSCCO), and Bi-Pb-Sr-Ca-Cu-O (BPSCCO) single crystals, were prepared using the starting materials of various compositions by the self-flux method. X-ray photoelectron spectroscopy (XPS) measurements from the air-cleaved surfaces of the single crystals were carried out. From the results of the surface composition analyses, we found that Pb occupied a part of the Bi site in the BPSCO single crystal and Ca occupied a part of the Sr site in the BSCCO single crystal. The chemical bond natures of Sr, Cu and O in the BSCO crystal seem to be changed by doping with Pb and/or Ca.