Thermal expansion of Bi2.2Sr1.8CaCu2Ox superconductor single crystals

Using a low-temperature stage X-ray diffractometer, we have investigated the temperature dependence of the lattice parameters of single crystal superconductors Bi₂Sr_1.8CaCu₂O_x. The experimental results show that (i) the lattice constants increase linearly with increasing temperature above about 100 K; (ii) thec-axis lattice parameter shows a kink in the superconducting transition region, while thea, b-axis parameters do not show any anomalous behavior in this region; (iii) botha, b, andc show negative thermal expansion coefficients below 40–50 K, which may be related to the characteristics of the Cu-O bond.     

Pinning potential transport measurements of Bi2Sr2CaCu2Ox thin films

Flux creep characteristics ofc-axis-oriented Bi₂Sr₂CaCu₂O _x thin films grown by the method of electron beam evaporation of stoichiometric target and subsequent annealing have been investigated in the absence of external magnetic field. A decrease of the pinning potential with decreasing temperature is obtained fromI–V curves. This result is explained by the presence of the spatially nonlinear pinning potential.     

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.     

Superconductivity of barium-doped Bi2Sr2CaCu2Oy

Sintered ceramic samples of Bi₂Sr_2–x Ba _x CaCu₂O_y with nominal barium fraction 0x0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Magnetic relaxation and critical current in Bi2Sr2CaCu2O8+x single crystals

The anisotropy of critical current densityJ _c in Bi₂Ba₂CaCu₂O_8+x single crystals has been investigated as a function both of the temperature and of the applied magnetic field. An anisotropic behavior ofJ _c has been found. The decay of the remanent magnetization has been studied for fields applied both parallel and perpendicular to thec axis. A logarithmic behavior was found. A pinning energyU ₀ of about 0.01 eV, independent of the direction of the applied field, was obtained.     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to 7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Can?1CunOx

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n–1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^– , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n–1Cu _n O _x systems.     

Superconductivity in epitaxial Bi2Sr2CuO6/Bi2Sr2CaCu2O8 superlattices: The superconducting behavior of ultrathin cuprate slabs

Utilizing atomic layer-by-layer molecular beam epitaxy (ALL-MBE), we have synthesized a series of high-quality superlattices in which ultrathin slabs (one-half unit cell thick) of the high-T _c superconductor Bi₂Sr₂CaCu₂O₈ alternate with up to five such layers of the low-T _c Bi₂Sr₂Cu₁O₆ phase. In all these superlattices we foundT _c to be essentially equal to that of the high-T _c Bi₂Sr₂CaCu₂O₈ phase itself, which indicates that this cuprate is a 2D superconductor insofar as the interslab coupling plays at best a secondary role. Furthermore, it is demonstrated thatT _c need not be reduced at heterostructure interfaces.     

Mechanical anisotropy of YBa2CU3Ox single crystals

The results of Vickers microhardness measurements on (100) and (001) surfaces respectively, of YBa₂Cu₃O_x single crystals are reported. It was found that the hardness ratio of (100) to (001) surfaces is 1.8 for the tetragonal phase and 1.2 for the orthorhombic phase.     

HREM surface profile images of Bi2Sr2CaCu2O8

Surface profile images of Bi₂Sr₂CaCu₂O₈ have been obtained using high-resolution electron microscopy. The cleaved (001) surface of the crystals terminates with a single Bi-O atomic layer. The modulated structure developed in this surface atomic layer was observed directly. The (hk0) surfaces were found to decompose in air into an amorphous coating layer. This coating layer was unlikely recrystallized into the original structure under electron beam irradiation. The amorphous layer on the (hk0) surface formed in pure Ar atmosphere was relatively thin and could be recrystallized into some secondary phases in which a Bi loss was observed. The original (001) surface might also be covered by an amorphous-like layer. This disordered layer could be recrystallized under electron beam irradiation into BiSr₂Ca₂Cu₃O₉, BiSr₂CuO₅, etc. which intergrow with the parent crystal perfectly on the (001) planes.The author thanks the EPSRC for financial support.     

Dielectric and ionic conduction properties in LiYF4 single crystals

A LiYF₄ single crystal was grown by the Czochralski technique in an Ar-gas atmosphere. The complex dielectric constants and complex impedances are measured in the range of 100 Hz–10 MHz and up to 200°C for a c-plate of a LiYF₄ single crystal. The LiYF₄ reveals a strong low frequency dielectric dispersion below 300 kHz, from room temperature to 200°C. It is believed that the strong dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of Li⁺-ions between neighboring sites, with an activation energy of Δ=0.49 eV. The conduction mechanism seems to be the non-interacting Debye type and the relaxation times satisfy the Arrhenius relation.     

Point contact investigation on Bi2Sr2CaCu2O8+y thin films

We have measured point contacts with a gold tip on Bi₂Sr₂CaCu₂O_8+y thin films and Bi₂Sr₂CaCu₂O_8+y/SrTiO₃ double layers. The results show tunneling or direct conductivity behaviour depending on the junction parameters and can be fitted by corresponding theoretical models. From fitting procedure of differential characteristics by modified Blonder-Tinkham-Klapwijk (BTK) theory the Fermi velocity v_F=5×10⁵ m/s, ratio 2(4.2K)/kT_c=7.9 and coherence length _ab=3.5 nm were obtained. The changing of the interface transmission by additional layers of different thickness corresponding to metallic or insulating behaviour is shown. In the frame of the inelastic scattering of quasiparticles the linear background of differential conductance is discussed.This work was supported by German BMFT under Contract No.l3N5924A and Slovak Grant Agency for Sciences (Grants Nos. 2/990125/93 and 2/999185/92).     

Transport and Magnetic Properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)y Superconductors

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y samples. All these properties have been compared with those of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiF) _y specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi₂Sr₂Ca₂Cu₃O_{10 +} high-temperature superconducting phase is maximum for y 0.02. The amount of LiCl in Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.     

Anisotropy in ac losses near the irreversibility field of Bi2Sr2CaCu2O8

The ac susceptibility under a biased dc field near the irreversibility field (H _irr) of a Bi₂Sr₂CaCu₂O₈ single crystal has been measured. The frequency dependence, the ac-power dependence, and the nearly lossless character of the vs.H _dc curve forHa-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.     

Transport properties of textured Bi2Sr2CaCu2O8+y, thin films

Superconducting films of the high-T _c compound Bi₂Sr₂CaCu₂O_8+y , have been grown on (111)-oriented gadolinium gallium garnet substrates by a liquid-phase technique. The films show a very high degree of preferential orientation with thec-axis perpendicular to the substrates. The onset of the resistive transition was 85 K while zero resistance was obtained at 78 K. Results concerning the critical current properties of the films are described. Measurements of the paraconductivity effects on the electrical resistivity above the superconducting transition due to thermodynamic fluctuations are also reported.Supported by Ansaldo S.p.A Divisione Ansaldo Ricerche, Via Corso Perrone 25, I-16100 Genova, Italy.     

Electronic Raman scattering of superconducting YBa2Cu3Oy single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^–1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^–1 fory7.0 to 550 cm^–1 fory6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

Localization effects in transition-metal doped Bi2Sr2CaCu2O8+y high-temperature superconductors

Doping Bi₂Sr₂Ca₁Cu₂O_8+y with Co causes a superconductor-insulator transition. We study correlations between changes in the electrical resistivity _ab(T) and the electronic bandstructure using identical single crystalline samples. For undoped samples the resistivity is linear in temperature and has a vanishing residual resistivity. In angle resolved photoemission these samples show dispersing band-like states. Co-doping decreases T_c and causes and increase in the residual resistivity. Above a threshold Co-concentration the resistivity is metallic (d_ab/dT>0) at room temperature, turns insulating below a characteristic temperature T_min and becomes superconducting at even lower temperature. These changes in the resistivity correlate with the disappearance of the dispersing band-like states in angle resolved photoemission. We show that Anderson localization caused by the impurity potential of the doped Co-atoms provides a consistent explanation of all experimental features. The coexistance of insulating (d_ab/dT <0) normal state behavior and superconductivity indicates that the superconducting ground state is formed out of spatially almost localized carriers.     

Domain patterns on ferroelectric Rh:BaTiO3 single crystals

Single crystal growth and domain structure of Rh:Barium titanate (BaTiO₃) have been investigated. Rh doping in BaTiO₃ is effective for the growth of bulk crystals without twin formation. Atomic force microscope (AFM) and optical microscope studies reveal the formation 180° and 90° domains on the grown crystals. It has been observed that the complex 180° domain structure with typical size of around 20 μm exists in the c-domain of {0 0 1} face of Rh doped BaTiO₃ crystals.     

A novel method for preparation of pure Bi2SiO5 crystals

Pure Bi₂SiO₅ crystals were firstly prepared by a novel method of melt-cooling process with Bi₂O₃ and SiO₂ as starting materials. In this study, the preparation and properties of the samples were characterized by thermo gravimetric (TG), differential scanning calorimeter (DSC), field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The results showed several endothermic valleys at 579.6 °C, 744.3 °C, 829.3 °C and 879.2 °C during heating process; and a distinct exothermic peak at 805.3 °C during the melt cooling process. XRD and SEM images revealed highly-ordered lamellar pure Bi₂SiO₅ crystals with coarse grains can be achieved by 2 h heating at 805.3 °C.     

Charge-transfer in YBa2Cu3Ox

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to 60 K and then to 90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.