Bi-O Planes Research Articles

Core-level X-ray photoemission studies of Bi2Sr2Ca1− xEuitxCu2Oy

A well-characterized Eu-doped Bi 2 Sr 2 Ca 1− x Eu x Cu 2 O y ( x=0 to 1) system has been studied using core-level photoe mission. The trivalent Eu is found to be incorporated into the lattice, causing an increase in the valence states of Bi, Sr and Ca and a decrease in the Cu valence. The observed superconductor-insulator transition in this system is found to be caused by an enhanced insulating nature of the Bi-O planes and a decrease in the O 2p hole concentration in the Cu-O planes. The results of Sr 3d, Ca 2p, Cu 2p and Bi 4f core-level spectra are discussed in detail.

Optical anisotropy of (2212)-type single crystals

Optical anisotropy in the ab plane has been studied in (2212)-type Bi(Pb)-Sr-Ca(Y, RE)-Cu-O single crystals and has been shown to be closely related to the microstructure of the samples. The single-phase samples were found to have different optical features characteristic for each phase. In a more general case, the crystals were composed of two (2212) phases with corresponding features. Such samples exhibited modulation of the lattice parameter in an electron diffraction pattern obtained in the ab plane. Optical anisotropy was mostly developed in the spectral range corresponding to electronic excitations in Bi-O planes.

The a b anisotropy in Bi 2Sr 2CaCu 2O 8 superconductor : An investigation by Raman spectroscopy

In the present paper, we report a study of the Raman active phonons of Bi 2Sr 2CaCu 2O 8 for frequencies extending down to 20 cm −1 and temperatures ranging from 77 to 673 K. In spite of important changes in the spectra occurring under heating, we show that the a b anisotropy, exemplified by Raman spectroscopy in Bi 2Sr 2CaCu 2O 8 and obvious on the macroscopic scale through the crystal morphology, still persists at 673 K. These changes are interpreted in terms of disordering effects in the BiO planes and speak in favor of an order-disorder transition around 500 K.

Transport properties of Bi 2Sr 2- xLa xCuO 6+ δ

Hall effect and thermoelectric power (TEP) measurements are reported for polycrystalline samples of Bi 2Sr 2- x La x CuO 6+ δ , which show superconductivity at relatively high hole densities. The Hall coefficient is positive and deviates strongly from the inverse of the hole concentration. The TEP shows a transition from positive to overall negative values with increasing hole density. Experimental evidence is presented which suggests that a part of the carriers is localized, probably in the BiO planes.

Statistical correlations on the BiO surface in Bi 2Sr 2CaCu 2O 8−x single crystals

Low temperature scanning tunneling microscope 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. Analysis of the correlation function in k-space reveals two modulating superstructures in the b direction. The most pronounced structure is the well known superstructure with period a 1 ≈ 27 Å. The other one has period a 2 ≈ 58 Å which is a little more than twice the period of the main superstructure.

Synthesis and 17O NMR characterization of the high Tc phase in the BiPbSrCaCuO superconductor

A single-phasic polycrystalline sample of superconductor Bi 1.8Pb 0.4- Sr 2Ca 2Cu 3O y ( T c = 108 K) was obtained by reaction of prereacted Bi 1.8Pb 0.2Sr 2Ca 2Cu 3O y with PbO under low oxygen pressure, which was fully characterized by X-ray diffraction and measurements of electrical resistivity and magnetic susceptibility. 17O NMR measurements have been performed for the compound at 300 K. Spectrum of the Bi 1.8Pb 0.4Sr 2Cu 3O y compound contained two distinctive peaks in the paramagnetic region, in which a broad resonance at 2150 ppm and a relatively sharp peak at 1600 ppm were assigned as the oxygens of two outer CuO 2 planes and a central CuO 2 layer, respectively. In the diamagnetic region a series of peaks were observed about 350 ppm, which are more likely to be associated with the oxygen resonance of the SrO plane. The BiO resonance expected from the BiO plane appears to be smeared out, which might be attributed to the incommensurate modulation and nonperiodic oxygen position of the Bi 2O 2 layers.

Chemistry and structural modulations in Bi2Sr2CuO6

A series of samples along the composition lines Bi2+xSr2−xCuOy, and Bi2Sr2−xCuOy have been used to study the structural modulation, chemistry, and superconducting properties of pseudo-tetragonal Bi2Sr2CuO6 (2201). The 2201 phase can be formed from crystallization of thin glassy platelets. The sample displayed a strong crystallographic (00l) orientation which made it possible to determine incommensurate modulations near (00l) reflections using a conventional x-ray θ-2θ scan. From the crystallization of the 2201 phase, it was found that structural modulation was intrinsic to the phase, and ordering of the structure required a long time at high temperature. High temperature in situ x-ray diffraction of a 2201 Bi2Sr1.85CuOy platelet showed that the modulation existed at 875 °C in O2 (Tmelt ≍ 892 °C in O2). These suggest that the structural modulation cannot be caused solely by oxygen ordering and that metal-ion displacement must be involved. By removing 0.04 to 0.05 oxygen atom per formula unit from Bi2Sr2CuOy and Bi2Sr1.85CuOy, the c* components of the modulation changed from 0.31 to 0.26 and from 0.38 to 0.31, respectively, while the b* component of the modulation remained approximately 0.2. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of bmod with respect to the change in oxygen content does not support the model that explained the modulation by inserting extra oxygen in the BiO plane. By varying metal-ion concentrations of Bi and Sr we found that both the lattice parameters and the modulation vectors depended more on the Bi/Sr ratio than on the Sr concentration alone. As the Bi/Sr ratio increased from 1.0 to 1.35, the modulation lines moved toward the (00l) reflections. The corresponding superstructural periodicities were calculated to vary from ∼1/5b* + 0.32 c* to ∼1/5 b* + 0.63 c*. Effects of oxygen content and metal-ion concentration on the 2201 phase formation and the superconducting properties will also be discussed.

Comment on ‘‘Angle-resolved-photoemission study ofBi2Sr2CaCu2O8+δ: Metallicity of the Bi-O plane’’

Comment on ‘‘Angle-resolved-photoemission study of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Bi</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CaCu</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn><mml:mo>+</mml:mo><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>: Metallicity of the Bi-O plane’’

Changes in the Cu K-edge XANES of Bi 2201 phase by N 2 annealing

Tc of the Bi 2201 system is known to increase by annealing in the absence of oxygen. In order to understand the changes of the geometric or electronic structure in CuO 2 plane, the energy structure of Bi system is studied by Cu K-edge XANES. For the single crystal of Bi 2212, where the CuO 2 layer has pyramidal structure, 1s-4p π transition splits into two peaks. This splitting is neither observed for the single crystal of Bi 2201 phase nor for the Nd- or the La- systems. When the Bi 2201 phase is annealed in the absence of oxygen, the splitting in the 1s-4p π transition is also observed. This may be due to the fact that the apex oxygen in 2201 phase moves into the direction from CuO 2 plane to BiO plane, and as a result, the octahedral symmetry breaks, giving similar energy structure to pyramidal CuO 2 plane.

Structural modulations of Bi2Sr2CuO6 studied by in situ x-ray diffraction

High-temperature x-ray diffraction has been carried out on a superconducting single-phase sample of pseudo-tetragonal Bi2Sr2CuO6+y, which was obtained from crystallization of thin glassy platelets. The sample displayed (00l) preferred orientation, which made it possible to determine the incommensurate modulation by conventional θ−2θ x-ray scans. The existence of the modulation peaks (800 °C) indicates that the modulation cannot be caused solely by oxygen ordering. As temperature increased, it was observed that a decrease in the intensity of modulation lines was associated with an increase in intensity of fundamental reflection, which was similar to the order-disorder phase transition. By removing 0.045(5) oxygen atom per formula unit, the components of modulation satellites along the b* and c* directions changed from bmod=0.183(9) and cmod=0.315(4) to 0.184(6) and 0.260(4), respectively. This demonstrates that oxygen, while not the sole cause, does play a role in the formation of the structural modulation. However, the invariance of bmod with respect to the change in oxygen content does not support the model which explained the modulation by inserting extra oxygen in BiO plane.

17O NMR characterisation of the oxygen sites in the Bi 2Sr 2Ca n−1Cu nO 4+2n (n = 1, 2, 3) high temperature superconductors

17O NMR measurements have been performed on the Bi 2Sr 2Ca n−1 Cu n O 4+2 n ( n = 1, 2, 3) phases allowing the different oxygen sites to be identified. Detailed study of the n=3 phase in different applied magnetic fields has allowed extraction of the shift and quadrupole interaction parameters at the oxygen sites. For this phase the two distinct sets of copper planes have markedly different oxygen shifts, 1590 ppm and 1285 ppm, shift anisotropies, 1300 ppm and < 50 ppm, and quadrupolar coupling constants, 2.8 MHz and <0.5 MHz, with the additional CuO 2 plane having the smaller shift and anisotropy. This implies that the local susceptibility is lower in this plane and that the 2p occupancy is less. There is only a small shift difference for the CuO 2 plane oxygens between the superconducting n=2 phase and the nonsuperconducting (for our samples) n=1 phase. Only one resonance, at 265 ppm, is observed in all three phases for the SrO, BiO planes. We suggest that this is from the SrO plane and that the electric field gradient at the oxygen site produced by the incommensurate modulation of the BiO distances broadens the line from the BiO plane beyond detection.

Electron-hole pairing and anomalous properties of layered high-Tc compounds.

Band-structure pictures for layered high-{ital T}{sub {ital c}} materials available in the literature show that, besides the dispersive broad band responsible for metallic properties, there are at least two additional bands having minima and maxima near the Fermi surface. These additional bands belong to different planes (for example, CuO planes and BiO planes in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}) or to planes and chains (in YBa{sub 2}Cu{sub 3}O{sub 7}). Provided the Coulomb repulsion is not very weak, pairing of electrons and holes belonging to these additional bands in different planes or planes and chains is possible. It is shown that, if this possibility is realized, a transition in the additional bands into a state of an excitonic dielectric occurs. The spin of an electron-hole pair can be both 0 and 1. Due to the fact that the electron and the hole of the pair belong to different planes, there are no charge- or spin-density waves. This excitonic insulator can serve as a polarizing substance and give a strong attraction between electrons of the metallic band even if the bare interaction is repulsive. It is also shown that some interesting gapless excitations exist. Provided there are impurities in the system thatmore » scatter from plane to plane, these excitations are coupled to the electrons of the metallic band. This effective interaction can be described in terms of an effective mode {ital P}({omega}) with Im{ital P}({omega}){similar to}{minus}sgn{omega}. As a result, one can obtain such properties of the normal state as a linear dependence of the resistivity on temperature, linear dependence of the density of states on energy, constant background in the Raman-scattering intensity, large nuclear relaxation rate, etc., which are very well known from experiments.« less

Optical excitations in Bi2Sr2CuO6 and Bi2Sr2CaCu2O8: Evidence for localized (excitonic) and delocalized charge-transfer gaps.

The optical properties of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$ (2:2:0:1) and ${\mathrm{Bi}}_{2}$${\mathrm{Sn}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ (2:2:1:2) have been investigated by transmission electron-energy-loss spectroscopy. At low energy, ${\mathit{E}}_{\mathrm{loss}}$\ensuremath{\le}10.0 eV, common features are observed in the spectra of these two materials at ${\mathit{E}}_{\mathrm{loss}}$=0.0,\ensuremath{\sim}2.7,3.6, and 4.6 eV. The effective number of charges associated with these excitations has been estimated using the optical-sum rule, and from the ratio of effective charges in the two materials, the origin of this feature has been inferred based on the relative number of Cu-${\mathrm{O}}_{2}$ and Bi-O layers per formula unit. The effective-charge ratio for the free carriers at zero energy loss, ${\mathit{N}}_{2:2:1:2}$/${\mathit{N}}_{2:2:0:1}$=5.0, could not be used to determine whether the carriers were in the Cu-${\mathrm{O}}_{2}$ or Bi-O planes because the oxygen doping in the two materials was not known. But at ${\mathit{E}}_{\mathrm{loss}}$=2.7 and 3.6 eV, the effective-charge ratio is 2.6 indicating that these transitions are associated with the Cu-${\mathrm{O}}_{2}$ planes since the ratio is close to 2:1. The effective-charge ratio is 0.9 for ${\mathit{E}}_{\mathrm{loss}}$=4.6 eV suggesting that this excitation is localized in the Bi-O planes. If the two excitations in the Cu-${\mathrm{O}}_{2}$ layers of 2:2:0:1 and 2:2:1:2 are identified with the delocalized and localized charge-transfer reactions [Mark S. Hybertsen, Michael Schluter, and Niels E. Christensen, Phys. Rev. B 39, 9028 (1989)], then the three-band Hubbard parameters \ensuremath{\varepsilon}=${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{p}}$-${\mathrm{\ensuremath{\varepsilon}}}_{\mathit{d}}$ and ${\mathit{U}}_{\mathit{p}\mathit{d}}$ (\ensuremath{\varepsilon} is the energy difference between the Cu 3d and O 2p levels and ${\mathit{U}}_{\mathit{p}\mathit{d}}$ is the Coulomb repulsion between two holes occupying adjacent Cu and O sites) are 1.8 and 0.9 eV, respectively, with an uncertainty of \ifmmode\pm\else\textpm\fi{}0.5 eV.

Relationships between x-ray photoelectron spectra and superconductivity in Bi-O thin films

Detailed X-ray photoelectron spectra (XPS) have been obtained from BiSrCaCuO thin films without lead doping (T c = 86 °K, 2212 phase) and from thin films with lead doping ( T c = 107 °K, mainly 2223 phase). Films were prepared by single target sputtering and post-annealed to synthesize superconducting films 1,2. The spectral features of the Cu 2p, O 1s, Sr 3p, Ca 2p and Bi 4f are considered in some detail. It is found that in general the XPS signatures of the Cu 2p peaks are consistent with the presence of a mixed oxide environment with a main contribution from a binding energy similar to that for CuO but also with a significant contribution from a component with a binding energy shifted up to 1.7 eV for the low T c phase and to 2 eV for the high T c phase consistent with some trivalent state of copper for lead doped films. The Cu 2p structure exhibits the main line and satellite peaks familiar from screening of the 2p hole by the two valence-band configurations 2p 3d 10 and 2p 3d 9 in the intensity ratio ~2:1. Line-shape decomposition of the O 1s emission suggests that the main line corresponds to emission from oxygen bonded to copper in the Cu-O planes while the satellite peak with the higher binding energy corresponds to oxygen bonded to bismuth in Bi-O planes. No oxygen species bonded to Ca and Sr is clearly identified by line-shape analysis. Such explanations agree with angular-resolved photoemission spectra on our c axis oriented thin films. The Ca 2p and Sr 3d core-level spectra do not exhibit two sets of doublets as reported in other papers but their corresponding binding energies are shifted in the opposite trend of that observed for oxides. The bismuth ionization state in unleaded films seems inert under different annealing conditions and is very similar to Bi 2O 3. It is equally shown that the valence band structures are quite sensitive to small changes of the surface conditions and lead doping. In essence, this strongly suggests that valence band spectra may be able to be used as a finger print for the surface of the superconducting films.

Electronic structure of Bi2Sr2CaCu2O8 by photoemission and brehmsstrahlung isochromat spectroscopy

The electronic structure of Bi 2 Sr 2 CaCu 2 O 8 both above and below the Fermi level has been investigated by photoemission and Bremsstrahlung isochromat (BIS) spectroscopy. A state 2.2 eV above the Fermi level is assigned to the Cu 3d 10 band. A comparison with recent optical reflectivity studies shows that most interband transitions greater than ≈ 2 eV may be assigned to excitations within the Bi-O planes. Significant energy shifts are observed in the reflectivity data due to a final state effect.

Atom- and cluster-assembled interfaces: Cr growth on Bi2Sr2-xCa1+xCu2O8+y.

X-ray photoemission results for Cr interaction on single-crystal Bi{sub 2}Sr{sub 2{minus}{ital x}}Ca{sub 1+{ital x}}Cu{sub 2}O{sub 8+{ital y}} demonstrate that interfaces nearly free of disruption can be formed by atom deposition at 20 K or by cluster assembly. For atom deposition at 20 K, a uniform metallic layer forms with only a slight reduction of Cu{sup 2+} bonding configurations, similar to that observed for nonreactive overlayers. This atom-assembled interface was stable when warmed to 300 K. Cluster assembly results in modification of the exposed Bi-O plane, but the Cu 2{ital p}{sub 3/2} emission shows no significant change because there was no change in Cu-O bonding. The irregular but complete layer of Cr on Bi{sub 2}Sr{sub 2{minus}{ital x}}Ca{sub 1+{ital x}}Cu{sub 2}O{sub 8+{ital y}} formed by cluster assembly exhibited limited reaction upon warming to 300 K. In contrast, interface formation at 300 K by atom deposition produced complete reduction of Cu{sup 2+}-like bonding within 30--50 A of the surface. These results demonstrate the competition between kinetic and thermodynamic processes.

Electronic structure of Bi-based copper oxide superconductors: A comparative photoemission study of Bi2Sr2CaCu2O8, Bi2Sr2CuO6, and Bi2Sr2CoO6+ delta.

Bi-based high-${\mathit{T}}_{\mathit{c}}$ copper oxide superconductors, ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$, ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$, and a related material ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CoO}}_{6+\mathrm{\ensuremath{\delta}}}$ have been studied by photoemission spectroscopy. While common prominent structures at high-binding energies (&gt;2 eV) are attributed to oxygen 2p states in the Bi-O planes, emission near the Fermi level (${\mathit{E}}_{\mathit{F}}$) is different from one other, indicating that these states are oxygen 2p states hybridizing with the transition-metal 3d states. For the states near ${\mathit{E}}_{\mathit{F}}$, the resonant behavior at the Cu 3p-3d absorption threshold and the photon energy dependence of the intensities are shown to be consistent with ${\mathrm{CuO}}_{3}$${\mathit{d}}^{9}$--O 2p hole local singlets formed within the ${\mathrm{CuO}}_{2}$ planes.

Neutron diffraction study of the modulated structure of Bi 2(Sr, Ca) 3Cu 2O 8+y

The modulated structure of the 80 K superconductor Bi 2(Sr, Ca) 3Cu 2O 8+y is refined from the single-crystal neutron diffraction data on a monodomain block of dimensions 1.5 × 1.0 × 0.03 mm 3. The modulation is recognized as commensurate with period 4.75 a, resulting in a supercell 19a × b × c (a = 5.397 (1), b = 5.401 (1), c = 30.716 (3) A ̊ ) and symmetry Pnaa. The essential feature of the structure is the periodic insertion of additional oxygen rows into BiO planes, which in turn causes large displacive modulation in all layers. The ideal oxygen stoichiometry is y = 4 19 ∽ 0.21 . Bonds in the BiO and SrO planes form characteristic chains running along the a-axis. The coordination of the Bi 3+ cations is uniform and consists of three mutually nearly perpendicular bonds to oxygens of 2.0–2.2 Å lengths. The Sr 2+ cations are bonded only to eight oxygen atoms at the most. About 7% of the Sr sites, located in the vicinity of the inserted oxygen, are possibly vacant. The Ca site situated between the CuO 2 planes is substituted to 30% by strontium.

Surface electronic properties on BiO plane of Bi2Sr2CaCu2O8 single crystal measured by scanning tunneling microscopy

Scanning tunneling microscope (STM) and tunneling spectroscopy were used to study the surface electronic states of a Bi2Sr2CaCu2O8 single crystal. We obtained an STM image of the BiO plane which showed a series of rows spaced by 2.6 nm. The density of electronic states of the BiO plane obtained from the tunneling spectra showed a depression around zero-bias voltage. From the present experimental results, it is concluded that the BiO plane is almost nonmetallic and therefore Bi2Sr2CaCu2O8 crystal is composed of an alternating stacking of metallic (CuO2 ) and nonmetallic (BiO) planes.

Upper critical field of Bi[sbnd]Pb[sbnd]Sr[sbnd]Ca[sbnd]Cu[sbnd]O system

Upper critical magnetic field of Bi1.6Pb0.4Sr2Ca2Cu2+xOy with x=0.8, 1.2, 1.6 and Bi1.9−xPbxSb0.1Ca2Cu3Oy with x=0.2, 0.3, 0.3, 0.5 were measured by magnetoresistance method under pulsed magnetic field. The Hc2 was very sensitive to the Cu composition and the substitution of Pb on Bi site, but the Tc and the structure change little. It is suggested that the coordination number of Cu and the BiO plane system influence the superconductivity strongly.