Study of the modulated structure of Bi2Sr2CaFe2Ox

The ceramics and single crystals of the Bi₂Sr₂CaFe₂O_x compound were synthesized. The X-ray diffraction data showed orthorhombic symmetry. The lattice parameters are equal toa=5.464 Å,b=5.453 Å,c=31.313 Å. The crystal structure is described by the primitive Bravais lattice with the possible space groups Pbmm. Pbm2 and Pb2₁m. The obtained X-ray patterns show the presence the incommensurate structural modulation with the following parameters: the modulation vector lies in the (100)-plane, the value of the wave-vector componentq _b=0.22(3). The comparison of the obtained results with data for Fe-doped Bi₂Sr₂CaCu₂O_x and Bi₂Sr₃Fe₂O_x are presented. No magnetic peculiarities of the compound were found. Mssbauer measurements of the ceramic samples indicate the presence mainly of Fe³⁺ oxidation state and Fe⁴⁺ (about 20%). The decrease of the quadrupole-splitting values for 2212Fe in comparison with Fe-doped 2212Cu was revealed that may be connected with higher symmetry of the local environment of Fe atoms in 2212Fe.     

a-b plane anisotropy of single-domain crystals of Bi2Sr2CaCu2O8

The polarized reflectance of thea-b plane of single-domain Bi₂Sr₂CaCu₂O₈ crystals is anisotropic above and belowT _c . The normal-state infrared conductivity is higher forEa whereas the high-frequency conductivity is higher alongb, particularly for transitions associated with the Bi–O layers. BelowT _c there is a definite anisotropy to the far-infrared absorption, with a finite absorption forEb down to 20 meV. This anisotropy of thea-b plane could be due either to anisotropy of the superconducting gap or to anisotropy of the midinfrared component to the conductivity.     

High-resolution electron microscopy and electron diffraction on Bi2Sr2-x La x Ca n Cu1+n O6+2n+δ

Solid solutions, Bi₂Sr_2-x La _x CuO₆₊ withx< 1.3 and Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ shows sometimes the absence of the 0.54 nm period alonga, whereas the modulation remains, suggesting both features not to be correlated. The investigation of the solid solution Bi₂Sr_2-x La _x CaCu₂O₈₊ with probablyx<1.3 are reported. Both solid solutions show considerable nonlinear changes in the unit cell dimensions and a change in the modulation vector from 4.7b to 3.5b forx=0 andx=1.2, respectively. It is shown that Bi₂Sr_2-x La _x CaCu₂O₈₊ is severly hampered by the presence of the n=0 structure in the reaction product when one starts with stoichiometric mixture. This can be overcome by adding extra Ca and heating directly just below the melting point. However addition of Ca to the mixture leads to changes in the solid solution range, because Ca can replace Sr.     

Modulated structure of the superconducting compounds Bi2Sr2Ca n-1Cu n O y withn=1 and 2

The incommensurate modulated structures of high-T _c superconducting phases Bi₂Sr₂Ca _n–1Cu _n O _y (n=1 and 2) have been studied using symmetry properties of four-dimensional super-spacegroups and electron diffraction as well as high resolution electron microscopy are used to describe the properties of the incommensurate modulated structure in the compounds. The main results are planar monoclinic symmetryP ^B _{2/b 11} orP B _{b 1} for Bi₂Sr₂CuO _y (n=1) and orthorhombic symmetryN ₁₁₁ ^Bbmb orN ₁₁₁ ^Bb2b for Bi₂Sr₂CaCu₂O _y (n=2). The temperature dependence of the modulated structure of Bi₂Sr₂CaCu₂O _y has been investigated from –190°C to 800°C. Some structure properties of Bi₂Sr₂Ca₂Cu₃O _y (n=3) are presented for comparison.     

Angle-resolved photoemission experiments on Bi2Sr2CaCu2O8+δ(001)

Core-level X-ray photoelectron-diffraction patterns have been measured from cleaved Bi₂Sr₂CaCu₂O₈₊ (001) surfaces for all elements present in this compound. The incommensurate modulation alongb ([010]) leads to a strong inequivalence ofa- andb-directions for Bi, Sr and Cu photoelectrons, while Ca and O emission show less effect. Ultraviolet-photoemission experiments recording the emission intensity at the Fermi energy over a large solid angle are also presented, providing a direct mapping of the Fermi surface. Ac(2×2) superstructure is observed on the Fermi surface suggesting antiferromagnetic correlations within the Cu–O planes. The effects of the lattice modulation are clearly observable at the Fermi energy, and they are enhanced for binding energies higher than a few tens of meV.     

Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Electrical properties of pulsed laser-deposited SrxBiyTa2O9 thin films on Bi/Sr ratios

The dependence of the electrical properties of Sr_xBi_yTa₂O₉ thin films on the (Bi/Sr ratio 1.6, 2.3, 2.8, 3.4) has been investigated. The Sr_xBi_yTa₂O₉ ferroelectric thin films were synthesized on Pt/Ti/SiO₂/Si substrates using the pulsed laser deposition method. Saturation polarization (P_st) and coercive field (2E_c) depend on the Bi/Sr ratio. P_st increases while E_c decreases with an increase in Bi/Sr ratio. Atomic Force Microscopy images show that the grain size grows with increasing Bi/Sr ratio. It is observed that the impedance behavior in Sr_xBi_yTa₂O₉ thin film, conforms to the Constant Phase Element (CPE) model. It is believed that the diffuse charges at the grain boundary build up a surface polarization because the impedance behavior close to pure capacitor with grain size increased.     

Electrical conductivity of (Er,U)O2+x and (Ce,U)O2+x

The electrical conductivity (σ) of (Er_yU_1−y)O_2+x (y=0.06, 0.20) and (Ce_yU_1−y)O_2+x (y=0.05, 0.15, 0.25) has been measured as a function of oxygen partial pressure in the temperature range of 1100≤T/°C≤1300 by a d.c. 4-probe method. Both of the oxides exhibited Po₂-regions where the electrical conductivity is independent of oxygen partial pressure, which indicates that doped Er and Ce exist as trivalent cations on uranium sites and fix the hole concentration by acting as electron acceptors, i.e. [h]=[Er′_U] and [h]=[Ce′_U], respectively. It is considered that strong oxidization tendency of uranium and reduction tendency of cerium simultaneously render the cerium ions exist exclusively as Ce³⁺ in the uranium dioxide. The electron-hole mobility of (Er_yU_1−y)O_2+x and (Ce_yU_1−y)O_2+x in the Po₂ region where σ is constant has been calculated by the combination of the electrical conductivity and charge carrier concentration; the activation energy (E_H) of each oxide has been obtained from the temperature dependence of the mobility. Small polaron hopping conduction mechanism was confirmed by small magnitude of the mobility (0.018-0.052 cm² V⁻¹ s⁻¹) and the activation energy (0.12-0.22 eV).     

Specific heat anomaly in Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The specific heat anomaly ΔC of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y was observed in the temperature range 80–100 K. It is estimated from ΔC that γ15 mJ/mol·K² and H_{c2(T=0)100 T} in this sample. The specific heat anomaly confirmed the occurrence of bulk superconductivity of the high-T_c phase in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y superconductor.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

In-plane London penetration depths near the critical temperature of Tl2Ba2Ca n−1Cu n O2n+4 and (Bi,Pb)2Sr2Ca n−1 Cu n O2n+4 (n=2,3)

From isothermalM(H) curves nearT _c , measured on polycrystalline Tl₂Ba₂Ca _n–1Cu _n O_2n +4 and (Bi, Pb)₂Sr₂Ca _n–1Cu _n O_2n+4 (n=2,3) samples, we deduce the in-plane penetration depths _ab as functions of temperature. An estimate according to the BCS weak-coupling clean-limit fit, which produces the data nearT _c very well, yields _ab(0)=3100 Å, 2320 Å 2210 Å, and 1960 Å for Bi₂Sr₂CaCu₂O₈, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀, Tl₂Ba₂CaCu₂O₈, and Tl₂Ba₂Ca₂Cu₃O₁₀, respectively. A comparison between strong-coupling and weak coupling fitting curves clearly favours the weak-coupling temperature dependence of _ab nearT _c .     

Electronic structure and Fermi surface of Bi2Sr2CaCu2O8

Applying angle-resolved photoemission spectroscopy of high angular and energy resolution we have measured the electronic structure of single crystals of Bi₂Sr₂CaCu₂O₈ near the Fermi energy. Along the high symmetry direction X a band is observed to disperse upwards and to cross the Fermi level, whose unoccupied part constitutes the hole-like carriers responsible for the superconductivity. From spectra along the M direction we find evidence for an electron pocket around the M point. The measured band widths appear to be drastically reduced compared with band structure calculations indicating strong electronic correlation effects. From the observation of Fermi-Dirac-like cut-offs in the spectra due to band crossings through the Fermi level we can confirm the existence and, in particular, the shape of the Fermi surface as calculated by band structure calculations.     

利用氘粒子活化分析测定高温超导体中的氧含量

详细介绍测量高温超导体含氧量的实验方法。测定了YBa₂Cu₃O_y和Bi₂Sr₂CaCuO_y两类超导体内氧的含量。发现在YBa₂Cu_3(1-X)O_y和Bi_2(1-x)Sr₂CaCu₂O_y中随着Cu缺位的增大,氧的变化趋势。用X     

Phonon spectra of La-containing Bi2Sr2CaCu2O8-based solid solutions measured by inelastic neutron scattering

Phonon spectra of Bi₂Sr_2?y Ca_1?x La _{x + y} Cu₂O_{8 + δ} solid solutions (x=y=0; x=0, y=0.25; x=0.25, y=0; x=0, y=0.5) were measured by inelastic neutron scattering on a DIN-2PI direct-geometry spectrometer. A full-profile analysis of the x-ray diffraction data yielded an estimate of the distribution of alkaline-earth (AE) and La atoms over the Ca and Sr positions in the Bi₂Sr₂CaCu₂O₈ structure. An assignment of the main features in the phonon spectrum to dominant vibrations of certain atoms was made. In particular, the correlation of the x-ray structural data with spectroscopic data implies that the feature due to cation vibrations in the Sr crystallographic positions of the basic 2212 structure (near 11 meV) is single-mode in behavior. Substitution of La for an AE atom modifies the high-frequency part (>40 meV) of the phonon spectrum (which corresponds to vibrations of oxygen in the SrO and CuO₂ layers of the Bi₂Sr₂CaCu₂O₈ structure) and affects the cut-off frequencies in the spectra of solid solutions of various compositions. A comparison of the results obtained in this study with earlier spectra of the 2212 compound, in which Ca was substituted for by Nd, indicates that the shape and intensity of the high-frequency vibration spectra of the samples studied are related to the average cation charge in the SrO layer of the Bi₂Sr₂CaCu₂O₈ structure.     

Application of151Eu Mössbauer spectroscopy to studies of electric field gradients in high-temperature superconductors

The rare-earth site of samples having the compositions (Fe,Cu)Sr₂(Eu,Ce) _n Cu₂O_4+2n+z (n=2 and 3) were studied by¹⁵¹Eu Mössbauer spectroscopy. The results obtained are compared with measurements of EuBa₂Cu₃O_7– (123) and Bi₂Sr₂(Ca_1–yEu_y)Cu₂O_8+E (2212) samples. The full quadrupole Hamiltonian of the 21.5-keV -transition was applied in analyzing the measured spectra. Simultaneous fitting of spectra recorded from crystallites oriented in various directions was used to determine the hyperfine parameters of then= 2 and 3 samples. Two different electric field gradients, not discernible in simple random crystallite measurements, were obtained for then=3 series.     

Polarized X-ray absorption study of Bi2Sr2CuO6 and Bi2Sr2CaCu2O8

Polarization dependent X-ray absorption measurements on single crystal Bi₂Sr₂CuO₆ (T _c =9 K) and Bi₂Sr₂CaCu₂O₈ (T _c =80 K) with one and two CuO₂ layers, respectively, show no energy shift of the Cu 2p main peak, and no relation between the amount of 3d(m=0, ±1) character and the critical temperatureT _c . At grazing incidence a structure in Bi₂Sr₂CaCu₂O₈ is found at 937 eV, which can be ascribed to a composite state of Cu 3d(z ²), Cu 4s ando–2p(z).     

Nb(Ba)加强Bi系中2223相生成作用研究

固相法合成系列名义组成为Bi_2-xNb_xSr₂Ca₂Cu₃O_y(x=0—1.0)的样品,电学和磁学测量表明,当掺入x=0.2—0.3的Nb有明显加强2223相生成的作用,配比为Bi_1.7Nb_0.3Sr₂Ca₂Cu₃O_y时最佳;当掺Ba以部分取代Sr后有明显协同Nb促进2223相生成的作用,比单掺Nb效果更为显著,其中以Bi_1.7Nb_0.3Sr_1.8Ba_0.2Ca₂Cu₃O_y为最优组成。热重分析(TGA)实验显示,掺Nb(Ba)后样品在～600°C出现吸氧增重;XPS中的O_1s峰显示掺Nb(Ba)后除～531eV的主峰外,在～529eV的肩峰更加明显;讨论了Nb(Ba)对晶体结构的可能影响。 关键词：     

Effect of microstructure on thermoelectric properties of CSD-grown Bi2Sr2Co2Oy thin films

Three Bi₂Sr₂Co₂O_y thin films with different microstructures have been prepared by chemical solution deposition on LaAlO₃(001) through varying the annealing temperature. With the decrease in the annealing temperature, both the size and c-axis alignment degree of grains in the film decrease as well, leading to an increase in the film resistivity. In addition, the decrease in the annealing temperature also results in a slight increase in the seebeck coefficient due to the enhanced energy filtering effect of small-grain film. The nanostructured Bi₂Sr₂Co₂O_y film with the average grain size of about 100 nm shows a power factor comparable to that of the films with larger grains. Since the thermal conductivity of the nanostrcutured films can be depressed due to the enhanced phonon scattering by grain boundary, a higher figure of merit is expected in Bi₂Sr₂Co₂O_y thin film with grains in nanometer size.     

μSr and vortex pinning in Bi2Sr2Ca1−x Gd x )Cu2O y

We present transverse fieldSR and bulk magnetisation studies on powders of Bi₂Sr₂(Ca_1{itxGd _x )Cu₂O _y for 0x<0.5. We have found that the effective pinning decreases rapidly withx even for values ofx for whichT _c remains unchanged. We have interpreted this decrease in terms of changes in _ab (obtained fromSR) which cause a decrease in vortex-vortex interactions. The field distribution spectra in these superconductors have been obtained using a maximum entropy analysis and are found to agree with the calculations by Harshmanet al.     

Bi2Sr2CaCu2O8+y掺Na样品的超导机制

用熔融法制备了Bi₂Sr₂CaCu₂O_8+y掺Ｎａ多晶样品，转变温度Ｔ_c提高到９０Ｋ以上，最大的零电阻温度达到９２．５Ｋ。在流动氧气退火后，未掺Ｎａ样品Ｔ_c显著降低，而掺Ｎａ样品Ｔ_c几乎不变，说明二者Ｔ_c提高具有不同的机制。据此我们认为Ｎａ替代了Ｂｉ而使Ｂｉｏ层电荷失去平衡，Ｃｕｏ面上电子转移到Ｂｉｏ层而形成空穴，空穴和电子的吸引相 关键词：