Epitaxial cuprate superconductor/ferroelectric heterostructures

Thin-film heterostructures of Bi(4)Ti(3)O(12)Bi(2)Sr(2)CuO(6+x), have been grown on single crystals of SrTiO(3), LaAlO(3), and MgAl(2)O(4) by pulsed laser deposition. X-ray diffraction studies show the presence of c-axis orientation only; Rutherford backscattering experiments show the composition to be close to the nominal stoichiometry. The films are ferroelectric and exhibit a symmetric hysteresis loop. The remanent polarization was 1.0 microcoulomb per square centimeter, and the coercive field was 2.0 x 10(5) volts per centimeter. Similar results were obtained with YBa(2)Cu(3)O(7-x) and Bi(2)Sr(2)CaCu(2)O(8+x), and single-crystal Bi(2)Sr(2)CuO(6+x)as the bottom electrodes. These films look promising for use as novel, lattice-matched, epitaxial ferroelectric film/electrode heterostructures in nonvolatile memory applications.     

Superconductivity in Alkaline Earth-Substituted La2CuO4-y

La(2)CuO4-y ceramics containing a few percent of Ca(2+), Sr(2+), and Ba(2+) ions have been prepared. Resistivity and susceptibility measurements exhibit superconductive onsets (as in earlier Ba(2+)-containing samples). The onset temperature La(2)CuO4-y with Sr(2+) is higher and its superconductivity-induced diamagnetism larger than that found with Ba(2+) and Ca(2+). This is proof that the electronic change resulting from alkaline earthdoping, rather than the size effect, is responsible for superconductivity. The ionic radius of Sr(2+) is close to that of La(3+) for which it presumably substitutes.     

Single-Crystal Epitaxial Thin Films of the Isotropic Metallic Oxides Sr1-xCaxRuO3 (0 le x le 1)

Single-crystal epitaxial thin films of the isotropic metallic oxides Sr1-xCaxRuO(3) (0 相似文献   

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y), with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 A, b= 5.414A, and c = 30.904 A. The structure contains copper-oxygen sheets as in La(2)CuO(4) and YBa(2)Cu(3)O(7), but the copper-oxygen chains present in YBa(2)Cu(3)O(7) do not occur in Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y). The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca(2+) and Sr(2+) cations between the adjacent Cu-O sheets; Sr(2+) cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi(2)Sr(3-x)Ca(x)Cu(2)O(8+y) show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.     

Photoemission evidence for a remnant fermi surface and a d-wave-like dispersion in insulating Ca2CuO2Cl2

An angle-resolved photoemission study is reported on Ca2CuO2Cl2, a parent compound of high-Tc superconductors. Analysis of the electron occupation probability, n(k), from the spectra shows a steep drop in spectral intensity across a contour that is close to the Fermi surface predicted by the band calculation. This analysis reveals a Fermi surface remnant, even though Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion with approximately the &cjs3539;coskxa - coskya&cjs3539; form along this remnant Fermi surface. Together with the data from Dy-doped Bi2Sr2CaCu2O8+delta, these results suggest that this d-wave-like dispersion of the insulator is the underlying reason for the pseudo gap in the underdoped regime.     

c-axis electrodynamics as evidence for the interlayer theory of high-temperature superconductivity

In the interlayer theory of high-temperature superconductivity, the interlayer pair tunneling energy (similar to the Josephson or Lawrence-Doniach energy) is the motivation for superconductivity. This connection requires two experimentally verifiable identities: the coherent normal-state conductance must be smaller than the "Josephson" coupling energy, and the Josephson coupling energy must be equal to the condensation energy of the superconductor. The first condition is well satisfied in the only case that is relevant, (La, Sr)2CuO4, but the second condition has been questioned. It is satisfied for all dopings in (La,Sr)2CuO4 and also in optimally doped Hg(Ba)2CuO5, which was measured recently, but seems to be strongly violated in measurements on single crystals of Tl2Ba2CuO6.     

An intrinsic bond-centered electronic glass with unidirectional domains in underdoped cuprates

Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral-weight transfers from the high-energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction-a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca(1.88)Na(0.12)CuO(2)Cl2 and Bi2Sr2Dy(0.2)Ca(0.8)Cu2O(8+delta). Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond-centered electronic pattern without long-range order but with 4a(0)-wide unidirectional electronic domains dispersed throughout (a(0): the Cu-O-Cu distance). The emerging picture is then of a partial hole localization within an intrinsic electronic glass evolving, at higher hole densities, into complete delocalization and highest-temperature superconductivity.     

Oxygen isotope effect in high-temperature oxide superconductors

The effect of oxygen isotope substitution on the superconducting transition temperature, T(c), has been measured for BaBi(0.25)Pb(0.75)O(3) (T(c), approximately 11 K) and Lal(1.85) Ca(0.15)CuO(4) (T(c) approximately 20 K), and is compared to the shifts observed for La(1.85)Sr(0.15)CuO(4) (T(c) approximately 37 K) and YBa(2)Cu(3)O(7) (T(c) approximately 92 K). For all four materials, the transition temperature is shifted to lower temperature upon substitution of oxygen-18 for oxygen-16. The observed shifts demonstrate that phonons are involved in the electron-pairing mechanism in these oxide superconductors.     

Bi_2Sr_2CuO_6超导性与价键关系的研究

本文研究了Bi2Sr2CuO6超导性与其价键结构间的关系。使用复杂晶体化学键理论,计算了Bi2Sr2CuO6超导体中的不同相结构的价键参数,通过比较不同相结构中的共价性关系,确定了此超导体中的超导成分为Bi3+Bi2+Sr2Cu3+O6,同时指出只有当Bi3+Bi2+Sr2Cu3+O6的含量≥61.6%时,Bi2Sr2CuO6具有超导性。     

The magnon pairing mechanism of superconductivity in cuprate ceramics

The magnon pairing mechanism is derived to explain the high-temperature superconductivity of both the La2-xSrxCu(1)O(4) and Y(1)Ba(2)Cu(3)O(7) systems. Critical features include (i) a one- or two-dimensional lattice of linear Cu-O-Cu bonds that contribute to large antiferromagnetic (superexchange) coupling of the Cu(II)(d(9)) orbitals; (ii) holes in the oxygen ppi bands [rather than Cu(III)(d(8))] leading to high mobility hole conduction; and (iii) strong ferromagnetic coupling between oxygen ppi holes and adjacent Cu(II)(d(9)) electrons. The ferromagnetic coupling of the conduction electrons with copper d spins induces the attractive interaction responsible for the superconductivity, leading to triplet-coupled pairs called "tripgems." The disordered Heisenberg lattice of antiferromagnetically coupled copper d spins serves a role analogous to the phonons in a conventional system. This leads to a maximum transition temperature of about 200 K. For La(1.85)Sr(0.15)Cu(1)O(4), the energy gap is in excellent agreement with experiment. For Y(1)Ba(2)Cu(3)O(7), we find that both the CuO sheets and the CuO chains can contribute to the supercurrent.     

A surface-tailored, purely electronic, mott metal-to-insulator transition

Mott transitions, which are metal-insulator transitions (MITs) driven by electron-electron interactions, are usually accompanied in bulk by structural phase transitions. In the layered perovskite Ca(1.9)Sr(0.1)RuO4, such a first-order Mott MIT occurs in the bulk at a temperature of 154 kelvin on cooling. In contrast, at the surface, an unusual inherent Mott MIT is observed at 130 kelvin, also on cooling but without a simultaneous lattice distortion. The broken translational symmetry at the surface causes a compressional stress that results in a 150% increase in the buckling of the Ca/Sr-O surface plane as compared to the bulk. The Ca/Sr ions are pulled toward the bulk, which stabilizes a phase more amenable to a Mott insulator ground state than does the bulk structure and also energetically prohibits the structural transition that accompanies the bulk MIT.     

A structural probe of the doped holes in cuprate superconductors

An unresolved issue concerning cuprate superconductors is whether the distribution of carriers in the CuO2 plane is uniform or inhomogeneous. Because the carriers comprise a small fraction of the total charge density and may be rapidly fluctuating, modulations are difficult to detect directly. We demonstrate that in anomalous x-ray scattering at the oxygen K edge of the cuprates, the contribution of carriers to the scattering amplitude is selectively magnified 82 times. This enhances diffraction from the doped holes by more than 10(3), permitting direct structural analysis of the superconducting ground state. Scattering from thin films of La2CuO4+delta (superconducting transition temperature = 39 K) at temperature = 50 +/- 5 kelvin on the reciprocal space intervals (0,0,0.21) --> (0,0,1.21) and (0,0,0.6) --> (0.3,0,0.6) shows a rounding of the carrier density near the substrate suggestive of a depletion zone or similar effect. The structure factor for off-specular scattering was less than 3 x 10(-7) electrons, suggesting an absence of in-plane hole ordering in this material.     

Ca2+和Sr2+联合处理对油菜抗氧化酶活性的影响

[目的]为了了解环境介质中同时存在Ca2+和Sr2+2种离子时油菜幼苗的抗氧化酶活性的变化。[方法]在石英砂和Hoagland营养液培养体系中,用浓度为0、10、20和40 mmol/L SrCl2及浓度为0、5和10 mmol/L Ca(NO3)2处理3叶龄油菜幼苗。在处理0、7、14和21 d后,分别取样测定POD、CAT和SOD的活性。[结果]在相同的Sr2+浓度下,随着Ca2+浓度的增加,POD活性增加,Ca2+明显缓解Sr2+对油菜幼苗的毒害作用。[结论]该研究从抗氧化角度初步阐明油菜具有较高富集Sr2+能力的机理,为利用油菜对锶污染环境进行植物修复提供理论依据。     

硫化温度和时间对SnxSy薄膜结构和成分的影响

用热蒸发技术在玻璃基片上沉积一层sn薄膜,在真空条件下,将其在150～300℃下硫化30．60min．对在不同温度和时间下硫化的薄膜进行结构、成分和表面形貌分析,结果表明：在不同温度和不同时间下硫化,所得到的薄膜在物相结构、成分和表面形貌上都存在差异．当硫化温度为240℃、硫化时间为45min时,所制得的薄膜为正交结构的SnS多晶薄膜,其均匀性、致密性以及对基片的附着力都较好,具有（111）方向优先生长,薄膜粒径在200～800nm,且晶格常数与标样的数值吻合很好．     

The effect of algal symbionts on the accuracy of Sr/Ca paleotemperatures from coral

The strontium-to-calcium ratio (Sr/Ca) of reef coral skeleton is commonly used as a paleothermometer to estimate sea surface temperatures (SSTs) at crucial times in Earth's climate history. However, these estimates are disputed, because uptake of Sr into coral skeleton is thought to be affected by algal symbionts (zooxanthellae) living in the host tissue. Here, we show that significant distortion of the Sr/Ca temperature record in coral skeleton occurs in the presence of algal symbionts. Seasonally resolved Sr/Ca in coral without symbionts reflects local SSTs with a temperature sensitivity equivalent to that of laboratory aragonite precipitated at equilibrium and the nighttime skeletal deposits of symbiotic reef corals. However, up to 65% of the Sr/Ca variability in symbiotic skeleton is related to symbiont activity and does not reflect water temperature.     

Tuning High-Tc Superconductors via Multistage Intercalation

Multistage intercalation has been used to tune the interaction between adjacent blocks of CuO(2) sheets in the bigh-T(c) (high superconducting transition temperature) superconductor Bi(2)Sr(2)CaCu(2)Ox. As revealed by atomic-resolution transmission electron microscopy images, foreign iodine atoms are intercalated into every nth BiO bilayer of the host crystal, resulting in structures of stoichiometry IBi2nSr2nCanCu2nOx with stage index n up to 4. An expansion of 3.6 angstroms for each intercalated BiO bilayer decouples the CuO(2) sheets in adjacent blocks. A comparison of the superconducting transition temperatures of the pristine host material and intercalated compounds of different stages suggests that the coupling between each pair of adjacent blocks contributes approximately 5 K to T(c) in Bi(2)Sr(2)CaCu(2)Ox.     

Direct Observation and Analysis of CuO2 Shear Defects in La2-xSrxCuO4

Direct observations of CuO(2) sheet defect structures in superconducting La(2-x)Sr(x)CuO(4), with x in the range 0.05 crystallographic directions, by a pure shear mechanism along the edge of the octahedral copper-oxygen units. The line defects are partial screw dislocations, with characteristic displacement vectors of the type <(a/2), 0, (c/6)>, bounding the stacking faults. The existence of this type of defect demonstrates that there is an oxygen deficiency within the CuO(2) layers. However, unlike the open ReO(3) type-related structures, the packing density of the K(2)NiF(4) structure necessarily requires that anion defects be accompanied by the loss of cations of the A type (lanthanum, strontium). Under identical synthesis conditions, no defects are observed in the parent compound La(2)CuO(4).     

Atomic Control of the SrTiO3 Crystal Surface

The atomically smooth SrTiO(3) (100) with steps one unit cell in height was obtained by treating the crystal surface with a pH-controlled NH(4)F-HF solution. The homoepitaxy of SrTiO(3) film on the crystal surface proceeds in a perfect layer-by-layer mode as verified by reflection high-energy electron diffraction and atomic force microscopy. Ion scattering spectroscopy revealed that the TiO(2) atomic plane terminated the as-treated clean surface and that the terminating atomic layer could be tuned to the SrO atomic plane by homooepitaxial growth. This technology provides a well-defined substrate surface for atomically regulated epitaxial growth of such perovskite oxide films as YBa(2)Cu(3)O(7-delta).     

Surface pinning as a determinant of the bulk flux-line lattice structure in copper oxide superconductors

Direct knowledge of crystal defects and their perturbation of magnetic flux lines is essential to understanding pinning and to devising approaches that enhance critical currents in superconductors with high critical temperatures (T(c)). Atomic force microscopy was used to simultaneously characterize crystal defects and the magnetic flux-line lattice in single crystals of Bi(2)Sr(2)CaCu(2)O(8). Images show that surface defects, which are present on all real samples, pin the flux-line lattice. Above a critical height, the pinning interaction is sufficiently strong to form grain boundaries in the bulk flux-line lattice. These results elucidate the structure of the defects that pin flux lines and demonstrate that surface pinning, through the formation of grain boundaries, can determine the bulk flux-line lattice structure in high-T(c) materials. The implications of these results to the bulk flux-line lattice structure observed in previous experiments and to enhancing critical currents are discussed.     

纳米氧化铜表面超疏水膜的制备

通过控制氧化法在铜基底表面制得了氧化铜纳米花瓣膜,然后分别用十二烷基硬脂酸、硬脂酸、十二烷基硫醇和线性低密度聚乙烯对其表面进行修饰.结果表明:试验得到了超疏水复合膜,表面接触角均超过150°,滚动角小于5°.其中,十二烷基硫醇修饰时,可在短时间内(1h)得到疏水性较高的纳米复合膜,表面的接触角达到165°.通过晶型分析讨论纳米氧化铜的形成机理,并用X-射线粉末衍射(XRD)、接触角测量、表面反射红外光谱(IR)及扫描电子显微镜(SEM)对复合膜进行了表征分析,结果表明,试验成功制备了具有不同形貌的超疏水性纳米结构复合膜.