High-Jc GdBa2Cu3O7−δ films fabricated by the metal–organic deposition process

We report a successful fabrication of high-J_c GdBa₂Cu₃O_7−δ (GdBCO) films by the metal–organic deposition process on the LaAlO₃ (LAO) (0 0 1) substrates. The coating solution was prepared by mixing Gd, Cu fluorine-free sources with Ba trifluoroacetate. Samples were dip-coated, pyrolized within 3 h at the temperature up to 400 °C in a humid oxygen atmosphere, and finally fired at various high temperatures in 100 ppm Ar/O₂ atmosphere. The GdBCO films fired at 775 and 800 °C exhibited J_c values of ∼2.1 MA/cm² at 77 K in a self-field, which are attributable to both high T_c,zero values of ∼89 K and high in-plane textures of 1.3–1.4°. Above 800 °C, however, the superconducting properties of GdBCO films are degraded due to the thermal decomposition of GdBCO film in 100 ppm Ar/O₂ atmosphere.     

Dielectric properties of Cu0.5Tl0.5Ba2Ca3Cu4O12−δ bulk superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12−δ superconductor samples were studied at 79 and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 kHz to 10 MHz. A negative capacitance (NC) phenomenon has been observed, which is most likely arising due to higher Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge located at the multiple insulator–superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 79 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Hall effect and sign reversal in Gd(Ba2−xNdx)Cu3O7+δ

We have investigated the Hall measurement of Gd(Ba_2−xNd_x)Cu₃O_7+δ with x=0, 0.05, 0.1, 0.15, and 0.2 in a magnetic field of 0–1 T. The nominal samples has been prepared by the conventional solid-state reaction technique. The iodometric titration experiment was carried out for samples. The Rietveld analysis of the X-ray diffraction patterns indicates that samples are mainly single phase. The normal state Hall coefficient behaves as 1/T in all samples. All samples with nominal compositions show single sign reversal with variation of magnetic field and temperature. The Hall resistivity of the samples with x=0 and 0.1 close to the superconducting transition temperature changes its sign with decreasing temperature and tends to a minimum −Δ_max, and then monotonically goes to zero. The absolute value of −Δ_max decreases with the increase of magnetic field. This can be qualitatively explained by a model calculation based on the time-dependent Ginsburg–Landau theory.     

Fermi-arc superconductivity, pseudogap and charge order in Bi2Sr2CaCu2O8+δ

We report STM/STS observations on the 4a×4a charge order in the pseudogap (PG) and superconducting (SC) states of Bi₂Sr₂CaCu₂O_8+δ, and suggest that the charge order is associated with incoherent quasiparticle or pair states around the antinodal fermi surface (FS), which are also responsible for the PG, and it can coexist with the superconductivity caused by the pairing of coherent quasiparticles on the nodal fermi arc. We also suggest that the nanometer-scale gap inhomogeneity in the SC state, reported previously [Pan, et al., Nature 413 (2001) 282; Mommo, et al., J. Phys. Soc. Jpn. 74 (2005) 2400; Hashimoto, et al., Phys. Rev. B74 (2006) 064508] arises from that in the PG state, which occurs around the antinodal FS.     

Sr doping effects on the transport and magnetic properties of GdBaCo2O5+δ

Studies on some new members of the cobalt perovskite Gd_1−xSr_xBaCo₂O_5+δ with low strontium concentrations (0<x<0.1) have been carried out with the aim of investigating possible metallization of the GdBaCo₂O_5+δ system by hole doping. Low temperature electrical resistivity, magnetic susceptibility and thermopower of the above system have been studied. The pristine compound with x=0 and δ∼0.5 exhibits a semiconductor-like behavior and two magnetic transitions below room temperature. Upon Sr²⁺ substitution, there is a fall in resistivity by 2-3 orders of magnitude at low temperature and also a dramatic reduction in the ferromagnetic to antiferromagnetic transition temperature. These changes can be explained on the basis of hole doping (and increase in the Co⁴⁺content). Evidence for an increase in Co⁴⁺ with Sr²⁺ substitution is provided by iodometric analysis.     

Superconducting gap of overdoped Tl2Ba2CuO6+δ observed by Raman scattering

We report Raman scattering spectra for single crystals of overdoped Tl₂Ba₂CuO_6+δ (Tl-2201) at low temperatures. It was observed that the pair-breaking peaks in A_1g and B_1g spectra radically shift to lower energy with carrier doping. We interpret it as s-wave component mixing into d-wave, although the crystal structure is tetragonal. Since similar phenomena were observed also in YBa₂Cu₃O_y and Bi₂Sr₂CaCu₂O_z, we conclude that s-wave mixing is a common property for overdoped high-T_c superconductors.     

Superconductivity and critical fields in undoped and Sn-doped MoSr2YCu2O8−δ

Samples with nominal compositions MoSn_xSr₂YCu₂O_8−δ (0 ? x ? 0.075) were synthesized and their superconducting and magnetotransport properties were investigated. It was established that the optimum Sn-doping (x = 0.02 and 0.03) increases the T_c of the undoped MoSr₂YCu₂O_8−δ. The upper critical fields of the samples were determined using magnetization, susceptibility and resistivity measurements. It was established that the Sn-doping weakly affects the first and the intragrain second critical field of Mo-1212 but enhances the extrapolated to T = 0 (i.e. the intergrain) upper critical field. The observed phenomena were discussed on the basis of the Josephson-junction-arrays model. A phenomenological expression describing the temperature dependency of the susceptibility of Mo-1212 was found.     

Magnetic behavior of MnCo2O4+δ spinel obtained by thermal decomposition of binary oxalates

Manganese cobaltites MnCo₂O_4.62 and MnCo₂O_4.275 having a spinel structure were studied by measuring magnetization, AC susceptibility and by XANES spectroscopy. These compounds were synthesized by decomposition of the binary oxalate Mn_1/3Co_2/3C₂O₄·2H₂O in air at 220 and 500 °C, respectively. It was found that the differences in magnetic characteristics of these cation-deficient spinels are due mainly to variations in the degree of oxidation of manganese. It was shown that the complex oxide MnCo₂O_4.62 formed right after decomposition of the binary oxalate contains about 5×10⁻⁴ mass% metallic cobalt, which determines the dependence of magnetic susceptibility χ on the magnetic field at 300 K. The magnetic transition peculiar to the stoichiometric spinel MnCo₂O₄ at 183 K decreases to 167.5 K for MnCo₂O_4.275 and 67.5 K for MnCo₂O_4.62.     

Y1−xCaxBa2Cu3O7−δ thin films: From phase fluctuations to a complex order parameter

This report presents a study of the temperature dependence of the penetration depth in Y_1−xCa_xBa₂Cu₃O_7−δ thin films, which shows that a contribution to the order parameter from phase fluctuations in optimally doped samples cannot be excluded, and demonstrates the appearance of an imaginary component in the order parameter in overdoped samples. Measurements were performed using two complementary techniques: the parallel plate resonator (PPR), operated at 10 GHz, and far infrared (FIR) transmission, where the absolute value of the penetration depth can be obtained.     

Anomalous Hall effect in sputter-deposited CoxTi1−xO2–δ films

Hall effect measurements were performed on epitaxial Co_xTi_1−xO_2–δ thin films grown on (0 0 1) LaAlO₃ by reactive RF magnetron co-sputter deposition. Magnetization measurements reveal ferromagnetic behavior in M–H loop at room temperature for Co_xTi_1−xO_2–δ thin films for which x?0.02. An anomalous Hall effect was observed for Co_0.10Ti_0.90O_2−δ films grown with the partial pressure of water P(H₂O)=4×10⁻⁴ Torr or less. These films exhibit a positive ordinary Hall coefficient and a positive magnetoresistance. X-ray diffraction on films grown under these conditions shows evidence for Ti_nO_2n−1 phase due to the deficiency of oxygen. In contrast, Hall measurements taken for undoped and Co-doped TiO₂ thin films grown under more oxidizing conditions show only the ordinary Hall effect with a negative Hall coefficient consistent with n-type conduction. For these films, the magnetoresistance was positive and increased monotonically with increasing magnetic field. The results suggest that Co-doped Ti_nO_2n−1 may be a dilute magnetic semiconducting oxide for which the carriers couple to the spin polarization.     

YBa2Cu3O6+δ的高频模及其随氧含量δ的变化

在中国原子能科学研究院新建的宽角(～30°)Be过滤探测器中子非弹性散射谱仪上,在入射中子能量从10直至150meV的范围内,测量了几种不同氧含量的YBa₂Cu₃O_6+δ样品的中子非弹性散射能谱。结果表明:(1)在δ=0和0.2时,在60至150meV能量范围内,发现有强的高频模存在,当其温度高到Nel点(～410K)以上直至466K时,没有观察出高频模强度的减弱。而δ=0.78和0.97时,在75至150meV能量范围内,散射强度在测量误 关键词：     

Critical RF losses in fine particles of La1−xAgyMnO3+δ: Prospects for temperature-controlled hyperthermia

Temperature and frequency dependencies of the real (χ′) and imaginary (χ″) parts of the dynamic magnetic susceptibility were studied experimentally in fine particles of La-Ag manganites prepared by various methods. The samples under study have the Curie points in the range T_C=42-48 °C, which is a medical hyperthermia range of interest. When approaching T_C from below, a critical peak of χ″ was revealed, followed by a steep drop while passing to the paramagnetic phase. The experiments on the magnetic radio-frequency (RF) heating of the manganite aqueous suspensions demonstrated good autostabilization of the temperature near T_C. Peculiar instability is found in the heating kinetics, caused by the observed critical behavior of the RF losses. The prospects of the La-Ag manganites as candidates for application in the temperature-controlled hyperthermia are discussed.     

High-temperature electronic transport properties of La1−xCaxMnO3+δ (0.0?x?1.0)

La_1−xCa_xMnO_3+δ (0.0?x?1.0) samples were prepared and their resistivity and Seebeck coefficients were measured in the high-temperature range. Ca doping changes the ratio of Mn³⁺/Mn⁴⁺ and influences the electronic transport behavior markedly. With the increase of Ca concentration, the samples change from a p-type semiconductor to an n-type one and Seebeck coefficient becomes increasingly negative. Low doping (x=0.2) and high doping (x=0.8) induces the drop of the resistivity compared with undoped LaMnO_3+δ and CaMnO_3+δ samples due to the rise of carrier concentration. However, the resistivity of moderate-doped samples (x=0.4, 0.6) is larger than low- and high-doped samples because dopant scattering decreases carrier mobility.     

First study of the B1g buckling phonon mode in optimally doped, de-twinned, YBa2Cu3O7−δ by inelastic X-ray scattering

We have measured the buckling, B_1g, phonon mode of optimally doped, de-twinned, YBa₂Cu₃O_7−δ using inelastic X-ray scattering (IXS) at BL35XU of SPring-8. Measurements of this mode, which has atomic motion transverse to the Cu-O planes, serve to demonstrate some of the advantages of the unique two-dimensional (2-D) analyzer array at BL35. Analysis based on fitting the entire spectra simultaneously at 10 and 100 K shows that the buckling mode is rather broad in the middle of the zone (near (0.3 0 0)). We see a consistent softening of the mode at 10 K as compared to 100 K for all measured wave-vectors (h 0 0), and a q-dependent softening as zone center is approached.     

NdBa2Cu3O7-δ/YBa2Cu3O7-δ多层膜体系的外延结构和磁通钉扎的研究

采用脉冲激光沉积技术,在单晶SrTiO₃基底上外延生长了一系列名义结构为p×(NdBa₂Cu₃O_7-δ(m)/YBa₂Cu₃O_7-δ(n))的多层膜和准多层膜(单元层NdBa₂Cu₃O_7-δ较厚而YBa₂Cu₃O_7-δ呈非连贯的岛状分布,m,n为激光脉冲数,p为重复周期).样品的超导转变温度在87—91 K范围,具体大小取决于不同的调制结构,多层膜的重复周期越大,层状界面越多,超导转变温度就越低.磁传输测量表明,准多层的样品不仅具有较高的超导转变温度,而且具有较强的磁通钉扎性能,77K零场下的临界电流密度高达4×10⁶ A/cm²,显示出良好的应用前景. 关键词： 2Cu₃O_7-δ')" href="#">NdBa₂Cu₃O_7-δ 多层膜 磁通钉扎 临界电流密度     

YBa2Cu3O6+δ/BaTiO3系复合功能陶瓷特性研究

报道了超导陶瓷ＹＢａ₂Ｃｕ₃Ｏ_6+δ与铁电陶瓷ＢａＴｉＯ₃进行复合的结果。研究了该复合功能陶瓷的物相、（超）导电性和低温电阻温度特性。结果表明，采用合理的合成工艺，可得到呈现混和分布的两相复合功能陶瓷材料；该复合材料的电导特征符合三维渗流导电行为，发现在较高ＹＢａ₂Ｃｕ₃Ｏ_6+δ含量时，样品呈超导电性，并对此作了初步讨论。 关键词：     

The epitaxial growth of BaSnO3 buffer layer on IBAD-MgO template and its effects on GdBa2Cu3O7−δ film: A preliminary study

We report a successful fabrication of c-axis oriented GdBa₂Cu₃O_7−δ (GdBCO) films on the BaSnO₃ (BSO) buffer layers on ion-beam assisted deposition (IBAD)-MgO template by pulsed-laser deposition (PLD). The (0 0 l) growth and in-plane textures of BSO buffer layers were found sensitive to the substrate temperature (T_s). With increasing the BSO layer thickness up to ∼165 nm, in-plane texture (Δ? ∼ 6.2°) of BSO layers was almost unaltered while completely c-axis oriented BSO layers were obtainable from samples with the thickness below ∼45 nm. On the BSO buffer layers showing in-plane texture of 6.2° and RMS surface roughness of ∼8.6 nm, GdBCO films were deposited at 780–800 °C. All GdBCO films exhibited Δ? values of 4.6–4.7°, T_c,zero of ∼91 K, and critical current density (J_c) over 1 MA/cm² at 77 K in a self-field. The highest J_c value of 1.82 MA/cm² (I_c of 51 A/cm-width) was achieved from the GdBCO film deposited at T_s of 790 °C. These results support that BSO can be a promising buffer layer on the IBAD-MgO template for obtaining high-J_c GdBCO coated conductors.     

La2−xSrxCuO4−δ superconducting samples prepared by the wet-chemical method

In this work, we report on the physical properties of good-quality polycrystalline superconducting samples of La_2−xSr_xCu_1−yZn_yO_4−δ (y=0, 0.02) prepared by a wet-chemical method, focusing on the temperature dependence of the critical current. Using the wet-chemical method, we were able to produce samples with improved homogeneity compared to the solid-state method. A complete set of samples with several carrier concentrations, ranging from the underdoped (strontium concentration x≈0.05) to the highly overdoped (x≈0.25) region, were prepared and investigated. The X-ray diffraction analysis, zero-field cooling magnetization and electrical resistivity measurements were reported on earlier. The structural parameters of the prepared samples seem to be slightly modified by the preparation method and their critical temperatures were lower than reported in the literature. The temperature dependence of the critical current was explained by a theoretical model which took the granular structure of the samples into account.     

Effect of granularity and inhomogeneity in excess conductivity of YBa2Cu3O7−δ+xBaTiO3 superconductor

Synthesis of polycrystalline YBCO+xBaTiO₃ (x=1.0, 2.5, 5.0) superconductor has been done and the effects of granularity and inhomogeneities due to inclusions of nano-BaTiO₃ in excess conductivity are reported in this work. The phase formation, texture and grain alignments were analyzed through XRD and SEM techniques. SEM results reveal that the grain size is reduced and morphology is improved with the incorporation of nano-BaTiO₃ particles. Superconducting order parameter fluctuation (SCOPF) studies on the electrical conductivity were investigated from the resistivity vs. temperature data in the experimental domain relatively above T_c. Log(Δσ) vs. log(ε) plots show that the 2D to 3D crossover temperature (T_LD) that demarcates dimensional nature of fluctuation inside the grains is influenced by BaTiO₃ incorporation in YBCO matrix. An upward shift of T_LD in the mean field region has been observed as a consequent dominance of 3D region with increase in 1 wt% BaTiO₃ in the composites as compared to higher inclusions. It has been analyzed that microscopic inhomogeneities produced as a result of diffusion of a fraction of Ti ions into the grains affect fluctuations in the excess conductive region. The interplay of microscopic inhomogeneities produced inside the grains and mesoscopic inhomogeneities in the grain boundaries on the excess conductivity has been explained in terms of thermal fluctuations for the composites.     

Structural investigation of the BaSnO3–YBa2Cu3O7−x system

We report the microstructure change of BaSnO₃ (BSO)–YBa₂Cu₃O_7−x (YBCO) thin film system grown on SrTiO₃ single crystal substrates by pulsed laser deposition with using the “surface-modified-target” and “mixture-target” methods. Although it was confirmed that the thick BSO nanorods incorporated to YBCO films act as strong artificial pinning centers, the formation mechanism of the nanorods is still unclear. The purpose of this work is to extend the structural investigation to higher contents of BSO (up to 71 vol.%) in order to enlighten the relationship among interfacial energy, morphology and pinning performance in binary BSO–YBCO films.