Analysis of thermoelectric power of some insulating high-T c superconductors using extended hubbard model

The thermoelectric power (TEP) for a one-dimensional lattice has been studied using the extended Hubbard model in the limitU ≠ ∞, whereU is the on-site Coulomb interaction. A new expression for TEP has been derived in this study. Our study shows that if theV-dependent term in the extended Hubbard model Hamiltonian is considered as attractive, the new expression for TEP could successfully reproduce the TEP results of high-T _c hole-doped insulating systems of Tl₂Ba₂Ca_1−x Y _x Cu₂O_8+y (Tl-2212) and Bi₂Sr₂Ca_−x Y _x Cu₂O_8+y (Bi-2212).     

Electron spin resonance in manganese-doped BSCCO (2212) crystals

The electron spin resonance (ESR) spectra of undoped and manganese-doped BSCCO (2212) crystals (nominal composition Bi₂Sr₂CaCu_2–x Mn_xO_y, where x=0.1) are compared. Zh. Tekh. Fiz. 67, 137–138 (October 1997)     

Superconductivity in Bi2.2Ca2.8−x Sr x Cu2O y system and effect of rare earth substitution

The oxide system Bi_2.2Ca_2.8−x Sr _x Cu₂O _y has been investigated for superconductivity as a function of Ca/Sr ratio withx=0.7, 1.0, 1.4 and 1.8. All these compositions are found to be superconducting with onset temperature in the range of 80–90 K implying a large homogeneity range between Sr and Ca for the superconductivity to occur in this system. Effect of partial replacement of Bi by rare earths has been studied and it is observed that 20% replacement of Bi by Nd and Eu does not significantly affect the superconducting transition temperature.     

The effect of Sb concentration on the superconducting properties and phase composition of Bi-Pb-Sb-Sr-Ca-Cu oxide materials

The effect of Sb superconducting materials with nominal composition Bi_1.6Pb_0.4–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.01–0.14) and Bi_1.5Pb_0.5–x Sb _x Sr₂Ca₂Cu₃O _y (x=0.025–0.12) has been investigated. These materials were subjected to DTA, ICP emission spectrometry, SEM, XRD and electric resistance measurements. It was found that the introduction of Sb raises the melting temperature of the material, it also raisesT _c (up to 110K) at several defined stoichiometric ratios. A certain lack of correspondence between the 2223 phase volume and theT _c value was noticed and a possible explanation has been proposed.     

A tunable green alkaline-earth silicon-oxynitride solid solution (Ca1? x Sr x )Si2O2N2:Eu2+ and its application in LED

A series of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ (x=0.0–0.97, y=0.03) phosphors were synthesized by high-temperature solid-state reaction. The XRD patterns confirm the formation of a solid solution of (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺. An intense tunable green light is observed with the increasing ratio of Sr/Ca. With an increase in x, the excitation and emission spectra show a redshift and blueshift, respectively, due to large centroid shift and small Stokes shift. The temperature dependent luminescence is also investigated in the temperature range of 77–450 K. The Huang–Rhys factor and the thermal-quenching temperature are determined. Intense green LEDs were successfully fabricated based on the (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ phosphor and near-ultraviolet (∼395 nm) GaN/blue (460 nm) InGaN chips. All the results indicate that the solid solution (Ca_1−x−y Sr _x )Si₂O₂N₂:yEu²⁺ is a promising phosphor applicable to near-UV and blue LEDs for solid-state lighting.     

Structure, electrical properties and temperature characteristics of?Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–Bi0.5Li0.5TiO3 lead-free piezoelectric ceramics

(1−x−y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃– yBi_0.5Li_0.5TiO₃ lead-free piezoelectric ceramics have been prepared by an ordinary sintering technique, and their structure, electrical properties, and temperature characteristics have been studied systematically. The ceramics can be well-sintered at 1050–1150 °C. The increase in K⁺ concentration decreases the grain-growth rate and promotes the formation of grains with a cubic shape, while the addition of Li⁺ decreases greatly the sintering temperature and assists in the densification of BNT-based ceramics. The results of XRD diffraction show that K⁺ and Li⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure. As x increases from 0.05 to 0.50, the ceramics transform gradually from rhombohedral phase to tetragonal phase and consequently a morphotropic phase boundary (MPB) is formed at 0.15≤x≤0.25. The concentration y of Li⁺ has no obvious influence on the crystal structure of the ceramics. Compared with pure Bi_0.5Na_0.5TiO₃, the partial substitution of K⁺ and Li⁺ for Na⁺ lowers greatly the coercive field E _c and increases the remanent polarization P _r of the ceramics. Because of the MPB, lower E _c and large P _r, the piezoelectricity of the ceramics is improved significantly. For the ceramics with the compositions near the MPB (x=0.15–0.25 and y=0.05–0.10), the piezoelectric properties become optimum: piezoelectric coefficient d ₃₃=147–231 pC/N and planar electromechanical coupling factor k _P=20.2–41.0%. In addition, the ceramics exhibit relaxor characteristic, which probably results from the cation disordering in the 12-fold coordination sites. The depolarization temperature T _d shows a strong dependence on the concentration x of K⁺ and reaches the lowest values at the MPB. The temperature dependences of the ferroelectric and dielectric properties at high temperatures may imply that the ceramics may contain both the polar and non-polar regions at temperatures above T _d.     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Measurements of the London penetration depth in Bi-based high-T c compounds

The muon spin relaxation was measured in the high-T _c superconductors Bi₂Sr_1.3Ca_0.7 CuO_6.15, Bi₂Sr₂CaCu₂O₈ and Bi₂Pb _x Sr₂Ca₂Cu₃O₁₀. We present our method to determine the London penetration depth perpendicular toc-axis λ_⊥ (0) taking also contributions from the anisotropic magnetization and nuclear dipoles to the muon frequency spectrum into account.     

Anomalous effect of magnetic field on the low-temperature thermal expansion of the Bi2Sr2?xLaxCuO6+δ and Nd2?xCexCuO4?δ HTSC systems in the non-superconducting phase

The thermal expansion of single crystals of the Bi₂Sr_2−x La _x CuO_6+δ high-temperature superconducting (HTSC) system in the insulating phase with compositions having no superconducting transition to a temperature of 1.8 K (x ≥ 0.8) is measured in an arbitrary direction in the (ab) plane in the temperature range of 7–50 K. Temperature regions of material compression upon heating are found. The study of anomalies in magnetic fields of 3 and 6 T, parallel and perpendicular to the c-axis revealed an anisotropic and nonmonotonic effect of the field on thermal expansion. Such anomalies for the n-type Nd_2−x Ce _x CuO_4−δ HTSC sample also having no superconducting transition are detected for the first time. The results show that the anomaly nature is caused by anisotropic electronic ordering, probably, by the charge density wave in the CuO₂ plane and superconductivity fluctuations in the insulating phase.     

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：     

Progress in the lithium insertion mechanism in Cu3P

Copper phosphide, Cu₃P has been synthesized using a ceramic route, and its electrochemical behaviour versus lithium has been studied studied galvanostatic and potentiodynamic measurements and in situ X-ray diffraction analysis. The insertion/extraction mechanism proceeds with the formation of at least three different Li_xCu_3−xP (x=1, 2, 3) phases. The electrochemical behaviour of Cu₃P samples obtained from ceramic and solvothermal syntheses are compared to further understanding of the complex redox mechanism occurring during insertion/extraction. First-principle electronic structure calculations show that discharge probably begins with the formation of a solid solution Li_xCu_3−yP (x<0.5). Paper presented at the Patras Conference on Solid State Ionics-Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

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.     

Synthesis,Mössbauer studies and superconductivity of substituted Bi 2201 phase

We report X-ray diffraction, resistivity and Mössbauer studies on several substituted 2201 Bi cuprates, Bi₂Sr₂Cu_1?xFe_xO_y(O≤x≤1) and Bi₂Sr_1.6La_0.4Cu_1?xFe_xO_y and Bi_1.6Pb_0.4Sr_1.6La_0.4Cu_1?xFe_xO_y (O≤x≤0.04). The first series is not superconducting and a new phase appears for x=1. For La substituted samples, T_c increases up to 35K and decreases strongly with x. All Mössbauer spectra were fitted well with four quadrupole doublets, with relative population depending on samples.     

Structure, ferroelectric and piezoelectric properties of (Bi1−x−yNa0.925−x−yLi0.075)0.5BaxSryTiO3 lead-free piezoelectric ceramics

Lead-free multi-component ceramics (Bi_1−x−yNa_{0.925−x−y}Li_0.075)_0.5Ba_xSr_yTiO₃ have been prepared by an ordinary sintering technique and their structure and electrical properties have been studied. All the ceramics can be well-sintered at 1100 °C. X-ray diffraction patterns shows that Li⁺, Ba²⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) is formed at 0.04 < x < 0.08. As compared to pure Bi_0.5Na_0.5TiO₃ ceramic, the coercive field E_C of the ceramics decreases greatly and the remanent polarization P_r of the ceramics increases significantly after the formation of the multi-component solid solution. Due to the MPB, lower E_C and higher P_r, the piezoelectricity of the ceramics is greatly improved. For the ceramics with the compositions near the MPB (x = 0.04–0.08 and y = 0.02–0.04), piezoelectric coefficient d₃₃ = 133–193 pC/N and planar electromechanical coupling factor k_P = 16.2–32.1%. The depolarization temperature T_d reaches a minimum value near the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions at temperatures near/above T_d.     

Superconductivity, Seebeck coefficient, and band structure transformation in Y1−x CaxBa2Cu3−x CoxOy (x=0–0.3)

The temperature dependences of the resistivity and of the Seebeck coefficient S is studied in three series of Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples (x=0–0.3) differing in oxygen content. It was found that the critical temperature decreases for y≈7.0, and S(T=300 K) increases with doping, whereas oxygen deficiency results in a nonmonotonic variation of these quantities with increasing x. The band structure parameters have been determined from an analysis of the S(T) relations using a phenomenological theory of electron transport. It was found that an increase in x results in a gradual increase in band asymmetry, which is caused by calcium-induced creation of additional states in the band responsible for conduction in the normal phase. An analysis has shown that high impurity concentrations in oxygen-deficient Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples bring about an additional ordering of the structure, which may be caused by formation of a cobalt superlattice. It has also been shown that, in the case of Ca and Co codoping, the dependence of critical temperature on effective conduction-band width coincides with the universal correlation relation observed in the YBa₂Cu₃O_y system with single substitutions in various lattice sites. Fiz. Tverd. Tela (St. Petersburg) 41, 1363–1371 (August 1999)     

Dynamics and distribution of doped holes in the CuO2 plane of slightly doped antiferromagnetic YBa2(Cu1 ? zLiz)3O6 + x (x < 0.1) studied by Cu(1) NQR

Incidence of doped holes in the CuO₂ plane on the AF state was studied by Cu(1) nuclear quadrupole resonance (NQR) in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} compounds. Inhomogeneous distribution of doped holes in the plane was detected in the low temperature measurements of transverse (1/T ₂) and longitudinal (1/T ₁) relaxation rates. We establish that at lower T the holes motion slows down and we estimate that the holes localize finally in restricted regions (∼3 lattice constants) in the Coulomb potential of the Li+ ions. Also we compared the hole behavior in slightly doped YBa₂(Cu_{1 − z} Li _z )₃O_{6 + x} samples with that in slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆. A stronger trapping potential of the in-plane Li⁺ impurities was concluded as compared to slightly doped Y_{1 − y} Ca _y Ba₂Cu₃O₆ compound with out-of-plane Ca²⁺ impurities.     

Crystallization process of Bi(Pb)-Sr-Ca-Cu-O glass

A crystallization study has been carried out for rapidly solidified Bi₂Pb_0.5Sr₂Ca₄Cu₅O_x glass. Glass transition temperature T _g, crystallized superconducting phases and microstructural changes were measured and analysed by differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The crystallization mechanism of the three superconducting phases — (2201) 20 K phase, (2212) 80 K phase, and (2223) 110 K phase — has been discussed, and a time-temperature-transformation diagram for the glass has been constructed.     

EPR search for vortex excitations in Bi2Sr2Ca1 ? xYxCu2O8 + y crystals above the critical temperature

Excitations of the vortex type in the regions of temperatures both below and above critical temperature T _c are found by studying the local magnetic fields on the surface of Bi₂Sr₂Ca_{1 − x} Y _x Cu₂O_{8 + y} single crystals by means of EPR. The narrow EPR signal of the surface paramagnetic probe allows us to observe weak distortions of the magnetic field that arise when T > T _c. The behavior of the signal broadening agrees with the hypothesis about the existence of vortices in this temperature region.     

Ultrasonic studies of aluminium-substituted Bi(Pb)-2223 superconductors

The compositional dependence of elastic properties of Al³⁺-substituted Bi(Pb)-2223 superconducting system with the general formula Bi_1.7−x Al_xPb_0.3Sr₂Ca₂Cu₃ O_y (x = 0.0, 0.1, 0.2 and 0.3) have been studied by means of ultrasonic pulse transmission (UPT) technique at 1 MHz (300 K). The elastic moduli of the specimens are computed and corrected to zero porosity. The observed variation of elastic constants with aluminium substitution has been explained on the basis of the strength of interatomic bonding. The applicability of heterogeneous metal mixture rule for estimating elastic constants and transition temperature has been tested.     

110 k superconductivity in crystallized Bi-Sr-Ca-Cu-O glasses

Superconducting transition temperatures near 110 K are observed in the Bi-Sr-Ca-Cu-O system with the nominal compositions Bi₂Sr₂Ca₃Cu₄O_x and Bi₂Sr₂Ca₄Cu₅O_x. Electrical resistivity, magnetization, and X-ray diffraction data are reported. Also discussed is the effect of heat treatment on the thermodynamic behavior of the superconducting phases in the rapidly quenched glasses.