Novel 1122 thallium cuprates showing high Tc superconductivity: TlCa1-xY,Ba2Cu2O y,T,1-xPb x CaSr2Cu2O y and TlCa0.5Ln0.5Sr2Cu2O y

Substitution of Ca by Y in TlCaBa₂Cu₂O _y does not favour superconductivity, but substitution of Tl by Pb or of Ca by Ln (Ln = Y or rare earth) in TlCaSr₂Cu₂O _y results in high T_c superconductivity (T_c π 60–90 K). TlCa_1-xLn _x Sr₂Cu₂O _y is a new series of high T_c superconductors, but the x = 0.0 composition does not exhibit bulk superconductivity.     

Hydrostatic pressure effect on Tc of new BiS2‐based Bi4O4S3 and NdO0.5F0.5BiS2 layered superconductors

We investigate the external hydrostatic pressure effect on the superconducting transition temperature (T_c) of new layered superconductors Bi₄O₄S₃ and NdO_0.5F_0.5BiS₂. Though the T_c is found to have a moderate decrease from 4.8 K to 4.3 K (dT_c^onset/dP = –0.28 K/GPa) for Bi₄O₄S₃ superconductor, the same increases from 4.6 K to 5 K (dT_c^onset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO_0.5F_0.5BiS₂ superconductor. The variation of T_c in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Electrical and infrared study of Bi2Sr2Ca1Cu2O8 in semiconducting, superconducting ceramic and superconducting glass ceramic state

We have studied the electrical and infrared properties of Bi₂Sr₂Ca₁Cu₂O₈ compound in three states. Electrical and IR measurements show that the pure powder state sample is a semiconductor, the ceramic Bi₂Sr₂Ca₁Cu₂O₈ sample prepared after annealing at 820°C for 240 hours shows a T_c of 85 K, whereas Bi₂Sr₂Ca₁Cu₂O₈ sample prepared through glassy route, i.e. melting at 1250°C and annealing at 820°C for 240 hours shows a drop of T_c by 5 K. The infrared spectra of superconducting ceramic and glass ceramic states in the available frequency range of measurement reveals the presence of three phonons. Since the vibrational mode around 595 cm^–1 is due to CuO₂ layers and as the CuO₂ layers are responsible for T_c in the ceramic superconductors, any change in these layers will affect the T_c. The shifting of the 595 cm^–1 mode towards lower frequencies in the glass ceramic due to different preparation process, indicates that there is a change in CuO₂ layers resulting in a change of T_c, which is confirmed by Four probe dc measurements.     

Copper Defects and Conductivity in Bi—Pb—Sr—Ca—Cu—O Glasses

Glass ceramics of the composition (Bi_0.8Pb_0.2)₄Sr₃Ca₃Cu₄O₈ prepared by the melt quenching technique and the crystalline phases produced by the rapid thermal annealing have been studied by electrical resistivity and electron paramagnetic resonance (EPR) measurements in the temperature range from liquid helium up to room temperature. The concentration of the EPR active Cu^2? paramagnetic centers decreases as conductivity increases for the glass ceramics and disappears after crystallization and the growth of superconducting phases, similar to bulk high-T_c superconductors. The KPR spectra of both glass and crystallized ceramics after short-time annealing indicate the coexistence of Cu^2? paramagnetic ions and the exchange coupled clusters.     

In situ preparation of Bi-Sr-Ca-Cu-O superconducting films by laser sputtering

Superconducting films of Bi-Sr-Ca-Cu-O on (100) MgO substrates have been fabricated by XeCl-excimer-laser sputtering from ceramic targets of Bi_2.5Sr₂Ca₂Cu₃O_y in O₂ atmosphere. The films were polycrystalline with the c-axis (30.80±0.02Å) preferably oriented normal to the substrate surface. Without post-annealing the films showed metallic resistance behavior with zero resistance temperatures of up to T_c(0) }- 79 K. The critical current density of the films had values of up to j_c(50K)}- 10⁴ A/cm².     

Neutron powder diffraction and superconductivity in Ca substituted Bi-0212

In the system Bi-0212 the carrier concentration can be changed by both cation substitution and oxygen content. The crystal structure of Ca substituted material was refined from neutron powder diffraction data for Bi_0.5Sr_1.5Ca_0.5Y_0.5Cu_1.5Fe_0.5O_y. It is shown that extra oxygen is introduced in the cation layers between the double sheets of Cu/O pyramids. In superconducting material the familar dependence ofT _c on the hole carrier concentration is observed. The higherT _c is situated at 75 K.     

含Sb的Bi系超导体的研究

Bi-Sr-Ca-Cu-O系中掺入适量的Sb,其高温相(110K相)含量明显多于不掺Sb样品。Sb具有加速高温相形成的作用。对于各义组分为Bi_2-xSb_xSr₂Ca_2.5Cu_3.7O_y的样品,x=0.1最有利于高温相的形成。用液氮中淬火后低温回火的方法,发现在865℃烧结时样品中首先形成的是低温相(80K相),然后再逐渐转变为高温相。T_c值随烧结时间的延 关键词：     

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.     

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.     

Superconductivity in the LnBaCuO system (Ln = Y,Ho, Er)

Superconductivity around 90K is observed in mixed and single phase system of LnBaCuO (Ln = Y, Ho, Er). Resistivity, X-ray powder diffraction and AC susceptibility measurements were carried out on samples of the mixed phase systems Ln_2−xBa_xCuO_4−y (Ln = Ho, Er), Y_1.2Ba_0.8Cu_1+xO_4+x−y(0<x<5) and single phase system Ba₂LnCu₃O₇ (Ln = Y, Ho, Er). Ln_2−xBa_xCuO_4−y consists of Ba₂LnCu₃O₇, BaCuLn₂O₅, Cu₂Ln₂O₅ and starting materials and shows a small diamagnetic signal below T_c in the temperature dependent AC susceptibility. Y_1.2Ba_0.8Cu_1+xO_4+x−y is a mixture of Ba₂YCu₃O₇, BaCuY₂O₅ and CuO and also indicates a small diamagnetic signal at 92K. Interesting resistivity anomalies occur in the Y_1.2Ba_0.8Cu₄O_7−y and Ho_0.8Ba_1.2CuO_4−y composites including large drops in the resistivity at ∼220 and ∼120K, respectively. The pure compounds, Ba₂LnCu₃O₇ (Ln = Y, Ho, Er) show sharp resistive superconducting transitions and bulk Meissner effects of 90%.     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Critical addition of Ti to maintain the Tc of (Bi1-zTiz)1.6Pb0.4Sr2Ca2Cu3Ox above 100 K

Influence of Ti addition (substitution for Bi) on T_c is studied for high T_c (Bi_1-zTi_z)_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. We found a critical Ti addition to keep high T_c (above 100 K) of the (Bi_1-zTi_z)_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. The high-T_c phase can be found up to 0.5 of z. It is explained by an abnormal T_c change which is found from 0.3 to 0.5 of z. Based on results of the electrical resistivity and x-ray diffraction, an abnormal T_c change is discussed.     

Effect of Li doping on the critical temperature and glass formation in the Bi-Sr-Ca-Cu-O system

A study has been made of the glass-forming ability, structure, and superconducting properties of Bi_2.2Sr_1.8Ca_1.05Cu_2.15Li_xO_y and Bi_2.2Sr_1.8Ca_1.05Cu_2.15−x Li_xO_y (x=0;0.3;0.5;0.7). The compounds were melted by rf at T=1300–1500 °C. Rapid quenching produces glassy alloys whose glass-forming ability is the highest when lithium is substituted for copper. Glass annealing at 700–800 °C results in the formation of the HTSC phase 2212 with a critical temperature of up to 91 K. In lithium-doped samples the HTSC phase forms at lower temperatures and shorter anneals and it depends on the cooling rate following the anneal. The composition and properties of the 2212 phase depend nonmonotonically on the anneal time. The lattice parameter C of the 2212 phase increases with increasing lithium content. Fiz. Tverd. Tela (St. Petersburg) 41, 18–21 (January 1999)     

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.     

Si doped Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductors

The Si doped (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25) superconductor samples have been synthesized to investigate the effect of mobile carriers on the critical temperature and magnitude of diamagnetism of this family of cuprates. The Si doped samples have tetragonal structure as observed from the X-ray diffraction spectra. The c-axis length of the unit cell of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ was increased after the increase of Si concentration, whereas the critical temperature and the magnitude of diamagnetism have been decreased. The decrease in T_c(0) and magnitude of diamagnetism is possibly due to the deficiency of carriers in CuO₂/SiO₂ planes caused by the +4 state of Si atoms. However, the post-annealing of these samples in oxygen atmosphere has increased the critical temperature and the magnitude of diamagnetism. The FTIR absorption measurements of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ samples have shown a softening of the apical oxygen mode of the type Cu(1)O_ACu(2) and TlO_ACu(2) with increased concentration of Si in the unit cell; the softening of this mode is directly linked with the increase in the c-axis lattice parameter of the (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ superconductor.     

Synthesis and superconducting properties of bscco including precipitates with high density

Bi₂Sr₂Ca₁Cu₂O _y (2212) superconductors including normal particles with high density were synthesized by a melt process, and the temperature and magnetic field dependence of the critical current density was estimated from the magnetization hysteresis. The temperature dependence of the irreversibility field, B_r, was expressed as (1-T/T_c)^3/2 above 50 K, and became weak below 50 K as compared with that in the single-crystalline (2212). The B_r, in the magnetic field parallel to the c-axis was about one order of magnitude lower than in a magnetic field normal to the c-axis.     

Neutron powder diffraction and superconductivity in Ca substituted Bi-0212

In the system Bi-0212 the carrier concentration can be changed by both cation substitution and oxygen content. The crystal structure of Ca substituted material was refined from neutron powder diffraction data for Bi_0.5Sr_1.5Ca_0.5Y_0.5Cu_1.5Fe_0.5O_y. It is shown that extra oxygen is introduced in the cation layers between the double sheets of Cu/O pyramids. In superconducting material the familar dependence ofT _c on the hole carrier concentration is observed. The higherT _c is situated at 75 K.     

Submillimeter transmission spectra of Sr2Ca2Cu4Oy

Transmission spectra of Sr₂Ca₂Cu₄O_y, which is a part of high T_c superconducting system Bi₂Sr₂Ca_n–1Cu_nO_z, and their temperature evolutions were investigated by monochromatic quasioptical submillimeter spectroscopy. Strong increase of high frequency conductivity in the range 80 to 350K was revealed. The obtained results were analyzed on basis of thermally activated carriers model.     

Temperature dependence of the critical current in YBa2Cu3O7−δ and Bi2Sr2Ca1Cu2O8 Josephson junctions

We have studied the stationary Josephson effect on YBa₂Cu₃O_7−δ (T_c=90 K) and Bi₂Sr₂Ca₁Cu₂ O₈ (T_c=80 Kand 87 Kfortwosamplesofdifferentorigin) ceramic based junctions. The temperature dependence of the critical current near T_c has been found as I_c≈(T_c-T) for the Y-Ba-Cu-O samples indicating that they should be classified as S-N-I-N-S type junctions. The I-V curves of the Bi-Sr-Ca-Cu samples show the typical behaviour of S-I-S structures. Using Ambegaokar-Baratoff's theory for Bi₂Sr₂Ca₁Cu₂O₈, the temperature dependence of the superconducting state gap Δ(T) was calculated and it was evaluated that 1.452Δ(0)/k_BT_c3.5.