high-Tc Phase Research Articles

Two Superconducting Phases and Their Characteristics in Layered BaTi2(Sb1−xBix)2O with x = 0.16

Two correlated superconducting phases are identified in the layered superconductor BaTi2(Sb1—xBix)2O (x = 0.16), with the superconducting transition temperatures of TC = 6 K (the high TC phase) and 3.4 K (the low TC Phase), respectively. The 6 K superconducting phase appears first in the as-prepared sample and can decay into the low TC phase by exposure to an ambient atmosphere for a certain duration. Specially, the high TC phase can reappear from the decayed sample with the low TC phase by vacuum annealing. It is also found that the CDW/SDW order occurs only with the 6 K superconducting phase. These notable features and alteration of the superconductivity due to the post-processing and external pressure can be explained by the scenario of electronic phase separation.

Effect of sintering time on Bi1.6Pb0.4Sr1.8Ba0.2Ca2Cu3O10+? compound quenched in air and liquid nitrogen

High temperature superconductor with nominal composition Bi1.6Pb0.4Sr1.8Ba0.2Ca2 Cu3O10+? was prepared by solid state reaction method. Two sets of samples have been prepared .The first one was quenched in air; the second set was quenched in liquid nitrogen. X-ray diffraction analyses showed an orthorhombic structure with two phases, high –Tc phase (2223) and low-Tc phase (2212) in addition to that impure phase was found. It has been observed that quenched in air samples display a sharp superconducting transition and a higher-Tc phase than that of the quenched in liquid nitrogen samples.

Effect of sintering time on Bi1.6Pb0.4Sr1.8Ba0.2Ca2Cu3O10+? compound quenched in air and liquid nitrogen

High temperature superconductor with nominal composition Bi1.6Pb0.4Sr1.8Ba0.2Ca2 Cu3O10+? was prepared by solid state reaction method. Two sets of samples have been prepared .The first one was quenched in air; the second set was quenched in liquid nitrogen. X-ray diffraction analyses showed an orthorhombic structure with two phases, high –Tc phase (2223) and low-Tc phase (2212) in addition to that impure phase was found. It has been observed that quenched in air samples display a sharp superconducting transition and a higher-Tc phase than that of the quenched in liquid nitrogen samples.

Remnant of stripe order.

At a phenomenological level, the superconducting phase of the cuprate high Tc phase diagram shown in Fig. 1A can be characterized as a BCS (named for three theorists: Bardeen, Cooper, and Schrieffer) superconductor with a d-wave gap. However, a suitable phenomenological description of the so-called “pseudogap” region indicated by the yellow shaded region in Fig. 1A has been missing. This region is characterized by the absence of low-energy single-particle states with momenta near the part of the Fermi surface, where a maximum d-wave gap opens in the superconducting phase, leading to it being called the pseudogap region. A variety of experiments show evidence of short-range unidirectional charge and in some cases spin order in this region. However, there remained questions of whether the boundary shown as the black T* curve in Fig. 1A marks a cross-over or an actual phase transition, and if the latter, what characterizes the pseudogap phase. In PNAS, Nie et al. (1) discuss a phenomenological Landau–Ginzburg–Wilson theory of an incommensurate unidirectional charge density wave (stripes) in the presence of weak quenched disorder. The authors find that although long-range stripe order, schematically illustrated in Fig. 1B, is destroyed by the disorder, a remnant nematic order, shown in Fig. 1C, can survive. As Nie et al. discuss, this theory provides a framework for interpreting a number of experiments, as well as the basis for experimental protocols that can provide further tests for nematic order.

The unusually high Tc in rare-earth-doped single crystalline CaFe2As2

In rare-earth-doped single crystalline CaFe2As2, the mysterious small volume fraction which superconducts up to 49 K, much higher than the bulk Tc ~ 30 s K, has prompted a long search for a hidden variable that could enhance the Tc by more than 30% in iron-based superconductors of the same structure. Here we report a chemical, structural and magnetic study of CaFe2As2 systematically doped with La, Ce, Pr and Nd. Coincident with the high Tc phase, we find extreme magnetic anisotropy, accompanied by an unexpected doping-independent Tc and equally unexpected superparamagnetic clusters associated with As vacancies. These observations lead us to conjecture that the tantalizing Tc enhancement may be associated with naturally occurring chemical interfaces and may thus provide a new paradigm in the search for superconductors with higher Tc.

Stabilization of High-Tc Phase of BiS2-Based Superconductor LaO0.5F0.5BiS2 Using High-Pressure Synthesis

High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc) of Tconset = 11.1 and Tczero = 8.5 K in the electrical resistivity measurements and Tconset = 11.5 and Tcirr = 9.4 K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.

Effect of Nb addition on magnetic, structural and superconducting properties of (Bi, Pb)-2223 superconductors

In this work, the effects of Nb2O5 addition with different ratios on the structural and magnetic properties of Bi1.7−xPb0.3NbxSr2Ca2Cu3Oy (x = 0.00–0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (Jc) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi1.7−xPb0.3NbxSr2Ca2Cu3Oy superconductor. Critical onset (T c on ) and loss peak temperatures (Tp) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108–98 K with increasing Nb addition (x = 0.00–0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (Jc) by means of the Bean’s model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca2PbO4 as the minor. It was also shown from XRD measurements that volume fraction of high-Tc phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.

Evidence of two superconducting phases in Ca1−xLaxFe2As2

Single crystals of Ca1−xLaxFe2As2 with x ranging from 0 to 0.25, have been grown and characterized by structural, transport, and magnetic measurements. Coexistence of two superconducting phases is observed, in which the phase with the lower superconducting transition temperature (Tc) has Tc ∼ 20 K and the higher Tc phase has Tc higher than 40 K. These data also delineate an x-T phase diagram in which the single magnetic/structural phase transition in undoped CaFe2As2 appears to split into two distinct phase transitions, both of which are suppressed with increasing La substitution. Superconductivity emerges when x is about 0.06 and coexists with the structural/magnetic transition until x is ∼ 0.13. With increasing concentration of La, the structural/magnetic transition is totally suppressed, and Tc reaches its maximum value of about 45 K for 0.15 ⩽ x ⩽ 0.19. A domelike superconducting region is not observed in the phase diagram, however, because no obvious over-doping region can be found. Two superconducting phases coexist in the x-T phase diagram of Ca1−xLaxFe2As2. The formation of the two separate phases and the origin of the high Tc in Ca1−xLaxFe2As2 have been studied and discussed in detail.

Annealing Time Effect Fluctuation Induced Excess Conductivity in Bi (Pb):2223 Superconductors

We report here on the effect of annealing time, in air and at 800°C for time periods of (6-30 h), on the fluctuation induced excess conductivity ∆σ in Bi2-xPbx: 2223 superconductors with various x values. The logarithmic plots of ∆σ and the reduced temperature e reveal three different exponents corresponding to two different crossover temperatures in the slope of each plot. The first exponent in the normal region is at a temperature of (Tc^(mf) ＜＜ T ＜ 2Tc ^(mf) ) and its values are close 2.5, in which the order parameter dimensionality (OPD) is not 0D. The second exponent is at a temperature of T ＞ Tc^(mf) , and its values are close to 1, in which the OPD is 2D. The third exponent is in the critical-field region at a temperature of (Tc ＜ T ≤ Tc^(mf)), and its values are close to 0.5, in which the OPD is 3D. Furthermore, the interlayer coupling J, Ginzburg-Landau coherence length ζc(0), temperature width (To-T^c(mf)), and resistivity slope (dp/dT) are also calculated for the two examined samples, and their values are affected by changing both the annealing time and Pb content. Our results are discussed in terms of the correlation between the carrier concentration and volume fraction of the high Tc phase for the considered samples.

EFFECT OF HEAT TREATMENT OF HoBa2Cu3O7-δ CERAMICS SUPERCONDUCTOR SYNTHESIZED FROM NANO-COPRECIPITATED POWDERS

Nano-sized metal oxalate precursors was thermally treated and investigated to produce high temperature superconducting ceramics with a formula of HoBa 2 Cu 3 O 7-δ ( Ho -123). The nano metal oxalate powders were subjected to different heat treatments at various temperatures for the phase formation study. The high-TC phase was observed for all samples as an evidence for the single step transition of (R–T) curves. The TC(R = 0) for Ho 123 is 91 K and 90 K for samples sintered at 920°C, 930°C and 88 K for samples sintered at 940°C and 950°C, respectively. XRD data showed single phase of an orthorhombic structure where a small amount of none superconducting phase Ho 2 BaCuO 5( Ho 211) phase was detected. The Ho 211 phase was increased as sintering temperature increased due to the decomposition process and affected the transport properties. SEM micrographs showed large grain sizes that are randomly distributed. However, the grain size decreased as the sintering temperature increased which resulted in the formation of weak-links and hence decreases the transport properties. Using nano-sized metal oxalate precursors simplified the formation of HTSC materials with less processing temperatures which could be beneficial in superconducting tape industry. In addition, control of the sintering temperature lead to the formation of Ho 123 phase with optimum amount of Ho 211 phase which acts as pinning centers and improves the transport properties.

Effect of CuF2 on the Synthesis and Superconducting Properties of (Bi,Pb)2Sr2Ca2Cu3O10 Bulk Ceramic Samples

Bulk polycrystalline samples with Bi1.72Pb0.34Sr1.87Ca1.91Cu3.13O9.83−x F x (0.00≤x≤0.51) were prepared by a solid-state route using CuF2 as a fluorine source. CuF2 was introduced in the precursor powders either by direct mixing with the other starting reagents or by addition to precalcined precursors. In both cases, CuF2 resulted in a clear lowering of the reaction temperature. The superconducting and secondary phase grains are larger in the CuF2 doped samples but the critical transition temperature and the lattice parameters of the high T c phase are not affected. EDS analysis shows that the fluorine is concentrated in SrF2 particles.

The effect of neutron irradiation on the properties of Tl0.6Pb0.3Cd0.1Ba2Ca2Cu3O9-? superconductors

The effect of neutron irradiation on phase purity, crystal structure, and electrical resistivity of Tl_{0.6}Pb_{0.3}Cd_{0.1} Ba_2Ca_2Cu_3O_{9-delta} superconductors has been studied before and after irradiation by electrical resistance measurements and X-ray diffraction techniques. The zero-resistance superconducting transition temperature, T_{c(offset)}, of samples systematically decreases by about 5 K with the increase of neutron irradiation fluence up 18.12 \times 10^{10} n cm^{-2}. The crystalline phases showed that the compounds have 2 phases, a high-Tc phase (1223) and low-Tc phase (1212), in addition to the impurity phase. The irradiation led to a decrease in oxygen content and volume fraction of high-Tc phase (1223).

Influence of isothermal hot pressing-doping treatment on the electrical and mechanical properties of bulk Bi-Sr-Ca-Cu-O

The influences of (isothermal hot pressing-doping) treatment on the microstructure, electrical and mechanical properties of Bi2Sr3-xCaxCu2O8+δ (BSCCO), x ranging from 0.0 to 2 are assessed. BSCCO pellets were prepared by isothermal hot pressing (IHP) technique. The experimental works in this study consists of dc electrical resistivity measurements for electrical and superconducting properties, microhardness tests for mechanical characteristics, powder x-ray diffraction (XRD) for phase analyses (phase ratio), differential thermal analysis (DTA) for melting behaviour and scanning electron microscopy (SEM) for microstructure investigations. The results indicated that the Ca2+ doping and isothermal hot pressing increased significantly the superconducting transition temperature (Toc), which promotes the formation of high-Tc phase. Additionally, Vickers microhardness (VHN), Young's modulus, fracture toughness, brittleness index and elastic stiffness coefficient were also improved. Moreover, IHP-Doping improved the microstructure and the density of the BSCCO studied samples. Microhardness was found to be load dependent. Otherwise, we evaluated the load independent hardness, Young's modulus, fracture toughness and brittleness index of BSCCO samples. The possible reasons for the observed enhancement in electrical and mechanical properties due to (i) the homogeneity of grain region distribution (ii) the enhancement of superconducting phases.

Synthesis and characterization of (Thallium-Tin) doped Bismuth based superconducting materials

Doping with combinations of Tl0.3-xSnx in a certain concentration range enhances the zero resistivity temperature Tc(0) from 104 to 108K. Both high-Tc (2223) and Low-Tc (2212) phases were detected by the X-ray diffraction analysis. However the low-Tc phase is found to dominate over the high-Tc phase with the increase in Sn content. Critical temperatures were determined by the electrical resistivity and ac magnetic susceptibility measurements. Amongst the doped samples studied here a sample containing Sn content of x = 0.06 exhibits comparatively the lowest value of ΔT = 5K, the lowest room temperature resistivity of ρRT = 0.01494 ohm-cm and the highest value of Tc(0) = 108K. Key words: Bismuth based high-Tc superconductors, X-ray diffraction, resistivity, susceptibility

Structural and magnetic properties of epitaxial In1–xMnxSb semiconductor alloys with x > 0.08

High temperature ferromagnetic In1−xMnxSb semiconductor alloys with a Curie temperature (TC) above 400 K were investigated. Alloys with x ranging from 0.08 to 0.22 deposited by metalorganic vapor phase epitaxy were examined. X-ray diffraction indicated alloys are primarily two phase consisting of a zinc blende InMnSb solid solution and hexagonal MnSb precipitates. Transmission electron microscopy analysis confirmed the presence of hex-MnSb nanoprecipitates as well as the presence of the additional minority phases Mn3Sb, metallic Mn, and MnAs1−xSbx. Magnetization measurements indicate that the alloy films are ferromagnetic, showing clear hysteresis in field dependent measurements from 5 to 400 K. Magnetization values as high as 47 emu/cm3 for an alloy with x = 0.22 were measured at room temperature. Irreversibility is observed between field-cooled and zero-field-cooled magnetization curves that is attributed to inhomogeneous magnetic order arising from randomly distributed ferromagnetic nanoprecipitates. Temperature dependent magnetization indicates at least two magnetic phases are present, one with a nominal TC of 300 K that is attributed to MnAs1−xSbx nanoprecipitates which form at the GaAs substrate interface, and a second with a TC > 400 K that is attributed to hex-MnSb nanoprecipitates and to the InMnSb matrix. The majority of magnetization arises from the high TC phase, where fitting the temperature dependent magnetization curve with a Brillouin function indicated a TC of 570 K. Magnetization measurements indicate that there is an interparticle magnetic interaction. Large saturation magnetization at 300 K and TC of 570 K make these multiphase InMnSb epitaxial films excellent candidates for ferromagnetic layers in semiconductor spintronic devices that operate at room temperature.

Influences of Sintering Time on the TC of Bi2-xCuxPb0.3Sr2Ca2Cu3O10+δ High Temperature Superconductors

Solid state thermodynamical reaction method involving mixing, calcinations and sintering were used to prepare a high temperature superconductor with a nominal composition Bi2-xCuxPb0.3Sr2Ca2Cu3O10+δ for Cu (0 d x d 0.5). The effect of sintering time and substituting Cu on Bi sites have been investigated by X-ray diffraction, resistance measurements and oxygen content to obtain the optimum conditions for the formation and stabilization of the high TC phase (2223-phase). The results showed that the optimum sintering time for the composition Bi2-xCuxPb0.3Sr2Ca2Cu3O10+δ is 140 hr at sintering temperature 860 oC. Substitution for Cu equals to 0.2 and 0.3 led to increase the transition temperature to TC =127K and 130K respectively but for 0.4, TC decreased to 125K. The X-ray diffraction (XRD) analysis showed that all samples have almost two phases: high-TC phase (2223-phase) and low-TC phase (2212-phase) in addition to the existing of very low phase in the sample x=0.4. It was found that the all samples having an orthorhombic.

B2223 High Temperature Superconductor Wires in Silver Sheath, Filament Diameter Effect on Critical Temperature and Current Density

B2223 High Temperature Superconductor (HTSC) has been prepared as a pellet by solid state reaction. Then, HTSC wires were fabricated from the prepared superconductor using powder in tube (PIT) method utilizing silver as the tube material. Superconductor powder was packed in the pure silver tube of about 4.5mm diameter, and 50mm long. The prepared wires are of three types; with monofilament MOF, 9 multifilament core (9MF) and 81 filaments core(81MF). Several cycles of mechanical are drawing and rolling process performed to the starting silver tube of 0.4cm diameter and 5cm length and 0.35mm thickness to minimize the filament diameter. The average filament diameter of MoC with 0.7mm SC core which was produce by the first step of drawing, the second 9 MF wire with nine multifilament wire of 0.175mm diameter for each filament,while the 81MFC wire filament diameter was about 25μm for each filament measured with an optical microscope. All the three types of wires were with same outer diameter of 0.9mm. Tc critical temperature for superconductivity is measured for the pellet and wires using four point probes techniques. The critical temperature for the pellet is 110 °K, for the monofilament MOF is 112.5 °K and for 9 filaments 9MF was found is 117°K; while the Tc the 81MF was 119.5°K. The critical current density for the 9MF and 81MF wires were found higher than that of the monofilament MOF in spite of the smaller filament diameters of the 9 MF and 81MF. X-ray diffraction pattern shows the high Tc phase of B2223 compounds in addition to low phase, of B2212 compounds. The I-V curves for the three different wires show higher current density for 81 MF wire, while the optical microscope picture shows more grains alignment . MOC and 9 MF wires show misalignment grains.

BPSCCO Superconducting Films Grown by Spray Pyrolysis Technique: Systematic Study of the Relationship between Pb Content and Annealing Conditions

Actually recent investigation in developing semiconducting-superconducting composites based in CdS and Bi-based superconductors has attracted interest in processing thin superconducting films. In this work are reported Bi-Pb-Sr-Ca-Cu-O (BPSCCO) thin films grown on MgO substrates by spray pyrolysis technique from a solution containing Bi(NO3)3, Pb(NO3)2, Sr(NO3)2, Ca(NO3)2 and Cu(NO3)2, with a subsequent solid state reaction for growing the Bi-based superconducting phases. Annealed films were characterized by X-ray diffraction, atomic absorption spectroscopy and resistance measurements. Interdependence between Pb content, annealing time and temperature, in the formation of superconducting phases was studied applying a fractional factorial design 3III4-2. Interrelation between Pb content, ta and Ta exists. The presence of Pb is necessary to stabilize the high-Tc phase but its content depends on the annealing conditions.

Time-Resolved Photoresponse Measurements of the Electrical Conductivity of the Quasi-Two-Dimensional Organic Superconductor β-(BEDT-TTF)2I3 Using a Nanosecond Laser Pulse

Time-resolved photoresponses in resistance have been measured following the nanosecond laser pulse excitation for the quasi-two-dimensional organic superconductors of hydrogenated and deuterated β-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], which show two different superconducting states with high-Tc and low-Tc, at temperatures near the critical temperatures. A transient increase of the resistance is induced by photoirradiation at all the temperatures, but a marked temperature dependence of the decay time is observed at temperatures close to the high-Tc phase transition temperature; the decay rate becomes faster and then becomes constant in both compounds, as the temperature decreases across the high-Tc phase transition temperature. The temperature dependence of the photoresponse intensity is different from the one expected from the bolometric effects, indicating the presence of the nonbolometric photoresponse. A possible mechanism explaining the photoresponse of the conductivity is discussed, based on the isotope effect on the photoresponse. A comparison is also made between β-(BEDT-TTF)2I3 and κ-(BEDT-TTF)2Cu[N(CN)2]Br for the transient photoresponse in resistance at temperatures across the metal-superconductor phase transition temperature.

Superconducting Properties of the (Bi0.8Pb0.2)2(Sr0.9Ba0.1)2 Ca2Cu3-x Nix O10+? System.

The effect of substitution of Ni on Cu in (Bi0.8Pb0.2)2(Sr0.9Ba0.1)2 Ca2Cu3-x Nix O10+? for (x=0,0.1….1,2,3) superconductor system and sintering time has been investigated .The samples were prepared by solid-state reaction methods. The results show that the optimum sintering temperature is equal to 850 ºC, and the sintering time is equal to 140 h. The highest transition temperature (Tc) obtained for (Bi0.8Pb0.2)2(Sr0.9Ba0.1)2 Ca2Cu3-x NixO10+? composition was 113 with x=0.8 Phase analyses of the samples by X-ray diffraction (XRD) analysis showed an orthorhombic structure with a high Tc phases (2223) as a dominant phase and low Tc phase (2212) in addition to some impurity phases.