Structure and Superconducting Properties of Cd0.8Ba2(Y0.7Ca0.4)Cu3.5Oy

A new Cd-containing superconductor with nominal composition ofCd_0.8Ba₂(Y_0.7Ca_0.4)Cu_3.5O _y was synthesized and investigated. The obtained Cd and Ca-doped 123 phase exhibits an orthorhombic (T _c=80 K) or tetragonal (T _c=65 K) modification depending on the reaction atmosphere. It was shown that the combined Cd and Ca substitution facilitates the 123 phase formation. The results of the EDX analysis, as well as the comparison of the obtained lattice parameters with those of undoped, Cd-doped, and Ca-doped 123 have shown that both Cd and Ca enter the 123 phase and form a new Cd–Ba–Y–Ca–Cu–O superconducting compound.     

Study on Incommensurate Modulation Structures in Ca-Doped Bi-2201 System

The Ca-doped Bi-2201 system with nominal composition Bi₂Sr_2?x Ca _x CuO_6+δ (x=0.1,0.3,0.5,0.7,0.9) was prepared by sol–gel method and investigated by X-ray diffraction, transmission electron microscopy and resistance measurements. It can be found that the Ca-doped Bi-2201 system was composed of Bi-2201 phase containing Ca (main phase) and a small quantity of Bi₁₆(Sr,Ca)₁₄O₃₈ (impurity). For the main phase, the b ^? component of the modulation wave vector decreases while the c ^? component increases with the increase of calcium content. The evolutional mechanism of the modulation wave vector, the correlation between superconductivity and modulation wave vector were also discussed.     

Superconducting and electrical properties of rhenium nitride and amorphous rhenium prepared by ion implantation

Different rhenium nitrides were prepared by homogeneous implantation of nitrogen ions into rhenium thin films at various temperatures as a function of N concentration. At 9 at % N new x-ray lines appeared in addition to the hcp rhenium phase. The formation of this phase, which could not be identified, was accompanied by a large increase in stress, of the order of 10¹¹ dynes/cm². Rhenium nitride with fcc structure was formed between 13 and 50 at %. For ReN a lattice parameter of 4.021±0.002 Å, a maximumT _cvalue of 4.5–5.0 K, and a resistivity ρ_RT of 250 µΩ-cm were obtained. Both rhenium nitride phases decomposed above 645 K. An x-ray amorphous phase was obtained after implantation of 40 at % N at LNT. This amorphous phase had aT _cvalue of 6.0 K and a ρ_RT value of 300 µΩ-cm. Transformation into an amorphous Re phase was also observed after implantation of 20 at % P. The maximumT _cvalue of this phase was 7 K. This phase decomposed at about 925 K.     

The Comparisons Between Y-123 and Y-124 Superconductor Substituted with Ca at the Cu-Site

The comparison between YBa₂Cu_3?x Ca _x O _δ and YBa₂Cu_4?x Ca _x O₈ superconductors substituted with Ca at the Cu-site was investigated. The concentration of Ca varied from x=0.00 to x=0.15. Resistivity and current density measurement (without magnetic field) were measured using four-probe method. The samples were characterized using X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). From the resistivity measurement, the critical temperature (T _c?zero) in x=0.00 was 84 K for Y-123 and 83 K for Y-124 superconductor system. As the Ca concentration increased to x=0.15, both superconductor systems showed a decrease in value whereby 71 K for Y-123 and 74 K for Y-124. This was a result of the decrease in the hole concentration. Critical current density (J _c ) decreased with the further increment of Ca concentration owing to grain connectivity and an increase of porosity. At 50 K, J _c decreased from 3.9790 A/cm² at x=0.00 to 3.5184 A/cm² at x=0.15 for Y-123, and from 3.6209 A/cm² at x=0.00 to 0.5243 A/cm² at x=0.15 for Y-124. The crystallographic crystal structure showed that both Y-123 and Y-124 superconductor systems exhibited an orthorhombic form. FESEM microscopy showed that the Y-123 sample had less porosity compared to Y-124 samples and the resulting Y-123 sample had a higher J _c compared to the Y-124 sample.     

Local Structural Properties Around Cu Atoms in Checkerboard Phased Ca2−x Na x CuO2Cl2

We examined the local structural properties of checkerboard phased Ca_2?x Na _x CuO₂Cl₂ (T _c =0, 15, 21, 27 K) using extended X-ray absorption fine structure (EXAFS) measurements at the Cu K edge. EXAFS revealed anomalous disorders existing in the bond lengths of Cu–Cu and Cu-Ca/Na pairs for all crystals below 50 K. A static disorder was observed even at room temperature. The disorders of the Cu–Cu in the a- and b-axes were independent of the T _c value whereas these of the Cu–Ca/Na pairs located about 30^° off from the Cu–O, ab-plane decreased, as T _c increased.     

Influence of aluminum and vanadium alloying elements on the resistivity of titanium

The experimental data on the resistivity of titanium-aluminum and titanium-vanadium alloys in the temperature ranges of 77–1700 and 77–350 K, respectively, at alloying component concentrations below 10 at % are generalized. At these concentrations, the isotherms of the dependences ρ(c) are linear in the temperature range of the α phase (77–500 K) and in the β phase. The numerical values of the concentration angular coefficients (dρ/dc) _T = b were obtained for the Ti-Al and Ti-V systems at temperatures of 77 and 300 K and Ti-Al alloys in the β phase at T = 1400 K. The temperature derivatives (dρ/dT) _T = b _T for the α phase near the Debye temperature and the β phase, where the dependences ρ(T) are close to linear, have been calculated for Ti-Al alloys. In both cases the concentration dependences of the temperature angular coefficients b _T (c) decrease with an increase in the Al concentration; however, their magnitudes differ by an order of magnitude.     

Electronic transport and specific heat of Ca-doped YBa2Cu3Ox-single crystals

Measurements of the anisotropic electrical resistivity, the Hall Effect, and the specific heat anomaly at T_c of Ca-doped YBa₂Cu₃O_x single crystals of various Ca and O contents are presented. The transport properties are discussed in terms of charge and spin separated quasiparticles and confirm that by doping Ca onto Y sites the hole concentration, n_h, can be extended beyond the value achieved with O-doping alone at x=7.0. The jump in the specific heat at T_c which increases monotonously with x up to x=7.0 in Ca-free crystals displays a maximum in highly oxygenated Ca-doped crystals. This maximum, however, is displaced from the T_c(n_h) maximum to higher values of n_h, consistent with the model of induced superconductivity in the CuO chains.This work was supported in part by the German-Israel Foundation for Scientific Research and Development under Grant No. I-210-206.7     

Electric,magnetic, and transport critical current density behavior of the 2201, 2212, and 2223 phases of ‘BSCCO’

We have studied the electrical properties of polycrystalline pellets of the high-T _c superconducting phases occurring in the Bi₂O₃-SrO-CaO-CuO (BSCCO) system, having nominal compositions Bi₂Sr₂Cu₁O_6+y, (2201 phase, withT _c = 10 K.) Bi₂Sr₂CaCu₂O_8+y (2212 phase, withT _c = 85 K), and Bi₂Sr₂Ca₂Cu₃O_10+y (2223 phase, withT _c = 110 K). Pellets containing the 2223 phase having zero resistance below 105 K were obtained. For the 2212 and 2223 phases, the transport critical current density was measured as a function of temperature and of the externally applied magnetic field. As previously found for YBa₂Cu₃O_7-x (YBCO), results are consistent with the occurrence of a weak superconducting coupling among the grains. Such coupling was, however, noticeably stronger for the 2223 phase than for the 2212 phase.     

Effect of Bi/Pb ratio and annealing temperature onT c and formation of high-T c phase in Bi-Pb-Sr-Ca-Cu-O superconductor

We studied the effect of Bi/Pb ratio and annealing temperature onT _c and formation of the high-T _c ; phase in Bi-Pb-Sr-Ca-Cu-O superconductor by the three-step reaction process. The optimum Bi/Pb ratio is about 1.8∶0.3 and the optimum annealing temperature is about 845–855°C. It is found that a variate high-T _c phase existed at the higher annealing temperature. The zero-resistance temperature of the variate high-T _c phase decreased when the annealing temperature increased, although the phase is isostructural with the 110 K phase.     

Superconductivity of a Calcium-Doped Graphite CaC30

We report the results of a dc magnetization study of the graphite doped with Ca atoms (CaC₃₀). A Ca-doped graphite was prepared in an ultra-high vacuum (UHV) chamber without lithium atoms used for making bulk CaC₆. The superconducting transition temperature (T _SC) was about 9.2 K, which is lower than that of CaC₆ (11.5 K). Further, at a 50 mT, the second transition occurs at higher temperatures in ZFC and FC, viz. 250 K in ZFC and 130 K in FC.     

Interplay Between Diamagnetic and Ferromagnetic Phase in (Bi2223)1−y (LSMO) y Composites Thick Films

Thick films of ((Bi,Pb)₂Sr₂Ca₂Cu₃O _x )_1?y (La_0.7Sr_0.3MnO₃) _y [(Bi2223)_1?y (LSMO) _y ] composites (y=0.01, 0.03, 0.05) are deposited by means of a simple melting-quenching-annealing method onto (001)-oriented LaAlO₃(LAO) substrates. The constituent compounds Bi2223 and LSMO are prepared by standard solid-state reaction and sol–gel method, respectively. Measurements of the dependence of the magnetization on the temperature show the presence of superconducting and ferromagnetic phase below ～54 K and ～370 K, respectively. Current–voltage measurements on composites with y=0.01, 0.03 show that the superconducting critical current drop dramatically from I _c≈350 mA at T=15 K to zero at T≈0.5T _c (～25 K). The dependence of the magnetization on the external magnetic field applied in-plane or out-of-plane at 5 K displays well defined hysteresis loops, which correspond to the superconducting, diamagnetic phase. For T>T _c, the ferromagnetic loop of LSMO is observed. The critical current density, J _c, was determined for samples with y=0.01 and 0.05 by applying Kim’s model to the superconducting, diamagnetic hysteresis recorded at 5 K. The calculated values for J _c(B) resulted to be smaller than those obtained for individual bulk Bi2223 samples due, probably, to the presence of LSMO particles whose ferromagnetic domains compete with the superconducting diamagnetic phase.     

Self-Consistent T-Matrix Approach to an Interacting Ultracold Fermi Gas with Mass Imbalance

We investigate the superfluid phase transition in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas with mass imbalance. In the presence of mass imbalance, it is known that the strong-coupling Gaussian fluctuation theory, as well as the ordinary non-self-consistent T-matrix theory, that have been extensively used to clarify various BCS-BEC crossover physics in the mass-balanced case, unphysically give double valued superfluid phase transition temperature T _c in the crossover region. In our previous paper (R. Hanai et al., J. Low Temp. Phys. 171:389, 2013), this difficulty was shown to be eliminated by an extended T-matrix approximation (ETMA). However, it was also found that this improved theory still gives vanishing T _c in the BCS regime, when the mass imbalance ratio remarkably deviates from unity. In this paper, further extending ETMA to include higher order pairing fluctuations to the fully self-consistent T-matrix level, we clarify whether the vanishing T _c obtained in ETMA is an intrinsic phenomenon or an artifact of this approximation. We show that the self-consistent T-matrix theory always gives a finite T _c, even in the region where ETMA predicts the absence of superfluid instability. The key to the recovery of T _c is found to be a consistent treatment with respect to the Fermi surface sizes of the light mass and heavy mass components, which ETMA lacks in. Since Fermi condensates with mass imbalance have been recently discussed in various systems, such as a ⁴⁰K–⁶Li Fermi gas, exciton-polariton condensate, as well as color superconductivity, our results would be useful in constructing a reliable strong-coupling theory to examine physical properties of these novel Fermi superfluids.     

Further Increase of T c in Y-Ba-Cu-O Superconductors

This paper presents the synthesis and characterization of three new Y-based high temperature-superconducting compounds having relatively higher transition temperature T _c compared with that of the Y-123, Y-124, and Y-247. The electrical resistivity measurements indicate the onset critical temperatures as high as 98 K. Atomic compositions of these new Y-Ba-Cu-O superconductors are Y₂Ba₃Cu_5.2O _x , Y₂Ba₅Cu₉O _x , and YBa₄Cu₅O _x . The X-ray analysis has shown that they have a similar crystalline structure as Y-123 phases.     

Investigation of the properties of superconducting niobium nitride films

Niobium nitride films with a value for T_c of up to 17.3 K have been prepared by reactive magnetron and diode sputtering in Ar and N₂ gas mixture. Alteration of T_c and N₂ ion implantation into NbN films was studied. It is shown that small doses of implanted ions cause an abrupt decrease of T_c to 12.8 K. Annealing at 900°C restores the high T_c By the tunnel effect, the magnitude of the energy gap is 3.05 Me V at 1.56 K for NbN with T_c = 17.1 K and $\text{22/kT}{}_{\mathrm{c}}\text{= 4.14}$. The existence of an undersurface layer has been discovered, whose width is of the order of the coherence length, with a lower T_c, of 12.0–12.8 K. The superconductivity of this layer above T_c is due to the proximity effect. NbN tunnel junctions are made with a density of the Josephson current i=(1.7-2.87) · 10³ A cm^?2.     

Mechanical and Transport Properties of Argon Annealed Y<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>:123 Superconductors

We report here the effects of argon heat treatment at 450 ^°C for 12 h on two similar sets of oxygenated Y_{1 ? x} Ca _x :123 superconducting samples prepared by two different heat treatments and oxygen purity as reported by Sedky and Abu-Ziad (Physica C 470, 659; 12). The oxygenated samples are called ssqHp, sssHp, ssqLp, and sssLp, and the Ar annealed samples are called ssqHpAr, sssHpAr, ssqLpAr, and sssLpAr. It is found that the c parameter of annealed samples is gradually increased with Ca addition for all samples, while orthorhombic distortion (OD) is decreased. It is also noted that Ar heat treatment decreased the link between superconducting grains for all samples, and a linear decrease in microhardness (Vickers hardness number (VHN)) with Ca addition is obtained. But the rate of increase/decrease of the c parameter, OD, and VHN against Ca content is different in ssqHpAr and sssLpAr samples as compared to sssHpAr and ssqLpAr samples. Interestingly, the sssHpAr and ssqLpAr samples show a considerable loss of oxygen and small decrease in T _c. While the ssqHpAr and sssLpAr samples show much smaller loss of oxygen and a gradual increase in T _c up to 16 and 17 K, respectively. These results are discussed in terms of the mechanism of oxygen loss by Ca in addition to ssqHpAr and sssLpAr samples as compared to sssHpAr and ssqLpAr samples.     

Synthesis and superconducting properties of CaC6

Among the superconducting graphite intercalation compounds, CaC₆ exhibits the highest critical temperature T_c=11.5 K. Bulk samples of CaC₆ are obtained by immersing highly oriented pyrographite pieces in a well-chosen liquid Li–Ca alloy for 10 days at 350 °C. The crystal structure of CaC₆ belongs to the space group. In order to study the superconducting properties of CaC₆, magnetisation was measured as a function of temperature and direction of magnetic field applied parallel or perpendicular to the c-axis. Meissner effect was evidenced, as well as a type II superconducting behaviour and a small anisotropy. In agreement with calculations, experimental results obtained from various techniques suggest that a classical electron-phonon mechanism is responsible for the superconductivity of CaC₆. Application of high pressure increases the T_c up to 15.1 K at 8 GPa.     

Influence of calcium content on the preparation of the high T c (110 K class) Bi-Pb-Sr-Ca-Cu-O thin film

The Bi-Pb-Sr-Ca-Cu-O thin films with various Ca/Sr ratios were prepared by r.f. magnetron sputtering using multi-targets (Bi_0.5Pb_0.50 _x , CaCu_0.750 _x , SrCu_0.75O_x. The high T _c phase (2223 phase) was obtained by firing these films. The film with the highest Ca/Sr ratio (Ca/Sr > 1.0) produced the largest amount of 2223 phase on firing at 850 °C. The highest Ca/Sr ratio (=1.17) film contained 91% volume ratio of 2223 phase compared with the low T _c phase (2212 phase) after 15 h firing. However, the 2223 phase decreased with increasing long-term firing (65 h). The resistance curve of the film with the highest Ca/Sr ratio fired for 65 h showed tailing, until the temperature fell below 20 K, suggesting that this phenomenon was due to the segregation of excess calcium and copper components around the grain, in the amorphous state.     

A new high-T c superconductor containing thallium and its crystal structure: The “1212” phase (Tl1-x Bi x )1.33Sr1.33Ca1.33Cu2O6.667+δ

Joining YBa₂Cu₃O_6.5+δ (123 phase) and Bi₄Sr₄Ca₂Cu₄O_16+δ (4424 phase) as structurally characterized high-T _c , superconductors, the thallium-containing superconductor (Tl_.75Bi_.25)_1.33Sr_1.33Ca_1.33Cu₂O_6.667+δ with the ideal stoichiometry (Tl,Bi)₁Sr₂Ca₁Cu₂O_6.5+δ (1212 phase) is reported here. As prepared from the component oxides, 1212 has an initial deviation from resistance linearity at 120 K, a superconducting onset temperature of 92 K, and zero resistance at 75 K. The tetragonal unit cell (P4/mmm, a=3.800 Å;c=12.072 Å, deduced from powder data) contains double copper oxygen sheets (like 4424 and 123) that alternate withsingle thallium-bismuth oxygen sheets (in contrast to 4424, which containsdouble bismuth oxygen sheets), resulting in a total of three stacked perovskite-like cells (as in 123). The copper oxide sheets (with intersheet spacing 3.38 Å) are separated by Ca²⁺ and the Cu oxide sheets and (Tl,Bi) oxide sheets (with spacing 4.35 Å) are separated by Sr²⁺, Ca²⁺, and excess (Tl,Bi)³⁺. The 1212 cell constitutes the building block for the centered, more complex 4424 cell. The 1212 structure persists to Bi contents as low as 1% and can also be stabilized by Pb instead of Bi; Tl cuprates also form other superconductors with lowerT _c .     

Superconducting Properties of the 3-K Phase of Sr2RuO4 near Hc2

The superconducting transition temperature, T _c , of the impurity-free, intrinsic Sr₂RuO₄ is as high as 1.50 K. However, we recently showed that T _c is remarkably increased up to 3 K in the Sr₂RuO₄–Ru eutectic system, in which plate-like microdomains of Ru metal are embedded in the primary-phase Sr₂RuO₄. The phase diagram of the anisotropic upper critical field of the 3-K phase indicates that H _c2 for the field parallel to the RuO₂ plane is strongly suppressed at low temperatures. We argue that the reorientation of the Cooper-pair spin direction near the Sr₂RuO₄–Ru interface may be responsible for this suppression. In addition, we observed unusual hysteresis in the out-of-plane resistivity, ρ _c , at low temperatures and near H _c2, only when the field was applied parallel to the RuO₂ plane.     

Precursor Influence on the Electrical Properties of Textured Bi-2212 Superconductors

Several synthetic methods, solid-state, sol-gel and polymer solution methods, have been used to prepare prereacted precursors, as well as a vitreous material obtained by melt quenching. The influence of the starting powder characteristics on the phase formation, microstructure, T _c and J _c of Bi-2212 textured rods prepared by directional crystallization from the melt has been analyzed. In all the cases, high transport critical current values (higher than 3000 A/cm² at 77 K) have been obtained, independently of the precursor type. Samples obtained by the polymer route show improved T _c values, associated to a lower oxygen content.