Complex Phase Separation in Oxygen-Doped Cuprates La2CuO4+ y Superconductors

The multiple-phase separation with coexistence of two different superconducting phases in the phase diagram of oxygen-doped La₂CuO_{4+ y} cuprates is interpreted with a modified Van der Waals scheme. The model includes the tendency of intercalated oxygen ions to form clusters at preferential densities. We obtain the coexistence and competition of two different superconducting phases with different T _c and interstitial oxygen content y, in agreement with experiments.     

Secondary ion emission of Fe-Ni alloys in the temperature range including the Curie point

The temperature dependence of secondary ion emission was investigated for Fe-Ni ferromagnetic alloys with different Curie points T_c and elemental composition: 35% Ni 65% Fe (T_c=240°C), 40% Ni 60% Fe (T_c=360°C), and 50% Ni 50% Fe (T_c=530°C). The alloy 79% Ni 16% Fe 5% Mo (T_c=345°C) was also studied. The spatial distribution of Ni⁺ and Fe⁺ secondary ions emitted from the (1 1 1) face of invar and permalloy single crystals was shown to be anisotropic with pronounced ion-yield maximum for both components in the 〈1 1 0〉 directions. The shape of the energy distribution of Ni⁺ and Fe⁺ ions was found to be virtually identical for all the alloys under investigation with a most probable energy at 7 eV and a width at half-maximum of 12 eV. The temperature dependence of the Ni⁺ and Fe⁺ emission has a maximum near the Curie point of the investigated alloys and another maximum at the Curie point of nickel which may indicate the precipitation of nickel into microscopic islands on the surface as a result of heating and sputtering. Auger analysis of the surface composition in the surface layers showed a variation in concentration of oxygen and carbon atoms when Fe-Ni alloys pass from the ferromagnetic to the paramagnetic state and this must affect also the secondary ion emission of alloy components.     

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.     

Synthesis and properties of o-containing chevrel phases,AxMo6S6O2 (A=Co,Ni, Cu and Pb)

Oxygen containing Chevrel phases, A_xMo₆S₆O₂ (A_x=Co₂,Ni₂,Cu₂,Pb) and Mo₆S₆O₂ have been synthesized and electrical and superconducting properties examined. These exhibit physical properties quite akin to the corresponding -S₈ phases. However, differences in ionic size and electronegativity between O and S bring about subtle changes in T_c and T_t after O-incorporation.     

Fluctuations of the Order Parameter's Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c ≤ T _c ≤ T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Current-carrying states in superconducting channels with nonhomogeneities

The stability of a current-carrying normal state in a superconducting channel with nonhomogeneities is considered at Tc. A value of the current is found below which the normal state becomes absolutely unstable with respect to nucleation and growth of a superconducting domain. Furthermore, the superconducting state appearing near a nonhomogeneity characterized by a critical temperatureT _c1 higher than the critical temperatureT _c of the main material is studied for the case ofT>T _c .     

Correlation Between Tc and Crystal Chemical Parameters of Cation Sublattice An+1(Cu)n in Perovskite Layer of High-Tc Cuprate Superconductors

It was found the empirical dependence of T _c of high-temperature superconducting cuprates (HTSC) from the ratio (J) of such parameters as the distances between Cu atoms in CuO₂ plane and the distances from CuO₂ plane to adjacent ones of A cations (A ? Ca, Sr, Ba, Y, La, and so on), also from the size and charge of A cations and doping atoms with effect on T _c is proved experimentally. All HTSC phases are divided only on two groups with an intrinsic dependence T _c (J): the phases formed by single CuO₂ plane and by several CuO₂ planes. The closer approximation of that dependence gives the equation of polynomial of third degree.     

CO2 laser annealing of plasma-deposited high-T c superconducting thick films

Recent advances in the fabrication of high-T _c superconducting thick films demand processing techniques which can eliminate film/substrate interdifiusion that occurs during subsequent post-annealing heat treatment after the film is deposited, thereby limiting the application of the thick films for devices. The present study evaluates laser annealing techniques for plasma-deposited Y-Ba-Cu-O thick films using a high-energy CO₂ laser (10.6Μm) in a continuous wave mode. The results are compared with those obtained by conventional furnace annealing techniques necessary for post-heat treatment of as-deposited superconducting thick films. The high-T _c superconducting phase is recovered by cationic diffusion during subsequent post-annealing heat treatment. Crystallographic phases and microstructural characterization have been performed using XRD, SEM, and EPMA analytical techniques. The significance of the technology lies in the elimination of film/substrate interdiffusion problems, thereby resulting in high-quality superconducting thick films. The technology will significantly reduce the post-annealing times usually required by conventional furnace annealing techniques.     

Superconductivity of barium-doped Bi2Sr2CaCu2O y

Sintered ceramic samples of Bi₂Sr_2?x Ba _x CaCu₂O_y with nominal barium fraction 0≤x≤0.3 have been prepared by the solid-state reaction method. WDS studies verified that barium enters the superconducting phase. For slowly cooled samples, the midpointT _c of the superconducting transition is significantly increased by barium doping, whereas for quenched samplesT _c is little affected. The increase ofT _c with increasing barium fraction is consistent with a decrease in the hole concentration in the superconducting layers.     

Phonons and Electron–Phonon Coupling in Nickel Borocarbides

We present first principles calculations of the bandstructure, the phonons and the electron–phonon interaction in superconducting YNi₂B₂C (T _c?=?15.5 K) within the framework of the local density approximation (LDA). The results are compared with those of nonsuperconducting reference systems LaNi₂B₂C and Y(Ni_.75Co_.25)₂B₂C.     

Study of Superconducting Properties in Bismuth-Based Cerium Doped High-T c Superconductors

Superconductivity in bismuth-based high-T _c superconducting materials attracts the researchers for their unique properties. Bismuth-based superconductors commonly called BSCCO have great importance among the superconducting family. These are divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on the calcium site of bismuth-based Bi(Pb)Sr(Ba)-2223 high-T _c superconductor. The nominal compositions of Bi_1.6Pb_0.4Sr_1.6Ba_0.4(Ca_1?x Ce _x )₂Cu₃O _x ceramic superconductor were prepared by the sol–gel method. X-ray diffraction (XRD) was done at room temperature for structural analysis and different parameters were calculated. Surface morphology was done by scanning electron microscopy (SEM). DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. Current density measurements were also taken by the same apparatus. The synthesized superconducting samples were also characterized by thermogravimetric analysis (TG) and Fourier transformation infrared radiations (FTIR). It is observed that the substitution of cerium on the calcium site favors the formation of single high-T _c 2223 phase.     

Effects of Ni Substitution in Bi-2212 Superconductors

In this study the samples were synthesized from a 2234 stoichiometric composition in order to obtain a large amount of pure 2212-BSCCO. The effects of Ni substitution on the properties of Bi-based Bi_2?x Ni _x Sr₂Ca₁Cu₂O _y superconductor with x=0, 0.05, 0.1 and 0.2 were investigated by means of X-ray analysis (XRD), DC electrical resistivity and magnetic-hysteresis loop measurements. It has been found that the T _c (onset) transition temperature does not change independently of Ni content. In addition, J _c values of the samples were calculated from the hysteresis loop measurement by using Bean??s model, showing that J _c decreases with increasing Ni substitution.     

Preparation of YBa2Cu3O7–x high-T c ceramic superconductors from melt

Fabrication of high-T _c ceramic superconductor in the system Y₂O₃-BaO-CuO by melting a mixture of component oxides has been investigated. The compositions of the resulting specimens and the effects of heat treatment have been investigated. It was determined that molten material was composed of phases including BaCuO₂, CuO, Y₂O₃, and Y₂BaCuO₅. A subsequent heat treatment in air produced a nominal amount of the high-T _c phase, while heat treatment in an O₂ atmosphere resulted in a significantly large percentage of the superconducting phase.     

Effect of Fe Substitution for Cu on Microstructure and Magnetic Properties of Laser Floating Zone (LFZ) Grown Bi-2212 Rods

In this paper, the laser floating zone (LFZ) technique has been used to fabricate Bi₂Sr₂Ca₁Cu_2?x Fe _x O (x=0,0.01,0.03,0.05,0.1) superconducting fibers. The effects of Fe substitution on grain alignment and superconducting properties of annealed fibers were studied using SEM/EDX microanalysis in addition to phase analysis and magnetic measurements realized through XRD, M–T, and M–H loops. In the same growth conditions, higher Fe contents lead to a more compact microstructure with lower porosity but also caused the formation of poor superconducting phases followed by the decrease of T _c and J _cmag. No evidence of enhanced pinning capability was found in the magnetic measurements. Annealed rods indicated a weakly ferromagnetic-like behavior at relatively high doping levels.     

Reduction of fault current peak in an inductive high-Tc superconducting fault current limiter

This paper dealt with current-limiting performances of an inductive high-T_c superconducting fault current limiter with an auxiliary coil. The fault current limiter mainly consists of the primary copper coil, secondary high-T_c superconducting rings, and auxiliary high-T_c superconducting coils, which are magnetically coupled through three-legged core. The superconducting fault current limiter as a series element in the power system is inserted to limit the fault current. The device presents fast variable-impedance features in the event of a fault condition. The fault current peak can become relatively large for certain ranges of the flux and the fault instant due to the core saturation. The auxiliary coil proposed in this paper was proven to increase the impedance of the SFCL up to more than 31% while preventing the core saturation.     

Resistivity and magnetic susceptibility studies of TlmCan−1Ba2CunOy before and after doping with LiF

Measurements of the superconducting resistance as a function of temperature were performed using the conventional four-probe method. The transition to complete superconductivity was recorded for samples of (Tl₂Ca₂Ba₂Cu₃O₁₀)_100?xLiF _x (2223) mixed with different LiF ratiosx=0, 2, 4, 5, 6, 8, 10, and 12 wt.%. It was found that the transition temperatureT _c was increased up to 5 wt.% of LiF. Further addition of LiF decreasesT _c . Therefore, 5 wt.% LiF is the optimum concentration giving a transition temperature of 130 K. Measurements of the superconducting resistance of all the samples except the (1111) compound show that the addition of 5 wt.% LiF increasesT _c and decreases the metastable phases. The real part of the a.c. magnetic susceptibility? _a.c. is studied using a zero-field cooled mechanism. The temperature dependence of? _a.c. for the prepared TlBaCaCuO having stoichiometric composition of (1111), (2223), (2234), and (3245) and that after doping with 5 wt.% LiF showed a broad feature. The transition to the complete diamagnetic state takes place in a broad transition region containing many transition steps, indicating the presence of metastable phases. The addition of LiF decreases the fluctuation in the transition region and its effect in reducing the number of multiphases.     

Electrical and dielectric properties of nano crystalline Ni–Co spinel ferrites

Nanocrystalline samples of Ni_xCo₁–_xFe₂O₄, where x = 1, 0.8, 0.6, 0.4, 0.2 and 0, were synthesized by chemical co-precipitation method. The spinel cubic phase formation of Ni–Co ferrite samples was confirmed by X-ray diffraction (XRD) data analysis. All the Bragg lines observed in XRD pattern belong to cubic spinel structure of ferrite. Scanning Electron Microscopy (SEM) technique was used to study the surface morphology of the Ni–Co ferrite samples. Nanocrystalline size of Ni–Co ferrite series was observed in SEM images. Pellets of Ni–Co ferrite were used to study the electrical and dielectric properties. The resistivity measurements were carried out on the samples in the temperature range 300–900 K. Ferrimagnetic to paramagnetic transition temperature (T_c) for all samples was noted from resistivity data. The activation energy below and above T_c was calculated. The dielectric constant (ɛ′) measurements with increasing temperature show two peaks in the temperature range of measurements for all samples under investigation. The peaks observed show frequency and compositional dependences as a function of temperature. Electrical and dielectric properties of nanocrystalline Ni_xCo₁–_xFe₂O₄ samples show unusual behavior in temperature range of 500–750 K. To our knowledge, nobody has discussed such anomalies for nanocrystalline Ni_xCo₁–_xFe₂O₄ at high temperature. Here, we discuss the mechanism responsible for electrical and dielectric behavior of nanocrystalline Ni_xCo₁–_xFe₂O₄ samples.     

Field-Induced Superconductivity in MoS2

Semiconducting TMDs are nowadays attracting great interest after the invention of the so-called “Scotch-tape method” established in graphene research. Semiconducting TMDs are front-runners of “post graphene” materials for their finite band gap crucial for device applications. MoS₂ is the most widely used TMD because of its application as a solid lubricant. Scientifically, it shows superconductivity after alkali or alkaline-earth doping with a highest T _c of around 7 K. Recently, we succeeded in inducing superconductivity in the MoS₂ transistor adopting electric double layer (EDL), a nanosized capacitor, as a gate dielectric. The field-induced superconducting transition of MoS₂ was realized with a maximum T _c around 11 K, the highest not only within a reported MoS₂ compound, but also among TMDs. This highest T _c lies in the carrier density region much smaller than a chemically doped compound; a low density region has never been successfully accessed by chemical methods. Combining a HfO₂ (high-k) back gate, quasi-continuous control of carrier density, and thus quantum phase, was demonstrated to unveil the phase diagram; the T _c exhibits strong carrier density dependence with a superconducting dome. Our result implies a common existence of the superconducting dome in 2D band insulators.     

On the Presence of 1D-Like Stripes in Superconducting Epitaxial La2 -x Sr x CuO4 thin Films

A large positive magnetoresistance (up to tens of percents) is observed in both underdoped (x?<?0.15) and overdoped superconducting La_{2? x}Sr_xCuO₄ (LSCO) epitaxial thin films, at temperatures far above the superconducting critical temperature T _c . In the underdoped regime, this magnetoresistance cannot be described by the Kohler rule and is due to the influence of superconducting fluctuations. On the other hand, in the overdoped regime, the Kohler rule does not seem to be violated. ?The strong magnetoresistance above T _c can be related to the preformed superconducting pairs existing well above T _c but forming a phase coherent superconducting state below T _c . The observations support the idea of a close relation between the pseudogap and the superconducting gap and provide evidence for the presence of pre-pairs above T _c . Both the observed fluctuations and the observed magnetoresistance are in accordance with the existence of 1D-like stripes. These results are further supported by recent high magnetic field measurements (up to 55 T) of the transverse magnetoconductivity σ _xy, which goes to zero for T→0 K.     

Effect of Ni Doping at Pd Site in Nb<Subscript>2</Subscript>PdS<Subscript>5</Subscript> Superconductor

The superconducting properties of Nb₂PdS₅ superconductor have been investigated with Ni doping at Pd site All the bulk polycrystalline Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.10) samples are crystallized in singlephase monoclinic structure. The electrical resistivity and magnetic measurements of Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.15) were carried out to study the variation of superconducting critical parameters with Ni doping. Superconductivity in Nb₂PdS₅ sample completely disappears for x ≥ 0.15. We observed that the ratio of upper critical field to transition temperature decreases with increasing Ni concentration. Also, the magnetization study of Nb₂Pd_1?xNi_x S ₅ (0 = x ≤ 0.15) samples shows similar superconducting behaviour. In summary, the superconductivity in Nb₂PdS₅ sample is slightly varying with partial doping of Ni at Pd site in Nb₂PdS₅ superconductor.