Issue No. 3, December

Abstract

We report superconductivity, which is achieved in Sr_1-xNa_xFe₂As₂ at T_c = 36.5 K for x = 0.47. Single crystals of Na-substituted SrFe₂As₂ up to 2 × 2 mm² were grown from NaF flux. The superconducting transition temperature with a narrow width less than 2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. The upper critical fields H^c_c2(0) = 87 T and H^ab_c2(0) = 217.5 T are extremely high. The anisotropy ratio g ~ 2.5 is close to that of other hole- or electron-doped 122 phases but lower than g ~ 4.3 – 5 of phases from the 1111 series and much lower than g ~ 7 – 10 of superconducting cuprates. Magnetic relaxation measurements, which are consistent with the collective pinning model, reveal that the flux-creep rate in Na substituted 122 SrFe₂As₂ is smaller than in 1111 type iron pnictides.

Abstract

Auger and SEM studies show that with increasing of MgB₂ manufacturing temperature from 600÷800 oC to 1050÷1100 oC the Mg-B-O nano-layers which are present in the MgB₂ matrix transform into distinct dispersed Mg-B-O inclusions. On the other hand the sizes of inclusions of higher magnesium borides (MgB_x, x=7 ÷ 25) which are also present in the MgB₂ matrix. The tendency is observed in a wide range of synthesis pressures (0.1 MPa -2 GPa). The described structural transformations are accompanied by an increase in critical current density, j_c, in low and medium magnetic fields and by transition from the grainboundary to the point pinning. The Ti addition results in a further increase in j_c due to: Ti promotes the formation of higher magnesium boride inclusions and localization (or segregation) of oxygen in MgB₂ matrix, and, hence, facilitates the formation of a homogeneous MgB₂ matrix with lower oxygen content, but with an increased number of Mg-B-O and MgB_x pinning centers. At low synthesis temperature Ti absorbs hydrogen forming titanium hydrides, thus preventing the formation of MgH₂ and provides the material densification. The positive effect of Ti addition is connected with the high ability of Ti to absorb hydrogen, oxygen, and magnesium. The results of the critical current and AC loss study by transformer method using rings from MgB₂ are discussed.

Abstract

Typically microwave properties of High Temperature Superconducting films are defined in terms of surface resistance Rs computed from Q-factor measurements obtained using one of resonant techniques. While NdBaCuO films are known to have higher TC and JC than YBaCuO films, not much is known about their loss at high frequencies. In this paper we report on microwave properties of NdBaCuO films of varying quality and thickness based on measurements with a 25GHz sapphire Hakki Coleman resonator and computations using two approaches: the surface resistance and the complex conductivity as primary variables for rigorous electromagnetic solutions.

Abstract

The relationship between critical current density (Jc) and pinning energy (Up) in partial melted superconductors was investigated. The partial melted Sm-Ba-Cu-O superconducting bulks were composed of SmBa₂Cu₃O_x (Sm-123) superconductor and Sm₂BaCuO_y (Sm-211) insulator. The pinning force and the pinning energy of the superconductor were measured by a system combining an electronic balance and permanent magnet. The pinning energy and the Jc of the samples were influenced by crystalline orientation. The Jc of the samples depends on the pinning energy. However, in the partial melting process, changes in retention temperature altered the Jc values but had little effect on the value of the pinning energy. The results indicated that Jc was influenced by the dispersion of the insulator acting as a pinning center with in the superconducting bulks.

Abstract

It is necessary to form pinning centers in superconductors to allow the flow of large currents through the specimens. To clarify the properties of pinning centers, it is preferable to investigate single crystals. In this study, heat treatment was used to dope various oxides into Bi₂Sr₂CaCu₂O_x (Bi-2212) single crystals prepared by self-flux methods and the critical current (Ic) was measured. The oxides used in this study were Al₂O₃ and the rare earth oxides Er₂O₃ and Nd₂O₃. At 77K, Nd₂O₃ and Er₂O₃ are magnetic, whereas Al₂O₃ is nonmagnetic. The Ic of the samples were measured as a current per width of 1 cm (Ics). The resulting Ics of the Bi-2212 single crystal was 2.8A/cm and that of the Al₂O₃ doped Bi-2212 sample was 4.5A/cm. Compared with these samples, doping the other rare earth oxides gave Ics values in excess 10A/cm. The results indicated that the doping oxides were effective in operating as pinning centers in the samples. We assumed the current path in a single crystal, and calculated the Ics by superconducting current simulation. The results indicated that the oxides permeated from a crystal surface in a porous shape. The oxides increase the current which flow in the Cu-O2 planes that are parallel to the a-b plane.

Abstract

Bulk YBa₂Cu₃O_y “Y-123” superconductors with high critical current density (J_c) have been grown by infiltrating a Ba₃Cu₅O_y “Y-035” liquid source into a Y₂BaCuO₅ “Y-211” precursor block. Magnetization measurements showed that similar T_c (onset) values and large variation in self field critical currents were observed when the samples were isothermally grown at different temperatures. We then adapted slow cooling process for the samples 27 mm in diameter and 5 mm in thickness using the growth profile which was optimized on the basis of isothermal growth conditions. The specimens cut from various locations of a large bulk material showed the uniform and sharp superconducting transition with T_c (onset) around 93.2 K. J_c values at 77 K and 0T was in the range of 150 - 175 kA/cm². Such high J_c values could be obtained without refining the particle size of Y-211 secondary phase. Atomic force microscopy measurements revealed that uniform nano-sized secondary Y-211 particles are embedded in the Y-123 matrix, which led to a dramatic improvement of the critical current performance at 77 K.

Abstract

A new scheme of density functional theory (DFT) for unconventional superconductivity is reviewed. To include the effect of charge fluctuations such as low-energy plasmons or excitons, we extend the conventional formalism of superconducting DFT where the dynamical structure of the screened Coulomb interaction is neglected. We applied the present method to fcc Li under high pressure. We show that the agreement between the theory and experiment is considerably improved. The present result indicates that plasmons can cooperate with phonons and enhance the pairing instability.

Abstract

We report on the fabrication and characterization of disk-shaped bulk MgB₂ superconductors with high performance as superconducting bulk magnets. Several samples of diameters 20 mm, 30 mm and 40 mm were fabricated using a solid state reaction in pure Ar atmosphere at 775°C for 3h. The magnetization measurements confirmed that all the samples exhibited a sharp superconducting transition with T_c (onset) at around 38.5 K. The samples showed respective critical current density (J_c) values of 176 kA cm^-2 and 55 kA cm^-2 at 20 K in self field and 1T. The J_c values increased further to 250 kA cm^-2 and 100 kA cm^-2 with decreasing temperature down to 10 K. MgB₂ samples 20 mm and 30 mm in diameter and 7 mm in thickness exhibited trapped field values of 1.15 T and 1.3 T at 25 K, respectively. Microstructural observations with scanning electron microscopy (SEM) revealed that the samples are highly porous. And hence, continuing development of large-sized bulk MgB₂ with higher density will lead to promising industrial applications.

Abstract

We address here the superconductivity quenching under an external magnetic field of amplitudes up to 1 T and in the so-called "thermal smallness" condition, when the microbridge width becomes smaller than the thermal diffusion length of both the own superconductor and its refrigerant (the substrate, in the case of thin films), which breaks their thermal dimensional scaling. Our results further support that when the current perturbations have characteristic times in the millisecond range the quenching is due to thermal instabilities associated with regular (nonsingular) flux-flow, and they also suggest how to optimize the refrigeration of practical superconductors.