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[en] We consider the superconducting transition in fermionic quantum critical systems. Assuming the validity of Migdal theorem, the gap equation can be written in terms of the retarded pair susceptibility. Instead of the usual BCS form, the pair susceptibility is now subject to scale invariance. The gap and transition temperature is thus of the algebraic form, totally different from the exponential behavior in BCS theory. Consequently, with reasonably small glue strength, we can get very large gap and transition temperature comparable to those discovered in cuprates. The ratio of the gap to retardation gets boosted by increasing retardation. We also find the upper critical field has a different scaling with the critical temperature. With a non-Lorentzian dynamical exponent, the upper critical field is greatly enhanced when approaching the critical point, though the critical temperature only changes modestly, in agreement with recent experiments on heavy fermions.
[en] As a phonon-media superconductor, the superconducting properties of MgB2 are very sensitive to the lattice parameters. Lattice strain has significant effects on the superconducting properties in MgB2 superconductor. We have systematically studied the relationship between lattice strain and superconducting properties (such as critical temperature, Tc, the upper critical field, Hc2, the irreversibility field, Hirr, and hence the critical current density, Jc) in pure and different chemical doped MgB2 samples. In contrast to the common practice of improving the Jc and Hc2 of MgB2 through chemical substitution, by taking advantage of residual thermal strains we are able to design a composite, which shows only a small drop in Tc and little increase in resistivity, but a significant improvement over the Jc and Hc2 of MgB2.
[en] Highlights: • Double peak in the imaginary part of the ac susceptibility χ(T) of Nb3Sn. • Explanation of the effect assuming a universal relationship χ″(χ′). • Different creep exponents apply within and outside the peak-effect region. -- Abstract: We performed a systematic study of the ac magnetic-susceptibility on a Nb3Sn single crystal which displays a strong peak effect near the upper critical field Hc2. In external magnetic fields above μ0H ≈ 3 T, the peak effect manifests itself in a single, distinct peak in the real part χ′(T) of the ac susceptibility as a function of temperature T, the size of which continuously increases with increasing magnetic field H. In the imaginary part χ″(T) of the ac susceptibility, on the other hand, a single peak initially grows with increasing H up to a well-defined value, and then splits into two sharp peaks which separate when H is further increased. We explain this surprising behavior by a flux-creep model and taking into account the enhancement of the critical-current density in the peak-effect region near Tc in which Bean’s critical-state model seems to apply. Outside this region, the crystal is clearly in a flux-creep regime with finite creep exponent n
[en] Highlights: • Chevrel phase are reviewed. • Some of the most important families are described. • Crystal structure and synthesis are recalled. • Potential applications are mentioned: batteries, catalysis, thermopower, etc. • Vortex lattices, granularity, current densities, anisotropy, unconventional superconductivity, universal scaling. - Abstract: The ternary molybdenum chalcogenides M_xMo_6X_8 (X = chalcogen), known as Chevrel phases, constitute an outstanding family of materials presenting numerous and spectacular properties. More than 100 examples of these compounds have been synthesized thanks to their versatile crystal structure. Numerous variants are found, from the binary material formed just by the molybdenum clusters Mo_6X_8 leaving a three-dimensional lattice where the third element M can be inserted, up to a condensation of clusters giving rise to a monodimensional material. The great interest in these compounds, discovered more than 40 years ago, came from their superconducting critical temperature and upper critical fields (15 K for the former, 50 Tesla at 4.2 K for the latter), both being reasonably high values at the time of their discovery thus opening enormous hopes for their use in the fabrication of magnets. Other fundamental features are found, such as the coexistence of magnetic order with the superconducting state. These features are still of interest for the scientific community, but other potential applications are now foreseen, such as their use in batteries, catalysis and thermopower technology. We recall herein some basic characteristic of Chevrel-phases, mentioning several important families, their crystal structure and mode of elaboration. This contribution being focused on the superconducting properties, we put an accent on some fundamental aspects, such as the structural and electronic transitions, the vortex lattice, their granular behavior, critical current densities, upper field and anisotropy, to finally discuss the so-called unconventional superconductivity, classifying these materials among the “exotic superconductors” and making a parallel with other superconductors which, in spite of their quite different electronic and crystal structures, present similar features. Chevrel phases have a long and incredible past as outstanding materials for basic and applied research but, in addition to that, they have a bright future ahead because of a large range of potentialities
[en] The magnetic field and surface currents induced within a superconductor are calculated assuming the field penetrates in it near sharp corners. Rounding the corners is used to keep the field less than a critical value. Analytical formulas for a corner radius are given for a wire of the rectangular cross-section and a cylinder in the external magnetic field. A boundary integral method is used to calculate the boundary of the Meissner domain when the external field penetrates deep into the superconductor. The effect of degree of penetration on the magnetic moment of superconducting cylinders and wires is discussed
[en] We present a method which allows to visualize the critical current when it appears suppressed and is unmeasurable. The method is based on the integration of the zero-bias peak observed in the differential conductance vs. bias curves. We have utilized this method for the measurement of those parts of the critical current I C(T) and I C(Φ) dependencies which are not measurable in the conventional way. The method helps us reveal phenomena related to the existence of 0-π-state in the Nb-Fe0.1Si0.9-Nb-Josephson junction. In general, the method can be used for the analysis of Josephson junctions under the conditions where the critical current is extremely small, like close to the critical temperature or in high applied magnetic fields
[en] We used preceramic polysiloxane polymers to fabricate superconducting MgB2 composites that are doped with carbon and nanosized inclusions to improve the pinning properties. The polysiloxanes were prepared by atom transfer radical polymerization and the composites were fabricated by the short time spark plasma sintering method. We found that the superconducting critical temperatures were higher than expected from the carbon content found from the X-ray diffraction analysis of the (1 1 0) peak of MgB2. To explain this finding we propose that the grains are unevenly doped, with a core-shell distribution. We also found that both, the upper critical fields and the critical current densities are higher in the preceramic-doped samples than in pure MgB2, in agreement with the carbon doping level. When ferrocene-grafted polysiloxane is used, the upper critical field is the largest, while the critical current density is the lowest. We attribute this fact to the fact that the polymer pyrolysis results in iron-based nanostructures which have a pair breaking effect.
[en] We report a simple method for the fabrication of Niobium superconducting (SC) tips for scanning tunneling microscopy which allow atomic resolution. The tips, formed in situ by the mechanical breaking of a niobium wire, reveal a clear SC gap of 1.5 meV and a critical temperature Tc = 9.2 ± 0.3 K, as deduced from Superconductor Insulator Normal metal (SIN) and Superconductor Insulator Superconductor (SIS) spectra. These match the values of bulk Nb samples. We systematically find an enhanced value of the critical magnetic field in which superconductivity in the tip is destroyed (around 1 T for some tips) up to five times larger than the critical field of bulk Nb (0.21 T). Such enhancement is attributed to a size effect at the tip apex
[en] The basic physical properties of MgB2 have been well established over the past two years of intensive research. At this point there is a general consensus about the values for the isotope shift, critical fields, most of the salient length scales, and general anisotropies. In this paper we will review the determination of these parameters and set the stage for further, more detailed discussions of specific aspects of the physics of MgB2
[en] Highlights: • Single crystals of K0.58Fe1.56Se2 were synthesized by a new single step process. • The anisotropy of upper critical field along ab-plane and c-axis is about 3.3. • High transport Jc of 1.7 × 104 A/cm2 at 5 K has been obtained in K0.58Fe1.56Se2. -- Abstract: Single crystals of K0.58Fe1.56Se2 were successfully synthesized by a new single step process with the onset superconducting transition temperature 31.9 K. The X-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. A possible modulation structure of Fe–vacancy along c-axis was observed. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3. It has also been shown that the critical current density of the K0.58Fe1.56Se2 is about 1.7 × 104 A/cm2 at 5 K