IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

https://cyberleninka.org/article/n/818074.pdf

Recent advances in iron-based superconductors toward applications

We establish the applicability to transport phenomena in the cuprate superconductors of a nearly antiferromagnetic Fermi liquid (NAFL) description of the magnetic interaction between planar quasiparticles by using it to obtain the doping- and temperature-dependent resistivity and Hall conductivity seen experimentally in the normal state. Following a perturbative calculation of the anisotropic (as one goes around the Fermi surface) quasiparticle lifetimes which are the hallmark of a NAFL, we obtain simple approximate expressions for the longitudinal, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{xx}}$, and Hall, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{xy}}$, conductivities which reflect the magnetic crossovers seen experimentally as one varies the doping level and temperature. We present a simple phenomenological model for the variation in the mean free path around the Fermi surface and use this to extract from experiments on ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{xx}}$ and ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{xy}}$ quasiparticle lifetimes in the hot (strongly coupled quasiparticle) and cold (weakly coupled quasiparticle) regions of the Fermi surface which are consistent with the perturbation theory estimates. We improve upon the latter by carrying out direct numerical (nonvariational) solutions of the Boltzmann equation for representative members of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathrm{x}}$ and ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ systems, with results for transport properties in quantitative agreement with experiment. Using the same numerical approach we study the influence of CuO chains on the a-b plane anisotropy and find results in agreement with experimental findings in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$.

A Theory of the Longitudinal and Hall Conductivities of Cuprate Superconductors

The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB2 crystallites.

Hot isostatic pressing of powder in tube MgB2 wires

SiO2 nanoparticles addition effect on microstructure and pinning properties in YBa2Cu3Oy

Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(SeTe x ) and FeSe samples by current–voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, whereas the activation energy as a function of temperature, is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.

https://iopscience.iop.org/article/10.1088/0953-2048/28/12/125001/pdf

Vortex pinning properties in Fe-chalcogenides

https://iopscience.iop.org/article/10.1088/1361-6668/aa9802/pdf

Evidence of pinning crossover and the role of twin boundaries in the peak effect in FeSeTe iron based superconductor

The magnetic behavior of an iron-based FeSe crystal sample has been studied by means of dc magnetization measurements as a function of the temperature (T), the dc magnetic field (H) and the time (t). The M(T) curves show a discrepancy in the determination of the onset of the critical temperature T C with respect to what is observed in the superconducting M(H) measurements obtained by subtracting the ferromagnetic background from the curves measured at various temperatures. By using magnetic relaxation measurements M(t), the correct value of T C has been obtained. Moreover, the superconducting M(H) loops show the presence of a noisy signal up to an anomalous 'peak effect' only found for positive and negative increasing fields. These features have been analyzed by fitting the temperature dependence of the critical current density J c(T), extracted from the M(H) loops, with the help of the J c(T) dependencies governing an S–N–S junction network. This analysis has allowed us to interpret the behavior found in the M(H) loops and to obtain the value of the intrinsic critical current density J 0 which is not influenced by the presence of the junctions.

https://iopscience.iop.org/article/10.1088/0953-2048/28/11/115005/pdf

Evaluation of the intragrain critical current density in a multidomain FeSe crystal by means of dc magnetic measurements

The measurements of DC magnetization as a function of the temperature M(T), magnetic field M(H), and time M(t) have been performed in order to compare the superconducting and pinning properties of an undoped FeSe0.94 sample and a silver doped FeSe0.94 + 6 wt% Ag sample. The M(T) curves indicate an improvement of the superconducting critical temperature and a reduction of the non-superconducting phase Fe7Se8 due to the silver doping. This is confirmed by the field and temperature dependent critical current density Jc(H,T) extracted from the superconducting hysteresis loops at different temperatures within the Bean critical state model. Moreover, the combined analysis of the Jc(T) and of the pinning force Fp(H/Hirr) indicate that the pinning mechanisms in both samples can be described in the framework of the collective pinning theory. The U*(T, J) curves show a pinning crossover from an elastic creep regime of intermediate size flux bundles, for low temperatures, to a plastic creep regime at higher temperatures for both the samples. Finally, the vortex hopping attempt time has been evaluated for both samples and the results are comparable with the values reported in the literature for high Tc materials.

https://iopscience.iop.org/article/10.1088/1361-6668/30/2/025013/pdf

Critical current and flux dynamics in Ag-doped FeSe superconductor

We investigated the superconducting transition and the pinning properties of undoped and Ag-doped FeSe0.94 at magnetic fields up to 14 T. We established that, due to Ag addition, the hexagonal phase formation in melted FeSe0.94 samples is suppressed and the grain connectivity is strongly improved. The obtained superconducting zero-field transition becomes sharp, with a transition width below 1 K. Tc and the upper critical field were found to increase, while the normal-state resistivity was significantly reduced, becoming comparable with that of FeSe single crystals. In addition, a considerable magnetoresistance was observed due to Ag doping. The resistive transition of undoped and Ag-doped FeSe0.94 is dominated by a thermally activated flux flow. From the activation energy U versus H dependence, we found a crossover from single-vortex pinning to a collective-creep pinning behavior by increasing the magnetic field.

https://iopscience.iop.org/article/10.1088/0953-2048/28/2/025013/pdf

E. Nazarova

Papers

Vortex pinning properties in Fe-chalcogenides

Evidence of pinning crossover and the role of twin boundaries in the peak effect in FeSeTe iron based superconductor

Evaluation of the intragrain critical current density in a multidomain FeSe crystal by means of dc magnetic measurements

Critical current and flux dynamics in Ag-doped FeSe superconductor

Transport and pinning properties of Ag-doped FeSe0.94