An overview of the behavior of Fe-based superconductors (FBSs) in magnetic fields is given with the emphasis on the materials features important for pinning of vortices, critical current densities, ...melting of the vortex structures and the upper critical fields. We also discuss how the multiband electronic structure of FBSs can result in extremely high upper critical fields tunable by doping or in the Fulde-Ferrel-Larkin-Ovchinnikov inhomogeneous state at high fields and low temperatures.
A theory of dissipative nonlinear conductivity, σ(1)(ω,H), of s-wave superconductors under strong electromagnetic fields at low temperatures is proposed. Closed-form expressions for σ(1)(H) and the ...surface resistance R(s)(ω,H) are obtained in the nonequilibrium dirty limit for which σ(1)(H) has a significant minimum as a function of a low-frequency (ħω ≪ k(B)T) magnetic field H. The calculated microwave suppression of R(s)(H) is in good agreement with recent experiments on alloyed Nb resonator cavities. It is shown that superimposed dc and ac fields, H = H(0) + H(a)cosωt, can be used to reduce ac dissipation in thin film nanostructures by tuning σ(1)(H(0)) with the dc field.
An overview of the theory of the upper critical field in dirty two-gap superconductors, with a particular emphasis on MgB
2 is given. We focus here on the maximum
H
c2 which may be achieved by ...increasing intraband scattering, and on the limitations imposed by weak interband scattering and paramagnetic effects. In particular, we discuss recent experiments which have demonstrated tenfold increase of
H
c2 in dirty carbon-doped films as compared to single crystals, so that
H
c2(0) parallel to the
ab planes may approach the BCS paramagnetic limit,
H
p
T
=
1.84
T
c
K
≃
60–70
T. New effects produced by weak interband scattering in the two-gap Ginzburg–Landau equations and features of
H
c2(
T) in ultrathin MgB
2 films are addressed.
We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance Rs of superconductors at ...low temperatures. Our measurements of Rs, combined with the temperature mapping and laser scanning of a 2.36-mm-thick Nb plate incorporated into a 3.3-GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, along with the observed hysteretic field dependence of Rs which can be tuned by the scanning laser beam, show that the hotspots in our Nb sample are due to trapped vortex bundles which contain ~ 10 super(6) vortices spread over regions ~ 0.1-1 cm. We calculated the frequency dependence of the rf power dissipated by oscillating vortex segments trapped between nanoscale pinning centers, taking into account all bending modes and the nonlocal line tension of the vortex driven by rf Meissner currents. We also calculated the temperature distributions caused by trapped vortex hotspots, and suggested a method of reconstructing the spatial distribution of vortex dissipation sources from the observed temperature maps. Vortex hotspots can dominate the residual surface resistance at low temperatures and give rise to a significant dependence of Rs on the rf field amplitude Hp, which can have important implications for the rf resonating cavities used in particle accelerators and forthin-film structures used in quantum computing and photon detectors.
Quantized magnetic vortices driven by electric current determine key electromagnetic properties of superconductors. While the dynamic behavior of slow vortices has been thoroughly investigated, the ...physics of ultrafast vortices under strong currents remains largely unexplored. Here, we use a nanoscale scanning superconducting quantum interference device to image vortices penetrating into a superconducting Pb film at rates of tens of GHz and moving with velocities of up to tens of km/s, which are not only much larger than the speed of sound but also exceed the pair-breaking speed limit of superconducting condensate. These experiments reveal formation of mesoscopic vortex channels which undergo cascades of bifurcations as the current and magnetic field increase. Our numerical simulations predict metamorphosis of fast Abrikosov vortices into mixed Abrikosov-Josephson vortices at even higher velocities. This work offers an insight into the fundamental physics of dynamic vortex states of superconductors at high current densities, crucial for many applications.Ultrafast vortex dynamics driven by strong currents define eletromagnetic properties of superconductors, but it remains unexplored. Here, Embon et al. use a unique scanning microscopy technique to image steady-state penetration of super-fast vortices into a superconducting Pb film at rates of tens of GHz and velocities up to tens of km/s.
The lion's share of bacteria in various environments cannot be cloned in the laboratory and thus cannot be sequenced using existing technologies. A major goal of single-cell genomics is to complement ...gene-centric metagenomic data with whole-genome assemblies of uncultivated organisms. Assembly of single-cell data is challenging because of highly non-uniform read coverage as well as elevated levels of sequencing errors and chimeric reads. We describe SPAdes, a new assembler for both single-cell and standard (multicell) assembly, and demonstrate that it improves on the recently released E+V-SC assembler (specialized for single-cell data) and on popular assemblers Velvet and SoapDeNovo (for multicell data). SPAdes generates single-cell assemblies, providing information about genomes of uncultivatable bacteria that vastly exceeds what may be obtained via traditional metagenomics studies. SPAdes is available online ( http://bioinf.spbau.ru/spades ). It is distributed as open source software.
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