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.
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.
Magnetic flux trapped during the cooldown of superconducting radio-frequency cavities through the transition temperature due to incomplete Meissner state is known to be a significant source of ...radio-frequency losses. The sensitivity of flux trapping depends on the distribution and the type of defects and impurities which pin vortices, as well as the cooldown dynamics when the cavity transitions from a normal to superconducting state. Here we present the results of measurements of the flux trapping sensitivity on 1.3 GHz elliptical cavities made from large-grain niobium with different purity for different cooldown dynamics and surface treatments. The results show that lower purity material results in a higher fraction of trapped flux and that the trapped flux sensitivity parameterSis significantly affected by surface treatments but without much change in the mean free pathl. We discuss our results within an overview of published data on the dependencies ofS(l,f)onland frequencyfusing theoretical models of rf losses of elastic vortex lines driven by weak rf currents in the cases of sparse strong pinning defects and collective pinning by many weak defects. Our analysis shows how multiscale pinning mechanisms in cavities can result in a maximum inS(l)similar to that observed by the FNAL and Cornell groups and how pinning characteristics can be extracted from the experimental data. Here the main contribution toScome from weak pinning regions at the cavity surface, where dissipative oscillations along trapped vortices perpendicular to the surface propagate into the bulk well beyond the layer of rf screening current. However, the analysis ofSas a function of only the mean free path is incomplete since cavity treatments change not onlylbut pinning characteristics as well. The effect of cavity treatments on pinning is primarily responsible for the change ofSwithout much effect onlobserved in this work. It also manifests itself in different magnitudes and peak positions inS(l), and scatter of theS-data coming from the measurements on different cavities which have undergone different treatments affecting bothland pinning. Optimizations of flux pinning to reduce flux sensitivity at low rf fields is discussed.
The recent synthesis of the superconductor LaFeAsO(0.89)F(0.11) with transition temperature T(c) approximately 26 K (refs 1-4) has been quickly followed by reports of even higher transition ...temperatures in related compounds: 41 K in CeFeAsO(0.84)F(0.16) (ref. 5), 43 K in SmFeAsO(0.9)F(0.1) (ref. 6), and 52 K in NdFeAsO(0.89)F(0.11) and PrFeAsO(0.89)F(0.11) (refs 7, 8). These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO(0.89)F(0.11) at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field B(c2) compared to values expected from the slopes dB(c2)/dT approximately 2 T K(-1) near T(c), particularly at low temperatures where the deduced B(c2)(0) approximately 63-65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with B(c2) values surpassing those of Nb(3)Sn, MgB(2) and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T(c) copper oxides.
Multiscale mechanisms of SRF breakdown Gurevich, A.
Physica. C, Superconductivity,
07/2006, Letnik:
441, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
Multiscale mechanisms of SRF breakdown and the dependence of the quality factor
Q(
H
0) on the rf field amplitude
H
0 are discussed. We first consider a basic nonlinear dependence of the surface ...resistance
R
s on the rf screening current density
J, which becomes crucial at rf fields of the order of the thermodynamic critical field
H
c
Nb
∼
200
mT
. The current-induced rf pairbreaking in the clean limit results in a quadratic field correction to
R
s(
H
0) at low
H
0 and an exponential increase of
R
s(
H
0) at
H
0
>
TH
c/
T
c. The effect of the nonlinear
R
s(
H
0) on the rf thermal breakdown is considered. The field dependence of
R
s(
H
0) reduces the breakdown field
H
b below
H
c and increases the medium and high field
Q slopes. Then a model of nonuniform thermal breakdown caused by macroscopic hotspots on the cavity surface is proposed. It is shown that hotspots expand as
H
0 increases, resulting in additional mechanism of field dependence of
Q(
H
0) which increases the
Q slope and reduces
H
b.
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