113 research outputs found
Interplay between superconductivity and itinerant magnetism in underdoped BaKFeAs ( 0.2) probed by the response to controlled point-like disorder
The response of superconductors to controlled introduction of point-like
disorder is an important tool to probe their microscopic electronic collective
behavior. In the case of iron-based superconductors (IBS), magnetic
fluctuations presumably play an important role in inducing high temperature
superconductivity. In some cases, these two seemingly incompatible orders
coexist microscopically. Therefore, understanding how this unique coexistence
state is affected by disorder can provide important information about the
microscopic mechanisms involved. In one of the most studied pnictide family,
hole-doped BaKFeAs (BaK122), this coexistence occurs over a
wide range of doping levels, 0.16~~0.25. We used
relativistic 2.5 MeV electrons to induce vacancy-interstitial (Frenkel) pairs
that act as efficient point-like scattering centers. Upon increasing dose of
irradiation, the superconducting transition temperature decreases
dramatically. In the absence of nodes in the order parameter this provides a
strong support for a sign-changing pairing. Simultaneously, in the
normal state, there is a strong violation of the Matthiessen's rule and a
decrease (surprisingly, at the same rate as ) of the magnetic transition
temperature , which indicates the itinerant nature of the long-range
magnetic order. Comparison of the hole-doped BaK122 with electron-doped
Ba(FeCo)As (FeCo122) with similar 110~K,
0.02, reveals significant differences in the normal states, with no
apparent Matthiessen's rule violation above on the electron-doped
side. We interpret these results in terms of the distinct impact of impurity
scattering on the competing itinerant antiferromagnetic and
superconducting orders
Vortex liquid correlations induced by in-plane field in underdoped Bi2Sr2CaCu2O8+d
By measuring the Josephson Plasma Resonance, we have probed the influence of
an in-plane magnetic field on the pancake vortex correlations along the c-axis
in heavily underdoped Bi2Sr2CaCu2O8+d (Tc = 72.4 +/- 0.6 K) single crystals
both in the vortex liquid and in the vortex solid phase. Whereas the in-plane
field enhances the interlayer phase coherence in the liquid state close to the
melting line, it slightly depresses it in the solid state. This is interpreted
as the result of an attractive force between pancake vortices and Josephson
vortices, apparently also present in the vortex liquid state. The results
unveil a boundary between a correlated vortex liquid in which pancakes adapt to
Josephson vortices, and the usual homogeneous liquid.Comment: 2 pages, submitted to the Proceedings of M2S HTSC VIII Dresde
Do columnar defects produce bulk pinning?
From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single
crystals, it is found that at fields and temperatures where strong single
vortex pinning by individual irradiation-induced amorphous columnar defects is
to be expected, vortex motion is limited by the nucleation of vortex kinks at
the specimen surface rather than by half-loop nucleation in the bulk. In the
material bulk, vortex motion occurs through (easy) kink sliding. Depinning in
the bulk determines the screening current only at fields comparable to or
larger than the matching field, at which the majority of moving vortices is not
trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter
Comparative study of the effects of electron irradiation and natural disorder in single crystals of SrFe(AsP) (0.35) superconductor
London penetration depth, , was measured in single crystals of
SrFe(AsP) (0.35) iron - based superconductor. The
influence of disorder on the transition temperature, , and on
was investigated. The effects of scattering controlled by the annealing of
as-grown crystals was compared with the effects of artificial disorder
introduced by 2.5~MeV electron irradiation. The low temperature behavior of
can be described by a power-law function, , with the exponent close to one in pristine annealed samples, as
expected for superconducting gap with line nodes. Upon
\ecm irradiation, the exponent increases rapidly exceeding a dirty limit
value of 2 implying that the nodes in the superconducting gap are
accidental and can be lifted by the disorder. The variation of the exponent
with is much stronger in the irradiated crystals compared to the crystals
in which disorder was controlled by the annealing of the growth defects. We
discuss the results in terms of different influence of different types of
disorder on intra- and inter- band scattering
Disorder-induced topological change of the superconducting gap structure in iron pnictides
In superconductors with unconventional pairing mechanisms, the energy gap in
the excitation spectrum often has nodes, which allow quasiparticle excitations
at low energies. In many cases, e.g. -wave cuprate superconductors, the
position and topology of nodes are imposed by the symmetry, and thus the
presence of gapless excitations is protected against disorder. Here we report
on the observation of distinct changes in the gap structure of iron-pnictide
superconductors with increasing impurity scattering. By the successive
introduction of nonmagnetic point defects into BaFe(AsP)
crystals via electron irradiation, we find from the low-temperature penetration
depth measurements that the nodal state changes to a nodeless state with fully
gapped excitations. Moreover, under further irradiation the gapped state
evolves into another gapless state, providing bulk evidence of unconventional
sign-changing -wave superconductivity. This demonstrates that the topology
of the superconducting gap can be controlled by disorder, which is a strikingly
unique feature of iron pnictides.Comment: 5 pages, 4 figure
Columnar defects and vortex fluctuations in layered superconductors
We investigate fluctuations of Josephson-coupled pancake vortices in layered
superconductors in the presence of columnar defects. We study the
thermodynamics of a single pancake stack pinned by columnar defects and obtain
the temperature dependence of localization length, pinning energy and critical
current. We study the creep regime and compute the crossover current between
line-like creep and pancake-like creep motion. We find that columnar defects
effectively increase interlayer Josephson coupling by suppressing thermal
fluctuations of pancakes. This leads to an upward shift in the decoupling line
most pronounced around the matching field.Comment: 5 pages, REVTeX, no figure
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