[Todos FBMC] Seminario en el DF - Marcin Konczykowski: Viernes 15/12, 14hs, Aula Seminario, 2do piso, Pab. I

Augusto Roncaglia augusto en df.uba.ar
Jue Dic 7 18:55:45 ART 2017


                        En el Aula Seminario, 2do piso, Pab. I,

                        Viernes 15/12, 14hs:

                        MARCIN KONCZYKOWSKI

                        LSI, Ecole Polytechnique

*Insight to pairing and vortex pining in iron-based superconductors from
irradiation induced disorder.*

In my talk I will overview two aspects of disorder related effects on
magnetism and
superconductivity in iron based superconductors. Pair breaking effect of
induced defects provides a test of nonconventional pairing. In fact
disorder affects
composition – temperature phase diagram of canonic 122 family
isovalently substituted  Ba(FeAs1-xPx)2 . In this part, will focus on the
effect of point-like
disorder introduced by low temperature electron irradiation. Beyond strong
suppression of critical temperature in optimally and overdoped materials,
magnetic Spin
Density Wave transition (SDW) is strongly suppressed by disorder. Evolution
of entire
phase composition – temperature phase diagram points to possible shift of
critical point by disorder. In the region close but below optimal doping,
sequence of
transitions on cooling from paramagnetic (PM) to antiferromagnetic and
superconducting state (SC), can be tuned by disorder to direct transition
from PM to SC
state. This implies change in the nature of vortex core and in pining.
Novel transition
inside of SC dome was identified in this composition range between two SC

Vortex pining by irradiation controlled disorder landscape will be the
second axis of my
talk. I will focus on optimally doped Ba1-xKx(FeAs)2 crystals. In pristine
samples strong
pinning regime prevails in wide magnetic field range. Introduction of point
disorder by
electron irradiation reveals weak collective pinning contribution with
fishtail feature and angular dependence. Correlated disorder introduced by
temperature heavy ion irradiation leads to strong anisotropic pinning with
for magnetic field parallel to traces. Magnetic relaxation in this
configuration reveals
crossover between two processes of flux creep identified by change of
energy barrier
versus current variation: from logarithmic at high temperature – low
current to power
law with exponent µ=1 at low temperature. Transition from collective
pinning of
vortices by assembly of columnar defects to individual pining and
localization on single
defect seems to be responsible for this change of creep process.
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