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

 

 

Marcin Konczykowski.

LSI, Ecole Polytechnique.

Viernes 15/12/2017, 14 hs. 

Aula Seminario, 2do piso, Pab. I. 

 

In my talk I will overview two aspects of disorder related effects on magnetism and
superconductivity in iron based superconductors. Pair breaking effect of irradiation-
induced defects provides a test of nonconventional pairing. In fact disorder affects
composition – temperature phase diagram of canonic 122 family superconductor,
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 quantum
critical point by disorder. In the region close but below optimal doping, sequence of
transitions on cooling from paramagnetic (PM) to antiferromagnetic and finally
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 states.
 
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 characteristic
fishtail feature and angular dependence. Correlated disorder introduced by low
temperature heavy ion irradiation leads to strong anisotropic pinning with maximum
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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