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

Marcin Konczykowski.

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(FeAs_1-xP_x)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 Ba_1-xK_x(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.