Tarih: 20 Eylül 2013
Yer: Ankara Üniversitesi Hızlandırıcı Teknolojileri Enstitüsü,                 Gölbaşı / Ankara
Salon: Engin Arık Konferans Salonu
Saat: 14.00

Seminer Başlığı ve Özeti:
“Electron Irradiation of Fe-Based Superconductors Using Pelletron Facility”
Jülich Centre for Neutron Science (JCNS), Forschungszentrum Jülich GmbH, Outstation at
FRM II, Lichtenbergstraße 1, 85747 Garching, Germany

While chemical heterogeneity and subsequent spatial variations of the superconducting
parameters, as well as crystalline disorder on the scale of several dozen nm determine strong
vortex pinning in iron-based superconductors at low fields [1,2,3], at higher magnetic fields the
critical current density Jc  is determined by quasi-particle scattering in the vortex cores. The
latter leads to the mean-free path variation pinning mechanism due to the local fluctuations of
the dopant atom distribution. Plausibly, the relevant scatterers are the dopant atoms themselves.
In order to test this hypothesis, we study the evolution of vortex pinning and creep in Ba(Fe1-
xCox)2As2 as function of dopant concentration x, using the magneto-optical imaging technique
and Hall probe array magnetometry. We also study the surface resistance as a function of
temperature for single crystalline Ba(Fe1-xCox)2As2 across the phase diagram, using the cavity
perturbation method. In order to test this premise irradiation of Co, Ni and P-doped 122-type
iron-based compounds with high-energy 2.5 MeV electrons is performed for several doping
levels of the materials and to different doses.[4] Such irradiation introduces atomic sized point-
like defects. In all cases, the differential magneto-optical technique is used to reveal large-scale
disorder, to discard single crystals with pathological defects, and to select use single crystals
with a spatially homogenous distribution of Tc .Following irradiation it appears that the critical
temperature Tc shows a similar depression for all studied materials. The weak collective
contribution to Jc in Co-doped is found to clearly increase. Moreover this contribution appears
after irradiation of the P-doped compound in which it was previously absent. This allows one to
confirm the role of atomic point-like pins as scatterers in Ni and Co-doped compounds, as well
as the hypothesis that these defects are at the origin of the weak collective pinning contribution
to Jc at larger fields.[4]
[1] S. Demirdis et al. Phys. Rev. B 84, 094517 (2011).
[2] C. J. van der Beek and S. Demirdis et al Physica B 407 1746–1749 (2012).
[3] S. Demirdis et. al. Phys. Rev. B  87 094506 (2013).
[4] C. J. van der Beek and S. Demirdis et al J. Phys.: Conf. Ser. (Proceedings of M2S
Washington DC 2012).