Doping dependence of the pinning efficiency in K-doped Ba122 single crystals prior to and after fast neutron irradiation

Abstract

A sharp peak was observed in the doping dependence of the critical current density, J c, in potassium doped Ba122 single crystals. This behavior is in contrast to the doping dependence of the transition temperature, T c, which varies much more smoothly around its maximum. We performed fast neutron irradiation on the crystals in order to find out whether the J c peak results from intrinsic properties or the particular defect landscape. Fast neutrons are known to introduce defects up to a size of a few nanometers, which have proven to be more efficient for flux pinning than the crystallographic defects in the pristine crystals. We find that the peak in J c shifts to higher doping levels after the irradiation, broadens, and roughly follows the shape of the T c curve. Moreover, a power law between J c and T c is observed in the irradiated crystals, which can be explained by relations between fundamental parameters and T c observed previously in iron-based superconductors. This power law does not hold for the pristine crystals which indicates that the doping dependence of J c results from an enhanced pinning efficiency in the under-doped area of the phase diagram.

Preface to the special issue ‘Focus on 10 Years of Iron-Based Superconductors’

“…Nanometric defects induced by fast neutron irradiation in (Ba,K)Fe 2 As 2 single crystals drastically change the pinning landscape that dominates the flux pinning properties, enhancing J c toward the depairing current density limit and modifying the doping dependence of J c , as shown by Kagerbauer et al [67]. Critical currents and pinning mechanisms were studied on Ba(Fe 1 −x Ni x ) 2 As 2 single crystals as a function of doping x and applied pressure p by Bioletti and coworkers [61].…”

“…Soon after the discovery of unconventional high-temperature superconductivity in iron-based compounds, the richness of possibilities to synthesize such compounds became apparent. Different families of iron-based superconductors are represented in this focus issue, both the most commonly studied ones, such as 122 chalcogenides [69] and pnictides [59][60][61][62]67], 1111 oxypnictides [66,70] and 11 chalcogenides [64,67,71], as well as the less-studied 21311 pnictides [72].…”

“…The richness of physical mechanisms in play is apparent in the non-monotonic dependence of J c on pressure and in a possible role of the proximity to a quantum critical point in the phase diagram. Uhrig and coworkers found that annealing of FeSe 1 −x Te x single crystals in air was a very simple strategy to increase T c from 7 to 14 K and the critical current density J c by up to one order of magnitude at all the applied magnetic fields [67]. The optimized annealing conditions were thickness dependent, and the related changes were attributed to the control of the interstitial excess iron by annealing, as well as to the emergence of a surface barrier, related to structural changes and oxide formation at the sample surface.…”

“…10 years after the discovery of superconducting properties in iron-based compounds, this special issue of Superconductor Science and Technology is focused on research development toward applications, with particular attention to the inter-and intra-granular critical current density [58][59][60][61][62][63][64][65][66] and the exploration of strategies to improve it [59,60,67,68].…”

“…A decade since the seminal work by the Hideo Hosono group, the technology is maturing in the fabrication of different kinds of samples, all of which are considered in this focus issue, namely thin films [58,59,66,70,71], coated conductors [64], single crystals [61,67,69], polycrystals [60,72] and tapes [62].…”