  Co-doped BaFe2As2 has been previously shown to have an unusually significant
improvement of Tc (up to 2 K, or almost 10%) with annealing 1-2 weeks at 700 or
800 C, where such annealing conditions are insufficient to allow significant
atomic diffusion. While confirming similar behavior in optimally Co-doped
SrFe2As2 samples, the influence on Tc of strain induced by grinding to ~50
micron sized particles, followed by pressing the powder into a pellet using 10
kbar pressure, was found to increase the annealed transition width of 1.5 K by
approximately a factor of ten. Also, the bulk discontinuity in the specific
heat at Tc, deltaC, on the same pellet sample was completely suppressed by
grinding. This evidence for a strong sensitivity of superconductivity to strain
was used to optimize single crystal growth of Co-doped BaFe2As2. This strong
dependence (both positive via annealing and negative via grinding) of
superconductivity on strain in these two iron based 122 structure
superconductors is compared to the unconventional heavy Fermion superconductor
UPt3, where grinding is known to completely suppress superconductivity, and to
recent reports of strong sensitivity of Tc to damage induced by
electron-irradiation-induced point defects in other 122 structure iron-based
superconductors, Ba(Fe0.76Ru0.24)2As2 and Ba1-xKxFe2As2. Both the electron
irradiation and the introduction of strain by grinding are believed to only
introduce non-magnetic defects, and argue for unconventional superconducting
pairing.
