Abstract
Large high-quality single crystals of hole-doped iron-based superconductor (BaK)FeAs were grown over a broad composition range by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals () with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the -axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity found doping-independent, constant within error bars, resistivity at room temperature, , in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFeAs based compositions. The shape of the temperature-dependent resistivity, , shows systematic doping-evolution, being close to in overdoped and revealing significant contribution of the -linear component at optimum doping. The slope of the upper critical field, , scales linearly with for both , , and , . The anisotropy of the upper critical field determined near zero-field increases from 2 to 4–5 with increasing K doping level from optimal to strongly overdoped .
7 More- Received 2 March 2014
- Revised 27 March 2014
DOI:https://doi.org/10.1103/PhysRevB.89.134504
©2014 American Physical Society