Transition from antiferromagnetism to superconductivity in the compensated metallic state of Ba1-xKx(Fe1-yCoy)2As2

We have investigated the electronic properties of Ba1-xKx(Fe1-yCoy)(2)As-2 (0 <= x <= 0.51,0 <= y <= 0.22) single crystals with a special focus on the carrier compensated regions (x = 2y). With increasing x and y under the x = 2y constraint, the ground state changes from an antiferromagnetic metal to a paramagnetic metal with an intervening superconducting phase. On the basis of the Hall coefficient measurements, we show that this transition occurs in a coherent alloy with a Fermi surface similar to BaFe2As2, and that the antiferromagnetic fluctuations are slightly weakened with increasing degree of alloying. We discuss the role of the randomness introduced by the substitution of Co2+ on this transition.