Pseudogap and its critical point in the heavily doped Ba(Fe1-xCox)2As2 from c-axis resistivity measurements

Temperature-dependent interplane resistivity, rho(c)(T), was used to characterize the normal state of the iron-arsenide superconductor Ba(Fe1-xCox)(2)As-2 over a broad doping range 0 <= x <= 0.50. The data were compared with in-plane resistivity, rho(a)(T), and magnetic susceptibility, chi(T), taken in H perpendicular to c, as well as Co NMR Knight shift, K-59, and spin-relaxation rate, 1/T1T. The interplane resistivity data show a clear correlation with the NMR Knight shift, assigned to the formation of the pseudogap. Evolution of rho(c)(T) with doping reveals two characteristic energy scales. The temperature of the crossover from nonmetallic, increasing on cooling, behavior of rho(c)(T) at high temperatures to metallic behavior at low temperatures, T*, correlates well with an anomaly in all three magnetic measurements. This characteristic temperature, equal to approximately 200 K in the parent compound, x = 0, decreases with doping and vanishes near x* approximate to 0.25. For doping levels x >= 0.166, an additional feature appears above T* with metallic behavior of rho(c)(T) found above the low-temperature resistivity increase. The characteristic temperature of this charge-gap formation, T-CG, vanishes at x(CG) similar or equal to 0.30, paving the way to metallic, T linear, rho(c)(T) close to x(CG) and superlinear T dependence for x > x(CG). None of these features are evident in the in-plane resistivity rho(c)(T). For doping levels x < x(CG), chi(T) shows a known, anomalous, T-linear dependence, which disappears for x > x(CG). These features are consistent with the existence of a charge gap, accompanying formation of the magnetic pseudogap, and its critical suppression with doping. The inferred c-axis charge gap reflects the three-dimensional character of the electronic structure and of the magnetism in the iron arsenides.