Pseudogap and its critical point in the heavily dopedBa(Fe1−xCox)2As2fromc-axis resistivity measurements

Abstract

Temperature-dependent interplane resistivity, ${\ensuremath{\rho}}_{c}(T)$, was used to characterize the normal state of the iron-arsenide superconductor $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ over a broad doping range $0\ensuremath{\le}x<0.50$. The data were compared with in-plane resistivity, ${\ensuremath{\rho}}_{a}(T)$, and magnetic susceptibility, $\ensuremath{\chi}(T)$, taken in $H\ensuremath{\perp}c$, as well as Co NMR Knight shift, $^{59}K$, and spin-relaxation rate, $1/{T}_{1}T$. The interplane resistivity data show a clear correlation with the NMR Knight shift, assigned to the formation of the pseudogap. Evolution of ${\ensuremath{\rho}}_{c}(T)$ with doping reveals two characteristic energy scales. The temperature of the crossover from nonmetallic, increasing on cooling, behavior of ${\ensuremath{\rho}}_{c}(T)$ at high temperatures to metallic behavior at low temperatures, ${T}^{\ensuremath{\ast}}$, 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 ${x}^{\ensuremath{\ast}}\ensuremath{\approx}0.25$. For doping levels $x\ensuremath{\ge}0.166$, an additional feature appears above ${T}^{\ensuremath{\ast}}$ with metallic behavior of ${\ensuremath{\rho}}_{c}(T)$ found above the low-temperature resistivity increase. The characteristic temperature of this charge-gap formation, ${T}_{\text{CG}}$, vanishes at ${x}_{\text{CG}}\ensuremath{\simeq}0.30$, paving the way to metallic, $T$ linear, ${\ensuremath{\rho}}_{c}(T)$ close to ${x}_{\text{CG}}$ and superlinear $T$ dependence for $x>{x}_{\text{CG}}$. None of these features are evident in the in-plane resistivity ${\ensuremath{\rho}}_{a}(T)$. For doping levels $x<{x}_{\text{CG}}$, $\ensuremath{\chi}(T)$ shows a known, anomalous, $T$-linear dependence, which disappears for $x>{x}_{\text{CG}}$. 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.