Pseudogap-like phase in Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ revealed by ${}^{75}$As NQR

We report ${}^{75}$As NQR measurements on single-crystalline Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($0\le x\le 0.09$). The nuclear spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant concentration $x$ reveals a gradual decrease of ${\left(T_{1}T\right)}^{-1}$ below a crossover temperature $T^{*}$ in the under- and optimally-doped region. Since there is no hint of a thermodynamic phase transition near $T^{*}$, this spin-gap behavior is attributed to the presence of a pseudogap-like novel state of electrons above $T_c$. The resulting $x$-$T$ phase diagram shows that, after suppression of the spin-density-wave order, $T^{*}$ intersects $T_c$ falling to zero rapidly near the optimal doping regime. Possible origins of the pseudogap behavior are discussed.

Figure 1

${}^{75}$As NQR spectra in Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($0\le x\le 0.09$) at room temperature. Both resonance frequency ${\nu }_Q$ and linewidth increase with increasing $x$.

Figure 2

Recovery of the ${}^{75}$As nuclear magnetization $M\left(t\right)$ as a function of time $t$ for optimally doped Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ ($x=0.063$). Single exponential function (solid lines) fits data quite well in the whole temperature range investigated.

Figure 3

Nuclear spin-lattice relaxation rate $T_1^{-1}$ versus temperature. Up arrows denote the SDW transition temperature $T_N$. $T_N$ is not observed at optimal $x=0.063$ and above.

Figure 4

$T$-$x$ phase diagram in Ca(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ obtained by ${\left(T_{1}T\right)}^{-1}$ measurements on the ${}^{75}$As NQR and by the uniform magnetic susceptibility ${\chi }_{\mathrm{DC}}$. FL and PG represent Fermi liquid and pseudogap, respectively. Superconducting volume fraction was estimated by the diamagnetic response of ${\chi }_{\mathrm{DC}}$.