Pseudogap-like phase in Ca(Fe1-xCox)2As2 revealed by 75As NQR

We report As-75 NQR measurements on single-crystalline Ca(Fe1-xCox)(2)As-2 (0 <= x <= 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 (T1T)(-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.