Antiferromagnetic spin correlations and pseudogaplike behavior inCa(Fe1−xCox)2As2studied by<

Abstract

We report 75 As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe1−xCox)2As2 (x = 0.023, 0.028, 0.033, and 0.059) annealed at 350• C for 7 days. From the observation of a characteristic shape of 75 As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of x = 0 (TN = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in x = 0.023 (TN = 106 K) and x = 0.028 (TN = 53 K). Through the temperature dependence of the Knight shifts and the nuclear spin lattice relaxation rates (1/T1), although stripe-type AFM spin fluctuations are realized in the paramagnetic state as in the case of other iron pnictide superconductors, we found a gradual decrease of the AFM spin fluctuations below a crossover temperature T * which was nearly independent of Co-substitution concentration, and is attributed to a pseudogap-like behavior in the spin excitation spectra of these systems. The T * feature finds correlation with features in the temperature-dependent inter-plane resistivity, ρc(T ), but not with the in-plane resistivity ρa(T ). The temperature evolution of anisotropic stripe-type AFM spin fluctuations are tracked in the paramagnetic and pseudogap phases by the 1/T1 data measured under magnetic fields parallel and perpendicular to the c axis. Based on our NMR data, we have added a pseudogap-like phase to the magnetic and electronic phase diagram of Ca(Fe1−xCox)2As2.

NMR study of the new magnetic superconductor CaK(Fe0.951Ni0.049)4As4 : Microscopic coexistence of the he

“…A similar T dependence of 1/T 1 T is observed in Ba(Fe 1−x Co x ) 2 As 2 which exhibits the coexistence of the stripe-type AFM and SC states [13]. The value of 1/T 1 T at low T , comparable to that at high T , indicates that significant AFM spin fluctuations persist in the AFM state [13]. This is completely different from the case of Co-doped CaFe 2 As 2 , where the AFM spin fluctuations are strongly suppressed in the AFM state, and the AFM and SC do not coexistence [13].…”

Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1−xCox)2Cui1, Wiecki2, Ran3 et al. 2016Phys. Rev. BSelf Cite8080View full textAdd to dashboardCite

“…As reported in Ref. 8, the temperaturedependent part of Knight shift K spin was reproduced by thermally activated behaviorK spin ∼ exp(-∆/k B T ) with a nearly x independent of ∆/k B ∼ 490 − 510 K. In order to extract K spin (T ), which is needed for the following Korringa ratio analysis, we plot K(T ) against the corresponding bulk static uniform magnetic susceptibility χ(T ) with T as an implicit parameter as shown in Fig A Korringa ratio analysis is applied to extract the character of spin fluctuations in Ca(Fe 1−x Co x ) 2 As 2 from 75 As NMR data as has been carried out for both the electron-doped Ba(Fe 1−x Co x ) 2 As 2 and hole-doped Ba 1−x K x Fe 2 As 2 families of iron-pnictide SCs.…”

“…Furthermore, in the case of Ca(Fe 1−x Co x ) 2 As 2 , pseudogap-like behavior 8 has been observed in the temperature dependence of 1/T 1 T and in-plane resistivity. The characteristic temperature of the pseudogap was reported to be nearly independent of Co substitution.…”

“…8. These single crystals were grown out of a FeAs/CoAs flux, 11,12 using conventional high temperature growth techniques.…”

Unconventional superconductivity

“…Based on NMR 1/T1 data, Cui et al [505] report a nearly composition independent pseudogap-like phase in Ca(Fe1-xCox)2As2, 0.023  x  0.059.…”