Doping dependence of the magnitude of fluctuating spin moments in the normal state of the pnictide superconductor $\mathrm{Sr}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$ inferred from photoemission spectroscopy

We report systematic temperature- and doping-dependent measurements of the $\mathrm{Fe}3s$ core-level photoemission spectra in the normal state of superconducting $\mathrm{Sr}{\left(\mathrm{F}{\mathrm{e}}_{1-x}\mathrm{C}{\mathrm{o}}_x\right)}_2\mathrm{A}{\mathrm{s}}_2$. The analysis of the $\mathrm{Fe}3s$ spectrum provides an element-specific determination of the mean value of the magnitude of the Fe spin moment measured on the fast $({10}^{-16}-{10}^{-15}\mathrm{s})$ timescale of the photoemission process. The data reveal the ubiquitous presence in the normal state of Fe spin moments with magnitude fluctuating on short timescales. The data reveal a significant reduction of the magnitude of the effective Fe spin moment on going from the parent to the optimal doped compound. The doping dependence of the magnitude of the spin moment at higher doping level is less clear, being either constant, or even nonmonotonic, depending on temperature. This phenomenology indicates the importance of the interaction between spin and itinerant degrees of freedom in shaping the properties of the normal state. These findings reaffirm the complexity of the normal state of 122 Fe-pnictides, which are typically viewed as the least correlated of the high-temperature unconventional superconductors.

Figure 1

Multiplet splittings in $\mathrm{Fe}3s$ core-level PES spectra: evidence of fluctuating spin moments on the Fe sites. $\mathrm{Fe}3s$ core-level PES spectra in $\mathrm{Sr}{\left(\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}\right)}_2\mathrm{A}{\mathrm{s}}_2$ recorded for different Co concentrations and different temperatures. A Shirley-type background has been subtracted from the data points (circles). The M-SP of the BE is clearly visible as a doublet structure consisting of a main line and a satellite peak at higher BE. The continuous line through the data points is the result of the two-component fit of the doublet. The distance between these two peaks maxima provides the multiplet separation $\mathrm{\Delta }{E}_{3s}$, with experimental uncertainty on $\mathrm{\Delta }{E}_{3s}$ of $\pm 150\mathrm{meV}$.

Figure 2

Multiplet energy separation $\mathrm{\Delta }{E}_{3s}$ and FSMs. Values for the multiplet energy separation $\mathrm{\Delta }{E}_{3s}$ (squares) as a function of temperature for different Co concentrations. The values of the FSMs (circles) have been extracted from the multiplet separation $\mathrm{\Delta }{E}_{3s}$ via the relation $\mathrm{\Delta }{E}_{3s}=0.94+1.01\times \left(2S_V+1\right)$ [11]. For clarity, the error bars are shown for all points for the FSM, but only for one point for the multiplet separation $\mathrm{\Delta }{E}_{3s}$. The uncertainties on the determination of the multiplet separation $\mathrm{\Delta }{E}_{3s}$ and the FSM $2S_V$ are $\pm 150\mathrm{meV}$ and $0.15{\mu }_{\mathrm{B}}$, respectively. The data indicate that the temperature dependence of the magnitude of the FSM is more pronounced in the ${\mathrm{OD}}_{17}$ and ${\mathrm{OD}}_{27}$ samples.

Figure 3

Dependence of the FSM on Co concentration. The dependence of the FSM on Co concentration plotted for different temperatures. The FSM has minimum values in correspondence with optimal doping.