SrFeAsF as a parent compound for iron pnictide superconductors

We have successfully synthesized the fluoroarsenide SrFeAsF, which is another parent phase with the ZrCuSiAs structure. The temperature dependence of resistivity and dc magnetization both reveal an anomaly at about $T_{\text{an}}=173\text{K}$, which may correspond to the structural and/or spin-density-wave (SDW) transition. Strong Hall effect and moderate magnetoresistance were observed below $T_{\text{an}}$. Interestingly, the Hall coefficient $R_H$ is positive below $T_{\text{an}}$, which is opposite to the cases of the two parent phases of FeAs-based systems known so far, i.e., LnFeAsO ($\text{Ln}=\text{rare}$-earth elements) and $\left(\text{Ba},\text{Sr}\right){\text{Fe}}_2{\text{As}}_2$, where the Hall coefficient $R_H$ is negative. This strongly suggests that the gapping of the Fermi surface induced by the SDW order leaves one of the hole pockets fully or partially ungapped in SrFeAsF. Our data show that it is possible for the parent phases of the arsenide superconductors to display dominant carriers that are either electronlike or holelike.

Figure 1

(Color online) X-ray diffraction patterns for the SrFeAsF sample. One can see that all the main peaks can be indexed to the tetragonal ZrCuSiAs-type structure. The asterisks indicate the little impurities from the ${\text{SrF}}_2$ phase.

Figure 2

(Color online) (a) Temperature dependence of resistivity for the SrFeAsF sample under magnetic fields of up to 9 T. A clear anomaly at about $T_{\text{an}}=173\text{K}$ can be observed. (b) Temperature dependence of dc magnetization for the zero-field-cooling (ZFC) process at a magnetic field of $H=5000\text{Oe}$. We can also see an anomaly at the same temperature in the $M\left(T\right)$ curve.

Figure 3

(Color online) Temperature dependence of Hall coefficient $R_H$ determined on the present sample SrFeAsF. One can see a monotonic decrease in $R_H$ in the temperature regime below about 160–170 K. Inset: the raw data of the Hall resistivity ${\rho }_{xy}$ versus the magnetic field ${\mu }_{0}H$ at different temperatures.

Figure 4

(Color online) Field dependence of MR for the present sample at different temperatures is shown in the top panel. A moderate MR effect of up to 9% is observed under the field of 9 T at 2 K. Kohler plot of MR is presented in the inset. In the bottom panel of this figure we show the temperature dependence of MR under the field of 9 T.