$G$-type magnetic order in ferropnictide $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{As}$ induced by hole doping on As sites

Strong antiferromagnetic (AFM) correlation has long been postulated to be closely related to the occurrence of unconventional high-temperature superconductivity observed in the cuprates, heavy fermions, and organic superconductors. The recently discovered Fe-based superconductors add another interesting member to the list. However, insufficient attention has been paid to the versatile nature of the magnetic correlation in these materials: some showing stripe ($C$-type) order, others double stripe ($E$-type) or block AFM order instead, implying potentially richer structures of the superconducting order. Here we report the observation of yet another AFM correlation in the family: a $G$-type AFM order as seen in the high-$T_{\mathrm{c}}$ cuprates, in $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{As}$ compounds isostructural to the LiFeAs superconductor. This study not only sheds light on the underlying mechanism of the rich magnetic correlations in the Fe-based superconductors, but also suggests the possibility of realizing a distinct pairing symmetry upon chemical doping or applying pressure.

Figure 1

(a) $G$-type antiferromagnetic structure, (b) collinear single-stripe antiferromagnetic structure, and (c) double-stripe antiferromagnetic structure. The red and blue arrows indicate the spin direction of Fe atoms in (a), (b), and (c). (d) Block checkboard antiferromagnetic structure, with the open squares representing the Fe vacancy, while the + and − symbols represent the spin direction of Fe atoms pointing to opposite directions along the $c$ axis. Magnetic structure of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{As}$ determined by neutron diffraction at 4 K (e) and 150 K (f), respectively. The spin direction at 150 K could not be uniquely determined due to the limitation of the data obtained.

Figure 2

(a) Specific heat of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{As}$ as a function of temperature measured during cooling from 250 to 2 K. The two purple arrows at 220 and 63 K correspond to the antiferromagnetic and weak ferromagnetic transitions, respectively. The inset shows an expanded view. (b) Magnetic susceptibility of $\mathrm{C}{\mathrm{u}}_x\mathrm{F}{\mathrm{e}}_{1-y}\mathrm{As}$ measured with a 1000 Oe magnetic field applied along both in-plane and out-of-plane directions after zero-field cooling.

Figure 3

Rocking scan of the $Q=\left(000.5\right)$ (a) and $Q=\left(100.5\right)$ (b) magnetic Bragg reflections at 4, 150, and 250 K. The neutron counting time is 30 and 480 s, respectively. (c) The temperature profile of the peak intensities of the $Q=\left(000.5\right)$ and (1 0 0.5) magnetic Bragg peaks with the counting time of 180 and 600 s, respectively. The dark blue and pink curves are guides to the eyes.

Figure 4

(a) Electronic band structure of $G$-type CuFeAs with highlighted Cu-$d$ orbital component, showing clearly that they are well below the Fermi energy (at zero) and fully occupied. (b) Illustration of strong next-nearest-neighbor ferromagnetic coupling of Fe atoms bridged by the spin-polarized $p_x$ or $p_y$ orbital of the As atom in the $G$-type antiferromagnetic configuration. (c) Comparison of the As $p_x$ partial density of states for $x=0$ and $x=1$ in $G$-type order, showing a large enhancement of the down-spin component above the Fermi level upon reduction of Cu (highlighted by the green circle). (d) and (e) The band structure for the case of $x=1$, with the highlighted As $p_x$ contribution showing a strong spin polarization.