Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in Layered ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x\mathrm{FeAs}$

A series of layered ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x\mathrm{FeAs}$ compounds with $x=0$ to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. $F$ doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high $T_c$ strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce $4f$ electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

Figure 1

X-ray powder diffraction patterns of the pure CeOFeAs and ${\mathrm{CeO}}_{0.84}{\mathrm{F}}_{0.16}\mathrm{FeAs}$ compounds.

Figure 2

(a) The electrical resistivity vs temperature for a series of ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x\mathrm{FeAs}$. (b) $T$-dependent resistivity in an expanded region for $x=0.16$ sample. The superconducting transition with sharp onset temperature at 41 K is seen. (c) Real part of $T$-dependent ac magnetic susceptibility.

Figure 3

(a) The phase diagram showing the anomaly (circle) and superconducting transition (square) temperatures as a function of $F$ content. (b) The resistivity vs temperature curves under selected magnetic fields.

Figure 4

The reflectance (a) and conductivity (b) spectra in the far-infrared region at different temperatures for the pure CeOFeAs sample.

Figure 5

(a) The plot of $C/T$ vs $T$ for the pure CeOFeAs sample at $H=0$ and 5 T, and the ${\mathrm{CeO}}_{0.84}{\mathrm{F}}_{0.16}\mathrm{FeAs}$ sample at $H=0$, respectively. The inset shows the plot in an expanded low temperature range. (b) The $T$-dependent magnetic susceptibility of pure CeOFeAs sample. The red (or gray) curve is a fit to the Curie-Weiss law. The sharp drop below 4 K is due to AFM ordering.