Coupling between the structural and magnetic transition in CeFeAsO

Using an adapted Sn-flux growth technique we obtained comparatively large CeFeAsO single crystals of better quality than previously reported polycrystals or single crystals, as evidenced by much sharper anomalies at the structural and magnetic phase transitions as well as a much higher residual resistivity ratio of 12. In the magnetically ordered phase we observe a very pronounced metallic behavior of the in-plane resistivity, which excludes a Mott insulator regime at low temperature. The separation $\Delta T=T_0-T_N$ between structural and magnetic ordering temperatures decreases with increasing sample quality, from 18 K in the initial reports to 6 K in the present single crystals, demonstrating that this separation is not an intrinsic property of the $R\text{FeAsO}$ systems. Our results indicate that the coupling between magnetic ordering and structural distortion is very similar in $A{\text{Fe}}_2{\text{As}}_2$ and $R\text{FeAsO}$ type of compounds, much more similar than previously thought. The implications of our experimental results give arguments both in favor and against the nematic phase model.

Figure 1

(Color online) Temperature dependence of specific heat (left scale) and electrical resistivity (right scale) of a cluster of CeFeAsO. The resistivity shows an almost steplike drop at $T=150\text{K}$ and a further decrease toward low temperatures, resulting in $\text{RRR}=12$. The specific heat shows an anomaly around $T\approx 145\text{K}$ with two distinct maxima, which can be attributed to the structural transition and the AFM ordering of Fe, and a peak at $T_N^{\text{Ce}}=3.7\text{K}$ corresponding to AFM ordering of Ce (Ref. 27).

Figure 2

(Color online) Temperature dependence of the electrical resistivity normalized to RT values for a small single crystal and a larger cluster of co-aligned single crystals of CeFeAsO. For comparison, the resistivity of polycrystalline CeFeAsO taken from McGuire et al. (Ref. 30) is included. The inset shows the magnetoresistance of single crystalline CeFeAsO with magnetic field applied along the crystallographic $c$ axis and electrical current in the $ab$ plane.

Figure 3

(Color online) Specific heat (upper panel) and derivative of the electrical resistivity normalized to RT (lower panel) of different CeFeAsO samples. While the onset of the structural transition is close to $T\approx 155\text{K}$ for all samples, the midpoint of this transition is slightly shifted to lower temperatures in the polycrystalline sample. In contrast, $T_N$ (peaks with dotted line) decrease significantly from the single crystal, to the cluster and the polycrystal. The data for the polycrystal were taken from (Ref. 30).

Figure 4

(Color online) Linear thermal-expansion coefficient for single crystalline CeFeAsO measured along a nonspecified in-plane direction ${\alpha }_{ab}$. The dotted line, interpolating between the data outside the peak anomaly, indicates the background expansivity. The inset shows ${\alpha }_{ab}$ for two different initial pressure values on the dilatometer cell (left scale) and ${\alpha }_c$ (right scale) in a representation ${\alpha }_i/T$ vs $T$. The derivative of the electrical resistivity in arbitrary units is shown for comparison. The vertical dashed lines indicate the transition temperatures of the structural transition, $T_0=151\text{K}$, and the AFM ordering of Fe moments, $T_N=145\text{K}$.