Comparative high-field magnetotransport of the oxypnictide superconductors $R{\text{FeAsO}}_{1-x}{\text{F}}_x$ ($R=\text{La}$, Nd) and ${\text{SmFeAsO}}_{1-\delta }$

We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO ($\text{Re}=\text{La}$, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between ${\text{MgB}}_2$ and YBCO.

Figure 1

(Color online) (a) Longitudinal resistivity ${\rho }_{xx}\left(T\right)$ at $B=0$ for three members of the ${\text{ReO}}_{1-x}{\text{F}}_x\text{FeAs}$ system: ${\text{LaFeAsO}}_{0.89}{\text{F}}_{0.11}$ with $T_c\simeq 28\text{K}$, ${\text{SmFeAsO}}_{0.85}$ with $T_c\simeq 53.5\text{K}$, and ${\text{NdFeAsO}}_{0.94}{\text{F}}_{0.06}$ with $T_c\simeq 50.5\text{K}$. Dashed lines show normal resistance ${\rho }_n\left(T\right)$ extrapolated below $T_c$. (b) ${\rho }_{xx}$ at various magnetic fields as a function of temperature for La compound at $B=0$, 2, 4, 6, 8, 12, 20, 25, 30, 40, and 45 T; (c) Nd compound at $B=0$, 2, 6, 8, 10, 15, 20, and 33 T; and (d) for Sm compound at $B=0$, 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 25, and 33 T. (e) Normalized resistivity ${\rho }_{xx}\left(B,T\right)/{\rho }_n\left(T\right)$ of Nd sample vs $B$ at various $T$ measured in dc resistive magnet up to 33 T (sweep rate 5 T/min) and pulsed magnetic field up to 54 T. The latter data exhibit saturation at lower temperatures which may result from eddy current heating in this relatively big sample. (f) ${\rho }_{xx}\left(B,T\right)/{\rho }_n\left(T\right)$ of Sm sample versus $B$ at various $T$ measured in dc resistive magnet up to 33 T.

Figure 2

(Color online) The Hall resistivity ${\rho }_{xy}$ versus magnetic field for: (a) La compound, (b) Sm compound and (c) Nd compound at $T>T_c$. Note that all ${\rho }_{xy}$ show a linear dependence on magnetic field. The low-temperature ${\rho }_{xy}$ for the La compound are also temperature independent, consistent with a dominant single-carrier conduction mechanism, while at higher $T$ all compounds show marked ${\rho }_{xy}\left(T\right)$ dependencies. (d) Hall coefficient determined from linear fits to ${\rho }_{xy}\left(H\right)$ for $-33<B<33\text{T}$ or $11.5<B<45\text{T}$ (solid squares) and at $B=9\text{T}$ (open triangles Ref. 3). No systematic departure from linearity was found regardless of temperature. Arrows show apparent inflections on $R_H\left(T\right)$ dependence around $T\approx 150\text{K}$, which may result from structural transitions.

Figure 3

(Color online) (a) Upper critical fields $H_{c2}$ versus temperature for the Sm, Nd, and La compounds. with $T_c=53.5$, 50.5, and 28 K respectively, as determined from 90% transitions; $T_c=52$, 47.5, and 25 K for 50% transitions and $T_c=51$, 46, and 23 K for 10% transitions. The data extracted from the results shown in Fig. 1 show the temperatures at which the resistance reaches 10%, 50%, and 90% of the normal-state resistance, as extrapolated linearly from the ${\rho }_n\left(T\right)$ temperature dependence above $T_c$. The solid lines guide the eye. (b) The same data divided by $T_c^2$ and plotted as a function of reduced temperature $T/T_c$.