Superconductivity above 50 K in Tb${}_{1-x}$Th${}_x$FeAsO

Abstract

We have successfully synthesized one member of LnFeAsO (Ln stands for lanthanides) with Ln=Tb. By partial substitution of Tb${}^{3+}$ by Th${}^{4+}$, superconductivity with onset $T_c$ up to 52 K was observed. In the undoped parent compound, the magnetic moments of Tb${}^{3+}$ ions order antiferromagnetically (AFM) at $T_N$ of 2.5 K, and there exists an anomaly in the resistivity at $T^{\ast }$ of about 124 K which corresponds to the structural phase transition and/or AFM ordering of magnetic moments of Fe${}^{2+}$ ions. This anomaly is severely suppressed by Th doping, similar to the cases of F doped LnFeAsO series. Thermopower measurements show that the charge carrier is electron-like for both undoped and Th doped compounds, and enhanced thermopower in Th doped samples was observed.

4 pages, 4 figures; minor changes, typos corrected; PRB (in press)