  Through partial substitution of Sm by U in SmFeAsO, a different member of the
family of iron-based superconductors was successfully synthesized. X-ray
diffraction measurements show that the lattice constants along the a and c axes
are both squeezed through U doping, indicating a successful substitution of U
at the Sm site. The parent compound shows a strong resistivity anomaly near 150
K, associated with spin-density-wave instability.U doping suppresses this
instability and leads to a transition to the superconducting state at
temperatures up to 49 K. Magnetic measurements confirm the bulk
superconductivity in this system. For the sample with a doping level of x =
0.2, the external magnetic field suppresses the onset temperature very slowly,
indicating a rather high upper critical field. In addition, the Hall effect
measurements show that U clearly dopes electrons into the material.
