Peter W. Stephens

Superconductivity inLaFe1−xCoxAsO

Here we report the synthesis and basic characterization of LaFe1-xCoxAsO for several values of x. The parent phase LaFeAsO orders antiferromagnetically (TN{approx}145 K). Replacing Fe with Co is expected both to electron dope and introduce disorder in the FeAs layer. For x=0.05 antiferromagnetic order is destroyed and superconductivity is observed at Tconset=11.2 K. For x=0.11 superconductivity is observed at Tconset=14.3 K and for x=0.15 it is observed at Tconset=6.0 K. For x=1, and the material appears to be ferromagnetic as judged by magnetization measurements. We conclude that Co is an effective dopant to induce superconductivity. Somewhat surprisingly, the system appears to tolerate considerable disorder in the FeAs planes.

Low-temperature phase transition inC60-n-pentane

In this paper we report the results of x-ray powder diffraction experiments on C[sub 60]-[ital n]-pentane. This material is a typical C[sub 60] clathrate and shows orthorhombic symmetry at room temperature. At low temperature an orthorhombic to monoclinic phase transition can be observed which involves the distortion of the base plane from rectangular. A microscopic picture of the phase transition is given by molecular-dynamics calculations.