Coordination motifs and large-scale structural organization in atomic clusters

The structure of nanoclusters is complex to describe due to their noncrystallinity, even though bonding and packing constraints limit the local atomic arrangements to only a few types. A computational scheme is presented to extract coordination motifs from sample atomic configurations. The method is based on a clustering analysis of multipole moments for atoms in the first coodination shell. Its power to capture large-scale structural properties is demonstrated by scanning through the ground state of the Lennard-Jones and C$_{60}$ clusters collected at the Cambridge Cluster Database.

Upper critical field, Hall effect and magnetoresistance in the iron-based layered superconductor LaFeAsO_{0.9}F_{0.1-delta}

By using a two-step method, we successfully synthesized the iron based new superconductor LaFeAsO_{0.9}F_{0.1-delta}$. The resistive transition curves under different magnetic fields were measured, leading to the determination of the upper critical field Hc2(T) of this new superconductor. The value of Hc2 at zero temperature is estimated to be about 50 Tesla roughly. In addition, the Hall effect and magnetoresistance were measured in wide temperature region. A negative Hall coefficient R_H has been found, implying a dominant conduction mainly by electron-like charge carriers in this material. The charge carrier density determined at 100 K is about 9.8E20cm^{-3}, which is close to the cuprate superconductors. It is further found that the magnetoresistance does not follow Kohlers law. Meanwhile, the different temperature dependence behaviors of resistivity, Hall coefficient, and magnetoresistance have anomalous properties at about 230 K, which may be induced by some exotic scattering mechanism.