Closed Line Of dislocations in graphene

Defects are valuables in materials science. They are purported to generate novel physics in unexpected ways. Low dimensional systems are more prone to defects. In graphene, structural defects can exist in zero or one D, called point defect or line defect, respectively. A line defect can have tremendous impact upon the transport properties of graphene. Very common is the dislocations. In this new work[PHYSICAL REVIEW B 83, 195425 (2011)], closed line of dislocations were observed. They look like flowers. They are topologically different from open dislocations. What kind of effects can be expected on electronic properties ? Let's see.

Topological defects can affect the physical properties of graphene in unexpected ways. Harnessing their influence may lead to enhanced control of both material strength and electrical properties. Here we present a class of topological defects in graphene composed of a rotating sequence of dislocations that close on themselves,
forming grain boundary loops that either conserve the number of atoms in the hexagonal lattice or accommodate vacancy or interstitial reconstruction, while leaving no unsatisfied bonds. One grain boundary loop is observed as a “flower” pattern in scanning tunneling microscopy studies of epitaxial graphene grown on SiC(0001).We show that the flower defect has the lowest energy per dislocation core of any known topological defect in graphene, providing a natural explanation for its growth via the coalescence of mobile dislocations.

Three-body states in dipolar molecules

Two-body bound states (Efimov states) are common: the earth-sun, the hydrogen atom, the double-atom molecu;es, cooper pairs, and so on. But three-body bound systems seem not that abundant (Why? I don't know yet). Now due to a theoretical work, the prospect is bright. According to it, such states should widely exist in dipolar molecules.

Tractable three-body problems are rare, which is why Vitaly Efimov’s study in 1970 proposing that bound states could exist between three interacting bosons was so intriguing. It took more than 30 years, though, to observe Efimov states in an ultracold gas of cesium atoms, in which interactions could be controlled with a magnetic field. Now, writing in Physical Review Letters, theorists suggest similar states should also exist between dipolar molecules.

In his prediction, Efimov assumed the interacting bosons were spherically symmetric. In their new work, Yujun Wang and colleagues at JILA, at the University of Colorado, Boulder, use numerical methods to look for bound states between molecules that have an electric dipole—an extended structure that greatly complicates the calculations. The group shows that such dipolar Efimov states are in fact long-lived and “universal,” meaning they don’t depend on the molecules’ detailed structure. (The states only exist when the separation between the molecules is large compared with the length of their dipole moment.)

Wang et al.’s prediction is timely, as it is only in the last two to three years that experimentalists have been able to cool the molecules in a gas to their absolute ground state and study and manipulate the dipole interactions between them. – Jessica Thomas [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.233201]