Effects of Pressure on the Electronic and Structural Properties of LaOFeAs

We studied the pressure effects on the electronic and structural properties of LaOFeAs by first-principles calculations. For the anti-ferromagnetic (AFM) phase with stripe- like aligned Fe spins, the electronic density of states at the Fermi level (N (EF)) slightly descends first with increasing applied pressure, then bounces up with further increasing pressure (or decreasing volume), and reaches its maximum at ~ 29.2 GPa with the volume ~ 80% of the ambient pressure value (V0). At this volume (V = 0.8V0), the LaOFeAs crystal undergoes a structural phase transition from the orthorhombic structure to the tetragonal one, which is accompanied by the disappearance of the long-ranged AFM order.Comment: 20 pages, 5 figure

Topological electronic states in holey graphyne

We unveil that the holey graphyne (HGY), a two-dimensional carbon allotrope where benzene rings are connected by two $-$C$\equiv$C$-$ bonds fabricated recently in a bottom-up way, exhibits topological electronic states. Using first-principles calculations and Wannier tight-binding modeling, we discover a higher-order topological invariant associated with $C_2$ symmetry of the material, and show that the resultant corner modes appear in nanoflakes matching to the structure of precursor reported previously, which are ready for direct experimental observations. In addition, we find that a band inversion between emergent $g$-like and $h$-like orbitals gives rise to a nontrivial topology characterized by $\mathbb{Z}_2$ invariant protected by an energy gap as large as 0.52 eV, manifesting helical edge states mimicking those in the prominent quantum spin Hall effect, which can be accessed experimentally after hydrogenation in HGY. We hope these findings trigger interests towards exploring the topological electronic states in HGY and related future electronics applications.Comment: 19+20 pages, 4+7 figure

Preparation and Self-assembly of Functionalized Nanocomposites and Nanomaterials – Relationship Between Structures and Properties

The recent progress in nanocomposites and nanomaterials is varied and occupies various fields. Nanocomposites can be prepared with a variety of special physical, thermal, and other unique properties. On the other hand, self-assembly technique is playing an important role in preparing well-defined multilevel nanostructures and the functionalized surface with the designed and controlled properties. In this chapter, various kinds of nanocomposites including gold nanoparticles, inorganic-organic hybrid composites, graphene oxide nanocomposites, and supramolecular gels via functionalized imide amphiphiles/binary mixtures have all been investigated and analyzed. We summarize main research contributions in recent years in three sections: preparation and self-assembly of some functionalized hybrid nanostructures; preparation and self-assembly of some graphene oxide nanocomposites; preparation and self-assembly of supramolecular gels based on some functionalized imide amphiphiles/binary mixtures. The above work may give the potential perspective for the design and fabrication of nanomaterials and composites. New nanocomposites and nanomaterials are emerging as sensitive study platforms based on unique optical and electrical properties. Future research on preparation of nanocomposites and nanomaterials will depend on the less-expensive processes in order to produce low-cost nanomaterials and devices