Spin texture on the Fermi surface of tensile strained HgTe

We present ab initio and k.p calculations of the spin texture on the Fermi surface of tensile strained HgTe, which is obtained by stretching the zincblende lattice along the (111) axis. Tensile strained HgTe is a semimetal with pointlike accidental degeneracies between a mirror symmetry protected twofold degenerate band and two nondegenerate bands near the Fermi level. The Fermi surface consists of two ellipsoids which contact at the point where the Fermi level crosses the twofold degenerate band along the (111) axis. However, the spin texture of occupied states indicates that neither ellipsoid carries a compensating Chern number. Consequently, the spin texture is locked in the plane perpendicular to the (111) axis, exhibits a nonzero winding number in that plane, and changes winding number from one end of the Fermi ellipsoids to the other. The change in the winding of the spin texture suggests the existence of singular points. An ordered alloy of HgTe with ZnTe has the same effect as stretching the zincblende lattice in the (111) direction. We present ab initio calculations of ordered Hg_xZn_1-xTe that confirm the existence of a spin texture locked in a 2D plane on the Fermi surface with different winding numbers on either end.Comment: 8 pages, 8 figure

Dirac semimetal in three dimensions

In a Dirac semimetal, the conduction and valence bands contact only at discrete (Dirac) points in the Brillouin zone (BZ) and disperse linearly in all directions around these critical points. Including spin, the low energy effective theory around each critical point is a four band Dirac Hamiltonian. In two dimensions (2D), this situation is realized in graphene without spin-orbit coupling. 3D Dirac points are predicted to exist at the phase transition between a topological and a normal insulator in the presence of inversion symmetry. Here we show that 3D Dirac points can also be protected by crystallographic symmetries in particular space-groups and enumerate the criteria necessary to identify these groups. This reveals the possibility of 3D analogs to graphene. We provide a systematic approach for identifying such materials and present ab initio calculations of metastable \beta-cristobalite BiO_2 which exhibits Dirac points at the three symmetry related X points of the BZ.Comment: 6 pages, 4 figure

Effects of mutations in the junction between helices 5 and 6 of the 16S rRNA upon 30S biogenesis

The ribosome is a large, compact RNA machine stabilized by conserved sequence motifs. The junction between helices 5 and 6 of the 16S 5’ domain undergoes transient structural rearrangements during in vitro assembly and the sequence of Right Angle motif consensus sequence but not the structure. In this work I will show that in vitro rRNA folding, r-protein binding, and RNP chemical footprinting reveal mild defects such as a greater dependence on Mg2+ when folding in low K+, approximately two-fold weaker r-protein association, and local structural perturbations. In vivo, the 30S ribosomes carrying mutations in this junction cannot support life, as they fail to mature. Neither an increase in growth temperature nor overexpression of ribosome assembly factors improves mutant pre-rRNA processing or polysomal localization. In vivo hydroxyl radical footprinting of wild type and mutant 16S rRNA revealed solvent exposures at helices 35-37 of the 3’ head domain and cleavages at helix 2. This suggests that late r-protein S2 and the native tertiary interactions with other domains of the 30S ribosome are not present in the J5/6 Triple mutant. These data are consistent with a hypothesis that the J5/6 mutants perturb central pseudoknot formation and proper docking of the 3’ head domain, perhaps communicated from the junction to helix 3 through helix 15 packing

Structural and electronic properties of the metal-metal intramolecular junctions of single-walled carbon nanotubes

Several intramolecular junctions (IMJs) connecting two metallic (11, 8) and (9, 6) carbon nanotubes along their common axis have been realized by using a layer-divided technique to the nanotubes and introducing the topological defects. Atomic structure of each IMJ configuration is optimized with a combination of density-functional theory (DFT) and the universal force field (UFF) method, based upon which a four-orbital tight-binding calculation is made on its electronic properties. Different topological defect structures and their distributions on the IMJ interfaces have been found, showing decisive effects on the localized density of states, while the sigma-pi coupling effect is negligible near Fermi energy (EF). Finally, a new IMJ model has been proposed, which probably reflects a real atomic structure of the M-M IMJ observed in the experiment [Science 291, 97 (2001)].Comment: 11 pages and 3 figure