12,342 research outputs found
Chemically gated electronic structure of a superconducting doped topological insulator system
Angle resolved photoemission spectroscopy is used to observe changes in the
electronic structure of bulk-doped topological insulator CuBiSe as
additional copper atoms are deposited onto the cleaved crystal surface. Carrier
density and surface-normal electrical field strength near the crystal surface
are estimated to consider the effect of chemical surface gating on atypical
superconducting properties associated with topological insulator order, such as
the dynamics of theoretically predicted Majorana Fermion vortices
Characterization of novel beta-galactosidase activity that contributes to glycoprotein degradation and virulence in Streptococcus pneumoniae.
The pneumococcus obtains its energy from the metabolism of host glycosides. Therefore, efficient degradation of host glycoproteins is integral to pneumococcal virulence. In search of novel pneumococcal glycosidases, we characterized the Streptococcus pneumoniae strain D39 protein encoded by SPD_0065 and found that this gene encodes a beta-galactosidase. The SPD_0065 recombinant protein released galactose from desialylated fetuin, which was used here as a model of glycoproteins found in vivo. A pneumococcal mutant with a mutation in SPD_0065 showed diminished beta-galactosidase activity, exhibited an extended lag period in mucin-containing defined medium, and cleaved significantly less galactose than the parental strain during growth on mucin. As pneumococcal beta-galactosidase activity had been previously attributed solely to SPD_0562 (bgaA), we evaluated the contribution of SPD_0065 and SPD_0562 to total beta-galactosidase activity. Mutation of either gene significantly reduced enzymatic activity, but beta-galactosidase activity in the double mutant, although significantly less than that in either of the single mutants, was not completely abolished. The expression of SPD_0065 in S. pneumoniae grown in mucin-containing medium or tissues harvested from infected animals was significantly upregulated compared to that in pneumococci from glucose-containing medium. The SPD_0065 mutant strain was found to be attenuated in virulence in a manner specific to the host tissue
Single-Dirac-Cone topological surface states in TlBiSe2 class of Topological Insulators
We have investigated several strong spin-orbit coupling ternary chalcogenides
related to the (Pb,Sn)Te series of compounds. Our first-principles calculations
predict the low temperature rhombohedral ordered phase in TlBiTe2, TlBiSe2, and
TlSbX2 (X=Te, Se, S) to be topologically Kane-Mele Z2 = -1 nontrivial. We
identify the specific surface termination that realizes the single Dirac cone
through first-principles surface state computations. This termination minimizes
effects of dangling bonds making it favorable for photoemission (ARPES)
experiments. Our analysis predicts that thin films of these materials would
harbor novel 2D quantum spin Hall states, and support odd-parity topological
superconductivity. For a related work also see arXiv:1003.2615v1. Experimental
ARPES results will be published elsewhere.Comment: Accepted for publication in Phys. Rev. Lett. (2010). Submitted March
201
Dynamics of the Solar Magnetic Network. II. Heating the Magnetized Chromosphere
We consider recent observations of the chromospheric network, and argue that
the bright network grains observed in the Ca II H & K lines are heated by an as
yet unidentified quasi-steady process. We propose that the heating is caused by
dissipation of short-period magnetoacoustic waves in magnetic flux tubes
(periods less than 100 s). Magnetohydrodynamic (MHD) models of such waves are
presented. We consider wave generation in the network due to two separate
processes: (a) by transverse motions at the base of the flux tube; and (b) by
the absorption of acoustic waves generated in the ambient medium. We find that
the former mechanism leads to an efficient heating of the chromosphere by slow
magnetoacoustic waves propagating along magnetic field lines. This heating is
produced by shock waves with a horizontal size of a few hundred kilometers. In
contrast, acoustic waves excited in the ambient medium are converted into
transverse fast modes that travel rapidly through the flux tube and do not form
shocks, unless the acoustic sources are located within 100 km from the tube
axis. We conclude that the magnetic network may be heated by magnetoacoustic
waves that are generated in or near the flux tubes.Comment: 30 pages, 8 figures, Accepted in Astrophysical Journa
- …