437 research outputs found
Evidence for the formation of a Mott state in potassium-intercalated pentacene
We investigate electronic transport through pentacene thin-films intercalated
with potassium. From temperature-dependent conductivity measurements we find
that potassium-intercalated pentacene shows metallic behavior in a broad range
of potassium concentrations. Surprisingly, the conductivity exhibits a
re-entrance into an insulating state when the potassium concentration is
increased past one atom per molecule. We analyze our observations theoretically
by means of electronic structure calculations, and we conclude that the
phenomenon originates from a Mott metal-insulator transition, driven by
electron-electron interactions.Comment: 8 pages, 6 figure
Pt-induced nanowires on Ge(001): a DFT study
We study formation of the nanowires formed after deposition of Pt on a
Ge(001) surface. The nanowires form spontaneously after high temperature
annealing. They are thermodynamically stable, only one atom wide and up to a
few hundred atoms long. Ab initio density functional theory calculations are
performed to identify possible structures of the Pt-Ge (001) surface with
nanowires on top. A large number of structures is studied. With nanowires that
are formed out of Pt or Ge dimers or mixed Pt-Ge dimers. By comparing simulated
scanning tunneling microscopy images with experimental ones we model the
formation of the nanowires and identify the geometries of the different phases
in the formation process. We find that the formation of nanowires on a
Pt-Ge(001) surface is a complex process based on increasing the Pt density in
the top layers of the Ge(001) surface. Most remarkably we find the nanowires to
consist of germanium dimers placed in troughs lined by mixed Pt-Ge dimer rows.Comment: 22 pages, 24 figure
De Novo sequences of Haloquadratum walsbyi from Lake Tyrrell, Australia, reveal a aariable genomic landscape
Hypersaline systems near salt saturation levels represent an extreme environment, in which organisms grow and survive near the limits of life. One of the abundant members of the microbial communities in hypersaline systems is the square archaeon, Haloquadratum walsbyi. Utilizing a short-read metagenome from Lake Tyrrell, a hypersaline ecosystem in Victoria, Australia, we performed a comparative genomic analysis of H. walsbyi to better understand the extent of variation between strains/subspecies. Results revealed that previously isolated strains/subspecies do not fully describe the complete repertoire of the genomic landscape present in H. walsbyi. Rearrangements, insertions, and deletions were observed for the Lake Tyrrell derived Haloquadratum genomes and were supported by environmental de novo sequences, including shifts in the dominant genomic landscape of the two most abundant strains. Analysis pertaining to halomucins indicated that homologs for this large protein are not a feature common for all species of Haloquadratum. Further, we analyzed ATP-binding cassette transporters (ABC-type transporters) for evidence of niche partitioning between different strains/subspecies. We were able to identify unique and variable transporter subunits from all five genomes analyzed and the de novo environmental sequences, suggesting that differences in nutrient and carbon source acquisition may play a role in maintaining distinct strains/subspecies.Funding for this was provided by the National Science Foundation (NSF) MCB Award no. 0626526 to J. Banfield, E. Allen, and K. Heidelberg
Graphite and graphene as perfect spin filters
Based upon the observations (i) that their in-plane lattice constants match
almost perfectly and (ii) that their electronic structures overlap in
reciprocal space for one spin direction only, we predict perfect spin filtering
for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. The spin
filtering is quite insensitive to roughness and disorder. The formation of a
chemical bond between graphite and the open -shell transition metals that
might complicate or even prevent spin injection into a single graphene sheet
can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu
while still preserving the ideal spin injection property
Pockets of Proterozoic hydrocarbons and implications for the Archaean
Precambrian biomarkers convey invaluable information
about the early evolution of life, ancient ecosystems, redox
conditions, climate and depositional environment and
prospective petroleum systems. They are however thermally
unstable, easily obliterated by contamination and thus
extremely difficult to find. This is particularly true if
conditions favourable for biomarker preservation had to
prevail for more than 2.5 billion years – the prerequisite for
finding Archaean biomarkers. Many organic geochemists
abandoned this hope after original discoveries of Archaean
biomarkers proved to be of younger origin [1,2] but our study
of ca. 550-825 Ma old sediments from the Centralian
Superbasin now shows that biomarkers can be preserved in
distinctive pockets in seemingly barren areas, even if sections
are metamorphosed in parts. Most Centralian sections seem
empty. Yet, eventually we identified intervals with preserved
biomarkers in three drill cores. A detailed investigation of 825
Ma sediments in drill core Mt Charlotte-1 revealed maturity
variations that are most likely due to hydrothermal influence
and in turn control the hydrocarbon preservation. Sediments
might appear metamorphosed after localized, subtle alteration
by hydrothermal fluids but protected intervals can still contain
biomarkers. The same might be true for Archaean sediments
and we might still find those protected intervals with
indigenous biomarkers that allow us to glimpse the early life
on earth
Electronic structure and optical properties of lightweight metal hydrides
We study the electronic structures and dielectric functions of the simple
hydrides LiH, NaH, MgH2 and AlH3, and the complex hydrides Li3AlH6, Na3AlH6,
LiAlH4, NaAlH4 and Mg(AlH4)2, using first principles density functional theory
and GW calculations. All these compounds are large gap insulators with GW
single particle band gaps varying from 3.5 eV in AlH3 to 6.5 eV in the MAlH4
compounds. The valence bands are dominated by the hydrogen atoms, whereas the
conduction bands have mixed contributions from the hydrogens and the metal
cations. The electronic structure of the aluminium compounds is determined
mainly by aluminium hydride complexes and their mutual interactions. Despite
considerable differences between the band structures and the band gaps of the
various compounds, their optical responses are qualitatively similar. In most
of the spectra the optical absorption rises sharply above 6 eV and has a strong
peak around 8 eV. The quantitative differences in the optical spectra are
interpreted in terms of the structure and the electronic structure of the
compounds.Comment: 13 pages, 10 figure
Germanene: the germanium analogue of graphene
Recently, several research groups have reported the growth of germanene, a new member of the graphene family. Germanene is in many aspects very similar to graphene, but in contrast to the planar graphene lattice, the germanene honeycomb lattice is buckled and composed of two vertically displaced sub-lattices. Density functional theory calculations have revealed that free-standing germanene is a 2D Dirac fermion system, i.e. the electrons behave as massless relativistic particles that are described by the Dirac equation, which is the relativistic variant of the Schrödinger equation. Germanene is a very appealing 2D material. The spin-orbit gap in germanene (~24 meV) is much larger than in graphene (<0.05 meV), which makes germanene the ideal candidate to exhibit the quantum spin Hall effect at experimentally accessible temperatures. Additionally, the germanene lattice offers the possibility to open a band gap via for instance an externally applied electrical field, adsorption of foreign atoms or coupling with a substrate. This opening of the band gap paves the way to the realization of germanene based field-effect devices. In this topical review we will (1) address the various methods to synthesize germanene (2) provide a brief overview of the key results that have been obtained by density functional theory calculations and (3) discuss the potential of germanene for future applications as well for fundamentally oriented studies
Novel diffusion mechanism on the GaAs(001) surface: the role of adatom-dimer interaction
Employing first principles total energy calculations we have studied the
behavior of Ga and Al adatoms on the GaAs(001)-beta2 surface. The adsorption
site and two relevant diffusion channels are identified. The channels are
characterized by different adatom-surface dimer interaction. Both affect in a
novel way the adatom migration: in one channel the diffusing adatom jumps
across the surface dimers and leaves the dimer bonds intact, in the other one
the surface dimer bonds are broken. The two channels are taken into account to
derive effective adatom diffusion barriers. From the diffusion barriers we
conclude a strong diffusion anisotropy for both Al and Ga adatoms with the
direction of fastest diffusion parallel to the surface dimers. In agreement
with experimental observations we find higher diffusion barriers for Al than
for Ga.Comment: 4 pages, 2 figures, Phys. Rev. Lett. 79 (1997). Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
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