1,380 research outputs found
STS Observations of Landau Levels at Graphite Surfaces
Scanning tunneling spectroscopy measurements were made on surfaces of two
different kinds of graphite samples, Kish graphite and highly oriented
pyrolytic graphite (HOPG), at very low temperatures and in high magnetic
fields. We observed a series of peaks in the tunnel spectra, which grow with
increasing field, both at positive and negative bias voltages. These are
associated with Landau quantization of the quasi two-dimensional electrons and
holes in graphite in magnetic fields perpendicular to the basal plane. Almost
field independent Landau levels fixed near the Fermi energy, which are
characteristic of the graphite crystalline structure, were directly observed
for the first time. Calculations of the local density of states at the graphite
surfaces allow us to identify Kish graphite as bulk graphite and HOPG as
graphite with finite thickness effectively
Brownian molecular motors driven by rotation-translation coupling
We investigated three models of Brownian motors which convert rotational
diffusion into directed translational motion by switching on and off a
potential. In the first model a spatially asymmetric potential generates
directed translational motion by rectifying rotational diffusion. It behaves
much like a conventional flashing ratchet. The second model utilizes both
rotational diffusion and drift to generate translational motion without spatial
asymmetry in the potential. This second model can be driven by a combination of
a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven)
depending on the chosen parameters. In the third model, elements of both the
Brownian motor and powerstroke mechanisms are combined by switching between
three distinct states. Relevance of the model to biological motor proteins is
discussed.Comment: 11 pages, 8 figure
Construction of a Versatile Ultra-Low Temperature Scanning Tunneling Microscope
We constructed a dilution-refrigerator (DR) based ultra-low temperature
scanning tunneling microscope (ULT-STM) which works at temperatures down to 30
mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these
extreme operation conditions, this STM has several unique features not
available in other DR based ULT-STMs. One can load STM tips as well as samples
with clean surfaces prepared in a UHV environment to an STM head keeping low
temperature and UHV conditions. After then, the system can be cooled back to
near the base temperature within 3 hours. Due to these capabilities, it has a
variety of applications not only for cleavable materials but also for almost
all conducting materials. The present ULT-STM has also an exceptionally high
stability in the presence of magnetic field and even during field sweep. We
describe details of its design, performance and applications for low
temperature physics.Comment: 6 pages, 9 figures. accepted for publication in Rev. Sci. Instru
Real-Space Imaging of Alternate Localization and Extension of Quasi Two-Dimensional Electronic States at Graphite Surfaces in Magnetic Fields
We measured the local density of states (LDOS) of a quasi two-dimensional
(2D) electron system near point defects on a surface of highly oriented
pyrolytic graphite (HOPG) with scanning tunneling microscopy and spectroscopy.
Differential tunnel conductance images taken at very low temperatures and in
high magnetic fields show a clear contrast between localized and extended
spatial distributions of the LDOS at the valley and peak energies of the Landau
level spectrum, respectively. The localized electronic state has a single
circular distribution around the defects with a radius comparable to the
magnetic length. The localized LDOS is in good agreement with a spatial
distribution of a calculated wave function for a single electron in 2D in a
Coulomb potential in magnetic fields.Comment: 4 pages, 4 figure
Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges
We measured the electronic local density of states (LDOS) of graphite
surfaces near monoatomic step edges, which consist of either the zigzag or
armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy
(STS) techniques. The STM data reveal that the and honeycomb superstructures coexist over a length scale of 3-4 nm
from both the edges. By comparing with density-functional derived nonorthogonal
tight-binding calculations, we show that the coexistence is due to a slight
admixing of the two types of edges at the graphite surfaces. In the STS
measurements, a clear peak in the LDOS at negative bias voltages from -100 to
-20 mV was observed near the zigzag edges, while such a peak was not observed
near the armchair edges. We concluded that this peak corresponds to the
graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J.
Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene
ribbons. The existence of the edge state only at the zigzag type edge was also
confirmed by our first-principles calculations with different edge
terminations.Comment: 20 pages, 11 figure
A DNA Damage-Induced, SOS-Independent Checkpoint Regulates Cell Division in Caulobacter crescentus
Cells must coordinate DNA replication with cell division, especially during episodes of DNA damage. The paradigm for cell division control following DNA damage in bacteria involves the SOS response where cleavage of the transcriptional repressor LexA induces a division inhibitor. However, in Caulobacter crescentus, cells lacking the primary SOS-regulated inhibitor, sidA, can often still delay division post-damage. Here we identify didA, a second cell division inhibitor that is induced by DNA damage, but in an SOS-independent manner. Together, DidA and SidA inhibit division, such that cells lacking both inhibitors divide prematurely following DNA damage, with lethal consequences. We show that DidA does not disrupt assembly of the division machinery and instead binds the essential division protein FtsN to block cytokinesis. Intriguingly, mutations in FtsW and FtsI, which drive the synthesis of septal cell wall material, can suppress the activity of both SidA and DidA, likely by causing the FtsW/I/N complex to hyperactively initiate cell division. Finally, we identify a transcription factor, DriD, that drives the SOS-independent transcription of didA following DNA damage.National Institutes of Health (U.S.) (Grant R01GM082899)National Science Foundation (U.S.). Graduate Research Fellowship Progra
Keldysh study of point-contact tunneling between superconductors
We revisit the problem of point-contact tunnel junctions involving
one-dimensional superconductors and present a simple scheme for computing the
full current-voltage characteristics within the framework of the
non-equilibrium Keldysh Green function formalism. We address the effects of
different pairing symmetries combined with magnetic fields and finite
temperatures at arbitrary bias voltages. We discuss extensively the importance
of these results for present-day experiments. In particular, we propose ways of
measuring the effects found when the two sides of the junction have dissimilar
superconducting gaps and when the symmetry of the superconducting states is not
the one of spin-singlet pairing. This last point is of relevance for the study
of the superconducting state of certain organic materials like the Bechgaard
salts and, to some extent, for ruthenium compounds.Comment: 10 pages, 4 figure
Scanning tunneling microscopy and spectroscopy studies of graphite edges
We studied experimentally and theoretically the electronic local density of
states (LDOS) near single step edges at the surface of exfoliated graphite. In
scanning tunneling microscopy measurements, we observed the and honeycomb superstructures extending over 34 nm
both from the zigzag and armchair edges. Calculations based on a
density-functional derived non-orthogonal tight-binding model show that these
superstructures can coexist if the two types of edges admix each other in real
graphite step edges. Scanning tunneling spectroscopy measurements near the
zigzag edge reveal a clear peak in the LDOS at an energy below the Fermi energy
by 20 meV. No such a peak was observed near the armchair edge. We concluded
that this peak corresponds to the "edge state" theoretically predicted for
graphene ribbons, since a similar prominent LDOS peak due to the edge state is
obtained by the first principles calculations.Comment: 4 pages, 6 figures, APF9, Appl. Surf. Sci. \bf{241}, 43 (2005
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