7,444 research outputs found
Fabrication of large addition energy quantum dots in graphene
We present a simple technique to fabricate graphene quantum dots in a
cryostat. It relies upon the controlled rupture of a suspended graphene sheet
subjected to the application of a large electron current. This results in the
in-situ formation of a clean and ultra-narrow constriction, which hosts one
quantum dot, and occasionally a few quantum dots in series. Conductance
spectroscopy indicates that individual quantum dots can possess an addition
energy as large as 180 meV and a level spacing as large as 25 meV. Our
technique has several assets: (i) the dot is suspended, thus the electrostatic
influence of the substrate is reduced, and (ii) contamination is minimized,
since the edges of the dot have only been exposed to the vacuum in the
cryostat.Comment: Improved version. To appear in Applied Physics Letter
Disordered hyperuniformity in two-component non-additive hard disk plasmas
We study the behavior of a two-component plasma made up of non-additive hard
disks with a logarithmic Coulomb interaction. Due to the Coulomb repulsion,
long-wavelength total density fluctuations are suppressed and the system is
globally hyperuniform. Short-range volume effects lead to phase separation or
to hetero-coordination for positive or negative non-additivities, respectively.
These effects compete with the hidden long-range order imposed by
hyperuniformity. As a result, the critical behavior of the mixture is modified,
with long-wavelength concentration fluctuations partially damped when the
system is charged. It is also shown that the decrease of configurational
entropy due to hyperuniformity originates from contributions beyond the
two-particle level. Finally, despite global hyperuniformity, we show that in
our system, the spatial configuration associated with each component separately
is not hyperuniform, i.e., the system is not "multihyperuniform.
High velocity structures in, and the X-ray emission from the LBV nebula around Eta Carinae
The Luminous Blue Variable star Eta Carinae is one of the most massive stars
known. It underwent a giant eruption in 1843 in which the Homunculus nebula was
created. ROSAT and ASCA data indicate the existence of a hard and a soft X-ray
component which appear to be spatially distinct: a softer diffuse shell of the
nebula around Eta Carinae and a harder point-like source centered on the star
Eta Car. Astonishingly the morphology of the X-ray emission is very different
from the optical appearance of the nebula. We present a comparative analysis of
optical morphology, the kinematics, and the diffuse soft X-ray structure of the
nebula around Eta Carinae. Our kinematic analysis of the nebula shows extremely
high expansion velocities. We find a strong correlation between the X-ray
emission and the knots in the nebula and the largest velocities, i.e. the X-ray
morphology of the nebula around Eta Carinae is determined by the interaction
between material streaming away from Eta Car and the ambient medium.Comment: 12 pages, 7 figures, A&A in press, same paper with images at full
resolution available from
http://www.ita.uni-heidelberg.de/~kweis/publications.htm
On the Connection Between 2d Topological Gravity and the Reduced Hermitian Matrix Model
We discuss how concepts such as geodesic length and the volume of space-time
can appear in 2d topological gravity. We then construct a detailed mapping
between the reduced Hermitian matrix model and 2d topological gravity at genus
zero. This leads to a complete solution of the counting problem for planar
graphs with vertices of even coordination number. The connection between
multi-critical matrix models and multi-critical topological gravity at genus
zero is studied in some detail.Comment: 29 pages, LaTe
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