18,261 research outputs found
Klein tunneling through an oblique barrier in graphene ribbons
We study a transmission coefficient of graphene nanoribbons with a top gate
which acts as an oblique barrier. Using a Green function method based on the
Dirac-like equation, scattering among transverse modes due to the oblique
barrier is taken into account numerically. In contrast to the 2-dimensional
graphene sheet, we find that the pattern of transmission in graphene ribbons
depends strongly on the electronic structure in the region of the barrier.
Consequently, irregular structures in the transmission coefficient are
predicted while perfect transmission is still calculated in the case of
metallic graphene independently of angle and length of the oblique barrier
The one-round Voronoi game replayed
We consider the one-round Voronoi game, where player one (``White'', called
``Wilma'') places a set of n points in a rectangular area of aspect ratio r
<=1, followed by the second player (``Black'', called ``Barney''), who places
the same number of points. Each player wins the fraction of the board closest
to one of his points, and the goal is to win more than half of the total area.
This problem has been studied by Cheong et al., who showed that for large
enough and r=1, Barney has a strategy that guarantees a fraction of 1/2+a,
for some small fixed a.
We resolve a number of open problems raised by that paper. In particular, we
give a precise characterization of the outcome of the game for optimal play: We
show that Barney has a winning strategy for n>2 and r>sqrt{2}/n, and for n=2
and r>sqrt{3}/2. Wilma wins in all remaining cases, i.e., for n>=3 and
r<=sqrt{2}/n, for n=2 and r<=sqrt{3}/2, and for n=1. We also discuss complexity
aspects of the game on more general boards, by proving that for a polygon with
holes, it is NP-hard to maximize the area Barney can win against a given set of
points by Wilma.Comment: 14 pages, 6 figures, Latex; revised for journal version, to appear in
Computational Geometry: Theory and Applications. Extended abstract version
appeared in Workshop on Algorithms and Data Structures, Springer Lecture
Notes in Computer Science, vol.2748, 2003, pp. 150-16
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Rapid Fabrication of Large-sized Solid Shape using Variable Lamination Manufacturing and Multi-functional Hotwire Cutting System
Rapid prototyping (RP) technologies have been widely used to reduce the lead-time and
development cost of new products. The VLM-ST process has been developed to overcome the
currently developed RP technologies such as a large building time, a high building cost, an
additional post-processing and a large apparatus cost. However, the VLM-ST process has the
limitation of fabricated model size (VLM300: 297Ă210 mm, VLM400: 420Ă297 mm) and the
limitation of slope angle when the large-sized model more than 600 Ă 600 Ă 600 mm or
axisymmetric shape is fabricated. The objective of this paper is to develop a multi-functional
hotwire cutting system (MHC) using EPS-foam block or sheet as the working material in order to
fabricate a large-sized shape more than 600 Ă 600 Ă 600 mm. Because the MHC apparatus
employs a four-axis synchronized hotwire cutter with the structure of two XY movable heads and
a turn-table, it allows the easy fabrication of various 3D shapes, such as (1) an axisymmetric
shape or a sweeping cross-sectioned pillar shape using the hot-strip in the form of sweeping
surface and EPS foam block on the turn-table, (2) a polyhedral complex shape using the hotwire
and EPS foam block on the turn-table, and (3) a ruled surface approximated freeform shape using
the hotwire and EPS foam sheet. In order to examine the applicability of the developed MHC
apparatus, an axisymmetric shape, a polyhedral shape and a large-sized freeform shape were
fabricated by the apparatus.Mechanical Engineerin
Efficient Schemes for Reducing Imperfect Collective Decoherences
We propose schemes that are efficient when each pair of qubits undergoes some
imperfect collective decoherence with different baths. In the proposed scheme,
each pair of qubits is first encoded in a decoherence-free subspace composed of
two qubits. Leakage out of the encoding space generated by the imperfection is
reduced by the quantum Zeno effect. Phase errors in the encoded bits generated
by the imperfection are reduced by concatenation of the decoherence-free
subspace with either a three-qubit quantum error correcting code that corrects
only phase errors or a two-qubit quantum error detecting code that detects only
phase errors, connected with the quantum Zeno effect again.Comment: no correction, 3 pages, RevTe
More on N=1 Matrix Model Curve for Arbitrary N
Using both the matrix model prescription and the strong-coupling approach, we
describe the intersections of n=0 and n=1 non-degenerated branches for quartic
(polynomial of adjoint matter) tree-level superpotential in N=1 supersymmetric
SO(N)/USp(2N) gauge theories with massless flavors. We also apply the method to
the degenerated branch. The general matrix model curve on the two cases we
obtain is valid for arbitrary N and extends the previous work from
strong-coupling approach. For SO(N) gauge theory with equal massive flavors, we
also obtain the matrix model curve on the degenerated branch for arbitrary N.
Finally we discuss on the intersections of n=0 and n=1 non-degenerated branches
for equal massive flavors.Comment: 36pp; to appear in JHE
Time-convolutionless reduced-density-operator theory of a noisy quantum channel: a two-bit quantum gate for quantum information processing
An exact reduced-density-operator for the output quantum states in
time-convolutionless form was derived by solving the quantum Liouville equation
which governs the dynamics of a noisy quantum channel by using a projection
operator method and both advanced and retarded propagators in time. The
formalism developed in this work is general enough to model a noisy quantum
channel provided specific forms of the Hamiltonians for the system, reservoir,
and the mutual interaction between the system and the reservoir are given.
Then, we apply the formulation to model a two-bit quantum gate composed of
coupled spin systems in which the Heisenberg coupling is controlled by the
tunneling barrier between neighboring quantum dots. Gate Characteristics
including the entropy, fidelity, and purity are calculated numerically for both
mixed and entangled initial states
Lyman alpha line formation in starbursting galaxies II. Extremely Thick, Dustless, and Static HI Media
The Lya line transfer in an extremely thick medium of neutral hydrogen is
investigated by adopting an accelerating scheme in our Monte Carlo code to skip
a large number of core or resonant scatterings. This scheme reduces computing
time significantly with no sacrifice in the accuracy of the results. We applied
this numerical method to the Lya transfer in a static, uniform, dustless, and
plane-parallel medium. Two types of photon sources have been considered, the
midplane source and the uniformly distributed sources. The emergent profiles
show double peaks and absorption trough at the line-center. We compared our
results with the analytic solutions derived by previous researchers, and
confirmed that both solutions are in good agreement with each other. We
investigated the directionality of the emergent Lya photons and found that limb
brightening is observed in slightly thick media while limb darkening appears in
extremely thick media. The behavior of the directionality is noted to follow
that of the Thomson scattered radiation in electron clouds, because both Lya
wing scattering and Thomson scattering share the same Rayleigh scattering phase
function. The mean number of wing scatterings just before escape is in exact
agreement with the prediction of the diffusion approximation. The Lya photons
constituting the inner part of the emergent profiles follow the relationship
derived from the diffusion approximation. We present a brief discussion on the
application of our results to the formation of Lya broad absorption troughs and
P-Cygni type Lya profiles seen in the UV spectra of starburst galaxies.Comment: 24 papges, 12 figures, The revised version submitted to Ap
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