1,696 research outputs found
Density of states as a probe of electrostatic confinement in graphene
We theoretically analyze the possibility to confine electrons in single-layer
graphene with the help of metallic gates, via the evaluation of the density of
states of such a gate-defined quantum dot in the presence of a ring-shaped
metallic contact. The possibility to electrostatically confine electrons in a
gate-defined ``quantum dot'' with finite-carrier density, surrounded by an
undoped graphene sheet, strongly depends on the integrability of the electron
dynamics in the quantum dot. With the present calculations we can
quantitatively compare confinement in dots with integrable and chaotic
dynamics, and verify the prediction that the Berry phase associated with the
pseudospin leads to partial confinement in situations where no confinement is
expected according to the arguments relying on the classical dynamics only.Comment: 9 pages, 7 figure
Semiclassical theory of the interaction correction to the conductance of antidot arrays
Electron-electron interactions are responsible for a correction to the
conductance of a diffusive metal, the "Altshuler-Aronov correction" . Here we study the counterpart of this correction for a ballistic
conductor, in which the electron motion is governed by chaotic classical
dynamics. In the ballistic conductance, the Ehrenfest time enters as
an additional time scale that determines the magnitude of quantum interference
effects. The Ehrenfest time effectively poses a short-time threshold for the
trajectories contributing to the interaction correction. As a consequence,
becomes exponentially suppressed if the Ehrenfest time is
larger than the dwell time or the inverse temperature. We discuss the explicit
dependence on Ehrenfest time in quasi-one and two-dimensional antidot arrays.
For strong interactions, the sign of may change as a function
of temperature for temperatures in the vicinity of .Comment: 20 pages, 10 figures, published versio
Interplay of Aharonov-Bohm and Berry phases in gate-defined graphene quantum dots
We study the influence of a magnetic flux tube on the possibility to
electrostatically confine electrons in a graphene quantum dot. Without magnetic
flux tube, the graphene pseudospin is responsible for a quantization of the
total angular momentum to half-integer values. On the other hand, with a flux
tube containing half a flux quantum, the Aharonov-Bohm phase and Berry phase
precisely cancel, and we find a state at zero angular momentum that cannot be
confined electrostatically. In this case, true bound states only exist in
regular geometries for which states without zero-angular-momentum component
exist, while non-integrable geometries lack confinement. We support these
arguments with a calculation of the two-terminal conductance of a gate-defined
graphene quantum dot, which shows resonances for a disc-shaped geometry and for
a stadium-shaped geometry without flux tube, but no resonances for a
stadium-shaped quantum dot with a -flux tube.Comment: 7 pages, 5 figure
Electronic transport in graphene with particle-hole-asymmetric disorder
We study the conductivity of graphene with a smooth but
particle-hole-asymmetric disorder potential. Using perturbation theory for the
weak-disorder regime and numerical calculations we investigate how the
particle-hole asymmetry shifts the position of the minimal conductivity away
from the Dirac point . We find that the conductivity minimum
is shifted in opposite directions for weak and strong disorder. For large
disorder strengths the conductivity minimum appears close to the doping level
for which electron and hole doped regions ("puddles") are equal in size
Nonexistence of exceptional imprimitive Q-polynomial association schemes with six classes
Suzuki (1998) showed that an imprimitive Q-polynomial association scheme with
first multiplicity at least three is either Q-bipartite, Q-antipodal, or with
four or six classes. The exceptional case with four classes has recently been
ruled out by Cerzo and Suzuki (2009). In this paper, we show the nonexistence
of the last case with six classes. Hence Suzuki's theorem now exactly mirrors
its well-known counterpart for imprimitive distance-regular graphs.Comment: 7 page
Novel Dexterous Robotic Finger Concept with Controlled Stiffness
This paper introduces a novel robotic finger concept for variable impedance grasping in unstructured tasks. The novel robotic finger combines three key features: minimal actuation, variable mechanical compliance and full manipulability. This combination of features allows for a minimal component design, while reducing control complexity and still providing required dexterity and grasping capabilities. The conceptual properties (such as variable compliance) are studied in a port-Hamiltonian framework
Strongly walk-regular graphs
We study a generalization of strongly regular graphs. We call a graph
strongly walk-regular if there is an such that the number of walks of
length from a vertex to another vertex depends only on whether the two
vertices are the same, adjacent, or not adjacent. We will show that a strongly
walk-regular graph must be an empty graph, a complete graph, a strongly regular
graph, a disjoint union of complete bipartite graphs of the same size and
isolated vertices, or a regular graph with four eigenvalues. Graphs from the
first three families in this list are indeed strongly -walk-regular for
all , whereas the graphs from the fourth family are -walk-regular
for every odd . The case of regular graphs with four eigenvalues is the
most interesting (and complicated) one. Such graphs cannot be strongly
-walk-regular for even . We will characterize the case that regular
four-eigenvalue graphs are strongly -walk-regular for every odd ,
in terms of the eigenvalues. There are several examples of infinite families of
such graphs. We will show that every other regular four-eigenvalue graph can be
strongly -walk-regular for at most one . There are several examples
of infinite families of such graphs that are strongly 3-walk-regular. It
however remains open whether there are any graphs that are strongly
-walk-regular for only one particular different from 3
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