4,726 research outputs found
Comparative study of the two-phonon Raman bands of silicene and graphene
We present a computational study of the two-phonon Raman spectra of silicene
and graphene within a density-functional non-orthogonal tight-binding model.
Due to the presence of linear bands close to the Fermi energy in the electronic
structure of both structures, the Raman scattering by phonons is resonant. We
find that the Raman spectra exhibit a crossover behavior for laser excitation
close to the \pi-plasmon energy. This phenomenon is explained by the
disappearance of certain paths for resonant Raman scattering and the appearance
of other paths beyond this energy. Besides that, the electronic joint density
of states is divergent at this energy, which is reflected on the behavior of
the Raman bands of the two structures in a qualitatively different way.
Additionally, a number of Raman bands, originating from divergent phonon
density of states at the M point and at points, inside the Brillouin zone, is
also predicted. The calculated spectra for graphene are in excellent agreement
with available experimental data. The obtained Raman bands can be used for
structural characterization of silicene and graphene samples by Raman
spectroscopy
Theoretical Raman fingerprints of -, -, and -graphyne
The novel graphene allotropes -, -, and -graphyne
derive from graphene by insertion of acetylenic groups. The three graphynes are
the only members of the graphyne family with the same hexagonal symmetry as
graphene itself, which has as a consequence similarity in their electronic and
vibrational properties. Here, we study the electronic band structure, phonon
dispersion, and Raman spectra of these graphynes within an
\textit{ab-initio}-based non-orthogonal tight-binding model. In particular, the
predicted Raman spectra exhibit a few intense resonant Raman lines, which can
be used for identification of the three graphynes by their Raman spectra for
future applications in nanoelectronics
Analysis of the radio-frequency single-electron transistor with large quality factor
We have analyzed the response and noise-limited sensitivity of the
radio-frequency single-electron transistor (RF-SET), extending the previously
developed theory to the case of arbitrary large quality factor Q of the RF-SET
tank circuit. It is shown that while the RF-SET response reaches the maximum at
Q roughly corresponding to the impedance matching condition, the RF-SET
sensitivity monotonically worsens with the increase of Q. Also, we propose a
novel operation mode of the RF-SET, in which an overtone of the incident rf
wave is in resonance with the tank circuit.Comment: 4 pages, submitted to Appl.Phys.Let
Model of collective modes in three-band superconductors with repulsive interband interactions
I consider a simple model of a three-band superconductor with repulsive
interband interactions. The frustration, associated with the odd number of
bands, leads to the possible existence of an intrinsically complex
time-reversal symmetry breaking (TRSB) order parameter. In such state the
fluctuations of the different gaps are strongly coupled, and this leads to the
development of novel collective modes, which mix phase and amplitude
oscillations. I study these fluctuations using a simple microscopic model and
derive the dispersion for two physically distinct modes, which are gapped by
energy less than twice the gap, and apparently present for all values of
interband couplings.Comment: 6 pages, 3 figure
Exactness of the Bogoliubov approximation in random external potentials
We investigate the validity of the Bogoliubov c-number approximation in the
case of interacting Bose-gas in a \textit{homogeneous random} media. To take
into account the possible occurence of type III generalized Bose-Einstein
condensation (i.e. the occurrence of condensation in an infinitesimal band of
low kinetic energy modes without macroscopic occupation of any of them) we
generalize the c-number substitution procedure to this band of modes with low
momentum. We show that, as in the case of the one-mode condensation for
translation-invariant interacting systems, this procedure has no effect on the
exact value of the pressure in the thermodynamic limit, assuming that the
c-numbers are chosen according to a suitable variational principle. We then
discuss the relation between these c-numbers and the (total) density of the
condensate
Partitioning a graph into highly connected subgraphs
Given , a -proper partition of a graph is a partition
of such that each part of induces a
-connected subgraph of . We prove that if is a graph of order
such that , then has a -proper partition with at
most parts. The bounds on the number of parts and the minimum
degree are both best possible. We then prove that If is a graph of order
with minimum degree , where
, then has a -proper partition into at most
parts. This improves a result of Ferrara, Magnant and
Wenger [Conditions for Families of Disjoint -connected Subgraphs in a Graph,
Discrete Math. 313 (2013), 760--764] and both the degree condition and the
number of parts are best possible up to the constant
Towards computing low-makespan solutions for multi-arm multi-task planning problems
We propose an approach to find low-makespan solutions to multi-robot
multi-task planning problems in environments where robots block each other from
completing tasks simultaneously. We introduce a formulation of the problem that
allows for an approach based on greedy descent with random restarts for
generation of the task assignment and task sequence. We then use a multi-agent
path planner to evaluate the makespan of a given assignment and sequence. The
planner decomposes the problem into multiple simple subproblems that only
contain a single robots and a single task, and can thus be solved quickly to
produce a solution for a fixed task sequence. The solutions to the subproblems
are then combined to form a valid solution to the original problem. We showcase
the approach on robotic stippling and robotic bin picking with up to 4 robot
arms. The makespan of the solutions found by our algorithm are up to 30% lower
compared to a greedy approach.Comment: Workshop for Planning and Robotics (PlanRob), International
Conference on Automated Planning and Scheduling (ICAPS), 202
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