27,617 research outputs found
Feedback local optimality principle applied to rocket vertical landing VTVL
Vertical landing is becoming popular in the last fifteen years, a technology known under the acronym VTVL, Vertical Takeoff and Vertical Landing [1,2]. The interest in such landing technology is dictated by possible cost reductions [3,4], that impose spaceship’s recycling. The rockets are not generally de- signed to perform landing operations, rather their design is aimed at takeoff operations, guaranteeing a very high forward acceleration to gain the velocity needed to escape the gravitational force. In this paper a new control method based on Feedback Local Optimality Principle, named FLOP is applied to the rocket landing problem. The FLOP belongs to a special class of optimal controllers, developed by the mechatronic and vehicle dynamics lab of Sapienza, named Variational Feedback Controllers - VFC, that are part of an ongoing research and are recently applied in different field: nonlinear system [5], marine and terrestrial autonomous vehicles [6,7,8], multi agents interactions and vibration control [9, 10]. The paper is devoted to show the robustness of the nonlinear controlled system, comparing the performances with the LQR, one of the most acknowledged methods in optimal control
Circuit QED and sudden phase switching in a superconducting qubit array
Superconducting qubits connected in an array can form quantum many-body
systems such as the quantum Ising model. By coupling the qubits to a
superconducting resonator, the combined system forms a circuit QED system.
Here, we study the nonlinear behavior in the many-body state of the qubit array
using a semiclassical approach. We show that sudden switchings as well as a
bistable regime between the ferromagnetic phase and the paramagnetic phase can
be observed in the qubit array. A superconducting circuit to implement this
system is presented with realistic parameters .Comment: 4 pages, 3 figures, submitted for publication
Consistent analysis of neutral- and charged-current neutrino scattering off carbon
Background: Good understanding of the cross sections for (anti)neutrino
scattering off nuclear targets in the few-GeV energy region is a prerequisite
for correct interpretation of results of ongoing and planned oscillation
experiments.
Purpose: Clarify possible source of disagreement between recent measurements
of the cross sections on carbon.
Method: Nuclear effects in (anti)neutrino scattering off carbon nucleus are
described using the spectral function approach. The effect of two- and
multi-nucleon final states is accounted for by applying an effective value of
the axial mass, fixed to 1.23 GeV. Neutral-current elastic (NCE) and
charged-current quasielastic (CCQE) processes are treated on equal footing.
Results: The differential and total cross sections for the energy ranging
from a few hundreds of MeV to 100 GeV are obtained and compared to the
available data from the BNL E734, MiniBooNE, and NOMAD experiments.
Conclusions: Nuclear effects in NCE and CCQE scattering seem to be very
similar. Within the spectral function approach, the axial mass from the shape
analysis of the MiniBooNE data is in good agreement with the results reported
by the BNL E734 and NOMAD Collaborations. However, the combined analysis of NCE
and CCQE data does not seem to support the contribution of multi-nucleon final
states being large enough to explain the normalization of the
MiniBooNE-reported cross sections.Comment: 14 pages, 9 figures, detailed discussion of the role of FSI is adde
Crossover from a pseudogap state to a superconducting state
On the basis of our calculation we deduce that the particular electronic
structure of cuprate superconductors confines Cooper pairs to be firstly formed
in the antinodal region which is far from the Fermi surface, and these pairs
are incoherent and result in the pseudogap state. With the change of doping or
temperature, some pairs are formed in the nodal region which locates the Fermi
surface, and these pairs are coherent and lead to superconductivity. Thus the
coexistence of the pseudogap and the superconducting gap is explained when the
two kinds of gaps are not all on the Fermi surface. It is also shown that the
symmetry of the pseudogap and the superconducting gap are determined by the
electronic structure, and non-s wave symmetry gap favors the high-temperature
superconductivity. Why the high-temperature superconductivity occurs in the
metal region near the Mott metal-insulator transition is also explained.Comment: 7 pages, 2 figure
Exceptional del Pezzo hypersurfaces
We compute global log canonical thresholds of a large class of quasismooth
well-formed del Pezzo weighted hypersurfaces in
. As a corollary we obtain the existence
of orbifold K\"ahler--Einstein metrics on many of them, and classify
exceptional and weakly exceptional quasismooth well-formed del Pezzo weighted
hypersurfaces in .Comment: 149 pages, one reference adde
Superconducting correlations in ultra-small metallic grains
To describe the crossover from the bulk BCS superconductivity to a
fluctuation-dominated regime in ultrasmall metallic grains, new order
parameters and correlation functions, such as ``parity gap'' and ``pair-mixing
correlation function'', have been recently introduced. In this paper, we
discuss the small-grain behaviour of the Penrose-Onsager-Yang off-diagonal
long-range order (ODLRO) parameter in a pseudo-spin representation. Relations
between the ODLRO parameter and those mentioned above are established through
analytical and numerical calculations.Comment: 7 pages, 1 figur
Integer quantum Hall effect and topological phase transitions in silicene
We numerically investigate the effects of disorder on the quantum Hall effect
(QHE) and the quantum phase transitions in silicene based on a lattice model.
It is shown that for a clean sample, silicene exhibits an unconventional QHE
near the band center, with plateaus developing at and
a conventional QHE near the band edges. In the presence of disorder, the Hall
plateaus can be destroyed through the float-up of extended levels toward the
band center, in which higher plateaus disappear first. However, the center
Hall plateau is more sensitive to disorder and disappears at a
relatively weak disorder strength. Moreover, the combination of an electric
field and the intrinsic spin-orbit interaction (SOI) can lead to quantum phase
transitions from a topological insulator to a band insulator at the charge
neutrality point (CNP), accompanied by additional quantum Hall conductivity
plateaus.Comment: 7 pages, 4 figure
On the Treves theorem for the AKNS equation
According to a theorem of Treves, the conserved functionals of the AKNS
equation vanish on all pairs of formal Laurent series of a specified form, both
of them with a pole of the first order. We propose a new and very simple proof
for this statement, based on the theory of B\"acklund transformations; using
the same method, we prove that the AKNS conserved functionals vanish on other
pairs of Laurent series. The spirit is the same of our previous paper on the
Treves theorem for the KdV, with some non trivial technical differences.Comment: LaTeX, 16 page
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