2,313 research outputs found
Effective time-independent description of optical lattices with periodic driving
For a periodically driven quantum system an effective time-independent
Hamiltonian is derived with an eigen-energy spectrum, which in the regime of
large driving frequencies approximates the quasi-energies of the corresponding
Floquet Hamiltonian. The effective Hamiltonian is evaluated for the case of
optical lattice models in the tight-binding regime subjected to strong periodic
driving. Three scenarios are considered: a periodically shifted one-dimensional
(1D) lattice, a two-dimensional (2D) square lattice with inversely phased
temporal modulation of the well depths of adjacent lattice sites, and a 2D
lattice subjected to an array of microscopic rotors commensurate with its
plaquette structure. In case of the 1D scenario the rescaling of the tunneling
energy, previously considered by Eckardt et al. in Phys. Rev. Lett. 95, 260404
(2005), is reproduced. The 2D lattice with well depth modulation turns out as a
generalization of the 1D case. In the 2D case with staggered rotation, the
expression previously found in the case of weak driving by Lim et al. in Phys.
Rev. Lett. 100, 130402 (2008) is generalized, such that its interpretation in
terms of an artificial staggered magnetic field can be extended into the regime
of strong driving.Comment: 10 pages, 5 figure
Hierarchical Beamforming: Resource Allocation, Fairness and Flow Level Performance
We consider hierarchical beamforming in wireless networks. For a given
population of flows, we propose computationally efficient algorithms for fair
rate allocation including proportional fairness and max-min fairness. We next
propose closed-form formulas for flow level performance, for both elastic (with
either proportional fairness and max-min fairness) and streaming traffic. We
further assess the performance of hierarchical beamforming using numerical
experiments. Since the proposed solutions have low complexity compared to
conventional beamforming, our work suggests that hierarchical beamforming is a
promising candidate for the implementation of beamforming in future cellular
networks.Comment: 34 page
Observation of Floquet-Bloch states on the surface of a topological insulator
The unique electronic properties of the surface electrons in a topological
insulator are protected by time-reversal symmetry. Circularly polarized light
naturally breaks time-reversal symmetry, which may lead to an exotic surface
quantum Hall state. Using time- and angle-resolved photoemission spectroscopy,
we show that an intense ultrashort mid-infrared pulse with energy below the
bulk band gap hybridizes with the surface Dirac fermions of a topological
insulator to form Floquet-Bloch bands. These photon dressed surface bands
exhibit polarization-dependent band gaps at avoided crossings. Circularly
polarized photons induce an additional gap at the Dirac point, which is a
signature of broken time-reversal symmetry on the surface. These observations
establish the Floquet-Bloch bands in solids and pave the way for optical
manipulation of topological quantum states of matter.Comment: 23 pages including supplementary material
Discrete-time output feedback sliding-mode control design for uncertain systems using linear matrix inequalities
An output feedback-based sliding-mode control design methodology for discrete-time systems is considered in this article. In previous work, it has been shown that by identifying a minimal set of current and past outputs, an augmented system can be obtained which permits the design of a sliding surface based upon output information only, if the invariant zeros of this augmented system are stable. In this work, a procedure for realising discrete-time controllers via a particular set of extended outputs is presented for non-square systems with uncertainties. This method is applicable when unstable invariant zeros are present in the original system. The conditions for existence of a sliding manifold guaranteeing a stable sliding motion are given. A procedure to obtain a Lyapunov matrix, which simultaneously satisfies both a Riccati inequality and a structural constraint, is used to formulate the corresponding control to solve the reachability problem. A numerical method using linear matrix inequalities is suggested to obtain the Lyapunov matrix. Finally, the design approach given in this article is applied to an aircraft problem and the use of the method as a reconfigurable control strategy in the presence of sensor failure is demonstrated
Resampling technique applied to the characterization of microsegregation
Characterization of short-range chemical heterogeneities in metallic materials, such as the so-called microsegregation resulting from solidification, is most often performed using EDS or WDS spot measurements. The most usual way is to perform countings on points located along a regular grid. Due to experimental limitation, the grid step is generally of the same order of magnitude than the characteristic distance(s) of the chemical heterogeneities under investigation. In such a case, the measurements can not be assumed to be independent one from each other, and the resulting interferences (correlations) preclude application of simple statistics to the solute distribution obtained. In the present work, this is clearly shown by using a resampling technique applied to "chemical" images obtained by phase field modelling
Analyse qualitative des rĂ©ponses Ă©pilinguistiques et mĂ©talinguistiques au test dâacceptabilitĂ© du THAM-3 par des Ă©tudiants italophones
ABSTRACT. Qualitative analysis of epilinguistic and metalinguistic answers to the THAMâ3 acceptability test by Italianâspeaking students. This article as two aims, methodological and descriptive. It analyses the grammatical and metalinguistic judgements of a group of italians learning French (19 adults) elicitated from the acceptability task of the THAM 3, a linguistic and metalinguistic tests conceived for native speakers. The goal is to show that some difficulties are due to the structure of these acceptabili ty tasks that influences langu
The spectrum of the B[e] star BAL224
We present optical spectroscopy of the emission line star BAL 224 (V=17.3,
B-V=0.46). This star also named KWBBE 485, [MA93]906 is located at the
periphery of the young SMC cluster NGC 330; it is known as a photometric
variable with a possible period around 1 day (Balona 1992). Furthermore it was
reported as the optical counterpart of the prominent mid-infrared source (MIR1)
by Kucinskas et al. (2000), indicating the presence of a dust shell. The star
was included in a sample of B-type stars observed using the ESO VLT-FLAMES
facilities. The presence of emission lines such as Fe II,[Fe II], [S II] make
this object like a B[e] star. The H, H and H lines show
an asymmetrical double-peaked emission profile suggesting the presence of an
accretion disk. Moreover the MACHO and OGLE light curves were analyzed; in
addition to a long-term variability ( 2300d), a short period very close
to 1 day has been detected using different methods, confirming the variability
previously reported by Balona (1992). Finally the nature of this object is
reconsidered.Comment: 4 page
Large Amplitude Harmonic Driving of Highly Coherent Flux Qubits
The device for the Josephson flux qubit (DJFQ) can be considered as a solid
state artificial atom with multiple energy levels. When a large amplitude
harmonic excitation is applied to the system, transitions at the energy levels
avoided crossings produce visible changes in the qubit population over many
driven periods that are accompanied by a rich pattern of interference
phenomena. We present a Floquet treatment of the periodically time-dependent
Schr\"odinger equation of the strongly driven qubit beyond the standard two
levels approach. For low amplitudes, the average probability of a given sign of
the persistent current qubit exhibits, as a function of the static flux
detuning and the driving amplitude, Landau-Zener-St\"uckelberg interference
patterns that evolve into complex diamond-like patterns for large amplitudes.
In the case of highly coherent flux qubits we find that the higher order
diamonds can not be simply described relying on a two-level approximations. In
addition we propose a new spectroscopic method based on starting the system in
the first excited state instead of in the ground state, which can give further
information on the energy level spectrum and dynamics in the case of highly
coherent flux qubits. We compare our numerical results with recent experiments
that perform amplitude spectroscopy to probe the energy spectrum of the
artificial atom.Comment: 12 Pages and 12 Figures Phys. Rev. B (in press
Le fricative dentali e alveo-palatali in francese e in italiano: differenze fonologiche e ricadute acquisizionali
French dental and palatal fricatives are not considered as a problem for italian
students because of the similarity of the two phonological systems. In
this study, we try to show that, instead of this current opinion, the different
phonological value of [s], [z], [â«] and [Ê] in italian can explain some acquisitional
errors such as âpositionâ [pozisjoÌ] > [posisjoÌ] or âmesureâ [mÉzyR] >
[mÉâ«yR], [mÉÊyR]
- âŠ