1,932 research outputs found
Dipolar fermions in a multilayer geometry
We investigate the behavior of identical dipolar fermions with aligned dipole
moments in two-dimensional multilayers at zero temperature. We consider density
instabilities that are driven by the attractive part of the dipolar interaction
and, for the case of bilayers, we elucidate the properties of the stripe phase
recently predicted to exist in this interaction regime. When the number of
layers is increased, we find that this "attractive" stripe phase exists for an
increasingly larger range of dipole angles, and if the interlayer distance is
sufficiently small, the stripe phase eventually spans the full range of angles,
including the situation where the dipole moments are aligned perpendicular to
the planes. In the limit of an infinite number of layers, we derive an analytic
expression for the interlayer effects in the density-density response function
and, using this result, we find that the stripe phase is replaced by a collapse
of the dipolar system.Comment: 9 pages, 8 figure
Effects of high subsonic flow on sound propagation in a variable-area duct
The propagation of sound in a converging-diverging duct containing a quasi-one-dimensional steady flow with a high subsonic throat Mach number was studied. The behavior of linearized acoustic theory at the throat of the duct was shown to be singular. This singularity implies that linearized acoustic theory is invalid. The explicit singular behavior was determined and used to sketch the development (by the method of matched asymptotic expansions) of a nonlinear theory for sound propagation in a sonic throat region
Nonlinear theory of shocked sound propagation in a nearly choked duct flow
The development of shocks in the sound field propagating through a nearly choked duct flow is analyzed by extending a quasi-one dimensional theory. The theory is applied to the case in which sound is introduced into the flow by an acoustic source located in the vicinity of a near-sonic throat. Analytical solutions for the field are obtained which illustrate the essential features of the nonlinear interaction between sound and flow. Numerical results are presented covering ranges of variation of source strength, throat Mach number, and frequency. It is found that the development of shocks leads to appreciable attenuation of acoustic power transmitted upstream through the near-sonic flow. It is possible, for example, that the power loss in the fundamental harmonic can be as much as 90% of that introduced at the source
On the inspiral of Massive Black Holes in gas-rich galaxy mergers
We present a study on the dynamics of massive BHs in galaxy mergers, obtained
from a series of high-resolution N-Body/SPH simulations. The presence of a
gaseous component is essential for the rapid formation of an eccentric
(Keplerian) BH binary, that resides at the center of a massive (~10^9 Msun)
turbulent nuclear disc. Using physically and/or numerically motivated recipes,
we follow the accretion history of the BHs during the merger. The mass of the
BHs increases as large central inflows of gas occur inside each galaxy, and
their mass ratio varies with time. Given the encountered strong degeneracy
between numerical resolution and physical assumptions, we suggest here three
possible paths followed by the galaxies and the BHs during a merger in order to
fulfill the M-sigma relation : Adjustment, Symbiosis, and BH Dominance. In an
extremely high resolution run, we resolved the turbulent gas pattern down to
parsec scales, and found that BH feedback is expected to be effective near the
end of the merger. We then trace the BH binary orbit down to a scale of 0.1 pc
modeling the nuclear disc as an equilibrium Mestel disc composed either of gas,
gas and stars, or just stars. Under the action of dynamical friction against
the rotating gaseous and/or stellar background the orbit circularizes. When
this occurs, each BH is endowed with its own small-size (~0.01 pc) accretion
disc comprising a few percent of the BH mass. Double AGN activity is expected
to occur on an estimated timescale of ~10 Myrs, comparable to the inspiral
time. The double nuclear point--like sources that may appear have typical
separations of ~10 pc, and are likely to be embedded in the still ongoing
starburst.Comment: 10 pages, 5 figures, Proceedings of the Conference "The Multicoloured
Landscape of Compact Objects and their Explosive Origins", Cefalu` 200
Surface Adsorbate Fluctuations and Noise in Nanoelectromechanical Systems
Physisorption on solid surfaces is important in both fundamental studies and technology. Adsorbates can also be critical for the performance of miniature electromechanical resonators and sensors. Advances in resonant nanoelectromechanical systems (NEMS), particularly mass sensitivity attaining the single-molecule level, make it possible to probe surface physics in a new regime, where a small number of adatoms cause a detectable frequency shift in a high quality factor (Q) NEMS resonator, and adsorbate fluctuations result in resonance frequency noise. Here we report measurements and analysis of the kinetics and fluctuations of physisorbed xenon (Xe) atoms on a high-Q NEMS resonator vibrating at 190.5 MHz. The measured adsorption spectrum and frequency noise, combined with analytic modeling of surface diffusion and adsorption−desorption processes, suggest that diffusion dominates the observed excess noise. This study also reveals new power laws of frequency noise induced by diffusion, which could be important in other low-dimensional nanoscale systems
Application of the Rotation Matrix Natural Invariants to Impedance Control of Rotational Parallel Robots
Force control of parallel robots with rotational degrees of freedom through impedance algorithms is considerably influenced by the representation method of the end-effector orientation. Using the natural invariants of the rotation matrix and the angular velocity vector in the impedance control law has some theoretical advantages, which derive from the Euclidean-geometric meaning of these entities. These benefits are particularly evident in case of robotic architectures with three rotational degrees of freedom (serial or parallel wrists with spherical motion). The behaviour of a 3-CPU parallel robot controlled by an impedance algorithm based on this concepts is assessed through multibody simulations, and the results confirm the effectiveness of the proposed approach
Growing Massive Black Hole Pairs in Minor Mergers of Disk Galaxies
We perform a suite of high-resolution smoothed particle hydrodynamics
simulations to investigate the orbital decay and mass evolution of massive
black hole (MBH) pairs down to scales of ~30 pc during minor mergers of disk
galaxies. Our simulation set includes star formation and accretion onto the
MBHs, as well as feedback from both processes. We consider 1:10 merger events
starting at z~3, with MBH masses in the sensitivity window of the Laser
Interferometer Space Antenna, and we follow the coupling between the merger
dynamics and the evolution of the MBH mass ratio until the satellite galaxy is
tidally disrupted. While the more massive MBH accretes in most cases as if the
galaxy were in isolation, the satellite MBH may undergo distinct episodes of
enhanced accretion, owing to strong tidal torques acting on its host galaxy and
to orbital circularization inside the disk of the primary galaxy. As a
consequence, the initial 1:10 mass ratio of the MBHs changes by the time the
satellite is disrupted. Depending on the initial fraction of cold gas in the
galactic disks and the geometry of the encounter, the mass ratios of the MBH
pairs at the time of satellite disruption can stay unchanged or become as large
as 1:2. Remarkably, the efficiency of MBH orbital decay correlates with the
final mass ratio of the pair itself: MBH pairs that increase significantly
their mass ratio are also expected to inspiral more promptly down to
nuclear-scale separations. These findings indicate that the mass ratios of MBH
pairs in galactic nuclei do not necessarily trace the mass ratios of their
merging host galaxies, but are determined by the complex interplay between gas
accretion and merger dynamics.Comment: 5 pages, 4 figures, replaced to match accepted version on Ap
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