Cooperative emission of a pulse train in an optically thick scattering medium

An optically thick cold atomic cloud emits a coherent flash of light in the forward direction when the phase of an incident probe field is abruptly changed. Because of cooperativity, the duration of this phenomena can be much shorter than the excited lifetime of a single atom. Repeating periodically the abrupt phase jump, we generate a train of pulses with short repetition time, high intensity contrast and high efficiency. In this regime, the emission is fully governed by cooperativity even if the cloud is dilute.Comment: 5 pages, 3 figure

Measurements of Grain Motion in a Dense, Three-Dimensional Granular Fluid

We have used an NMR technique to measure the short-time, three-dimensional displacement of grains in a system of mustard seeds vibrated vertically at 15g. The technique averages over a time interval in which the grains move ballistically, giving a direct measurement of the granular temperature profile. The dense, lower portion of the sample is well described by a recent hydrodynamic theory for inelastic hard spheres. Near the free upper surface the mean free path is longer than the particle diameter and the hydrodynamic description fails.Comment: 4 pages, 4 figure

Revealing local failed supernovae with neutrino telescopes

We study the detectability of neutrino bursts from nearby direct black hole-forming collapses (failed supernovae) at Megaton detectors. Due to their high energetics, these bursts could be identified - by the time coincidence of N >= 2 or N >= 3 events within a ~ 1 s time window - from as far as ~ 4-5 Mpc away. This distance encloses several supernova-rich galaxies, so that failed supernova bursts could be detected at a rate of up to one per decade, comparable to the expected rate of the more common, but less energetic, neutron star-forming collapses. Thus, the detection of a failed supernova within the lifetime of a Mt detector is realistic. It might give the first evidence of direct black hole formation, with important implications on the physics of this phenomenon.Comment: LaTeX, 4 pages, 4 figures; minor changes to the text, results unchange

The structural, mechanical, electronic, optical and thermodynamic properties of t-X3_{3}As4_{4} (X == Si, Ge and Sn) by first-principles calculations

The structural, mechanical, electronic, optical and thermodynamic properties of the t-X$_{\mathrm{3}}$As$_{\mathrm{4}}$ (X $=$ Si, Ge and Sn) with tetragonal structure have been investigated by first principles calculations. Our calculated results show that these compounds are mechanically and dynamically stable. By the study of elastic anisotropy, it is found that the anisotropic of the t-Sn$_{\mathrm{3}}$As$_{\mathrm{4}}$ is stronger than that of t-Si$_{\mathrm{3}}$As$_{\mathrm{4}}$ and t-Ge$_{\mathrm{3}}$As$_{\mathrm{4}}$. The band structures and density of states show that the t-X$_{\mathrm{3}}$As$_{\mathrm{4}}$ (Si, Ge and Sn) are semiconductors with narrow band gaps. Based on the analyses of electron density difference, in t-X$_{\mathrm{3}}$As$_{\mathrm{4}}$ As atoms get electrons, X atoms lose electrons. The calculated static dielectric constants, $\varepsilon_{1} (0)$, are 15.5, 20.0 and 15.1 eV for t-X$_{\mathrm{3}}$As$_{\mathrm{4}}$ (X $=$ Si, Ge and Sn), respectively. The Dulong-Petit limit of t-X$_{\mathrm{3}}$As$_{\mathrm{4}}$ is about 10 J mol$^{\mathrm{-1}}$K$^{\mathrm{-1}}$. The thermodynamic stability successively decreases from t-Si$_{\mathrm{3}}$As$_{\mathrm{4}}$ to t-Ge$_{\mathrm{3}}$As$_{\mathrm{4}}$ to t-Sn$_{\mathrm{3}}$As$_{\mathrm{4}}$.Comment: 14 pages, 10 figures, 6 table

Stable embedded solitons

Stable embedded solitons are discovered in the generalized third-order nonlinear Schroedinger equation. When this equation can be reduced to a perturbed complex modified KdV equation, we developed a soliton perturbation theory which shows that a continuous family of sech-shaped embedded solitons exist and are nonlinearly stable. These analytical results are confirmed by our numerical simulations. These results establish that, contrary to previous beliefs, embedded solitons can be robust despite being in resonance with the linear spectrum.Comment: 2 figures. To appear in Phys. Rev. Let

Surface roughness and interfacial slip boundary condition for quartz crystal microbalances

The response of a quartz crystal microbalance (QCM) is considered using a wave equation for the substrate and the Navier-Stokes equations for a finite liquid layer under a slip boundary condition. It is shown that when the slip length to shear wave penetration depth is small, the first order effect of slip is only present in the frequency response. Importantly, in this approximation the frequency response satisfies an additivity relation with a net response equal to a Kanazawa liquid term plus an additional Sauerbrey "rigid" liquid mass. For the slip length to result in an enhanced frequency decrease compared to a no-slip boundary condition, it is shown that the slip length must be negative so that the slip plane is located on the liquid side of the interface. It is argued that the physical application of such a negative slip length could be to the liquid phase response of a QCM with a completely wetted rough surface. Effectively, the model recovers the starting assumption of additivity used in the trapped mass model for the liquid phase response of a QCM having a rough surface. When applying the slip boundary condition to the rough surface problem, slip is not at a molecular level, but is a formal hydrodynamic boundary condition which relates the response of the QCM to that expected from a QCM with a smooth surface. Finally, possible interpretations of the results in terms of acoustic reflectivity are developed and the potential limitations of the additivity result should vapour trapping occur are discussed