Tachyons via supersonics

An apparent inadequacy of the velocity addition theorem in handling a Fizeau (like) experiment beyond the resonance frequence is shown not to be a true failing. The proof consists in showing that the invariance of the signal velocity is unrelated to its maximality. Next, Lorentz transformations which connect frames moving with supersignal velocity to those of subsignal velocity are derived, taking a cue from the acoustic case. Methods are also suggested for laboratory verification of the theory.     

Theory and experimental realization of negative refraction in a metallic helix array

We developed a theory to compute and interpret the photonic band structure of a periodic array of metallic helices. Interesting features of the band structure include longitudinal and circularly polarized eigenmodes and wide polarization gap. The helical symmetry also implies unusual features such as negative group velocity bands at both sides of the polarization gap and band crossings pinned at the zone boundary. A direct proof of negative refraction via a chiral route is achieved for the first time by measuring the spatial beam shift through a slab of the three-dimensional helices.     

掺杂光子晶体光纤自发辐射与掺杂激活杂质的光增益透射谱研究

借助光子晶体中二能级原子的自发辐射理论证明缺陷态的局域场存在的必然性以及局域场基本性质,为研究光子晶体光纤掺杂自发辐射的内在规律提供了理论依据。且将自发辐射理论与数值模拟相结合,在缺陷介质中掺激活杂质时,研究了光子晶体光纤的掺杂局域场特征以及受激辐射增强和透射率大于1现象与光子带隙群速度异常和掺杂层复有效折射率成负的虚部之间的内在关系。由此说明光子晶体光纤的缺陷介质中掺入激活杂质时,光子禁带中能出现品质因子非常高的杂质态,具有很大的态密度,较强的受激辐射放大。     

Time ordering and the Lorentz group

A simplified definition of point local clocks and the relationship between an inertial reference frame and a class of such clocks, at rest with respect to each other, are used for an algebraic determination of the geometry of Minkowski's space-time on the set of point events. The group of all automorphisms that preserve the time ordering induced by the set of all equivalent local clocks is shown to be generated by the inhomogeneous orthochronous Lorentz group and dilatations, consistently with a well-known result of E. C. Zeeman. The existence of a universal limit velocity and of an invariant clock cone through any event is among the implications of our proof, thus effectively exploiting a suggestion due to F. Severi.     

Global existence for a model Boltzmann equation

We give a global existence proof for a class of inhomogeneous continuous velocity Boltzmann models with stochastic scattering kernels.     

Active damping control unit using a small scale proof mass electrodynamic actuator

This paper presents a study on the design and use of a small scale proof mass electrodynamic actuator, with a low mounting resonance frequency, for velocity feedback control on a thin rectangular panel. A stability-performance formula is derived, which can be effectively used to assess the down scaling effects on the stability and control performance of the feedback loop. The design and tests of a velocity feedback loop with a prototype small scale proof mass actuator are also presented. When a feedback control having a gain margin of about 6 dB is implemented, so that there is little control spillover effect around the fundamental resonance of the actuator, reductions of vibration between 5 dB and 10 dB in the frequency band between 80 Hz and 250 Hz have been measured at the control position.     

Observation of anomalous phonon softening in bilayer graphene

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W. Kohn, Phys. Rev. Lett. 2, 393 (1959)], the observed phonon anomaly exhibits a logarithmic singularity in the optical-phonon energy. Arising from a resonant electron-phonon coupling effect, the anomaly was also expected, albeit not observed, in monolayer graphene. We propose an explanation for why it is easier to observe in bilayer samples.     

Bounds on the Growth of the Velocity Support for the Solutions of the Vlasov–Poisson Equation in a Torus

A bound on the growth of the velocity for the Vlasov–Poisson equation in a torus is given in one and two dimensions. The main tool used in the proof is a partition into fast and slow particles and the ergodic property of the free motion in a torus.     

EXAFS measurement of iron bcc-to-hcp phase transformation in nanosecond-laser shocks

Extended x-ray absorption fine structure (EXAFS) measurements have demonstrated the phase transformation from body-centered-cubic (bcc) to hexagonal-close-packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometry. This is a direct, atomic-level, and in situ proof of shock-induced transformation in iron, as opposed to the previous indirect proof based on shock-wave splitting.     

Continuity of Velocity Gradients in Suspensions of Rod–like Molecules

We investigate the Doi model for suspensions of rod–like molecules in the dilute regime. For certain parameter values, the velocity gradient vs. stress relation defined by the stationary and homogeneous flow is not rank–one monotone. We then consider the evolution of possibly large perturbations of stationary flows. We prove that, even in the absence of a microscopic cut–off, discontinuities in the velocity gradient cannot occur in finite time. The proof relies on a novel type of estimate for the Smoluchowski equation.     

From Discrete Velocity Boltzmann Equations to Gas Dynamics Before Shocks

This article is devoted to the proof of the hydrodynamic limit for a discrete velocity Boltzmann equation before appearance of shocks in the limit system. Partially supported by the National Science Foundation Grants DMS-0607053, DMS-0555272 and DMS-0757227, and by the program “Pythagoras” of the Greek Secretariat of Research.     

The axially symmetric flow-through problem for the Navier-Stokes equations in variables “vorticity-stream function”

The axially symmetric steady-state motion of a viscous incompressible fluid in a region of the type of a spherical layer is considered. The boundary problem for the Navier-Stokes equations is formulated in terms of the stream function and vorticity. The solvability of this problem was previously established provided that the fluid flow rate through each layer boundary is sufficiently small. The main result of this study is in proving the solvability of the axially symmetric flow-through problem without limitations on the magnitude of the flow rate. The proof is based on the a priori Dirichlet estimate of the flow velocity field. The asymptotic properties of the solution near the symmetry axis are established conditionally. The proof procedure admits the generalization to the case of axially symmetric problems in multiply-connected domains.     

On the Propagation of a Perturbation in an Anharmonic System

We give a not trivial upper bound on the velocity of disturbances in an infinitely extended anharmonic system at thermal equilibrium. The proof is achieved by combining a control on the non equilibrium dynamics with an explicit use of the state invariance with respect to the time evolution.     

A simple property of the Weyl tensor for a shear,vorticity and acceleration-free velocity field

We prove that, in a space-time of dimension \(n>3\) with a velocity field that is shear-free, vorticity-free and acceleration-free, the covariant divergence of the Weyl tensor is zero if and only if the contraction of the Weyl tensor with the velocity is zero. This extends a property found in generalised Robertson–Walker spacetimes, where the velocity is also eigenvector of the Ricci tensor. Despite the simplicity of the statement, the proof is involved. As a product of the same calculation, we introduce a curvature tensor with an interesting recurrence property.     

Superdiffusive Behaviour of a Passive Ornstein–Uhlenbeck Tracer in a Turbulent Shear Flow

In this paper, we are interested in the study of the diffusion of a passive particle with positive mass by a divergence free velocity field. We consider here the very simple turbulent shear flow case, in which we will prove the superdiffusive behaviour of the motion for large enough values of the energy spectrum of the velocity field. For small values, the proof of the diffusive behaviour of the model is also new, and it is shown that this diffusion is strictly greater than the one obtained with a non-massive particle. One interesting point to insist on is that we are able to obtain explicit hydrodynamic equations without even having the stationary measure of the studied processes     

Features of transition and Cherenkov radiations and the correlation between them

It is shown that transition radiation arising at the boundary of two media is being emitted as a Cherenkov one, if the phase velocity of transition radiation waves in the medium of transition radiation propagation becomes equal to the velocity of the moving radiating particle (the necessary condition for the Cherenkov radiation). The proof of this statement is based on the analysis of the transition radiation formation zone, which may become large enough and provide interference between the field of transition radiation and the own Coulomb field of the moving particle, in case when the Cherenkov radiation condition is fulfilled. As a result, the transition radiation field transforms into the Cherenkov field. The problem is considered for cases of both a waveguide and free space.     

Some further comments on the Selleri-Tarozzi proof of the nonlocality of quantum mechanics

A recent proof, by Selleri and Tarozzi, of the nonlocality of quantum mechanics is reexamined within a general ensemble interpretation. It is concluded that there is no reason at all to doubt the compatibility between quantum mechanics and Einstein locality in the sense of Einstein's relativity principle (propagation of physical influences with finite velocity).     

The positivity of gravitational energy and global supersymmetry

The concept of gravitational energy and the proof of its positivity are reviewed. The relationship between Witten's proof of the positivity of mass and supergravity is explained with reference to the group of global supersymmetries of a spacetime. A formula for the mass is given, in terms of the change of the supercharge under global supersymmetry, which has a simple positivity theorem and which reduces to Witten's expression. An interpretation of Witten's constraint on the spinors used in his proof is given.     

On Symmetric Water Waves with Constant Vorticity

We prove that a solution to the gravity water wave problem with constant vorticity, whose wave profile as well as its horizontal velocity component at the free surface are symmetric at any instant of time, is given by a traveling wave. The proof is based on maximum principles and structural properties of the governing equations.     

低温热泵蒸发器结霜特性实验的研究

实验研究了不同的空气参数如空气相对湿度、温度以及空气流速对低温热泵室外侧蒸发器结霜特性的影响。研究发现空气入口相对湿度等参数对蒸发器结霜以及换热性能有很大的影响。实验结果可以为热泵除霜以及翅片管式蒸发器优化设计提供可靠的实验依据。