[en] Short communication

[en] We use the compact U(1) theory formulated on a euclidean lattice to calculate the strength of electromagnetic coupling to magnetic monopoles. We find that the coupling strength goes as exp(-π/α). (orig.)

[en] Interacting surface waves, parametrically excited by two commensurate frequencies (Faraday waves), yield a rich family of nonlinear states, which result from a variety of three-wave resonant interactions. By perturbing the system with a third frequency, we selectively favor different nonlinear wave interactions. Where quadratic nonlinearities are dominant, the only observed patterns correspond to 'grid' states. Grid states are superlattices in which two corotated sets of critical wave vectors are spanned by a sublattice whose basis states are linearly stable modes. Specific driving phase combinations govern the selection of different grid states

[en] We present an experimental study of highly localized, soliton-like structures that propagate on the two dimensional surface of highly dissipative fluids. Like the well known Faraday instability, these highly dissipative structures are driven by means of the spatially uniform, vertical acceleration of a thin fluid layer. These structures, harmonically coupled to the external driving frequency, are observed above it critical intrinsic dissipation in the system (i.e. the ratio of the viscous boundary layer height to the depth of the fluid layer) for a wide range of fluid viscosities and system parameters. These highly localized nonlinear states, unlike classical solitons, propagate at a single constant velocity for given fluid parameters and their existence is dependent on the highly dissipative character of the system. The properties of these states are discussed and examples of bound states and two state interactions are presented

[en] Full Text:The study of parametrically driven surface waves has been of much recent interest. In Particular, recent experiments where the system is driven by two commensurate frequencies has revealed a number of interesting nonlinear states. Among the is an 'unlocked' state, which is excited in the near vicinity of the system's fir bifurcation, and exhibits highly disordered behavior in both space and time. O current research is focused on both the characterization and control of this state. V find that this disordered state can be stabilized and a rapid switching to spatial ordered patterns can be accomplished by the addition of a small amplitude 3 frequency excitation. The spatial symmetry of the selected pattern is governed I the temporal symmetry of the third frequency used

[en] We investigate the transition from weakly nonlinear to nonlinear traveling-wave states. Pattern selection due to a transition from convective to absolute instability conditions is found, in good agreement with theory. While the linear properties depend on the (boundary-dependent) threshold of convection, the weakly nonlinear properties refer back to the threshold of an infinite system

[en] Interacting surface waves, parametrically excited by two commensurate frequencies, yield a number of nonlinear states. Near the system's bicritical point, a state, highly disordered in space and time, results from competition between nonlinear states. Experimentally, this disordered state can be rapidly stabilized to a variety of nonlinear states via open-loop control with a small-amplitude third frequency excitation, whose temporal symmetry governs the temporal and the spatial symmetry of the selected nonlinear state. This technique also excites rapid switching between nonlinear states

[en] Full Text:Though studied for hundreds of years by names as distinct as Leonardo da Vinci, Coulomb, and Hertz many aspects in the study of solid friction remain as fresh today as they were was five hundred years ago. One such aspect is the onset of slip. First described by Coulomb and Amontons as the transition from static to dynamic friction, the onset of frictional slip is central to fields as diverse as physics tribology, mechanics of earthquakes and fracture. We perform real-time measurements of the net contact area between two blocks of like material at the onset of frictional slip. We show that the process of interface detachment, which immediately precedes the inception of frictional sliding, is governed by three different types of detachment fronts. These crack-like detachment fronts differ by both their propagation velocities and by the amount of net contact surface reduction caused by their passage. Two of these which propagate at Sub-Rayleigh and intersonic velocities, have beau tile subject of intensive recent interest. The most rapid fronts, propagating at intersonic velocities, generate a negligible reduction in contact area across the interface. Sub-Rayleigh fronts are crack-like modes which propagate at velocities up to the Rayleigh wave speed, VR, and give rise to an approximate 10% reduction in net contact area. We show that a new third type of front, which propagates au order of magnitude more slowly and give rise to an approximate 20% reduction in net contact area is the dominant mechanism for interface detachment. No over all sliding occurs until either of the slower two fronts traverses the entire interface

[en] Understanding the dynamics of frictional motion is essential to fields ranging from nano-machines to the study of earthquakes. Frictional motion involves a huge range of time and length scales, coupling the elastic fields of two blocks under stress to the dynamics of the myriad interlocking microscopic contacts that form the interface at their plane of separation. In spite of the immense practical and fundamental importance of friction, many aspects of the basic physics of the problem are still not well understood. One such aspect is the nucleation of frictional motion commonly referred to as the transition from static to dynamic friction. Here we review experimental studies of dynamical aspects of frictional sliding. We focus mainly on recent advances in real-time visualization of the real area of contact along large spatially extended interfaces and the importance of rapid fracture-like processes that appear at the onset of frictional instability.