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One-dimensional "sonic vacuum" type phononic crystals were assembled from a chain of polytetrafluoroethylene ͑PTFE,Teflon͒ spheres with different diameters in a Teflon holder. It was demonstrated that this polymer-based sonic vacuum, with exceptionally low elastic modulus of particles, supports propagation of strongly nonlinear solitary waves with a very low speed. These solitary waves can be described using the classical nonlinear Hertz law despite the viscoelastic nature of the polymer and high strain rate deformation of the contact area. The experimentally measured speeds of solitary waves at high amplitudes are close to the theoretically estimated values with a Young's modulus of 1.46 GPa obtained from shock wave experiments. This is significantly higher than the Young's modulus of PTFE from ultrasonic measurements. Trains of strongly nonlinear solitary waves excited by an impact were investigated experimentally and were found to be in reasonable agreement with numerical calculations based on Hertz interaction law though exhibiting a significant dissipation.

One-dimensional strongly nonlinear phononic crystals were assembled from chains of PTFE ͑polytetrafluo-roethylene͒ and stainless-steel spheres with gauges installed inside the beads. Trains of strongly nonlinear solitary waves were excited by impacts. A significant modification of the signal shape and an increase of solitary wave speed up to two times ͑at the same magnitude of dynamic contact force͒ were achieved through a noncontact magnetically induced precompression of the chains. The data for the PTFE based chains are presented for the first time and the data for the stainless-steel beads chains are extended into a range of maximum dynamic forces more than one order of magnitude lower than previously reported. Experimental results agreed reasonably well with the long-wave approximation and numerical calculations based on the Hertz interaction law for particles interactions.

Granular materials exhibit a strongly nonlinear behavior affecting the propagation of energy and information. Dynamically self-organized strongly nonlinear solitary waves are the main information carriers in granular chains. We report the first experimental observation of the dramatic change of solitary wave reflectivity from the interface of two granular media triggered by a magnetically induced precompression. It may be appropriate to name this phenomenon the ''acoustic diode'' effect. We explain this effect by the high gradient of particle velocity near the interface. DOI: 10.1103/PhysRevLett.95.158702 PACS numbers: 05.65.+b, 43.25.+y, 45.70.2n, 46.40.Cd Strongly nonlinear granular chains are ''sonic vacuum'' (SV) type systems that support a new type of solitary wave with parameters determined by the interaction force [1][2][3]. These solitary waves are qualitatively different from the well known weakly nonlinear solitary waves of the Korteweg-de Vries equation [4,5] which were first discovered experimentally by Russel [6] in 1834. The concept of a SV was proposed to emphasize the uniqueness of the types of materials that do not support sound waves without initial prestress [3,7]. The unique property of these materials is that a single parameter (initial prestress) is able to tune the response from a linear to a strongly nonlinear regime. One of the main features of strongly nonlinear solitary waves is that the speed is strongly influenced by interrelated potential and kinetic energies [3]. A granular chain with particles interacting according to Hertz law [8] is just one example of strongly nonlinear behavior which can support the new type of solitary wave [3]. Different groups investigated numerically and experimentally the properties of these waves [1,2,9-20] and found a good agreement with the theoretical predictions based upon a long wave approximation. Nonlinear dynamic properties can be extended to other metamaterials including the propagation of electrical or other types of signals. Interesting applications of this new area of wave dynamics have been proposed, e.g., for the creation of nanodroplets [21].Another intriguing property of these materials is the reflection of the solitary waves at the interface of two SV materials [3,7,20,22], from imperfections [11] or from a wall [19]. Based on the former, the novel concept of impulse trapping inside a protecting granular laminar layer has been proposed [23,24].In this Letter we present the first experimental and numerical observation of strongly nonlinear wave interaction with the interface of two SV-type systems resulting in anomalous reflected compression and transmitted rarefaction waves when the magnetically induced prestress is applied.We propose a new method of noncontact precompression based on the magnetic interaction of the first magnetic particle in the chain with a Nd-Fe-B ring magnet [25]. The magnetic force (2.38 N) is practically independent of the motion of the magnetic particle.In experiments we placed a chain of 20 nonmagnetic stain...