This paper reports on the study of the dynamics of 1D magneto-granular phononic crystals composed of a chain of spherical steel beads inside a properly designed magnetic field. This field is induced by an array of permanent magnets, located in a holder at a given distance from the chain. The theoretical and experimental results of the band gap structure are displayed, including all six degrees of freedom for the beads, i.e. three translations and three rotations. Experimental evidence of transverse-rotational modes of propagation is presented; moreover, by changing the strength of the magnetic field, the dynamic response of the granular chain is tuned. The combination of non-contact tunability with the potentially strong nonlinear behavior of granular systems ensures the suitability of magneto-granular phononic crystals as nonlinear, tunable mechanical metamaterials for use in controlling elastic wave propagation.The ability to control the propagation of elastic waves has been widely investigated in phononic crystals, which are a class of engineered media composed of periodic arrays of scattering inclusions in a homogeneous host material 1 . The propagation of sound in phononic crystals is driven by the interference between Bragg scattered waves. Similar to the electronic band structure of semiconductors and the electromagnetic band structure of photonic crystals, the phononic crystal structure does not allow certain frequency ranges to propagate within it. The existence of these forbidden bands makes phononic crystals suitable for direct applications, such as mechanical frequency filtering and sound insulation. In addition, the removal of inclusions along some pathways produces acoustic waveguides, demultiplexers and other elastic wave devices. To foresee the next generation of elastic devices, it would be necessary to introduce a certain degree of frequency tunability in the phononic properties. Greater interest has been shown along these lines over the last few years, and many solutions have been proposed by a number of authors; these include geometric changes to the structure by applying external stresses 2 and changes to the elastic characteristics of the constitutive materials through application of external stimuli, like an electric field 3 , temperature changes 4 or a magnetic field 5 . Among the various proposed phononic crystals, granular crystals 6 , which are closely-packed ordered arrangements of elastic particles (mainly spheres) in contact, have been the topic of a large body of recent work. Due to the Hertzian contact interaction between particles 7 , the dynamic vibration response of these structures may be nonlinear and tunable. These features make granular chains a perfect medium for studying fundamental wave structures, including solitary waves with a highly localized waveform 8,9 or discrete breathers 10 , as well as in engineering applications, e.g. tunable vibration filters 11 , impulse energy protectors 12 , acoustic lenses 13 , acoustic rectifiers 14 , tunable functional switches 15 . Pr...

We present the design of negative-refractive-index acoustic metamaterials operating at near-megahertz frequencies, intended for the eventual aim of enabling enhanced acoustic transmission through highimpedance-contrast biological layers. Leveraging the concept of complementary acoustic metamaterials, the negative effective properties of the metamaterials are designed to match the magnitude of an ultrasound-blocking, high-impedance-contrast layer's properties. The negative properties are obtained using a linear array of unit cells containing Helmholtz resonators and membranes. Using finite-element and analytical models, we calculate the band structure and the effective modulus and density of the proposed negative metamaterials. Using the full three-dimensional model of the metamaterials, we then simulate the enhancement of ultrasound transmission, through layers with high-impedance contrast to water. For instance, we see an improvement from 80% transmission through a high-impedance layer alone to 98% transmission through the metamaterial-plus-high-impedance layer combination. Scaling arguments are provided to estimate the system dimensions needed to provide higher operational frequencies appropriate for imaging and high-intensity ultrasound applications. Finally, as a proof of feasibility, a preliminary experimental realization of the unit-cell structure, created using the nanofabrication approach, is presented and tested in the near-megahertz regime. These results provide a step toward metamaterial-enhanced devices for noninvasive biomedical ultrasonic imaging and therapy.

We report on the design and operation of a 1D magneto-granular phononic crystal composed of a chain of steel spherical beads on top of permanent magnets. The magnetic field of the permanent magnets induces forces in the granular structure. By changing its strength, we can tune the dynamic response of the granular structure. We present experimental results with evidence of coupled transversal-rotational modes, and zero group velocities modes. These observations are well supported by a proposed model taking into account the mechanical coupling between the beads and the magnets by linear stiffnesses and including all degrees of freedom in translations and rotations.

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We theoretically investigate and experimentally demonstrate the existence of topological edge states in a mechanical analog of the Kitaev chain with a non-zero chemical potential. Our system is a one-dimensional monomer system involving two coupled degrees of freedom, i.e., transverse displacement and rotation of elastic elements. Due to the particle-hole symmetry, a topologically nontrivial bulk leads to the emergence of edge states in a finite chain with fixed boundaries. In contrast, a topologically trivial bulk also leads to the emergence of edge states in a finite chain, but with free boundaries. We unravel a duality in our system that predicts the existence of the latter edge states. This duality involves the iso-spectrality of a subspace for finite chains, and as a consequence, a free chain with topologically trivial bulk maps to a fixed chain with a nontrivial bulk. Lastly, we provide the conditions under which the system can exhibit perfectly degenerate in-gap modes, akin to Majorana zero modes. These findings suggest that mechanical systems with fine-tuned degrees of freedoms can be fertile test beds for exploring the intricacies of Majorana physics.

We propose a mechanical graphene analogue which is made of stainless steel beads placed in a periodic magnetic field by a proper design. A stable, free of mechanical borders granular structure with well-predicted wave dynamics is experimentally constructed. First we report the dispersion relation in conjunction with the evidence of the Dirac points. Theoretical analysis shows that, compared to genuine or other artificial graphene analogues, unconventional edge modes exist in the free zigzag and armchair boundaries together with novel bulk modes composed of in-plane extended translations but localized rotations at the edges. We observe the existence of edge modes in free zigzag boundary, and we reveal an experimental robust turning effect of edge waves from the zigzag to the armchair/zigzag boundary, even in the absence of a full band gap for bulk modes. Our work shows that granular graphene can serve as an excellent experimental platform to study novel Dirac, topological and nonlinear wave phenomena.

Ultrasound neuromodulation has rapidly developed over the past decade, a consequence of the discovery of strain‐sensitive structures in the membrane and organelles of cells extending into the brain, heart, and other organs. A key limitation to its use in the brain is the formation of standing waves within the skull. In standing acoustic waves, the maximum ultrasound intensity spatially varies from near zero to double the mean in one‐half of a wavelength, and has led to localized tissue damage and disruption of normal brain function while attempting to evoke a broader response. This phenomenon also produces a large spatial variation in the actual ultrasound exposure in tissue, leading to heterogeneous results. One approach to overcome this limitation is presented here: transducer‐mounted diffusers that result in spatiotemporally incoherent ultrasound. It is shown through experiment and analysis that adding a diffuser to the transducer leads to a twofold increase in ultrasound responsiveness of transient receptor potential ankyrin 1 (TRPA1)‐transfected human embryonic kidney cells. Furthermore, it is shown that the diffuser produces a uniform spatial distribution of pressure within the rodent skull. The approach offers uniform ultrasound delivery into irregular cavities for sonogenetics.

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