Surface phononic graphene.
TL;DR: The demonstrated fully integrated artificial phononic graphene platform here constitutes a step towards on-chip quantum simulators of graphene and unique monolithic electro-acoustic integrated circuits.
Abstract: Strategic manipulation of wave and particle transport in various media is the key driving force for modern information processing and communication. In a strongly scattering medium, waves and particles exhibit versatile transport characteristics such as localization, tunnelling with exponential decay, ballistic, and diffusion behaviours due to dynamical multiple scattering from strong scatters or impurities. Recent investigations of graphene have offered a unique approach, from a quantum point of view, to design the dispersion of electrons on demand, enabling relativistic massless Dirac quasiparticles, and thus inducing low-loss transport either ballistically or diffusively. Here, we report an experimental demonstration of an artificial phononic graphene tailored for surface phonons on a LiNbO3 integrated platform. The system exhibits Dirac quasiparticle-like transport, that is, pseudo-diffusion at the Dirac point, which gives rise to a thickness-independent temporal beating for transmitted pulses, an analogue of Zitterbewegung effects. The demonstrated fully integrated artificial phononic graphene platform here constitutes a step towards on-chip quantum simulators of graphene and unique monolithic electro-acoustic integrated circuits.
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TL;DR: In this paper, the authors constructed an unpaired Dirac point at the Brillouin zone center as the intersection of the second and third bands corresponding to the monopole and dipole excitations.
Abstract: Photonic pseudospin-$\frac{1}{2}$ systems which exhibit Dirac cone dispersion at Brillouin zone corners in analogy to graphene, have been extensively studied in recent years. However, it is known that a linear band crossing of two bands cannot emerge at the center of a Brillouin zone in a two-dimensional photonic system respecting time-reversal symmetry. Using a square lattice of elliptical magneto-optical cylinders, we constructed an unpaired Dirac point at the Brillouin zone center as the intersection of the second and third bands corresponding to the monopole and dipole excitations. Effective medium theory can be applied to the two linearly crossed bands with the effective constitutive parameters numerically calculated using the boundary effective medium approach. It is shown that only the effective permittivity approaches zero, while the determinant of the nonzero effective permeability vanishes at the Dirac point frequency, showing a different behavior from the double-zero-index metamaterials obtained from the pseudospin-1 triply degenerate points for time-reversal symmetric systems. Exotic phenomena, such as the Klein tunneling and Zitterbewegung, in the pseudospin-$\frac{1}{2}$ system can be well understood from the effective medium description. When the Dirac point is lifted, the edge-state dispersion near the \ensuremath{\Gamma} point can be accurately predicted by the effective constitutive parameters. We also further realized magneto-optical complex conjugate metamaterials for a wide frequency range by introducing a particular type of non-Hermitian perturbations which make the two linear bands coalescence to form exceptional points at the real frequency.
10 citations
TL;DR: In this article, the authors demonstrate a technological means of realizing slow on-chip SAWs that is relevant for practical rf signal processing, gyrometers, sensing, and transduction.
Abstract: Strategically reducing the speed of waves, which greatly improves both the energy density and information capacity of carrier signals in space, is a key enabling factor for signal-processing devices. Among these devices, especially in the prosperous wireless communication industry, surface acoustic wave (SAW) devices based on interdigital transducers (IDTs) currently hold an essential status. However, velocity reduction in traditional IDT-based SAW devices can be achieved only by using specific substrate materials that are generally of lower hardness, which inevitably leads to an increase in device size and less-optimal electromechanical coupling coefficients. Here, we demonstrate a technological means of realizing slow on-chip SAWs that is relevant for practical rf signal processing, gyrometers, sensing, and transduction. This method takes advantage of the gradual flattening of a Rayleigh-type dispersion band due to the spatial lattice evolution of a surface phononic crystal. In our experiment, the speed of an ultraslow SAW is measured to be approximately 200 m/s, which is even slower than the speed of sound in air and equivalent to 1/17.4 of the speed of the original Rayleigh waves in ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$. Such ultraslow SAWs may have promising applications in time-dependent SAW modulation, high-sensitivity SAW sensors, and SAW nonlinear even quantum-dynamic systems. Additionally, our technique can be similarly applied to a broad range of other two-dimensional or quasi-two-dimensional wave structures, e.g., in electronic, optical, acoustic, and thermal systems.
10 citations
TL;DR: In this paper, the authors identify a wide, hypersonic surface phononic band gap in a pillar-based surface PnC and propose a low-loss, CMOS-compatible platform.
Abstract: The promise of surface phononic crystals (PnCs) for $e.g.$ rf signal processing in wireless communication (like your mobile phone) has not been realized, due to the complexity of acoustic wave propagation in such structures, plus the lack of a low-loss, CMOS-compatible platform. The authors identify a wide, hypersonic surface phononic band gap in a pillar-based surface PnC. The significance of their platform lies in the reduced phononic material loss in dielectric pillars, and the use of CMOS-compatible AlN. This allows dense integration of low-loss hypersonic devices with electronics on the same die, enabling many exciting applications of surface PnCs.
10 citations
TL;DR: In this paper, the authors demonstrate the self-collimated propagation and slow sound effect of spoof acoustic surface waves over a thin solid slab with partially embedded spherical cavities in a square lattice.
Abstract: Self-collimated propagation and slow-sound effect of spoof acoustic surface waves over a thin solid slab with partially embedded spherical cavities in a square lattice are numerically and experimentally demonstrated. Band structure calculations via the Finite-Element Method reveal that a single spoof surface wave band appears below the air-line, which flattens as the spheres are embedded deeper, leveraging the observation of self-collimated slow spoof modes. For a radius-to-lattice constant ratio of 0.45 and embedding depth of 60% of the radius, the surface band is such that non-diffractive guiding of spoof waves along the [11] direction can be achieved. Persistent self-collimated propagation of spoof surface waves over long distances is demonstrated through frequency-domain Finite-Element Method simulations. Plane waves incident from air can couple to the self-collimated modes for a wide range of azimuthal angle of incidence up to 60°, where the polar angle of incidence can be in the range of ±15°. Self-collimation of spoof waves is experimentally realized by employing a plane-wave source incident from air. In addition, when the embedding depth is higher than 85%, self-collimated slow spoof modes with group indices higher than 15 can be obtained. The observed phenomena can be utilized in two-dimensional acoustic systems such as logic circuits and interferometric sensing devices.
9 citations
TL;DR: The reported ASHE paves a new way to exploiting signal routing and unidirectional excitation controlled by the helical directions of the acoustic helical wave.
Abstract: Because of the spin-less nature of sound, acoustic helical wave with different helical directions can be taken as a "spin-like" degree of freedom. In this Letter, we examine the pseudospin-orbit coupling effect in acoustics when an acoustic helical wave emitter interacts with the acoustic hyperbolic metamaterial (AHMM). The acoustic helical wave emitter is situated at the boundary of the AHMM, which gives rise to the unidirectional excitation with the trajectory controlled by the helical directions, and hence the acoustic spin Hall-like effect (ASHE) is observed. The ASHE is further demonstrated for the string-type and the membrane-type AHMM based on the hyperbolic dispersion. The reported ASHE paves a new way to exploiting signal routing and unidirectional excitation controlled by the helical directions of the acoustic helical wave.
9 citations
References
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TL;DR: This study reports an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation and reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions.
Abstract: Quantum electrodynamics (resulting from the merger of quantum mechanics and relativity theory) has provided a clear understanding of phenomena ranging from particle physics to cosmology and from astrophysics to quantum chemistry. The ideas underlying quantum electrodynamics also influence the theory of condensed matter, but quantum relativistic effects are usually minute in the known experimental systems that can be described accurately by the non-relativistic Schrodinger equation. Here we report an experimental study of a condensed-matter system (graphene, a single atomic layer of carbon) in which electron transport is essentially governed by Dirac's (relativistic) equation. The charge carriers in graphene mimic relativistic particles with zero rest mass and have an effective 'speed of light' c* approximately 10(6) m s(-1). Our study reveals a variety of unusual phenomena that are characteristic of two-dimensional Dirac fermions. In particular we have observed the following: first, graphene's conductivity never falls below a minimum value corresponding to the quantum unit of conductance, even when concentrations of charge carriers tend to zero; second, the integer quantum Hall effect in graphene is anomalous in that it occurs at half-integer filling factors; and third, the cyclotron mass m(c) of massless carriers in graphene is described by E = m(c)c*2. This two-dimensional system is not only interesting in itself but also allows access to the subtle and rich physics of quantum electrodynamics in a bench-top experiment.
18,958 citations
TL;DR: This work reviews recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.
Abstract: Recent years have witnessed many breakthroughs in research on graphene (the first two-dimensional atomic crystal) as well as a significant advance in the mass production of this material. This one-atom-thick fabric of carbon uniquely combines extreme mechanical strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of production methods, and critically analyse the feasibility of various graphene applications.
7,987 citations
TL;DR: In this paper, it was shown that the Klein paradox can be tested in a conceptually simple condensed-matter experiment using electrostatic barriers in single and bi-layer graphene, showing that quantum tunnelling in these materials becomes highly anisotropic, qualitatively different from the case of normal, non-relativistic electrons.
Abstract: The so-called Klein paradox—unimpeded penetration of relativistic particles through high and wide potential barriers—is one of the most exotic and counterintuitive consequences of quantum electrodynamics. The phenomenon is discussed in many contexts in particle, nuclear and astro-physics but direct tests of the Klein paradox using elementary particles have so far proved impossible. Here we show that the effect can be tested in a conceptually simple condensed-matter experiment using electrostatic barriers in single- and bi-layer graphene. Owing to the chiral nature of their quasiparticles, quantum tunnelling in these materials becomes highly anisotropic, qualitatively different from the case of normal, non-relativistic electrons. Massless Dirac fermions in graphene allow a close realization of Klein’s gedanken experiment, whereas massive chiral fermions in bilayer graphene offer an interesting complementary system that elucidates the basic physics involved.
3,402 citations
TL;DR: In this article, the authors consider the specific effects of a bias on anomalous diffusion, and discuss the generalizations of Einstein's relation in the presence of disorder, and illustrate the theoretical models by describing many physical situations where anomalous (non-Brownian) diffusion laws have been observed or could be observed.
3,383 citations
TL;DR: This work shows that the fluctuations are significantly reduced in suspended graphene samples and reports low-temperature mobility approaching 200,000 cm2 V-1 s-1 for carrier densities below 5 x 109 cm-2, which cannot be attained in semiconductors or non-suspended graphene.
Abstract: The discovery of graphene1,2 raises the prospect of a new class of nanoelectronic devices based on the extraordinary physical properties3,4,5,6 of this one-atom-thick layer of carbon. Unlike two-dimensional electron layers in semiconductors, where the charge carriers become immobile at low densities, the carrier mobility in graphene can remain high, even when their density vanishes at the Dirac point. However, when the graphene sample is supported on an insulating substrate, potential fluctuations induce charge puddles that obscure the Dirac point physics. Here we show that the fluctuations are significantly reduced in suspended graphene samples and we report low-temperature mobility approaching 200,000 cm2 V−1 s−1 for carrier densities below 5 × 109 cm−2. Such values cannot be attained in semiconductors or non-suspended graphene. Moreover, unlike graphene samples supported by a substrate, the conductivity of suspended graphene at the Dirac point is strongly dependent on temperature and approaches ballistic values at liquid helium temperatures. At higher temperatures, above 100 K, we observe the onset of thermally induced long-range scattering. The novel electronic properties of graphene can be compromised when it is supported on an insulating substrate. However, suspended graphene samples can display low-temperature mobility values that cannot be attained in semiconductors or non-suspended graphene, and the conductivity approaches ballistic values at liquid-helium temperatures.
2,977 citations