In-plane Wave Research Articles

Sound Propagation in Lipid Membranes

It has recently been shown that in-plane sound waves can propagate over long distances in lipid monolayers [1]. Earlier it has been proposed that the propagation of nerve signals can be described by sound phenomena called solitons [2]. The implications of sound propagation in lipid membranes for signaling in biology are far reaching and insight into this is essential for further investigation. Particularly interesting are propagation properties in the vicinity of the biologically relevant lipid melting transition, where mechanical and thermodynamical properties of the system change drastically. We have theoretically addressed the properties of sound propagation in lipid membranes throughout the lipid melting transition. We explored dispersion and attenuation for low frequency sound propagation, a regime previously unexplored. We find that dispersion and attenuation is closely related to the relaxation and the state of lipid membranes [3]. Interestingly, the vast significant changes of dispersion and attenuation occur on timescales similar to ion channel open times and the temporal length of the nerve pulse. [1] Griesbauer et al., PRL, 108, 198103 (2012). [2] Heimburg & Jackson, PNAS, 102, 9790 (2005). [3] Mosgaard et al., Adv. Planar Lipid Bilayers Liposomes, 16 (2012).

Ballistic thermal transport contributed by the in-plane waves in a quantum wire modulated with an acoustic nanocavity

We investigate ballistic thermal transport contributed by the in-plane waves in a quantum wire modulated with an acoustic nanocavity. Here, the numerical calculations for two transmitted components (P wave and SV wave) are made to consider the mode conversion. Results show for the P wave component that the universal quantum thermal conductance can be observed in spite of structural details in the low temperature limit, and then the thermal conductance decreases with increasing temperature. However, for the SV wave component, the thermal conductance increases from zero monotonically with temperature. These indicate the mode conversion is directly proportional to the temperature. In addition, it is found that the mode conversion sensitively depend upon the incident frequency, the index of the modes as well as structural parameters. A brief analysis of these results is given.

In-Plane and Transverse Eigenmodes of High-Speed Rotating Composite Disks

We apply Hamilton's principle and model the coupled in-plane and transverse vibrations of high-speed spinning disks, which are fiber-reinforced circumferentially. We search for eigenmodes in the linear regime using a collocation scheme, and compare the mode shapes of composite and isotropic disks. As the azimuthal wavenumber varies, the radial nodes of in-plane waves are remarkably displaced in isotropic disks while they resist such displacements in composite disks. The reverse of this phenomenon happens for transversal waves and the radial nodes move toward the outer disk edge as the azimuthal wavenumber is increased in composite disks. This result is in accordance with the predictions of Nowinski's theory, and therefore, it is independent of the magnitude of the spinning velocity and the mode frequency. Although there are notable differences between the form of governing equations derived from Hamilton's principle and Nowinski's theory, transverse eigenmodes differ a little in the two approaches. We argue that the application of orthotropic composites as the core material of the next generation of hard disk drives, together with an angular velocity controller, can enhance the data access rate.

Band gap structures in two-dimensional super porous phononic crystals

As one kind of new linear cellular alloys (LCAs), Kagome honeycombs, which are constituted by triangular and hexagonal cells, attract great attention due to the excellent performance compared to the ordinary ones. Instead of mechanical investigation, the in-plane elastic wave dispersion in Kagome structures are analyzed in this paper aiming to the multi-functional application of the materials. Firstly, the band structures in the common two-dimensional (2D) porous phononic structures (triangular or hexagonal honeycombs) are discussed. Then, based on these results, the wave dispersion in Kagome honeycombs is given. Through the component cell porosity controlling, the effects of component cells on the whole responses of the structures are investigated. The intrinsic relation between the component cell porosity and the critical porosity of Kagome honeycombs is established. These results will provide an important guidance in the band structure design of super porous phononic crystals.

Comparison of waveguide properties of curved versus straight planar elastic layers

Dispersion equations are solved for the in-plane and anti-plane wave propagation in planar elastic layer with constant curvature. The classical Lamé formulation of displacements via elastic potentials is applied and appropriate simplifications are employed. The dispersion diagrams in each case are compared with their counterparts for a straight layer, e.g., the classical Rayleigh–Lamb solution. The curvature-induced symmetry-breaking effects are investigated for layers with symmetric boundary conditions. The role of curvature is also investigated in the cases, when the boundary conditions are not symmetrical. The elementary Bernoulli–Euler theory is employed to analyze the wave guide properties of a curved planar elastic beam in its in-plane deformation. The validity range of the Bernoulli–Euler theory is assessed via comparison of dispersion diagrams.

Spin-wave excitations and superconducting resonant mode in CsxFe2−ySe2

We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with previous measurements on Rb(x)Fe(2-y)Se2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating A(x)Fe(2-y)Se2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave vector of (0.25, 0.5). The resonance peak increases below Tc = 27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of Rb(x)Fe(2-y)Se2 and now of Cs(x)Fe(2-y)Se2 suggests that this is a common feature of superconductivity in this family. The low energy spin-wave excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases.

Bandgap calculation for mixed in-plane waves in 2D phononic crystals based on Dirichlet-to-Neumann map

In this paper, a method based on the Dirichletto-Neumann map is developed for bandgap calculation of mixed in-plane waves propagating in 2D phononic crystals with square and triangular lattices. The method expresses the scattered fields in a unit cell as the cylindrical wave expansions and imposes the Bloch condition on the boundary of the unit cell. The Dirichlet-to-Neumann (DtN) map is applied to obtain a linear eigenvalue equation, from which the Bloch wave vectors along the irreducible Brillouin zone are calculated for a given frequency. Compared with other methods, the present method is memory-saving and time-saving. It can yield accurate results with fast convergence for various material combinations including those with large acoustic mismatch without extra computational cost. The method is also efficient for mixed fluid-solid systems because it considers the different wave modes in the fluid and solid as well as the proper fluid-solid interface condition.

First-principles study of tunneling magnetoresistance in Fe/MgAl2O4/Fe(001) magnetic tunnel junctions

We investigated the spin-dependent transport properties of Fe/MgAl2O4/Fe(001) magnetic tunneling junctions (MTJs) on the basis of first-principles calculations of the electronic structures and the ballistic conductance. The calculated tunneling magnetoresistance (TMR) ratio of a Fe/MgAl2O4/Fe(001) MTJ was about 160%, which was much smaller than that of a Fe/MgO/Fe(001) MTJ (1600%) for the same barrier thickness. However, there was an evanescent state with delta 1 symmetry in the energy gap around the Fermi level of normal spinel MgAl2O4, indicating the possibility of a large TMR in Fe/MgAl2O4/Fe(001) MTJs. The small TMR ratio of the Fe/MgAl2O4/Fe(001) MTJ was due to new conductive channels in the minority spin states resulting from a band-folding effect in the two-dimensional (2-D) Brillouin zone of the in-plane wave vector (k//) of the Fe electrode. Since the in-plane cell size of MgAl2O4 is twice that of the primitive in-plane cell size of bcc Fe, the bands in the boundary edges are folded, and minority-spin states coupled with the delta 1 evanescent state in the MgAl2O4 barrier appear at k//=0, which reduces the TMR ratio of the MTJs significantly.

Neutron scattering studies of spin excitations in superconducting Rb0.82Fe1.68Se2

We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = (PI, 0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy, and occur at the expense of spin waves associated with the coexisting sqrt(5)\timessqrt(5) block AF phase. Since angle resolved photoemission experiments reveal no evidence for hole-like Fermi surface at Gamma(0, 0), our results suggest that the Q = (PI, 0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.

Giant reflection band and anomalous negative transmission in a resonant dielectric grating slab: Application to a planar cavity

The fundamental optical effects that are at basis of giant reflection band and anomalous negative transmission in a self-sustained rectangular dielectric grating slab in P polarization and for incidence angle not very far from the Brewster's angle of the equivalent slab, are investigated. Notice, that the self sustained dielectric grating slab is the simplest system that, due to the Bragg diffraction, can show both the former optical effects. A systematic study of its optical response is performed by an analytical exact solution of the Maxwell equations for a general incidence geometry. At variance of the well known broad reflection bands in high contrast dielectric grating slab in the sub-wavelength regime, obtained by the destructive interference between the travelling fundamental wave and the first diffracted wave (a generalization of the so called second kind Wood's anomalies), the giant reflection band is a subtle effect due to the interplay, as well as among the travelling fundamental wave and the first quasi-guided diffracted one, also among the higher in-plane wave- vector components of the evanescent/divergent waves. To better describe this effect we will compare the optical response of the self-sustained high contrast dielectric grating slab with a system composed by an equivalent homogeneous slab with a thin rectangular high contrast dielectric grating engraved in one of the two surfaces, usually taken as a prototype for the second kind Wood's anomalies generation. Finally, the electromagnetic field confinement in a patterned planar cavity, where the mirrors are two self-sustained rectangular dielectric grating slabs, is briefly discussed.

Entangled photons from the polariton vacuum in a switchable optical cavity

We study theoretically the entanglement of two-photon states in the ground state of the intersubband cavity system, the so-called polariton vacuum. The system consists of a sequence of doped quantum wells located inside a microcavity and the photons can interact with intersubband excitations inside the quantum wells. Using an explicit solution for the ground state of the system, operated in the ultrastrong coupling regime, a post-selection is introduced, where only certain two-photon states are considered and analyzed for mode entanglement. We find that a fast quench of the coupling creates entangled photons and that the degree of entanglement depends on the absolute values of the in-plane wave vectors of the photons. Maximally entangled states can be generated by choosing the appropriate modes in the post-selection.

Accurate absorbing boundary conditions for anisotropic elastic media. Part 1: Elliptic anisotropy

With the ultimate goal of devising effective absorbing boundary conditions (ABCs) for heterogeneous anisotropic elastic media, we investigate the accuracy aspects of local ABCs designed for tilted elliptic anisotropy in the frequency domain (time-harmonic case). Such media support both anti-plane and in-plane wavemodes with opposing signs of phase and group velocities (cpxcgx<0) that have long posed a significant challenge to the design of accurate (and stable) local ABCs. By first considering the simpler case of scalar anti-plane waves, we show that it is possible to overcome the challenges posed by cpxcgx<0 by simply utilizing the inevitable reflections occurring at the truncation boundaries. This understanding helps us to explain the ability of a recently developed local ABC – the perfectly matched discrete layer (PMDL) – to handle the challenges posed by cpxcgx<0without the need of intervening space–time transformations. PMDL is a simple variant of perfectly matched layers (PML) that is also equivalent to rational approximation-based local ABCs (rational ABCs); it inherits the straightforward approximation properties of rational ABCs along with the versatility of PML. The approximation properties of PMDL quantified through its reflection matrix is used to derive simple bounds on the PMDL parameters necessary for the accurate absorption of all outgoing anti-plane and in-plane wavemodes – including those with cpxcgx<0. Beyond the previously derived bound on the real parameters of PMDL sufficient for the absorption of outgoing propagating anti-plane wavemodes, we present bounds on the complex parameters of PMDL necessary for the absorption of outgoing propagating and evanescent wavemodes for both anti-plane and coupled in-plane pressure and shear waves. The validity of this work is demonstrated through a series of numerical experiments.

Waveguide characteristics of coupled in-plane waves

In-plane waves in a waveguide made from a thin plate are described by a superposition of a set of orthogonal functions that satisfy the edge conditions of the waveguide. Due to the Poisson and shear effects, the displacement components of the in-plane waves along the two in-plane orthogonal coordinates are coupled and this coupling affects the propagation and spatial properties of the waveguide modes. The orthogonal functions and their associated wavenumbers represent the characteristics of the uncoupled modes of the waveguide where the above mentioned couplings are ignored. This study demonstrates that the characteristics of the waveguide modes are determined by the couplings of the uncoupled mode pairs, which become significant when the pairs satisfy the conditions of spatial coincidence. At some frequencies, certain waveguide modes can be determined by a single pair of uncoupled modes. For this case, the analytical solution for the waveguide modes exists and provides both a qualitative and quantitative interpretation of the characteristics of the waveguide mode.

Localized Modes in Metamaterial-Dielectric Photonic Crystals with a Dielectric-Superconductor Pair Defect

Transmission coefficient and dispersion relation are calculated for a one-dimensional photonic crystal of alternating dielectric-metamaterial slabs with a dielectric-superconductor pair defect. The presence of the superconductor slab in the pair defect layer leads to a strong depletion of the TM transmission coefficient in a frequency range close to the edges of the non-Bragg gap and around the characteristic resonance frequency of the superconductor and introduces TM electromagnetic modes inside such gap. The features of the arising modes depend on the relation between the thicknesses of the layers involved in the defect and a low-frequency mode can arise from the low-frequency continuum of the bulk metamaterial modes at a non-zero in-plane wave vector.

Screened exciton properties of narrow-gap lead salt finite confining potential semiconductor quantum well

A formalism for studying the screened exciton states in the realistic narrow-gap finite confining potential quantum well is developed theoretically with taking into account the quantum well and barrier material parameter's strong contrast across the heteroboundaries. The typical band-nonparabolicity effect of narrow-gap semiconductor has been considered. In the presence of inhomogeneous dielectric polarization of the realistic structure, the suitable in-plane wave vector ranges are established for the average value of the Q2D screened Coulomb potential obtained by Rytova previously. For the two different regimes of Coulomb potential behavior, namely “logarithmical” and “exponential”, the problem of screened exciton binding energy has been investigated variationally at the first time. The analytical and numerical analysis depend on the specifics of realistic narrow-gap EuS/PbS/EuS quantum well are performed. The strong enhancement of the screened exciton binding energy (∼10meV) compared to the tiny unscreened bulk value (∼0.019meV) is received in the presence of the logarithmical potential for thin enough QW, demonstrating that quantum and dielectric confinements together overbalance to the Q2D screening effect. Conversely, in case of exponential potential the binding energy decreases monotonically with the increase of density/ temperature ratio parameter until saturation for the negligible energy values (<1meW). The acceptable intervals of noted ratio parameter in correlation with QW width is also established.

Hyperelastic cloaking theory: transformation elasticity with pre-stressed solids

Transformation elasticity, by analogy with transformation acoustics and optics, converts material domains without altering wave properties, thereby enabling cloaking and related effects. By noting the similarity between transformation elasticity and the theory of incremental motion superimposed on finite pre-strain, it is shown that the constitutive parameters of transformation elasticity correspond to the density and moduli of small-on-large theory. The formal equivalence indicates that transformation elasticity can be achieved by selecting a particular finite (hyperelastic) strain energy function, which for isotropic elasticity is semilinear strain energy. The associated elastic transformation is restricted by the requirement of statically equilibrated pre-stress. This constraint can be cast as tr F = constant, where F is the deformation gradient, subject to symmetry constraints, and its consequences are explored both analytically and through numerical examples of cloaking of anti-plane and in-plane wave motion.

Spin splitting of electron states in (110) quantum wells: Symmetry analysis and k·p theory versus microscopic calculations

Spin splittings in quantum wells have attracted considerable attention over the past decade due to potential application of semiconductor spin properties to ``spintronic devices.'' Recent experimental results stimulate theoretical investigations of new physical situations like unconventional growth directions. Here we focus on electron spin properties in (110)-oriented quantum wells that are of particular interest because qualitative symmetry analysis shows that spin relaxation by the D'yakonov-Perel' mechanism should be strongly suppressed in this geometry. We combine symmetry analysis, envelope function theory, and tight-binding calculation and obtain quantitative description of the in-plane wave vector, well width, and applied electric field dependence of the spin structure of electron subbands in $(110)$ quantum wells.

Optical anisotropy of InAs/GaSb broken-gap quantum wells

We investigate in detail the optical anisotropy of absorption of linearly polarized light in InAs/GaSb quantum wells grown on GaSb along the [001] direction, which can be used as an active region of different laser structures. The energy level positions, the wave functions, the optical matrix elements, and the absorption coefficients are calculated using the eight-band k center dot p model and the Burt-Foreman envelope function theory. In these calculations, the Schrodinger and Poisson equations are solved self-consistently taking the lattice-mismatched strain into account. We find that a realistic Hamiltonian, which has the C (2v) symmetry, results in considerable anisotropy of optical matrix elements for different directions of light polarization and different directions of the initial-state in-plane wave vector, including low-symmetry directions. We trace how the optical matrix elements and absorption are modified when spin-orbit interaction and important symmetry breaking mechanisms are taken into account (structural inversion asymmetry, bulk inversion asymmetry, and interface Hamiltonian). These mechanisms result in an almost 100% anisotropy of the absorption coefficients as the light polarization vector rotates in the plane of the structure and in a plane normal to the interfaces.

Two-dimensional phononic crystal sensor based on a cavity mode

Abstract Phononic crystals offer an innovative platform for acoustic liquid sensors. Based on a longitudinal cavity mode, we introduce an acoustic sensor system using a two dimensional phononic crystal with in-plane wave incidence. The phononic crystal is made up of a steel plate having two regular arrays of holes and a cavity in-between. The holes and the cavity are filled with the liquid of interest. We both theoretically and experimentally demonstrate that the transmission peak caused by the cavity mode can be used for sensor purpose. Theoretical simulation and experimental measurement show good consistency in the response of the transmission peak frequency. This frequency is primarily sensitive to the speed of sound thereby sensitive to the composition of the liquid mixture.

Theoretical Estimation and Modeling for Constraint-Tuning Ultrasonic Actuator

ABSTRACTUsing a piezoelectric unimorph vibrator with constraint-tuning modified-mode (CTMM) mechanism, a novel design of a thin-disc ultrasonic actuator was developed to drive an optical sled. The theoretical estimation of in-plane wave propagation on a thin disc is introduced to explain the novel actuating mechanism, via a modal expansion technique and modal participation factors. Furthermore, the approximate wave propagations could be illustrated by the practical estimated wave equations in this study. Applying four screws at the exact distribution of angles on the thin-disc vibrator, the actuating mechanism of ultrasonic modified modes is generated and propagated. The in-plane vibration modes could be tuned by these desired screw constraints on the piezoelectric vibrator. The ultrasonic actuator offers the output force to drive an optical sled by friction contact in bilateral motions. To implement the equilibrium structure force in bilateral directions, natural and forced analysis as well as impedance comparison of FEM software ANSYS are also introduced into the constrained design. Hence, there are two various modified modes chosen at the different resonant frequencies within the electromechanical coupling of piezoelectric material to pursue more efficiency in energy conversion. Experimental results have demonstrated consistency with the approximate theoretical approach of the in-plane wave propagation and simulations based on the concept of constrained-tuning modified modes.