Two-dimensional Yukawa liquids: correlation and dynamics

A combined theoretical and molecular dynamics (MD) simulation study of the collective modes and their dispersion in a two-dimensional Yukawa system in the strongly coupled liquid state is presented. The theoretical analysis relies upon the quasilocalized charge approximation; the MD simulation generates static pair correlation functions and dynamical current-current correlation spectra.     

Phonon anomalies due to collective stripe modes in high T(c) cuprates

Phonon anomalies observed in various high T(c) cuprates by neutron experiments are analyzed theoretically in terms of the stripe concept. The phonon self-energy correction is evaluated by taking into account the charge collective modes of stripes, giving rise to dispersion gap, or kink and shadow phonon modes at twice the wave number of spin stripe. These features coincide precisely with observations. The gapped branches of the phonon are found to be in-phase and out-of-phase oscillations relative to the charge collective mode.     

Collective excitations in electron-hole bilayers

We report a combined analytic and molecular dynamics analysis of the collective mode spectrum of a bipolar (electron-hole) bilayer in the strong coupling classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles. In the in-phase spectra we identify longitudinal and transverse acoustic modes wholly maintained by correlations. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed.     

Axial instability of double-nanobeam-systems

This Letter considers the axial instability of double-nanobeam-systems. Eringen's nonlocal elasticity is utilized for modelling the double-nanobeam-systems. The nonlocal theory accounts for the small-scale effects arising at the nanoscale. The small-scale effects substantially influence the instability (or buckling) of double-nanobeam-systems. Results reveal that the small-scale effects are higher with increasing values of nonlocal parameter for the case of in-phase (synchronous) buckling modes than the out-of-phase (asynchronous) buckling modes. The increase of the stiffness of the coupling elastic medium in double-nanobeam-system reduces the small-scale effects during the out-of-phase (asynchronous) buckling modes. Analysis of the scale effects in higher buckling loads of double-nanobeam-system with synchronous and asynchronous modes is also discussed in this Letter. The theoretical development presented herein may serve as a reference for nonlocal theories as applied to the instability analysis of complex-nanobeam-system such as complex carbon nanotube system.     

Defect solitons supported by parity-time symmetric defects in superlattices

The existence and stability of defect solitons supported by parity-time (PT) symmetric defects in superlattices are investigated. In the semi-infinite gap, in-phase solitons are found to exist stably for positive defects, zero defects, and negative defects. In the first gap, out-of-phase solitons are stable for positive defects or zero defects, whereas in-phase solitons are stable for negative defects. For both the in-phase and out-of-phase solitons with the positive defect and in-phase solitons with negative defect in the first gap, there exists a cutoff point of the propagation constant below which the defect solitons vanish. The value of the cutoff point depends on the depth of defect and the imaginary parts of the PT symmetric defect potentials. The influence of the imaginary part of the PT symmetric defect potentials on soliton stability is revealed.     

Plasmon modes in 3-layer graphene structures: Inhomogeneity effects

We calculate the plasmon frequency and damping rate in a 3-layer graphene system made of parallel monolayer and bilayer graphene sheets using the random-phase-approximation dielectric function and taking into account the inhomogeneity of the dielectric background of the system. Numerical results show that two out-of-phase acoustic and one in-phase optical plasmon modes can be found from the zeroes of dynamical dielectric function of the structure. Plasmon frequencies and damping rate of plasma oscillations depend significantly on the inhomogeneity of environment, so plasmon curves become more distinctive from each other in single-particle excitation region, compared to the case of homogeneous medium. Finally, Plasmon dispersion patterns depend remarkably on the number (but not order) of bilayer graphene sheet constructing to the system.     

Interface kink solitons in defocusing saturable nonlinear media

We report on the dynamics of semi-localized nonlinear optical modes supported by an interface separating a uniform defocusing saturable medium and an imprinted semi-infinite photonic lattice. Out-of-phase and in-phase kink solitons composed by dark-soliton-like pedestals and oscillatory tails are found. Two branches of out-of-phase kink solitons exist in shallow lattices. Saturable nonlinearity enhances the pedestal height and renormalized energy flow of kink solitons evidently. While in-phase kink solitons are always unstable, out-of-phase kink solitons will be completely stable provided that lattice depth exceeds a critical value. Furthermore, stable kink solitons in the higher band gaps are also possible. Our results may give a helpful hint for understanding the dynamics of kink solitons with high pedestals in other fields.     

Delay-coupled discrete maps: Synchronization, bistability, and quasiperiodicity

The synchronization transition is studied in delay-coupled logistic maps. For low coupling, in-phase and out-of-phase synchronous dynamics coexist, and with increasing coupling there is a regime of quasiperiodicity before eventual attraction to a fixed point at a critical value of coupling that depends on the nonlinearity. The presence of a region of asynchrony separating two synchronized regimes—termed anomalous behaviour—has been observed earlier in continuous systems and is shown here to occur in delay mappings as well. There are regions of in-phase, anti-phase, and out-of-phase dynamics of periodic as well as chaotic attractors.     

Experimental evidence of time-delay-induced death in coupled limit-cycle oscillators

Experimental observations of time-delay-induced amplitude death in two coupled nonlinear electronic circuits that are individually capable of exhibiting limit-cycle oscillations are described. The existence of multiply connected death islands in the parameter space of coupling strength and time delay for coupled identical oscillators is established. The existence of such regions was predicted earlier on theoretical grounds [Phys. Rev. Lett. 80, 5109 (1998); Physica (Amsterdam) 129D, 15 (1999)]. The experiments also reveal the occurrence of multiple frequency states, frequency suppression of oscillations with increased time delay, and the onset of both in-phase and antiphase collective oscillations.     

Collision Dynamics of Dissipative Matter-Wave Solitons in a Perturbed Optical Lattice

We investigate the stability and collision dynamics of dissipative matter-wave solitons formed in a quasi-onedimensional Bose-Einstein condensate with linear gain and three-body recombination loss perturbed by a weak optical lattice.It is shown that the linear gain can modify the stability of the single dissipative soliton moving in the optical lattice.The collision dynamics of two individual dissipative matter-wave solitons explicitly depend on the linear gain parameter,and they display different dynamical behaviors in both the in-phase and out-of-phase interaction regimes.     

Higher-order solitons in amplitude-disordered waveguide arrays

We investigate the existence and stability of different families of spatial solitons in optical waveguide arrays whose amplitudes obey a disordered distribution. The competition between focusing nonlinearity and linearly disordered refractive index modulation results in the formation of spatial localized nonlinear states. Solitons originating from Anderson modes with few nodes are robust during propagation. While multi-peaked solitons with in-phase neighboring components are completely unstable, multipole-mode solitons whose neighboring components are out-of-phase can propagate stably in wide parameter regions provided that their power exceeds a critical value. Our findings, thus, provide the first example of stable higher-order nonlinear states in disordered systems.     

Dipolar drag in bilayer harmonically trapped gases

We consider two separated pancake-shaped trapped gases interacting with a dipolar (either magnetic or electric) force. We study how the center of mass motion propagates from one cloud to the other as a consequence of the long-range nature of the interaction. The corresponding dynamics is fixed by the frequency difference between the in-phase and the out-of-phase center of mass modes of the two clouds, whose dependence on the dipolar interaction strength and the cloud separation is explicitly investigated. We discuss Fermi gases in the degenerate as well as in the classical limit and comment on the case of Bose-Einstein condensed gases.     

Hysteresis loops of fiber-reinforced ceramic-matrix composites under in-phase/out-of-phase thermomechanical and isothermal cyclic loading

In this paper, the stress?strain hysteresis loops of fiber-reinforced ceramic-matrix composites (CMCs) under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been investigated. The thermomechanical hysteresis loops models have been developed considering synergistic effects of thermal temperature cycling, stress levels and fiber/matrix interface debonding. The relationships between thermal cyclic temperatures, peak stress, fiber/matrix interface shear stress and stress?strain hysteresis loops under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been established. The effects of fiber volume fraction, peak stress, matrix crack spacing, interface frictional coefficient, interface debonded energy and temperature range on the stress?strain hysteresis loops under in-phase/out-of-phase thermomechanical and isothermal cyclic loading have been analyzed. The hysteresis loops of cross-ply SiC/magnesium aluminosilicate (MAS) composite under in-phase/out-of-phase thermomechanical and isothermal fatigue loading have been predicted.     

Synchronization of coupled metronomes on two layers

Coupled metronomes serve as a paradigmatic model for exploring the collective behaviors of complex dynamical systems, as well as a classical setup for classroom demonstrations of synchronization phenomena. Whereas previous studies of metronome synchronization have been concentrating on symmetric coupling schemes, here we consider the asymmetric case by adopting the scheme of layered metronomes. Specifically, we place two metronomes on each layer, and couple two layers by placing one on top of the other. By varying the initial conditions of the metronomes and adjusting the friction between the two layers, a variety of synchronous patterns are observed in experiment, including the splay synchronization (SS) state, the generalized splay synchronization (GSS) state, the anti-phase synchronization (APS) state, the in-phase delay synchronization (IPDS) state, and the in-phase synchronization (IPS) state. In particular, the IPDS state, in which the metronomes on each layer are synchronized in phase but are of a constant phase delay to metronomes on the other layer, is observed for the first time. In addition, a new technique based on audio signals is proposed for pattern detection, which is more convenient and easier to apply than the existing acquisition techniques. Furthermore, a theoretical model is developed to explain the experimental observations, and is employed to explore the dynamical properties of the patterns, including the basin distributions and the pattern transitions. Our study sheds new lights on the collective behaviors of coupled metronomes, and the developed setup can be used in the classroom for demonstration purposes.     

Enhanced signal intensities obtained by out-of-phase rapid-passage EPR for samples with long electron spin relaxation times

To understand the signals that are observed under rapid-passage conditions for samples with long electron spin relaxation times, the E' defect in irradiated vitreous SiO(2) was studied. For these samples at room temperature, T(1) is 200 mciro s and T(2) ranged from 35 to 200 micro s, depending on spin concentration. At X band with 100-kHz modulation frequency and 1-G modulation amplitude there was minimal lineshape difference between the low-power, in-phase spectra and high-power spectra detected 90 degrees out-of-phase with respect to the magnetic field modulation. Signal enhancement, defined as the ratio of the intensities of the out-of-phase to the in-phase signals when B(1) for both observation modes is adjusted to give maximum signal, was 3.4 to 9.5 at room temperature. The origin of the out-of-phase signal was modeled by numerical integration of the Bloch equations including magnetic field modulation. The waveforms for the E' signal, prior to phase sensitive detection, were simulated by summing the contributions of many individual spin packets. Good agreement was obtained between experimental and calculated waveforms. At low B(1) the experimental values of T(1) and T(2) were used in the simulations. However, at higher B(1), T(2) was adjusted to match the experimental signal intensity and increased with increasing B(1). At high B(1), T(2)=T(1), consistent with Redfield's and Hyde's models. For the spin concentrations examined, the out-of-phase signals at very high power (B(1) approximately 0.33 G) displayed a linear relationship between peak-to-peak signal amplitude and spin concentration. Under the conditions used for spin quantitation the signal-to-noise for these spectra was up to 5 times higher than for the in-phase signal, which greatly facilitates quantitation for these types of samples. For samples in which T(2) is dominated by electron spin-spin interaction, lower spin concentration results in longer T(2) and the enhancement is increased.     

Anisotropic plasmon dispersion and damping in multilayer 8-Pmmn borophene structures

We investigate the collective plasma oscillations theoretically in multilayer 8-Pmmn borophene structures, where the tilted Dirac electrons in spatially separated layers are coupled via the Coulomb interaction. We calculate the energy dispersions and Landau dampings of the multilayer plasmon excitations as a function of the total number of layers, the interlayer separation, and the different orientations. Like multilayer graphene, the plasmon spectrum in multilayer borophene consists of one in-phase optical mode and N - 1 out-of-phase acoustical modes. We show that the plasmon modes possess kinks at the boundary of the interband single-particle continuum and the apparent anisotropic behavior. All the plasmon modes approach the same dispersion at a sufficiently large interlayer spacing in the short-wavelength limit. Especially along specific orientations, the optical mode could touch an energy maximum in the nondamping region, which shows non-monotonous behavior. Our work provides an understanding of the multilayer borophene plasmon and may pave the way for multilayer borophene-based plasmonic devices.     

Measurements of ultrasound velocity and attenuation in numerical anisotropic porous media compared to Biot’s and multiple scattering models

This article quantitatively investigates ultrasound propagation in numerical anisotropic porous media with finite-difference simulations in 3D. The propagation media consist of clusters of ellipsoidal scatterers randomly distributed in water, mimicking the anisotropic structure of cancellous bone. Velocities and attenuation coefficients of the ensemble-averaged transmitted wave (also known as the coherent wave) are measured in various configurations. As in real cancellous bone, one or two longitudinal modes emerge, depending on the micro-structure. The results are confronted with two standard theoretical approaches: Biot’s theory, usually invoked in porous media, and the Independent Scattering Approximation (ISA), a classical first-order approach of multiple scattering theory. On the one hand, when only one longitudinal wave is observed, it is found that at porosities higher than 90% the ISA successfully predicts the attenuation coefficient (unlike Biot’s theory), as well as the existence of negative dispersion. On the other hand, the ISA is not well suited to study two-wave propagation, unlike Biot’s model, at least as far as wave speeds are concerned. No free fitting parameters were used for the application of Biot’s theory. Finally we investigate the phase-shift between waves in the fluid and the solid structure, and compare them to Biot’s predictions of in-phase and out-of-phase motions.     

Phase evolution of the reemitted field in the semiconductor quantum wells under the femtosecond pulse train intersubband excitation

Property of the phase of the reemitted field in the semiconductor quantum wells (QWs) excited by femtosecond pulse train is investigated. It is shown that the phase evolution of the reemitted field is controlled by the relative phase between the successive pulses of the incident train. For all the odd pulses excitation,the reemitted field is from out-of-phase to in-phase, then again to out-of-phase with the incident pulses,whereas for all the even pulses excitation, the situation is the opposite, i.e., it is from in-phase to out-of-phase, then again to in-phase with the incident pulses.     

Analysis of damping-induced phase flips of plasmonic nanowire modes

We launch surface plasmons from one end of a silver nanowire by asymmetric illumination with white light and use plasmon-to-light scattering at the nanowire ends to probe spectroscopically the plasmonic Fabry-Perot wire modes. The spectral positions of the maxima and minima in the scattered intensity from both nanowire ends are found to be either in-phase or out-of-phase, depending on the nanowire length and the spectral range. This behavior can be explained by a generalized Fabry-Perot model. The turnover point between the two regimes is sensitive to the surface plasmon round trip losses and thus opens a new possibility for detecting changes of the optical absorption in the nanowire environment.     

Nonlinear dynamical stability of gap solitons in Bose-Einstein condensate loaded in a deformed honeycomb optical lattice

We investigate the existence and dynamical stability of multipole gap solitons in Bose-Einstein condensate loaded in a deformed honeycomb optical lattice. Honeycomb lattices possess a unique band structure, the first and second bands intersect at a set of so-called Dirac points. Deformation can result in the merging and disappearance of the Dirac points, and support the gap solitons. We find that the two-dimensional honeycomb optical lattices admit multipole gap solitons. These multipoles can have their bright solitary structures being in-phase or out-of-phase. We also investigate the linear stabilities and nonlinear stabilities of these gap solitons. These results have applications of the localized structures in nonlinear optics, and may helpful for exploiting topological properties of a deformed lattice.