Anomalous Oscillations Research Articles

Scale-free memory model for multiagent reinforcement learning. Mean field approximation and rock-paper-scissors dynamics

A continuous time model for multiagent systems governed by reinforcement learning with scale-free memory is developed. The agents are assumed to act independently of one another in optimizing their choice of possible actions via trial-and-error search. To gain awareness about the action value the agents accumulate in their memory the rewards obtained from taking a specific action at each moment of time. The contribution of the rewards in the past to the agent current perception of action value is described by an integral operator with a power-law kernel. Finally a fractional differential equation governing the system dynamics is obtained. The agents are considered to interact with one another implicitly via the reward of one agent depending on the choice of the other agents. The pairwise interaction model is adopted to describe this effect. As a specific example of systems with non-transitive interactions, a two agent and three agent systems of the rock-paper-scissors type are analyzed in detail, including the stability analysis and numerical simulation. Scale-free memory is demonstrated to cause complex dynamics of the systems at hand. In particular, it is shown that there can be simultaneously two modes of the system instability undergoing subcritical and supercritical bifurcation, with the latter one exhibiting anomalous oscillations with the amplitude and period growing with time. Besides, the instability onset via this supercritical mode may be regarded as “altruism self-organization”. For the three agent system the instability dynamics is found to be rather irregular and can be composed of alternate fragments of oscillations different in their properties.

Angle-dependent magnetoresistance oscillations and magnetic breakdown in [formula omitted

We present results of stereographic measurements of interlayer magnetoresistance in the quasi-two-dimensional (Q2D) organic conductor, α ‐ ( BEDT ‐ TTF ) 2 KHg ( SCN ) 4 , at around charge density wave (CDW) transition temperature T c ∼ 8 K . At 2 K in the CDW phase, there exists the anomalous Lebed resonance pattern, the amplitude of which is modulated by Danner–Chaikin oscillations in case of applied magnetic field close to the Q2D conducting plane. On the contrary, at 7 K just below T c , we have found that the Kajita oscillation of the cylindrical Fermi surfaces appears superposed on the anomalous Lebed resonance. A quantum model of interlayer tunneling demonstrates these behaviors qualitatively. These results suggest that magnetic breakdown plays a key role in anomalous angle-dependent magnetoresistance oscillations.

The anomalous negative acoustoelectric current in single-electron transport devices

We investigate the anomalous negative oscillations present in the frequency dependence of an acoustoelectric current by using surface acoustic wave devices. The time domain responses of the transmission performances for our devices show that the Bragg reflections from the second transducer have a strong effect. On the basis of the results measured, a new model is advanced for explaining the anomalous negative oscillations present in the I ( f ) characteristics. In addition, in the negative direction, the acoustoelectric current measured as a function of the gate voltage is observed to be quantized. And the interpretation for the negative quantized plateaus present in the I ( V g ) characteristics is also given.

Interface states and anomalous quantum oscillations in hybrid graphene structures

One- and two-layer graphene have recently been shown to feature new physical phenomena such as unconventional quantum Hall effects and prospects of supporting a non-silicon technological platform using epitaxial graphene. While both one- and two-layer graphene have been studied extensively, continuous sheets of graphene possessing both parts have not yet been explored. Here we report a study of such graphene hybrid structures. In a bulk hybrid featuring two large-area one- and two-layer graphene and an interface between them, two sets of Landau levels and features related to the interface were found. In edge hybrids featuring a large two-layer graphene with narrow one-layer graphene edges, we observed an anomalous suppression in quantum oscillation amplitude due to the locking of one- and two-layer graphene Fermi energies and emergent chiral interface states. These findings demonstrate the importance of these hybrid structures whose unique interface states and related phenomena deserve further studies.

Oscillations of the intertropical convergence zone and the genesis of easterly waves. Part I: diagnostics and theory

We examine the mean and transient state of the intertropical convergence zone (ITCZ) by analyzing data and using simple theory. We concentrate on the tropical eastern Pacific Ocean noting that there exists in this region a well-developed mean ITCZ. Furthermore, it is a region where there has been considerable discussion in the literature of whether easterly waves develop in situ or propagate westwards from the Atlantic Ocean. The region is typical of tropical regions where there is a strong cross-equatorial pressure gradient (CEPG): mean convection well removed from the equator but located equatorward of the maximum sea-surface temperature (SST) and minimum sea level pressure (MSLP). Further to the west, near the dateline where the CEPG is much smaller, convection is weaker and collocated with SST and MSLP extrema. It is argued that in regions of significant CEPG that the near-equatorial tropical system is inertially unstable and that the rectification of the instability for a given CEPG determines the location and intensity of the climatological ITCZ. Using simple theoretical arguments, we develop an expression for the mean latitude of the ITCZ as a function of the CEPG. We note on a day-by-day basis that the ITCZ is highly transient state with variability occurring on 3–8 day time scales. Transients with amplitudes about half of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere as anomalous meridional oscillations, eventually amplifying convection in the vicinity of the mean ITCZ. It is argued that in these longitudes of strong CEPG the mean ITCZ is continually inertially unstable with advections of anticylonic vorticity across the equator resulting in the creation of an oscillating divergence–convergence doublet. The low-level convergence produces convection and the resultant vortex tube stretching generates cyclonic vorticity which counteracts the northward advection of anticylonic vorticity. During a cycle, the mid-troposphere heating near 10oN oscillates between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. As a result, there exists an anomalous and shallower, oscillating meridional circulation with a magnitude about 50% of the mean ITCZ associated with the stable state following the generation of anticylonic vorticity. Further, it is argued that the instabilities of the ITCZ are directly associated with in situ development of easterly waves which occur with the inertial period of the latitude of the mean ITCZ. The dynamical sequences and the genesis of easterly waves are absent in the regions further to the east where the CEPG is much weaker or absent altogether. In a companion study (Part II), numerical experiments are conducted to test the hypothesis raised in the present study.

Investigation of C60 Epitaxial Growth Mechanism on GaAs Substrates

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs (111)B, (111)A, and (001) substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by repeating nucleation and step-flow growth mode, as is the case with GaAs and other semiconductor materials. Anomalous oscillations are observed in the initial stage of C60 layer growth on a GaAs (111)B surface with (2×2) structure. These oscillations indicate that the growth of the first C60 layer is completed at the point of approximately 0.5 monolayer coverage by C60 molecules. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. A similar relationship between C60 adsorption and surface reconstruction is observed on (001) GaAs substrates, i.e., the first C60 layer on the (2×4) surface is terminated at approximately 0.5 monolayer coverage, while full coverage is needed of the c(4×4) surface. These observed results are strongly supported by the reported results of scanning tunnel microscopy.

RHEED intensity oscillation of C 60 layer epitaxial growth

Intensity oscillations of reflection high-energy electron diffraction are observed during a C 60 layer epitaxial growth on GaAs (1 1 1)B, (1 1 4)A and (1 1 4)B substrates. Frequencies of the oscillations coincide well with growth rates of C 60 layers, suggesting that C 60 layers grow by layer-by-layer growth mode as with GaAs and other semiconductor materials. Anomalous oscillations are observed in the initial stage of a C 60 layer growth on GaAs (1 1 1)B surface with (2×2) reconstruction. These oscillations indicate that the C 60 first-layer growth is completed at approximately half monolayer coverage. This phenomenon is explained by a model that C 60 adsorption sites are limited due to As-trimers adsorbed on GaAs surface. Clear oscillations are observed during a C 60 layer growth on GaAs (1 1 4)A substrate, and X-ray diffraction peaks of the layer are sharp. In contrast, no oscillation is detected during the growth on the (1 1 4)B substrate, and these layers exhibit poor X-ray diffraction characteristics. Thus, the C 60 epitaxial layer growth on GaAs substrates is strongly affected by the GaAs surface reconstruction and polarity.

Anomalous Conductance Oscillations and Half-Metallicity in Atomic Ag-O Chains

Using spin density functional theory, we study the electronic and magnetic properties of atomically thin, suspended chains containing silver and oxygen atoms in an alternating sequence. Chains longer than 4 atoms develop a half-metallic ground state implying fully spin-polarized charge carriers. The conductances of the chains exhibit weak even-odd oscillations around an anomalously low value of 0.1G0 (G0=2e2/h) which coincide with the averaged experimental conductance in the long chain limit. The unusual conductance properties are explained in terms of a resonating-chain model, which takes the reflection probability and phase shift of a single bulk-chain interface as the only input. The model also explains the conductance oscillations for other metallic chains.

Quantum jumps of saturation level rigidity and anomalous oscillations of level number variance in the semiclassical spectrum of a modified Kepler problem

We observe numerically, and explain analytically, a previously unknown phenomenon of quantum-Hall-like jumps in saturation spectral rigidity in the semiclassical spectrum of a modified Kepler problem as a function of the interval center. These jumps correspond to integer decreases of the radial winding numbers in classical periodic motion. We also observe and explain single-harmonic-dominated oscillations of the level number variance with the width of the energy interval. The level number variance becomes effectively zero for the interval widths defined by the frequency of the shortest periodic orbit. This signifies that there are virtually no variations from sample to sample in the number of levels on such intervals.

Landau levels in a simple hexagonal bilayer graphene

A new approach, which makes the Hamiltonian of the Peierls tight-binding model change into a band matrix, is used to investigate the Landau levels in a AA-stacked bilayer graphene. The interlayer atomic hoppings could induce an energy gap, the asymmetry of the Landau levels about the chemical potential, the random variation in the level spacing, more fourfold degenerate Landau levels at low energy, and the oscillatory Landau levels and the complicated state degeneracies at moderate energy. For the low-energy Landau levels, their dependence on the quantum number and the field strength cannot be well characterized by a simple power law. They exhibit a anomalous oscillation during the variation of the magnetic field. The main features of the magnetoelectronic states are directly reflected in density of states.

Grazing-incidence diffraction anomalous fine structure: Application to the structural investigation of group-III nitride quantum dots

The relevance of grazing-incidence anomalous diffraction as a tool to investigate the strain and structure of small-size embedded nano-objects is examined. Multiple scattering effects, originating from the grazing-incidence setup, are analyzed with a special emphasis on the cusp of the diffraction anomalous spectrum and the extended diffraction anomalous fine-structure oscillations. It is shown that even for grazing-incidence angle, a Born approximation treatment is justified for quantum dots (QDs) on top of a thin wetting layer. The discussion focuses on the overgrowth of AlN on top of GaN QDs. Both the in-plane and out-of-plane strains in the dots can be specifically determined, by extracting the Ga partial scattering amplitude from measurements of the scattered intensity along both the in- and out-of-plane directions, close to the $(30\overline{3}0)$ and $(30\overline{3}2)$ reflections, at several energies across the Ga $K$ edge. The study is complemented by the analysis of the local environment of Ga atoms in the dots through the measurement of the fine-structure oscillations in diffraction condition. The oscillations are found almost insensitive to the grazing-incidence multiple-scattering effects. Accordingly, the out-of-plane strain and possible intermixing specifically in the dots can be deduced. The QDs are shown to remain pure GaN all along the capping process. The QDs strain state exhibits a larger strain relaxation than expected from an elastic model, suggesting the presence of a plastic strain relaxation, possibly through dislocations at the vicinity of the QDs. Finally, the influence of the substrate as regards strain relaxation in the QDs is discussed by comparing our results to those we previously obtained for a series of samples grown on AlN/sapphire.

The anomalous orbital velocity effect and high frequency quasi-periodic oscillations in accreting black holes

The recently discovered anomalous orbital velocity effect, which implies a reduced velocity with decreasing radius for an orbiting test particle, is present in extreme Kerr black holes with spin > 0.9953 and radii between 1.05 and 1.78 gravitational radii. In this region a 3:1 resonance between the vertical and the radial epicyclic frequencies occurs, which may lead to quasi-peridoc light variations with two periods differing by a factor of 3. Such variations may have been observed from the Galactic Center black hole Sgr A*. At the same spin but at a larger radius a 3:2 resonance between the two epicyclic frequencies is found as well. Such 3:2 frequencies have been observed in four galactic microquasars. In either case, i.e. the 3:1 or the 3:2 resonance, the black hole mass determined from this model agrees very well with the dynamical mass for Sgr A* and each of the four microquasars. Meanwhile the microquasar GRS 1915+105 has been classified by its X-ray spectrum as an extreme Kerr black hole with a spin > 0.98 [1], which agrees quite nicely with our prediction for the spin, i.e. a = 0.996. Model and observational data are presented and discussed.

Anomalous voltage oscillations and metastability in the strongly nonlinear I– V regime

We report unusual voltage oscillations, in real time, caused by the dc radial current in the Meissner phase of a thick amorphous Mo x Si 1− x film with the Corbino-disk contact geometry. We observe voltage pulses that oscillate almost periodically at temperature immediately below the transition temperature, where current–voltage characteristics show strong nonlinearity. The results indicate the presence of metastable vortex states. The origin of the oscillation is most likely attributed to large number-fluctuations of vortices and antivortices.

Anomalous oscillations in the spectra of light backscattered by inhomogeneous microparticles

We report anomalous oscillatory features in the spectra of cross-polarized backscattered light from inhomogeneous dielectric microparticles. Numerical experiments based on the finite-difference-time-domain method demonstrate that cross-polarized backscattered spectra exhibit oscillation frequencies with two a priori surprising features. First, the oscillation frequencies decrease as the correlation length (Lc) of the particle's refractive index increases. Second, high-frequency oscillations exist even for Lc much smaller than the optical wavelength. These findings are exactly opposite to what is observed in co-polarized backscattering spectra, and are not expected from conventional optical scattering theory. We explain this anomalous behavior by examining the path-length distributions of the backscattered photons in the cross polarization.

The Effect of 3He Impurities on Anomalous Oscillations in the Expansion of Solid 4He

The regular periodic intensity bursts recently observed in the expansion of solid 4He into vacuum have anomalies that have been attributed to a transition to some new solid phase induced by excess vacancies. Here it is shown that a small concentration of 3He, from 1% down to 0.1%, added to the 4He solid is sufficient to remove all the anomalies. The origin of the observed anomalies is discussed in the light of these new experiments.

Dynamics of ultrasonically induced birefringence of in rod-like colloidal solutions

This review summarized our experimental studies of ultrasonically induced birefringence on aqueous solution of rigid rod-like colloids and rod-like micelles from the view point of dynamics of particle orientation. For rigid rod-like colloids in dilute concentration, the orientational relaxation time was described in terms of the Debye–Einstein equation. This indicated that the ultrasonically induced birefringence could be one of useful tools for determining the size of particles of anisotropic shape. For rod-like micelles, the birefringence showed anomalous damping oscillation when the rod-like micelle was in an entangling state. The mechanism of damping oscillation was discussed using the results of the rheological measurements.

Elastic properties of [formula omitted

The clathrate compound Eu 8 Ga 16 Ge 30 with the type-I cubic structure is expected to show rattling motion of Eu atom which is an anomalous off-center oscillation. To investigate an influence of rattling motion to the lattice system of Eu 8 Ga 16 Ge 30 , we have measured temperature T dependence of elastic modulus ( C 11 - C 12 ) / 2 and ultrasonic attenuation α of a single-crystalline sample in the T range between 4.2 and 150 K for ultrasonic frequencies from 20 to 150 MHz . We found ultrasonic frequency dependence in both ( C 11 - C 12 ) / 2 and α , suggesting rattling motion of Eu atoms between 50 and 100 K . We obtained a relaxation time 6.0 × 10 - 11 s and an excitation energy 420 K of rattling motion with Γ 3 symmetry.

Revisiting the anomalous shelf water oscillation of Buckles Bay, Macquarie Island

Historic observations of sea-level on Macquarie Island recorded an ∼ 6 min oscillation with a perceived beat of ∼ 3 h , interpreted as two counter-rotating edge waves of slightly different frequencies. Alternative evidence is presented here that the oscillations are consistent with the fundamental shelf period of Buckles Bay; an envelope characteristic of local coast and shelf features. New data from a shore-mounted tide gauge, analyzed using fourier and wavelet techniques, show an ∼ 6.6 min phenomenon occurring in bursts every 1–4 h. A simple model shows that the fundamental resonant frequency may spread and comply with maximum and minimum coast and shelf features.

Elastic properties of [formula omitted

The clathrate compound Sr 8 Ga 16 Ge 30 with the type-I cubic structure is expected to show the rattling motion of Sr atom which is an anomalous off-center oscillation. To investigate an influence of the rattling motion to the lattice system, we have measured temperature T dependence of elastic modulus C 44 and ultrasonic attenuation α 44 of a single-crystalline sample in the T range between 4.2 and 150 K. We found a frequency dependence in C 44 and α 44 suggesting the rattling motion of Sr atoms between 20 and 60 K. We obtained the relaxation time 7.0 × 10 - 12 s and the excitation energy 285 K of the rattling motion.

Experimental and numerical study of the recovery softness and overvoltage dependence on p–i–n diode design

This work offers a reliability-oriented characterization of power p–i–n diodes turn-off transients. The softness of the turn-off and the snap-off voltage (defined as the threshold beyond which large, anomalous reverse overvoltages develop across the diode at turn-off) are investigated both experimentally and numerically for a wide set of diodes with different drift region width, resistivity and lifetime. In particular, lifetime control is obtained by electron irradiation at different doses. As a result, guidelines emerge for the design of the snubberless diode with optimum trade-off between switching speed and softness. It is also suggested that, for complete diode characterization, the well-known softness factor be accompanied by the snap-off voltage, i.e. the peak reverse voltage triggering anomalous oscillations in the turn-off transient.