Anomalous Oscillations Research Articles

Abnormal Absorption and Energy Flow of Electromagnetic Wave in Ultrathin Metal Films

Recently nonlocal optical responses of metal nanostructures have been investigated to better describe the abnormal optical properties of structures within a feature length of 10 nm. However, most of the investigations are based on hydrodynamical Drude model. In the present work, on the basis of the quantum nonlocal response, we study properties of s-polarization electromagnetic waves in metal films. In the studies of the dependence of absorption on frequency and thin film thickness, we find that in the case of no plasmon excitations and any scattering experienced by electrons in the system, the nonlocal response theory still gives a considerable energy absorption caused by electron–hole pair excitations. However, both the local Drude model and hydrodynamical Drude model only present zero absorption in this case. In addition, our calculation shows that the energy flow of the nonlocal-response theory displays an anomalous oscillation in the film, which is ascribed to the negative work done by electric field in some regions of the film. This is just the manifestation for the nonlocal response of current to the field.

Crossover between two-dimensional surface state and three-dimensional bulk phase in Fe-doped Bi2Te3

In Fe-doped Bi2Te3, we have observed higher mobility, larger linear magnetoresistance, and anomalous quantum oscillations. The angle dependence of Shubnikov-de Haas (SdH) oscillations gives two different periodicities depending on the angle from the c-axis. The low-angle SdH period is identified with a surface origin, while the high-angle period is against the surface origin. The high-angle SdH period well agrees with the de Haas-van Alphen (dHvA) period with a bulk origin. The physical parameters obtained from the quantum oscillations support the crossover between two-dimensional surface state and three-dimensional bulk phase by Fe doping in Bi2Te3.

A theoretical study of pump–probe experiment in single-layer, bilayer and multilayer graphene

The pump–probe experiment is typically used to study relaxation phenomena in nonlinear optical systems. Here we use it as a tool to study the phenomenon of anomalous Rabi oscillations in graphene that was predicted recently in single-layer graphene. Unlike conventional Rabi oscillations, anomalous Rabi oscillations are unique to graphene (and possibly to surface states of topological insulators (TIs)), attributable to the pseudospin (conventional spin for TI) degree of freedom and Dirac-fermion character of the graphene system. A pump pulse of a finite duration long enough to contain a large number of cycles induces a current density that oscillates with the frequency of the pump pulse. The amplitude associated with these fast oscillations is seen to exhibit much slower oscillations with a frequency given by ${ 2 \omega ^2_{\mathrm {R}} }/{ \omega } $ – the anomalous Rabi frequency, where ω R is the conventional Rabi frequency and ω is the frequency of the external pump field. This effect is easily probed by a probe pulse subsequent to the pump, where it manifests itself as periodic oscillations of the probe susceptibility as a function of pump duration at each probe frequency. Alternatively, it is also seen as an oscillatory function of the pump–probe delay with other variables remaining fixed. This period corresponds to the anomalous Rabi frequency. An analysis of the previously reported experimental data confirms the presence of anomalous Rabi oscillations in graphene.

Light sterile neutrinos in particle physics: Experimental status

Abstract Most of the neutrino oscillation results can be explained by the three-neutrino paradigm. However several anomalies in short baseline oscillation data, corresponding to an L/E of about 1 m/MeV, could be interpreted by invoking a hypothetical fourth neutrino. This new state would be separated from the three standard neutrinos by a squared mass difference Δ m n e w 2 ∼ 0.1 – 1 eV 2 and would have mixing angles of sin 2 2 θ e e ≳ 0.01 and sin 2 2 θ μ e ≳ 0.001 , in the electron disappearance and appearance channels, respectively. This new neutrino, often called sterile, would not feel standard model interactions but mix with the others. Such a scenario calling for new physics beyond the standard model has to be either ruled out or confirmed with new data. After a brief review of the anomalous oscillation results we discuss the forthcoming laboratory experiments aiming to clarify the situation.

Numerical prediction of an anomalous biased oscillation regime in vortex-induced vibrations of two tandem cylinders

Dynamics of vortex-induced vibrations (VIV) of one and two elastically mounted circular cylinders in tandem is numerically studied via a lattice Boltzmann method. The structural damping is set to zero and the cylinders are of one degree of freedom. The Reynolds number is in the range of Re = [5, 100], covering the sub- and super-critical regions. The effects of reduced velocity UR = [4, 10], center-to-center spacing between the two cylinders S/D = [1.1, 4] and mass ratio M = [0.5, 4] on the cylinder motion and flow structures are investigated. The VIV at subcritical Reynolds number exists extensively for both one- and two-cylinder systems. Besides the traditional symmetrical vibration regime, an anomalous biased vibration regime is observed for the case of two tandem cylinders. It is the first time that the existence of such an asymmetric vibration regime has been confirmed. The biased oscillation regime mainly exists in the region of subcritical Reynolds number, large reduced velocity, small spacing, and low mass ratio. As the reduced velocity is more than 7, such an asymmetric periodic vibration regime even occurs for Reynolds number as low as 10. Based on the positions of the oscillation axes and the flow structures, the biased vibration regime is classified into two subregimes and the mechanisms of the biased vibration subregimes are analysed separately.

Quantum Rabi oscillations in graphene

Graphene has been theoretically shown to exhibit anomalous Rabi oscillations (AROs) far from resonance in addition to conventional Rabi oscillations close to resonance (classical light frequency matches particle-hole frequency). The ARO has been attributed to the pseudospin degree of freedom that is unique to graphene-like systems. In this work, we show the same phenomenon also occurs in the single-photon limit or even in a vacuum. This is to be expected for conventional Rabi oscillations; however, the prediction that AROs also occur in the single-photon situation means that this notion of ARO is robust and not an artifact of approximations. We also study collapse and revival of both conventional and AROs in response to a coherent radiation field and extract the collapse and revival times in both cases.

MODULATIONS IN LOW-TEMPERATURE TRANSIENT REFLECTIVITY MEASUREMENTS

Periodic modulations that appear in the low-temperature transient reflectivity signal of a GaAsP / AlGaAs single quantum well is studied. Similar anomalous oscillations are also reported in layered manganite [K. Kouyama et al., J. Phys. Soc. Jpn.76 (2007) 123702(1–3).] We show that such periodic modulations are caused by changes in the linear reflectivity of the sample during transient reflectivity measurements. Studies carried out on reflectivity of different materials under identical conditions show that these modulations on the true transient reflectivity signal are caused by condensation of residual gases on the surface of quantum well. Methods to obtain reliable transient reflectivity data are also described.

Ionic liquid gated electric-double-layer transistors based on Mg-doped InN epitaxial films

Taking advantages of broad tunability of carrier density in electric-double-layer transistors (EDLTs) with ionic-liquid gating, we demonstrate evidence of parallel conduction from both p-type bulk and n-type surface in Mg-doped InN EDLTs by comparing their transport properties, especially Hall effect, with those in non-doped InN. Large anomalous oscillation in Hall coefficients with decreasing gate bias was observed in Mg-doped samples, which can be well simulated by two-carrier model. Our results provide the proof for the p-type bulk conduction in Mg-doped InN by showing its prominent effects on electrical transport.

Coherent non-linear optical response in SU(2) symmetry broken single and bilayer graphene

Anomalous Rabi oscillations in single and bilayer graphene, in the absence of time-reversal symmetry, are described. The main findings of this work are that intra-layer sublattice space asymmetry has a remarkable effect on anomalous Rabi frequency in single and bilayer graphene, namely it is offset by the asymmetry parameter. However, the conventional Rabi frequency is nearly independent of the asymmetry parameter. Inter-layer asymmetry in bilayer graphene has an even more significant effect on anomalous Rabi frequency. When inter-layer asymmetry is taken into account, the anomalous Rabi frequency versus the external field goes through a minimum. The induced current in the frequency domain in these systems shows a finite threshold behavior even for vanishingly small applied fields. These offset oscillations are attributable to the asymmetry parameter in these systems, and are observable only for weak applied fields. For stronger applied fields these phenomena tend towards those without asymmetry.

Magnetotransport along a boundary: from coherent electron focusing to edge channel transport

We study theoretically how electrons, coherently injected at one point on the boundary of a two-dimensional electron system, are focused by a perpendicular magnetic field B onto another point on the boundary. Using the non-equilibrium Green's function approach, we calculate the generalized four-point Hall resistance Rxy as a function of B. In weak fields, Rxy shows the characteristic equidistant peaks observed in the experiment and explained by classical cyclotron motion along the boundary. In strong fields, Rxy shows a single extended plateau reflecting the quantum Hall effect. In intermediate fields, we find superimposed upon the lower Hall plateaus anomalous oscillations, which are neither periodic in 1/B (quantum Hall effect) nor in B (classical cyclotron motion). The oscillations are explained by the interference between the occupied edge channels, which causes beatings in Rxy. In the case of two occupied edge channels, these beatings constitute a new commensurability between the magnetic flux enclosed within the edge channels and the flux quantum. Introducing decoherence and a partially specular boundary shows that this new effect is quite robust.

Magneto-Phonon Effect in the Longitudinal Magnetoresistance of Bismuth

Anomalous longitudinal Shubnikov-de Haas oscillation of bismuth has been observed and an explanation is made as a new kind of magneto-phonon effect. The phenomenon is that the Shubnikov-de Haas oscillation amplitude increases remarkably with the temperature rise at lower temperature, saturates and decreases slowly at higher temperature. This anomalous effect is explained by considering two relaxation mechanisms for electron orbit, i.e., intra-Landau level and inter-Landau level scatterings due to phonons, and the latter takes the main role in the longitudinal magneto-oscillatory resistance. Phonons responsible for the inter-level scatterings must have equal momenta to the distances between magnetically quantized peripheries of the Fermi surface. These phonons are less excited at lower temperature and give the observed anomalous effect. A numerical estimate is presented.

Quasiperiodic oscillation and possible Second Law violation in a nanosystem

Simulation of a virus-like particle reveals persistent oscillation about a free-energy minimizing structure. For an icosahedral structure of 12 human papillomavirus (HPV) L1 protein pentamers, the period is about 70 picoseconds and has amplitude of about 4Å at 300K and pH 7. The pentamers move radially and out-of-phase with their neighbors. As temperature increases the amplitude and period decrease. Since the dynamics are shown to be friction-dominated and free-energy driven, the oscillations are noninertial. These anomalous oscillations are an apparent violation of the Second Law mediated by fluctuations accompanying nanosystem behavior.

The Perturbed Puma Model

The puma model on the basis of the Lorentz and CPT violation may bring an economical interpretation to the conventional neutrinos oscillation and part of the anomalous oscillations. We study the effect of the perturbation to the puma model. In the case of the first-order perturbation which keeps the (23) interchange symmetry, the mixing matrix element Ue3 is always zero. The nonzero mixing matrix element Ue3 is obtained in the second-order perturbation that breaks the (23) interchange symmetry.

Striatal NMDA receptors gate cortico-pallidal synchronization in a rat model of Parkinson's disease

Anomalous patterns of synchronization between basal ganglia and cortex underlie the symptoms of Parkinson's disease. Computational modeling studies suggest that changes in cortical feedback loops involving trans-striatal and trans-subthalamic circuits bring up this anomalous synchronization. We asked whether striatal outflow synchronizes globus pallidus neurons with cortical activity in a rat model of Parkinson's disease. We found that striatal firing is highly increased in rats with chronic nigrostriatal lesion and that this hyperactivity can be reduced by locally infusing a competitive NMDA receptor antagonist. Moreover, NMDA receptor-dependent striatal output had frequency dependent effects on distinct pathological patterns of cortico-pallidal coupling. Blockade of striatal NMDA receptors almost completely abolished an anomalous ~1Hz cortico-pallidal anti-phase synchronization induced by nigrostriatal degeneration. Moreover, under striatal NMDA receptor blockade, synchronization with 2.5–5Hz cortical oscillations falls to negligible levels and oscillations at 10–20Hz are markedly attenuated, whereas beta synchronization (with a peak at ~26Hz) is marginally reduced. Thus, tonic activation of striatal NMDA receptors allows different forms of anomalous oscillations along the cortico-striato-pallidal axis. Moreover, the frequency dependent effects of NMDA receptors suggest that low and high frequency parkinsonian oscillations stem from partially different mechanisms. Finally, our results may help to reconcile views about the contributions of changes in firing rate and oscillatory synchronization to Parkinson's disease symptoms by showing that they are related to each other.

Magnetoresistance oscillations arising from edge-localized electrons in low-defect graphene antidot-lattices

The so-called zigzag edge of graphenes has localized and strongly spin-polarized electrons. However, magnetoresistance (MR) behavior associated with the edge electrons has not been reported in graphenes. Here, we measure MR of graphene antidot-lattices, honeycomb-like arrays of hexagonal antidots with a large ensemble of hydrogen-terminated and low-defect antidot edges, prepared by a nonlithographic method using nanoporous alumina templates. We find anomalous MR oscillations arising from localized electron spins existing at the antidot edges. These are promising for realization of spintronic devices.

Geochemical evidence of the 8.2 ka event and other Holocene environmental changes recorded in paleolagoon sediments, southeastern Brazil

The paleoclimatic record of Juréia Paleolagoon, coastal southeastern Brazil, includes cyclic and gradual changes with different intensities and frequencies through geological time, and it is controlled by astronomical, geophysical, and geological phenomena. These variations are not due to one single cause, but they result from the interaction of several factors, which act at different temporal and spatial scales. Here, we describe paleoenvironmental evidence regarding climatic and sea level changes from the last 9400calyr BP at the Juréia Paleolagoon — one of the main groups of protected South Atlantic ecosystems. Geochemical evidences were used to identify anomalies from multi-proxy analyses of a paleolagoon sediment core. The anomalies of centennial scale were correlated to climate and transgression–regression cycles from the Holocene period. Decadal scale anomalous oscillations in the Quaternary paleolagoon sediments occur between 9400 and 7500calyr BP, correlated with long- and short-term natural events, which generated high sedimentation rates, mainly between 8385 and 8375calyr BP (10cm/yr). Our results suggest that a modern-day short-duration North Atlantic climatic event, such as the 8.2ka event, could affect the environmental equilibrium in South America and intensify the South American Summer Monsoon.

Diffraction Anomalous Fine Structure study and atomistic simulation of Ge/Si nanoislands

We applied Grazing Incidence Diffraction Anomalous Fine Structure to the study of the structure of Ge dome-shaped nanoislands, grown by Molecular Beam Epitaxy on Si (001) substrates at a temperature of 650°C. We determined the vertical composition of the islands showing the presence of a strong Ge/Si intermixing that is nearly constant from bottom to top. In particular, an abrupt change is found at the substrate interface where the composition switches from pure Si to Ge0.6Si0.4. The analysis of the Diffraction Anomalous Fine Structure oscillations of the spectra is crucial to obtain the true composition profile. We performed atomistic simulations to investigate the role of the strained substrate underneath the dome on the diffraction results and to quantify the resolution of our method. Anomalous Diffraction spectra and Diffraction Anomalous Fine Structure oscillations have been simulated for a real size and real shape cluster including faceting, giving a more detailed data interpretation and understanding of the Ge–Si intermixing mechanism.

Anomalous Bloch oscillations in one-dimensional parity-breaking periodic potentials

We investigate the dynamics of a wave packet in a parity-breaking one-dimensional periodic potential slowly varied in time and perturbed by a linear potential. Parity is broken by considering an asymmetric double well per unit cell. By comparing the prediction of the semiclassical dynamics with the full Schr\"odinger solution, we show that Bloch oscillations are strongly affected by anomalous velocity corrections related to Berry's phase. We characterize how these effects depend on the degree of parity breaking of the potential and on the modulation parameters. We also discuss how to measure the effects of the anomalous velocity in current experiments with noninteracting Bose-Einstein condensates in bichromatic optical lattices, under the effect of gravity.

Growth and characterization of C60/GaAs interfaces and C60 doped GaAs

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by layer-by-layer growth mode. 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. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. C60 uniformly doped and δ-doped GaAs and AlGaAs layers are grown by migration enhanced epitaxy method. Crystalline and electrical characteristics of the layers are investigated by transmission electron microscopy (TEM) and electrochemical capacitance voltage (ECV) measurements. The layers are confirmed to have no defect and well-defined δ-doped structures by TEM measurement, and ECV profiles of C60 δ-doped GaAs and AlGaAs layers suggest that C60 molecules in GaAs and AlGaAs lattice produce deep traps, which can be charged or discharged by applied electrical fields.

Quantum dynamics of a microwave driven superconducting phase qubit coupled to a two-level system

We present an analytical and comprehensive description of the quantum dynamics of a microwave resonantly driven superconducting phase qubit coupled to a microscopic two-level system (TLS), covering a wide range of the external microwave field strength. Our model predicts several interesting phenomena in such an ac driven four-level bipartite system including anomalous Rabi oscillations, high-contrast beatings of Rabi oscillations, and extraordinary two-photon transitions. Our experimental results in a coupled qubit-TLS system agree quantitatively very well with the predictions of the theoretical model.