Peaks In Oscillations Research Articles

Surface levels of organic conductors in a tilted in-plane magnetic field

Abstract Surface quantum states in quasi-two dimensional organic conductors induced by an external magnetic field tilted in the plane of the layers are obtained and analyzed. In tilted magnetic fields, these states arise from the transitions of the electrons between the closed orbits on the sides of the Fermi surface determined by the electron momentum along the magnetic field direction p B and the coordinate of the center of electron revolution Z. By far, in organic conductors, the surface states have not been studied for tilted magnetic fields. In this work, we have performed detail analyses of the surface states in a tilted in-plane magnetic field by calculating the surface energy spectrum and surface wave functions in order to address their properties and features. We find that, in a tilted magnetic field, the surface levels have higher energies compared to those at zero tilt angle but can be observed only up to a certain tilt angle. The resonant magnetic field and angular values at which the peaks in the surface oscillations should be observed are obtained. Further theoretical and new experimental studies of the surface states in a tilted magnetic field might give new insights into the surface properties of quasi-two dimensional organic conductors. Additionally, they may reveal new information about the parameters of the Fermi surface of organic conductors necessary for its reconstruction.

Effects of delay, noises and fractional-order on the peaks in antiperiodic oscillations in Duffing oscillator

In this paper, we numerically analyze the effects of the time-delay feedback position, power noises, and fractional order on the peaks of antiperiodic oscillations (APOs) in a forced Duffing equation. It is found that the stability of the system depends on the sign of the gain coefficient about the delay. It is also found that the number, the shape, and the amplitude of the peaks of APO depend on the value of the time delay. When the system is subject to a stochastic perturbation, the number of peaks is not periodic. The effects of fractional order on the APOs are also studied. The results show that the number when the value of fractional-order derivative decreases, the number of the peaks of APO also decreases and the APO disappears for a certain value of fractional-order derivative.

Magnetotransport of a two-dimensional electron gas with anisotropic Rashba interaction at the LaAlO3/SrTiO3 interface

We investigate the magnetotransport properties of a two-dimensional electron gas with anisotropic k-cubic Rashba interaction at the $\rm{LaAlO_3}$/$\rm{SrTiO_3}$ interface. The Landau levels and density of states of the system as well as the magnetotransport coefficients are evaluated. A somehow anomalous beating pattern in low magnetic field regime is found both in the density profile and magnetoresistivity. We discuss the impact of electron density, Landau level broadening and Rashba spin-orbit constant on the appearance of the beatings in low magnetic fields and find that at low electron concentrations and not very strong spin-orbit interactions the beatings smooth out. On the other hand, as the magnetic field increases, the Zeeman term becomes the dominant splitting mechanism leading to the spin-split peaks in SdH oscillations. We also show that the observation of the beatings in low magnetic fields needs a system with rather higher carrier concentration so that the beatings persist up to sufficiently large fields where the oscillations are not smoothed out by Landau level broadening. The quantum Hall plateaus are evaluated and we show the Chern number with both even and odd values is replaced by the odd numbers when two subband energies are close with spin degenerate energy levels. Along with the numerical evaluation of the magnetotransport properties, a perturbative calculation is also performed which can be used in the case of low densities and not very large filling factors.

Efficient N2 laser pumped by nanosecond diffuse discharge

Abstract Laser action on C3 Π u → B3 Π g transition of nitrogen pumped by a runaway electron preionized discharge (REP DD) was investigated. Emission parameters of diffuse discharge formed by runaway electrons in gaps between long blade electrodes are studied. It was shown that sufficient uniformity of REP DD allows its application as an efficient pumping source of gas lasers. Model of nitrogen laser on N2 – SF6 mixture pumped by nanosecond diffuse discharge was developed. Results of calculations were close to experimental ones and predicted two laser operation modes. In mixture of SF6 – N2 electrical efficiency of 0.2% was obtained. Ultimate laser efficiency of 0.23% was achieved in triple mixtures He–N 2 – SF6. Lasing on N2 molecules with 2 or 3 peaks in successive discharge current oscillations was obtained for the first time in N2 – SF6 – (He) mixture. It was shown that addition of helium to the mixture allows to change energy and spectral distribution on separate laser peaks. Promising prospects of REP DD employment for pumping nitrogen gas lasers was demonstrated which suggest that diffuse discharge can be efficient for pumping a wide range of gas lasers operating on gas mixtures with electro-negative gases, for example, excimer and non-chain chemical lasers.

Run-away electron preionized diffuse discharge as a source of efficient laser emission in the IR, UV, VUV

REP DD was suggested as an excitation source of various gas lasers. The efficient lasing was obtained in the IR, UV, and VUV spectral ranges. The ultimate intrinsic efficiency of non-chain chemical lasers on HF(DF) molecules was achieved. REP DD pumped N2 laser with an ultimate electrical efficiency of 0.2% was developed. Lasing on N2 molecules with 2 or 3 peaks in successive REP DD current oscillations was obtained for the first time. The laser action on F2* at 157 nm and rare gas fluorides (KrF*, XeF*) under REP DD pumping was obtained for the first time, as well. It has been shown that the volume stage of REP DD in mixtures with fluorine can last over 50 ns during several current half-cycles. Therewith, the efficiency and the pulse duration of lasers on rare gas fluorides and VUV F2* laser parameters under REP DD excitation are comparable with those obtained in suitable transverse discharges. The results allow the conclusion that the REP DD homogeneity in mixtures with F2 and SF6 is high enough for attaining high laser efficiency.

Evidence for massive bulk Dirac fermions in Pb1−xSnxSe from Nernst and thermopower experiments

Topological surface states protected by mirror symmetry are of interest for spintronic applications. Such states were predicted to exist in the rocksalt IV-VI semiconductors, and several groups have observed the surface states in (Pb,Sn)Te, (Pb,Sn)Se and SnTe using photoemission. An underlying assumption in the theory is that the surface states arise from bulk states describable as massive Dirac states, but this assumption is untested. Here we show that the thermoelectric response of the bulk states displays features specific to the Dirac spectrum. By relating the carrier density to the peaks in the quantum oscillations, we show that the first (N=0) Landau level is non-degenerate. This finding provides robust evidence that the bulk states are indeed massive Dirac states. In the lowest Landau level, Sxx displays a striking linear increase versus magnetic field characteristic of massive Dirac fermions. In addition, the Nernst signal displays a sign anomaly in the gap-inverted phase at low temperatures.

Intricate routes to chaos in the Mackey–Glass delayed feedback system

We describe some remarkable continuous deformations which create and destroy peaks in periodic oscillations of the Mackey–Glass equation, a paradigmatic example of a delayed feedback system. Peak creation and destruction results in richer bifurcation diagrams which, in addition to the familiar branches arising from period-doubling and peak-adding bifurcations, may also display arbitrary combinations of doubling and adding, leading to highly complex mosaics of stability domains in control parameter space. In addition, we show that the onset of higher dimensionality does not alter the prevailing dynamics instantaneously and, remarkably, even may have no effect at all, a result that cannot be predicted analytically with standard methods.

Resonant tunneling through edge states bounded around an antidot

Resonant tunneling through edge states bounded around an antidot in the quantum Hall (QH) regime has been investigated and we have found split peaks in a resonant tunneling oscillations through an antidot. We have measured the resistance as a function of magnetic field and front gate voltage and found a periodic oscillation in the ν = 2 to 1 transition regime. It is related to the fact that discrete energy levels are formed in the edge states around the antidot (Science 267 (1995) 1010). Not only single peaks but also some split peaks are seen in our data, which is considered to be related to the electrostatic coupling between the outer and inner edge states.

Glutamate hyperexcitability and seizure-like activity throughout the brain and spinal cord upon relief from chronic glutamate receptor blockade in culture

Cortical structures such as the hippocampus and cerebral cortex are considered to be particularly susceptible to seizures and epileptiform electrical activity and, as such, are the focus of intense investigation relative to hyperexcitability. To determine whether parallel glutamate-mediated hyperexcitability and seizure-like activity in the rat can be generated by neurons irrespective of their origin within the CNS, we maintained cells from the spinal cord, hippocampus, olfactory bulb, striatum, hypothalamus, and cortex in the long-term presence of glutamate receptor antagonists 2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2,3-dione. After removal of chronic (3–11 weeks) glutamate receptor block, whole-cell patch-clamp recordings from current-clamped neurons ( n=94) revealed an immediate increase in large excitatory postsynaptic potentials and a depolarization of 20–35 mV that was often sustained for recording periods lasting 5 min (54% of 66 neurons from all six areas). The intense activity was not seen in age-matched control neurons not subjected to chronic glutamate receptor block. Selective blockade of ionotropic glutamate receptors showed that the hyperexcitability was due to an enhanced response through both AMPA/kainate and N-methyl- d-aspartate receptors. Relief from chronic glutamate receptor block also increased inhibitory activity, as revealed by an increase in inhibitory postsynaptic currents while neurons were voltage-clamped at −25 mV. These inhibitory postsynaptic currents could be blocked with bicuculline, indicating that they were mediated by an enhanced GABA release. This enhanced GABA activity reduced, but did not eliminate, the glutamate-mediated hyperactivity, shown by an increase in both intracellular Ca 2+ and excitatory electrical activity when bicuculline was added. When the glutamate receptor block was removed, cells ( n>1000) from all six regions showed exaggerated Ca 2+ activity, characterized by abnormally high increases in intracellular Ca 2+, rising from basal levels of 50–100 nM up to 150–1600 nM. Cd 2+ eliminated the hyperexcitability by blocking Ca 2+ channels, and reducing excitatory transmitter release and response. Fura-2 digital imaging revealed Ca 2+ oscillations with periods ranging from 4 to 60 s. Ca 2+ peaks in oscillations were synchronized among most neurons recorded simultaneously. That synchronization was dependent on a mechanism involving voltage-dependent Na+ channels was demonstrated with experiments with tetrodotoxin that blocked Ca 2+ rises and synchronous cellular behavior. Removal of the glutamate receptor antagonists resulted in the glutamate-mediated death of 44% of the chronically blocked cells after 23 days of chronic block and 82% cell death after 40 days of chronic block. Nimodipine substantially reduced cell death, indicating that one mechanism responsible for the enhanced cell death after relief from chronic glutamate receptor block was increased intracellular Ca 2+ entry through l-type voltage-gated calcium channels. These data indicate that glutamate is released by neurons from all areas studied, including the spinal cord. Sufficient amounts of glutamate can be released from axon terminals from all areas to cause cell death in postsynaptic neurons. Glutamate-mediated seizure-like activity can be generated not only by hippocampal and cortical neurons, but also by neurons from any of the brain regions tested after chronic deprivation of glutamate receptor stimulation during development. This hyperexcitability is mediated by glutamatergic mechanisms independent of the specific excitatory connections existing in vivo. The epileptiform activity of neurons from one region is indistinguishable from that of another in culture, underlining the importance of synaptic connections in vivo that define the responses characteristic of neurons from different brain regions.

Single neurons are differently involved in stimulus-specific oscillations in cat visual cortex.

Synchronised oscillatory population events (35-80 Hz; 60-300 ms) can be induced in the visual cortex of cats by specific visual stimulation. The oscillatory events are most prominent in local slow wave field potentials (LFP) and multiple unit spikes (MUA). We investigated how and when single cortical neurons are involved in such oscillatory population events. Simultaneous recordings of single cell spikes, LFP and MUA were made with up to seven microelectrodes. Three states of single cell participation in oscillations were distinguished in spike triggered averages of LFP or MUA from the same electrode: (1) Rhythmic states were characterised by the presence of rhythmicity in single cell spike patterns (35-80 Hz). These rhythms were correlated with LFP and MUA oscillations. (2) Lock-in states lacked rhythmic components in single cell spike patterns, while spikes were phase-coupled with LFP or MUA oscillations. (3) During non-participation states LFP or MUA oscillations were present, but single cell spike trains were neither rhythmic nor phase coupled to these oscillations. Stimulus manipulations (from "optimal" to "suboptimal" for the generation of oscillations) often led to systematic transitions between these states (from rhythmic to lock-in to non-participation). Single cell spike coupling was generally associated with negative peaks in LFP oscillations, irrespective of the cortical separation of single cell and population signals (0-6 mm). Our results suggest that oscillatory cortical population activities are not only supported by local and distant neurons with rhythmic spike patterns, but also by those with irregular patterns in which some spikes occur phase-locked to oscillatory events.

Interaction-induced transition at low densities in silicon inversion layer

Intravalley exchange and correlation are found to induce a first-order transition at low densities in the inversion layer, lowering the valley degeneracy. For the n-channel on Si(100) surface, the reduction from two valleys to one occurs roughly below 3 × 10 11 cm −2. The one-valley phase with domain structure yields activated conductivity, Hall coefficient measuring the full density, and extra peaks in Shubnikov-De Haas oscillations. We, therefore, suggest this phase as a possible explanation of the low density behavior, presently ascribed to the Mott-Anderson localization or the pinned Wigner lattice.

Anisotropy of Spin Splitting and the Band Structure Parameters of HgSe from Shubnikov‐De Haas Experiments

AbstractMeasurements are made of the Shubnikov‐de Haas (SdH) effect at 4.2 K up to 180 kOe in HgSe samples with electron concentration in the range 1.7 × 1017 to 2.2 × 1018 cm−3. The theoretical analysis is based on the Pidgeon and Brown model. The spin splitting of SdH oscillations shows an angular dependence. The anisotropy of the spin splitting exceeds 15% between < 100 > and < 111 > direction and depends on energy and the quantum number of the Landau levels. The theory gives satisfactory agreement with the experimental data. A new set of band structure parameters is established. Additional peaks in SdH oscillations, which are not predicted by the model, are also observed.

Absence of spectral peaks in short-period oscillations from the Chilean Earthquake

On the occasion of the great Chilean earthquake of May 1960, various modes of free oscillations of the earth were observed: spheroidal oscillations of wave number n = 2 (period 53 min) to 38 (3.7 min) [Benioff et al., 1961; Ness et al., 1961; Alsop et al., 1961; Bogert, 1961] and torsional oscillations of n up to 24 (5.2 min) [Bolt and Marussi, 1962; Brune et al., 1961]. From a rather negative result that the shorter-period oscillations were not well observed, we try here to deduce some conclusions regarding the earth's upper mantle structure or the oscillation mechanism of the earthquake. The result may be due to horizontal inhomogeneities of the upper mantle of the earth making the resolution of spectral peaks in shorter-period oscillations difficult, or it may be due to weak excitation of shorter-period oscillations by the source.