Excitation Region Research Articles

Spiral waves within a bistability parameter region of an excitable medium

Spiral waves are a well-known and intensively studied dynamic phenomenon in excitable media of various types. Most studies have considered an excitable medium with a single stable resting state. However, spiral waves can be maintained in an excitable medium with bistability. Our calculations, performed using the widely used Barkley model, clearly show that spiral waves in the bistability region exhibit unique properties. For example, a spiral wave can either rotate around a core that is in an unexcited state, or the tip of the spiral wave describes a circular trajectory located inside an excited region. The boundaries of the parameter regions with positive and ‘negative’ cores have been defined numerically and analytically evaluated. It is also shown that the creation of a positive or ‘negative’ core may depend on the initial conditions, which leads to hysteresis of spiral waves. In addition, the influence of gradient flow on the dynamics of the spiral wave, which is related to the tension of the scroll wave filaments in a three-dimensional medium, is studied.

Energy transfer properties of a single-phase Na3Gd(PO4)2:Eu2+,Mn2+ phosphor with excellent thermal stability for w-LEDs.

A high temperature solid state method was used to prepare Na3Gd(PO4)2:Eu2+,Mn2+ phosphors with good thermal stability. The phosphor shows a broadband excitation region of 250-430 nm, which can be matched with the emissions of ultraviolet (UV)/near-ultraviolet (NUV) LED chips for white light emitting diodes (w-LEDs). The energy transfer efficiency is 74.46% from the sensitizer Eu2+ ions to the activator Mn2+ ions, which enhances the intensities of the Na3Gd(PO4)2-based phosphor. In addition, by increasing the Mn2+ doping level in the phosphor, the Na3Gd(PO4)2:Eu2+,Mn2+ phosphor first shows blue light, then turns to white light, and finally emits red light under 365 nm excitation. Besides, the temperature-dependent photoluminescence measurements indicate that the prepared phosphors exhibit good thermal stability. W-LEDs fabricated by combining a 365 nm chip with the Na3Gd(PO4)2:Eu2+,Mn2+ phosphor exhibit bright white light, which has a high color rendering index (CRI) = 91.5, and a relatively low correlated color temperature (CCT) = 5198 K. Moreover, the CIE point is calculated to be at (0.3337, 0.3465), which is located in the white light region. These results indicate that the as-prepared phosphors can be considered as potential candidates for UV/NUV light-excited w-LED applications.

The trade-off between pulse duration and power in optical excitation of midbrain dopamine neurons approximates Bloch’s law

Optogenetic experiments reveal functional roles of specific neurons. However, functional inferences have been limited by widespread adoption of a restricted set of stimulation parameters. Broader exploration of the parameter space can deepen insight into the mapping between selective neural activity and behavior. In this way, characteristics of the activated neural circuit, such as temporal integration, can be inferred. Our objective was to determine whether an equal-energy principle accounts for the interaction of pulse duration and optical power in optogenetic excitation. Six male TH::Cre rats worked for optogenetic (ChannelRhodopsin-2) stimulation of VTA dopamine neurons. We used a within-subject design to describe the trade-off between pulse duration and optical power in determining reward seeking. Parameters were customized for each subject based on behavioral effectiveness. Within a useful range of powers (~12.6–31.6 mW) the product of optical power and pulse duration required to produce a given level of reward seeking was roughly constant. Such reciprocity is consistent with Bloch’s law, which posits an equal-energy principle of temporal summation over short durations in human vision. The trade-off between pulse duration and power broke down at higher powers. Thus, optical power and duration can be adjusted reciprocally for brief durations and lower powers, and power can be substituted for pulse duration to scale the region of excitation in behavioral optogenetic experiments. The findings demonstrate the utility of within-subject and trade-off designs in optogenetics and of parameter adjustment based on functional endpoints instead of physical properties of the stimulation.

Radiation of argon clustered flow particles outside the exciting electron beam

The spectral study results of a supersonic clustered argon flow stimulated emission in the region of particle excitation (on an electron beam) and downstream are presented. Anomalies in the luminescence of the traditional spindle-shaped jet and the flow of heavy clusters (“wake”) are discussed. Possible causes of the observed anomalous phenomena are presented on the basis of the given comparisons of the results obtained.

Precise measurement of the D2 S1(0) vibrational transition frequency

We report the absolute frequency of the vibrational transition in the electronic ground state of D. Deuterium molecules in the state in a molecular beam are excited to using a narrow-linewidth continuous-wave laser stabilised to an optical frequency comb; a strong static electric field in the excitation region induces a transition dipole moment, greatly enhancing the transition strength. Molecules excited to are state-selectively ionised using a pulsed ultraviolet laser and mass-selectively detected. Spectra recorded by scanning the infrared laser frequency are fit with a multi-Gaussian model that accounts for the intensities and relative frequencies of all hyperfine components at the static electric field strength used in the measurement. Zero-field absolute transition frequencies were determined from fits of 28 spectra recorded at nine different field strengths. Statistical uncertainty and systematic uncertainties due to the field plate spacing and relative intensities of the hyperfine components comprise the bulk of the overall uncertainty. We determine an absolute S(0) transition frequency of 94,925,100,487(17) kHz after correcting for the photon recoil and second-order Doppler shifts. This value agrees with other recent experimental measurements and a theoretical prediction, but with a lower fractional uncertainty of only 0.2 ppb.

Decoherent quench dynamics across quantum phase transitions

We present a formulation for investigating quench dynamics across quantum phase transitions in the presence of decoherence. We formulate decoherent dynamics induced by continuous quantum non-demolition measurements of the instantaneous Hamiltonian. We generalize the well-studied universal Kibble-Zurek behavior for linear temporal drive across the critical point. We identify a strong decoherence regime wherein the decoherence time is shorter than the standard correlation time, which varies as the inverse gap above the groundstate. In this regime, we find that the freeze-out time \bar{t}\sim\tau^{{2\nu z}/({1+2\nu z})}t-∼τ2νz/(1+2νz) for when the system falls out of equilibrium and the associated freeze-out length \bar{\xi}\sim\tau^{\nu/({1+2\nu z})}ξ‾∼τν/(1+2νz) show power-law scaling with respect to the quench rate 1/\tau1/τ, where the exponents depend on the correlation length exponent \nuν and the dynamical exponent zz associated with the transition. The universal exponents differ from those of standard Kibble-Zurek scaling. We explicitly demonstrate this scaling behavior in the instance of a topological transition in a Chern insulator system. We show that the freeze-out time scale can be probed from the relaxation of the Hall conductivity. Furthermore, on introducing disorder to break translational invariance, we demonstrate how quenching results in regions of imbalanced excitation density characterized by an emergent length scale which also shows universal scaling. We perform numerical simulations to confirm our analytical predictions and corroborate the scaling arguments that we postulate as universal to a host of systems.

A Drive Topology for High-Speed SRM With Bidirectional Energy Flow and Fast Demagnetization Voltage

A drive topology composed of a T-type single-phase three-level voltage source rectifier (VSR) and a four-level switched reluctance motor (SRM) converter is proposed in this article. The commonly used diode bridge rectifier presenting degraded power quality is replaced by the VSR to drive the SRM converter. Compared with the conventional split-dc converter, the four-level SRM converter of the proposed drive topology adopts one more switch and one more diode for each phase, but two more operation modes are formed. With the double-voltage demagnetization mode of the four-level SRM converter, the tailing time of demagnetization current is significantly reduced, which increases the utilization of inductance increasing region. All the operation modes of this converter can be implemented successfully, even in the multiphase excitation region. Moreover, a centralized control strategy is proposed to regulate the VSR and SRM together, within which the speed and grid-side power factor are regulated by the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis component of grid-side current in VSR, respectively. Most importantly, the bidirectional energy flow is realized. An idea-proofed testbench is constructed. The results obtained from the comparative experiments confirm the validity of the proposed drive topology and its control strategy.

Heat Transfer in Multilayered Thin Film Materials Applied to Photothermal Deflection Spectroscopy

Heat transfer from a modulated laser incident on a multilayered thin film material is theoretically analyzed as a continuum system accounting for thermal contact resistance between the layers and anisotropy of thermal conductivity of the films. Formulations are obtained for the intensity-averaged deflection of a probe beam (PB) laser bounced on the surface as it passes through the thermally excited region of the gas. The PB formulations are the basis for nonintrusively measuring material properties. An algorithm is used to determine property values by matching measured and simulated PB deflections with selected properties as fitting parameters. Experiments are reported to provide data for validating the PB formulations for bulk standard reference and film/substrate materials. For the latter, a virtual film bonded to a substrate of the same material was used. Results show that predicted and measured PB deflections are in good agreement for thin films when the modulation frequency positions the thermal penetration depth inside the film. It is also shown that for increasing thermal contact resistance, anisotropy of thermal conductivity of the film materials, and modulated beam radius and frequency, the PB deflections are reduced through the effect of these variables on heat flow in the solid and gas.

The Isoscalar Monopole Strength of $$^{13}$$C

To identify the 3alpha BEC state with the excess neutron, we have investigated the monopole strength of the excited states of 13C by using the theoretical framework of the real-time evolution method. The calculations have revealed several candidates of the Hoyle-analog states in a highly excited region.

Selective excitation CARS spectroscopy: An efficient method for analysis of vibrational coupling and intramolecular vibrational energy transfer

Selective excitation of the high-frequency CH stretching vibrational modes around 3000 cm−1 and the low-frequency vibrational modes around 1500 cm−1 of toluene in the narrow femtosecond time- and frequency-resolved CARS spectrum induces the transfer of the intramolecular vibrational energy from the vibrational modes in the selectively-excited region to the vibrational modes outside the excitation region. The vibrational energy transfer from high-frequency parent modes (vibrational modes are selectively excited by pump and Stokes laser pulses) to low-frequency daughter modes (vibrational modes are indirectly excited by energy transfer) via vibrational coupling and the dynamic information of vibrational modes involved in coupling are identified by fast Fourier Transformation (FFT) analysis. The lifetime of the vibrational modes is described, and the energy transfer pathway is confirmed. When the majority of toluene vibrational modes are collectively excited in the multiplex femtosecond time- and frequency-resolved CARS, the vibrational coupling information can be traced from the time-domain CARS spectrum and its frequency-domain FFT power spectrum. Vibrational energy transfer cannot be detected in the multiplex CARS spectrum of toluene. According to experimental results from the selective excitation CARS spectrum and multiplex CARS spectrum, the selective excitation CARS spectroscopy is an efficient method to track intramolecular vibrational energy transfer from parent modes to daughter modes and identify the energy transfer pathway.

Quenching effect of upper laser level lifetime of Fe2+ ions in ZnSe by free electrons

We theoretically and experimentally examined the assumption that the lifetime excited state of Fe2+ in ZnSe:Fe crystal is quenched by free electrons. The effect of the electron beam current value on the temporal characteristics of the luminescence was studied. Three variants, including a sample in the electronic circuit in the case of Fe2+ ions luminescence excitation in a ZnSe:Fe crystal by a beam of accelerated electrons, were first considered theoretically. These variants differ in external electric field magnitude in the sample, which affects the concentration of free electrons in the excited region of the sample. Equations describing the time dependence of the sample's electron density were derived. The luminescence decay kinetics of Fe2+ ions in ZnSe:Fe crystal excited by electron pulse with different electron beam current values and short-pulse optical radiation were obtained. The values of the quantities that determine the temporal characteristics of the luminescence decay depending on the electron beam current were found.

COMBINED CONTROL OF VEHICLE SUSPENSION WITH FRACTIONAL-ORDER MAGNETORHEOLOGICAL FLUID DAMPER MODEL

The fractional-order Bingham model of magnetorheological fluid damper has simple structure and can better describe the hysteretic characteristics of the system. The vibration control of a nonlinear vehicle suspension system with magnetorheological fluid damper under harmonic excitation is studied, where the single-degree-of-freedom 1/4 vehicle suspension system with fractional-order Bingham model of magnetorheological fluid damper is considered. The primary resonance response of suspension system with fractional-order Bingham model under sky-hook damping semi-active control is analyzed, and the approximate analytical solution is obtained by means of averaging method. The amplitude-frequency response equation of the steady-state solution of the suspension system is obtained, and the stability condition of the suspension system is also obtained according to Lyapunov’s stability theory. By comparing the amplitude-frequency response curves of the numerical solution and approximate analytical solution, the accuracy of the approximate analytical solution has been verified. The influence of semi-active control on the ride comfort of the vehicle is illustrated by the root mean square values of acceleration of the sprung mass in the vertical direction, it is found that the semi-active control strategy of sky-hook damping can not improve the ride comfort of vehicle in low frequency excitation region of road. Therefore, a combined control strategy of passive control and semi-active control is proposed, and the influence of semi-active control parameter on the vibration control effect is analyzed. The results show that the combined control strategy can not only improve the ride comfort of the vehicle, but also effectively suppress the primary resonance vibration amplitude of suspension system.

Intensity distribution, evolution, and dispersion of discrete spin wave modes in nanoscale spin-torque oscillator

Spin wave dynamics form the foundation of spin-torque oscillator (STO) modulation. In addition to the uniform spin wave mode with wave vector k = 0, multiple spin wave modes with different wave vectors and frequencies coexist in the nanoscale STO. To characterize these spin wave modes and the interactions between them, the distribution and evolution of spin wave intensity in k space need to be investigated together with spin wave dispersion of the individual mode, stressing energy degeneracy. To this end, using micromagnetic simulation, we studied the dynamics of the discrete spin wave modes in a nanoscale STO with structure enhancing spin wave propagation. Simulation using the object oriented micromagnetic framework demonstrated that although they were generated with different spin currents, spin wave modes evolved similarly in k space, according to the wave vector. Furthermore, magnetization pinning at the corners of yttrium iron garnet led to two-magnon scattering. This interaction affects spin wave dynamics in two ways: multivalued dispersion occurs with two-magnon scattering and the uniform mode can become weaker than the near-uniform modes (spin wave modes with wave vector close to zero) in the strong excitation region. The latter phenomenon is supported by the results of studies on the spin wave dispersion of the individual mode, which demonstrate the energy degeneracy between the uniform mode and the near-uniform modes.

Regularities of Temperature Changes in IR Reflection Spectra of a Bi0.8Sb1.2Te3 Crystal in the Region of Excitation of Free-Carrier Plasma Oscillations and Interband Transitions

Regularities of Temperature Changes in IR Reflection Spectra of a Bi0.8Sb1.2Te3 Crystal in the Region of Excitation of Free-Carrier Plasma Oscillations and Interband Transitions

Fast widefield scan provides tunable and uniform illumination optimizing super-resolution microscopy on large fields

Non-uniform illumination limits quantitative analyses of fluorescence imaging techniques. In particular, single molecule localization microscopy (SMLM) relies on high irradiances, but conventional Gaussian-shaped laser illumination restricts the usable field of view to around 40 µm × 40 µm. We present Adaptable Scanning for Tunable Excitation Regions (ASTER), a versatile illumination technique that generates uniform and adaptable illumination. ASTER is also highly compatible with optical sectioning techniques such as total internal reflection fluorescence (TIRF). For SMLM, ASTER delivers homogeneous blinking kinetics at reasonable laser power over fields-of-view up to 200 µm × 200 µm. We demonstrate that ASTER improves clustering analysis and nanoscopic size measurements by imaging nanorulers, microtubules and clathrin-coated pits in COS-7 cells, and β2-spectrin in neurons. ASTER’s sharp and quantitative illumination paves the way for high-throughput quantification of biological structures and processes in classical and super-resolution fluorescence microscopies.

Hunger-promoting AgRP neurons trigger an astrocyte-mediated feed-forward autoactivation loop in mice.

Hypothalamic feeding circuits have been identified as having innate synaptic plasticity, mediating adaption to the changing metabolic milieu by controlling responses to feeding and obesity. However, less is known about the regulatory principles underlying the dynamic changes in agouti-related protein (AgRP) perikarya, a region crucial for gating of neural excitation and, hence, feeding. Here we show that AgRP neurons activated by food deprivation, ghrelin administration, or chemogenetics decreased their own inhibitory tone while triggering mitochondrial adaptations in neighboring astrocytes. We found that it was the inhibitory neurotransmitter GABA released by AgRP neurons that evoked this astrocytic response; this in turn resulted in increased glial ensheetment of AgRP perikarya by glial processes and increased excitability of AgRP neurons. We also identified astrocyte-derived prostaglandin E2, which directly activated - via EP2 receptors - AgRP neurons. Taken together, these observations unmasked a feed-forward, self-exciting loop in AgRP neuronal control mediated by astrocytes, a mechanism directly relevant for hunger, feeding, and overfeeding.

Flogging a Dead Salmon? Reduced Dose Posterior to Prostate Correlates With Increased PSA Progression in Voxel-Based Analysis of 3 Randomized Phase 3 Trials

Flogging a Dead Salmon? Reduced Dose Posterior to Prostate Correlates With Increased PSA Progression in Voxel-Based Analysis of 3 Randomized Phase 3 Trials

Capturing structural changes of the S1 to S2 transition of photosystem II using time-resolved serial femtosecond crystallography.

Photosystem II (PSII) catalyzes light-induced water oxidation through an S i -state cycle, leading to the generation of di-oxygen, protons and electrons. Pump-probe time-resolved serial femtosecond crystallography (TR-SFX) has been used to capture structural dynamics of light-sensitive proteins. In this approach, it is crucial to avoid light contamination in the samples when analyzing a particular reaction intermediate. Here, a method for determining a condition that avoids light contamination of the PSII microcrystals while minimizing sample consumption in TR-SFX is described. By swapping the pump and probe pulses with a very short delay between them, the structural changes that occur during the S1-to-S2 transition were examined and a boundary of the excitation region was accurately determined. With the sample flow rate and concomitant illumination conditions determined, the S2-state structure of PSII could be analyzed at room temperature, revealing the structural changes that occur during the S1-to-S2 transition at ambient temperature. Though the structure of the manganese cluster was similar to previous studies, the behaviors of the water molecules in the two channels (O1 and O4 channels) were found to be different. By comparing with the previous studies performed at low temperature or with a different delay time, the possible channels for water inlet and structural changes important for the water-splitting reaction were revealed.

Open-aperture Z-scan study for absorption saturation: accurate measurement of saturation intensity in YLF:Yb for optical refrigeration

Knowledge of saturation intensity of gain or absorption plays a fundamental role in a variety of applications ranging from lasers to many nonlinear optical processes. Here, we present an analytical expression for open-aperture Z-scan transmission for accurately measuring the saturation intensity in the low absorbance samples but at arbitrary pump intensities. We exploit this formalism to investigate the absorption saturation of LiYF4:Yb3+ (YLF:Yb) in the anti-Stokes excitation region for optical refrigeration at high pump intensities. An absorption saturation intensity of 14.5±1kW/cm2 was measured in YLF:Yb at 1020 nm (E||c) at room temperature.

Luminescence Semiconductor Quantum Dots in Chemical Analysis

Luminescence semiconductor quantum dots (QDs) offer a rather new type of luminescent labels used for the visual and instrumental determination of analytes in various versions of chemical analysis. The high quantum yield, photostability, narrow emission spectra, and wide excitation regions of these species make them ideal labels for analysis and ensure the development of procedures for the simultaneous determination of several substances and the attainment of low limits of detection. The widespread introduction of QDs in analytical practice is limited by the multistage character and laboriousness of some approaches to the synthesis of QDs and reagents on their basis, their trend to nonspecific interactions, and a probability of fluorescence reabsorption at the improper selection of the concentration of QDs. The review covers the main approaches to the synthesis, modification, and use of luminescence QDs in chemical analysis. Approaches to the sensitization and multiplicity of analysis and methods for regulating selectivity based on both the properties of QDs and, in addition, the receptors used are discussed.