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Chemical vapor deposition (CVD) is a crucial technique to prepare high-quality graphene because of its controllability. In the research, we perform a systematic computational fluid dynamics numerical investigation on the effect of gas-phase reaction dynamics on the graphene growth in a horizontal tube CVD reactor. The research results indicate that the gas-phase chemical reactions in the CVD reactor are in a nonequilibrium state, as evidenced by the comparison of species mole fraction distributions during the CVD process and under chemical equilibrium conditions. The effect of gas-phase reaction dynamics on the deposition rate of graphene under different conditions is studied, and our research shows that the main causes of change in graphene growth rates under different conditions are gas-phase reaction dynamics and active species transport. The results of numerical simulation agree well with the experimental phenomena. The research results also indicate that, for methane, the main limiting factor of graphene growth is the surface kinetic reaction rate. Conversely, for active species, the main limiting factor of graphene growth is species transport. Our research suggests that the growth rate of graphene can be regulated from the perspective of the gas reaction mechanism. This method has theoretical guiding significance and can be extended to the preparation of large-area graphene. [ABSTRACT FROM AUTHOR]

We present a simple model for a patch antenna based on metamaterial concepts operating in the microwave region with a bandwidth of 1.9-2.6 GHz. The small size and the relatively large operational band make this device exploitable for electromagnetic energy harvesting in urban environments. Describing the antenna by an equivalent circuit composed of a transmission line having right/left hand properties with a RC load, we can properly account for the phase dynamics in correspondence of resonances characterized by a specific handedness. The model is validated measuring the response of the reflection scattering parameter S1,1 for a copper prototype fabricated on a FR4 printed circuit board. [ABSTRACT FROM AUTHOR]

time dependent thermal model for a superconducting constriction based weak-link (WL) is discussed for investigating the deterministic dynamics of its temperature and phase. A new dynamic regime is found where a non-zero voltage exists across the WL, and its temperature stabilizes between the bath temperature and superconductor's critical temperature. This regime exists over a limited bias current range and gives rise to a new hysteretic regime in current-voltage characteristics. We also discuss the effect of fluctuations on the current-voltage characteristics and experimental implications of this dynamic regime. [ABSTRACT FROM AUTHOR]

Analyzes the Fiske resonance of one-dimensional parallel arrays of underdamped Josephson tunnel junctions. Appearance in current voltage characteristics as resonant current singularities; Application of magnetic fields to array cells; Measurements of current step amplitudes; Numerical simulations of the phase dynamics in small parallel arrays.

We have studied both theoretically and experimentally how an LC series circuit connected in parallel to a Josephson junction influences the Josephson dynamics. The presence of the shell circuit introduces two energy scales, which in specific cases, can strongly differ from the plasma frequency of the isolated junction. Josephson junctions were manufactured using the Nb/Al-AlOx/Nb fabrication technology with various on-chip LC shunt circuits. Spectroscopic measurements in the quantum limit show excellent agreement with theory taking into account the shunt inductance and capacitance in the resistively and capacitively shunted junction model. The results clearly show that the dynamics of the system are two-dimensional, resulting in two resonant modes of the system. These findings have important implications for the design and operation of Josephson junction based quantum bits. [ABSTRACT FROM AUTHOR]

We report a systematic study of the transport properties of high critical temperature superconductor (HTS) biepitaxial Josephson junctions in the submicron range. Junction performances point to more uniform and reproducible devices and to better control of d-wave intrinsic properties. Outcomes promote novel insights into the transport mechanisms across grain boundaries and encourage further developments in the control of dissipation in HTS devices. The application of nanotechnology to HTS could be an additional tool to properly engineer the junction properties to match specific circuit design also in view of the integration into hybrid quantum circuits. [ABSTRACT FROM AUTHOR]

We investigate the phase dynamics of moderately damped NbN/AlN/NbN Josephson junctions and we present experimental results on detailed aspects of phase diffusion processes. We measure both single escape and multiple escape and retrapping events obtaining a crossover temperature T* from Kramers to phase diffusion switching. We observe a clear dependence of the crossover temperature T* by the bias current ramp rate, while the damping factor Q remains the same. The measured effect is in strong agreement with theoretical predictions reported by Fenton and Warburton. [ABSTRACT FROM AUTHOR]

The dynamics of laser generated plasma plumes expanding within confining surfaces display a two-phase nature. Early phase enhancement due to hydrodynamic containment results in higher temperatures, densities, and average charges states in comparison to freely expanding plasma plumes. Later phase dynamics result in rapid decay of the plasma plume due to lossy plasma-surface interactions. This paper examines laser plasma generation and expansion within rectangular aluminium cavities biased to high voltages (Vbias = ±9 kV). 'Hydro-electro-dynamic' confinement of the laser plasma plumes and the expansion dynamics were studied via space and time resolved visible emission spectroscopy. The charged confining cavities displayed enhanced emission, higher electron densities (Ne) and longer emission durations compared to those of an unbiased cavity. This behavior is attributed to the influence of the electric fields in the cavity on the charged particle dynamics within the cavity volume. The degree of enhancement depended strongly on the applied polarity. [ABSTRACT FROM AUTHOR]

The magnetization orientation of the polarizing reference layer would fluctuate between the parallel and antiparallel states when the reference layer loses stability. In this paper, a combined analytical and simulation study is presented to predict the magnetic dynamics of the spin valve element with single-domain magnets as the free and reference layers. We acquire a complete phase diagram that includes the normal spin-torque switching and random magnetization fluctuations by tuning the spin torque strength between the free and reference layers. The phase dynamics strongly depends on the magnetization state of the reference layer, showing that the instability of the reference layer could be responsible for the random fluctuation events. [ABSTRACT FROM AUTHOR]

Synchronization is common in both nature and human physiology, often used to illustrate nonlinear dynamics. Interestingly, one can initiate their comprehension of this phenomenon from pure linear systems. In this Tutorial, we begin with a theoretical exploration of coupled oscillators' dynamic behavior, enabling us to discern and contrast the unique attributes of dissipative coupling as opposed to commonly observed coherent coupling. We then examine synchronization in two dissipative coupled linear systems: one with two pendulums mutually linked via the Lenz effect and the other with two RLC oscillators coupled via a resistor. This Tutorial is designed to serve as a concise starting point for researchers interested in exploring synchronization phenomena using a simplified model driven solely by dissipative coupling. [ABSTRACT FROM AUTHOR]

Nonlinear dynamical systems such as coupled oscillators are being actively investigated as Ising machines for solving computationally hard problems in combinatorial optimization. Prior works have established the equivalence between the global minima of the cost function describing the coupled oscillator system and the ground state of the Ising Hamiltonian. However, the properties of the oscillator Ising machine (OIM) from a nonlinear control viewpoint, such as the stability of the OIM solutions, remain unexplored. Therefore, in this work, using nonlinear control-theoretic analysis, we (i) identify the conditions required to ensure the functionality of the coupled oscillators as an Ising machine, (ii) show that all globally optimal phase configurations may not always be stable, resulting in some configurations being more favored over others and, thus, creating a biased OIM, and (iii) elucidate the impact of the stability of locally optimal phase configurations on the quality of the solution computed by the system. Our work, fostered through the unique convergence between nonlinear control theory and analog systems for computing, provides a new toolbox for the design and implementation of dynamical system-based computing platforms. [ABSTRACT FROM AUTHOR]

Portable terahertz (THz) frequency comb sources are highly desired for applications in rotational molecular spectroscopy and sensing. To date, direct THz quantum cascade laser (QCL) frequency comb generation is not achievable at room temperature. However, THz comb generation based on intracavity difference frequency generation (DFG) in mid-infrared (mid-IR) QCLs is a promising alternative. Here, we present a numerical study of THz DFG-QCL comb formation in mid-IR QCLs based on a self-consistent multi-domain simulation approach. The dynamical simulations are performed using our open-source software tool mbsolve, which provides a flexible and efficient codebase for solving the generalized full-wave Maxwell–Bloch equations. Here, DFG in the active region of a dual-wavelength mid-IR QCL is considered for the generation of THz radiation. The mixing process and, thus, THz generation require a high second-order intersubband nonlinear susceptibility in the QCL active region and can be obtained by targeted quantum engineering. The associated nonlinear effects are included in the Hamiltonian of our Maxwell–Bloch simulation approach. All necessary input parameters for the description of the quantum system are determined self-consistently using our in-house ensemble Monte Carlo software tool for stationary carrier transport simulations. Notably, such simulations require a full-wave Maxwell–Bloch solver that does not employ the common rotating wave approximation, as a broadband optical field extending from the THz to the mid-IR region is investigated. Our modeling approach and the obtained simulation results for two THz DFG-QCL comb setups are validated against experimental data, showing reasonable agreement. Furthermore, we obtain a locked frequency modulated comb state for mid-IR and THz regimes. [ABSTRACT FROM AUTHOR]

Understanding the transport of photogenerated charge carriers in semiconductors is crucial for applications in photovoltaics, optoelectronics, and photo-detectors. While recent experimental studies using scanning ultrafast electron microscopy (SUEM) have demonstrated that the local change in the secondary electron emission induced by photoexcitation enables direct visualization of the photocarrier dynamics in space and time, the origin of the corresponding image contrast still remains unclear. Here, we investigate the impact of photoexcitation on secondary electron emissions from semiconductors using a Monte Carlo simulation aided by time-dependent density functional theory. Particularly, we examine two photoinduced effects: the generation of photocarriers in the sample bulk and the surface photovoltage (SPV) effect. Using doped silicon as a model system and focusing on primary electron energies below 1 keV, we found that both the hot photocarrier effect immediately after photoexcitation and the SPV effect play dominant roles in changing the secondary electron yield (SEY), while the distribution of photocarriers in the bulk leads to a negligible change in SEY. Our work provides insights into electron–matter interaction under photo-illumination and paves the way toward a quantitative interpretation of the SUEM contrasts. [ABSTRACT FROM AUTHOR]

Superconducting weak link (WL), acting as a Josephson junction (JJ), is one of the widely used elements in superconductor science and quantum circuits. A hysteretic JJ with robust switching between its superconducting and resistive state is an excellent candidate for single-photon detection. However, the ubiquitous fluctuations in the junction strongly influence the stability of the states and, thus, the transition from one to the other. Here, we present an experimental study of switching statistics of critical and retrapping currents of a JJ based on niobium WL in its hysteretic regime. The mean lifetimes of the two metastable states, namely, the zero-voltage superconducting state and finite-voltage resistive state, are estimated from the distributions. Further, close to the hysteresis crossover temperature, observed telegraphic noise in the time domain due to random switching between the states provides their lifetimes directly. We present a thermal model introducing a double-well (bistable) feature with an intriguing quantity with respect to the devices' temperature states. The effects of temperature fluctuations on the stability of the states are shown. We discuss our results toward further improvement of the efficiency of superconducting WL or nanowire single-photon detectors. [ABSTRACT FROM AUTHOR]

The resistance hysteresis of vanadium dioxide (V O 2 ) is a key feature in revealing mechanisms of a phase transition as well as emerging applications. In this study, a dynamical model based on random-resistor networks is developed to simulate the transport properties of V O 2 thin films. The reversible metal–insulator phase transition of each microscopic domain is captured by a modified Landau-type functional. The proposed model enables analysis of not only the formation of conducting filaments driven by an electric field, but also the thermal-driving reversal curves of resistance hysteresis. It is shown that the appearance of a hysteresis loop as well as the aggregation of metallic domains can be tuned via the interactions of each domain with its neighbors and the substrate. The interaction effects are vital for the persistence of metallic domains, which can re-trigger the insulating-to-metallic transition by a subthreshold voltage bias with the delay time much longer than the transition switching time. These results are in agreement with experimental observations and can be helpful in developing V O 2 -based key components ranging from infrared bolometers to the volatile resistive switches for neuromorphic computing. [ABSTRACT FROM AUTHOR]

Thermal energy storage is being actively investigated for grid, industrial, and building applications for realizing an all-renewable energy world. Phase change materials (PCMs), which are commonly used in thermal energy storage applications, are difficult to design because they require excellent energy density and thermal transport, both of which are difficult to predict from simple physics-based models. In this Perspective, we describe recent advances in the understanding of the equilibrium and transport properties of PCM materials that can help accelerate technology development. We then emphasize how the microscopic phonon picture of both liquids and solids enables a better understanding of novel PCM systems and their predictive power. We then show how this microscopic picture can be used to understand kinetic processes, such as supercooling, and how it can impact the thermal power output in thermal energy storage systems. [ABSTRACT FROM AUTHOR]

Electrothermal instability is responsible for degrading numerous applications of pulsed-power technology, yet the initial conditions from which it grows are not well understood. For the first time, metal surfaces have been tracked from characterization to self-emission. This reveals no clear correlation between non-uniform thermal emissions and surface metallurgical defects or crystallographic grains, while correlations are observed with surface topography for 5N metal but not 6061 metal. For 5N metal, surfaces with average roughness as small as 5 nm still admit thermal perturbations with δ T / T > 0.1. [ABSTRACT FROM AUTHOR]

In plasma–liquid interactions, the phenomenon of induced liquid flow that originates at the plasma–liquid contact point is important in that it influences mass, charge, and heat transport from the source to the surrounding bulk fluid. Such stimulated flows have been observed in 1 atm glows with a liquid anode. Because the plasma contact point in such discharges is patterned, a natural question is what is the relationship between the observed self-organized patterns and the induced flow field? It is, therefore, of great interest to investigate the coupling mechanism between the self-organization patterns in an atmospheric pressure dc helium glow discharge with a liquid anode and the induced flow circulation. Particle imaging velocimetry is used to probe the flow fields in the plane normal and parallel to the plasma–liquid interface. A strong ascending flow with maximum speed up to 1.5 cm/s and circulation vortices nearby are observed in the plane normal to the interface centered at the plasma attachment. The experiment results suggested that the ascending flow is caused by water evaporation and the vortices are formed by viscous stress. With a self-organization pattern formed, the flow structures become non-static and the circulation vortices are observed to periodically form and decay. In the plane parallel to the interface, a strong swirl flow was found to exist only when the plasma attachment is self-organized. The analysis revealed that the driving mechanism could be the electrohydrodynamics force. Averaged flow velocity over time in the field of view was found to scale linearly with increasing input power and increasing liquid conductivity. [ABSTRACT FROM AUTHOR]

Hybrid dynamic systems have recently gained interest with respect to both fundamental physics and device applications, particularly with their potential for coherent information processing. In this perspective, we will focus on the recent rapid developments of magnon-based hybrid systems, which seek to combine magnonic excitations with diverse excitations for transformative applications in devices, circuits, and information processing. Key to their promising potentials is that magnons are highly tunable excitations and can be easily engineered to couple with various dynamic media and platforms. The capability of reaching strong coupling with many different excitations has positioned magnons well for studying solid-state coherent dynamics and exploiting unique functionality. In addition, with their gigahertz frequency bandwidth and the ease of fabrication and miniaturization, magnonic devices and systems can be conveniently integrated into microwave circuits for mimicking a broad range of device concepts that have been applied in microwave electronics, photonics, and quantum information. We will discuss a few potential directions for advancing magnon hybrid systems, including on-chip geometry, novel coherent magnonic functionality, and coherent transduction between different platforms. As a future outlook, we will discuss the opportunities and challenges of magnonic hybrid systems for their applications in quantum information and magnonic logic. [ABSTRACT FROM AUTHOR]

Electromagnetic radiation at frequencies in the terahertz (THz) range is highly interesting for potential applications in biology or medical diagnostics, high-bandwidth communication, security and defence, nondestructive evaluation, and other fields. However, particularly, the frequency range between 0.5 and 1.5 THz has turned out to be difficult to fill with powerful coherent solid-state sources. Suitably patterned structures made of the high critical temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 + δ can operate in this frequency range, and the best devices presently reach an output power around 100 μ W. In this tutorial, we will give an introduction into the basic physics of this type of oscillator and into the challenges that appear. We will explain how the generators are fabricated, which measurement techniques are used for characterization, and which models are used to describe their dynamics. We will also give some examples demonstrating potential applications in the fields of THz imaging and THz spectroscopy. [ABSTRACT FROM AUTHOR]

With the growing demand for wearable computers, there is a need for material systems that can perform computational tasks without relying on external electrical power. Using theory and simulation, we design a material system that “computes” by integrating the inherent behavior of self-oscillating gels undergoing the Belousov–Zhabotinsky (BZ) reaction and piezoelectric (PZ) plates. These “BZ-PZ” units are connected electrically to form a coupled oscillator network, which displays specific modes of synchronization. We exploit this attribute in employing multiple BZ-PZ networks to perform pattern matching on complex multi-dimensional data, such as colored images. By decomposing a colored image into sets of binary vectors, we use each BZ-PZ network, or “channel,” to store distinct information about the color and the shape of the image and perform the pattern matching operation. Our simulation results indicate that the multi-channel BZ-PZ device can detect subtle differences between the input and stored patterns, such as the color variation of one pixel or a small change in the shape of an object. To demonstrate a practical application, we utilize our system to process a colored Quick Response code and show its potential in cryptography and steganography. [ABSTRACT FROM AUTHOR]

The evidence for macroscopic quantum tunneling (MQT) in Josephson junctions at low temperatures has been reassessed. Swept bias escape distributions have been modeled with an algorithm-based simulation and the results compared with data from representative published experiments. Signatures expected of a crossover to MQT are not found in the analyzed data. [ABSTRACT FROM AUTHOR]

We study several parasitic effects on the implementation of a Josephson radiation comb generator based on a dc superconducting quantum interference device (SQUID) driven by an external magnetic field. This system can be used as a radiation generator similarly to what is done in optics and metrology, and allows one to generate up to several hundreds of harmonics of the driving frequency. First we take into account how the assumption of a finite loop geometrical inductance and junction capacitance in each SQUID may alter the operation of the devices. Then, we estimate the effect of imperfections in the fabrication of an array of SQUIDs, which is an unavoidable source of errors in practical situations. We show that the role of the junction capacitance is, in general, negligible, whereas the geometrical inductance has a beneficial effect on the performance of the device. The errors on the areas and junction resistance asymmetries may deteriorate the performance, but their effect can be limited to a large extent by a suitable choice of fabrication parameters. [ABSTRACT FROM AUTHOR]

Water management is one of the key issues in proton exchange membrane fuel cells. Fundamentally, it is related to dynamic behaviors of droplets on a gas diffusion layer (GDL) surface, and consequently they are investigated in this work. A two-dimensional hybrid method is employed to implement numerical simulations, in which the flow field is solved by using the lattice Boltzmann method and the interface between droplet and gas is captured by solving the Cahn-Hilliard equation directly. One or two liquid droplets are initially placed on the GDL surface of a gas channel, which is driven by the fully developed Poiseuille flow. At a fixed channel size, the effects of viscosity ratio of droplet to gas (μ*), Capillary number (Ca, ratio of gas viscosity to surface tension), and droplet interaction on the dynamic behaviors of droplets are systematically studied. By decreasing viscosity ratio or increasing Capillary number, the single droplet can detach from the GDL surface easily. On the other hand, when two identical droplets stay close to each other or a larger droplet is placed in front of a smaller droplet, the removal of two droplets is promoted. [ABSTRACT FROM AUTHOR]

The fundamental function of any oscillator is to produce a waveform with a stable frequency. Here, we show a method of frequency stabilization for spin-torque nano-oscillators (STNOs) that relies on coupling with an adjacent nanomagnet through the magnetic dipole-dipole interaction. It is numerically demonstrated that highly stable oscillations occur as a result of mutual feedback between an STNO and a nanomagnet. The nanomagnet acts as a nonlinear resonator for the STNO. This method is based on the nonlinear behavior of the resonator and can be considered as a magnetic analogue of an optimization scheme in nanoelectromechanical systems. The oscillation frequency is most stabilized when the nanomagnet is driven at a special feedback point at which the feedback noise between the STNO and resonator is completely eliminated. [ABSTRACT FROM AUTHOR]

We propose a scheme to measure both the current-phase relation and differential conductance dI/dV of a superconducting junction, in the normal and the superconducting states. This is done using a dc Superconducting Quantum Interference Device with two Josephson junctions in parallel with the device under investigation and three contacts. As a demonstration, we measure the current-phase relation and dI/dV of a small Josephson junction and a carbon nanotube junction. In this latter case, in a regime where the nanotube is well conducting, we show that the non-sinusoidal current phase relation we find is consistent with the theory for a weak link, using the transmission extracted from the differential conductance in the normal state. This method holds great promise for future investigations of the current-phase relation of more exotic junctions. [ABSTRACT FROM AUTHOR]

The spatio-temporally periodic (STP) potential is interesting in Physics due to the intimate coupling between its time and spatial components. In this paper, we begin with a brief discussion of the dynamical behaviors of a single particle in a STP potential and then examine the dynamics of multiple particles interacting in a STP potential via the electric Coulomb potential. For the multiple particles' case, we focus on the occurrence of bifurcations when the amplitude of the STP potential varies. It is found that the particle concentration of the system plays an important role; the type of bifurcations that occur and the number of attractors present in the Poincaré sections depend on whether the number of particles in the simulation is even or odd. In addition to the nonlinear dynamical approach, we also discuss dependence of the squared fractional deviation of particles' kinetic energy of the multiple particle system on the amplitude of the STP potential which can be used to elucidate certain transitions of states; this approach is simple and useful particularly for experimental studies of complicated interacting systems. [ABSTRACT FROM AUTHOR]

Certain high-Tc cuprate superconductors, which naturally realize a stack of Josephson junctions, thus can be used to generate electromagnetic waves in the terahertz region. A plate-like single crystal with 104 junctions without cavity resonance was proposed to achieve strong radiation. For this purpose, it is required to synchronize the Josephson plasma oscillation in all junctions. In this work, we propose to use two stacks of junctions shunted in parallel to achieve synchronization. The two stacks are mutually synchronized in the whole IV curve, and there is a phase shift between the plasma oscillation in the two stacks. The phase shift is nonzero when the number of junctions in different stacks is the same, while it can be arbitrary when the number of junctions is different. This phase shift can be tuned continuously by applying a magnetic field when all the junctions are connected by superconducting wires. [ABSTRACT FROM AUTHOR]

There has been much recent interest in engineering the phenomenon of synchronization in coupled micro-/nano-scale oscillators for applications ranging from precision time and frequency references to new approaches to information processing. This paper presents descriptive modelling detail and further experimental validation of the phenomenon of mutual synchronization in coupled MEMS oscillators building upon recent experimental validation of this concept by the present authors. In particular, the underlying dependence of the observation of synchronization on system parameters is studied through numerical and analytical modelling while considering essential nonlinearities in both the resonator and circuit domain. Experimental results demonstrating synchronized response are elaborated based on the realization of electrically coupled MEMS resonator based square-wave oscillators. The experimental results on frequency entrainment are found to be in general agreement with results obtained through analytical modeling and numerical simulation. The concept presented here is scalable and could be used to investigate the dynamics of large-arrays of coupled MEMS oscillators. [ABSTRACT FROM AUTHOR]

Reactive gases such as oxygen and water vapor modify the surface morphology of BaO dispenser cathodes and degrade the electron emission properties. For vacuum cathodes operating at fixed temperature, the emission current drops rapidly when oxygen adsorbs on top of the low work function surface. Previous experiments have shown that plasma cathodes are more resistant to oxygen poisoning and can operate with O2 partial pressures one to two orders of magnitude higher than vacuum cathodes before the onset of poisoning occurs. Plasma cathodes used for electric thrusters are typically operated with xenon; however, gas phase barium, oxygen, and tungsten species may be found in small concentrations. The densities of these minor species are small compared with the plasma density, and thus, their presence in the discharge does not significantly alter the xenon plasma parameters. It is important, however, to consider the transport of these minor species as they may deposit on the emitter surface and affect the electron emission properties. In this work, we present the results of a material transport model used to predict oxygen fluxes to the cathode surface by solving the species conservation equations in a cathode with a 2.25mm diameter orifice operated at a discharge current of 15A, a Xe flow rate of 3.7 sccm, and 100 ppm of O2. The dominant ionization process for O2 is resonant charge exchange with xenon ions. Ba is effectively recycled in the plasma; however, BaO and O2 are not. The model shows that the oxygen flux to the surface is not diffusion-limited, therefore, the high resistance to oxygen poisoning observed in plasma cathodes likely results from surface processes not considered here. [ABSTRACT FROM AUTHOR]

We propose a method for inducing magnetization reversal using an AC spin current polarized perpendicular to the equilibrium magnetization of the free magnetic layer. We show that the critical AC spin current is significantly smaller than the corresponding DC one. The effect is understood as a consequence of the underdamped nature of the spin-torque oscillators. It allows to use the kinetic inertia to overcome the residual energy barrier, rather than suppressing the latter by a large spin current. The effect is similar to a swing which may be set into high amplitude motion by a weak near-resonant push. The optimal AC frequency is identified as the upper bifurcation frequency of the corresponding driven nonlinear oscillator. Together with fast switching times it makes the perpendicular AC method to be the most efficient way to realize spin-torque memory valve. [ABSTRACT FROM AUTHOR]

We have studied the effect of metastable, irreversibility induced by repeated thermal cycles on the electric transport and magnetization of polycrystalline samples of La1-xCaxMnO3 (0.48 ≤ x ≤ 0.55) close to charge ordering. With time and thermal cycling (T < 300 K) there is an irreversible transformation of the low-temperature phase from a partially ferromagnetic and metallic to one that is less ferromagnetic and highly resistive for the composition close to charge ordering (x = 0.50 and 0.52). Irrespective of the actual ground state of the compound, the effect of thermal cycling is toward an increase of the amount of the insulating phase. We have observed the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We observed changes in the resistivity and magnetization as the revived metastable state is cycled. The time changes in the magnetization are logarithmic in general and activation energies are consistent with those expected for electron transfer between Mn ions. Changes induced by thermal cycling can be inhibited by applying magnetic field. These results suggest that oxygen nonstoichiometry results in mechanical strains in this two-phase system, leading to the development of frustrated metastable states which relax toward the more stable charge-ordered and antiferromagnetic microdomains. Our results also suggest that the growth and coexistence of phases gives rise to microstructural tracks and strain accommodation, producing the observed irreversibility. [ABSTRACT FROM AUTHOR]

The dynamics of the plasma sheath layer and its velocity enhancement have been studied in a low energy (4.9 kJ) Mather-type plasma focus device. Experiments were performed to study the effect of the Lorentz force variation on the current sheath expansion and movement, as well as the existence of traction between all parts of the sheath layer. Two different shape of anodes (cylindrical and step) along with an axial magnetic probe were used to investigate the effects of various experimental conditions, namely different charging voltages and gas pressures. In order to explore the upper limit of the current sheath velocity, a comparison has been made between the experimental data gathered by the probe and the Lee's computational model. The limitations governing the enhancement of the current sheath velocity that can lead to the deterioration of a good focusing phenomenon were also investigated. The increase of the current sheath velocity due to the usage of the step anode on ion generation and hard x-ray emissions have been demonstrated by means of an ion collector and a hard x-ray detector. [ABSTRACT FROM AUTHOR]

The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects. [ABSTRACT FROM AUTHOR]

This paper presents a mathematical model for determining the stable phase-lock areas of microwave-biased superconductor-normal metal- superconductor Josephson junctions with low normal state resistance and small junction capacitance. The calculations are based on the resistively shunted junction model. The theoretically determined phase-lock areas are in accordance with the experimental results on Nb/HfTi/Nb junctions. [ABSTRACT FROM AUTHOR]

The paper describes the theory and the experimental results for the synthesis of precision waveforms by using the pulse-width modulation of the power of the microwave, which drives the superconductor–insulator–normal metal–insulator–superconductor Josephson array. The synthesized sine waveform has a high precision. Its harmonic content was determined to be less than 0.003%. This degree of harmonic content is in agreement with the theory. [ABSTRACT FROM AUTHOR]

We describe a relatively simple, reliable, and reproducible preparation technique, the precursor oxidation process, for making VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5–7 h at 370–415 °C in 0.2 Torr O2 to form 22–220 nm VO2. The strength [resistivity ratio, RR=ρS/ρM] and sharpness (hysteresis width ΔTC) of T-dependent semiconductor-to-metal hysteretic phase transition in VO2 were our most immediate and relevant quality indicators. In 200-nm-range films, the process was optimized to yield RR=(1–2)×103, ΔTC∼11 °C and absolute resistivity in a semiconducting phase ρS=0.4±0.2 Ω m, close to resistivity in bulk single crystals of VO2. Films were characterized by scanning electron microscopy, atomic force microscopy, grazing-incidence x-ray diffraction, and Raman spectroscopy, and found to be polycrystalline single-phase VO2. We also measured optical reflectivity RT(λ) from 200 to 1100 nm, and Rλ(T) from 20 to 100 °C. RT(λ) measured in thin-film interference structures allowed us to calculate the index of refraction in the two phases, which agrees well with the published data and, together with structural measurements, confirms that our films are essentially pure VO2. The limited study of these films in terms of stability, aging, lithographic processing, and thermal cycling shows that they can be used in applications. [ABSTRACT FROM AUTHOR]

The paper discusses the dynamics of different types of overdamped Josephson junctions biased by a sinusoidal microwave current. The results clarify the theoretical background for optimizing the single junction and the junction array parameters for their use as alternating current (ac) wave form synthesizers. The determined phase lock areas demonstrate the degree of overlap for Shapiro steps [S. Shapiro, Phys. Rev. Lett. 11, 80 (1963)] of different orders as a function of the normalized frequency μ=2πfωc-1, where f is the microwave frequency and ωc is the Josephson junction characteristic frequency. For μ<=0.7, which is a typical operation range for superconductor-insulator-superconductor Josephson junctions in the conventional direct current (dc) voltage standard at the bias frequency range from 10 to 100 GHz, the Shapiro steps overlap completely. For μ>=1.4 [typical for superconductor-normal-superconductor junctions at the same frequency range], the steps do not overlap. This allows for ac voltage synthesis by means of binary arrays and pulse-driven systems. For 0.7<=μ<=1.4 there is a range of partially overlapping Shapiro steps, which allows a rapid switching of the Josephson dc voltages for ac synthesis by microwave power modulation. Experimentally we have achieved this range through properly designed superconductor-insulator-normal-insulator-superconductor junctions. [ABSTRACT FROM AUTHOR]

A time-frequency-domain formalism for analysis of mode-locked quantum dot laser dynamics is developed. A potential for generating subpicosecond pulses in an actively mode-locked construction is predicted. Low chirp pulses are predicted, correlating with a low linewidth enhancement factor calculated. Strong hole burning associated with quantum dot active media is shown to broaden the mode-locking spectra noticeably, leading to shorter pulses. [ABSTRACT FROM AUTHOR]

Metal nanoparticles on oxide matrices have gained a markedly increasing consideration with regard to both scientific and applicative purposes, thanks to the possibility of tailoring the system characteristics by a proper choice of the preparation route and the processing conditions. In the present work, Au/SiO2 nanocomposites were prepared by radio-frequency (rf) sputtering of gold from Ar plasmas on amorphous silica substrates. Particular attention was devoted to the influence of the synthesis parameters on the chemicophysical properties of the final nanosystems. To this regard, both in situ and ex situ characterization techniques were adopted. In particular, laser reflection interferometry was employed for an in situ monitoring of growth processes, while ex situ analyses were specifically dedicated to the investigation of Au/SiO2 nanostructure, chemical composition, optical properties, and surface morphology (glancing-incidence x-ray diffraction, transmission electron microscopy, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, ultraviolet-visible absorption, and atomic force microscopy). The obtained results showed the possibility of tailoring the Au/SiO2 morphology from clusterlike to islandlike systems or continuous films, with consequent modification of the optical properties, by a proper choice of total pressure, rf power, deposition time, and growth temperature. [ABSTRACT FROM AUTHOR]

The first step of this work is the determination of the elastic and inelastic ion-molecule collision cross sections for the main ions (N[sup +, sub 2], O[sub 2, sup +], CO[sub 2, sup +], H[sub 2]O[sup +] and O[sup -]) usually present either in the air or flue gas discharges. The obtained cross section sets, given for ion kinetic energies not exceeding 100 eV, correspond to the interactions of each ion with its parent molecule (symmetric case) or nonparent molecule (asymmetric case). Then by using these different cross section sets, it is possible to obtain the ion swarm data for the different gas mixtures involving N[sub 2], CO[sub 2], H[sub 2]O and O[sub 2] molecules whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in gas mixtures. This also allows us to clearly show that the classical linear approximations usually applied for the ion swarm data in mixtures such as Blanc's law are far to be valid. Then, the ion swarm data are given in three cases of gas mixtures: a dry air (80% N[sub 2], 20% O[sub 2]), a ternary gas mixture (82% N[sub 2], 12% CO[sub 2], 6% O[sub 2]) and a typical flue gas (76% N[sub 2], 12% CO[sub 2], 6% O[sub 2], 6% H[sub 2]O). From these reliable ion swarm data, electrical discharge modeling for a wire to plane electrode configuration has been carried out in these three mixtures at the atmospheric pressure for different applied voltages. Under the same discharge conditions, large discrepancies in the streamer formation and propagation have been observed in these three mixture cases. They are due to the deviations existing not only between the different effective electron-molecule ionization rates but also between the ion transport properties mainly because of the presence of a highly polar molecule such as H[sub 2]O. This emphasizes the necessity to properly consider the ion transport in the discharge modeling. [ABSTRACT FROM AUTHOR]

We have fabricated and characterized double-layer-type electroluminescent devices with the structure of indium-tin-oxide-coated glass/poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (CzEH-PPV)/tris(8-hydroxyquinoline) aluminum (Alq[sub 3])/Li:Al, in which CzEH-PPV was used as a hole transport medium, and neutral and ionized cluster beam deposition (NCBD and ICBD) methods were applied to deposit Alq[sub 3]. The surface morphology observed by atomic force microscopy shows that NCBD and especially ICBD methods are more efficient in producing flat and smooth thin film surfaces in comparison to the conventional physical vapor deposition method. Studies of photoluminescence, electroluminescence (EL), and device characteristics demonstrate that the polymeric thin film is susceptible to ion radiation damage and the NCBD-based devices show better device performance, including lower threshold and turn-on voltages, improved EL intensity-voltage, current density-voltage, and external quantum efficiency (EQE)-current characteristics. In addition, the doping effect of the highly fluorescent dye molecule 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran into the Alq[sub 3] layer reveals a complete energy transfer, color-tuning capability and enhanced EQEs. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Proposes a two-stage two-dimensional gas-dynamic model of laser ablation in an ambient gas atmosphere. Compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas found; Relation between the initial state of laser ablation and the ambient atmosphere.

Investigates the spatial distributions of copper atoms sputtered by He+, Ar+, and Xe+ ions incident. Ionized physical vapor deposition; Estimated forward directed resputtering; Total yield as a function of angle of incidence.

Deals with a study which examined copper oxide films of high chemical and crystal purity grown on magnesium oxide substrates using pulsed laser ablation of copper in conjunction with a pulsed oxidizing gas source. Experimental geometry in which the gas pulse and ablation plume cross close to their respective origins; Analysis of the properties of the films using x-ray diffraction and scanning electron microscopy; Growth rates.

Presents a study that investigated the frequency selection and transient dynamics in single-mode semiconductor lasers with optical feedback. Analysis of the individual switch-on events in the laser; Calculation of the steady-state spectra; Examination of the transient intensity and frequency statistics.

Presents information on a study which proposed a model of the hydrodynamics, kinetics and spectroscopy of the transient species formed following the detonation of lead azide. Results of high-speed photography; Transmission measurements; Chemiluminescence results; Hydrodynamic models.

We present here the influence of the phase transformation on the magnetic and magnetocaloric properties of a typical first order system (MnAs) having mixed-phase state near the transition temperature. We use chemical doping to introduce various field and temperature dependent thermodynamic functions into the system. From the nature of the phase transformation with different thermodynamics and the magnetic entropy behavior treating Maxwell's relation on magnetization data, we modeled the mixed-phase state of the system with magnetization. This lead to distinguish the contribution of entropy influenced by phase transformation obtained from incorrect application of Maxwell's relation on the nonequilibrium system and mixed state and a realistic entropy value can be obtained subjected to simple correction to the experimental nonequilibrium magnetization data and using Maxwell's relation. [ABSTRACT FROM AUTHOR]

We study decoherence processes of an S=1/2 localized spin coupled to conduction band electrons in a metal or a semiconductor via an Ising-like interaction. We derive master equations for the density matrix of the localized spin by tracing out all degrees of freedom in the conduction electron system based on the linked-cluster-expansion technique. It is found that the decoherence occurs more rapidly for the metallic case than for semiconducting case. [ABSTRACT FROM AUTHOR]