International Journal of Modern Physics B

The main component of magnetically selected cesium frequency standard is electron multiplier. Its working principle is to magnify cesium beam signal by emitting secondary electron through surface of material. Magnesium oxide (MgO) has been adopted due to its excellent capability to emit secondary electron. There are many ways to prepare MgO thin film, including evaporation, pulsed laser deposition and MOCVD. It has been studied that doping can be used to enhance its emission capability and decrease its working potential of the thin film. In this paper, Ti-doped MgO thin films were deposited by magnetron sputtering method. Titanium metal and MgO ceramic were co-sputtered in oxygen containing argon ambient with DC and RF power source, respectively. XRD results show that the deposited film has the texture of (100) and (110) orientation. The ratio of Ti/Mg were consistent in XPS and EDS. SEM and AFM show that the film is polycrystalline. Electron multiplier device was assembled, and the secondary electron emission coefficient of the Ti-doped MgO film is greater than that of the undoped one, especially 36% higher at low primary electron energy of 200 eV which is meaningful to cesium atomic frequency standard.

During the development of intelligent transportation systems, traffic data has the characteristics of streaming, high dimension and uncertainty. In order to realize the query of uncertain traffic data streams in a distributed environment, the authors design the algorithm of Uncertain Traffic Data Stream Parallel Continuous Query algorithm (UTDSPCQ). Firstly, the sliding window mode is applied to realize the data receiving and buffering in the data stream environment, so as to adapt to the MapReduce computing framework of the Hadoop distributed structure. Then, the impact of the high dimensionality and uncertainty of the data on the feature analysis of the dataset is reduced, through the dimension reduction and data rewriting. Finally, a multi-attribute data point RePoint is newly defined, to solve the problem of data dimension increase caused by data rewriting. Experiments show that this algorithm optimizes the traditional density-based clustering algorithm, and make it more adaptable to parallel continuous queries for uncertain traffic data streams, and can fully consider the newly generated streaming traffic data.

Terahertz (THz) oscillations in n${}^{+}$nn${}^{+}$ In${}_{0.53}$Ga${}_{0.47}$As diodes have been simulated with the use of a time-dependent hydrodynamic model. Under proper biased voltage and doping concentration, THz self-oscillations show up. The current self-oscillations originate from the formation and propagation of electric field domains in In${}_{0.53}$Ga${}_{0.47}$As diodes. The In${}_{0.53}$Ga${}_{0.47}$As device studied here may be presented as an excellent candidate as a solid-state THz source for monolithic integration.

By combining the density functional theory (DFT) with the nonequilibrium Green’s function, we studied the transmission properties of Si₂C₂ clusters (the linear form M1 and the rhombic form M2). The result shows that the I-V characteristics of the cluster are closely related to the structure of the cluster. The change in the structure of the cluster causes a change in the coupling process between the cluster and the electrode, resulting in a change in its transmission characteristics. Compared with the linear form M1 system, the rhombic form M2 system has obvious rectification and negative differential resistance (NDR) effects, which makes it more advantageous as a candidate for molecular devices.

In many cases, the correlation between time series has a certain lag effect. To study the lag correlation between two time series variables, we select London Metal Exchange (LME) nickel futures and spot prices from 3 January 2008 to 29 December 2017 as sample data to carry out stationarity tests, cointegration tests and Granger causality tests to determine the stationarity and correlation of two time series. Then, we use the method of combining the distributed lag model and sliding window method to construct a network. We select the best sliding window length through a sensitivity test. The time series is reconstructed into a complex network by taking the types of patterns as the nodes and the conduction relationship between the patterns as the edges. The number of transitions between patterns is defined as the weight of the edge. The results show that the spot price changes are caused by the change in nickel futures price and that the optimal sliding window length is 64. Additionally, 12 types of patterns account for a large proportion of the patterns in the network. Six patterns are the main intermediaries of pattern transmission and appear centrally with the change in the market environment. Therefore, the relationship model between these futures and spot prices has remained stable for a long time. Combining the positive and negative news of the market, we identify the timing of the change in the relationship model and can use this approach to improve the accuracy of early warning methods. This study provides a method to construct a complex network using a distributed lag model, which can help analyze two real time series variables with lag correlation.

In this paper, by employing both analytical and numerical methods, global dynamic responses including subharmonic bifurcations and chaos are investigated for a carbon nanotube supported by a Winkler and Pasternak foundation. The criteria of chaos arising from transverse intersections for stable and unstable manifolds of homoclinic orbits are proposed with the Melnikov method. The critical curves separating the chaotic and nonchaotic regions are plotted in the parameter plane. The parameter conditions for subharmonic bifurcations are also obtained by the subharmonic Melnikov method. It is proved rigorously that the route to chaos for this model is infinite subharmonic bifurcations. The stability of subharmonic bifurcations is also studied by the characteristic multipliers. Numerical simulations are given to confirm the analytical results.

Low-frequency and broadband are the critical challenges in real-life applications. Here, we try to tackle the challenges by proposing a reconfigurable acoustic metasurface (AM) composed of the membrane-type metamaterial (MAM) structure of deep sub-wavelength scale. By employing the external air pumping system into each individual unit cell of the AM, the tension of the membrane can be readily tailored by the system with little interference from other unit cells. Two strategies of the constant pressure method (CPM) and constant volume method (CVM) are reported to design the MAM. And the CVM is adopted as the ultimate design strategy by comparing both methods from aspects of the dimension, operating frequency, and structure complexity. In order to validate the low-frequency and broadband performances of the AM, the Airy-like beams and the acoustic converging based on two identical Airy-like beams are introduced and proof-of-concept simulations are performed with the finite element method. The simulated results agree well with the theoretical predictions. Our design provides the little-interference active design method for the low-frequency and broadband AM to manipulate the wave front, and may have practical engineering applications in areas of the aerospace, high-speed train, marine vessel, and power transmission and transformation project.

Solar energy has been widely used as a kind of renewable power generation in recent years. In a photovoltaic (PV) system, polyethylene terephthalate (PET) is the core component of backsheet and affects the reliability of the system. In this paper, the effect of temperature on partial discharge (PD) of PET is studied. As shown by the PD measurement system, the amplitude of PD of PET films decreases significantly with temperature increasing. The mechanism of PD degradation, which is closely related to the operational temperature and the chemical change of the insulation backsheet, has been investigated.

In this paper, we present the multivariate distribution of independent Lévy flight jump diffusion molecules that cause water pollution. We consider that the waiting time of this jump has a Gaussian distribution. Rather than studying the statistical properties of this distribution in water, we estimate the length of the jump distance parameters for each molecule. These estimated jump distances of the molecules are used to predict the proportion of pollution in a large area of the sea.

In social networks, individuals are usually but not exactly divided into communities such that within each community people are friendly to each other while being hostile towards other communities. This is in line with structural balance theory which enables a comprehensive understanding of the stability and tensions of social systems. Yet, there may be some conflicts such as the intra-community negative edges or inter-community positive edges that affect the balancedness of the social system. This raises an interesting question of how to partition a signed network for minimal conflicts, i.e., maximum balancedness. In this paper, by analyzing the relationship between balancedness and spectrum space, we find that each eigenvector can be an indicator of dichotomous structure of networks. Incorporating the leader mechanism, we partition signed networks to maximize the balancedness with top-k eigenvectors. Moreover, we design an optimizing segment to further improve the balancedness of the network. Experimental data both from real social and synthetic networks demonstrate that the spectral algorithm has higher efficiency, robustness and scientificity.

Perovskites CaPd₃B₄O${}_{12}$ (B = Ti, V) are studied theoretically using generalized gradient approximation (GGA), GGA-modified Becke–Johnson (GGA-mBJ), GGA with spin-orbit coupling (GGA + SOC) and hybrid functional (HF) in the domain of density functional theory (DFT). The estimated structural parameters are reliable with the experimentally reported data. Cohesive energy and enthalpy show that these compounds are stable thermodynamically. Bonding nature makes known that the chemical bond between Ca/Ti–O is ionic, Pd/V–O is covalent and Ti/V–Ti/V is metallic. The mechanical properties show that these compounds are stable, elastically anisotropic and ductile in nature. CaPd₃Ti₄O${}_{12}$ is a 2.94 eV direct-wide bandgap semiconductor through GGA-mBJ and consistent with experiments. The optical properties show that CaPd₃Ti₄O${}_{12}$ is a good dielectric material. The dense electronic states and the wide-gap semiconductor nature of CaPd₃Ti₄O${}_{12}$ suggest that it can be used as a good thermoelectric material.

The spatiotemporal synchronous activity of delay coupled chaotic systems in a network is very important to understand the communications through neurons of the human body. In this paper, we present a homogeneous one-dimensional delay coupled neuron model in a complex network. We design a network model in which the links are stochastically updated at every time step. In this model, the nodes of the network are bidirectionally asymmetrically coupled instead of usual symmetric coupling. We further analyze the impacts of dynamic random updating of links and delay term on the synchronization phenomenon of the network. Synchronized fixed points are found in the networks of neurons numerically. Analytically, the stability range of synchronized fixed point and period-2 orbits are determined which are in very good agreement with the numerical simulation results. Moreover, numerical simulation results have been presented and are found to be in very good concurrence with our analytical result. Interestingly, we have found that in our model synchronized period-2 and period-4 dynamics appear for weak coupling which were not observed earlier. In addition, we have plotted the time series data and maximum Lyapunov exponent to confirm the existence of synchronized chaos.

The structures, adsorption mechanisms and electronic attributes of the O₂Fe₃N clusters are calculated at Perdew, Burke and Ernzerhof (PBE) functional. The results show that two O atoms prefer to be located at the Fe–Fe bridge site of Fe–N molecule which form the ground-state O₂Fe₃N cluster, respectively. It means that O₂ molecule is dissociated by Fe₃N molecule. Compared to the isomer (3)–(6), it indicates that an O₂ molecule is preferentially adsorbed on the top site of Fe atom which is close to N atom of the Fe₃N molecule in the vertical direction. The adsorptions of Fe₃N with O₂ are the exothermic before endothermic reaction. All the O₂Fe₃N clusters possess higher kinetic activity. The average spin magnetic moments of the O₂Fe₃N clusters are as follows: isomer (6) $>$ isomer (1) $>$ isomer (3) $>$ isomer (5) $>$ ground-state $>$ isomer (2) $>$ isomer (4). Compared to the external charge transfer of the O₂Fe₃N clusters, the transfer of electrons between 4s and 3d, 4p orbitals in the same atom is significantly higher.

The superposition principle is one of the cornerstones of physics. In low-dimensional systems, it is routinely used to model the potential profile. That is the case of coupled $\delta$-doped quantum wells, for which, several works have studied the transport and optoelectronic properties. However, the Poisson equation determines the potential profile is not linear, and the superposition principle is not at all valid. The aim of this work is to correct some of the inconsistencies of the mentioned models for coupled $\delta$-doped quantum wells. In the framework of Thomas–Fermi approximation, we calculated the potential profile, the wave functions, the energy values and the relative absorption coefficient for the double system compared to an isolated delta system in terms of impurity density and distance between $\delta$-wells. We found a red shifting in the absorption coefficient when the interlayer distances increase, in addition, an enhancement in the absorption coefficient is detected for a specific separation distance. Our results agree with ab-initio calculations reported for the electronic structure.