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Evolution of innovative behaviors on scale-free networks
Ying-Ting Lin, Xiao-Pu Han, Bo-Kui Chen, Jun Zhou, Bing-Hong Wang

Frontiers of Physics. 2018, 13 (4 ): 130308-.
https://doi.org/10.1007/s11467-018-0767-1
Innovation, which involves technological transformation and management reorganization, brings about significant changes in modern society. In this paper, to investigate how innovations can be promoted, we propose a game-based model to study the co-evolutionary dynamics of human innovative behaviors. A simulation on scale-free networks is conducted, in which the innovative behavior of each node is determined and updated based on the feedback regarding its innovation, namely the diffusion of the innovation status. Numerical simulations of the model generate a series of patterns, which is consistent with people’s daily experiences and perceptions as regards real-world innovative behaviors. Specifically, various scaling spatiotemporal properties and rich structural impacts on dynamics can be observed. This model provides a novel approach to understand the evolution of innovative behaviors and provides insight for strategy studies of innovation promotion.

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Self-trapping under two-dimensional spin-orbit coupling and spatially growing repulsive nonlinearity
Rong-Xuan Zhong, Zhao-Pin Chen, Chun-Qing Huang, Zhi-Huan Luo, Hai-Shu Tan, Boris A. Malomed, Yong-Yao Li

Frontiers of Physics. 2018, 13 (4 ): 130311-.
https://doi.org/10.1007/s11467-018-0778-y
We develop a method for creating two- and one-dimensional (2D and 1D) self-trapped modes in binary spin-orbit-coupled Bose–Einstein condensates with the contact repulsive interaction, whose local strength grows sufficiently rapidly from the center to the periphery. In particular, an exact semi-vortex (SV) solution is found for the anti-Gaussian radial modulation profile. The exact modes are included in the numerically produced family of SV solitons. Other families, in the form of mixed modes (MMs), as well as excited states of SVs and MMs, are also produced. Although the excited states are unstable in all previously studied models, they are partially stable in the present one. In the 1D version of the system, exact solutions for the counterpart of SVs, namely, semi-dipole solitons, are also found. Families of semi-dipoles, as well as the 1D version of MMs, are produced numerically.

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All-dielectric bowtie waveguide with deep subwavelength mode confinement
Wen-Cheng Yue, Pei-Jun Yao, Li-Xin Xu, Hai Ming

Frontiers of Physics. 2018, 13 (4 ): 134207-.
https://doi.org/10.1007/s11467-018-0803-1
Plasmonic waveguides and conventional dielectric waveguides have favorable characteristics in photonic integrated circuits. Typically, plasmonic waveguides can provide subwavelength mode confinement, as shown by their small mode area, whereas conventional dielectric waveguides guide light with low loss, as shown by their long propagation length. However, the simultaneous achievement of subwavelength mode confinement and low-loss propagation remains limited. In this paper, we propose a novel design of an alldielectric bowtie waveguide, which simultaneously exhibits both subwavelength mode confinement and theoretically lossless propagation. Contrary to traditional dielectric waveguides, where the guidance of light is based on total internal reflection, the principle of the all-dielectric bowtie waveguide is based on the combined use of the conservation of the normal component of the electric displacement and the tangential component of the electric field, such that it can achieve a mode area comparable to its plasmonic counterparts. The mode distribution in the all-dielectric bowtie waveguide can be precisely controlled by manipulating the geometric design. Our work shows that it is possible to achieve extreme light confinement by using dielectric instead of lossy metals.

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Photon and positron generation by ultrahigh intensity laser interaction with electron beams
Muhammad Ali Bake, Aimierding Aimidula, Arkin Zakir, Nuriman Abdukerim, Abduleziz Ablat

Frontiers of Physics. 2018, 13 (4 ): 135202-.
https://doi.org/10.1007/s11467-018-0788-9
This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particlein- cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.

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Geometric field theory and weak Euler–Lagrange equation for classical relativistic particle-field systems
Peifeng Fan, Hong Qin, Jian Liu, Nong Xiang, Zhi Yu

Frontiers of Physics. 2018, 13 (4 ): 135203-.
https://doi.org/10.1007/s11467-018-0793-z
A manifestly covariant, or geometric, field theory of relativistic classical particle-field systems is developed. The connection between the space-time symmetry and energy-momentum conservation laws of the system is established geometrically without splitting the space and time coordinates; i.e., spacetime is treated as one entity without choosing a coordinate system. To achieve this goal, we need to overcome two difficulties. The first difficulty arises from the fact that the particles and the field reside on different manifolds. As a result, the geometric Lagrangian density of the system is a function of the 4-potential of the electromagnetic fields and also a functional of the particles’ world lines. The other difficulty associated with the geometric setting results from the mass-shell constraint. The standard Euler–Lagrange (EL) equation for a particle is generalized into the geometric EL equation when the mass-shell constraint is imposed. For the particle-field system, the geometric EL equation is further generalized into a weak geometric EL equation for particles. With the EL equation for the field and the geometric weak EL equation for particles, the symmetries and conservation laws can be established geometrically. A geometric expression for the particle energy-momentum tensor is derived for the first time, which recovers the non-geometric form in the literature for a chosen coordinate system.

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Analytical approach to quantum phase transitions of ultracold Bose gases in bipartite optical lattices using the generalized Green’s function method
Zhi Lin, Jun Zhang, Ying Jiang

Frontiers of Physics. 2018, 13 (4 ): 136401-.
https://doi.org/10.1007/s11467-018-0751-9
In order to investigate the quantum phase transitions and the time-of-flight absorption pictures analytically in a systematic way for ultracold Bose gases in bipartite optical lattices, we present a generalized Green’s function method. Utilizing this method, we study the quantum phase transitions of ultracold Bose gases in two types of bipartite optical lattices, i.e., a hexagonal lattice with normal Bose–Hubbard interaction and a d -dimensional hypercubic optical lattice with extended Bose–Hubbard interaction. Furthermore, the time-of-flight absorption pictures of ultracold Bose gases in these two types of lattices are also calculated analytically. In hexagonal lattice, the time-of-flight interference patterns of ultracold Bose gases obtained by our analytical method are in good qualitative agreement with the experimental results of Soltan-Panahi, et al . [Nat. Phys . 7, 434 (2011)]. In square optical lattice, the emergence of peaks at (±$\frac{\mathrm{\pi}}{a}$ ,±$\frac{\mathrm{\pi}}{a}$ ) in the time-of-flight absorption pictures, which is believed to be a sort of evidence of the existence of a supersolid phase, is clearly seen when the system enters the compressible phase from charge-density-wave phase.

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Polarons in alkaline-earth-like atoms with multiple background Fermi surfaces
Jin-Ge Chen, Yue-Ran Shi, Xiang Zhang, Wei Zhang

Frontiers of Physics. 2018, 13 (4 ): 136702-.
https://doi.org/10.1007/s11467-018-0802-2
We study the impurity problem in a Fermi gas of ^{173} Yb atoms near an orbital Feshbach resonance (OFR), where a single moving particle in the ^{3} P _{0} state interacts with two background Fermi seas of particles in different nuclear states of the ground ^{1} S _{0} manifold. By employing wave function ansatz to molecule and polaron states, we investigate various properties of the molecule, the attractive polaron, and the repulsive polaron states. In comparison to the case where only one Fermi sea is populated, we find that the presence of an additional Fermi sea acts as an energy shift between the two channels of the OFR. In addition, quantum fluctuations near the Fermi level can also induce sizable effects to various properties of the attractive and repulsive polarons.

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Tunable electronic structure and magnetic coupling in strained two-dimensional semiconductor MnPSe_{3}
Qi Pei, Xiao-Cha Wang, Ji-Jun Zou, Wen-Bo Mi

Frontiers of Physics. 2018, 13 (4 ): 137105-.
https://doi.org/10.1007/s11467-018-0796-9
The electronic structures and magnetic properties of strained monolayer MnPSe_{3} are investigated systematically via first-principles calculations. It is found that the magnetic ground state of monolayer MnPSe_{3} can be significantly affected by biaxial strain engineering, while the semiconducting characteristics are well-preserved. Owing to the sensitivity of the magnetic coupling towards structural deformation, a biaxial tensile strain of approximately 13% can lead to an antiferromagnetic (AFM)- ferromagnetic (FM) transition. The strain-dependent magnetic stability is mainly attributed to the competition of the direct AFM interaction and indirect FM superexchange interaction between the two nearest-neighbor Mn atoms. In addition, we find that FM MnPSe_{3} is an intrinsic half semiconductor with large spin exchange splitting in the conduction bands, which is crucial for the spin-polarized carrier injection and detection. The sensitive interdependence among the external stimuli, electronic structure, and magnetic coupling makes monolayer MnPSe_{3} a promising candidate for spintronics.

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Electron mass enhancement and magnetic phase separation near the Mott transition in double-layer ruthenates
Jin Peng, X. M. Gu, G. T. Zhou, W. Wang, J. Y. Liu, Yu Wang, Z. Q. Mao, X. S. Wu, Shuai Dong

Frontiers of Physics. 2018, 13 (4 ): 137108-.
https://doi.org/10.1007/s11467-018-0813-z
We present a detailed investigation of the specific heat of Ca3(Ru_{1−x} M_{x} )_{2} O_{7} (M= Ti, Fe, Mn) single crystals. Depending on the dopant and doping level, three distinct regions are present: a quasitwo- dimensional metallic state with antiferromagnetic (AFM) order formed by ferromagnetic bilayers (AFM-b), a Mott insulating state with G-type AFM order (G-AFM), and a localized state with a mixed AFM-b and G-AFM phase. Our specific heat data provide deep insights into the Mott transitions induced by Ti and Mn doping. We observed not only an anomalous large mass enhancement, but also an additional term in the specific heat, i.e., C ∝ T ^{2} , in the localized region. The C ∝ T ^{2} term is most likely due to long-wavelength excitations with both FM and AFM components. A decrease in the Debye temperature is observed in the G-type AFM region, indicating lattice softening associated with the Mott transition.

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Spin-dependent transport properties and Seebeck effects for a crossed graphene superlattice p-n junction with armchair edge
Ben-Hu Zhou, Ben-Liang Zhou, Yang-Su Zeng, Man-Yi Duan, Guang-Hui Zhou

Frontiers of Physics. 2018, 13 (4 ): 137304-.
https://doi.org/10.1007/s11467-018-0770-6
Using the nonequilibrium Green’s function method combined with the tight-binding Hamiltonian, we theoretically investigate the spin-dependent transmission probability and spin Seebeck coefficient of a crossed armchair-edge graphene nanoribbon (AGNR) superlattice p-n junction under a perpendicular magnetic field with a ferromagnetic insulator, where junction widths W1 of 40 and 41 are considered to exemplify the effect of semiconducting and metallic AGNRs, respectively. A pristine AGNR system is metallic when the transverse layer m = 3j + 2 with a positive integer j and an insulator otherwise. When stubs are present, a semiconducting AGNR junction with width W _{1} = 40 always shows metallic behavior regardless of the potential drop magnitude, magnetization strength, stub length, and perpendicular magnetic field strength. However, metallic or semiconducting behavior can be obtained from a metallic AGNR junction with W _{1} = 41 by adjusting these physical parameters. Furthermore, a metal-to-semiconductor transition can be obtained for both superlattice p-n junctions by adjusting the number of periods of the superlattice. In addition, the spin-dependent Seebeck coefficient and spin Seebeck coefficient of the two systems are of the same order of magnitude owing to the appearance of a transmission gap, and the maximum absolute value of the spin Seebeck coefficient reaches 370 μV/K when the optimized parameters are used. The calculated results offer new possibilities for designing electronic or heat-spintronic nanodevices based on the graphene superlattice p-n junction.

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Evolution of individual quantum Hall edge states in the presence of disorder
Kai-Tong Wang, Fuming Xu, Yanxia Xing, Hong-Kang Zhao

Frontiers of Physics. 2018, 13 (4 ): 137306-.
https://doi.org/10.1007/s11467-018-0784-0
By using the Bloch eigenmode matching approach, we numerically study the evolution of individual quantum Hall edge states with respect to disorder. As demonstrated by the two-parameter renormalization group flow of the Hall and Thouless conductances, quantum Hall edge states with high Chern number n are completely different from that of the n = 1 case. Two categories of individual edge modes are evaluated in a quantum Hall system with high Chern number. Edge states from the lowest Landau level have similar eigenfunctions that are well localized at the system edge and independent of the Fermi energy. On the other hand, at fixed Fermi energy, the edge state from higher Landau levels exhibit larger expansion, which results in less stable quantum Hall states at high Fermi energies. By presenting the local current density distribution, the effect of disorder on eigenmode-resolved edge states is distinctly demonstrated.

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Electronic and optical properties of single-layer MoS_{2}
Hai-Ming Dong, San-Dong Guo, Yi-Feng Duan, Fei Huang, Wen Xu, Jin Zhang

Frontiers of Physics. 2018, 13 (4 ): 137307-.
https://doi.org/10.1007/s11467-018-0797-8
The electronic structures of a MoS_{2} monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS_{2} is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS_{2} . Moreover, the strong optical absorption up to 10^{5} cm^{−1} and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS_{2} as an advanced photoelectronic device.

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Quantum confinement effect in β -SiC nanowires
Gang Peng (彭刚), Xiaoyan Yu (于晓燕), Yan-Lan He (何焰兰), Gong-Yi Li (李公义), Yi-Xing Liu (刘一星), Xinfang Zhang (张鑫方), Xue-Ao Zhang (张学骜)

Frontiers of Physics. 2018, 13 (4 ): 137802-.
https://doi.org/10.1007/s11467-018-0768-0
The quantum confinement effect is important in nanoelectronics and optoelectronics applications; however, there is a discrepancy between the theory of quantum confinement, which indicates that band-gap widening occurs only at small sizes, and experimental observations of band-gap widening in large-diameter nanowires (NWs). This paper reports an obvious blue shift of the absorption edge in the UV-visible absorption spectra of SiC NWs with diameters of 50–300 nm. On the basis of quantum confinement theory and high-resolution transmission electron microscopy images of SiC NWs, band-gap widening in SiC NWs with diameters of up to hundreds of nanometers is fully explained; the results could help to explain similar band-gap widening in other NWs with large diameters.

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Vertically aligned γ -AlOOH nanosheets on Al foils as flexible and reusable substrates for NH_{3} adsorption
Chen Yang, Ying Chen, Dan Liu, Jinfeng Wang, Cheng Chen, Jiemin Wang, Ye Fan, Shaoming Huang, Weiwei Lei

Frontiers of Physics. 2018, 13 (4 ): 138101-.
https://doi.org/10.1007/s11467-018-0747-5
Vertically aligned γ -AlOOH nanosheets (NSs) have been successfully fabricated on flexible Al foils via a solvothermal route without morphology-directing agents. Three different reaction temperature (25, 80, and 120 ?C) and time (30 min, 45 min, and 24 h) are discussed for the growth period, which efficiently tune the density and size of the γ -AlOOH NSs. Meanwhile, the growth speed of the nanosheets confirms that dominant growth stage is seen in the initial 45 min. Furthermore, the interlayer of the γ -AlOOH NSs displays an average height of 140 nm and superhydrophilicity. By dynamic adsorption, the assynthesized γ -AlOOH NSs exhibit an outstanding NH_{3} adsorption capacity of up to 146 mg/g and stably excellent regeneration for 5 cycles. The mechanism of NH3 adsorption on the in-plane of the γ -AlOOH NSs is explained by the Lewis acid/base theory. The H-bond interactions among the NH3 molecules and the edge groups (-OH) further improve the capture ability of the nanosheets.

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Two-dimensional materials: Emerging toolkit for construction of ultrathin high-efficiency microwave shield and absorber
Mingjun Hu, Naibo Zhang, Guangcun Shan, Jiefeng Gao, Jinzhang Liu, Robert K. Y. Li

Frontiers of Physics. 2018, 13 (4 ): 138113-.
https://doi.org/10.1007/s11467-018-0809-8
Two-dimensional (2D) materials generally have unusual physical and chemical properties owing to the confined electro-strong interaction in a plane and can exhibit obvious anisotropy and a significant quantum-confinement effect, thus showing great promise in many fields. Some 2D materials, such as graphene and MXenes, have recently exhibited extraordinary electromagnetic-wave shielding and absorbing performance, which is attributed to their special electrical behavior, large specific surface area, and low mass density. Compared with traditional microwave attenuating materials, 2D materials have several obvious inherent advantages. First, similar to other nanomaterials, 2D materials have a very large specific surface area and can provide numerous interfaces for the enhanced interfacial polarization as well as the reflection and scattering of electromagnetic waves. Second, 2D materials have a particular 2D morphology with ultrasmall thickness, which is not only beneficial for the penetration and dissipation of electromagnetic waves through the 2D nanosheets, giving rise to multiple reflections and the dissipation of electromagnetic energy, but is also conducive to the design and fabrication of various well-defined structures, such as layer-by-layer assemblies, core–shell particles, and porous foam, for broadband attenuation of electromagnetic waves. Third, owing to their good processability, 2D materials can be integrated into various multifunctional composites for multimode attenuation of electromagnetic energy. In addition to behaving as microwave reflectors and absorbers, 2D materials can act as impedance regulators and provide structural support for good impedance matching and setup of the optimal structure. Numerous studies indicate that 2D materials are among the most promising microwave attenuation materials. In view of the rapid development and enormous advancement of 2D materials in shielding and absorbing electromagnetic wave, there is a strong need to summarize the recent research results in this field for presenting a comprehensive view and providing helpful suggestions for future development.

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Monolayered semiconducting GeAsSe and SnSbTe with ultrahigh hole mobility
Yu Guo, Nan Gao, Yizhen Bai, Jijun Zhao, Xiao Cheng Zeng

Frontiers of Physics. 2018, 13 (4 ): 138117-.
https://doi.org/10.1007/s11467-018-0810-2
High carrier mobility and a direct semiconducting band gap are two key properties of materials for electronic device applications. Using first-principles calculations, we predict two types of two-dimensional semiconductors, ultrathin GeAsSe and SnSbTe nanosheets, with desirable electronic and optical properties. Both GeAsSe and SnSbTe sheets are energetically favorable, with formation energies of −0.19 and −0.09 eV/atom, respectively, and have excellent dynamical and thermal stability, as determined by phonon dispersion calculations and Born–Oppenheimer molecular dynamics simulations. The relatively weak interlayer binding energies suggest that these monolayer sheets can be easily exfoliated from the bulk crystals. Importantly, monolayer GeAsSe and SnSbTe possess direct band gaps (2.56 and 1.96 eV, respectively) and superior hole mobility (~20 000 cm^{2} ·V^{−1} ·s^{−1} ), and both exhibit notable absorption in the visible region. A comparison of the band edge positions with the redox potentials of water reveals that layered GeAsSe and SnSbTe are potential photocatalysts for water splitting. These exceptional properties make layered GeAsSe and SnSbTe promising candidates for use in future high-speed electronic and optoelectronic devices.

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