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Cover illustration

Volume 9, Number 2

The amplitudes of different pairing gap components induced by J1, J2 with J2'=0 at electron doping =0.51. sNNN is the s wave resulting from NNN AFM coupling, sx2+y2 and dx2-y2 from NN AFM coupling. The pairing strength of sNNN is peaked around the doping level x = 0.5, which is consistent with experimental observation. sNNN is also very robust against spin-orbital coupling. Moreover, it can be distinguished from conventional s-wave pairing by measuring and comparing superconducting gaps of different Fermi surfaces. For more details, please refer to the article “Pairing symmetry in layered BiS2 compounds driven by electron–electron correlation” by Yi Liang et al., pp 194–199. [Photo credits: Yi Liang, Institute of Physics, Chinese Academy of Sciences]

Review Article

Quantum tunneling of ultracold atoms in optical traps

Jian-Hua Wu, Ran Qi, An-Chun Ji, Wu-Ming Liu

We review our theoretical advances in quantum tunneling of Bose–Einstein condensates in optical traps and in microcavities. By employing a real physical system, the frequencies of the pseudo Goldstone modes in different phases between two optical traps are studied respectively, which are the crucial feature of the non-Abelian Josephson effect. When the optical lattices are under gravity, we investigate the quantum tunneling in the “Wannier–Stark localization” regime and “Landau– Zener tunneling” regime. We finally get the total decay rate and the rate is valid over the entire range of temperatures. At high temperatures, we show how the decay rate reduces to the appropriate results for the classical thermal activation. At intermediate temperatures, the results of the total decay rate are consistent with the thermally assisted tunneling. At low temperatures, we obtain the pure quantum tunneling ultimately. And we study the alternating-current and direct-current (ac and dc) photonic Josephson effects in two weakly linked microcavities containing ultracold two-level atoms, which allows for direct observation of the effects. This enables new investigations of the effect of many-body physics in strongly coupled atom-cavity systems and provides a strategy for constructing novel interference devices of coherent photons. In addition, we propose the experimental protocols to observe these quantum tunneling of Bose–Einstein condensates.


Front. Phys., 2014, 9(2): 137
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Research Article

Phase regulated suppression and enhancement switches of four-wave mixing and fluorescence

Zhi-Guo Wang, Peng Ying, Pei-Ying Li, Hua-Yan Lan, He-Qing Huang, Hao Tian, Jian-Ping Song, Yan-Peng Zhang

We experimentally study the phase regulated switch between electromagnetically induced transparency and electromagnetically induced absorption in probe transmission signal and the conversion between enhancement and suppression in four-wave mixing and fluorescence signals for the first time. By changing the relative phase, electromagnetically induced transparency can be converted into electromagnetically induced absorption. In this process, the conversion from suppression to enhancement is also obtained in four-wave mixing and fluorescence signals. This research can be applied in non-linear optical device like optical switch and optical wavelength convertor.

Front. Phys., 2014, 9(2): 153
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Research Article

Electron-positron pair production in a strong asymmetric laser electric field

Obulkasim Oluk, Bai-Song Xie, Muhmmad Ali Bake, Sayipjamal Dulat

By solving the quantum Vlasov equation, electron–positron pair production in a strong electric field with asymmetric laser pulses has been investigated. We consider three different situations of subcycle, cycle and supercycle laser pulses. It is found that in asymmetric laser pulse field, i.e., when the pulse length of one rising or falling side is fixed while the pulse length of the other side is changed, the pair production rate and number density can be significantly modified comparable to symmetric situation. For each case of these three different cycle pulses, when one side pulse length is constant and the other side pulse length becomes shorter, i.e., the whole pulse is compressed, the more pairs can be produced than that in the vice versa case, i.e., the whole pulse is elongated. In compressed pulse case there exists an optimum pulse length ratio of asymmetric pulse lengths which makes the pair number density maximum. Moreover, the created maximum pair number density by subcycle pulse is larger than that by cycle or/and supercycle pulse. In elongated pulse case, however, only for supercycle laser pulse the created pairs is enhanced and there exists also an optimum asymmetric pulse length ratio that maximizes the pair number density. On the other hand, surprisingly, in both cases of subcycle and cycle elongated laser pulses, the pair number density is monotonically decreasing as the asymmetry of pulse increases.

Front. Phys., 2014, 9(2): 157
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Research Article

Anharmonic effect of adiabatic quantum pumping

Wei-Yin Deng, Ke-Ju Zhong, Rui Zhu, Wen-Ji Deng

Based on the scattering matrix approach, we systematically investigate the anharmonic effect of the pumped current in double-barrier structures with adiabatic time-modulation of two sinusoidal AC driven potential heights. The pumped current as a function of the phase difference between the two driven potentials looks like to be sinusoidal, but actually it contains sine functions of double and more phase difference. It is found that this kind of anharmonic effect of the pumped current is determined combinedly by the Berry curvature and parameter variation loop trajectory. Therefore small ratio of the driving amplitude and the static amplitude is not necessary for harmonic pattern in the pumped current to dominate for smooth Berry curvature on the surface within the parameter variation loop.

Front. Phys., 2014, 9(2): 164
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Review Article

Single photon sources with single semiconductor quantum dots

Guang-Cun Shan, Zhang-Qi Yin, Chan Hung Shek, Wei Huang

In this contribution, we briefly recall the basic concepts of quantum optics and properties of semiconductor quantum dot (QD) which are necessary to the understanding of the physics of single-photon generation with single QDs. Firstly, we address the theory of quantum emitter-cavity system, the fluorescence and optical properties of semiconductor QDs, and the photon statistics as well as optical properties of the QDs. We then review the localization of single semiconductor QDs in quantum confined optical microcavity systems to achieve their overall optical properties and performances in terms of strong coupling regime, efficiency, directionality, and polarization control. Furthermore, we will discuss the recent progress on the fabrication of single photon sources, and various approaches for embedding single QDs into microcavities or photonic crystal nanocavities and show how to extend the wavelength range. We focus in particular on new generations of electrically driven QD single photon source leading to high repetition rates, strong coupling regime, and high collection efficiencies at elevated temperature operation. Besides, new developments of room temperature single photon emission in the strong coupling regime are reviewed. The generation of indistinguishable photons and remaining challenges for practical single-photon sources are also discussed.

Front. Phys., 2014, 9(2): 170
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Research Article

Pairing symmetry in layered BiS2 compounds driven by electron–electron correlation

Yi Liang, Xianxin Wu, Wei-Feng Tsai, Jiangping Hu

We investigate the pairing symmetry of layered BiS2 compounds by assuming that electron-electron correlation is still important so that the pairing is rather short range. We find that the extended s-wave pairing symmetry always wins over d-wave when the pairing is confined between two short range sites up to next nearest neighbors. The pairing strength is peaked around the doping level x = 0.5, which is consistent with experimental observation. The extended s-wave pairing symmetry is very robust against spin–orbital coupling because it is mainly determined by the structure of Fermi surfaces. Moreover, the extended s-wave pairing can be distinguished from conventional s-wave pairing by measuring and comparing superconducting gaps of different Fermi surfaces.

Front. Phys., 2014, 9(2): 194
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Research Article

Effects of various defects on the electronic properties of single-walled carbon nanotubes: A first principle study

Qing-Xiao Zhou, Chao-Yang Wang, Zhi-Bing Fu, Yong-Jian Tang, Hong Zhang

The geometries, formation energies and electronic band structures of (8, 0) and (14, 0) single-walled carbon nanotubes (SWCNTs) with various defects, including vacancy, Stone–Wales defect, and octagon–pentagon pair defect, have been investigated within the framework of the density-functional theory (DFT), and the influence of the concentration within the same style of defect on the physical and chemical properties of SWCNTs is also studied. The results suggest that the existence of vacancy and octagon–pentagon pair defect both reduce the band gap, whereas the SW-defect induces a band gap opening in CNTs. More interestingly, the band gaps of (8, 0) and (14, 0) SWCNTs configurations with two octagon–pentagon pair defect presents 0.517 eV and 0.163 eV, which are a little smaller than the perfect CNTs. Furthermore, with the concentration of defects increasing, there is a decreasing of band gap making the two types of SWCNTs change from a semiconductor to a metallic conductor.

Front. Phys., 2014, 9(2): 200
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Research Article

First-principles study on the structure, electronic and magnetic properties of HoSin (n = 1–12, 20) clusters

Tai-Gang Liu, Wen-Qing Zhang, Yan-Li Li

Abstract: The structure, electronic and magnetic properties of HoSin (n = 1−12, 20) clusters have been widely investigated by first-principles calculation method based on density functional theory (DFT). From our calculation results, we find that for HoSin (n = 1−12) clusters except n = 7, 10, the most stable structures are a replacement of Si atom in the corresponding pure Sin+1 clusters by Ho atom. The doping of Ho atom makes the stability of Si clusters enhance remarkably, and HoSin (n = 2, 5, 8, 11) clusters are more stable than their neighboring clusters. The magnetic moment of Ho atom in HoSin (n = 1− 12, 20) clusters mainly comes from 4f electron of Ho, and never quenches.

Front. Phys., 2014, 9(2): 210
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Research Article

Concentration-dependent crystal structure, elastic constants and electronic structure of ZrxTi1–x alloys under high pressure

Xiao-Li Yuan, Mi-An Xue, Wen Chen, Tian-Qing An

The physical properties of ZrxTi1–x (x = 0.0, 0.33, 0.5, 0.67, 0.75 and 1.00) alloys were simulated by virtual crystal approximation (VCA) methods which is generally used for disordered solid solutions modeling. The elastic constant, electronic structure and thermal Equation of state (EOS) of disordered ZrxTi1–x alloys under pressure are investigated by plane-wave pseudo-potential method. Our simulations reveal increasement of variations of the calculated equilibrium volumes and decreasement of Bulk modulus as a function of the alloy compositions. Lattice parameters a and c of alloys with different Zr concentrations decrease linearly with pressure increasing, but the c/avalues are increasing as pressure increases, indicating no phase transitions under pressure from 0 GPa to 100 GPa. The elastic constants and the Bulk modulus to the Shear modulus ratios (B/G) indicate good ductility of Zr, Zr0.33Ti0.67, Zr0.5Ti0.5, Zr0.75Ti0.25 and Ti, but the Zr0.67Ti0.33 alloy is brittle under 0 K and 0 GPa. The metallic behavior of these alloys was also proved by analyzing partial and total DOS.

Front. Phys., 2014, 9(2): 219
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Research Article

Current neutralization and plasma polarization for intense ion beams propagating through magnetized background plasmas in a two-dimensional slab approximation

Zhang-Hu Hu, Mao-Du Chen, You-Nian Wang

A two-dimensional electromagnetic Particle-in-Cell (PIC) simulation model is proposed to study the propagation of intense ion beams with beam width wb small compared to the electron skin depth c/ωpe through background plasmas in the presence of external applied magnetic fields. The effective electron gyroradius wge is found to be an important parameter for ion beam transport in the presence of magnetic fields. In the beam regions, the background plasmas respond differently to the ion beam of width wb < wge and wb > wge for the given magnetic field and beam energy. For the case of beam width wb < wge with relative weak external magnetic fields, the rotation effects of plasma electrons are found to be significant and contributes to the significant enhancement of the self-electric and self-magnetic fields. While for the case of beam width wb > wge with relative strong external magnetic fields, the rotation effects of plasma electrons are strongly inhibited and a well neutralization of ion beam current can be found. Finally, the influences of different beam widths, beam energies and magnetic fields on the neutralization of ion beam current are summarized for the cases of wb < wge < c/ωpe, wge < wb < c/ωpe and wb < c/ωpe < wge.

Front. Phys., 2014, 9(2): 226
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Research Article

A higher-dimensional model of the nucleon– nucleon central potential

Eric R. Hedin

Based on a theory of extra dimensional confinement of quantum particles [E. R. Hedin, Physics Essays, 2012, 25(2): 177], a simple model of a nucleon–nucleon (NN) central potential is derived which quantitatively reproduces the radial profile of other models, without adjusting any free parameters. It is postulated that a higher-dimensional simple harmonic oscillator confining potential localizes particles into three-dimensional (3D) space, but allows for an evanescent penetration of the particles into two higher spatial dimensions. Producing an effect identical with the relativistic quantum phenomenon of zitterbewegung, the higher-dimensional oscillations of amplitude ħ/(mc) can be alternatively viewed as a localized curvature of 3D space back and forth into the higher dimensions. The overall spatial curvature is proportional to the particle’s extra-dimensional ground state wave function in the higher-dimensional harmonic confining potential well. Minimizing the overlapping curvature (proportional to the energy) of two particles in proximity to each other, subject to the constraint that for the two particles to occupy the same spatial location one of them must be excited into the 1st excited state of the harmonic potential well, gives the desired NN potential. Specifying only the nucleon masses, the resulting potential well and repulsive core reproduces the radial profile of several published NN central potential models. In addition, the predicted height of the repulsive core, when used to estimate the maximum neutron star mass, matches well with the best estimates from relativistic theory incorporating standard nuclear matter equations of state. Nucleon spin, Coulomb interactions, and internal nucleon structure are not considered in the theory as presented in this article.

Front. Phys., 2014, 9(2): 234
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Research Article

A unified dynamic scaling property for the unified hybrid network theory framework

Qiang Liu, Jin-Qing Fang, Yong Li

Abstract: In this article, we present a new type of unified dynamic scaling property for synchronizability, which can describe the scaling relationship between dynamic synchronizability and four hybrid ratios under the unified hybrid network theory framework (UHNTF). Our theory results can not only be applied to judge and analyze dynamic synchronizability for most of complex networks associated with the UHNTF, but also we can flexibly adjust and design different hybrid ratios and scaling exponent to meet actual requirement for the dynamic characteristics of the UHNTF.

Front. Phys., 2014, 9(2): 240
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Research Article

Two dimensional Multiple-Relaxation-Time Lzttice Boltzmann model for compressible and incompressible flows

Feng Chen, Ai-Guo Xu, Guang-Cai Zhang, Yong-Long Wang

In the paper we extend the Multiple-Relaxation-Time (MRT) Lattice Boltzmann (LB) model proposed in [Europhys. Lett., 2010, 90: 54003] so that it is suitable also for incompressible flows. To decrease the artificial oscillations, the convection term is discretized by the flux limiter scheme with splitting technique. A new model is validated by some well-known benchmark tests, including Riemann problem and Couette flow, and satisfying agreements are obtained between the simulation results and analytical ones. In order to show the merit of LB model over traditional methods, the non-equilibrium characteristics of system are solved. The simulation results are consistent with the physical analysis.

Front. Phys., 2014, 9(2): 246
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