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Quantum tunneling of ultracold atoms in optical traps
Jian-Hua Wu, Ran Qi, An-Chun Ji, Wu-Ming Liu
Frontiers of Physics. 2014, 9 (2 ): 137-152.
https://doi.org/10.1007/s11467-013-0359-z
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.
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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
Frontiers of Physics. 2014, 9 (2 ): 153-156.
https://doi.org/10.1007/s11467-013-0402-0
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.
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Electron–positron pair production in a strong asymmetric laser electric field
Obulkasim Oluk, Bai-Song Xie, Muhmmad Ali Bake, Sayipjamal Dulat
Frontiers of Physics. 2014, 9 (2 ): 157-163.
https://doi.org/10.1007/s11467-013-0379-8
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.
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Single photon sources with single semiconductor quantum dots
Guang-Cun Shan, Zhang-Qi Yin, Chan Hung Shek, Wei Huang
Frontiers of Physics. 2014, 9 (2 ): 170-193.
https://doi.org/10.1007/s11467-013-0360-6
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.
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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
Frontiers of Physics. 2014, 9 (2 ): 200-209.
https://doi.org/10.1007/s11467-013-0409-6
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.
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First-principles study on the structure, electronic and magnetic properties of HoSi n (n= 1–12, 20) clusters
Tai-Gang Liu, Wen-Qing Zhang, Yan-Li Li
Frontiers of Physics. 2014, 9 (2 ): 210-218.
https://doi.org/10.1007/s11467-013-0398-5
The structure, electronic and magnetic properties of HoSi n ( 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 HoSi n ( n = 1-12) clusters except n = 7, 10, the most stable structures are a replacement of Si atom in the corresponding pure Si n+1 clusters by Ho atom. The doping of Ho atom makes the stability of Si clusters enhance remarkably, and HoSi n ( n = 2, 5, 8, 11) clusters are more stable than their neighboring clusters. The magnetic moment of Ho atom in HoSi n ( n = 1-12, 20) clusters mainly comes from 4 f electron of Ho, and never quenches.
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Concentration-dependent crystal structure, elastic constants and electronic structure of Zr x Ti1- x alloys under high pressure
Xiao-Li Yuan, Mi-An Xue, Wen Chen, Tian-Qing An
Frontiers of Physics. 2014, 9 (2 ): 219-225.
https://doi.org/10.1007/s11467-013-0391-z
The physical properties of Zr x Ti1- 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 Zr x Ti1- 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, Zr0.33 Ti0.67 , Zr0.5 Ti0.5 , Zr0.75 Ti0.25 and Ti, but the Zr0.67 Ti0.33 alloy is brittle under 0 K and 0 GPa. The metallic behavior of these alloys was also proved by analyzing partial and total DOS.
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A higher-dimensional model of the nucleon–nucleon central potential
Eric R. Hedin
Frontiers of Physics. 2014, 9 (2 ): 234-239.
https://doi.org/10.1007/s11467-013-0393-x
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 ? /( 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 1 st 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.
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Two-dimensional Multiple-Relaxation-Time Lattice Boltzmann model for compressible and incompressible flows
Feng Chen, Ai-Guo Xu, Guang-Cai Zhang, Yong-Long Wang
Frontiers of Physics. 2014, 9 (2 ): 246-254.
https://doi.org/10.1007/s11467-013-0368-y
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.
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