Oct 2013, Volume 8 Issue 5

Cover illustration

  • The current issue is dedicated to celebrating Centennial Physics at Peking University. The cover picture in the background shows the Western Gate of Peking University, and four small images are taken from four papers in the current issue respectively. For more detailed information, please refer to the current issue. [Photo credits: Ya-Nan Song, School of Physics, Peking University]

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    Xin-Cheng Xie
    Yi-Wen Hu, Yun-Feng Xiao, Yong-Chun Liu, Qihuang Gong

    The coupling between optical and mechanical degrees of freedom has been of broad interest for a long time. However, it is only until recently, with the rapid development of optical microcavity research, that we are able to manipulate and utilize this coupling process. When a high Q microcavity couples to a mechanical resonator, they can consolidate into an optomechanical system. Benefitting from the unique characteristics offered by optomechanical coupling, this hybrid system has become a promising platform for ultrasensitive sensors to detect displacement, mass, force and acceleration. In this review, we introduce the basic physical concepts of cavity optomechanics, and describe some of the most typical experimental cavity optomechanical systems for sensing applications. Finally, we discuss the noise arising from various sources and show the potentiality of optomechanical sensing towards quantum-noise-limited detection.

    Ying Xing (邢颖), Yi Sun (孙祎), Meenakshi Singh, Yan-Fei Zhao (赵弇斐), Moses H. W. Chan, Jian Wang (王健)

    In this review, we present a summary of some recent experiments on topological insulators (TIs) and superconducting nanowires and films. Electron-electron interaction (EEI), weak anti-localization (WAL) and anisotropic magneto-resistance (AMR) effect found in topological insulator films by transport measurements are reported. Then, transport properties of superconducting films, bridges and nanowires and proximity effect in non-superconducting nanowires are described. Finally, the interplay between topological insulators and superconductors (SCs) is also discussed.

    Xue-Wen Fu, Qiang Fu, Liang-Zhi Kou, Xin-Li Zhu, Rui Zhu, Jun Xu, Zhi-Min Liao, Qing Zhao, Wan-Lin Guo, Da-Peng Yu

    We conduct systematical cathodoluminescence study on red-shift of near-band-edge emission energy in elastic bent ZnO nanowires with diameters within the exciton diffusion length (~ 200 nm) in liquid nitrogen temperature (81 K). By charactering the emission spectra of the nanowires with different local curvatures, we find a linear relationship between strain-gradient and the red-shift of near-band-edge emission photon energy, an elastic strain-gradient effect in semiconductor similar to the famous flexoelectric effect in liquid crystals. Our results provide a new route to understand the inhomogeneous strain effect on the energy bands and optical properties of semiconductors and should be useful for designing advanced nano-optoelectronic devices.

    Yang Hu (胡杨), You-Kai Wang (王由凯), Peng-Fei Yin (殷鹏飞), Shou-Hua Zhu (朱守华)

    In this review we do not try to cover all the aspects of physics beyond the standard model (BSM), instead our latest understandingon the BSM will be presented: i) The Higgs sector is likely related to BSM, which can be confirmed at current running large hadron collider (LHC) or the future colliders. Furthermore we pointed out that spontaneous CP violation can be closely related to the lightness of the Higgs boson. ii) Top quark forward-backward asymmetry, which was measured by Tevatron, might be the sign of BSM. We proposed a new color-octet particle ZC to account for the observation and ZC can be further studied at the LHC. iii) If dark matter (DM) is utilized to accommodate astrophysical observations, it ought to be observed at the high energy LHC and DM produced at colliders should be the smoking gun signal. iv) Lithium puzzle might also be the sign of the BSM. We briefly review the newly proposed solution to Lithium puzzle, i.e., the existence of non-thermal component during the big bang nuclei-synthesis (BBN). The possible origins of the non-thermal component can be dark matter or the new accelerating mechanism of normal particles.

    Han-Qing Zheng

    The establishment of production representation of partial wave scattering amplitudes is reviewed in the context of quantum field theory. Its relation to the production representation, or Ning Hu representation in quantum mechanical scattering theory is pointed out. One of the most important application of the production representation is the physics of the f0(600) and K(700) scalar hadron resonances, on which we also give a brief review. It is emphasized that all evidences accumulated so far are consistent with the picture that the f0(600) meson is the chiral partner of the Nambu-Goldstone bosons in a linear realization of chiral symmetry.

    Zhi-Huan Li, Jian-Ling Lou, Qi-Te Li, Yu-Cheng Ge, Zhe-Wei Yin, Yan-Lin Ye

    Particle detection technologies have been largely advanced in our laboratory over the past decade. A neutron sphere was built to detect the decay neutron emitted from the implanted unstable nucleus, whereas a multi-neutron correlation spectrometer was implemented to detect the forward moving neutrons resulting from breakup reactions. Charged particle telescopes are now equipped with double sided Silicon strip detectors which have excellent energy and position resolutions. Large size gas chambers, such as resistive plate chambers, have been developed in order to achieve high performances related to timing or position measurements. The advances of these technologies contribute substantially to such large science project, as LHC-CMS, and to the experiments with the radioactive nucleus beams.

    Li-Xin Li

    A relativistically expanding sphere exists in many explosive astrophysical systems, including gammaray bursts, neutron star mergers, and some supernovae. In this paper we investigate the photon diffusion process in a relativistically expanding sphere, which is important for understanding the energetic and radiative characters of the above mentioned explosive systems. The following contents are discussed in the frame work of special relativity: random walks of photons by scattering with electrons, photospheres, photon diffusion, and the energy flux density emerging from the surface of the expanding sphere. Some of the results are also applicable to the Universe since the Universe is also a spherical expanding system.

    Ke-Xin Liu, Yu-Gang Wang, Tie-Shuan Fan, Guo-Hui Zhang, Jia-Er Chen

    Electrostatic accelerator is a powerful tool in many research fields, such as nuclear physics, radiation biology, material science, archaeology and earth sciences. Two electrostatic accelerators, one is the single stage Van de Graaff with terminal voltage of 4.5 MV and another one is the EN tandem with terminal voltage of 6 MV, were installed in 1980s and had been put into operation since the early 1990s at the Institute of Heavy Ion Physics. Many applications have been carried out since then. These two accelerators are described and summaries of the most important applications on neutron physics and technology, radiation biology and material science, as well as accelerator mass spectrometry (AMS) are presented.

    Xue-Qing Yan, Chen Lin, Hai-Yang Lu, Kun Zhu, Yu-Bin Zou, Hong-Yong Wang, Bing Liu, Shuan Zhao, Jiao Zhu, Yi-Xing Geng, He-Zheng Fu, Yong Shang, Chao Cao, Yin-Ren Shou, Wei Song, Yuan-Rong Lu, Zhong-Xi Yuan, Zhi-Yu Guo, Xian-Tu He, Jia-Er Chen

    Recently, radiation pressure acceleration (RPA) has been proposed and extensively studied, which shows that circularly polarized (CP) laser pulses can accelerate mono-energetic ion bunches in a phase-stable-acceleration (PSA) way from ultrathin foils. It is found that self-organizing proton beam can be stably accelerated to GeV in the interaction of a CP laser with a planar target at 1022 W/cm2. A project called Compact LAser Plasma proton Accelerator (CLAPA) is approved by MOST in China recently. A prototype of laser driven proton accelerator (1 to 15 MeV/1 Hz) based on the PSA mechanism and plasma lens is going to be built at Peking University in the next five years. It will be upgraded to 200 MeV later for applications such as cancer therapy, plasma imaging and fast ignition for inertial confine fusion.

    Xiao-Gang Wang (王晓钢), Qi-Bin Luan (栾其斌)

    Whistler waves generated in fast magnetic reconnection processes of collisionless high beta plasmas are reviewed in experiments and satellite observations, as well as in theory and simulation, and further studied in the two-fluid theory. It is found that low frequency whistler waves can be excited in the ion inertial range of the reconnection region. The wave is found right-handed polarized with a quadrupolar out-of-plane magnetic perturbation, in accord with satellite observations in the geomagnetosphere.

    Xin-Yu Wen, Yong-Yun Hu, Ji-Ping Liu

    The Northern-Hemisphere high-latitude continents experienced extremely cold weathers in winter 2009-2010. In the present paper, we show that the cold winter was associated with the activity of the Arctic oscillation (AO), which demonstrated the strongest negative polarity over the past six decades and persisted from December, 2009 to March, 2010. It is found that variations of the surface AO was closely linked to stratospheric polar vortex anomalies, and that the surface AO phases followed downward propagation of stratospheric Northern-Hemisphere Annular mode (NAM) anomalies during the winter. The case of 2009-2010 winter provides us with a typical example that anomalous stratospheric signals can be used to improve skills of long-range weather forecast and intra-seasonal climate prediction in winter time. We also show that the El N?no event, which started developing from May 2009, might contribute the formation of exceptionally negative and persistent AO and stratospheric NAM, particularly over North Pacific and North America.