The nitrogen-vacancy colour centre in diamond

The nitrogen-vacancy (NV) colour centre in diamond is an important physical system for emergent quantum technologies, including quantum metrology, information processing and communications, as well as for various nanotechnologies, such as biological and sub-diffraction limit imaging, and for tests of entanglement in quantum mechanics. Given this array of existing and potential applications and the almost 50 years of NV research, one would expect that the physics of the centre is well understood, however, the study of the NV centre has proved challenging, with many early assertions now believed false and many remaining issues yet to be resolved. This review represents the first time that the key empirical and ab initio results have been extracted from the extensive NV literature and assembled into one consistent picture of the current understanding of the centre. As a result, the key unresolved issues concerning the NV centre are identified and the possible avenues for their resolution are examined.     

Uniaxial stress measurements on the 1039.8 meV zero-phonon line in irradiated silicon.

A vibronic band in Si with its zero-phonon line at 1039.8 meV is a product of neutron irradiation and annealing at 500 to 800 K. We show that uniaxial stresses perturb the zero-phonon line as expected for a tetragonal centre. Since the TA(X) phonons are particularly important in forming the phonon sideband, we suggest that the tetragonal symmetry is a result of a naturally axial centre, and is not a Jahn-Teller distortion of a T_d centre. No stress-induced reorientation of the centre occurs at small stress perturbations, but when the zero-phonon line splitting is comparable with the TA(X) energies the high energy stress-split component decreases rapidly in intensity. The zero-phonon line is negligibly affected by magnetic fields of up to 5 Tesla.     

Pressure-dependent studies of the DX centre in Si- and Sn-doped n+GaAs

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the effect of hydrostatic pressure on the free electron concentration, mobility, and the occupancy of the DX centre in MBE grown n⁺GaAs heavily doped with either Si or Sn. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The position and occupancy of the DX centre are calculated using Fermi-Dirac statistics. For the Si-doped samples comparison with local vibrational mode measurements indicate that the DX level can be identified with Si_Ga.     

boldmath Study of electronic structures and absorption bands of BaMgF4 crystal with F colour centre

The electronic structures of BaMgF₄ crystals containing an F colour centre are studied within the framework of the fully relativistic self-consistent Direc-Slater theory, using a numerically discrete variational (DV-Xα) method. It is concluded from the calculated results that the energy levels of the F colour centre are located in the forbidden band. The optical transition energy from the ground state to the excited state for the F colour centre is about 5.12 eV, which corresponds to the 242-nm absorption band. These calculated results can explain the origin of the absorption bands.     

A novel four-legged loaded element thick-screen frequency selective surface with a stable performance

<正>A thick-screen frequency selective surface(FSS) has not only a broad bandwidth but also the advantages of overcoming the multilayer FSS shortcoming of complex structure and low transmittance of centre frequency due to the cascade of FSSs,and this means it could potentially be applied in a stealth curved streamlined radome.However,there is an unsteadiness of centre frequency in a wide range of incident angles and another unsteadiness of polarization in a big incident angle.In order to solve these problems,in this paper we provide a novel four-legged loaded element thick-screen FSS.The structure is analysed and simulated using the mode matching method and moment method.The centre frequency,the transmittance of centre frequency,and bandwidth of the structure are investigated when some parameters including the polarization at a big incident angle and the incident angles of TE & TM waves are changed. The novel four-legged loaded element thick-screen FSS has better transmission properties with a better steadiness of polarization and incident angle independence.The novel structure of the four-legged loaded element thick-screen FSS provides a valuable reference for their application in a stealth curved streamlined radome.     

Photocapacitance quenching effect for “oxygen” in GaAs

New experimental data about the persistent photocapacitance quenching observed at 77 K in GaAs are reported. The effect is demonstrated to arise on the so-called oxygen centre in this material. Thermal annealing is described. The results are interpreted assuming that the O centre when filled with an electron has two possible states: the stable one and a metastable state with a large lattice relaxation.     

Clan structure in high energypn,pp andp\bar p interactions

We have analysed ourpn data at 400 GeV (corresponding to a centre-of-mass energy \(\sqrt s = 27.5\) GeV) together with otherpp and \(p\bar p\) data in the centre of mass energy range 11.5–900 GeV. The fits of all these distributions to a negative binomial distribution are performed. The variations in the fitted average multiplicity and in the inverse of the fitted parameterk are obtained with respect to the centre of mass energy. An assessment of the single diffractive component in the multiplicity distribution is made and its variation with the centre of mass energy discussed. The average number of clans and the average charged multiplicity per clan are determined and the centre of mass energy dependences shown.     

Canonical quantization of interacting WZW theories

Using canonical quantization we find the Virasoro centre for a class of conformally invariant interacting Wess-Zumino-Witten theories. The theories have a group structure similar to that of Toda theories (both abelian and non-abelian) but the usual Toda constraints on the coupling constants are relaxed and the theories are not necessarily integrable. The general formula for the Virasoro centre is compared to that derived by BRST methods in the Toda case, and helps to explain the structure of the latter.     

乙烷在中孔分子筛MCM-41中吸附的计算机分子模拟

采用计算机模拟方法研究不同温度下乙烷在不同孔径的MCM-41中的吸附.其中乙烷分子采用两个LJ中心的势模型表征，乙烷分子与MCM-41孔壁的相互作用采用一个连续的势模型表示.除考察了温度、孔径对吸附量的影响外，还研究这些量对乙烷分子在孔中的甲基和质心的分布，以及它们对乙烷分子在孔中的排列方向的影响.GCMC模拟结果发现，在180K和300K时壁面处都有较多的乙烷分子倾向于沿着壁面排列，同时在180K时其余的流体分子倾向于垂直于壁面排列，而在300K时其余的分子并不像在180K时一样倾向于垂直于壁面排列.模拟结果还表明，除壁面附近外，即使是在180K的较低温度下，乙烷分子的排列也是混乱和无序的.     

Electrical characteristics of ion-implanted laser-annealed silicon

The present paper summarizes recent experiments at the JRC, aiming to determine the relative damage effects produced in a-SiO2 by particles of different mass and energy, so as to correlate typical defects with specific production mechanisms. By systematic analysis of optical absorption spectra, the main colour centres that are currently associated with primary irradiation effects were characterized. Centres responsible for the B₂-band (5.04 eV) and E'-band (5.78 eV) were quantitatively investigated with respect to particle type, production yield and saturation behaviour. Arguments taken from energy loss and dpa seem to suggest that the E' centre is based on a stable structural substrate, which is not reconciled with simple ideas of a broken bond. The B₂ centre is confirmed to be associated with atomic defects produced by collisions. New determinations in the vacuum u.v., performed on heavy-ion irradiated samples, seem to put this centre in quantitative relationship to a centre responsible for a 7.15 eV absorption.     

Calculation of the Lyman-α asymmetry in a dense,partially-ionized hydrogen plasma

The asymmetry of the hydrogen resonance line Lyman-α is calculated. The electron perturbers are treated by the impact approximation and the ion perturbers by the quasistatic approximation. Contributions to the quasi-static ion broadening from first- and second-order asymmetry terms are included in the calculations, i.e. contributions from the quadrupolar ion-atom interaction and the quadratic Stark effect.The asymmetry at small separations from the line centre is unaffected by the inclusion of the second-order correction and remains slightly larger than the experimentally determined asymmetry. At larger separations from the line centre, the inclusion of the second-order correction causes a decrease in asymmetry due primarily to the quadratic Stark effect. For these larger separations from the line centre, rather poor agreement is obtained between theory and the experimental results of Boldt and Cooper.     

Charge-state effects of deep centres in semiconductors on non-radiative capture of carriers by multiphonon processes

A set of simplified analytical expressions for carrier capture coefficients, including quantatively the charge-dependent effect, have been obtained for easy physical examination and comparison with experiments. The temperature-related charge-state-dependent factorF(T) thus calculated could be used to present more accurately the nature and magnitude of the charge state of a trap centre. The ranges of values ofF(T) valid for attractive, repulsive and neutral centres are also obtained. In addition, we show that the thermal ionization energy for theB centre in GaAs is a function of temperature. The importance of the data of capture cross-section at low temperatures in determining the charge state and characteristic of a deep centre is also manifested. Both the absolute magnitude and the temperature-dependent behaviour of the calculated capture cross-section are well-supported by the very good fits to the experimental electron cross-sections forA andB centres in GaAs reported by Lang [7] and Wang et al. [22] and for Cu centre in Ge reported by Zhdanova and Kalashnikov [23].     

Remarks on Radial Centres of Convex Bodies

The paper concerns a family of selectors for convex bodies in Rⁿ, radial centre maps, defined in the article of M. Moszyńska, Looking for selectors of star bodies, Geom. Dedicata 81 (2000), 131–147. A radial centre of a convex body A is the maximizer of a suitable generalized dual volume of A. We give physical interpretations of the notion of radial centre and study its geometric properties. We prove that these selectors are continuous with respect to the Hausdorff metric and solve the problem of direct additivity for radial centre of order α, which corresponds to the dual volume of order α. Mathematics Subject Classifications (2005) 52A20, 52A40, 51P05, 85A25, 86A20.     

高精度光学中心偏测量仪主要技术指标的检测

简要阐述高精度光学中心偏测量仪主要技术指标，即仪器基准轴运动误差、测微目镜值误差、测量系统倍率误差和偏心测量误差的检测原理，最后人出计算实例。     

Graphs for the design of laser mirrors at normal incidence

The results of computer calculations concerning the reflectivity in the stop-band at normal incidence are given for periodic multilayer systems made out of layers of ZnS and MgF₂ of equal optical thickness. General formulae are derived to distinguish between the maxima and minima of the reflectivity at the centre of the stop-bands and for the calculation of the width of the stop-band of higher order mirrors as a function of the wavelength.The bandwidth of higher order mirrors and the reflectivity at the centre of the stop-band are presented in a number of graphs for systems with up to 18 layers on substrates with varying refractive index. Quarter-wave layer anti-reflection coatings are treated as special cases. The process of designing laser mirrors can be simplified using the graphs. They may also be useful during the evaporation process when thickness monitoring is done by optical methods. An example of a partially reflecting mirror is given. This uses a flat minimum of reflectivity at the centre of the stop-band to obtain an almost constant reflectivity over the total width of the stop-band.     

均匀横向磁场条件下抛物量子阱结构中的共振隧穿

采用简单的数值计算方法研究了均匀横向磁场条件下通过抛物量子阱结构的共振隧穿。计算了几种GaAs-AlxGa1-xAs-GaAs抛物阱结构的透射系数。结果表明：磁场增加时，共振峰向高能区移动，新的共振峰出现；同时讨论了回旋中心在不同位置时，抛物阱结构的J-V 特性。我们发现回旋中心在抛物阱中心时更有助于解释实验，并且能够得到和实验一致的结果：磁场增加，电流峰值减少且向高偏压移动。     

Vibrations of a beam with an absorber consisting of a viscoelastic beam and a spring-viscous damper

The effectiveness of the dynamic vibration absorber which consists of a double-cantilever viscoelastic beam and a spring-viscous damper is studied. The absorber is attached to the centre of the main beam. The ends of the main beam are built in and excited sinusoidally by the base motion. In the numerical example, the displacement transmissibility, i.e., the ratio of the displacement at the centre of the main beam to that of the base, is investigated. Variations of the resonant peaks are shown when the absorber parameters are changed. Values of the optimum tuning design parameters are presented, and it is verified that two of the main beam resonances are optimized simultaneously.     

Modellmäßige und numerische Beschreibung eines toroidalen Lichtbogens

This paper deals with a wallstabilized, toroidal electric arc at atmospheric pressure. Contrary to cylindrical arcs there appear asymmetrical thermal, electrical and magnetical values. The displacement of the maximum of temperature out of the centre of cross-section to the centre of curvature may be a criterion for this asymmetry. The effects of curvature are realized analytically in this paper by a model theory. Further-more, a theory of conform mapping of the numerical results of a cylindrically-symmetric arc to a torus geometry yields to values, which are in good correspondence with numerical solutions of the energy balance of the toroidal geometry. In addition convection — caused by the asymmetry of the Lorentz forces — are pointed out.     

Synthesis, Characterization, Luminescence and Defect Centres in CaYAl3O7:Eu3+ Red Phosphor

CaYAl(3)O(7):Eu(3+) phosphor was prepared at furnace temperatures as low as 550°C by a solution combustion method. The formation of crystalline CaYAl(3)O(7):Eu(3+) was confirmed by powder X-Ray diffraction pattern. The prepared phosphor was characterized by SEM, FT-IR and photoluminescence techniques. Photoluminescence measurements indicated that emission spectrum is dominated by the red peak located at 618 nm due to the (5)D(0)-(7)F(2) electric dipole transition of Eu(3+) ions. Electron Spin Resonance (ESR) studies were carried out to identify the centres responsible for the thermoluminescence (TL) peaks. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0126 is identified as an O(-) ion while centre II with an isotropic g-factor 2.0060 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F(+) centre appears to correlate with the observed high temperature TL peak in CaYAl(3)O(7):Eu(3+) phosphor.     

Luminescence bands in natural brown diamonds

We report on new luminescence bands exhibited by natural brown/mauve diamonds. We describe detailed absorption and luminescence measurements in the visible spectral region. The structured bands with ZPL at 2.424 eV, 2.114 eV, 1.819 eV and 1.707 eV, and a broad band with a maximum at 1.8 eV are shown to be the main components of the visible luminescence. Bands with ZPL at 2.114 eV, 1.819 eV and 1.707 eV are shown to correspond to forbidden transitions with lifetimes of the order of msec. Energy absorbed by the 2.424 eV centre is transferred into the 1.819 eV centre.