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《Radiation measurements》2007,42(4-5):719-722
Luminescence and luminescence excitation under VUV radiation of (, K, Cs; , In) double phosphates are studied. Two emission bands peaking near 330 and 420 nm are common for investigated crystals. The band structure and partial densities of electronic states of perfect , and crystals are calculated by the full-potential linear-augmented-plane-wave (FLAPW) method. It is found that the structures of the conduction bands of crystals, which have different B cations, are appreciably different. Experimental results are compared with results of calculations of the electronic structure. Assumptions concerning the origin of luminescence in double phosphates are made. 相似文献
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The present work is divided into three parts. First we study the null hypersurfaces of the Minkowski space , classifying all rotation null hypersurfaces in . In the second part we start our analysis of the submanifold geometry of the null hypersurfaces. In the particular case of the -dimensional light cone, we characterize its totally umbilical spacelike hypersurfaces, show the existence of non-totally umbilical ones and give a uniqueness result for the minimal spacelike rotation surfaces in the -dimensional light cone. In the third and final part we consider an isolated umbilical point on a spacelike surface immersed in the 3-dimensional light cone of and obtain the differential equation of the principal configuration associated to this point, showing that every classical generic Darbouxian principal configuration appears in this context. 相似文献
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In Cr-doped MgAl2O4 most of the Cr3+-ions occupy normal B-sites. In addition to this axial center we report here the presence of an orthorhombic Cr3+-center. The ESR-spectra of this new center can be described by the spin Hamiltonian:, with S=3/2. The principal axes of the center are along [111]=Z, and . The values of the parameters are: gz=1·950±0·005, gx=gy=1·915±0·015, D=+0·903cm?1±0·003 cm?1 and E=+0·084 cm?1±0·005 cm?1. The intensity of the lines is about ten times lower than that of the axial Cr3+-center. 相似文献
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At weak coupling, we investigate the frustrated Hubbard chain including nearest-neighbor and next-nearest-neighbor antiferromagnetic exchange with easy-plane anisotropy and . At half filling, we obtain three different phases in the – plane: a transverse spin density wave phase, a bond order wave phase and a Luther–Emery metallic phase characterized by the coexistence of singlet superconductivity and charge density wave correlations. The frustrating exchange accounts for the spin-gapped phases. 相似文献
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-monopoles are solutions to gauge theoretical equations on noncompact -manifolds of holonomy. We shall study this equation on the Bryant–Salamon manifolds. We construct examples of -monopoles on two of these manifolds, namely the total space of the bundle of anti-self-dual two forms over the and . These are the first nontrivial examples of -monopoles.Associated with each monopole there is a parameter , known as the mass of the monopole. We prove that under a symmetry assumption, for each given there is a unique monopole with mass . We also find explicit irreducible -instantons on and on .The third Bryant–Salamon -metric lives on the spinor bundle over the -sphere. In this case we produce a vanishing theorem for monopoles. 相似文献
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《Journal of Electrostatics》2007,65(3):174-181
This paper presents a numerical model of the negative-corona discharge in oxygen under different pressures at room temperature. The model assumes the presence of two charged species: electrons and ions. The boundary element and finite-element methods have been used to calculate the electric field and the method of characteristics has been employed for the densities of electrons and ions. The simulation has been performed for an axisymmetrical point-plane geometry with a hyperboloidal needle perpendicular to infinitely large conducting plate. The results of computations show the effect of pressure on the electron and negative-ion densities along the axial direction, electric field distribution along the axis of symmetry and current density on the ground plane. 相似文献
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《Radiation measurements》2007,42(4-5):644-647
The tetragonal : persistent luminescence materials were prepared by a solid state reaction. The UV excited and persistent luminescence was observed in the green region centred at 535 nm. Both luminescence phenomena are due to the same ion occupying the single site in the host lattice. The codoping usually reduced the persistent luminescence of :, which differs from the : and : materials. Only the ion enhanced slightly the persistent luminescence. With the aid of synchrotron radiation, the band gap energy of : was found to be about 7 eV that is very similar to those of the : materials. Thermoluminescence results suggested that the ions might act as electron traps, but only the TL peaks created by and can be found in the temperature range accessible. Lattice defects (e.g. oxygen vacancies) are also important, since the same main thermoluminescence peak was observed at about with and without codoping. 相似文献
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Negative thermal expansion materials can experience significant stresses when they are used in composites. Under ambient conditions Zr2(WO4)(PO4)2 displays anisotropic negative thermal expansion (NTE) (, , , at 0 GPa). The effect of hydrostatic pressure on its thermal expansion characteristics was investigated by neutron diffraction between 300 and 60 K at pressures up to 0.3 GPa. No phase transitions were observed in the pressure and temperature range examined. The material was found to have a bulk modulus, B0, of 61.3(8) GPa at ambient temperature, and unlike some other NTE materials, pressure had no detectable effect on thermal expansion (, , , at 0.3 GPa). 相似文献