Metastability of a circular o-ring due to intrinsic curvature

An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that . This model can be generalized to the case where the bend is induced by a concentration field which follows the variations of the curvature. Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997     

On some natural and model 2D bimodal random cellular structures

The topological and metric properties of a few natural 2D random cellular structures, namely an armadillo shell structure and young soap froths, which are formed from two classes of cells, large and small, have been characterized. The topological properties of a model generated from a Kagome tiling, which mimics such random binary structures, have also been exactly calculated. The distribution of the number of cell sides is bimodal for the structures investigated. In contrast to the classical Aboav-Weaire law for homogeneous 2D random cellular structures, nm(n), the mean total number of edges of neighbouring cells of cells with n sides does not vary linearly with n. Only the nm(i, n) (i=1,2) determined separately for every class of cells are linear in n for all investigated structures. Topological properties and correlations between metric and topological properties are finally compared with the predictions of various literature models. Received: 24 December 1997 / Revised: 7 April 1998 / Accepted: 20 April 1998     

Extremal coupled map lattices

Multifractal critical phenomena with infinite-temperature critical point and with complex coexistence of the infinite and finite temperature critical points are considered and it is shown that strange attractors generated by cascades of period-doubling bifurcations (Feigenbaum scenario) as well as fields of velocity differences in fluid turbulence belong to the former subclass of the multifractal critical phenomena, while the real traffic processes and real currency exchange processes belong to the last (complex) subclass of the multifractal critical phenomena. Data obtained by different authors are used for this purpose. Received 5 February 1999     

En">Characters and Composition Factor Multiplicities for the Lie Superalgebra $${\mathfrak{g}}{\mathfrak{l}}$$ ( m / n )

The multiplicities a of simple modules L in the composition series of Kac modules V lambda for the Lie superalgebra (m/n ) were described by Serganova, leading to her solution of the character problem for (m/n ). In Serganova's algorithm all with nonzero a are determined for a given this algorithm, turns out to be rather complicated. In this Letter, a simple rule is conjectured to find all nonzero a for any given weight . In particular, we claim that for an r-fold atypical weight there are 2r distinct weights such that a = 1, and a = 0 for all other weights . Some related properties on the multiplicities a are proved, and arguments in favour of our main conjecture are given. Finally, an extension of the conjecture describing the inverse of the matrix of Kazhdan–Lusztig polynomials is discussed.     

Positive and negative Hanbury-Brown and Twiss correlations in normal metal-superconducting devices

In the light of the recent analogs of the Hanbury-Brown and Twiss experiments [1] in mesoscopic beam splitters, negative current noise correlations are recalled to be the consequence of an exclusion principle. Here, positive (bosonic) correlations are shown to exist in a fermionic system, composed of a superconductor connected to two normal reservoirs. In the Andreev regime, the correlations can either be positive or negative depending on the reflection coefficient of the beam splitter. For biases beyond the gap, the transmission of quasiparticles favors fermionic correlations. The presence of disorder enhances positive noise correlations. Potential experimental applications are discussed. Received 1 June 1999     

798 nm半导体激光激发下Yb3+, Tm3+:ZBLAN玻璃的上转换发光

在用７９８ｎｍ半导体激光直接激发Ｔｍ＾３＋离子至＾３Ｆ４能级时,Ｔｍ＾３＋和Ｙｂ＾３＋共掺的锆系氟化物（Ｔｍ＾３＋,Ｙｂ＾３＋;ＺＢＬＡＮ）玻璃中观测到了较强的上转换蓝光。分析了发光机理：Ｔｍ＾３＋离子把能量传递给Ｙｂ＾２＋离子,被激发的Ｙｂ＾３＋离子又把能量传递给Ｔｍ＾３＋离子,从而把Ｔｍ＾３＋激发至发射蓝光的能级＾１Ｇ４和＾１Ｄ２。对这种泵浦机制的特点进行了讨论。     

Particle Creation in Cosmological Models with Varying Gravitational and Cosmological “Constants”

Einstein's field equations with variablegravitational coupling G = G(t) and decaying vacuumenergy density = (t) are considered asdescribing matter creation in a cosmological framework.The particle creation rate is determined by thevariation rate of both G and . By consideringsimple phenomenological evolution laws for G and Lambda,an exact solution of the gravitational field equationsfor a flat Friedmann-Robertson-Walker (FRW)space-time is obtained leading to a self-consistentcosmological model describing matter and entropygeneration in the very early Universe.     

多模干涉型GaAs光功分器的研制

采用导模传输分析法设计了多模干涉型１×４ 光功分器。根据所确定的器件结构参数, 结合器件设计和制作过程中可能遇到的因素, 详细分析了器件中自映像效应多模波导的宽度和长度、波导的折射率等结构参数对器件性能的影响。进行了多模干涉型 Ｇａ Ａｓ １×４ 光功分器的制作和测试。测试结果表明, 所研制的器件实现了１×４ 光功分器功能。针对实验过程和测试结果中遇到的问题进行了讨论, 并提出了进一步完善器件性能的方法。     

Luminescent characteristics of Tm3+/Tb3+/Eu3+ tri-doped Na5Y9F32 single crystals for white emission with high thermal stability

By using an improved Bridgman method, 0.3 mol% Tm$^{3+}/0.6$ mol% Tb$^{3+}/y$ mol% Eu$^{3+}$ ($y = 0$, 0.4, 0.6, 0.8) doped Na$_{5}$Y$_{9}$F$_{32}$ single crystals were prepared. The x-ray diffraction, excitation spectra, emission spectra and fluorescence decay curves were used to explore the crystal structure and optical performance of the obtained samples. When excited by 362 nm light, the cool white emission was realized by Na$_{5}$Y$_{9}$F$_{32}$ single crystal triply-doped with 0.3 mol% Tm$^{3+}/0.6$ mol% Tb$^{3+}/0.8$ mol% Eu$^{3+}$, in which the Commission Internationale de l'Eclairage (CIE) chromaticity coordinate was (0.2995, 0.3298) and the correlated color temperature (CCT) was 6586 K. The integrated normalized emission intensity of the tri-doped single crystal at 448 K could keep 62% of that at 298 K. The internal quantum yield (QY) was calculated to be $\sim 15.16$% by integrating spheres. These results suggested that the single crystals tri-doped with Tm$^{3+}$, Tb$^{3+}$ and Eu$^{3+}$ ions have a promising potential application for white light-emitting diodes (w-LEDs).     

Ion beam mixing and sputtering of Kr-irradiated TiN films measured with RBS,RNRA, and PIXE

Thin titanium nitride films of 10–300 nm thickness were irradiated with ⁸⁴Kr ions of 80–700 keV energy and fluences ranging from 10¹⁶ cm² to 2×10¹⁷ cm². Sputter yields (Y=0.4–1.0) and mixing rates (k=0.05–0.5 nm⁴) were determined using the depth profiling methods RBS, RNRA, and PIXE. While the sputter yields agree well with the modified Sigmund theory, the energy dependence of the mixing rates cannot be explained by standard models.