Anomalous elastic behaviour of monoclinic potassium hydrogen carbonate,KHCO3, in the vicinity of the phase transition at 318 K

The elastic constants of monoclinic KHCO₃ have been determined from resonance frequencies of thick plates at ca. 15 MHz in the range between 250 and 370 K. A strong elastic anisotropy is present with a maximum of the longitudinal stiffness near the direction of the hydrogen bonds within the (HCO₃)^2-₂ dimers. The shear stiffness c₆₆ related to transversal waves propagating along the twofold axis [0 1 0]′ and [1 0 0]′ with displacement vectors nearly parallel to [1 0 0]′ and [0 1 0]′, respectively, shows a drastic softening approaching 318 K from below and from above, indicating a ferroelastic transition. All other elastic constants behave normally except in the close vicinity of the transition temperature where in some cases a small discontinuity is visible. The components of the thermal expansion tensor undergo a discontinuous change at the transition. The pressure derivative of the transition temperature is 2.3 K kbar^-1. A comparison with alkali cyanides and KH₂PO₄ reveals that the interactions driving the transition are rather originating from order-disorder processes of protons as in KH₂PO₄ than from an orientational molecular disorder as in alkali cyanides.     

Levels of118Sn populated in the decay of118In and118Sb isomers

Gamma-rays in the disintegration of 5.0 s (1⁺), 8.5 s (7^?, 8^?), and 4.4 min (4⁺, 5⁺)¹¹⁸In isomers and of 3.5 min (1⁺) and 5.1 h (8^?)¹¹⁸Sb isomers have been investigated using Ge(Li) detectors and Ge(Li)-NaI(Tl) coincidence spectrometers. The decay schemes of these isomers have been constructed incorporating 17 levels in the product nucleus,¹¹⁸Sn, at the following energies (in keV): 0 (0⁺), 1229.5 (2⁺), 1757.5 (0⁺), 2043.1 (2⁺), 2056.4 (0⁺), 2280.3 (4⁺), 2321.3 (5^?), 2326.4 (1⁺, 2⁺), 2402.7 (4⁺), 2488.8 (4⁺), 2496.5 (0⁺), 2575.2 (7^?), 2677.3, 2733.7 (2⁺), 2929.8, 2963.5 (4⁺), and 3137.1 (0⁺). The structure of¹¹⁸Sn is briefly discussed in view of results from recent quasiparticle-model calculations.     

Birefringence in tetragonal antiferromagnets

It is shown that measurements made in the flop phase of antiferromagnets can lead to an unambiguous determination of the relative importance of the possible microscopic origins of the magnetic linear birefringence. From our experiments we conclude that in GdVO₄ exchangestriction is the dominant contribution.     

Birefringence in leucosapphire plates

Modification of leucosapphire by inhomogeneous plastic deformation makes it possible to obtain plates with a varying inclination angle of the optical axis of the crystal to the surface. We have considered the light refraction in a plate upon perpendicular incidence of a parallel beam of rays.     

Spin fluctuations in the vicinity of a charged particle in solid3He

At low temperature the positively charged muon (μ⁺) in solid³He is localized in a polaron which, unlike a chemically bound complex, is held together with polarizational attraction (van der Waals force). The dynamic effects in the muon relaxation are determined by the spin exchange of the³He atoms in the polaron. In crystals with large molar volumes (melting pressureP _m<60 bar) the rate of magnetic field fluctuations at the muon site is at least one order of magnitude lower than in an unperturbed crystal. In an external electric field the μ⁺ produces an anisotropic local distortion which reduces the rate of the local³He spin exchange and leads to an increase of the muon spin relaxation rate.     

Birefringence of the high-energy γ-quanta in monocrystals

It has been shown that a crystal becomes bireringent in the hard γ-region due to the channeling pair production. This makes it possible to obtain polarized beams of γ-quanta of tens and hundreds of GeV and effectively change their polarization states.     

Birefringence of porous silicon

The paper reports the observation and studies of the birefringence in porous silicon samples obtained onto different crystallographic planes of silicon single crystals.     

Birefringence of muscovite mica

Birefringence of muscovite mica (n_e−n_o) is usually calculated from the dispersion relations of the indices n_e and n_o. As small differences between relatively large experimentally-measured values, the few available data for birefringence of mica are widely distinct and sometimes contradict basic experimental facts. A different procedure is adopted in this work to measure birefringence directly as a single quantity in terms of the thicknesses of mica plates and the wavelengths at which they act as quarterwave or halfwave phase retarders. Birefringence is found to decrease gradually from −0.0040 at 420 nm to −0.0046 at 700 nm in conformity with pertinent independent experimental results.     

Birefringence of Irregular-Microstructure Fibre

Birefringence of a microstructure fibre with irregular structures is investigated using the finite element method. The birefringence of each sample is statistically analysed, and the relation between the birefringence and the irregular structure of the microstructure fibre is derived. The results indicate that the irregular structure of the symmetrical microstructure fibre result in the birefringence which is more strongly affected by disorder of air holes than the nonuniformity. Moreover, lower birefringence can be obtained with smaller air holes and larger pitch in the fibre structure.     

Beta decay of neutron-rich 118Rh and the lowest excited states in 118Pd

Beta decay of a refractory isotope ¹¹⁸Rh produced in symmetric fission and mass separated by the ion guide technique has been applied for the study of low-lying excited states of ¹¹⁸Pd. The yrast band in ¹¹⁸Pd has been observed up to a 6⁺ state and the lowest states of the asymmetric γ-band have been identified. The measured half-life of ¹¹⁸Rh is (300±60)ms. The systematics of the excited states in neutron-rich Pd-isotopes implies the saturation towards an O(6) symmetry at N = 70. Received: 19 June 2000 / Accepted: 1 August 2000     

Birefringence in electromagnetically induced transparency

(4)He 2 (3)S(1) --> 2 (3)P transitions at 1083 nm were laser probed in the presence of radiation that coupled the 2 (3)P level with the 3 (3)S(1) level in a ladder scheme. The metastable helium that was used was produced in a rf discharge. Significant atomic birefringence was detected, in addition to complete electromagnetically induced transparency. A simple theoretical model explains the experimental results.     

光盘盘基应力双折射的研究

从理论和实验讨论了光盘盘基在不同入射角江束通过时所产生的相位延迟值的改变。用起偏器－调制器－补偿器－样品－检偏器结构形式的偏振相位调制方法对玻璃和聚碳酸酯两种材料光盘盘基进行测量，结果表明聚碳酸酯盘基的双折射远大于玻璃盘基的双折射。     

微孔光纤双折射的研究

采用有限元方法对新型微孔光纤的熊猫与微孔熊猫型微孔光纤双折射给出计算和特性分析，并着重介绍微孔光纤应力双折射和几何双折射的理论与实验研究的最新成果。     

偶氮苯共聚PMMA薄膜的光致双折射特性研究

研究了偶氮共聚聚合物的光致双折射特性,利用幂指数和高斯函数拟合双折射过程中不同的物理过程,对各种不同的浓度、泵浦功率、主链运动及实验温度对双折射特性的影响进行了讨论,并在光致双折射过程中对样品的饱和回落进行了观察和分析,随着样品浓度的增加,尽管双折射的绝对值增大,但残留比例减小.     

Birefringence in porous anodic aluminum oxide

Birefringence properties of a disordered two-dimensional photonic crystal are demonstrated experimentally for the first time. It is proposed to use porous anodic aluminum oxide, which has anisotropy on the scale of the wavelength of light, as a new birefringent material in the optical range. An original method for studying the birefringence properties of two-dimensional photonic crystals based on the use of monochromatic light is described.     

Birefringence compensation for single-shot 3D profilometry using a full-Stokes imaging polarimeter

Optical Review - We demonstrate a single-shot uniaxial 3D profilometry system that illuminates a sample with a structured polarization pattern and measures the reflected full-Stokes polarimetric...     

偏振保持光纤的模式双折射

偏振保持光纤是单模光纤的一种特殊类型,其模式双折射与温度、芯和皮折射率差及椭圆度、热膨胀系数、光纤直径、弹性模量、泊松比、频率、波长、拉丝张力等诸多因素有关,对与偏振保持光纤模式双折射相关的上述因素进行分析,指出光纤模式双折射越高,光纤的偏振态保持就越好,为更好地设计新型保偏光纤或合理选择保偏光纤类型奠定了基础.