Investigations on the soft-mode mechanism of the ferroelectric phase transition in

Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B / B g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles strongly that of monoclinic . Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June 1998     

Enhancement and generation of sum squeezing in two-mode light in mixing with coherent light using a beam splitter

Recently, we showed [J. Phys. B: At. Mol. Opt. Phys. 38, 665 (2005)] that the `amplitude-squared squeezing', a non-classical feature, can be enhanced in mixing with a coherent light beam using a beam splitter. Here, we show that the sum squeezing in a two-uncorrelated-mode light beam (one mode is in Gaussian state and the other one is in coherent state), which is degenerate limit of amplitude-squared squeezing, may be generated or enhanced in mixing with a two-mode coherent light beam using a beam splitter.     

An equation of state for anisotropic solids under shock loading

An anisotropic equation of state is proposed for accurate extrapolation of high-pressure shock Hugoniot states to other thermodynamics states for shocked single crystals and polycrystalline alloys. The proposed equation of state represents mathematical and physical generalization of the Mie-Grüneisen equation of state for isotropic material and reduces to this equation in the limit of isotropy. Using an anisotropic nonlinear continuum framework and generalized decomposition of a stress tensor [Int. J. Plasticity 24, 140 (2008)], the shock waves propagation along arbitrary directions in anisotropic solids of any symmetry can be examined. The non-associated strength model includes the distortion effect of the yield surface which can be used to describe the anisotropic strength differential effect. A numerical calculation showed that the general pulse shape, Hugoniot Elastic Limits (HELs), and Hugoniot stress levels for aluminum alloy 7010-T6 agree with the experimental data. The results are presented and discussed, and future studies are outlined.     

SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

Soft Dynamics simulation. 1. Normal approach of two deformable particles in a viscous fluid and optimal-approach strategy

Discrete simulation methods are efficient tools to investigate the behaviors of complex fluids such as dry granular materials or dilute suspensions of hard particles. By contrast, materials made of soft and/or concentrated units (emulsions, foams, vesicles, dense suspensions) can exhibit both significant elastic particle deflections (Hertz-like response) and strong viscous forces (squeezed liquid). We point out that the gap between two particles is then not determined solely by the positions of their centers, but rather exhibits its own dynamics. We provide the first ingredients of a new discrete numerical method, named Soft Dynamics, to simulate the combined dynamics of particles and contacts. As an illustration, we present the results for the approach of two particles. We recover the scaling behaviors expected in three limits: the Stokes limit for very large gaps, the Poiseuille-lubricated limit for small gaps and even smaller surface deflections, and the Hertz limit for significant surface deflections. We find that for each gap value, an optimal force achieves the fastest approach velocity. The principle of larger-scale simulations with this new method is provided. They will consitute a promising tool for investigating the collective behaviors of many complex materials.     

Structural study of the ferroelectric instability in SnPSe

The structural change occurring in between the paraelectric and the ferroelectric phases is investigated by means of X-ray diffraction. Details of the structure in both phases are obtained and the role of the lone pair is discussed in the light of structural data. In agreement with the 2/ m to m symmetry lowering, polar displacements are found within the m plane away from a particular crystallographic direction but antiparallel displacements occur also. These results are discussed in the frame of the phenomenological theory which predicts a particular temperature dependence of the dielectric polarization in this crystal. Received: 10 July 1998     

Control of extraordinary light transmission through perforated metal films using liquid crystals

We calculate the effective dielectric tensor of a metal film penetrated by cylindrical holes filled with a nematic liquid crystal (NLC). We assume that the director of the NLC is parallel to the film, and that its direction within the plane can be controlled by a static magnetic field, via the Freedericksz effect. To calculate the effective dielectric tensor, we consider both randomly distributed holes (using a Maxwell-Garnett approximation) and a square lattice of holes (using a Fourier technique). Both the holes and the lattice constant of the square lattice are assumed small compared to the wavelength. The films are found to exhibit extraordinary light transmission at special frequencies related to the surface plasmon resonances of the composite film. Furthermore, the frequencies of peak transmission are found to be substantially split when the dielectric in the holes is anisotropic. For typical NLC parameters, the splitting is of order 5–10% of the metal plasma frequency. Thus, the extraordinary transmission can be controlled by a static magnetic or electric field whose direction can be rotated to orient the director of the NLC. Finally, as a practical means of producing the NLC-filled holes, we consider the case where the entire perforated metal film is dipped into a pool of NLC, so that all the holes are filled with the NLC, and there are also homogeneous slabs of NLC on both sides of the film. The transmission in this geometry is shown to have similar characteristics to that in which the NLC-filled screen is placed in air.     

F2-laser ablation patterning of dielectric layers

Spatially defined patterning of multi-layer dielectric optical systems by laser-induced ablation is demonstrated. A 49-layer high-reflectivity mirror for 193-nm light was irradiated with F₂-laser light through the CaF₂-substrate to cleanly remove the whole dielectric stack by rear-sided ablation. The 157-nm light is absorbed efficiently by dielectric layers such as SiO₂ and Al₂O₃ that are typically used for ultraviolet (UV) transmission at 193-nm and longer wavelengths. Thus it is possible to ablate highly reflective UV-laser mirrors (HR 193 nm) and to create dielectric masks that withstand high power levels at 193 nm. A single 157-nm pulse with a fluence of less than 500 mJ/cm² is sufficient to cleanly ablate the whole layer stack with sharp edges and without debris deposition. Received: 31 October 2000 / Accepted: 14 November 2000 / Published online: 10 January 2001     

On the harmonic analysis of non-linear dielectric effects

We investigate the nonlinear dielectric effects in a polar viscous liquid, propylene carbonate, by analyzing the voltage and current traces obtained for a sinusoidal electric field at a frequency of 1 kHz and field amplitudes as high as 162 kV/cm. The main source of non-linear behavior results from the energy absorbed from the field and is understood quantitatively. However, there is a qualitative difference in the behavior of the field induced change, Δε^′′(E), and the third harmonic amplitude of the current, I_3ω. Although both Δε^′′(E) and I_3ω are considered reliable measures of non-linear behavior, we show here that the third harmonic signal reflects only those non-linear responses that are instantaneous on the time scale of the test frequency.     

Physicochemical studies of PVdF–HFP-based polymer–ionic liquid composite electrolytes

Polymer–ionic liquid composite electrolytes based on poly (vinylidenefluoride-co-hexafluoropropylene) (PVdF–HFP) and room temperature ionic liquid: 2,3-dimethyl-1-octylimidazolium hexafluorophosphate (DMOImPF₆) have been synthesized and studied. The addition of dimethylacetamide (DMA) and propylene carbonate (PC), both with high dielectric constant and low viscosity, to polymer electrolytes has been found to result in an enhancement of conductivity by one order of magnitude. Composite polymer electrolytes containing ionic liquid have been found to be thermally stable upto 300°C. Motional narrowing observed in the variation of line width of ¹H and ¹⁹F NMR peaks with temperature suggests that both cations and anions are mobile in these electrolytes.     

Lehmann effect in a compensated cholesteric liquid crystal: Experimental evidence with fixed and gliding boundary conditions

In a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of éber and Jánossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). These results prove that one must take into account the chirality of the molecules and the absence of inversion symmetry at the macroscopic scale when deriving the constitutive equations of the phase at the compensation temperature. In this paper, we present the details of our experimental work and a new experiment performed in a sample treated for planar gliding anchoring. The latter experiment, coupled with a numerical simulation, supports the existence of a thermomechanical coupling in a compensated cholesteric.     

Optical properties and photonic bands of GaAs photonic crystal waveguides with tilted square lattice

Reflectance measurements at variable angle of incidence are performed on GaAs photonic crystal waveguides with unconventional square lattices. The technique yields the dispersion of photonic bands for the investigated lattices, as first shown by Astratov et al. [Phys. Rev. B 60, R16225 (1999)]. A sample with a square lattice of air rings and small air fraction yields narrow resonant structures and a dispersion of photonic modes close to that of free photons. Another sample with a square lattice of dielectric squares and large air fraction leads to broader structures and to a dispersion of photonic modes which differs strongly for the two polarizations of light: this sample has a pseudo-gap around 1 micron wavelength. The experimental results are in good agreement with theoretical calculations of the reflectance and of the photonic mode dispersion in the photonic crystal slabs. Received 16 January 2002     

Onset of the nonlinear dielectric response of glasses in the two-level system model

We have calculated the real part of the nonlinear dielectric susceptibility of amorphous insulators in the kHz range, by using the two-level system model and a nonperturbative numerical quantum approach. At low temperature T, it is first shown that the standard two-level model should lead to a decrease of when the measuring field E is raised, since raising E increases the population of the upper level and induces Rabi oscillations cancelling the ones induced from the ground level. This predicted E-induced decrease of is at odds with experiments. However, a better, though still not perfect, agreement with low-frequency experimental nonlinear data is recovered if, in our fully quantum simulations, interactions between defects are taken into account by a new relaxation rate whose efficiency increases as , as was proposed recently by Burin et al. [Phys. Rev. Lett. 86, 5616 (2001)]. In this approach, the behavior of at low T is mainly explained by the efficiency of this new relaxation channel. Since a quantitative understanding of glasses is still missing, we finally discuss experiments whose results should yield a refined understanding of this new relaxation mechanism: i) a completely new nonlinear behavior should be found for samples whose thickness is ≃ 10 nm; ii) a decrease of nonequilibrium effects should be found when E is increased. Received 19 September 2002 / Received in final form 4 December 2002 Published online 14 March 2003     

绝缘颗粒液体主体基质复合介质的非线性光学性质

利用Maxwell-Garnett近似，并结合谱表示方法，理论研究了具有一般非线性的绝缘颗粒，无规则浸入液体主体基质复合体系的非线性光学性质-在弱非线性条件下，数值研究了体系的有效线性介电函数和光学非线性随体积分数的变化，并发现体系的线性介电函数比光学非线性更依赖于绝缘颗粒组分的体积分数-在一般非线性条件下，研究体系的有效介电函数随入射光强度的依赖关系，还研究了s方向和p方向极化的总反射率的行为- 关键词： 复合介质 谱表示 非线性光学性质     

Light scattering by longitudinal acoustic modes in molecular supercooled liquids I: phenomenological approach

We derive expressions for the intensity of the Brillouin polarized spectrum of a molecular liquid formed of axially symmetric molecules. These expressions take into account both the molecular dielectric anisotropy and the modulation of the local polarisability by density fluctuations. They also incorporate all the retardation effects which occur in such liquids. We show that the spectrum splits into a q-independent rotational contribution and q-dependent term, which reflects the propagation of longitudinal acoustic modes. In the latter, the two light scattering mechanisms enter on an equal footing and generate three scattering channels. We study the influence of the two new channels and show that they may substantially modify the Brillouin line-shape when the relaxation time of the supercooled liquid and the period of the acoustic excitation are of the same order of magnitude. Received 14 August 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: franosch@hmi.de     

On the high-pressure phase transition in

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO₄ at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a =7.306?, b =5.887? and c =5.124?. Received: 7 October 1997 / Received in final form: 17 November 1997 / Accepted: 18 November 1997     

Photon channelling in foams

Experiments by Gittings, Bandyopadhyay and Durian (Europhys. Lett. 65, 414 (2004)) demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first derive a central relation in the work of Gitting et al. without any free parameters. It links the photon's path-length fraction f in the liquid phase to the liquid fraction ɛ. We then construct two-dimensional Voronoi foams, replace the cell edges by channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling only occurs in a finite range of ɛ.     

Polarization-Independent Directional Beaming of Light by a Subwavelength Metal Slit

We present the directional beaming effect of light at the terahertz frequency by using a subwavelength slit in the metal film. The metal is dressed with anisotropic dielectric so that both the transverse electric （TE） and transverse magnetic （TM） polarized waves can be well guided on the metal surface and reach the phase matching. By using a periodical array of dielectric ridges and grooves around the slit, the guided waves can be scattered out of the slit and interfere with the transmitted light directly through the slit. The results performed by finite-difference at time-domain computations indicate that the directional beaming of light can be obtained simultaneously for both the TE and TM polarized waves after optimizing the geometric parameters. The structure may find great applications in polarization-independent optical devices such as couplers, connectors, beam collimator, and etc.     

Slow light in photonic crystal with combinations of gradient dielectric constant

A line defect waveguide formed by a new kind of two-dimensional photonic crystal with combinations of gradient varied dielectric was designed and the properties of slow light will been studied by using the Plane Wave expansion Method (PWM). The result shows that the group velocity is slower when the corresponding combination of gradient dielectric constants are with higher dielectric constant and lower interval. In our new structure of photonic crystal, the corresponding group velocity can reach as low as 0.06c (c: light velocity in vacuum), two orders magnitude lower than light velocity. Meanwhile, the group velocity dispersion effect is relatively small.