Atomistic simulations of a thermotropic biaxial liquid crystal

We have performed molecular dynamics simulations of a 2,5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole mesogen (ODBP-Ph-C(7)) at a fully atomistic level for a range of temperatures within the region that has experimentally been assigned to a biaxial nematic phase. Analysis of the data shows that the simulated nematic phase is biaxial but that the degree of biaxiality is small. The simulations show also the formation of ferroelectric domains in the nematic where the molecular short axis is aligned with the oxadiazole dipoles parallel to each other. Removal of electrostatic interactions leads to destabilization of ferroelectric domains and destabilization of the biaxiality. An additional simulation shows the slow growth of a mesophase directly from the isotropic fluid over a period of approximately 50 ns. This is the first time this has been achieved within the framework of an all-atom model.     

Biaxial modeling of the structure of the chevron interface in smectic liquid crystals

We have included the inherent molecular biaxiality of the smectic C phase in a model of the chevron structure. This molecular biaxiality is related to a hindered rotation about the molecular long axis which for chiral, polar molecules induces a spontaneous polarization. Through the coupling between biaxiality and the smectic cone angle, continuity of the molecular distribution at the chevron interface leads to changes in the cone angle. Under certain approximations we are able to find analytic expressions for the chevron structure and consequently estimate the width of the chevron interface. There are in fact two correlation lengths which govern variations in the cone angle and the biaxiality.     

分子双轴性与向列型液晶在各向同性相的最低过冷温度

在分子场近似下,研究分子双轴性对向列型液晶在各向同性相最低过冷温度T^*的影响.理论结果表明,通过考虑分子双轴性,各向同性相最低过冷温度T^*与向列相—各向同性相相变温度T_c的比值向实验值靠近.当分子双轴性参数δ取到0.3时,T^*/T_c达到0.9817,已很接近0.99的实验值. 关键词：     

Search for optical biaxiality in discotic liquid crystals

Using a high-finesses Fabry-Perot structure, we examine the behaviour of discotic liquid crystals for possible biaxiality. This is done by confining the discotic liquid crystals in the Fabry-Perot cavity and examining the mode structure in presence and absence of an applied electric field. It is concluded that at least in the discotic liquid crystal that we have examined, there is no evidence of biaxiality.     

Order and dynamics of molecular reorientations in norbornene: What μSR,NMR and neutrons can say

We present results from μSR and NMR on norbornene single crystals. The first two techniques provide a measure of the only non-zero rank-2 molecular order parameter in the plastic phase. Its non-vanishing value proves that molecular reorientations are not isotropic, its uniqueness indicates negligible biaxiality and its temperature dependence suggests anisotropic rotational diffusion. Preliminary analysis of single crystal IQNS supports this picture.     

Biaxial nematic phase in bent-core thermotropic mesogens

A biaxial nematic phase had been predicted with D(2h) symmetry, wherein the mesogen's long and short transverse axes are simultaneously aligned along the two orthogonal, primary and secondary directors, n and m, respectively. The unique low-angle x-ray diffraction patterns in the nematic phases exhibited by three rigid bent-core mesogens clearly reveal their biaxiality. The results of x-ray diffraction can be readily reproduced by ab initio calculations that explicitly include the bent-core shape in the form factor and assume short-range positional correlations.     

Spatial reorientation of azobenzene side groups of a liquid crystalline polymer induced by linearly polarized light

The photoinduced 3D orientational structures in films of a liquid crystalline polyester, containing azobenzene side groups, are studied both experimentally and theoretically. By using the null ellipsometry and the UV/Vis absorption spectroscopy, the preferential in-plane alignment of the azobenzene fragments and in-plane reorientation under irradiation with linearly polarized UV light are established. The uniaxial and biaxial orientational order of the azobenzene chromophores are detected. The biaxiality is observed in the intermediate stages of irradiation, whereas the uniaxial structure is maintained in the photosaturated state of the photo-orientation process. The components of the order parameter tensor of the azobenzene fragments are estimated for the initial state and after different doses of irradiation. The proposed theory takes into account biaxiality of the induced structures. Numerical analysis of the master equations for the order parameter tensor is found to yield the results that are in good agreement with the experimental dependencies of the order parameter components on the illumination time. Received 23 April 2001 and Received in final form 1 August 2001     

Josephson plasmon and inhomogeneous superconducting state in La2-xSrxCuO4

We report on the interlayer far infrared response for a series of La2-xSrxCuO4 crystals with 0.08相似文献   

Biaxialite critique d'un smectique C

Measurements of the biaxiality of a smectic C as a function of the order parameter are presented for n-heptyloxybenzylidene-n-heptylanisol at the second order transition smectic A to smectic C. The experimental set up is a variation of the classical conoscopy method of interference.     

Metal nanoplasmas as bright sources of hard X-ray pulses

We report significant enhancements in light coupling to intense-laser-created solid plasmas via surface plasmon and "lightning rod" effects. We demonstrate this in metal nanoparticle-coated solid targets irradiated with 100 fs, 806 nm laser pulses, focused to intensities approximately 10(14)-10(15) W cm(-2). Our experiments show a 13-fold enhancement in hard x-ray yield (10-200 keV) emitted by copper nanoparticle plasmas formed at the focal volume. A simple model explains the observed enhancement quantitatively and provides pointers to the design of structured surfaces for maximizing such emissions.     

Analysis of non-photonic electrons in Cu + Cu collisions at $\sqrt{s_{NN}}=200$ GeV

Heavy-flavor semileptonic decays are expected to dominate the spectrum of non-photonic electrons produced from collisions at the energies of the Relativistic Heavy Ion Collider. The non-photonic electron yield is suppressed by approximately a factor of 5 in central Au + Au events at  GeV relative to p+p events with the same collision energy. Most theoretical models predict less non-photonic-electron suppression than is observed experimentally. We present a preliminary measurement of the yield of non-photonic electrons in Cu + Cu events at  GeV, as well as the nuclear modification factor.     

Quenching of Myosin Intrinsic Fluorescence Unravels the Existence of a High Affinity Binding Site for Decavanadate

Decavanadate, one of the aggregated species of vanadate, is a potent inhibitor of several enzymes, including skeletal muscle myosin. However, its putative binding sites in myosin are largely unknown. Titration of the intrinsic fluorescence of myosin, purified from rabbit skeletal muscle, have been carried out in 0.3 M KCl, 5 mM CaCl₂ and 25 mM Tris-HCl (pH 7.0), with 0.1 mg/ml myosin. In the 0-200 M total vanadate concentration range, decavanadate produced approximately 25% quenching of the intrinsic fluorescence of myosin, with an apparent dissociation constant in the micromolar range. This effect was found to be specific of decavanadate, because titration with metavanadate up to 200 M did not produce a significant quenching of the intrinsic fluorescence of myosin. This quenching was accompanied by a parallel decrease of the accessibility of myosin tryptophans to the water-soluble collisional quencher KI, with an apparent dissociation constant also in the micromolar range. It is concluded that the binding of decavanadate to high-affinity sites in myosin produces local conformational change(s) near the tryptophans more accessible to water in the three-dimensional structure of this protein.     

Diffusion and reactions of hydrogen in F2-laser-irradiated SiO2 glass

The diffusion and reactions of hydrogenous species generated by single-pulsed F2 laser photolysis of SiO-H bond in SiO2 glass were studied in situ between 10 and 330 K. Experimental evidence indicates that atomic hydrogen (H0) becomes mobile even at temperatures as low as approximately 30 K. A sizable number of H0 dimerize by a diffusion-limited reaction into molecular hydrogen (H2) that may migrate above approximately 200 K. Activation energies for the diffusion, inherently scattered due to the structural disorder in glass, are separated into three bands centered at approximately 0.1 eV for free H0, approximately 0.2 eV presumably for shallow-trapped H0, and approximately 0.4 eV for H2.     

Emergent biaxiality in nematic microflows illuminated by a laser beam

Anisotropic fluids (e.g. liquid crystals) offer a remarkable promise as optofluidic materials owing to the directional, tunable, and coupled interactions between the material, flow, and the optical fields. Here we present a comprehensive in silico treatment of this anisotropic interaction by performing nonequilibrium molecular dynamics simulations. We quantify the response of a nematic liquid crystal (NLC) undergoing a Poiseuille flow in the Stokes regime, while being illuminated by a laser beam incident perpendicular to the flow direction. We adopt a minimalistic model to capture the interactions, accounting for two features: first, the laser heats up the NLC locally; and second, the laser polarises the NLC and exerts an optical torque that tends to reorient molecules of the nematic phase. Because of this reorientation the liquid crystal exhibits small regions of biaxiality, where the nematic director is one symmetry axis and the axis of rotation for the reorientation of the molecules is the other one. We find that the relative strength of the viscous and the optical torques mediates the flow-induced response of the biaxial regions, thereby tuning the emergence, shape and location of the regions of enhanced biaxiality. The mechanistic framework presented here promises experimentally tractable routes toward novel optofluidic applications based on material-flow-light interactions.     

Nematic-wetted colloids in the isotropic phase: pairwise interaction, biaxiality, and defects

We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.     

Optical anomalies and residual stresses in basal-plane-faceted ribbons of Stepanov-grown sapphire crystals

The effects of an anomalous biaxiality and a residual-stress field in basal-plane-faceted single-crystal sapphire ribbons were studied by an optical polarization method using a conoscopic light beam. Formulas that relate the difference between the principal stresses to the angle of optical-axis divergence are obtained. It is shown that the central regions of the ribbons undergo compressive stresses whereas their edges undergo tensile stresses. As compared to block-free ribbons, the stresses in block-containing ribbons increase more intensely with distance from the seed.     

Intensity discrimination, increment detection, and magnitude estimation for 1-kHz tones

Intensity difference limens (DLs) were measured over a wide intensity range for 200-ms, 1-kHz gated tones and for 200-ms increments in continuous 1-kHz tones. Magnitude estimates also were obtained for the gated tones over a comparable intensity range. The discrimination data are in general agreement with those from earlier studies but they extend them by showing: (1) good discrimination for gated tones over at least a 115-dB dynamic range; (2) a slight increase in the relative DL (delta I/I) as intensity increases above 95 dB SPL; (3) smaller DLs for increments than for gated tones, with the difference approximately independent of intensity; (4) negligible "negative masking" when thresholds are expressed as intensity differences (delta I). For two of the three subjects, magnitude estimates do not conform to a single-exponent power law for suprathreshold intensities. Over the middle range of intensities where a single exponent is appropriate, the value of the exponent is less than 0.1 for all subjects.     

Mössbauer studies of baotite and bafertisite

The Mössbauer effect was used to study silicate minerals of baotite and bafertisite at 298 K and 95 K. Each spectrum of baotite at 298 K and 95 K consists of two doublets, and they are contributed from Fe²⁺ and Fe³⁺ in the octahedral Tisites, respectively. Each spectrum of bafertisite at 298 K and 95 K is composed of two doublets, and they are mainly caused by Fe²⁺ in the octahedral Fe(I) and Fe(II)sites, respectively. The average effective ionic radii of the Ti sites in baotite and the Fe(I) and Fe(II)sites in bafertisite were estimated based on the correlation of the isomer shifts with the average effective ionic radii in silicates, and they are 0.56 , 0.73 and 0.73 , respectively.     

Source breakup dynamics in Au + Au collisions at sqrt[s(NN)]=200 GeV via three-dimensional two-pion source imaging

A three-dimensional correlation function obtained from midrapidity, low p(T), pion pairs in central Au+Au collisions at sqrt[s(NN)]=200 GeV is studied. The extracted model-independent source function indicates a long range tail in the directions of the pion pair transverse momentum (out) and the beam (long). A proper breakup time tau(0) ~ 9 fm/c and a mean proper emission duration Delta tau ~ 2 fm/c, leading to sizable emission time differences ({|Delta t(LCM)|} approximately 12 fm/c), are required to allow models to be successfully matched to these tails. The model comparisons also suggest an outside-in "burning" of the emission source reminiscent of many hydrodynamical models.     

液晶SmC*相自发极化的统计理论

采用格胞模型，定义在格胞中心上的极化序参量．选用双轴性并具有横向电矩的分子作用势，求得系统的自由能，得到自发极化强度的大小和螺旋变化螺距的表达式．计算了DOBAMBC和3M2CPOOB等4种典型SmC^*相液晶的自发极化强度大小和螺距随温度的变化及其受分子横向电矩的影响，结果与实验相符．表明SmC^*相的自发极化强度主要产生于分子的双轴性和横向电矩． 关键词：