共查询到20条相似文献,搜索用时 15 毫秒
1.
S. Faetti M. Nobili I. Raggi 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):445-453
In this paper we report an experimental investigation on the dynamics of the azimuthal director reorientation at a nematic-solid
interface. Three qualitatively different kinds of substrates have been investigated: I) intrinsically anisotropic SiO-substrates
(-evaporation), II) isotropic SiO-substrates (-evaporation) and III) rubbed PVA-substrates. In the case II), an in-plane anisotropy was induced cooling slowly the thermotropic
nematic liquid crystal (NLC) from the isotropic phase in the presence of a 0.75 T magnetic field. The reorientation dynamic
of the surface azimuthal director angle at the switching-on and off of a magnetic (or electric) field has been investigated.
All the substrates show comparable azimuthal anchoring energies and two dynamic regimes: a fast dynamic response, driven by
the bulk director reorientation and an extremely slow reorientation. The slow dynamics is explained in terms of anisotropic
adsorption of NLC molecules on the solid substrate and is well represented by a stretched exponential.
Received 7 December 1998 相似文献
2.
Gunyakov VA Parshin AM Shabanov VF 《The European physical journal. E, Soft matter》2006,20(4):467-473
Specific features of the anisotropic interaction between a nematic mixture and a polar surface of a ferroelectric triglycine
sulfate crystal have been studied over a wide temperature range including the substrate's Curie point Tc. The mixture was composed of two nematic liquid crystals, 60% of p-methoxybenzylidene-p-n-butylaniline (MBBA) and 40% of p-ethoxybenzylidene-p-n-butylaniline (EBBA), and doped with a small amount of a dichroic dye. The temperature dependence of the polarized components
of optical density Dj of the dye absorption band for the nematic and isotropic phases of the MBBA+EBBA mixture has been obtained using polarization
optic techniques. The temperature-induced structural changes in the nematic layer near Tc were found to be related to the changes in the orientational part of the tensor order parameter Qik. The experimental data have been interpreted using the model, in which the dispersive van der Waals forces of the substrate
stabilize the planar orientation of the nematic in the bulk competing with the short-range anchoring forces in the vicinity
of Tc. At the same time, the anisotropic part of the surface energy has two terms with the orthogonal easy axes. The nature of
the surface electric field and its effect on the director alignment at the interface have been clarified. Taking into account
the known relation between anchoring strength and the nematic order parameter, the effective anchoring energy weff for the studied system has been determined as a function of temperature. 相似文献
3.
G. Barbero L.R. Evangelista N.V Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):327-331
We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties
of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director
with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric
coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed
ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We
also discuss the director distortion when the anchoring energy of the surface is finite.
Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997 相似文献
4.
R. Barberi I. Dozov M. Giocondo M. Iovane P. Martinot-Lagarde D. Stoenescu S. Tonchev L.V. Tsonev 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):83-91
We measure the azimuthal anchoring energy of the nematic 5CB on sinusoidal holographic unidimensional and bidimensional gratings.
We find that the anchoring strength is almost the same for all of the gratings and up to one order of magnitude stronger than
expected by the topographic mechanism proposed by Berreman. We observe strong memory effects which dominate both the elastic
anchoring and the easy axis gliding. We show that these memory effects limit from below the anchoring energy and are the main
obstacle in the realization of substrates with weak anchoring conditions.
Received: 29 April 1998 / Accepted: 23 June 1998 相似文献
5.
Summary A new reflectometric method to measure the azimuthal anchoring energy at the interface between a nematic liquid crystal and
an isotropic method is proposed. The spurious contribution to the reflectivity coefficients due to the presence of a director-field
distortion near the interface is calculated by using the numerical Berreman algorithm and is shown to be negligible for magnetic
fields small enough (H<10 kG). Preliminary measurements of the azimuthal anchoring energy at the interface between the NLC 5CB and a rubbed glass
plate are given.
Research supported in part by MPI and in part by CNR. 相似文献
6.
S. Scandolo F. Bassani V. Lucarini 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(3):319-323
We describe a procedure to take into account the spatial dispersion of the optical excitations in the susceptibility sum rules.
We show that this implies that relativistic corrections of the same order must be considered. The final result is a decrease
of the total oscillator strength equal to the ratio of the average electron kinetic energy with mc2. We propose experiments with synchrotron radiation sources on crystals of heavy elements to observe the described effect.
Received 5 June 2001 相似文献
7.
The effect of a surface electric field produced by ionic adsorption on the molecular orientation of a nematic liquid crystal
sample is analyzed. The eigenvalue problem for a semi-infinite medium is analytically solved both for strong and weak anchoring
situations. The threshold instabilities are numerically determined and it is shown that the homeotropic pattern can be destabilized
also in the situation of strong anchoring. The dependence of the threshold field on the anchoring strength and on the surface
polarization is determined by taking into account also the coupling of the quadrupolar component of the flexoelectric coefficient
with the field gradient.
Received 4 November 1999 and Received in final form 4 April 2000 相似文献
8.
A. Ahmad A. Alsaad 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(1):41-46
Zinc oxide thin films were sputter deposited on (100) silicon substrates at
250 ○C substrates temperature via reactive unbalanced dc magnetron
process using pure zinc target and argon/oxygen gases. The influence of the
applied dc sputtering power (between 100 to 250 Watts, step 50 Watts) on the
optical properties of the grown films was systematically investigated by
variable angle of incidence spectroscopic ellipsometry (VASE) technique. The
refractive indices were found to follow the second-order Sellmeier
dispersion relation. However, Cauchy-like dispersion model was formulated to
account for the absorption tail and excitonic structure near the direct band
gap. The optical properties such as refractive indices, extinction
coefficients, optical band gaps, Urbach's energies, excitonic binding
structure and absorption coefficients of the grown films were reported as a
function of dc power in the photon energy range between 1.2 eV and 4.2 eV.
The films were found to be polycrystalline with (002) preferred orientation. 相似文献
9.
A.J. Jin M.R. Fisch M.P. Mahajan K.A. Crandall P. Chu C.-Y. Huang V. Percec R.G. Petschek C. Rosenblatt 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):251-255
Magnetic and electrical Freedericksz measurements were performed on the second generation monodendritic liquid crystal G2(OH). The deduced elastic constants were found to be exceptionally anisotropic: the splay elastic constant K11 is more than an order of magnitude larger than the bend elastic constant K33, and the twist constant K22 is approximately twice K33. The results are discussed in terms of molecular conformations.
Received: 23 February 1998 / Revised and Accepted: 26 May 1998 相似文献
10.
Y. Galerne P. Hubert 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(2):245-249
The active oxygen gas arising from a plasma reactor is used to realize progressive chemical modifications onto silane coatings
that could be particularly interesting as alignment layers for liquid crystal display applications. Depending on the oxygen
density grafted onto the substrate, these alignment layers provide different zenithal anchoring angles, or pretilt angles,
with anchoring transitions, for polar and non-polar nematic liquid crystals as 5CB and MBBA, respectively. The anchoring transitions
are found to be smoother with the polar nematics. Such a behavior is discussed in terms of the differential wetting model
by adding a cosine term to the interaction energy between the nematic and the substrate. A local justification is proposed
for this symmetry breaking term.
Received: 18 May 1998 相似文献
11.
M. Petit M. Nobili P. Barois 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):341-345
We report the first observation of an electroclinical effect at the TGB-TGB transition induced by an external DC electric field applied perpendicular to the pitch direction. Upon increasing the field,
the smectic layers rather than the director field tilt over relative to the helical axis, allowing to detect the effect by
X-ray scattering from well aligned samples. The observations are qualitatively interpreted in the frame of a mean field phenomenological
model of a helically modulated electroclinical effect.
Received: 24 April 1998 / Revised: 22 June 1998 / Accepted: 31 July 1998 相似文献
12.
C. Chevallard M. Clerc P. Coullet J.-M. Gilli 《The European physical journal. E, Soft matter》2000,1(2-3):179-188
We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal
layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation
for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is
characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have
also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance
of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns
which have been both experimentally and theoretically pointed out.
Received 5 August 1999 and Received in final form 13 September 1999 相似文献
13.
Arkhipkin VG Gunyakov VA Myslivets SA Zyryanov VY Shabanov VF 《The European physical journal. E, Soft matter》2007,24(3):297-302
A one-dimensional ZrO2/SiO2 photonic crystal with a 4-n -pentyl-4' -cyanobiphenyl (5CB) nematic defect layer was used to investigate the transmission spectra of light polarized
parallel and perpendicular to the liquid-crystal director at different angles of incidence. The spectra of the photonic crystal
were shown to split into four polarized components Tij at oblique incidence. When the incident angle increased, the bandgap edges and the defect modes shifted towards short wavelengths,
while the amplitudes of the defect modes increased for the transverse magnetic polarization and decreased for the transverse
electric polarization. The observed discrepancy between the defect mode amplitudes in the center and near the edges of the
photonic bandgap was found to be related to the radiation losses inside the defect layer of a non-ideal photonic crystal.
The simulated transmission spectra obtained using recurrence relations and taking into account the decay of defect modes are
in good agreement with the experimental data. 相似文献
14.
R. Granger J.T. Benhlal O. Ndap R. Triboulet 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(3):429-436
The dielectric function of ZnSe has been deduced from ellipsometric measurements between 20 K and 380 K. is analysed around each critical point with the standard critical point model. The variations of the different parameters
characterising each transition with temperature are presented and analysed. The temperature coefficients of the energies of
the critical transitions are given. is essentially governed by the Coulomb interaction near the fundamental gap. Thanks to the high binding energy of the exciton
and the low spectral width of the ellipsometer, the fundamental state of the exciton is found completely separated from the
first excited states and the continuum at low temperature. In return the strong transition E1 near the L points of the Brillouin zone can be described equally well with a 2D or an excitonic transition.
Received 5 February 1999 and Received in final form 15 June 1999 相似文献
15.
N. Petit V. Garnier V. Ta Phuoc R. Caillard A.-M. Frelin A. Ruyter I. Laffez J.-C. Soret A. Maignan F. Gervais 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(4):423-429
The reflectivity spectra of an oriented ceramic of Bi-2223 has been investigated by polarized infrared reflectivity spectroscopy
in the energy range 0.005-2.2 eV. It is shown that the data for the polarization parallel to the c axis cannot be fitted with a one-component Drude or extended-Drude model. The conductivity spectrum is then obtained from
the best fit of a “double-damping Drude” model to reflectivity spectra, itself derived from the factorized form of the dielectric
function, and by a Kramers-Kronig inversion as well. The data and their analysis give a new insight of the 2D character of
the system.
Received 26 April 2001 and Received in final form 28 August 2001 相似文献
16.
R.A. Ganeev A.I. Ryasnyansky M.K. Kodirov S.R. Kamalov V.A. Li R.I. Tugushev T. Usmanov 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,20(1):129-131
The results of third-order nonlinear susceptibilities studies of Fe- and Zn-doped polyvinylpyrrolidone (PVP) aqueous solution
in processes of third harmonic generation of Nd:YAG laser radiation are presented. Nonlinear susceptibilities of PVP:Fe and
PVP:Zn were calculated to be 5×10-13 esu and 3×10-13 esu respectively. Third harmonic conversion efficiencies in these metallocomplexes were measured to be 8×10-7 and 5×10-7 respectively. The Z-scan method was applied to determine Kerr effect influence on frequency conversion process. The value
of nonlinear refractive index of PVP:Fe at the wavelength of λ = 1064 nm was measured to be n
2 = - 6.7×10-13 esu.
Received 30 November 2001 / Received in final form 27 March 2002 Published online 28 June 2002 相似文献
17.
N. Richard 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(3):411-422
This paper describes the magneto-optical effects and the reflectivity behaviors of bilayers based on magnetic and isotropic
()/anisotropic () layers under the condition of total internal reflection. In the framework of Green's dyadic technique, we show accurately
the optical properties of anisotropic layers deposited on a substrate. We present numerical simulations which account for
the variation of angle of incidence at the HeNe laser wavelength. The Kerr rotation is found to increase significantly around
the optical modes in total reflection. We also discuss the importance of anisotropic effects due to the crystallization of
the dielectric material () which occur in the reflectivity and Kerr rotation spectra.
Received 26 January 2000 相似文献
18.
C. Benjamin 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,52(3):403-410
Reflectance measurements from p-type GaSb:Zn epitaxial films with different hole concentrations (1017–1018 cm-3) have been investigated over the frequency region of
100–1000 cm-1. A minimum broadening feature corresponding to the hole plasmon was observed in the reflectance spectra. The experimental
infrared spectra were well fitted using a
Lorentz-Drude dispersion model. The real part ε1 of the dielectric function decreases with increasing hole concentration. However, the imaginary part ε2
increases with hole concentration in the far-infrared region.
This indicates that the acoustic- and optic-phonons mainly
participate in the free carrier absorption processes. The hole
mobility obtained from Hall-effect measurements is slightly
larger than that derived from optical measurements and the
average ratio of mobilities is estimated to be 1.33. Owing to
overdamping effects, the upper branch of longitudinal-optical
phonon plasmon (LPP) coupled modes was observed. The upper
LPP+ frequency increases with hole concentration and it
shows a transition from phonon-like to plasmon-like behavior. A
theoretical analysis with solutions in the complex frequency
plane describes these experimental results. 相似文献
19.
W. X. Shu Z. Ren H. L. Luo F. Li 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(3):541-546
We systematically investigate the reflection and refraction of an
electromagnetic wave between two semi-infinite anisotropic
magnetoelectric materials. Using the integral formulation of Hertz
vectors and the principle of superposition, we generalize the
extinction theorem and derive the propagation characteristics of
wave. Applying the results obtained, we find a general origin of
Brewster effect. We also show that, through choosing appropriate
material parameters, oblique or omnidirectional total transmission
can occur to TE and TM waves. Compared to the traditional method,
the method used here discloses the underlying mechanism of wave
propagation between two arbitrary anisotropic materials and can be
applied to other problems of propagation. 相似文献
20.
R. Ferrini G. Guizzetti M. Patrini A. Parisini L. Tarricone B. Valenti 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):449-458
In0.49Ga0.51P films, both undoped and doped n- and p-type (up to 1018 cm-3), were grown lattice matched on GaAs substrates, with different miscut angles, by Metal-Organic Vapour Phase Epitaxy (MOVPE)
at different temperatures. The shift of the fundamental gap E0, caused by “ordering effect” was measured as a function of temperature by photoluminescence. The complex refractive index
= n + ik and the dielectric function = ɛ
1 + iɛ
2 at room temperature were determined from 0.01 to 5.5 eV by using complementary data from fast-Fourier-transform far-infrared
(FFT-FIR), dispersive, and ellipsometric spectroscopies. The effect of the native oxide was accounted for and the self-consistency
of the optical functions was checked in the framework of the Kramers-Kronig causality relations. In the restrahlen region
the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below E0, the refractive index was modelled by a Sellmeier dispersion relation; in the interband region the dielectric function was
well reproduced by analytical lineshapes associated to seven critical points. Thus parametrized analytical expressions were
obtained for the optical functions all over the spectral range, without discontinuities, to be used in the modelling and characterization
of multi-layer structures, also on opaque substrates.
Received 13 December 2001 Published online 25 June 2002 相似文献