共查询到20条相似文献,搜索用时 406 毫秒
1.
Ismaili M Anakkar A Joly G Isaert N Nguyen HT 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(1):519-525
We report here the composition-temperature phase diagram of a mixture between n=9 and n=16 terms of a chiral tolans series shortly noted nF(2)BTFO1M7. This diagram constitutes an experimental illustration of one of the three theoretical phase diagrams predicted by the Renn and Lubensky's model. The pure compounds n=9 and n=16 exhibit, respectively, the phase sequences Cry-Sm-C*-Sm-A-TGB(A)-N*-BP-I and Cry-Sm-C*-N*-BP-I, where TGB refers to a twist grain boundary phase. Phases identification and transition temperatures, at atmospheric pressure, have been determined by both optical microscopy and photothermal methods. The experimental phase diagram shows the disappearance of Sm-A and TGB(A) mesophases and the appearance of a TGB(C) phase when the n=16 composition increases. Four of the five multicritical points theoretically anticipated by Renn and Lubensky are pointed out. 相似文献
2.
B. Wantusiak S. Zalewski P. Suchodolski Mirosława D. Ossowska-Chruściel 《Phase Transitions》2013,86(5):444-452
This article reports the results on phase transitions, mesomorphic and electro-optical (EO) properties of new bis [4-(4′-alkylphenylthiocarbonyl)phenyl] isophtalates from a homologous series of banana-shaped thioesters, referred to as IFOSn, when n?=?8, synthesized by us. The IFOS8 compound has two enantiotropic phases: B2 with antiferroelectric order and B6. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarized light optical microscopy and transmitted light intensity. EO measurements were also carried out. 相似文献
3.
We discuss the coupling between flow and other hydrodynamic variables that can occur in the optically isotropic tetrahedratic
phase (Td) characterized by a third rank tensor order parameter Tijk. We point out that an applied electric field or an applied temperature gradient will lead to flow. Reciprocally we predict
that, for example, a shear flow applied to a tetrahedratic phase leads to an induced electric field and a temperature gradient.
Similarities to recent experimental observations in the vicinity of the isotropic-B7 phase transitions in materials formed
by banana-shaped molecules are discussed.
Received 31 July 2001 相似文献
4.
E. S. Larin 《Physics of the Solid State》2004,46(8):1560-1568
The phase diagrams of transitions from an isotropic phase to nematic and smectic phases are investigated within a simple phenomenological model of the Landau thermodynamic potential. The conditions of the isomorphic phase transition between two uniaxial smectic phases and the direct transition from the isotropic phase to the uniaxial and biaxial smectic phases are determined. The behavior of the order parameters is described along different thermodynamic paths. The theoretical results are discussed using the example of liquid-crystal phases in compounds with banana-shaped achiral molecules. 相似文献
5.
M.Cvetinov D.Z.Obadovic M.Stojanovic A.Vajda K.Fodor-Csorba N.Eber I.Ristic 《中国物理 B》2014,(9):341-346
In this work, five mixtures with different concentrations of banana-shaped and calamitic compounds have been prepared and subsequently studied by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction on non-oriented samples. The phase sequences and molecular parameters of the binary systems are presented. 相似文献
6.
The first experimental evidence for triclinic symmetry of bulk smectic liquid-crystal samples of achiral banana-shaped molecules is presented. This phase corresponds to the so-called Sm-CG phase consisting of biaxial molecules and characterized by two tilt directions with respect to the layer normal: tilt of the molecular plane (clinic) and tilt of the molecular kink direction (leaning). Each smectic layer has a polarization component normal to the smectic layers (C1 symmetry). The observations suggest that the phase tentatively labeled as B7 is identical with the Sm-CG phase. 相似文献
7.
B K Sadashiva 《Pramana》1999,53(1):213-222
We describe briefly some results obtained on both chiral and achiral compounds exhibiting chiral mesophases. We report the
first example of a single component system exhibiting the undulated twist grain boundary C* or UTGB
C* phase. Preliminary results concerning a few achiral compounds composed of banana-shaped molecules exhibiting a mesophase
is reported. They have been investigated by polarized light optical microscopy, differential scanning calorimetry and x-ray
diffraction studies. 相似文献
8.
We show that defect-free splay-bend textures are less energetic compared to uniform states in liquid-crystalline phases that
possess both quadrupolar (nematic) and octupolar (tetrahedratic) order. This is because, in such systems, there is a symmetry-allowed
linear gradient term in the energy. Another unusual feature of these splay-bend textures is the fact that they have a non-homogeneous,
space-dependent free-energy density. These results may help clarify some mysterious features noted for the B7 liquid-crystal
phase formed by achiral banana-shaped molecules. 相似文献
9.
Ultrasound tagging of light provides a unique way to probe photon density inside turbid media. We show that this technique allows one to probe the well-known banana-shaped photon density noninvasively, giving rise to a new tool for modeling diffusive photon propagation. Moreover, we show that this technique is quantitative and allows one to get a precise determination of the absorbing constituents inside the turbid medium. 相似文献
10.
We discuss the symmetry properties and the macroscopic behavior of a nematic liquid crystal phase with D2d symmetry. Such a phase is a prime candidate for nematic phases made from banana-shaped molecules where the usual quadrupolar
order coexists with octupolar (tetrahedratic) order. The resulting nematic phase is nonpolar. While this phase could resemble
the classic D
∞h nematic in the polarizing microscope, it has many static as well as reversible and irreversible properties unknown to nonpolar
nematics without octupolar order. In particular, there is a linear gradient term in the free energy that selects parity leading
to ambidextrously helical ground states when the molecules are achiral. In addition, there are static and irreversible coupling
terms of a type only met otherwise in macroscopically chiral liquid crystals, e.g. the ambidextrous analogues of Lehmann-type effects known from cholesteric liquid crystals. We also discuss the role of hydrodynamic
rotations about the nematic director. For example, we show how strong external fields could alter the D2d symmetry, and describe the non-hydrodynamic aspects of the dynamics, if the two order structures, the nematic and the tetrahedratic
one, rotate relative to each other. Finally, we discuss certain nonlinear aspects of the dynamics related to the non-commutativity
of three-dimensional finite rotations as well as other structural nonlinear hydrodynamic effects. 相似文献
11.
In this paper we present a review of five-rings banana-shaped molecules derived from isophthalic acids. This study deals with
about a hundred compounds and most of them have not been published. By a combination of several linking groups and different
selected substituents either on the outer rings or on the central core, several mesophases with switching properties are induced.
The study of homologous series underlines the importance of the length and nature of the terminal chains. X-ray analysis reveals
several new structures. 相似文献
12.
H.R. Brand P.E. Cladis H. Pleiner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):347-353
We discuss the macroscopic behavior of smectic CG liquid crystals. Smectic CG is the most general tilted smectic phase that is fluid in the layers. It is characterized by global C1 symmetry. Consequently, it is ferroelectric, pyroelectric and piezoelectric, opening up a number of possible applications
for such a phase. As smectic CG-phase has a macroscopic hand due to its structure, it is a natural candidate to explain the recent experimental observations
of left and right-handed helices in a system composed of achiral molecules. We also discuss critically to what extent smectic
CG could be important for liquid crystalline phases formed by banana-shaped molecules. Phase transitions involving a smectic
CG phase and defects of its in-plane director are briefly discussed.
Received: 25 March 1998 / Revised: 15 June 1998 / Accepted: 15 July 1998 相似文献
13.
As a remarkable orientationally ordered soft-condensed matter system, the smectic-C*(alpha) phase exhibits an incommensurate nanoscale helical pitch of more than four smectic layers. Recent theoretical advances predict the existence of a new phase showing a helical pitch of less than four smectic layers (<16 nm). By applying a novel optical technique to the study of freestanding films with increments in thickness of one smectic layer, we have measured the size of this pitch using 633 nm HeNe laser light. Our results confirm the existence of the newly predicted phase in one unique compound. 相似文献
14.
ABSTRACTA broad-frequency dielectric spectroscopy (100 Hz to 10 MHz) was used to investigate newly synthesized fluorinated compound. This compound exhibits a wide-temperature ferroelectric phase. Due to the fact that the compound does not crystallize at temperatures available in our DSC equipment, additional measurements were done at the very low temperatures (down to ?100 °C) to find the crystallization temperature. Finally, we did not manage to crystallize the investigated compound. Compound under studies seems to be overcooled. Observed dielectric modes are described and their parameters are shown in this paper. Additionally, DC field influence on dielectric response is shown and analyzed here. Both modes observed at the low temperature are recognized as the molecular ones. 相似文献
15.
Sławomir Zalewski 《Phase Transitions》2016,89(4):383-392
ABSTRACTIn this article we present results concerning phase transitions and physical properties of the ferroelectric phase of the compound (S)-4-(1-methylheptyloxy)biphenyl–4'-(heptyloxy phenyl)-4-carboxylate (MHOBOPO7). The compound has the following phases: smectic ferroelectric C (SmC*), chiral nematic N*, and two defected phases, TGBC and blue phase. The mesomorphic properties were investigated by means of three complementary methods: differential scanning calorimetry, polarizing light optical microscopy, and transmitted light intensity. The electro-optical measurements were carried out on an ordered sample in a middle electric field during very slow cooling from the nematic phase to the ferroelectric phase. 相似文献
16.
消色差谱相位延迟器的光谱特性测试研究 总被引:6,自引:1,他引:5
本文应用归一化偏振调制原理建立的测试系统,对常规菲涅耳菱体,改进型菲涅菱体,斜入射型以及直角棱镜组合型的消色差相位延迟的光谱特性进行了测试研究,得到了各延迟器的消色差特性曲线,并对测试结果进行了分析。结果表明改进型菲涅耳菱体以及直色棱镜组合型型消色差相位延迟器,是目前较理想的实用型消色相位延迟器。 相似文献
17.
Wang ST Liu ZQ McCoy BK Pindak R Caliebe W Nguyen HT Huang CC 《Physical review letters》2006,96(9):097801
Employing both null transmission ellipsometry and resonant x-ray diffraction, we confirmed the SmC*FI2-SmC* phase sequence reversal in one liquid crystal compound and specially prepared binary mixtures. This phase sequence reversal was predicted by two recent theoretical advances. Moreover, the temperature range for the SmC*FI2 phase increases significantly in the mixture suggesting that such a phase sequence may exist in other compounds. 相似文献
18.
消色差相位延迟器的光谱特性测试研究 总被引:1,自引:0,他引:1
本文应用归一化偏振调制原理建立的测试系统 ,对常规菲涅耳菱体、改进型菲涅耳菱体、斜入射型以及直角棱镜组合型的消色差相位延迟器的光谱特性进行了测试研究 ,得到了各延迟器的消色差特性曲线 ,并对测试结果进行了分析。结果表明改进型菲涅耳菱体以及直角棱镜组合型消色差相位延迟器 ,是目前较理想的实用型消色差相位延迟器。 相似文献
19.
We discuss changes in the symmetry and physical properties of an isotropic phase which has initially tetrahedral symmetry characterized by four unit vectors. In its undeformed state, these four vectors are at the tetrahedral angle (
) to each other. We find that this optically isotropic phase becomes uniaxial under the influence of an external electric field,
, resulting in a phase with C3v symmetry. For an applied simple shear flow, the system becomes biaxial and a time-dependent state with C1 symmetry arises. We discuss to what extent deformations induced by external forces and flows on this optically isotropic phase, which we call a "deformable tetrahedratic phase", are consistent with observations at the isotropic-B7 transition found recently in compounds composed of banana-shaped molecules and suggest a number of experiments to test the conclusions of this model.Received: 28 June 2002, Published online: 15 July 2003PACS:
61.30.Gd Orientational order of liquid crystals; electric
and magnetic field effects on order - 64.70.Md Transitions in liquid crystals - 05.70.Ln Nonequilibrium irreversible thermodynamics 相似文献
20.
Using a density functional approach, we study the structural and magnetic properties of the pyrox-ene compound NaTiSi2O6. While all previous workers are taking that NaTiSi2O6 is a quasi-one-dimensional S=1/2 system, our theoretical results indicate that this is a Haldane S=1 chain compound below the phase transition at 210 K. A good agreement is obtained between the calculated and the measured Ti-Ti distances in the dimerized low temperature phase. We present a simple explanation of the flow of the unusual phase transition which is taking place in this compound. 相似文献