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1.
Inspired by the observations of a remarkably stable biaxial nematic phase [van?den?Pol et?al., Phys. Rev. Lett. 103, 258301 (2009)], we investigate the effect of size polydispersity on the phase behavior of a suspension of boardlike particles. By means of Onsager theory within the restricted orientation (Zwanzig) model we show that polydispersity induces a novel topology in the phase diagram, with two Landau tetracritical points in between which oblate uniaxial nematic order is favored over the expected prolate order. Additionally, this phenomenon causes the opening of a huge stable biaxiality regime in between uniaxial nematic and smectic states. 相似文献
2.
Robert A. Skutnik Louis Lehmann Sergej Püschel-Schlotthauer George Jackson Martin Schoen 《Molecular physics》2019,117(20):2830-2845
ABSTRACTMonte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species. 相似文献
3.
External magnetic field-dependent behavior of tricritical points of uniaxial–biaxial nematic phase transition in a mixture of prolate and oblate molecules has been studied using the Landau phenomenological theory. Topological classification of phase diagrams in the field-temperature coordinates is performed using a sixth-order term in order parameter tensor. For both prolate and oblate molecules, the tricritical order parameters corresponding to the uniaxial–biaxial increase with the intensity of the external magnetic field. Field-induced paranematic phase has only been reported for prolate molecules, but for oblate molecules, field-induced splitting of the order parameter has been reported. 相似文献
4.
We show that quantum and thermal fluctuations in spin-2 Bose-Einstein condensates lift the accidental degeneracy of the mean-field phase diagram. Fluctuations select the uniaxial (square biaxial) nematic state for scattering lengths a4>a2 (a4相似文献
5.
We show that the newly discovered 52Cr Bose condensate in zero magnetic field can be a spin nematic of the following kind: a "maximum" polar state, a "colinear" polar state, or a biaxial nematic ferromagnetic state. We also present the phase diagram with a magnetic field in the interaction subspace containing the chromium condensate. It contains many uniaxial and biaxial spin nematic phases, which often but not always break time reversal symmetry, and can exist with or without spontaneous magnetization. 相似文献
6.
M. A. Aliev N. Yu. Kuz’minykh 《Russian Journal of Physical Chemistry B, Focus on Physics》2013,7(1):50-61
Weak segregation theory is applied to studying the phase behavior of a melt of a binary multigraft copolymer with polydisperse side chains regularly spaced along the backbone. The resulting phase diagrams of the melt show an increase of the temperature of transition to an ordered state with increasing coefficient of polydispersity of side chains. The effects of the average length of the side chains and the distance between the points of their grafting on the phase diagram of the melt are examined. 相似文献
7.
We numerically observe the effect of homogeneous magnetic field on the modulationally stable case of polar phase in F = 2 spinor Bose-Einstein condensates (BECs). Also we investigate the modulational instability of uniaxial and biaxial (BN) states of polar phase. Our observations show that the magnetic field triggers the modulational instability and demonstrate that irrespective of the magnetic field effect the uniaxial and biaxial nematic phases show modulational instability. 相似文献
8.
Phase Transition and Band Structure Tuned by Strains in Al_(1/2)Ga_(1/2)N Alloy of Complex Structure
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《中国物理快报》2016,(7)
Phase transition and band structure tuned by uniaxial and biaxial strains are systematically investigated based on the density-functional theory for ordered Al_(1/2)Ga_(1/2)N alloys of complex structures.Although the structural transformations to graphite-like from wurtzite are energetically favorable for both types of strain,the phase transitions are different in nature:the second-order transition induced by uniaxial strain is jointly driven by the mechanical and dynamical instabilities and the first-order transition by biaxial strain only by the mechanical instability.The wurtzite phase always shows the direct band gap,while the band gap of the graphite-like phase is always indirect.Furthermore,the band gaps of the wurtzite phase can be reduced by both types of strain,while that of the graphite-like phase is enhanced by uniaxial strain and is suppressed by biaxial strain. 相似文献
9.
We analyze the global phase diagram of a Maier-Saupe lattice model with the inclusion of shape-disordered degrees of freedom
to mimic a mixture of oblate and prolate molecules (discs and cylinders). In the neighborhood of a Landau multicritical point,
solutions of the statistical problem can be written as a Landau-de Gennes expansion for the free energy. If the shape-disordered
degrees of freedom are quenched, we confirm the existence of a biaxial nematic structure. If orientational and disorder degrees
of freedom are allowed to thermalize, this biaxial solution becomes thermodynamically unstable. Also, we use a two-temperature
formalism to mimic the presence of two distinct relaxation times, and show that a slight departure from complete thermalization
is enough to stabilize a biaxial nematic phase. 相似文献
10.
Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase. 相似文献
11.
Braga WS Kimura NM Luders DD Sampaio AR Santoro PA Palangana AJ 《The European physical journal. E, Soft matter》2007,24(3):247-250
The phase diagram is an interesting field of research, particularly in lyotropic liquid crystals (LLC). In this way, one of the most important phase diagrams of this LLC system was reported by Yu and Saupe. Two uniaxial (calamitic--N(C) and discotic--N(D)) and one biaxial nematic (N(B)) phases were determined by these authors. Furthermore, in this phase diagram the classical isotropic phase (I was observed at high temperature as well as a reentrant isotropic phase (I(RE)) which takes place at lower temperature. Later, this phase diagram was also studied by several authors and in all cases the I(RE)-N(C)-I phase transitions were not observed. In this work, we present a study of this phase diagram through digital image processing and refractometry optical techniques. The occurrence of these phase transitions is investigated and characterized. In addition, the order parameter is obtained based on the Vuks hypothesis from a particular point, in the range of the N(C) phase, where the absolute value of the optical birefringence (Deltan is maximum. 相似文献
12.
A model nematogenic mixture of rod-like and plate-like particles has been studied using Monte Carlo computer simulation. The mixture is found to separate into two uniaxial phases and does not form a biaxial nematic as previously proposed. 相似文献
13.
R. Micnas 《Physica A》1977,89(3):431-460
The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained.Using the standard-basis operator method, the collective excitation spectrum is studied in detail; moreover the softening of the excitations at the phase boundaries is discussed. A self-consistent version of RPA with emphasis on the role of kinematic restrictions with regard to the standard-basis operators is analyzed.Moreover, the phase transitions are considered in the Ising model, where a line of tricritical points occurs, and the planar model, where a line of bicritical points, two lines of tricritical points and a line of triple points, occur. 相似文献
14.
We determine the bifurcation phase diagrams with isotropic (I), uniaxial (N(U)) and biaxial (N(B)) nematic phases for model bent-core mesogens using Onsager-type theory. The molecules comprise two or three Gay-Berne interacting ellipsoids of uniaxial and biaxial shape and a transverse central dipole. The Landau point is found to turn into an I-N(B) line for the three-center model with a large dipole moment. For the biaxial ellipsoids, a line of Landau points is observed even in the absence of the dipoles. 相似文献
15.
We study here a mean field model to obtain the phase diagram (concentration versus temperature) near the NAC* point in a binary mixture of liquid crystal. We have fitted our phase line equations to the experimental data for the mixture of SCE9 + SCE10 liquid crystals. We deduce from our analysis that there should exist a tricritical point close to the NAC* point on the AC* phase line. 相似文献
16.
17.
Symmetry of a nematic liquid crystal phase is broken by an anchoring wall and also by an external field. Nematic system sandwiched
between biaxial anchoring walls is introduced as a correspondent to a bulk nematic system exposed to a couple of fields, an
electric field and a magnetic one in directions perpendicular to each other, and thermal behaviours of the system are studied.
The crossover between a homeotropic structure and homogeneous one occurs, similarly to the bulk system in the fields, in which
the anchoring condition of coexistence is shown to have the same expression as the one at the bulk. As to a characteristic
phenomenon at the sandwiched system, it is proved that an appearance of a biaxial nematic order suppresses a uniaxial nematic
order. A surface transition, i.e., a wetting phenomenon is shown to occur also in the biaxial nematics, even though the uniaxial
order is suppressed therein. 相似文献
18.
We report on variable-angle deuterium NMR measurements of nematic liquid-crystalline side-chain polymers, where we have found proof of the existence of a biaxial nematic phase in a system with a side-on attachment of the mesogenic group to the polymer backbone. This provides efficient coupling to the polymer backbone and thus stabilizes the as yet elusive biaxial phase. The experimental approach is validated on a uniaxial end-on nematic polymer, and the reliability of the results is investigated in detail using two-dimensional correlation spectra, providing information on director distribution effects. 相似文献
19.
Chaudhuri P Karmakar S Dasgupta C Krishnamurthy HR Sood AK 《Physical review letters》2005,95(24):248301
The phase diagram of a polydisperse hard-sphere system is examined by numerical minimization of a discretized form of the Ramakrishnan-Yussouff free-energy functional. Crystalline and glassy local minima of the free energy are located and the phase diagram in the density-polydispersity plane is mapped out by comparing the free energies of different local minima. The crystalline phase disappears and the glass becomes the equilibrium phase beyond a "terminal" value of the polydispersity. A crystal-to-glass transition is also observed as the density is increased at high polydispersity. The phase diagram obtained in our study is qualitatively similar to that of hard spheres in a quenched random potential. 相似文献
20.
D. Ď. Obadović A. Vajda M. Stančić S. Jovanović-šanta J. Petrović D. Lazar 《Phase Transitions》2013,86(4):509-519
The synthesis of new chiral seco-estrone derivatives is presented, as well as their influence on the phase transition of binary mixtures of cholesteryc liquid crystals. The new chiral derivatives do not posses any liquid crystalline phases and were synthesized in several synthetic steps, starting from estrone. We have studied the mixtures of cholesteryl non-anoate (40%) with cholesteryl myristate (40%) and addition of new chiral derivatives 3 4, or 5 (20%). It was concluded that the addition of chiral derivative 3 to the binary mixture stabilizes smectic A and cholesteric phase and shifts the phase transition temperature with respect to pure binary mixture for about 5°C towards lower temperatures. The extension of the temperature range of the cholesteric phase from 5°C to 15°C was established in the case when the derivatives 3 and 4 are added to the binary mixture of cholesteryl nonanoate with cholesteryl myristate. The phase diagrams of investigated compounds are formed on the basis of data obtained by the optical microscopy. Using X-ray diffraction on the crystalline powder of unoriented samples we have determined the molecular parameters: the thickness of smectic and cholesteric layers and average distance between the long axes of neighboring molecules. 相似文献