First-order Orientational Transition Research Articles

Optical nonlinearity of a dual-frequency nematic liquid crystal via temperature-mediated mapping of dielectric anisotropy.

Liquid crystals are of great interest in the field of nonlinear optics due to their efficient response to low intensity light fields. Here we present a new, to the best of our knowledge, mechanism of a nonlinear optical response which is observed for a dye-doped dual-frequency nematic liquid crystal. The local increase in temperature caused by the absorption of light beam in the liquid crystal medium leads to spatial variation and inversion of the sign of the dielectric anisotropy. When an alternating current electric field with a frequency close to the cross-over frequency is applied to the liquid crystal cell, the planar orientation sustains at the beam periphery, but elastic deformation occurs in the irradiation region. In the case of a dye dopant with negative absorption dichroism, a first-order orientational transition with large bistability region is obtained.

Orientational phase transition in methane-intercalated fullerite C60

The known first-order orientational phase transition in pure ${\mathrm{C}}_{60}$ occurs at 260 K and is accompanied by the change from face-centered cubic $(Fm3m)$ to simple cubic unit cell $(Pa\overline{3})$. While ${\mathrm{C}}_{60}$ molecules rotate freely above 260 K, they are fixed in inversion centers below 260 K. The phase transition in methane-intercalated ${\mathrm{C}}_{60}$, ${(\mathrm{C}{\mathrm{H}}_{4})}_{x}({\mathrm{C}}_{60})$, occurs at $236\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. It was studied by DSC and single crystal x-ray diffraction. Above 236 K, the intercalate adopts the face-centered cubic structure which is characterized by freely rotating ${\mathrm{C}}_{60}$ and disordered ${\mathrm{CH}}_{4}$ molecules. The ordered low-temperature crystal structure possesses simple cubic unit cell in the space group $Pa\overline{3}$ in which both ${\mathrm{C}}_{60}$ and ${\mathrm{CH}}_{4}$ molecules are fixed on threefold inversion axes. In the structure of ${(\mathrm{C}{\mathrm{H}}_{4})}_{0.72}{\mathrm{C}}_{60}$ at 100 K, a ${\mathrm{C}}_{60}$ molecule is fixed by its 12 ${\mathrm{C}}_{60}$ neighbors due to donor-acceptor van der Waals interactions between electron-rich 6:6 C-C bonds and electron-deficient cage pentagons. At 200 K, the second orientation of a ${\mathrm{C}}_{60}$ molecule is present (with 7.5% occupancy) in which 6:6 C-C bonds are directed to cage hexagons. The methane molecules are situated in octahedral holes and statistically disordered over two orientations around inversion centers. The phase transition temperature of $(\mathrm{C}{\mathrm{H}}_{4})\phantom{\rule{0.16em}{0ex}}{\mathrm{C}}_{60}$ intercalate is lower than in ${\mathrm{C}}_{60}$ by $\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ due to the increased ${\mathrm{C}}_{60}\ensuremath{\cdots}{\mathrm{C}}_{60}$ separation. This study provides a reliable structural information about the low-temperature structure of the ${(\mathrm{C}{\mathrm{H}}_{4})}_{0.7}({\mathrm{C}}_{60})$ intercalate thus demonstrating significant differences between orientational phase transitions in pure fullerite ${\mathrm{C}}_{60}$ and ${\mathrm{C}}_{60}$ intercalated with methane. In addition, the molecular structure of ${\mathrm{C}}_{60}$ fullerene in the ${(\mathrm{C}{\mathrm{H}}_{4})}_{0.7}{\mathrm{C}}_{60}$ intercalate is distinguished by a high precision which is superior to the former experimental data for ${\mathrm{C}}_{60}$ and its adducts reported in the literature.

Influence of Ferromagnetic Carbon Nanotubes on Magnetic Transitions in Liquid Crystals

Magnetic field-induced orientational transitions in suspensions of carbon nanotubes (CNTs) in nematic liquid crystals have been studied in the framework of a continuum theory. Ferromagnetic CNTs are considered that possess, in addition to a large anisotropy of diamagnetic susceptibility, an additional magnetic moment due to ferromagnetic particles either situated on the CNT surface or encapsulated inside. It is established that, depending on the energy of coupling between disperse CNTs and liquid-crystalline matrix, the external magnetic field induces various sequences of orientational transitions. A threshold value of the coupling energy is determined, above which a transition from the initial homogeneous planar phase to inhomogeneous state (angular phase) becomes possible. At coupling energies below the threshold, the increasing magnetic field induces the following sequence of orientational transitions: homogeneous planar phase–inhomogeneous (angular) phase–homogeneous homeotropic phase–inhomogeneous angular phase. It is established that, depending on the intensity of segregation phenomena, these transitions can be of the first or second order. Analytical expressions for the fields of second-order transitions between phases are found. On an example of the optical phase difference between the ordinary and extraordinary rays of light passing through the cell of CNT suspension, it is shown that the suspension can exhibit optical bistability during the first-order orientational transition.

Light-induced first-order orientational transitions in a nematic liquid crystal in the presence of an ordinary wave

The effects of light-induced reorientation of the director of a nematic liquid crystal doped with dendrimer are investigated experimentally. The influence of light polarisation on the firstorder orientational transition that occurs with changing the light beam power is studied. An irreversible first-order orientational transition when changing the polarisation of light is discovered and examined. A theoretical description of the observed effects is presented.

Optical bistability of the director field of the dendrimer-doped nematic liquid crystal

It was found that the second-generation carbosilane dendrimers with statistically distributed terminal aliphatic and azobenzene fragments induce negative orientational nonlinearity in nematic liquid crystals (NLC) (the NLC director turns perpendicular to the light field, decreasing the refractive index of an extraordinary wave). The light-induced Freedericksz transition in planar oriented NLCs is the first-order orientational phase transition characterized by a wide bistability region.

Vortex lattice transformation in d-wave superconductors with Pauli paramagnetic effect

By self-consistently solving the Eilenberger equations in a vortex state, we quantitatively evaluate the free energy for various lattice configurations of the d-wave superconductor under applied magnetic fields along to the c-axis, and discuss the dependence of stable configuration on the paramagnetic effect. By the paramagnetic effect, though the distribution of zero energy local density of states does not extend to the nodal direction above the middle fields, it still has good connection with vortex cores at the nodal directions. Then it makes square vortex lattice stable. We show that there is a first-order orientational transition before rocking into the square vortex lattice at low fields.

Simulation study of the phase behavior of a planar Maier-Saupe nematogenic liquid

Using extensive Monte Carlo simulations and a simple approximation in density functional theory, we study the phase behavior of a fluid of nematogenic molecules with centers of mass constrained to lie in a plane but with axes free to rotate in any direction, both with and without an external disorienting field perpendicular to the plane. We find that simulation predicts the existence of an order-disorder phase transition belonging to the Berezinskii-Kosterlitz-Thouless type, along with a low temperature gas-liquid transition. In contrast to the simulation results, density functional theory predicts a first-order orientational phase transition coupled continuously with a first-order gas-liquid transition. The approximate theoretical approach qualitatively reproduces the field dependence of the order-disorder and gas-liquid transitions but is far from quantitative.

Specific Features of Magnetoacoustic Waves in Fe[sub 3]BO[sub 6

The specific features of magnetoacoustic waves propagating in an orthorhombic antiferromagnet with weak ferromagnetism, namely, the Fe3BO6 orthoborate, are investigated experimentally in the vicinity of the spontaneous, first-order orientational phase transition. It is revealed that, at the phase transition point, the amplitude of active acoustic waves interacting with spin waves increases significantly. A phenomenological theory of magnetoelastic waves in orthoferrites is proposed. This theory allows for an intermediate domain structure existing in the range of a first-order orientational phase transition and offers a satisfactory explanation of the experimentally observed anomaly in the amplitude of active acoustic waves.

The dynamics of solid cubane

Molecular solids are an intriguing class of materials in which molecules rather than individual atoms make up the underlying lattice. Because molecules possess rotational degrees of freedom that single atoms do not, molecular solids can exhibit orientational phase transitions. At sufficiently high temperatures the molecules can enter into a phase where they rotate about one or more axes, instead of librating about a fixed angular orientation as they do at low temperatures. Perhaps the best known molecular solid is solid C60 which is composed of nearly spherical molecules that exhibit a first-order orientational phase transition precisely because the C60 molecular symmetry deviates slightly from that of a sphere.

Liquid-Crystal Phase Transitions Induced by Microtextured Substrates.

We show that novel orientational states and temperature-dependent tilt angle of the director may be induced in nematic liquid crystals by microtextured substrates, i.e., spatially mixed alignment potentials. In particular, for an alternating stripe pattern of planar and homeotropic substrate potentials, there is a first-order orientational transition between two states with bulk directors orthogonal to each other. This transition can be observed by varying either the temperature or the texturing periodicity, or through the application of an electric field normal to the substrate.

Microscopic Description of Anchoring Transitions

Abstract The anchoring direction taken by a nematic liquid crystal at the surface of a substrate can be varied, leading to anchoring transitions. Several examples of such transitions have been observed at the surface of mica when the system is put in presence of water and alcohol vapour. A macroscopic model, characterizing the liquid crystal-mica interface by its energy as a function of the liquid-crystal orientation, can reproduce all the observed transitions but does not give any information about the microscopic mechanisms leading to these transitions. More insight on these mechanisms is obtained from studies of the orientational distribution of the liquid crystal molecules in contact with the substrate performed using optical second harmonic generation. These studies show that the bulk reorientation characterizing the first-order anchoring transitions observed on mica is driven by a first-order orientational transition in the surface liquid crystalline layer. More generally, these studies provide the boundary conditions on the liquid crystalline orientational order necessary for a microscopic description of the order close to a substrate surface.

Orientational order in dense molecular hydrogen: A first-principles path-integral Monte Carlo calculation.

A quantum-mechanical simulation of orientational order in solid molecular hydrogen is presented, based on path-integral Monte Carlo calculations. The effective potential between nearest-neighbor hydrogen molecules is derived from first-principles total energy calculations, taking full account of the symmetries of the Hamiltonian within a hexagonal close-packed arrangement of the molecule centers. A first-order orientational order transition is found by studying short- and long-range angular correlations and the behavior of the corresponding order parameters. A pressure-temperature phase diagram is constructed and compared to relevant experimental and theoretical results.

Anchoring of nematic liquid crystals on mica in the presence of volatile molecules.

Using optical second-harmonic generation, we have studied the orientational order of cyanobiphenyl liquid-crystal monolayers at the interface between a mica crystal and a liquid-crystal film put in the presence of ethylene glycol vapor. These small, volatile molecules adsorb at the mica surface and significantly affect the orientational order of the surface liquid-crystal molecules. As the chemical potential of the volatile molecules was increased, this orientational order first varied continuously, and at a certain stage, exhibited a discontinuous change corresponding to a first-order orientational transition. This surface transition drives the first-order anchoring transition (i.e., the reorientation of the bulk liquid crystal) previously observed in these systems.

Bulk reorientation driven by orientational transition in a liquid crystal monolayer.

Optical second-harmonic generation has been used to study the effect of coadsorbed small volatile molecules on the orientational order of a liquid crystal monolayer at the interface between a mica surface and a liquid crystal film. As the chemical potential of the volatile molecules was increased, a first-order orientational transition was observed in the monolayer. This surface transition is shown to drive the reorientation of the bulk liquid crystal previously observed in these systems.

Weak first-order orientational transition in the Lebwohl-Lasher model for liquid crystals.

The nature of the orientational phase transition in the three-dimensional Lebwohl-Lasher model of liquid crystals has been studied by computer simulation using reweighting techniques and finite-size scaling analysis. Unambiguous numerical evidence is found in favor of a weak first-order transition and the presence of pseudospinodal points, ${\mathit{T}}_{\ifmmode\pm\else\textpm\fi{}}^{\mathrm{*}}$, which are extremely close to the equilibrium transition temperature, \ensuremath{\Vert}${\mathit{T}}_{\mathit{c}}$-${\mathit{T}}_{\ifmmode\pm\else\textpm\fi{}}^{\mathrm{*}}$\ensuremath{\Vert}/${\mathit{T}}_{\mathit{c}}$\ensuremath{\lesssim}0.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$, in good agreement with experimental data for the nematic-isotropic transition.

Optical spectroscopy studies of orientational phase transitions in antiferromagnetic dielectrics

Abstract Optical spectroscopy studies of magnetic phase transitions have been performed on the biaxial antiferromagnet CsMnCl3 · 2H2O and the uniaxial antiferromagnet MnF2. It is shown that at a critical value of the magnetic field Hc ≈ 1.8 T oriented along the “easy” b-axis in antiferromagnet CsMnCl3 · 2H2O a first-order orientational phase transition (spin-flop transition) occurs. As a result of the spin-flop transition the antiferromagnetism vector l is oriented along the “intermediate” c-axis. The maximum field deflection angle Ψc from the b-axis in the bc-plane at which the first-order phase transition disappears is determined. The magnetic Hamiltonian parameters obtained are responsible for the energy of the “interionic” (5.04 · 10−2 T) and “intraionic” (1.05 · 10−1 T) magnetic anisotropy. The antiferromagnetism vector l direction is determined in antiferromagnet MnF2 in the spin-flop phase (l||[100]) for an exact field orientation along the “easy” C4-axis. Magnetic anisotropy parameters in the basa...

Cluster variation expansion for solid ortho-hydrogen

The cluster variation method is applied to the orientational ordering of ortho-hydrogen molecules on a rigid f.c.c. lattice. In the three-spin cluster approximation with nearest neighbor quadrupole-quadrupole coupling a first-order orientational transition is found. A substantial amount of short-range order persists above the transition.