Monodomain State Research Articles

Dynamics of the director reorientation in confined nematic liquid crystals imposed by a strong orthogonal electric field

The dynamics of the periodic distortions in confined nematic liquid crystals (LCs) has been investigated theoretically basing on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in confined LC film, only in response to the suddenly applied strong electric field. It has been shown that there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values of the amplitude dominate the uniform mode. During the turn-off process the reorientation of the director to the direction preferred by the surfaces is characterized by the complex destruction of the initially periodic structure to a monodomain state.

A(2+) Cation Control of Chiral Domain Formation in A(TiO)Cu4(PO4)4 (A = Ba, Sr).

Single crystals of two novel tetragonal chiral materials, A(TiO)Cu4(PO4)4 (A = Ba, Sr), were grown from Na2Mo2O7 flux, and their crystal and chiral domain structures were characterized. Polarized-light microscopy studies of the chiral domain structures in the crystals show that Ba(TiO)Cu4(PO4)4 mostly hosts a multidomain state, while a monodomain state predominantly appears in Sr(TiO)Cu4(PO4)4. To explain this striking difference, we quantified the chirality strength of these materials by comparing atomic positions in the chiral and nearest-achiral crystal structures, revealing larger chirality strength in Sr(TiO)Cu4(PO4)4 than in Ba(TiO)Cu4(PO4)4. Our proposed mechanisms linking the chirality strength and domain formation can account for the different occurrence frequency of chiral domains in this system.

A glimpse at topological structures in multiferroic materials

A glimpse at topological structures in multiferroic materials

新物質<i>A</i>（TiO）Cu<sub>4</sub>（PO<sub>4</sub>）<sub>4</sub>（<i>A</i>＝Ba, Sr）における結晶カイラリティの強度とドメイン構造の相関

Crystal chirality, a label defining the handedness of a system without improper symmetry, has long been an intensive subject in a variety of research fields such as chemistry, biology, and solid-state physics. Its great importance in solid-state physics lies in the potential unique functionalities of chiral materials such as optical activity, piezoelectricity, and recently discovered unconventional magnetism and associated magneto-transport and magnetoelectric phenomena. An important step toward further development of the chirality research would be a quantification of chirality and a clarification of its relationship to physical properties. Here, we report the discovery of two novel chiral materials, A(TiO)Cu4(PO4)4 (A＝Ba, Sr), and the detailed characterization of their crystal and chiral domain structures. We identify that the chiral crystal structure of this system can be characterized by an antiferrorotative distortion of structural units, indicating that the rotation angle is a good quantitative measure of the chirality strength. Interestingly, the rotation angle and thus the chirality strength can be tuned by changing A site cations. We observe striking differences in the occurrence frequency of chiral domains in the two materials：weakly chiral Ba(TiO)Cu4(PO4)4 mostly hosts a multidomain state, while strongly chiral Sr(TiO)Cu4(PO4)4 predominantly shows monodomain state. On the basis of these results, we discuss the relationship between the chirality strength and the occurrence frequency of chiral domains in this system.

Emergence of Negative Capacitance in Multidomain Ferroelectric-Paraelectric Nanocapacitors at Finite Bias.

The emergence of negative capacitance in an ultrathin ferroelectric/paraelectric bilayer capacitor under electrical bias is examined using first-principles simulation. An antiferroelectric-like behavior is predicted, and negative capacitance is shown to emerge when the monodomain state becomes stable after bias application. The polydomain-monodomain transition is also shown to be a source of capacitance enhancement.

Multidomain switching in the ferroelectric nanodots

Controlling the polarization switching in the ferroelectric nanocrystals, nanowires and nanodots has an inherent specificity related to the emergence of depolarization field that is associated with the spontaneous polarization. This field splits the finite-size ferroelectric sample into polarization domains. Here, based on 3D numerical simulations, we study the formation of 180° polarization domains in a nanoplatelet, made of uniaxial ferroelectric material, and show that in addition to the polarized monodomain state, the multidomain structures, notably of stripe and cylindrical shapes, can arise and compete during the switching process. The multibit switching protocol between these configurations may be realized by temperature and field variations.

Utilizing mechanical loads and flexoelectricity to induce and control complicated evolution of domain patterns in ferroelectric nanofilms

We have conducted a systematical investigation to reveal the stability and evolution path of various ferroelectric domain patterns in nanofilms subjected to mechanical loads and related flexoelectric field. Within a rigorous framework of flexoelectricity, a phase-field approach has been established for simulating the domain structure of ferroelectric nanofilms. The electromechanical fields of the nanofilms are numerically solved by a fast Fourier transform technique (FFT) based on the combination of Khachaturyan's microscopic elastic theory and Stroh's formalism of anisotropic elasticity. Using this approach, we simulate eight types of domain patterns that can be stabilized in the nanofilms. It is further demonstrated that these domain patterns can be significantly affected by the mechanical loads and related flexoelectric field and exhibit fruitful evolution paths. To adapt the applied mechanical strain and strain gradient, the domain pattern may remain stable, evolve into another polydomain pattern, or become a monodomain state (an effect of domain erasing). The domain fraction, detailed domain morphology, average stresses in the nanofilms, average polarization and temporal evolution characteristics of the domain patterns under various mechanical loads and sources of flexoelectric field have been analyzed. This investigation should provide instructive information for the practical application of ferroelectric nanofilms under complex and changeable mechanical conditions.

Dynamics of the periodic distortions in confined nematic liquid crystals imposed by a strong orthogonal electric field

The dynamics of the periodic distortions in confined nematic liquid crystals (LCs) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in confined LC film, only in response to the suddenly applied strong electric field. It has been shown that there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the non uniform rotation mode rather than the uniform one, whereas the lower values of the amplitude dominate the uniform mode. During the turn-off process, the reorientation of the director to the direction preferred by the surfaces is characterized by the complex destruction of the initially periodic structure to a monodomain state. Analysis of evolution of the director distribution in the LC film under the influence of the pretransitional fluctuations when the temperature goes to the bulk nematic-smectic A transition temperature TNA shows that there is two scenarios of the evolution. First, in the reduced temperature range -5⩽logTTNA-1⩽-2, dominate the non uniform rotation mode and the growing pretransitional fluctuations does not destructed the initially periodic structure, whereas at the lower temperature towards TNA, the reorientation of the director field is characterized by the complex destruction of the initially periodic structure to a monodomain state with the director field aligned parallel to the direction preferred by the surfaces.

Finite size effects in ferroelectric-semiconductor thin films under open-circuit electric boundary conditions

General features of finite size effects in the ferroelectric-semiconductor film under open-circuit electric boundary conditions are analyzed using Landau-Ginzburg-Devonshire theory and continuum media electrostatics. The temperature dependence of the film critical thickness, spontaneous polarization, and depolarization field profiles of the open-circuited films are found to be significantly different from the characteristics of short-circuited ones. In particular, we predict the re-entrant type transition boundary between the mono-domain and poly-domain ferroelectric states due to reduced internal screening efficiency and analyzed possible experimental scenarios created by this mechanism. Performed analysis is relevant for the quantitative description of free-standing ferroelectric films phase diagrams and polar properties. Also our results can be useful for the explanation of the scanning-probe microscopy experiments on free ferroelectric surfaces.

Dual relaxation and structural changes under uniaxial strain in main-chain smectic-C liquid crystal elastomer.

The relationship between strain-dependent macroscopic elastic behavior and the changes in microscopic structure of the smectic-C liquid crystal elastomer (LCE), C11MeHQSi8 were investigated using synchrotron X-ray studies. At very low strains ε ≤ 0.2, the smectic layers are randomly oriented. As the strain increases beyond 0.2, the smectic layers reorient and become parallel to the direction of the applied strain. The polydomain to monodomain (P-M) transition accompanied by the formation of chevron structure ensues for ε > 0.2 and is nearly complete for ε = 0.7. The chevron structure relaxes after the applied strain changes, with a time constant τα ∼ 45 min while the orientation order parameters of the mesogenic and elastomeric components gradually increase and saturate at 0.83 and 0.4, respectively at ε = 1.7 which is near the end of the plateau region. Relaxation rates τα for the tilt angle and τd corresponding to the smectic layer spacing both become about 10 times faster when the strain exceeds 0.7. The LCE remains "locked" into the monodomain state and retains 90% and 80% values of α and S, respectively for 24 hours after the applied strain is removed. The viscoelastic properties of the liquid crystal appear to dominate the equilibration process at low strains while the elastomeric properties control the system's response at high strains.

Coplanar switching of polarization in thin films of vinylidene fluoride oligomers

Switching of vinylidene fluoride oligomer thin with molecular chains aligned normal to the substrate and exhibiting a preferential in-plane have been investigated using coplanar geometry of inter-digital via high-resolution It has been shown that in-plane switching proceeds via non-180° rotation of dipoles mediated by non-stochastic expansion, and coalescence of domains. As-grown multidomain configuration is found to be strongly pinned aided by charged and the electrically induced (in-plane) mono-domain states relax to the as-grown state. The observed (approximately 0.6 MV/m) is two to three orders of magnitude smaller than that for the oligomer with out-of-plane It is suggested that the low steric hindrance to the rotation of molecular dipoles gives rise to the observed low

Spontaneous Growth of Strain-Free Magnetite Nanocrystals via Temperature-Driven Dewetting

Temperature-driven dewetting of a self-template layer has been exploited to spontaneously grow epitaxial magnetic Fe3O4 nanocrystals by pulsed laser deposition. A 10 unit cell thick Fe3O4 template layer was deposited at 400 °C on a perovskite SrTiO3(001) substrate, followed by heating at 1100 °C, which induced dewetting of the Fe3O4 template layer, forming three-dimensional islands that were uniformly distributed over the substrate surface area. These islands were used as seed crystals for the growth of spatially separated quasi-epitaxial Fe3O4 nanopyramids. Structural analysis by scanning transmission electron microscopy and X-ray diffraction revealed incoherent growth of fully relaxed (001)-oriented Fe3O4 nanocrystals on the SrTiO3 (001) substrate. Higher-order epitaxial matching of the substrate and film lattices appears to be responsible for the well-defined in-plane orientation of the Fe3O4(001) nanopyramids despite the large nominal lattice mismatch of −7.5%. The nanopyramid structure strongly affected the magnetic characteristics of magnetite, dramatically reducing the coercive field compared to conventional magnetite thin films. The coercivity of the Fe3O4 nanopyramids was also strongly size dependent. This is attributed to a transition between poly- and monodomain states. A superparamagnetic phase was detected for the smallest pyramid sizes.

Dynamics of the modulated distortions in confined nematic liquid crystals

The peculiarities in the dynamics of the director reorientation in confined nematic liquid crystals (LCs) under the influence of a strong electric field E have been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in confined LC film, only in response to the suddenly applied strong E. It has been shown that there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values of the amplitude dominate the uniform mode. During the turn-off process, the reorientation of the director to the direction preferred by the surfaces is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Field-induced director dynamics in confined nematic liquid crystals imposed by a strong orthogonal electric field

The field-induced director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion with appropriate boundary and initial conditions. Analysis of the numerical results for the turn-on process provides evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4'-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage of 200 V across the 194.7 μm LC film and the magnetic field of 7.05 T directed at the angle α=1.57(~89.99°) between two fields, there is a threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provides the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α [<1.565(~88.81°)] dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destruction of the initially periodic structure to a monodomain state.

Field-induced non-uniform director reorientation for a low molar mass nematic imposed by a strong orthogonal electric field

The director dynamics for a low molar mass nematic liquid crystal (LC) has been investigated theoretically based on the hydrodynamic theory including the director motion. Analysis of the numerical results for the turn-on process provides an evidence for the appearance of the spatially periodic patterns in 4-n-pentyl-4′-cyanobiphenyl LC film, only in response to the suddenly applied strong electric field orthogonal to the magnetic field. It has been shown that at the values of the voltage in 200V across the 194.7μm LC film and the magnetic field of 7.05T directed at the angle α=1.57(~89.99°) between two fields there is the threshold value of the amplitude of the thermal fluctuations of the director over the LC sample which provide the nonuniform rotation mode rather than the uniform one, whereas the lower values both of the amplitude and the angle α (<1.565(~88.81°)) dominate the uniform mode. During the turn-off process, the reorientation of the director to its equilibrium orientation is characterized by the complex destroying of the initially periodic structure to a monodomain state.

Modeling the polydomain-monodomain transition of liquid crystal elastomers.

We study the mechanism of the polydomain-monodomain transition in liquid crystalline elastomers at the molecular scale. A coarse-grained model is proposed in which mesogens are described as ellipsoidal particles. Molecular dynamics simulations are used to examine the transition from a polydomain state to a monodomain state in the presence of uniaxial strain. Our model demonstrates soft elasticity, similar to that exhibited by side-chain elastomers in the literature. By analyzing the growth dynamics of nematic domains during uniaxial extension, we provide direct evidence that at a molecular level the polydomain-monodomain transition proceeds through cluster rotation and domain growth.

Dielectric and Piezoelectric Properties of K(Ta0.51Nb0.49)O3 Single Crystal

Dielectric and piezoelectric properties of K(Ta0.51Nb0.49)O3 [KTN] single crystals along [100], [001] and [011] pseudo-cubic crystallographic orientations are reported and discussed. Temperature dependences of the dielectric permittivity in tetragonal phase of monodomain and polydomain crystals of these orientations are compared. A strong dielectric anisotropy is found in the tetragonal phase, with an ϵ[100] / ϵ[001] ratio equal to 16 at 300 K. Room temperature piezoelectric coefficients for the monodomain state of KTN are reported.

A stochastic principle behind polar properties of condensed molecular matter

A statistical mechanics view leads to the conclusion that polar molecules allowed to populate a degree of freedom for orientational disorder in a condensed phase thermalize into a bi-polar state featuring zero net polarity. In cases of orientational disorder polar order of condensed molecular matter can only exist in corresponding sectors of opposite average polarities. Channel type inclusion compounds, single component molecular crystals, solid solutions, optically anomalous crystals, inorganic ionic crystals, biomimetic crystals and biological tissues investigated by scanning pyroelectric and phase sensitive second harmonic generation microscopy all showed domains of opposite polarities in their final grown state. For reported polar molecular crystal structures it is assumed that kinetic hindrance along one direction of the polar axis is preventing the formation of a bi-polar state, thus allowing for a kinetically controlled mono-domain state. In this review we summarize theoretical and experimental findings leading to far reaching conclusions on the polar state of solid molecular matter. “… no stationary state … of a system has an electrical dipole moment.” P. W. Anderson, Science, 1972, 177, 393.

Some aspects of antiferromagnetic ordering in LiMnP0.85V0.15O4: Neutron diffraction and DC-magnetization studies

V-substituted LiMnPO4 has been synthesized by the solid state reaction route. Combined Rietveld refinement of neutron and X-ray data revealed that all vanadium ions are located in the same positions as the phosphorus ions. The magnetic structure of LiMnP0.85V0.15O4 was found to be the same as that described for undoped antiferromagnetic LiMnPO4 (TN=34.5K). DC-magnetization measurements were carried out to study the peculiarities of magnetic ordering in LiMnP0.85V0.15O4. An irreversible behavior of DC magnetization was found at magnetic fields less than ∼2kOe. It was demonstrated that an increase in the magnitude of the applied field leads to a reduction of the discrepancy between zero-field-cooled and field-cooled processes. These effects were explained by the movement of domain walls and by transition of the system to a monodomain state. The anomaly in the magnetization vs field dependence attributed presumably to the spin-flop transition was observed at ∼40kOe. The existence of magnetic inhomogeneity in the paramagnetic phase of LiMnP0.85V0.15O4 was proved by the analysis of the χ⁎T product. It was concluded that the observed changes in χ⁎T are indicative of a competition between ferromagnetic and antiferromagnetic correlations at temperatures slightly exceeding TN.

Study on High-Frequency 3–D Magnetization Precession Modes of Circular Magnetic Nano-Dots Using Coplanar Wave Guide Vector Network Analyzer Ferromagnetic Resonance

Ferromagnetic resonance (FMR) modes for Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">70</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">30</sub> magnetic nano-dots of 100 nm in diameter in a mono-domain state are studied under different in-plane and out-of-plane magnetic fields using a reflection measurement from a vector network analyzer with a coplanar wave guide (CPW VNA FMR) and a micromagnetics simulation. The observed real and imaginary parts of the reflection signal become a combination of symmetric and antisymmetric Lorentzian forms in FMR. The combinations are given different susceptibility components of χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> and χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">zx</sub> by changing the sample position on the CPW. The magnetic dots exhibit triple modes in FMR spectra from an influence of the demagnetization field distribution in the mono-domain state. The magnetic dots also show a different variation of FMR amplitude in χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">xx</sub> and χ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">zx</sub> for an out-of-plane field increase.