Anomalous dielectric behavior in the nematic and isotropic phases of a strongly polar–weakly polar binary system

We report permittivity studies in the isotropic liquid and anisotropic liquid crystalline (nematic or N) phases of a binary system, in which one component has molecules with a strongly polar longitudinal group, and the other has a weakly polar transverse group. The dielectric constant in the isotropic phase as well as its anisotropy in the N phase show, surprisingly, an anomalous non-monotonic dependence with concentration having a peak-like behavior for 10.2 mol% concentration of the weakly polar constituent. The transition between the two phases, being weakly first order, exhibits pretransitional effects signifying the appearance of the N-like regions in the isotropic phase. The coordinates of the maximum point of the convex shaped temperature-dependence of the dielectric constant in the isotropic phase, characteristic of strongly polar systems, also exhibit a non-monotonic dependence with concentration. Possible causes for these observations are discussed.     

Near equilibrium dynamics and one-dimensional spatial–temporal structures of polar active liquid crystals

We systematically explore near equilibrium, flow-driven, and flow-activity coupled dynamics of polar active liquid crystals using a continuum model. Firstly, we re-derive the hydrodynamic model to ensure the thermodynamic laws are obeyed and elastic stresses and forces are consistently accounted. We then carry out a linear stability analysis about constant steady states to study near equilibrium dynamics around the steady states, revealing long-wave instability inherent in this model system and how active parameters in the model affect the instability. We then study model predictions for onedimensional(1D) spatial–temporal structures of active liquid crystals in a channel subject to physical boundary conditions.We discuss the model prediction in two selected regimes, one is the viscous stress dominated regime, also known as the flow-driven regime, while the other is the full regime, in which all active mechanisms are included. In the viscous stress dominated regime, the polarity vector is driven by the prescribed flow field. Dynamics depend sensitively on the physical boundary condition and the type of the driven flow field. Bulk-dominated temporal periodic states and spatially homogeneous states are possible under weak anchoring conditions while spatially inhomogeneous states exist under strong anchoring conditions. In the full model, flow-orientation interaction generates a host of planar as well as out-of-plane spatial–temporal structures related to the spontaneous flows due to the molecular self-propelled motion. These results provide contact with the recent literature on active nematic suspensions. In addition, symmetry breaking patterns emerge as the additional active viscous stress due to the polarity vector is included in the force balance. The inertia effect is found to limit the long-time survival of spatial structures to those with small wave numbers, i.e., an asymptotic coarsening to long wave structures. A rich set of mechanisms for generating and limiting the flow structures as well as the spatial–temporal structures predicted by the model are displayed.     

Rotational Rydberg states of polar molecules: Hund’s classification and Zeeman effect

The rotational Rydberg states of polar molecules, which arise as a result of the interaction of a Rydberg electron with core rotations, are considered. A large number of angular momenta in the core-electron system lead to a considerably greater number of possible coupling schemes of these momenta compared to the number of schemes determined by the classical five Hund’s cases for lower excited electron states of molecules. As a result of such detailed Hund’s classification, more than 30 different coupling schemes (Hund’s subcases) are constructed for rotational Rydberg states of molecules. The conditions of their realization are indicated in terms of the relative quantities of intramolecular interactions, for which analytical estimates are presented. For a large number of subcases, analytical expressions for the molecular matrix elements are found. These expressions can be useful in classification of the experimental spectra of highly excited molecules. As an application, for each of the subcases considered, analytical expressions are given, which describe the linear Zeeman effect and the Paschen-Back effect.     

Electrically driven motion,destruction, and chirality change of polar vortices in oxide superlattices

Topological polar vortices, which are electric analogs of magnetic objects, present great potential in applications of future nanoelectronics because of their nanometer size, anomalous dielectric response, and chirality. To enable the functionalities, it is prerequisite to manipulate the polar states and chirality by using external stimuli. Here, we probe the evolutions of polar state and chirality evolutions of topological polar vortices in Pb TiO;/Sr TiO;superlattices under an electric field by using atomically resolved in situ scanning transmission electron microscopy and phase-field simulations. We find that, under electric field, the chiral vortex cores can be moved laterally to form close-pair structures, transform into a/c domain stripes, and finally become a nonchiral c-domain. Such transition is reversible and spontaneous after bias removal. Interestingly, during switching and backswitching events, the vortex rotation can be changed, offering a potential strategy to manipulate vortex chirality. The revealed dynamic behavior of individual polar vortices at the atomic scale provides fundamentals for future device applications.     

Optical vibration modes and electron－phonon interaction in ternary mixed crystals of polar semiconductors

Optical vibrations of the lattice and the electron－phonon interaction in polar ternary mixed crystals are studied in the framework of the continuum model of Born and Huang and the random-element-isodisplacement model. A normal-coordinate system to describe the optical vibration in ternary mixed crystals is correctly adopted to derive a new Fr?hlich-like Hamiltonian for the electron－phonon interaction including the unit-cell volume variation influence. The numerical results for the phonon modes, the electron－phonon coupling constants and the polaronic energies for several typical materials are obtained. It is verified that the nonlinearity of the electron－phonon coupling effects with the composition is essential and the unit-cell volume effects cannot be neglected for most ternary mixed crystals.     

Smectic polymorphism of 4-decyloxybenzylidene- 4′-alkyloxyanilines and their mixtures with polar standards of mesophases

This article describes liquid crystalline properties of the homologous series of 4-decyloxybenzylidene-4′-alkyloxyanilines. Based on the polarization microscopy (POM and TOA methods) and the calorimetric (DSC) measurements a rich polymorphism has been detected. Investigated compounds exhibit nematic, smectic A, smectic C, smectic B, smectic I and G (smectic G) mesophases with their characteristic configurations. The presence of these mesophases was confirmed by the miscibility method, using 4-heptyloxybenxylidene-4′-pentylaniline as a mesophase reference. Nine other smectic standards with different polarity have been applied in order to investigate their influence on the mesophase stability.     

The supersymmetric WKB approximation of ring-shaped oscillator potential in r and θ dimensions with spherical polar coordinates

According to calculation of the energy spectrum of ring-shape oscillator potential by using the supersymmetric WKB approximation, it is shown that the energy spectrum of some noncentral separable potentials can be exactly obtained in r and θ dimensions by above method.     

Giant and controllable Goos–H?nchen shift of a reflective beam off a hyperbolic metasurface of polar crystals

We conduct a theoretical analysis of the massive and tunable Goos–H¨anchen(GH) shift on a polar crystal covered with periodical black phosphorus(BP)-patches in the THz range. The surface plasmon phonon polaritons(SPPPs), which are coupled by the surface phonon polaritons(SPh Ps) and surface plasmon polaritons(SPPs), can greatly increase GH shifts.Based on the in-plane anisotropy of BP, two typical metasurface models are designed and investigated. An enormous GH shift of about-7565.58 λ₀     

Interface phonon-polaritons in quantum well systems of polar ternary mixed crystals

The interface phonon-polaritons in quantum well systems consisting of polar ternary mixed crystals are investi-gated. The numerical results of the interface phonon-polariton frequencies in the GaAs/AlxGa1-xAs, ZnSxSe1-x/ZnS, and ZnxCd1-xSe/ ZnSe quantum well systems are obtained and discussed. It is shown that there are six branches of interface phonon-polariton modes distributed in three bulk phonon-polariton forbidden bands in the systems. The electric fields of interface phonon-polaritons are also presented and show the interface locality of the modes. The effects of the ’two-mode’ and ’one-mode’ behaviours of the ternary mixed crystals on the interface phonon-polariton modes are shown in the dispersion curves.     

Electrostatic surface guiding of cold polar molecules with double charged wires

We propose a novel scheme to guide cold polar molecules on the surface of an insulating substrate (i.e., a chip) using a static electric field generated by the combination of a pair of parallel charged wires and a grounded metal plate. We calculate the spatial distributions of the electric fields from the above charged-wire layout and their Stark potentials for cold CO molecules, and analyze the relationships between the electric field and the parameters of the charged-wire layout. The result shows that this charged-wire scheme can be used to guide cold polar molecules in the weak-field-seeking state and to form various molecule-optical elements, even to realize a single-mode molecular waveguide on a molecule chip under certain conditions.     

Critical behavior in the vicinity of nematic to smectic A (N–SmA) phase transition of two polar–polar binary liquid crystalline systems

Phase diagram, critical behavior and order of the nematic (N)–smectic A (SmA) phase transition of two polar–polar binary systems (i) 4-n-heptyloxy-4′-cyanobiphenyl (7OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB); (ii) 4-n-octyloxy-4′-cyanobiphenyl (8OCB) and 4-n-nonyloxy-4′-cyanobiphenyl (9OCB) by means of a high-resolution temperature scanning measurement of birefringence have been reported in this work. A simple power law analysis has been adopted to extract the specific heat critical exponent (α′) at N–SmA transition from birefringence data. The α′ for N–SmA transition indicates a uniform crossover behavior and has appeared to be non-universal in nature. With increasing concentration of the higher homologues for both the binary systems, the N–SmA transition reveals a strong tendency to be driven towards the tricritical nature. The 3D-XY limit (i.e. α′ = ?0.007) for N–SmA transition reaches at the concentration x_8OCB = 0.28 corresponding to the McMillan ratio 0.914, whereas the tricritical point has been found to appear near x_9OCB = 1.0 corresponding to McMillan ratio 0.992.     

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3（001） and polar（111） surfaces

The influence of surface polarity on the structural properties of BiFeO3 （BFO） thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 （STO） （100） and polar （111） surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar （001） or polar （111） surfaces perform very differently. BFO/STO （001）is a fully strained tetragonal phase with orientation relationship （001）[100]BFOII（001）[100]STO, while BFO/STO （111） is a rhombohedral phase. BFO/STO （111） has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO （001） and STO （111） are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.     

Quenching efficiency of “hot" polar molecules by He buffer gas at ultralow energies: quantum results for MgH and LiH rotations

Rotationally superelastic collisions are computed from quantum dynamical calculations for the two title systems, down to the very low collision energies of interest in cold trap experiments. The results are obtained via ab initio interaction potentials, the one for the MgH-He system being given here for the first time and discussed in details in the present paper. The calculations show marked differences in behaviour for both cooling efficiency rates at vanishing temperatures and individual cross sections between levels, and their origin is related to target structural properties and with specific features of the two potentials.     

Magnetic and piezoelectric properties of the Bi1 − x La x FeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi_{1 ? x} La _x FeO₃ solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.     

Surface phonon–polaritons in rectangular quantum wires of polar ternary mixed crystals

In this paper, the dispersion relations of surface phonon–polaritons in freestanding rectangular quantum wire systems of polar ternary mixed crystals are derived. The numerical calculations for Al_xGa_1−xAs and Zn_xCd_1−xSe quantum wire systems are performed. The results reveal that the frequencies of surface phonon–polariton modes are sensitive to the geometric structures of the quantum wires, the wave-vectors in z-direction, and the compositions of the ternary mixed crystal materials. The effects of the “two-mode” and “one-mode” behaviors of the ternary mixed crystals on the surface phonon–polariton modes are also discussed.     

Anisotropic polarization response of polar nanostructures in relaxor-type lead lanthanum zirconate titanate ferroelectric ceramics

Polar nanoregions (PNRs) in relaxor-type PLZT ferroelectric ceramics and their response to the out-of-plane, in-plane electric fields were investigated by the high-resolution piezoresponse force microscopy. Anisotropic polarization switching kinetic behaviors of PNRs were found in relaxor PLZT ceramics. Normal, stable ferroelectric domain states were formed through the coalescence of PNRs in the out-of-plane field, while in the high in-plane electric field, the polarization dynamic behaviors of PNRs show spatial inhomogeneity due to the anisotropy random fields in the ferroelectric PLZT ceramics.     

Water in the Moon’s polar areas: Results of LEND neutron telescope mapping

The results of LEND neutron telescope mapping of the lunar surface are presented. The estimations obtained show that the water content in the regolith in the polar areas of the Moon reaches 0.4 % by weight and not directly determined by the illuminated surface.