Sign Of Anisotropy Research Articles

Surface bound states in the continuum in Dyakonov structures

Surface bound states in the continuum (SBICs) have been found to occur in diverse settings, but so far always at the interface of nonhomogeneous media, such as discrete lattices or periodic systems. Here, we show that they can also exist at the interface of homogeneous media, resulting in unique SBICs. Specifically, we found that, contrary to general belief, leaky Dyakonov states exist at the interface between materials that exhibit opposite signs of anisotropy. In addition, properly breaking the anisotropy symmetry leads to the formation of both guided states and also SBICs embedded within the continuum. A direct implication of our finding is the possibility to create SBICs and Dyakonov states in a whole new class of materials and metamaterials.

Experimental evidence of a change of exchange anisotropy sign with temperature in Zn-substituted Cu2OSeO3

We report small-angle neutron scattering from the conical state in a single crystal of Zn-substituted Cu2OSeO3. Using a 3D vector-field magnet to reorient the conical wavevector, our measurements show that the magnitude of the conical wavevector changes as a function of crystallographic direction. These changes are caused by the anisotropic exchange interaction (AEI), whose magnitude transitions from a maxima to a minima along the <111> and <100> crystallographic directions respectively. We further find that the AEI constant undergoes a change of sign from positive to negative with decreasing temperature. Unlike in the related compound FeGe, where similar behaviour of the AEI induces a reorientation of the helical wavevector, we show that the zero field helical wavevector in (Cu0.98Zn0.02)2OSeO3 remains along the <100> directions at all temperatures due to the competing fourth-order magnetocrystalline anisotropy becoming dominant at lower temperatures.

A comparable study of the EPR spectra and local structures for 3d1‐ and 3d9‐doped halo‐borate glasses

AbstractThe spin Hamiltonian parameters (SHPs) of Cu2+‐doped halo‐glasses PbX2–PbO–B2O3 (X = F and Br) are investigated by means of the perturbation method where the impact of the halides and the local structure of the dopants, that is, the [CuO6]10− clusters suffering the tetragonal elongation distortions, is involved. By comparing with the obtained results of the 3d1 system (VO2+), Cu2+ and V4+ are complementary state, which show opposite distortions. Furthermore, the d‐d transition bands and the sign of anisotropy g factors also show whole opposite concentration variation trends with similar fluctuations due to the increase of non‐bridging oxygen (NBO). The values of the optical basicity Λth referring to the Pauling electronegativity, whose influence on E1 and g factors are also discussed. The above correlations would be beneficial to analyze and predict the properties of analogous borate glasses and ceramics when doped with different transition metal ions.

Dielectric properties of the system: 4-n-pentyloxybenzoic acid– N-(4-n-butyloxybenzylidene)-4᾽-methylaniline

Objectives. Our aim was to study the dielectric properties of the 4-n-pentyloxybenzoic acid– N-(4-n-butyloxybenzylidene)-4’-methylaniline system and reveal how different concentrations of N-(4-n-butyloxybenzylidene)-4’-methylaniline additives affect the dielectric properties of 4-n-pentyloxybenzoic acid.Methods. System properties were investigated using polarization thermomicroscopy and dielcometry.Results. We found that dielectric anisotropy changes its sign from positive to negative at the transition temperature of the high-temperature nematic subphase to the low-temperature one. The anisotropy of the dielectric constant of N-4-n-butoxybenzylidene-4’-methylaniline has a positive value and increases as to the system approaches the crystalline phase. The crystal structure of the 4-n-pentyloxybenzoic acid contains dimers formed by two independent molecules due to a pair of hydrogen bonds. The crystal structure of N-(4-n-butoxybenzylidene)-4’-methylaniline contains associates formed by orientational interactions of two independent molecules. 4-n-Pentyloxybenzoic acid dimers (270 nm) and associates of N-4-n-butoxybenzylidene-4’- methylaniline (250 nm) proved to have approximately the identical length. Considering the close length values of the structural units of both compounds and the dielectric anisotropy sign, we assume that the N-4-n-butoxybenzylidene-4’-methylaniline associates are incorporated into the supramolecular structure of the 4-n-pentyloxybenzoic acid. The specific electrical conductivity of the compounds under study lies between 10−7 and 10−12 S∙cm−1. The relationship between the specific electrical conductivity anisotropy and the system composition in the nematic phase at the identical reduced temperature, obtained between 100 and 1000 Hz is symbatic. However, the electrical conductivity anisotropy values of the system obtained at 1000 Hz are lower compared to those obtained at 100 Hz. At N-(4-n-butoxybenzylidene)-4’-methylaniline concentrations between 30 and 60 mol %, the electrical conductivity anisotropy values are higher than those of the individual component.Conclusions. A change in the sign of the dielectric constant anisotropy of the 4-n-pentyloxybenzoic acid during nematic subphase transitions was established. We showed that the system has the highest dielectric constant anisotropy value when components have an equal number of moles. Highest electrical conductivity anisotropy values are observed when the concentration of the N-4-n-butoxybenzylidene-4᾽-methylaniline system lies between 30 and 60 mol %.

Cortical bone drilling: A time series experimental analysis of thermal characteristics

Bone drilling is an essential and technically demanding procedure in many forms of surgery. The thermal characteristics in the drilling process directly affect thermal damage to the bone and postoperative recovery. In this study, a comprehensive experimental investigation was conducted to understand the thermal characteristics during cortical bone drilling using a bone cutting system coupled with a state-of-the-art infrared thermography and a dynamometer. The time-series temperature distributions, temperature history at different locations, and quasi-three-dimensional maximum temperature distributions were established and analyzed. The results show that the chisel edge, cutting lips, and margin are the main heat sources during bone drilling. The maximum temperature was generated by the cutting lip. Drilling in three feeding directions was performed to investigate the bone anisotropy. However, no sign of anisotropy on the temperature distribution was observed. But the maximum temperature and overheat duration exhibited a significant variation because of differences in the removal mechanism and bone density. Moreover, the effects of processing parameters on the thermal characteristics were analyzed in terms of the failure mechanism of bone. The drilling process duration may be the governing factor that can finally determine the thermal damage. This is the first study to reveal the thermal characteristics in full detail at the drill-exit with respect to the bone properties, processing parameters, drill-bit geometries, and inclined drilling. These results provide a fundamental understanding of the thermal characteristics in bone drilling and subsequently contribute to the formulation of various strategies for reducing the thermal damage to the bone.

Noncoplanar multiple- Q spin textures by itinerant frustration: Effects of single-ion anisotropy and bond-dependent anisotropy

We theoretically investigate multiple-$Q$ spin textures, which are composed of superpositions of spin density waves with different wave numbers, for an effective spin model of centrosymmetric itinerant magnets. Our focus is on the interplay between biquadratic interactions arising from the spin-charge coupling and magnetic anisotropy caused by the spin-orbit coupling. Taking into account two types of the magnetic anisotropy, single-ion anisotropy and bond-dependent anisotropy, we elucidate magnetic phase diagrams for an archetypal triangular-lattice system in the absence and presence of an external magnetic field. In the case of the single-ion anisotropy, we find a plethora of multiple-$Q$ instabilities depending on the strength and the sign of the anisotropy (easy plane or easy axis), including a noncoplanar triple-$Q$ state regarded as a skyrmion crystal with topological number of two, and coplanar and noncoplanar double-$Q$ states. In an external magnetic field, we find that another noncoplanar triple-$Q$ state, a skyrmion crystal with topological number of one, is stabilized by the in-plane (out-of-plane) magnetic field under the easy-plane (easy-axis) anisotropy. A part of the results, especially for the relatively large biquadratic interaction, qualitatively reproduce those in the Kondo lattice model, which explicitly includes itinerant electrons [S. Hayami and Y. Motome, Phys. Rev. B 99, 094420 (2019)]. We also examine the stability of the field-induced skyrmion crystal by rotating the field direction. As a by-product, we show that a different triple-$Q$ state with nonzero chirality appears in the presence of the biquadratic interaction and the easy-axis anisotropy. Meanwhile, we find that the bond-dependent anisotropy also stabilizes both types of skyrmion crystals. We show that, however, for the skyrmion crystal with topological number of one, Bloch- and N\'eel-type skyrmion crystals are selectively realized depending on the sign of the bond-dependent anisotropy, since this anisotropy selects a particular set of the helicity and vorticity. Moreover, we find yet another multiple-$Q$ states with nonzero spin scalar chirality, including two types of meron crystals with the skyrmion numbers of one and two. The systematic investigation of multiple-$Q$ instabilities in triangular itinerant magnets will provide a reference to complex magnetic textures in centrosymmetric magnetic metals.

Switching of easy-axis to easy-plane anisotropy in cobalt(ii) complexes

In situmicrocalorimetry monitored assembly and coligand induced switching of the magnetic anisotropy sign have been observed in a β-diketonate-Co(ii) system.

Polarized Fluorescence in Indole under Two-Photon Excitation by Femtosecond Laser Pulses

We have analyzed the decay of fluorescence intensity in indole dissolved in propylene glycol under two-photon excitation by linearly and circularly polarized femtosecond laser pulses in the wavelength range 475–510 nm. The dependences of fluorescence intensity I0, anisotropy r, and parameter Ω on the excitation energy have been determined and analyzed. In particular, a nonmonotonic behavior of I0 indicating the increase of indole excited state density in the excitation energy range above 5.1 eV and the anisotropy sign reversal due to the variation of the symmetry of indole vibronic states have been observed and interpreted theoretically.

Dynamic control of cylindrical vector beams via anisotropy.

We demonstrate that the spatially diffractive properties of cylindrical vector beams could be controlled via linear interactions with anisotropic crystals. It is the first time to show experimentally that the diffraction of the vector beams can be either suppressed or enhanced significantly during propagation, depending on the sign of anisotropy. Importantly, it is also possible to create a linear non-spreading and shape-preserving vector beam, by vanishing its diffraction during propagation via strong anisotropy in a crystal. The manageable diffractive effect enables manipulating propagation dynamics of the circular Airy vector beams, i.e., their propagation trajectories can be dynamically controlled by weakening or enhancing self-acceleration of the Airy beam. We further demonstrate that the cylindrical vector beams with initially zero orbital angular momentum can be rotated either clockwise or anticlockwise, relying on the sign of the anisotropy.

Unusual electro-optical behaviour of the nematic polyacrylate

ABSTRACTHigh sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties.The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated.

Temperature-dependent dielectric function of nickel

Confirming historical results by Ornstein and Koefoed (1938), the authors found an anomaly in the optical constants at 1.96eV for bulk nickel near the Curie temperature through careful high-precision spectroscopic ellipsometry measurements from 80 to 800K. The anomaly is only seen in sweeps with increasing temperature if the sample carries a net magnetization. In decreasing temperature sweeps or for unmagnetized samples, the anomaly is absent. The sign of the anomaly in the optical conductivity at 1.96eV is in contrast to the sign of the anomaly in the electrical DC conductivity. The anomaly is rather large and therefore explained with changes in the on-diagonal Drude–Lorentz portion of the dielectric tensor. No sign of anisotropy (polar magneto-optical Kerr effect) is found in the data.

Anisotropy of magnetic properties of Fe1+yTe

The magnetic properties of Fe1+yTe single crystals (y ≃ 0.1 ÷ 0.18) were studied at temperatures 4.2 ÷ 300 K. At an ambient pressure, with decreasing temperature a drastic drop in χ(T) was confirmed at T ≃ 60 ÷ 65 K, which appears to be closely related to the antiferromagnetic (AFM) ordering. It is found that the magnitudes of the anisotropy of magnetic susceptibility Δχ in the AFM phase are close in the studied samples, whereas the sign of the anisotropy apparently depends on the small variations of the excess iron y in Fe1+yTe samples. The performed DFT calculations of the electronic structure and magnetic properties for the stoichiometric FeTe compound indicate the presence of frustrated AFM ground states. There are very close energies and magnetic moments for the double stripe configurations, with the AFM axes oriented either on the basal plane or along the [0 0 1] direction. Presumably, both these configurations can be realized in Fe1+yTe single crystals, depending on the variations of the excess iron. This can provide different signs of magnetic anisotropy in the AFM phase, presently observed in the Fe1+yTe samples. For these types of AFM configuration, the calculations for the FeTe values of Δχ are consistent with our experimental data.

The sensitivity of ferronematics to external magnetic fields

The stable colloidal suspensions of nematic liquid crystals with magnetic nanoparticles are called ferronematics. Their behaviour in magnetic field depends on an anchoring energy, volume concentrations of magnetic nanoparticles and sign of anisotropy of diamagnetic susceptibility of liquid crystal as well as on the initial orientation between director (n) of liquid crystal and magnetic moment (m) of magnetic nanoparticles. In this work we present structural changes in ferronematics based on two kinds of liquid crystals: 6CHBT (positive anisotropy of diamagnetic susceptibility) and ZLI1695 (negative anisotropy of diamagnetic susceptibility) exposed to high magnetic fields. In both cases the parallel initial orientation between director and magnetic moments is fulfilled. The density of anchoring energy between liquid crystal molecules and magnetic particles was determinated by Burylov and Raikher's theory. In the case of 6CHBT-based ferronematics the decrease while in the case of ZLI1695-based ferronematics the increase of the critical magnetic field were observed.

Searching for planar signatures in WMAP

We search for planar deviations of statistical isotropy in the Wilkinson Microwave Anisotropy Probe (WMAP) data by applying a recently introduced angular-planar statistics both to full-sky and to masked temperature maps, includingin our analysis the effect of the residual foreground contamination and systematics in the foreground removing process as sources of error. We confirm earlier findings that full-sky maps exhibit anomalies at the planar (l) and angular (ℓ) scales (l,ℓ) = (2,5),(4,7), and (6,8), which seem to be due to unremoved foregrounds since this features are present in the full-sky map but not in the masked maps. On the other hand, our test detects slightly anomalous results at the scales (l,ℓ) = (10,8) and (2,9) in the masked maps but not in the full-sky one, indicating that the foreground cleaning procedure (used to generate the full-sky map) could not only be creating false anomalies but also hiding existing ones. We also find a significant trace of an anomaly in the full-sky map at the scale (l,ℓ) = (10,5), which is still present when we consider galactic cuts of 18.3% and 28.4%. As regards the quadrupole (ℓ = 2), we find a coherent over-modulation over the whole celestial sphere, for all full-sky and cut-sky maps.Overall, our results seem to indicate that current CMB maps derived from WMAPdata do not show significant signs of anisotropies, as measured by ourangular-planar estimator. However, we have detected a curious coherence ofplanar modulations at angular scales of the order of the galaxy's plane, whichmay be an indication of residual contaminations in the full- and cut-sky maps.

Optical Phase Modulation Using Dual-Frequency Nematic Liquid Crystals

Phase modulation characteristics of dual-frequency liquid crystal materials possessing the dielectric anisotropy sign inversion have been studied in detail under the quasi-static and dynamic electric field control. It was found that the main obstacle in development of dual-frequency liquid crystal modulators having simultaneously short response time and large switchable values of phase retardation is related to an electrohydrodynamic instability, which disturbs the director reorientation. We are discussing general conditions at which a stable 2π phase modulation is possible at highest switching frequencies. In particular, for the developed dual-frequency liquid crystal material we have established conditions at which the stable phase retardation dynamic range of 2π at switching time ∼ 2 ms is realized at room temperature.

Bistability in dual-frequency nematic liquid crystals

AbstractDifferent modes of bistable switching in liquid crystals with frequency inversion of the dielectric anisotropy sign are discussed. The study is performed by numerical simulation and experimentally. It is shown that dual frequency driving can be effectively used to control switching between topologically equivalent and non-equivalent director field distributions. The experimental results on temperature performance of the dual-frequency switching and possible driving methods for energy consumption and expanding the temperature range are presented.

P-100: Electro-Optic Response of Dual-Frequency LCD

Displays with the twist structure of the LC layer hold a leading position among the LCDs. With low frequency inversion of the dielectric anisotropy sign, the use of double frequency driving in cells filled with LC material allows both switch-on and switch-off times of the electro-optic mode to be reduced. In our paper oscillograms of electrooptic response vs. high frequency (HF) voltage are presented for such LC cells. The bounce, which is induced by the back flow in strongly deformed twist cells, gradually decreases in magnitude with increasing HF voltage. It is suggested that the HF voltage suppresses the back flow in the vicinity of the cell's substrate surface. At the HF voltage that follows the disappearance of the bounce, the short response mode arises.

Antiferromagnetic resonance and phase diagrams of gadolinium ferroborate GdFe3(BO3)4

Antiferromagnetic resonance in single crystals of rhombohedral gadolinium ferroborate GdFe3(BO3)4 was studied. The frequency-field dependences of antiferromagnetic resonance over the frequency range 26–70 GHz and the temperature dependences of resonance parameters for magnetic fields oriented along the crystal axis and in the basal plane were determined. It was found that the iron subsystem, which can be treated as a two-sublattice antiferromagnet with anisotropy of the easy-plane type, experienced ordering at T=38 K. At temperatures below 20 K, the gadolinium subsystem with the opposite anisotropy sign strongly influenced the anisotropic properties of the crystal. This resulted in a spontaneous spin-reorientation transition from the easy-plane to the easy-axis state at 10 K. Below 10 K, magnetic field-induced transitions between the states were observed. Experimental phase diagrams on the temperature-magnetic field plane were constructed for fields oriented along the crystal axis and in the basal plane. A simple model was used to calculate the critical transition fields. The results were in close agreement with the experimental values measured at T=4.2 K for both field orientations.

Nonlinear polarization rotation and orthogonal polarization generation experienced in a single-beam configuration

Nonlinear polarization rotation and generation of a polarization component orthogonal to the input beam were observed along fourfold axes of YVO4 and BaF2 crystals. We demonstrate experimentally that in both crystals the angle of rotation is proportional, at low intensities, to the square of the product of the input intensity and the crystal length and is the result of simultaneous action of two third-order processes. This type of nonlinear polarization rotation is driven by the real part of the cubic susceptibility. The recorded energy exchange between the two orthogonal components can exceed 10%. It is to our knowledge the highest energy-conversion efficiency achieved in a single beam nonresonant χ(3) interaction. A simple theoretical model is elaborated to describe the dependence of nonlinear polarization rotation and orthogonal polarization generation on the intensity of the input beam at both low- and high-intensity levels. It reveals the potential contributions from the real and the imaginary parts of the susceptibility tensor. Moreover, this kind of measurement is designed to permit the determination of the magnitude and the sign of the anisotropy of the real part of third-order nonlinearity in crystals with cubic or tetragonal symmetry on the basis of polarization-rotation measurements. The χxxxx(3) component of the third-order susceptibility tensor and its anisotropy sign and amplitude value for BaF2 and YVO4 crystals are estimated and discussed.

Dielectric anisotropy of 5CB liquid crystal in a decimeter wavelength range

Parallel and perpendicular components of the dielectric constant for 5CB nematic liquid crystal are measured in a relaxation region. Two frequencies f1≈30 and f2≈350 MHz are determined at which the inversion of the dielectric anisotropy sign is observed. It is shown that the observed sizable high-frequency shift of f2 with respect to the value calculated according the Debye theory is caused by the dielectric resonance near the relaxation region.