Polar Axis Of Crystal Research Articles

High vacuum equipment for the measurement of the macroscopic dipole moment of polar molecular crystals.

Polar crystals placed into a charged plate capacitor may experience a mechanical torque. Mounting crystals on a spiral spring allows us to measure the momentum and thus the macroscopic dipole moment of a crystal object. To exclude the influence of ambient conditions (air, ions, electrons), experiments are performed in a high vacuum. Following text book knowledge, polar crystals show a surface charge density especially for faces involving the polar crystal axis. For ambient conditions, it is assumed that external charge carriers compensate the surface charge. Here, we present unique equipment by which the dipole moment of crystals can be measured also when the charge carrying parts of a crystal were cut off by two metallic blades. Present measurements for α-resorcinol (mm2) allow us to conclude that (a) as grown polar crystals measured under the high vacuum condition show a macroscopic dipole moment, (b) the weighted dipole moment increases after cutting the main polar faces, and (c) the weighted dipole moment decreased after exposure to ambient air.

Multi-excitonic emission from Stranski-Krastanov GaN/AlN quantum dots inside a nanoscale tip

Single-dot time-resolved micro-photoluminescence spectroscopy and correlated electron tomography (ET) have been performed on self-assembled GaN/AlN quantum dots isolated within a field-emission nanoscale tip by focused ion beam (FIB). Despite the effect of the FIB, the system conserves the capability of emitting light through multi-excitonic complexes. The optical spectroscopy data have then been correlated with the electronic structure and lifetime parameters that could be extracted using the structural parameters obtained by ET via a 6 band k.p model. A biexciton-exciton cascade could be identified and thoroughly analysed. The biexciton-exciton states exhibit a non-negligible polarization component along the [0001] polar crystal axis, indicating a significant valence band mixing, while the relationship between exciton energy and biexciton binding energy is consistent with a hybrid character of the biexciton.

Acoustic and dielectric anomalies in the temperature range of a ferroelectric phase transition in glycine phosphite crystals

The acoustical and dielectric properties of glycine phosphite crystals are investigated in the temperature range of a ferroelectric phase transition. The acoustic anomalies for longitudinal waves along the X, Y, and Z crystallographic axes (where Y is the spontaneous polarization axis) are analyzed in the framework of the Landau theory. It is shown that pronounced acoustic anomalies of the velocity can be quantitatively described within the pseudoproper ferroelectric phase transition model with due regard for the long-range dipole-dipole interaction. For longitudinal acoustic waves propagating along the polar crystal axis, the striction anomaly of the velocity is only partly suppressed by the long-range dipole-dipole forces and an abrupt jump in the velocity is observed in the vicinity of the phase transition. The temperature coefficients of the velocity in the paraphase are determined. The striction contribution and the contributions biquadratic in the order parameter and in the strain to the velocity anomaly are separated.

X-ray Crystal Structures, Molecular Mechanics Calculations, and Calculations of the Nonlinear Polarizabilities (β and γ) of Dicyanovinylbenzene and Its Methoxy Derivatives, and Comparison with Experimental Values of β

X-ray single-crystal study, molecular mechanics calculations, and calculations of the static nonlinear polarizabilities (β and γ) were performed for dicyanovinylbenzene and series of its mono- and dimethoxy-substituted derivatives. X-ray analysis has been done for dicyanovinylbenzene, its o- and p-methoxy- and 2,4-dimethoxy-substituted derivatives together with corresponding EFISH measurements of the β values for these compounds. Nonlinear polarizabilities were calculated for all series of the mono- and disubstituted methoxy derivatives of the parent compound with modified finite-field formalism that included calculation of polarization values versus static electric fields using semiempirical Hamiltonian, polynomial fit of all tensor elements of β and γ on the former data, and evaluation of the numerical instability of the calculations. All calculations were performed with optimized molecular geometries taken from X-ray data, molecular mechanics, ab initio, and semiempirical quantum chemical results. Good correlation was found between the predicted static and experimental molecular values of β. A factor of 2.0 was found to be a probable adjustment parameter to account for the solvent (1,4-dioxane) and dispersion (at 1064 nm) effects. Crystal packing analysis of the X-ray structures studied together with energetic calculations revealed the factors responsible for formation of centrosymmetric crystals. Only o-methoxydicyanovinylbenzene (also known as DIVA) forms acentric crystals (space group P21) among the experimentally studied compounds, and it was found that its molecular dipole moment orientation with respect to the polar crystal axis is close to the “optimal” for a manifestation of the high NLO responses.

Determination of crystal polarity in the Electron Microscope

A few methods were proposed for the determination of crystal polarity using electron diffraction. The method by Taftø and Spence is based on the coupling between ZOLZ and FOLZ reflections, requiring exact tilting to excite weak high- and odd-indexed reflections. The method by Spellward and James relies on the comparison of experimentally observed and calculated CBED patterns of fairly thick crystals, where the features in the CBED discs are compared. In this paper we present a method for polarity determination readily evident from ZOLZ reflections in zone-axis microdiffraction patterns. The method is based on zero-layer interactions from thin crystals with thickness t smaller than the extinction length of the primary beam. Then the diffraction discs appear homogeneous, and the breakdown of Friedel's law is noticable in a difference of the intensities of the reflections g and -g which are not related to a mirror plane (Bijvoet-related reflections).The method can be applied using any transmission electron microscope which is designed for obtaining convergent-beam microdiffraction patterns and in principle, specimen-cooling is not necessary. The crystals have to be oriented to a low-indexed zone axis, where the diffraction plane contains a polar crystal axis.

Degenerate stimulated parametric scattering in LiNbO_3:Fe

Nonlinear photoinduced light scattering at 414 nm in LiNbO3:Fe caused by degenerate stimulated parametric scattering and its competition process with the photorefractive noise phase gratings have been observed. The phase-matching scattered light was located in a plane normal to the polar axis of crystal.

Linear Magnetoelectric Effects in Gallium Iron Oxide at Low Temperatures and in High Magnetic Fields

The magnetoelectric (ME) experiments1 on monocrystals of the piezoelectric ferromagnet Ga2−xFexO3 (x≈1) are extended to low temperatures (T≳4.2°K) and high magnetic fields (H≲150 kOe). It is found that: (1) The linear ME susceptibility α, which has a maximum1 at some T=Tmax, exhibits a minimum at some T=Tmin satisfying 4.2°K<Tmin<Tmax; moreover, the value of α is almost as large at 4.2°K as at Tmax. (2) If H is sufficiently large so that the direction u=Ms/Ms of the spontaneous magnetization Ms is parallel to H, then the electric polarization Pb induced at constant T along the polar crystal axis (b axis) is given by Pb=ηu+αuH+βu H2, where βuH≪α for H≲150 kOe. The ``spontaneous ME polarization'' ηu is strongly u-dependent. However, αu is approximately u-independent, its u-dependent part being only about 2% at 145.2°K and 77.4°K and about 10% at 4.2°K. Since the (approximate) u-independence of αu indicates that the details of the spin canting assumed in the previous theoretical model1 are inapplicable to Ga2−xFexO3, a new thermodynamic description of ME effects is presented which does apply to ferromagnets having u ∥ H and satisfactorily describes (2). Finally, a thermodynamic calculation is given of a new mechanism of linear ME effects, namely the combined action of forced magnetostriction (i.e., a kind of piezomagnetism) and piezoelectricity. This mechanism, which ultimately arises from the strain dependence of exchange (and other) interactions, must exist in all unclamped piezoelectric ferromagnets having u ∥ H, and is probably significant in Ga2−xFexO3 near the Curie point.