Preferred Polarization Direction Research Articles

Clarification of the spontaneous polarization direction in crystals with wurtzite structure

The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically very relevant III-N compounds (AlN, GaN, and InN). In particular, the spontaneous polarization has been reported to either point up or down for the same unit cell orientation, depending on the literature source—with important implications for, e.g., the carrier type and density expected at interfaces of heterostructures involving materials with the wurtzite structure. This perspective aims to resolve this ambiguity by reviewing available reports on the direction of the energetically preferred polarization direction in the presence of external electric fields as well as atomically resolved scanning transmission electron microscopy images. While we use ferroelectric wurtzite Al1−xScxN as a key example, our conclusions are generalizable to other compounds with the same crystal structure. We demonstrate that a metal-polar unit cell must be associated with an upward polarization vector—which is contrary to long-standing conventional wisdom.

Vortex-Oriented Ferroelectric Domains in SnTe/PbTe Monolayer Lateral Heterostructures.

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

Direct Observation of Large Atomic Polar Displacements in Epitaxial Barium Titanate Thin Films

AbstractThe development of ferroelectric perovskite oxides having a controlled polarization direction is an ongoing and challenging topic of research. Here direct observation of large atomic polar displacements, which correspond to a polar density of ≈0.9 C m−2 pointing upward, in an epitaxial BaTiO3 film grown by molecular beam epitaxy on a SrTiO3 substrate is reported. Aberration‐corrected scanning transmission electron microscopy is used to map the polarization displacement with unit‐cell resolution. Oxygen vacancies and other types of defects are examined and mapped using electron energy‐loss near‐edge structure analysis. The contributions from strain, strain gradient, and defects are quantitatively modeled in order to explain the large polarization. Calculations show that strain (through a defect dipole‐enhanced polarization) creates the large atomic polar displacements, and strain gradient (through inverse Vegard electrochemical strain effect) compensates the polarization. These two effects may explain the preferred polarization direction and the anomalous flexoelectric effect in ferroelectric thin films.

Piezoelectric Response in Hybrid Micropillar Arrays of Poly(Vinylidene Fluoride) and Reduced Graphene Oxide

This study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α, β, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from −120° to 20°–40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d33eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response.

Reversed polarized emission in highly straineda-plane GaN/AlN multiple quantum wells

The polarization of the emission from a set of highly strained $a$-plane GaN/AlN multiple quantum wells of varying well widths has been studied. A single photoluminescence peak is observed that shifts to higher energies as the quantum well thickness decreases due to quantum confinement. The emitted light is linearly polarized. For the thinnest samples the preferential polarization direction is perpendicular to the wurtzite $c$ axis with a degree of polarization that decreases with increasing well width. However, for the thickest well the preferred polarization direction is parallel to the $c$ axis. Raman scattering, x-ray diffraction, and transmission electron microscopy studies have been performed to determine the three components of the strain tensor in the active region. Moreover, the experimental results have been compared with the strain values computed by means of a model based on the elastic continuum theory. A high anisotropic compressive in-plane strain has been found, namely, $\ensuremath{-}0.6\mathrm{%}$ and $\ensuremath{-}2.8\mathrm{%}$ along the in-plane directions $[1\overline{1}00]$ and [0001], respectively, for the thickest quantum well. The oscillator strength of the lowest optical transition has been calculated within the framework of a multiband envelope function model for various quantum well widths and strain values. The influence of confinement and strain on the degree of polarization is discussed and compared with experiment considering various sets of material parameters.

Polarized Emission From Twin Microdisk Photonic Molecules

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We investigate the polarization of emission from pairs of coupled whispering-gallery-mode semiconductor microdisks. The finite-difference time-domain method is used to carry out an electromagnetic field profile analysis of four classes of bonding and antibonding modes. For coupled circular microdisks, our simulations predict distinct in-plane polarization directions for the bonding and antibonding modes, in agreement with experimental measurements. For pairs of elliptical microdisks coupled along either the major or the minor axes, our simulations do not predict any polarized emission. Experimental measurements of elliptical microdisks coupled along the minor axis appear to be consistent with these predictions, with no preferred polarization direction for the bonding and antibonding modes. However, elliptical microdisks coupled along the major axis often show a preferred polarization at 45<formula formulatype="inline"> <tex Notation="TeX">$^{\circ} $</tex></formula> to the major axis, inconsistent with both the simulations and with simple symmetry considerations. This suggests that the polarization characteristics of the emission in elliptical geometries may be very sensitive to extrinsic symmetry-breaking factors such as patterning imperfections. </para>

Magnetic signatures of medium‐scale traveling ionospheric disturbances as observed by CHAMP

In this work we analyze the global distribution and physical characteristics of nighttime midlatitude magnetic field fluctuations (MMFs) as observed by the CHAMP satellite from 2001 to 2002 (solar maximum) and from 2006 to 2007 (solar minimum). MMFs are defined as medium‐scale magnetic fluctuations perpendicular to the mean field, which are not accompanied by plasma density irregularities at the CHAMP altitude (∼400 km). MMFs occur at 15°–40° invariant latitude in the ionospheric F region. The occurrence is rare above the southern Atlantic ocean, and bears little connection to geomagnetic activity. The global MMF occurrence rate depends on season. The occurrence is generally low in equinox, maximizes around east Asia/Oceania and Europe/northern Atlantic Ocean in June solstice, and peaks above the American continents in December solstice. As the solar cycle declines, the detected MMF occurrence rate also decreases. The MMF occurrence peaks around 2100 LT and slowly decreases toward midnight. In the postmidnight sector, events are practically absent. The MMF occurrence is generally consistent with known features of nighttime medium‐scale traveling ionospheric disturbances (MSTIDs), such as the conjugate climatology, and premidnight occurrence peak in the east Asia/Oceania region. But differences in their distributions also exist, implying that factors other than MSTIDs, e.g., ionospheric conductivity, sporadic E layer or plasma instabilities, may play a nonnegligible role in generating MMFs. MMFs have a preferred direction of polarization, which is consistent with that of MSTIDs and again corroborates the close connection between these two phenomena. We interpret the observed magnetic deflections in terms of field‐aligned currents (FACs). The estimated wavelength range (∼200–500 km) of associated FAC pairs also agrees well with the size of MSTID density structures.

Distribution and character of water in a surge‐type glacier revealed by multifrequency and multipolarization ground‐penetrating radar

We have investigated the distribution and character of radar scatterers in the glacier Bakaninbreen, Svalbard, using ground‐penetrating radar (GPR). The GPR profiles showed scattering regions with an undulating upper boundary in both along‐ and across‐glacier orientations. The simplest interpretation of such scattering regions would be a layer of randomly distributed scatterers with an undulating upper boundary, underlying a layer without scatterers. We propose an alternative model of scatterer distribution, in which the scatterers are confined to obliquely dipping planes. This model is shown to be able to reproduce the shape of the scattering regions observed in the field data and is consistent with previous observations of dipping thrust features in Bakaninbreen from radar data and in outcrop. Multifrequency profiles (50, 100, and 200 MHz) contained scatterers with similar measured power, indicating that the scatterers have decimeter‐scale dimensions. Multipolarized antenna configurations were used to detect a preferred polarization direction for scattered radiation, which was approximately across glacier, suggesting that the scatterers are nonspherical and aligned across glacier. The scatterers are thought to be large water bodies confined to dipping planar features and elongated in the strike orientation of the planes.

Characterization of the Piezoelectric Properties of Pb0.98Ba0.02(Mg1/3Nb2/3)O3–PbTiO3 Epitaxial Thin Films

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) (70/30) thin films were deposited by pulsed laser deposition using two growth strategies: adsorption controlled deposition from lead‐rich targets (∼25–30 mass%) and lower‐temperature deposition (Td≤600°C) from targets containing a small amount of excess lead oxide (≤3 mass %). The substrates used were (001) SrRuO3/LaAlO3. Typical remanent polarization values ranged between 12 and 14 μC/cm2 for these films. The longitudinal piezoelectric coefficient (d33,f) was measured using in situ four‐circle X‐ray diffraction, and the transverse coefficient (d31,f or e31,f) was measured using the wafer flexure method. d33,f and e31,f coefficients of ∼300–350 pm/V and ∼−11 C/m2 were calculated, respectively. In general, the piezoelectric coefficients and aging rates were strongly asymmetric, suggesting the presence of a polarization bias. The large, extremely stable piezoelectric response that results from poling parallel to the preferred polarization direction is attractive for miniaturized sensors and actuators.

Analysis of the long-term polarization behaviour of BL Lac

An analysis is performed of polarimetric and photometric observations of BL Lac carried out in 1969–1991 at the Astronomical Institute of St. Petersburg State University. The distribution of polarization directions certainly points to the existence of the preferred direction of polarization ($\theta= 22\degr$) close to the direction of the jet observed by VLBI. High polarization degree and colorimetric data are evidence of the synchrotron nature of the variable source. The relative Stokes parameters are distributed normally over some mean value. The correlations between different parameters characterizing the flux density and polarization of the central point source allow us to consider that in BL Lac there is a continually acting source of polarized radiation on which the sources with randomly distributed polarization directions are superimposed.

X-ray scattering evidence for the structural nature of fatigue in epitaxial Pb(Zr, Ti)O3 films

We have probed the microscopic distribution of 180° domains as a function of switching history in 40 nm epitaxial films of Pb(Zr0.30Ti0.70)O3 by analyzing interference effects in the x-ray scattering profiles. These as-grown films exhibit voltage offsets (imprint) in the polarization hysteresis loops, coupled with a strongly preferred polarization direction in the virgin state. Our x-ray results are consistent with models attributing the loss of switchable polarization to the inhibition of the formation of oppositely polarized domains in a unipolar matrix. Using such model epitaxial films, we demonstrate that different microscopic ensembles of domains resulting from, for example, fatigue, may be resolved by this technique.

Piezoelectric properties of sputtered PbTiO3 films: Growth temperature and poling treatment effects

Thin films of lead titanate (PbTiO3) were prepared on silicon substrates by sputtering. Two processes have been developed; (a) the in situ process (the growth temperature was 440 °C) and (b) the deposition at room temperature followed by a conventional annealing at 680 °C. The dielectric constant, the ferroelectric, and the piezoelectric properties were evaluated and compared. The dielectric constant of films deposited in situ was higher than those of postannealed films likely due to the dense microstructure with fine and homogeneous grains. The embedded beam method and the Berlincourt piezometer were used to measure the e31 and the d33 piezoelectric coefficients, respectively. The PbTiO3 films were naturally polarized, in particular the films grown in situ have large piezoelectric constants: e31=−0.49 C/m2 and d33=20 pC/N. These values agree broadly with data of bulk ceramics, however, with bulk material a poling treatment is necessary to attain these values. The poling of the postannealed films leads to a substantial increase of their piezoelectric properties; e31=−0.26 C/m2 and d33=8 pC/N for virgin film and e31=−0.41 C/m2−d33=18 pC/N for an applied electric field of 110 kV/cm (close to saturation) and a poling time of 30 min. The piezoelectric response was found to depend on the direction of the poling field. Note d33 and e31 decreased when the poling field was applied against the preferred polarization direction; the polar domains were oriented from the film surface to the substrate.

Shear wave splitting observations and implications on stress regimes in the Los Angeles basin, California

A systematic analysis of three‐component seismograms recorded at 15 stations from earthquakes occurring at depths of 5 to 18 km beneath the Los Angeles basin and adjacent areas during the period between 1988 and 1994 shows 20 to 160 ms shear wave splitting. Shallow events exhibit little splitting, while deeper events show progressively greater splitting with depth. The preferred polarization direction of the fast shear wave is nearly N‐S, consistent with the direction of the regional maximum horizontal compressive stress but independent of the azimuth between the event and station. We interpret that the shear wave splitting is caused by fluid‐filled crustal microcracks and macrofractures aligned in the N‐S direction. The shear wave splitting observations of 2.8 to 7.8 ms/km can be explained in terms of an anisotropic crust containing vertical cracks with the apparent crack density of 0.023–0.08. On a regional basis, the crack density may vary from station to station, but we find that the apparent crack density in the strike‐slip region of the Newport‐Inglewood fault and the Whittier fault is higher than in the reverse‐thrusting Santa Monica Mountains and Palos Verdes Hills. No systematic change of shear wave splitting in the Los Angeles basin is found in this study.

Fabrication and characterization of PZT thin films for micromotors

Piezoelectric membranes consisting of PbZrxTi1-xO3 (PZT) films on silicon diaphragms have been fabricated, investigated, and applied for micromotors. The PZT films were deposited by sputtering and sol gel techniques. Resonance amplitudes of up to 1000 nm/V have been measured for 16 mu m thick, 2 mm diameter membranes with 0.6 mu m PZT. The PZT films show a preferred direction of polarization, which seems to depend on the deposition technique. Membranes of 4 mm diameter have been successfully applied as a vibrator of a micro motor, allowing for the first time the characterization of a PZT thin film micro motor. The motor could be operated with voltages as low as 1.0 V-rms, which is much less than has been obtained with ZnO micro motors and is sufficiently low for standard battery and IC supply voltages. A 100 h degradation test of frequency and amplitude has revealed a 5 % drop in amplitude, possibly due to depolarization.

Stress orientation and anisotropy based on shear-wave splitting observations in the Cerro Prieto fault area, Baja California, Mexico

Digital seismograms continuously recorded from 1988 to 1992 by two stations of the RESNOM seismic network in northern Baja California, Mexico, were used to search for probable shear-wave anisotropic characteristics in the region of the Cerro Prieto fault. Shear-wave splitting was identified in many of the three-component records analyzed. We measured the polarization direction of the leadingS wave inside theS-wave window as well as the delay times between fast and slow phases on those records displaying shear-wave splitting. For station CPX, which is nearest the Imperial Valley region to the north, the preferred polarization direction found in this study (azimuth 180°±10°) coincides with the direction of the regional maximum compressive stress determined for the region. This polarization direction can be interpreted in terms of the “Extensive Dilatancy Anisotropy” model as the effect of vertical parallel aligned cracks. The preferred polarization direction measured at LMX, however, gives an azimuth of 45°±5°. Thus, it appears that faults and fractures aligned oblique to the main tectonic trend have a greater influence on the anisotropic characteristics of the crust south of Cerro Prieto volcano than that of the regional stress field. Time delays between slow and fastS waves observed at CPX appear constant from 1988 to 1992 while delays measured at LMX for the same interval indicate a small increase with time which cannot be attributed to azimuthal variations of paths.

Long-Term Optical Behavior of BL Lac Objects

On the basis of results of optical photometry and polarimetry of OJ287 and Mrk421 since 1981, we find two states of activities for both objects. In OJ287, a color-magnitude relation which is consistent with the standard jet-shock model is seen after the outburst in 1983, while before 1982 the optical colors were almost constant. In Mrk421, the colors became redder in 1985–87, and the change of the preferred direction of polarization was associated on smaller time scale in 1987.

A survey of the milliarcsecond polarization properties of BL Lacertae objects at 5 GHz

Consideration is given to millarcsecond-resolution total-intensity and linear polarization maps at 5 GHz presented for 11 BL Lacertae objects. In every BL Lacertae object in which polarization structure was detected, the polarization position angle of the knots is nearly parallel to the VLBI structural axis. The direction of the polarization in the cores of these sources appears to be random. The preferred polarization direction in the jets is explained by the fact that plane perpendicular shock waves are common in these sources; the origin of the absence of the preferred polarization direction in the core components is unclear. These results support an early conclusion that it cannot be the case that a significant number of BL Lacertae objects are gravitationally microlensed images of more distant quasars, since the characteristic VLBI polarization structures observed in these two types of objects are very different.

Polarimetric and photometric studies of BL Lac. Analysis of the observational data. II

Analysis of the results of polarization and photometric observations of BL Lac during 1969-1982 published earlier is continued. It is found that the degree of polarization is weakly correlated on the average with the brightness but that there is a definite connection between the direction of polarization and the brightness, only directions close to the preferred direction being observed when the object is faint. This agrees with the idea that BL Lac objects have a continually acting polarized source with direction of polarization that determines the preferred direction. It is established that in individual time intervals the behavior of BL Lac is determined by variable sources with degree of polarization approx. 50%. The synchrotron nature of these sources is not in doubt. The existence of a preferred direction of polarization then indicates that BL Lac has a stable magnetic field.

Photometric and polarimetric study of two BL Lac objects

The results are given of polarimetric and photometric observations of the BL Lac objects OI 090.4 over the period 1979-1982 and B2 1418 + 54 over the period 1980-1982. For both objects it was noted that there was variability on a time scale of from several years to several days. The comparison of the polarimetric parameters indicates the existence of a preferred direction of polarization both for B2 1418 + 54 (theta /sub O/ = 120/sup 0/), and for OI 090.4 (theta /sub O/ = 50/sup 0/), which points to a stable magnetic field. For OI 090.4 it was observed that there was a certain dependence of the polarization parameters on the wavelength.

Order parameter fluctuations of a laser with polarization symmetry

We consider a single-mode laser with no preferred direction of polarization and analyze its fluctuations theoretically. The mode amplitude plays the usual role of an order parameter, but the basic symmetry group of this order parameter is here the non-abelian group SO(4) rather than the usual U(1) group of a laser with preferred direction of polarization. The usual phase-diffusion of the mode-amplitude above threshold is extended therefore to diffusion on a four-dimensional sphere, corresponding to a joint diffusion of the phase and the state of polarization of the emitted field. As a result this diffusion becomes visible in intensity correlation functions.