Polarization Reorientation Research Articles

RelaxorPb(Mg1/3Nb2/3)O3: A Ferroelectric with Multiple Inhomogeneities

Despite intensive studies on Pb(Mg(1/3)Nb(2/3))O(3) (PMN) relaxor, understanding the exact nature of its giant dielectric response and of its physical ground state is a fundamental issue that has remained unresolved for decades. Here, we report a comprehensive study of PMN relaxor crystal, and show that (i) its anomalous dielectric behavior in a broad temperature range results from the reorientation of polarization in the crystal, and (ii) the PMN relaxor is essentially a nanosized ferroelectric material with multiscale inhomogeneities of domain structure in addition to the well-known inhomogeneities of chemical composition and local symmetry. Such inhomogeneities are believed to play a crucial role in producing the huge and enigmatic physical effects in relaxor system, and may be used to design other new systems with giant effects such as a relaxor system.

Mechanical stress induced polarization reorientation in polycrystalline Bi 3.25La 0.75Ti 3O 12 films

A wafer bending stage and a scanning probe microscope are combined to investigate the in situ domain pattern evolution in polycrystalline Bi 3.25La 0.75Ti 3O 12 films under stress. We observe the stress induced polarization reorientation that sensitively depends on the relative alignment of the BLT unit cell with respect to the stress.

Z-SCAN MEASUREMENTS OF OPTICAL NONLINEARITIES OF DYE-DOPED LIQUID CRYSTALS

Nematic liquid crystal (NLC) is a material that features a large nonlinear optical response. The nonlinearity can be enhanced by doping a dye agent into a NLC host. In this paper, we review the nonlinear phenomena recently observed for dye-doped liquid crystal (DDLC) films measured using the Z-scan technique, which is a simple, yet powerful method for measuring nonlinear refractive indices (optical Kerr constant) n2 and nonlinear absorption coefficients β. The nonlinear effects of liquid crystals (LCs) include the electronic polarization effect, electrostriction effect, thermal effect and reorientation effect. When LCs are doped with dye agents, the dye-induced orientational effect, photoisomerization effect, and light-induced thermal effect come about to enhance LC nonlinearity. The dominant effect depends strongly upon the spatial, temporal and polarization properties of the excitation laser beam. Generally speaking, the nonlinear effects resulted from the electronic polarization and photoisomerization effects arise from a laser pulse on picosecond duration scale, while electrostriction, thermal and reorientation effects occur on nanosecond ~ microsecond, nanosecond ~ millisecond and millisecond ~ second duration scales, respectively. In this paper, the causation of nonlinear refractive index n2 into various different experimental conditions is summarized, including the changes of the external electric field, external optical field, polarization of pump beam, temperature and the use of deuterated materials. The mechanisms, which influence the nonlinear effect in dye-doped liquid crystal films, are also discussed.

Tau-induced microtubules polarity reorientation rather than depolymerization leads to axonal traffic jams, ultrastructural modifications and neurodegeneration

Tau-induced microtubules polarity reorientation rather than depolymerization leads to axonal traffic jams, ultrastructural modifications and neurodegeneration

Superparaelectric phase in the ensemble of noninteracting ferroelectric nanoparticles

We predict the conditions of superparaelectric phase appearance in the ensemble of noninteracting spherical ferroelectric nanoparticles. The superparaelectricity in the ensemble of nanoparticles was defined by analogy with superparamagnetism, obtained earlier in small nanoparticles made of magnetic material. Calculations of correlation radius, energetic barriers of polarization reorientation, and polarization response to external electric field, performed within Landau-Ginzburg phenomenological approach for perovskites $\text{Pb}(\text{Zr},\text{Ti}){\text{O}}_{3}$, ${\text{BiFeO}}_{3}$, uniaxial ferroelectrics Rochelle salt, and triglycine sulfate, proved that under the favorable conditions ensemble of noninteracting nanoparticles possesses superparaelectric features. The main favorable conditions for the superparaelectricity observation in ferroelectric nanoparticles are the radius smaller than the correlation radius, surface screening of depolarization field, small Curie-Weiss constant, and high nonlinear coefficients, which guarantee that the barrier of the particle polarization orientation will be smaller than the thermal energy and the particle is single domain. The theoretical forecast is waiting for experimental revealing.

Electric-Field-Induced Spin Flop inBiFeO3Single Crystals at Room Temperature

Bismuth ferrite, BiFeO3, is the only known room-temperature magnetic ferroelectric material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric order parameters are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarization reorientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than in thin films where the cycloid is absent.

Structural and dielectric characterizations of relaxor/ferroelectric superlattice films Pb(Sc1/2Nb1/2)O3/PbTiO3 fabricated on a single-lattice scale

Relaxor/ferroelectric superlattice (SL) films Pb(Sc1/2Nb1/2)O3(PSN)/PbTiO3(PT) with nine different PT concentrations x are fabricated by a pulsed laser deposition method, which keeps the total film thickness at 100 nm. The structure of the SL films is examined by out-of-plane and in-plane x-ray diffractions. All films are epitaxial and their SL structures are coherent over the whole sample. SL reflections up to the fourth order are observed. Lattice constants of the PT and PSN layers in the SL are determined, and the PT lattice constant exhibits a marked increase at xm=32%. At this PT concentration, this phenomenon is due to a polarization reorientation from the a-axis oriented to the c-axis oriented domain. The complex dielectric constants of all samples are measured in the frequency region from 1 to 106 Hz, and the intrinsic dielectric constant ε of the SL is derived by an analysis using the equivalent electric circuit model. ε takes the maximum at xm. The possible origin of this peculiar phenomenon is discussed.

Crystallographic and dielectric properties of highly oriented BaTiO3 films: Influence of oxygen pressure utilized during pulsed laser deposition

The crystal structure of BaTiO3 thin films grown by pulsed laser deposition on MgO substrates was found to be strongly influenced by the oxygen pressure used during growth. Low pressures produced epitaxial films with highly strained out-of-plane lattice parameter c compared to in-plane parameter a, while increasing oxygen pressure resulted in the ratio c/a < 1 with a concomitant increase in polycrystallinity. The dielectric properties varied with changing crystal structure reaching a maximum permittivity value in films near the minimum point of tetragonal distortion and exhibiting relaxor-like behavior in c/a < 1 films close to this point. Hysteresis observed in dielectric tuning loops pointed to the presence of the ferroelectric phase in all films at room temperature. As a result of high electric field poling treatment at 300 °C, the tunability generally increased and initially symmetric tuning curves became asymmetric. The tuning curve in c/a < 1 samples became nearly linear, supporting the premise of polarization reorientation with changing deposition condition. Phase transitions to the paraelectric phase were highly suppressed and shifted upwards in temperature from the bulk transition temperature of 130 to ∼250 °C in strongly c-oriented films. Moderate shifts in oxygen working pressure were demonstrated to produce films with very different properties thereby offering convenient means for strain engineering and control of preferred crystal orientation and polarization direction of highly oriented BTO films.

Local self‐assembly mechanisms underlie the differential transformation of the proximal and distal cut axonal ends into functional and aberrant growth cones

Following axotomy, both the proximal and distal cut axonal ends transform into growth cones (GCs). Whereas the GCs formed by the tip of the proximal segment branch to form neurites, the structure formed by the distal cut end fails to grow. The mechanisms underlying the formation of an aberrant GC by the distal cut end are not understood. Earlier we described the cascade that transforms the tip of the proximal cut axon into a GC. This involves microtubule (MT) polar reorientation, which culminates in the formation of two MT-based vesicle traps, one for Golgi-derived vesicles and the other that retains retrogradely transported vesicles. The formation of these traps is the outcome of local interactions between dynamically repolymerizing MTs and molecular motors. The concentration of Golgi-derived vesicles in the plus-end trap is essential for the successful generation of a functional GC. By using online confocal imaging of transected cultured Aplysia neurons, we analyzed here the restructuring of the distal cut end after axotomy. We found that initially the proximal and distal cut ends undergo identical alterations. Nevertheless, in contrast to the proximal end, the distal cut axon forms only a minus-end MT-based trap that concentrates endocytotic vesicles driven by minus-end oriented motors. Whereas the MTs forming the trap polymerize pointing their plus-ends centrifugally to form finger-like protrusions, the trapped vesicles cannot translocate out to fuse with the plasma membrane. Thus, the structure formed at the distal cut axon is incompetent to support growth processes.

Tau‐Induced Traffic Jams Reflect Organelles Accumulation at Points of Microtubule Polar Mismatching

It is currently accepted that tau overexpression leads to impaired organelle transport and thus to neuronal degeneration. Nevertheless, the underlying mechanisms that lead to impaired organelle transport are not entirely clear. Using cultured Aplysia neurons and online confocal imaging of human tau, microtubules (MTs), the plus-end tracking protein - end-binding protein 3, retrogradely and anterogradely transported organelles, we found that overexpression of tau generates the hallmarks of human tau pathogenesis. Nevertheless, in contrast to earlier reports, we found that the tau-induced impairment of organelle transport is because of polar reorientation of the MTs along the axon or their displacement to submembrane domains. 'Traffic jams' reflect the accumulation of organelles at points of MT polar discontinuations or polar mismatching rather than because of MT depolymerization. Our findings offer a new mechanistic explanation for earlier observations, which established that tau overexpression leads to impaired retrograde and anterograde organelle transport, while the MT skeleton appeared intact.

Morphological Control of Polar Orientation in Single-Crystal Ferroelectric Nanowires

In an attempt to explore and understand domain configurations that occur in idealized ferroelectric nanowires, a focused ion beam microscope has been used to directly cut columns in a variety of sizes from single-crystal barium titanate. The scanning transmission electron microscope has then been used to image the ferroelectric domain patterns evident after cooling through the Curie temperature in the vacuum environment of the electron microscope. As the overall length of the wires is physically constrained, the observed length-conserving packets of 90° domains with {110}pseudocubic domain-wall orientations can easily be rationalized. Such domain structures necessarily dictate that although half of the domains can be such that their polarization direction lies parallel to the axis of the wire, the other half of the domains will have polarization directions approximately perpendicular to the wire axis (nonaxial). This situation introduces a depolarizing field and associated energy, which is minimized when the nonaxial polarization is oriented perpendicular to the smallest dimension of the column. Locally changing the aspect ratio of the column dimensions therefore allows local variations in the direction of polarization to be introduced. This was demonstrated by fabricating wire structures in which dimensions were varied along their length. Such wires did indeed show morphologically controlled polar reorientation. The study suggests that shape engineering alone could be used to create complex heterogeneous dipole configurations in ferroelectrics at the nanoscale without the need for externally applied poling electric fields. Possibilities for the further development of this observation might include introducing chiral variations in wire thickness, for example, to create dipole helices.

Rac1 is required for reorientation of polarity and lumen formation through a PI 3-kinase-dependent pathway

Epithelial cells are characterized by the ability to form sheets of cells that surround fluid-filled lumens. Cells in these sheets exhibit a characteristic subcellular polarity, with an apical pole that faces the lumen and a basolateral pole that is in contact with other cells and the extracellular matrix (ECM). To investigate the signaling events required for polarization and lumen formation, we have taken advantage of the ability of Madin-Darby canine kidney (MDCK) cells to dynamically remodel their polarity in response to changes in ECM cues. When MDCK cells are grown in suspension culture, they form multicellular "inside-out" cysts with apical proteins found on the peripheral surface and basolateral markers on the interior surface. When these inside-out cysts are embedded in ECM, they rapidly reorient their polarity: apical proteins become localized to the inside surface, and basolateral proteins are found on the surface that contacts ECM. Here we have characterized the signaling requirements for these early molecular reorientation events. Specifically, expression of a dominant-negative form of Rac1 (DN-Rac1) blocks the reorientation of polarity. Phosphoinositide 3'-kinase is required for apical membrane protein remodeling from the initial apical membrane surface. Cells expressing DN-Rac1 fail to detectably activate the PI 3-kinase/protein kinase B pathway. Last, we found that atypical protein kinase C (aPKC) is also required for reorientation of polarity, since an inhibitor of atypical PKC blocks reorientation. This effect cannot be overcome by constitutively active Rac1, demonstrating that both Rac1 and atypical PKC are required for reorientation of cellular polarity.

Induction of ferroelectric transitions by the noncollinear exchange-modulated magnetic structure and magnetic fields in terbium manganate

The magnetic transition from the exchange-modulated collinear to the noncollinear state in terbium manganate, accompanied by the appearance of electric polarization, is explained within the Landau theory of phase transitions. The experimentally observed reorientation or disappearance of electric polarization in magnetic fields along the Y and Z axes are explained by the spin-flop transitions of manganese spins from the incommensurate noncollinear to the commensurate magnetic phase.

Ferroelectric lithography

The fact that domain polarization affects the surface properties suggests a method to direct chemical reactions on ferroelectric substrates. In combination with domain manipulation at small scales, a new lithography process is developed to assemble several classes of nanostructures. Three domain patterning techniques, which employ contact electrodes, SPM and e-beam are introduced, with focus on the physical interactions between electrons and ferroelectrics. The effects of electron beam parameters on polarization reorientation are quantified and it is shown that both positive and negative polarization can be achieved depending on conditions. Potential applications of ferroelectric lithography on fabrication of complex structures are illustrated.

Scrib Controls Cdc42 Localization and Activity to Promote Cell Polarization during Astrocyte Migration

Mammalian Scribble (Scrib) plays a conserved role in polarization of epithelial and neuronal cells. Polarization is essential for migration of a variety of cell types; however, the function of Scrib in this context remains unclear. Scrib has been shown to interact with betaPIX, a guanine nucleotide exchange factor for the small GTPases Rac and Cdc42. Cdc42 controls cell polarity from yeast to mammals during asymmetric cell division and epithelial cell polarization, as well as during cell migration. Cdc42 is, in particular, required for polarization and orientation of astrocytes in a scratch-induced polarized migration assay. Using this assay, we characterized Scrib function during polarized cell migration. Depletion of Scrib by siRNA or expression of dominant-negative constructs inhibits astrocyte polarization. Like Cdc42, Scrib controls protrusion formation, cytoskeleton polarization, and centrosome and Golgi reorientation. Scrib interacts and colocalizes with betaPIX at the front edge of polarizing astrocytes. Perturbation of Scrib localization or of Scrib-betaPIX interaction inhibits betaPIX polarized recruitment. We further show that betaPIX is required for astrocyte polarization and that both the Scrib-binding motif and the GEF activity of betaPIX are essential for its function. Scrib and betaPIX control Cdc42 activation and localization during astrocyte polarization. Thereby, Scrib regulates Cdc42-dependent APC and Dlg1 recruitment to the leading edge to promote cell orientation. We conclude that Scrib plays a key role in the establishment of cell polarity during migration. By interacting with betaPIX, Scrib controls localization and activation of the small GTPase Cdc42 and regulates Cdc42-dependent polarization pathways.

Orientation Dependence of Nonlinearity and Hysteresis in PZN-4.5%PT Single Crystals II: Bipolar Electromechanical Response

Relaxor ferroelectric single crystals exhibit orientation dependent hysteresis. In this study, combined bipolar electric field and compressive stress are applied to relaxor single crystals of [Pb(Zn1/3Nb2/3)O3]0.955-[PbTiO3]0.045 (PZN-4.5%PT) in a series of crystal orientations between h001i and h111i. The resulting bipolar electromechanical response is presented. The observed hysteresis and nonlinear phenomena related to polarization reorientation and phase transitions are discussed.

Electric-Field-Induced Polar Biaxial Order in a Nontilted Smectic Phase of an Asymmetric Bent-Core Liquid Crystal

A polarization reorientation process has been studied by means of optical second-harmonic generation in an optically uniaxial smectic phase of an asymmetric bent-core liquid crystal. A nontilted polar smectic order with a biaxial order is induced by applying an electric field to the uniaxial nonpolar smectic phase. This phenomenon was well simulated by the two-dimensional Langevin process, i.e., electric-field-induced continuous molecular reorientation against thermal agitation. The simulation suggests that about 200s of molecules form a polar domain and cooperatively respond to the applied field. The existence of the polar domains and their reorientation are consistent to the dielectric measurement; a high dielectric constant of about 60 at the kHz range is markedly suppressed by applying a bias field.

Local inversion of magnetic anomalies: Implication for Mars’ crustal evolution

Martian magnetic anomalies have been revealed by the Mars Global Surveyor (MGS) mission in the south hemisphere of Mars. The present study models anomalies located in the ancient Terra Sirenum area between latitudes 26°S and 40°S and longitudes 185°E and 210°E using forward and inverse approaches. While the high-altitude measurements reveal the presence of two main magnetic anomalies, three are detected by low-altitude data. They are modeled as uncorrelated dipolar sources. Forward models predict large magnetizations between 30 and 60 A/m. A generalized non-linear inversion is used to determine the characteristics of the dipoles, based on different subsets of data. Low-altitude measurements inversion leads to more reliable results than those obtained by the inversion of high-altitude measurements only. Inversion of both low- and high-altitude data together provides with three dipoles that explain more than 57% of the signal, within this 10 6 km 2 area. All dipoles have large magnetizations. Serpentinization of the early martian crust can explain such remanent magnetizations. Two resulting dipoles are 56 km deep, which suggests a locally thick martian crust. The last one is shallower (31 km). This indicates different origins and/or magnetization processes. Paleomagnetic poles are calculated and located around the Tharsis bulge. It suggests that Tharsis formed at high latitudes and moved toward its present location by polar reorientation.

23Na NMR in the relaxor ferroelectric Na1/2Bi1/2TiO3

The 23Na NMR spectra of a Na1/2Bi1/2TiO3 crystal are studied at frequencies of 79.4 and 15.7 MHz in the temperature range 150–720 K. It is revealed that, at all temperatures, the crystal contains regions with a nearly cubic matrix and polar clusters. The temperature dependence of the local distortion of the Na environment in the clusters is determined. The dynamics of the reorientation of the local cluster polarization in the tetragonal and trigonal NBT phases is analyzed.

90° switching of polarization in La3+-doped SrBi2Ta2O9 thin films

The crystal structure and polarization switching behavior of SrBi1.4La0.6Ta2O9 (SBLT) thin films have been studied by x-ray diffraction and piezoresponse force microscopy (PFM), respectively. Compared with SrBi2Ta2O9 (SBT), SBLT thin films show a reduced orthorhombic distortion. The polarization rotation of SBLT thin film, which is driven by negative and positive direct current (dc) biases, has been investigated by a combination of vertical and lateral PFM (VPFM and LPFM, respectively). After dc bias applications, the VPFM image is hardly changed, whereas the LPFM image experiences an obvious variation. It is believed that such difference is caused by 90° polarization switching. However, this kind of switching can be only realized by the exchange of a axis and b axis. By virtue of the reduced orthorhombic distortion, the a-b exchange in SBLT is easier than that in SBT. Unfortunately, stress is created due to the 90° polarization switching in SBLT thin films. The internal stress is found to increase with the repeated switching cycles, and so the polarization reorientation in SBLT is constrained. Thus, the fatigue resistance of SBLT thin films is not thought to be as good as that of SBT.