Flexoelectric MEMS: towards an electromechanical strain diode

Piezoelectricity and flexoelectricity are two independent but not incompatible forms of electromechanical response exhibited by nanoscale ferroelectrics. Here, we show that flexoelectricity can either enhance or suppress the piezoelectric response of the cantilever depending on the ferroelectric polarity and lead to a diode-like asymmetric (two-state) electromechanical response.

Mechanical Tuning of LaAlO3/SrTiO3 Interface Conductivity

In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage.

Mechanical writing of ferroelectric polarization

Ferroelectric materials are characterized by a permanent electric dipole that can be reversed through the application of an external voltage, but a strong intrinsic coupling between polarization and deformation also causes all ferroelectrics to be piezoelectric, leading to applications in sensors and high-displacement actuators. A less explored property is flexoelectricity, the coupling between polarization and a strain gradient. We demonstrate that the stress gradient generated by the tip of an atomic force microscope can mechanically switch the polarization in the nanoscale volume of a ferroelectric film. Pure mechanical force can therefore be used as a dynamic tool for polarization control and may enable applications in which memory bits are written mechanically and read electrically.

Magnetotransport at domain walls in BiFeO3

Domain walls in multiferroics can exhibit intriguing behaviors that are significantly different from the bulk of the material. We investigate strong magnetoresistance in domain walls of the model multiferroic BiFeO3 by probing ordered arrays of 109° domain walls with temperature- and magnetic-field-dependent transport. We observe temperature-dependent variations in the transport mechanism and magnetoresistances as large as 60%. These results suggest that by locally breaking the symmetry of a material, such as at domain walls and structural interfaces, one can induce emergent behavior with properties that deviate significantly from the bulk.

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: I. Strain analysis and a static order parameter

The structural evolution of Pb(Mg(1/3)Nb(2/3))O(3) (PMN) has been reviewed in terms of characteristic temperatures, length scales and timescales, with a view to considering the overall relaxor behaviour from the perspectives of strain and elasticity. A conventional analysis of lattice parameter data in terms of spontaneous strain and strain/order parameter coupling shows that even though a normal phase transition does not occur the relaxor ordering process is accompanied by a significant volume strain which follows the pattern of a static order parameter evolving according to that expected for a tricritical phase transition with T(c) ≈ 350 K. This matches the evolution of the intensity of the elastic central peak in neutron scattering spectra, and reflects the development of static (or quasistatic) polar nanoregions (PNRs) as if by a mean-field phase transition. Use of a Landau free energy expansion, which includes Γ4(-) order parameter components to describe ferroelectric contributions and an R1(+) order parameter to describe cation ordering together with their formal coupling with strain, then allows the pattern of elastic softening expected for a cubic → rhombohedral phase transition to be anticipated. The extent to which observed softening differs from this static mean-field pattern serves to highlight the additional roles of local heterogeneity and relaxation dynamics in determining the relaxor properties of PMN.

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: II. Strain-order parameter coupling and dynamic softening mechanisms

Elastic and anelastic behaviour of single crystal and ceramic samples of Pb(Mg(1/3)Nb(2/3))O(3) has been investigated at frequencies of ~0.1-1.2 MHz through the temperature interval 10-800 K by resonant ultrasound spectroscopy (RUS). Comparison with data from the literature shows that softening of the shear modulus between the Burns temperature and the freezing interval is independent of frequency. The softening is attributed to coupling between acoustic modes and the relaxation mode(s) responsible for central peaks in Raman and neutron scattering spectra below the Burns temperature, and can be described with Vogel-Fulcher parameters. Shear elastic compliance and dielectric permittivity show similar patterns of temperature dependence through the freezing interval, demonstrating strong coupling between ferroelectric polarization and strain such that the response to applied stress is more or less the same as the response to an applied electric field, with a frequency dependence consistent with Vogel-Fulcher-like freezing in both cases. Differences in detail show, however, that shearing induces flipping between different twin orientations, in comparison with the influence of an electric field, which induces 180° flipping: the activation energy barrier for the former appears to be higher than for the latter. Below the freezing interval, the anelastic loss also has a similar pattern of evolution to the dielectric loss, signifying again that essentially the same mechanism is involved in the freezing process. Overall softening at low temperatures is attributed to the contributions of strain relaxations due to coupling with the local ferroelectric order parameter and of coupling between acoustic modes and continuing relaxational modes of the polar nanostructure. Dissipation is attributed to movement of boundaries between PNRs or between correlated clusters of PNRs. Overall, strain coupling is fundamental to the development of the characteristic strain, dielectric and elastic properties of relaxors.

Flexoelectric rotation of polarization in ferroelectric thin films

Strain engineering enables modification of the properties of thin films using the stress from the substrates on which they are grown. Strain may be relaxed, however, and this can also modify the properties thanks to the coupling between strain gradient and polarization known as flexoelectricity. Here we have studied the strain distribution inside epitaxial films of the archetypal ferroelectric PbTiO(3), where the mismatch with the substrate is relaxed through the formation of domains (twins). Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal an intricate strain distribution, with gradients in both the vertical and, unexpectedly, the horizontal direction. These gradients generate a horizontal flexoelectricity that forces the spontaneous polarization to rotate away from the normal. Polar rotations are a characteristic of compositionally engineered morphotropic phase boundary ferroelectrics with high piezoelectricity; flexoelectricity provides an alternative route for generating such rotations in standard ferroelectrics using purely physical means.

The flexoelectricity of barium and strontium titanates from first principles

We present ab initio calculations of the longitudinal flexoelectricity for BaTiO(3) and SrTiO(3) using a direct approach. The calculated value for SrTiO(3) agrees with recently reported measurements. For BaTiO(3), however, the theoretical values are smaller than the measured ones; possible reasons for the discrepancy are discussed.

Domains in ferroelectric nanodots

Almost free-standing single crystal mesoscale and nanoscale dots of ferroelectric BaTiO(3) have been made by direct focused ion beam patterning of bulk single crystal material. The domain structures which appear in these single crystal dots, after cooling through the Curie temperature, were observed to form into quadrants, with each quadrant consisting of fine 90 degrees stripe domains. The reason that these rather complex domain configurations form is uncertain, but we consider and discuss three possibilities for their genesis: first, that the quadrant features initially form to facilitate field-closure, but then develop 90 degrees shape compensating stripe domains in order to accommodate disclination stresses; second, that they are the result of the impingement of domain packets which nucleate at the sidewalls of the dots forming "Forsbergh" patterns (essentially the result of phase transition kinetics); and third, that 90 degrees domains form to conserve the shape of the nanodot as it is cooled through the Curie temperature but arrange into quadrant packets in order to minimize the energy associated with uncompensated surface charges (thus representing an equilibrium state). While the third model is the preferred one, we note that the second and third models are not mutually exclusive.

Epitaxial TbMnO(3) thin films on SrTiO(3) substrates: a structural study

TbMnO(3) films have been grown under compressive strain on (001)-oriented SrTiO(3) crystals. They have an orthorhombic structure and display the (001) orientation. With increasing thickness, the structure evolves from a more symmetric (tetragonal) to a less symmetric (bulk-like orthorhombic) structure, while keeping constant the in-plane compression, thereby leaving the out-of-plane lattice spacing unchanged. The domain microstructure of the films is also revealed, showing an increasing number of orthorhombic domains as the thickness is decreased: we directly observe ferroelastic domains as narrow as 4 nm. The high density of domain walls may explain the induced ferromagnetism observed in the films, while both the decreased anisotropy and the small size of the domains could account for the absence of a ferroelectric spin spiral phase.

Conduction at domain walls in oxide multiferroics

Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO(3). The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.

Fractal dimension and size scaling of domains in thin films of multiferroic BiFeO3

Domains in ferroelectric films are usually smooth, stripelike, very thin compared with magnetic ones, and satisfy the Landau-Lifshitz-Kittel scaling law (width proportional to square root of film thickness). However, the ferroelectric domains in very thin films of multiferroic BiFeO3 have irregular domain walls characterized by a roughness exponent 0.5-0.6 and in-plane fractal Hausdorff dimension H||=1.4+/-0.1, and the domain size scales with an exponent 0.59+/-0.08 rather than 1/2. The domains are significantly larger than those of other ferroelectrics of the same thickness, and closer in size to those of magnetic materials, which is consistent with a strong magnetoelectric coupling at the walls. A general model is proposed for ferroelectrics, ferroelastics or ferromagnetic domains which relates the fractal dimension of the walls to domain size scaling.

Strain-gradient-induced polarization in SrTiO3 single crystals

Piezoelectricity is inherent only in noncentrosymmetric materials, but a piezoelectric response can also be obtained in centrosymmetric crystals if subjected to inhomogeneous deformation. This phenomenon, known as flexoelectricity, can significantly affect the functional properties of insulators, particularly thin films of high permittivity materials. We have measured strain-gradient-induced polarization in single crystals of paraelectric SrTiO3 as a function of temperature and orientation down to and below the 105 K phase transition. Estimates were obtained for all the components of the flexoelectric tensor, and calculations based on these indicate that local polarization around defects in SrTiO3 may exceed the largest ferroelectric polarizations. A sign reversal of the flexoelectric response detected below the phase transition suggests that the ferroelastic domain walls of SrTiO3 may be polar.

Polar domains in lead titanate films under tensile strain

Thin films of PbTiO3, a classical ferroelectric, have been grown under tensile strain on single-crystal substrates of DyScO3. The films, of only 5 nm thickness, grow fully coherent with the substrate, as evidenced by synchrotron x-ray diffraction. A mapping of the reciprocal space reveals intensity modulations (satellites) due to regularly spaced polar domains in which the polarization appears rotated away from the substrate normal, characterizing a low-symmetry phase not observed in the bulk material. This could have important practical implications since these phases are known to be responsible for ultrahigh piezoelectric responses in complex systems.

Management of pulmonary venous obstruction after correction of TAPVC: risk factors for adverse outcome

OBJECTIVE

Recurrent pulmonary venous obstruction (PVO) occurs in 0-18% of infants undergoing correction of total anomalous pulmonary venous connection (TAPVC). Limited published data suggest that PVO usually develops within 6 months of primary repair, and that outcomes of reoperations are poor. This study aimed to review our experience of reoperations for PVO post-TAPVC repair and to identify risk factors for adverse outcome.

METHODS

Twenty patients underwent reoperation for PVO between 1982 and 2002. Clinical data were reviewed. TAPVC was mostly infracardiac (11 patients). TAPVC was obstructed in nine patients. PVO developed early (<6 months) in seven patients, and late in 13 (>6 months). Time of presentation was unrelated to type of PVO (anastomotic vs. ostial). Repair was accomplished using various techniques (anastomotic enlargement with native atrial tissue, enlargement with pericardium, free or in situ, or other prosthetic material). Follow-up ranged from 1 month to 15 years (average 44 months).

RESULTS

Thirteen patients received one reoperation, while seven had multiple reoperations. In 13 patients, PVO was defined as new onset (no obstruction post-TAPVC repair), and in seven patients as residual (minimal obstructive changes post-TAPVC repair that progressed to PVO). Ten patients presented with anastomotic PVO, six with anastomotic and ostial PVO (involving the PVs), three with ostial PVO, and one with coronary sinus-left atrial junction stenosis. Mortality was 25% (5/20). Six of the ten patients with anastomotic PVO underwent one reoperation (2/6 died); the other four developed ostial PVO after reoperation, requiring multiple procedures (2/4 died). Mode of presentation (new onset vs. residual), site of obstruction (anastomotic vs. ostial), preoperative RV pressure (<0.8 vs. >0.8 systemic), number of reoperations (single vs. multiple), residual obstruction (presence or absence), and operative approach (Gore-tex or not) did not seem to affect outcomes. Risk factors for death were early presentation (<6 months) and persistence of pulmonary hypertension after reoperation; early presentation was also a risk factor for multiple reoperations.

CONCLUSIONS

Our findings support the conclusion that early presentation and postoperative pulmonary hypertension have the greatest adverse impact on outcome. Of these, failure to achieve a low-pressure pulmonary vascular system seems to be the variable that most strongly prevents survival. In our series, neither ostial PVO nor multiple re-interventions significantly increased surgical risk. The negative impact of postoperative residual obstruction on outcome was not striking. However, an aggressive surgical approach to this disease is still warranted. Although the role of each technique in obtaining long-lasting relief of PVO remains to be established, the use of artificial material seems unwise.

Heterogeneity in Klippel-Feil syndrome: a new classification

BACKGROUND

Klippel-Feil syndrome (KFS) is characterised by congenital vertebral fusion of the cervical spine and a wide spectrum of associated anomalies. KFS has often been considered a sporadic syndrome. However, since the publication of the original KFS classification early this century, a number of KFS families have indicated heterogeneity complicated by a broad range of variable expression.

OBJECTIVE

The two major objectives of this study were (1) to identify differences and similarities in the postnatal appearance, morphology, position and inheritance of vertebral fusions within and between KFS families and (2) to establish a new KFS classification focussed on KFS aetiology.

MATERIALS AND METHODS

Vertebral fusions were assessed via spinal radiography. Chromosomal karyotypes were performed using routine cytogenetics.

RESULTS

The medical histories of three KFS families are presented. The postnatal time, position and appearance of vertebral fusions, associated anomalies and mode of inheritance were different for the three KFS families. Four classes of KFS are described in a comprehensive classification table that allays much of the uncertainty arising from KFS heterogeneity and variable expression.

CONCLUSION

We have described four different KFS classes (KF1-4) within a comprehensive classification that addresses KFS genetic heterogeneity. The position of vertebral fusions in the cervical spine and their incidence within affected families are delineating features of KFS.

Image structure from mirrors with print-through undulations: theoretical analysis

The image structure produced by a periodic hexagonal pattern of mirror surface undulations has been analyzed. Such undulations form a two-dimensional phase grating that can result from the polishing of honeycomb mirrors or, for example, meniscus mirrors with a hexagonal pattern of axial supports. For monochromatic light of wavelength lambda, undulations having uniform peak-to-valley amplitude H ? lambda and period L cause a decrease in the central intensity of the point spread function (PSF), and a fraction, ~13(H/lambda)(2), of the total power is diffracted into an infinite hexagonal array of satellite images. These have angular separations of 2lambda/ radical3L and intensity profiles in the form of perfect diffraction limited PSF's, but with intensities decreasing with increasing diffraction order. The six innermost (first-order) satellites each have central intensities approximately 2(H/lambda)(2) times that of the central image. If the amplitudes of the surface bumps are of random size with a normal frequency distribution, then the intensity of the diffracted orders decreases, and an additional weak structure appears over the image plane; the positions and heights of the peaks in this grasslike structure depend on the particular two-dimensional distribution of the random bumps. When the input is polychromatic, the diffracted orders other than zero give images that are elongated radially and decrease outward in intensity with a 1/lambda(4) dependence.

Design method of an astronomical telescope with reduced sensitivity to misalignment

Approximate formulas are derived for the axial coma resulting from tilt and decenter of a surface and for the spherical aberration resulting from a change in its axial position. These expressions include terms that represent aberrations induced by the subsystem preceding the surface in addition to other terms that are intrinsic contributions from the misaligned surface itself. This separation of the terms gives a simple method of designing a system that is insensitive to a misalignment at a given surface. The method is illustrated by applying it to a two-mirror astronomical telescope with corrector. Two examples are given-one for tilt and the other for despace. In both examples an appreciable reduction in the sensitivity is obtained. The limitations of these solutions and the problem of simultaneous correction for two types of misalignment are examined.