Mesoscale flux-closure domain formation in single-crystal BaTiO3

Over 60 years ago, Charles Kittel predicted that quadrant domains should spontaneously form in small ferromagnetic platelets. He expected that the direction of magnetization within each quadrant should lie parallel to the platelet surface, minimizing demagnetizing fields,and that magnetic moments should be configured into an overall closed loop, or flux-closure arrangement. Although now a ubiquitous observation in ferromagnets, obvious flux-closure patterns have been somewhat elusive in ferroelectric materials. This is despite the analogous behaviour between these two ferroic subgroups and the recent prediction of dipole closure states by atomistic simulations research. Here we show Piezoresponse Force Microscopy images of mesoscopic dipole closure patterns in free-standing, single-crystal lamellae of BaTiO3. Formation of these patterns is a dynamical process resulting from system relaxation after the BaTiO3 has been poled with a uniform electric field. The flux-closure states are composed of shape conserving 90° stripe domains which minimize disclination stresses

Dynamics of ganglioside headgroup in lipid environment: Molecular dynamics simulations of GM1 embedded in dodecylphosphocholine micelle

Recognition of membrane-anchored glycosphingolipid receptors by various ligands is the key event in several biological phenomena. However, mere presence of these cell-surface receptors does not always ensure their recognition and binding by their respective ligands, a phenomenon termed "crypticity". Earlier studies have suggested that different glycan headgroup, orientations exposing and/or masking different epitopes may explain the crypticity of glycolipids. The effect of lipid environment on the orientation and conformation of GM1 ganglioside has been investigated in the present study by MD simulation technique in an attempt to understand the structural basis of crypticity. In addition, the effect of the length of the ceramide hydrocarbon tails on the headgroup conformation has also been investigated. MD simulations for 1 ns were performed with explicit water molecules for both GM 1 headgroup (GM 1-Os) and GM1 embedded in dodecylphosphocholine micelle. The simulations show that the conformations of the hydrocarbon tails, ceramide-saccharide linkages, and the headgroup are inter-related and are affected by micellar packing considerations. The conformations of GalNAc-beta1 -->4-Gal and Neu5Ac-alpha2 -->3-Gal linkages were found to be restricted when GM I is embedded in the micelle compared to that in GM1-Os. The GalNAc-beta1 -->4-Gal linkage being a branch point, affects the orientation/accessibility of all the residues linked to GalNAc. If such a disallowed glycan conformation is necessary for recognition by, and binding to, their ligands, then the glycans may become cryptic. The ceramide with 8-carbon hydrocarbon tails was found to adopt a "surface-bound" configuration. In contrast, ceramide with 12- or 16-carbon tails was found to adopt "micelle-inserted" configuration. The effect of lipid environment was found to be the least on the oligosaccharide linked to the ceramide with hexadecyl tails. Thus, the headgroup conformation in this case is essentially the same as that of the free headgroup. This probably explains the lesser crypticity of glycolipids with longer hydrocarbon tails

Conformation, orientation and dynamics of dodecylphosphocholine in micellar aggregate: A 3.2 ns molecular dynamics simulation study

A dodecylphosphocholine micelle of 86 monomers with 5776 water molecules has been simulated under NPT conditions for 3.2 ns using GROMACS2.0. The micelle was found to be very dynamic. Some of the C-C bonds, independent of their position in the DPC monomer, adopt gauche conformation and the trans gauche transitions are quite frequent, An average of about 11% of the C-C bonds in the micelle are observed to be in the gauche conformation (i.e., /dihedral angle/< 120degrees). The terminal methyl groups are randomly distributed all over the micelle whereas the nitrogen atom of phosphocholine headgroup atoms is restricted to the interface region. Some of the monomers were found to lie on the surface. The shape of micelle, influenced by the packing considerations, shows deviations from spherical shape. The phosphocholine headgroup is well solvated and there is no water penetration into the micelle core. The overall features of the micelle of 86 DPC monomers conforms to the lattice model of micelle proposed by Dill and Flory [Dill K A, Flory P J (1981) Proc Natl Acad Sci USA 78, 676-680] and is similar to DPC micelles of smaller aggregate sizes except for the positional preference of the C-C bonds for the gauche conformation and the trans-gauche transition times [Tieleman D P, van der Spoel D, Berendsen H J C (2000) J Phys Chem B 104, 6380-6388; Wymore T, Gao X F, Wong T C (1999) J Mol Struct (Theochem) 485-486, 195-210]. It appears that packing considerations play a predominant role in determining the shape and dynamics of the micelle

Molecular dynamics simulations of alpha 2 -> 8-linked disialoside: Conformational analysis and implications for binding to proteins

Computational methods have played a key, role in elucidating the various three-dimensional structures of oligosaccharides. Such structural information, together with other experimental data, leads to a better understanding of the role of oligosaccharide in various biological processes. The disialoside Neu5Ac-alpha2-->8-Neu5Ac appears as the terminal glycan in glycoproteins and glycolipids, and is known to play an important role in various events of cellular communication. Neurotoxins such as botulinum and tetanus require Neu5Ac-alpha2 --> 8-Neu5Ac for infecting the host. Glycoconjugates containing this disialoside and the enzymes catalyzing their biosynthesis are also regulated during cell growth. development, and differentiation. Unlike other biologically relevant disaccharides that have only two linkage bonds, the alpha2-8-linked disialoside has four: C2-O, O-C8', C8'-C7', and C7'-C6'. The present report describes the results from nine 1 ns MD simulations of alpha2-->8-linked disialoside (Neu5Ac-alpha2-->8-Neu5Ac); simulations were run using GROMOS96 by explicitly considering the solvent molecules. Conformations around the O-C8' bond are restricted to the +sc/+ap regions due to stereochemical reasons. In contrast. conformations around the C2-O and C8'-C7' bonds werefound to be largely unrestricted and all the three staggered regions are accessible. The conformations around the C7'-C6' bond were found to be in either the - sc or the anti region. These results are in excellent agreement with the available NMR and potential energy calculation studies. Overall, the disaccharide is flexible and adopts mainly two ensembles of conformations differing in the conformation around the C7'-C6' bond. The flexibility associated with this disaccharide allows for better optimization of intermolecular contacts while binding to proteins and this may partially compensate for the loss of conformational entropy that may be incurred due to disaccharide's flexibility. (C) 2002 John Wiley Sons, Inc

Higher order harmonic detection for exploring nonlinear interactions with nanoscale resolution

Nonlinear dynamics underpin a vast array of physical phenomena ranging from interfacial motion to jamming transitions. In many cases, insight into the nonlinear behavior can be gleaned through exploration of higher order harmonics. Here, a method using band excitation scanning probe microscopy (SPM) to investigate higher order harmonics of the electromechanical response, with nanometer scale spatial resolution is presented. The technique is demonstrated by probing the first three harmonics of strain for a Pb(Zr(1-x)Ti(x))O(3) (PZT) ferroelectric capacitor. It is shown that the second order harmonic response is correlated with the first harmonic response, whereas the third harmonic is not. Additionally, measurements of the second harmonic reveal significant deviations from Rayleigh-type models in the form of a much more complicated field dependence than is observed in the spatially averaged data. These results illustrate the versatility of n(th) order harmonic SPM detection methods in exploring nonlinear phenomena in nanoscale materials

Bias assisted scanning probe microscopy direct write lithography enables local oxygen enrichment of lanthanum cuprates thin films

Scanning probe bias techniques have been used as a method to locally dope thin epitaxial films of La<inf>2</inf>CuO<inf>4</inf> (LCO) fabricated by pulsed laser deposition. The local electrochemical oxidation of LCO very efficiently introduces interstitial oxygen defects in the thin film. Details on the influence of the tip voltage bias and environmental conditions on the surface morphology have been investigated. The results show that a local uptake of oxygen occurs in the oxidized films. © 2015 IOP Publishing Ltd