Suppression of creep-regime dynamics in epitaxial ferroelectric BiFeO3 films.

Switching dynamics of ferroelectric materials are governed by the response of domain walls to applied electric field. In epitaxial ferroelectric films, thermally-activated 'creep' motion plays a significant role in domain wall dynamics, and accordingly, detailed understanding of the system's switching properties requires that this creep motion be taken into account. Despite this importance, few studies have investigated creep motion in ferroelectric films under ac-driven force. Here, we explore ac hysteretic dynamics in epitaxial BiFeO3 thin films, through ferroelectric hysteresis measurements, and stroboscopic piezoresponse force microscopy. We reveal that identically-fabricated BiFeO3 films on SrRuO3 or La0.67Sr0.33MnO3 bottom electrodes exhibit markedly different switching behaviour, with BiFeO3/SrRuO3 presenting essentially creep-free dynamics. This unprecedented result arises from the distinctive spatial inhomogeneities of the internal fields, these being influenced by the bottom electrode's surface morphology. Our findings further highlight the importance of controlling interface and defect characteristics, to engineer ferroelectric devices with optimised performance.

Giant flexoelectric effect in ferroelectric epitaxial thin films.

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

DNA methyltransferase-mediated transcriptional silencing in malignant glioma: a combined whole-genome microarray and promoter array analysis.

Epigenetic inactivation of tumor suppressor genes is a common feature in human cancer. Promoter hypermethylation and histone deacetylation are reversible epigenetic mechanisms associated with transcriptional regulation. DNA methyltransferases (DNMT1 and DNMT3b) regulate and maintain promoter methylation and are overexpressed in human cancer. We performed whole-genome microarray analysis to identify genes with altered expression after RNAi-induced suppression of DNMT in a glioblastoma multiforme (GBM) cell line. We then identified genes with both decreased expression and evidence of promoter CpG island hypermethylation in GBM tissue samples using a combined whole-genome microarray transcriptome analysis in conjunction with a promoter array analysis after DNA immunoprecipitation with anti-5-methylcytidine. DNMT1 and 3b knockdown resulted in the restored expression of 308 genes that also contained promoter region hypermethylation. Of these, 43 were also found to be downregulated in GBM tissue samples. Three downregulated genes with hypermethylated promoters and restored expression in response to acute DNMT suppression were assayed for methylation changes using bisulfite sequence analysis of the promoter region after chronic DNMT suppression. Restoration of gene expression was not associated with changes in promoter region methylation, but rather with changes in histone methylation and chromatin conformation. Two of the identified genes exhibited growth suppressive activity in in vitro assays. Combining targeted genetic manipulations with comprehensive genomic and expression analyses provides a potentially powerful new approach for identifying epigenetically regulated genes in GBM.

Nonlinear dynamics of domain-wall propagation in epitaxial ferroelectric thin films.

We investigated the ferroelectric domain-wall propagation in epitaxial Pb(Zr,Ti)O3 thin film over a wide temperature range (3-300 K). We measured the domain-wall velocity under various electric fields and found that the velocity data is strongly nonlinear with electric fields, especially at low temperature. We found that, as one of surface growth issues, our domain-wall velocity data from ferroelectric epitaxial film could be classified into the creep, depinning, and flow regimes due to competition between disorder and elasticity. The measured values of velocity and dynamical exponents indicate that the ferroelectric domain walls in the epitaxial films are fractal and pinned by a disorder-induced local field.

Domain switching kinetics in disordered ferroelectric thin films.

We investigated domain kinetics by measuring the polarization switching behaviors of (111)-preferred polycrystalline Pb(Zr,Ti)O3 films, which are widely used in ferroelectric memories. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of logarithmic domain-switching times. We suggested that the local field variation due to dipole defects at domain pinning sites could explain the Lorentzian distribution.

Polarization switching dynamics governed by the thermodynamic nucleation process in ultrathin ferroelectric films.

In most ferroelectrics, the domain nucleation barrier (U*) is thermally insurmountable; this is called "Landauer's paradox." However, we showed that, in ultrathin films, the large depolarization fields could lower U* to a level comparable to thermal energy (k(B)T), resulting in power-law decay of polarization. We empirically found a universal relation between the power-law decay exponent and U*/k(B)T. This relation will provide a practical but fundamental limit for capacitor-type ferroelectric devices, analogous to the superparamagnetic limit for magnetic memory devices.

Lipopolysaccharide and CpG DNA synergize for tumor necrosis factor-alpha production through activation of NF-kappaB.

Unmethylated CpG motifs in bacterial DNA (CpG DNA) activate host innate immune responses synergistically with some other microbial products, such as endotoxins, and may contribute to disease pathogenesis through excessive production of proinflammatory cytokines. Because monocyte-derived tumor necrosis factor (TNF)-alpha is an important mediator of disease, we investigated whether CpG DNA and lipopolysaccharide (LPS) synergize for inducing TNF-alpha biosynthesis. CpG DNA and LPS synergistically induce TNF-alpha production in RAW264.7 cells and J774 cells through activation of NF-kappaB. Furthermore, transient transfection with a super-repressive mutant of IkappaBalpha (IkappaBalpha-AA) demonstrated that NF-kappaB plays a critical role in CpG DNA-mediated TNF-alpha expression. Like NF-kappaB activation, CpG DNA-induced activation of mitogen-activated protein kinases (MAPK) regulates TNF-alpha production. Both extracellular receptor kinase (ERK) and p38 can regulate TNF-alpha gene transcription induced by CpG DNA. Although CpG DNA at the higher concentration slightly enhanced LPS-mediated phosphorylation of ERK, it did not alter the LPS-mediated activation of c-Jun N-terminal kinase and p38. In addition, CpG DNA showed little or no enhancement of LPS-mediated AP-1 activation. These results suggest that CpG DNA- and LPS-mediated signals converge at or above the level of NF-kappaB and ERK, and that there are distinct, as well as common, signaling pathways which are utilized by both CpG DNA and LPS for activating various transcription factors and MAPK.

Design and feasibility of quick EXAFS scans for a 'broomstick' double-crystal monochromator at PLS beamline.

A data-collection technique for quick extended X-ray absorption fine-structure spectroscopy (QEXAFS) was developed with a new 'broomstick' double-crystal monochromator, which has been installed for X-ray absorption fine-structure (XAFS) applications at the Pohang Light Source. The monochromator operates in a fixed-exit scan mode as the Bragg angle is varied from 8 to 80 degrees, corresponding to 2-14 keV, using an Si(111) crystal. The monochromator scan capability was investigated by analysing EXAFS data quality from step-scan and from continuous rotation of the Bragg crystal reflection angle. In our fast continuous-scan design, the electronic pulsing speed of the step motor is adjustable to avoid the monochromatic beam instability caused by serious mechanical resonance. The feasibility of QEXAFS scanning is demonstrated by a typical EXAFS scan (e.g. 1 keV range) being taken within 1 min.

Immunocytochemical localization of Diploptera punctata allatostatin-like peptide in Drosophila melanogaster.

Allatostatins isolated from the cockroach Diploptera punctata are a family of neuropeptides that inhibit juvenile hormone synthesis in cockroaches and related insects but not in flies. In cockroaches, these widely distributed peptides have been shown to have other functions. This report provides evidence for the presence of allatostatin-like peptides in Drosophila melanogaster by demonstration of allatostatic activity of extracts of central nervous system from larvae and adults on corpora allata of Diploptera and by immunocytochemical localization of peptides in Drosophila with monoclonal antibody against Diploptera allatostatin I. Extract of adult central nervous system showed four times more allatostatic activity than that of the larva or twice the activity per unit volume of central nervous system. This is reflected in an increase in number and arborization of immunoreactive neurons in the adult. The immunoreactive neurons in the central nervous system appear to be interneurons, with the exception of motoneurons in the last abdominal neuromere that project to muscles of the hindgut, a pair of peripheral cells in each of two thoracic segments in the larva and on nerves to wings and halteres in the adult, and endocrine cells of the midgut epithelium. Nerves to the corpus allatum were not immunoreactive. The presence of Diploptera allatostatin-like peptides in interneurons and motoneurons, in the neurohemal networks, and in endocrine cells of the midgut and their absence in nerves to the corpus allatum in Drosophila suggests that these peptides may function as neuromodulators, myomodulators, and neurohormones and not as regulators of the corpus allatum.