Improvement of dielectric loss of doped Ba0.5Sr0.5TiO3 thin films for tunable microwave devices

Al2O3-Ba0.5Sr0.5TiO3 (Al2O3-BST) thin films, with different Al2O3 contents, were deposited on (100) LaAlO3 substrate by pulsed laser deposition (PLD) technique. The Al2O3-BST films was demosnstrated to be a suitable systems to fabricate ferroelectric thin films with low dielectric loss and higher figure of merit for tunable microwave devices. Pure BST thin films were also fabricated for comparison purpose. The films' structure and morphology were analyzed by X-ray diffractiopn and scanning electron microscopy, respectively; nad showed that the surface roughness for the Al2O3-BST films increased with the Al2O3 content. Apart from that, the broadening in the intensity peak in XRD result indicating the grain size of the Al2O3-BST films reduced with the increasing of Al2O3 dopant. We measured the dielctric properties of Al2O3-BST films with a home-made non-destructive dual resonator method at frequency ~ 7.7 GHZ. The effect of doped Al2O3 into BST thin films significantly reduced the dielectric constant, dielectric loss and tunability compare to pure BST thin film. Our result shows the figure of merit (K), used to compare the films with varied dielectric properties, increased with the Al2O3 content. Therefore Al2O3-BST films show the potential to be exploited in tunable microwave devices.Comment: 8 pages, 4 figures, 1 table. Accepted & tentatively for Feb 15 2004 issue, Journal of Applied Physic

Evaluation of Glucosidase Inhibitory and Cytotoxic Potential of Five Selected Edible and Medicinal Ferns

Purpose: To evaluate the glucosidase inhibitory and cytotoxic activities of five selected edible and medicinal ferns, namely, Blechnum orientale, Davallia denticulata, Diplazium esculentum, Nephrolepis biserrata, and Pteris vittata.Methods: The aqueous extracts of the five ferns were prepared by water extraction at 90 ºC for 1 h. Antiglucosidase assay was used to determine the effect of each extract on yeast alpha-glucosidase activity in vitro. Cytotoxicity was evaluated using methylthiazol tetrazolium assay on chronic myelogenous leukaemia cell line (K562). The phenolic, hydroxycinnamic acid, flavonoid and proanthocyanidin contents of the extracts were also determined.Results: The α-glucosidase inhibitory activity of D. esculentum (half maximal effective concentration, EC50 = 6.85 μg/ml) was considerably stronger than that of myricetin (EC50 = 53.21 μg/ml). B. orientale, D. esculentum, N. biserrata, and P. vittata were cytotoxic to K562 cells. P. vittata had the strongest cytotoxicity, although it was less potent than 5-fluorouracil. D. denticulata had the highest phenolic, hydroxycinnamic acid and flavonoid contents of all the extracts while B. orientale had the highest proanthocyanidin content.Conclusion: Among the five ferns evaluated, D. esculentum is a potential source of an antidiabetic agent and is recommended for further investigation in this regard. All the fern extracts, except D. denticulata, exhibited dose-dependent cytotoxicity against K562 cells.Keywords: Medicinal fern, α-Glucosidase inhibition, Cytotoxicity, Blechnum orientale, Davallia denticulata, Diplazium esculentum, Nephrolepis biserrata, Pteris vittat

Codon 249 mutation of the p53 gene is a rare event in hepatocellular carcinomas from ethnic Chinese in Singapore.

The present study characterised p53 mutations in 44 hepatocellular carcinomas (HCCs) from Chinese patients residing in a high-incidence area. Twelve point mutations (27%) were detected in tumour tissues using single-strand conformation polymorphism analysis followed by direct DNA sequencing. Remarkably, no mutations were observed at codon 249. This is in contrast to HCCs from other high HCC incidence areas with endemic aflatoxin exposures, in which codon 249 is a mutational hot spot. It is therefore suggested that risk factors other than dietary exposure to aflatoxin may contribute to the high HCC incidence in Singapore

Geometric Approach to Lyapunov Analysis in Hamiltonian Dynamics

As is widely recognized in Lyapunov analysis, linearized Hamilton's equations of motion have two marginal directions for which the Lyapunov exponents vanish. Those directions are the tangent one to a Hamiltonian flow and the gradient one of the Hamiltonian function. To separate out these two directions and to apply Lyapunov analysis effectively in directions for which Lyapunov exponents are not trivial, a geometric method is proposed for natural Hamiltonian systems, in particular. In this geometric method, Hamiltonian flows of a natural Hamiltonian system are regarded as geodesic flows on the cotangent bundle of a Riemannian manifold with a suitable metric. Stability/instability of the geodesic flows is then analyzed by linearized equations of motion which are related to the Jacobi equations on the Riemannian manifold. On some geometric setting on the cotangent bundle, it is shown that along a geodesic flow in question, there exist Lyapunov vectors such that two of them are in the two marginal directions and the others orthogonal to the marginal directions. It is also pointed out that Lyapunov vectors with such properties can not be obtained in general by the usual method which uses linearized Hamilton's equations of motion. Furthermore, it is observed from numerical calculation for a model system that Lyapunov exponents calculated in both methods, geometric and usual, coincide with each other, independently of the choice of the methods.Comment: 22 pages, 14 figures, REVTeX

NbSe3: Effect of Uniaxial Stress on the Threshold Field and Fermiology

We have measured the effect of uniaxial stress on the threshold field ET for the motion of the upper CDW in NbSe3. ET exhibits a critical behavior, ET ~ (1 - e/ec)^g, wher e is the strain, and ec is about 2.6% and g ~ 1.2. This ecpression remains valid over more than two decades of ET, up to the highest fields of about 1.5keV/m. Neither g nor ec is very sensitive to the impurity concentraction. The CDW transition temperature Tp decreases linearly with e at a rate dTp/de = -10K/%, and it does not show any anomaly near ec. Shubnikov de-Haas measurements show that the extremal area of the Fermi surface decreases with increasing strain. The results suggest that there is an intimate relationship between pinning of the upper CDW and the Fermiology of NbSe3.Comment: 4 pages, 5 figure

Chemical Instability of the Cobalt Oxyhydrate Superconductor under Ambient Conditions

The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping.Comment: 10 pages and 4 figure

Particle-hole symmetry and transport properties of the flux state in underdoped cuprates

Transport properties are studied for the flux state with the gauge flux $\phi$ per plaquett, which may model the underdoped cuprates, with the emphasis on the particle-hole and parity/chiral symmetries.This model is reduced to the Dirac fermions in (2+1)D with a mass gap introduced by the antiferromagnetic (AF) long range order and/or the stripe formation. Without the mass gap, the Hall constant $R_H$ and the thermopower $S$ obey two-parameter scaling laws, and show the strong temperature dependence due to the recovery of the particle-hole symmetry at high temperature. The $x$-dependences of $\sigma_{xx} (\propto \sqrt{x})$ and $\sigma_{xy}$ (independent of $x$) are in a sharp contradiction with the experiments. (Here $x$ is the hole concentration.) Therefore there is no signature of the particle-hole symmetry or the massless Dirac fermions in the underdoped cuprates even above the Neel temperature $T_N$. With the mass gap introduced by the AF order, there occurs the parity anomaly for each of the Dirac fermions. However the contributions from different valleys and spins cancel with each other to result in no spontaneous Hall effect even if the time-reversal symmetry is broken with $\phi \ne \pi$. The effects of the stripes are also studied. The diagonal and vertical (horizontal) stripes have quite different influence on the transport properties. The suppression of $R_H$ occurs at low temperature only when (i) both the AF order and the vertical (horizontal) stripe coexist, and (ii) the average over the in-plane direction is taken. Discussions on the recent experiments are given from the viewpoint of these theoretical results.Comment: RevTeX, 14 pages, 11 figure