Phase coherent transport in SrTiO3/LaAlO3 interfaces

The two dimensional electron gas formed between the two band insulators SrTiO3 and LaAlO3 exhibits a variety of interesting physical properties which make it an appealing material for use in future spintronics and/or quantum computing devices. For this kind of applications electrons have to retain their phase memory for sufficiently long times or length. Using a mesoscopic size device we were able to extract the phase coherence length, and its temperature variation. We find the dephasing rate to have a power law dependence on temperature. The power depends on the temperature range studied and sheet resistance as expected from dephasing due to strong electron-electron interactions.Comment: Submitted to Phys. Rev

Antibiofilm forming, antimicrobial activity and some biochemical properties of Vaccinium vitis idaea leaf and berry extracts on Staphylococcus aureus

Infections caused by Staphylococcus genus bacteria remain a relevant problem due to the high percentage of antibio­tic-resistant biofilm-forming strains of isolates of this genus. Herbs are a promising source for many biologically active compounds with antimicrobial properties. The aim of the research was to study the antimicrobial and antibiofilm formation activity of berry and leaf extracts of Vaccinium vitis-idaea L. upon clinical isolates of S. aureus, and the main biochemical properties of these extracts. For the purpose of analysis, we used S. aureus isolated from the mouth cavities and pharynx of human patients suffering from inflammatory diseases. The plants for the study were gathered in Pylypets, Mizhhiria rayon, Zakarpatska oblast (Transcarpathia). From Vaccinium vitis-idaea L., leaf and berry extracts were produced. To determine the chemical properties of the extracts, the following constituents were investigated: total tannin, flavonoids, total phenols, anthocyanins (by spectrophotometric method), and the total amount of vitamin C in berry extract (chromatographically). The antimicrobial activity was studied by diffusion-into-agar method and determination of minimum inhibitory concentrations. The antibiofilm activity of the extracts was tested in standard 96-well microtitration plates. The main chemical composition of ethyl extracts of Vaccinium vitis-idaea L. berries and leaves was identified. The level of tannins in leaf extracts was established to be higher than in fruit extracts (3.50% and 0.26% per 100 g of extract, respectively). It was shown that extracts of V. vitis-idaea berries and leaves demonstrate high antimicrobial activity against clinical isolates of S. aureus. Further it was established that leaf extracts had high ability to destroy the bacterial biofilm of S. aureus. Leaf extracts were also able to destroy the formed biofilm. Even in the 0.01% concentration, leaf extract inhibited the formation of the biofilm by 69.9% and caused the destruction of the formed biofilm by 62.5%. Thereby, the obtained results show good prospects for the use of V. vitis-idaea leaf extracts as an anti-staphylococcal remedy with antibiofilm forming properties

Mean curvature flow and quasilocal mass for two-surfaces in Hamiltonian General Relativity

A family of quasilocal mass definitions that includes as special cases the Hawking mass and the Brown-York ``rest mass'' energy is derived for spacelike 2-surfaces in spacetime. The definitions involve an integral of powers of the norm of the spacetime mean curvature vector of the 2-surface, whose properties are connected with apparent horizons. In particular, for any spacelike 2-surface, the direction of mean curvature is orthogonal (dual in the normal space) to a unique normal direction in which the 2-surface has vanishing expansion in spacetime. The quasilocal mass definitions are obtained by an analysis of boundary terms arising in the gravitational ADM Hamiltonian on hypersurfaces with a spacelike 2-surface boundary, using a geometric time-flow chosen proportional to the dualized mean curvature vector field at the boundary surface. A similar analysis is made choosing a geometric rotational flow given in terms of the twist covector of the dual pair of mean curvature vector fields, which leads to a family of quasilocal angular momentum definitions involving the squared norm of the twist. The large sphere limit of these definitions is shown to yield the ADM mass and angular momentum in asymptotically flat spacetimes, while at apparent horizons a quasilocal version of the Gibbons-Penrose inequality is derived. Finally, some results concerning positivity are proved for the quasilocal masses, motivated by consideration of spacelike mean curvature flow of 2-surfaces in spacetime.Comment: Revised version, includes an analysis of null flows with applications to mass and angular momentum for apparent horizon

From the Birkhoff-Gustavson normalization to the Bertrand-Darboux integrability condition

The Bertrand-Darboux integrability condition for a certain class of perturbed harmonic oscillators is studied from the viewpoint of the Birkhoff-Gustavson(BG)-normalization: By solving an inverse problem of the BG-normalization on computer algebra, it is shown that if the perturbed harmonic oscillators with a homogeneous-{\it cubic} polynomial potential and with a homogeneous-{\it quartic} polynomial potentials admit the same BG-normalization up to degree-4 then both oscillators satisfy the Bertrand-Darboux integrability condition.Comment: 23 pages, LaTeX (iop.sty), typos and Appendix adde

Critical disorder effects in Josephson-coupled quasi-one-dimensional superconductors

Effects of non-magnetic randomness on the critical temperature T_c and diamagnetism are studied in a class of quasi-one dimensional superconductors. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between the wires and T_c vanishes discontinuously when the randomness reaches a critical value. The parallel and transverse components of the penetration depth are found to diverge at different critical temperatures T_c^{(1)} and T_c, which correspond to pair-breaking and phase-coherence breaking. The interplay between disorder and quantum phase fluctuations results in quantum critical behavior at T=0, manifesting itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.Comment: 4 pages, 2 figure

Generalized Paraxial Ray Trace Procedure Derived from Geodesic Deviation

Paraxial ray tracing procedures have become widely accepted techniques for acoustic models in seismology and underwater acoustics. To date a generic form of these procedures including fluid motion and time dependence has not appeared in the literature. A detailed investigation of the characteristic curves of the equations of hydrodynamics allows for an immediate generalization of the procedure to be extracted from the equation form geodesic deviation. The general paraxial ray trace equations serve as an ideal supplement to ordinary ray tracing in predicting the deformation of acoustic beams in random environments. The general procedure is derived in terms of affine parameterization and in a coordinate time parameterization ideal for application to physical acoustic ray propagation. The formalism is applied to layered media, where the deviation equation reduces to a second order differential equation for a single field with a general solution in terms of a depth integral along the ray path. Some features are illustrated through special cases which lead to exact solutions in terms of either ordinary or special functions.Comment: Original; 40 pages (double spaced), 1 figure Replaced version; 36 pages single spaced, 7 figures. Expanded content; Complete derivation of the equations from the equations of hydrodynamics, introduction of an auxiliary basis for three dimensional wave-front modeling. Typos in text and equations correcte

Proximity-induced superconductivity in graphene

We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T_c, can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T<<T_c, and zero magnetic field, the density of states is characterized by a small gap E_g<T_c resulting from the collective proximity effect. Transverse magnetic field H_g(T) E_g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H_{g2}(T).Comment: 4 pages, 3 figure

Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in sub-ablation conditions

An investigation of ultrashort pulsed laser induced surface modification due to conditions that result in a superheated melted liquid layer and material evaporation are considered. To describe the surface modification occurring after cooling and resolidification of the melted layer and understand the underlying physical fundamental mechanisms, a unified model is presented to account for crater and subwavelength ripple formation based on a synergy of electron excitation and capillary waves solidification. The proposed theoretical framework aims to address the laser-material interaction in sub-ablation conditions and thus minimal mass removal in combination with a hydrodynamics-based scenario of the crater creation and ripple formation following surface irradiation with single and multiple pulses, respectively. The development of the periodic structures is attributed to the interference of the incident wave with a surface plasmon wave. Details of the surface morphology attained are elaborated as a function of the imposed conditions and results are tested against experimental data

Mesoscopic mechanism of exchange interaction in magnetic multilayers

We discuss a mesoscopic mechanism of exchange interaction in ferromagnet-normal metal-ferromagnet multilayers. We show that in the case when the metal's thickness is larger than the electron mean free path, the relative orientation of magnetizations in the ferromagnets is perpendicular. The exchange energy between ferromagnets decays with the metal thickness as a power law

Loss of Pi-Junction Behaviour in an Interacting Impurity Josephson Junction

Using a generalization of the non-crossing approximation which incorporates Andreev reflection, we study the properties of an infinite-U Anderson impurity coupled to two superconducting leads. In the regime where $\Delta$ and $T_K$ are comparable, we find that the position of the sub-gap resonance in the impurity spectral function develops a strong anomalous phase dependence-- its energy is a minimum when the phase difference between the superconductors is equal to $\pi$. Calculating the Josephson current through the impurity, we find that $\pi$-junction behaviour is lost as the position of the bound-state moves above the Fermi energy.Comment: 4 pages, 4 figures; labelling of Fig. 3 corrected; final published form, only trivial change