Discrete solitons in coupled active lasing cavities

We examine the existence and stability of discrete spatial solitons in coupled nonlinear lasing cavities (waveguide resonators), addressing the case of active defocusing media, where the gain exceeds damping in the low-amplitude limit. A new family of stable localized structures is found: these are bright and grey cavity solitons representing the connections between homogeneous and inhomogeneous states. Solitons of this type can be controlled by the discrete diffraction and are stable when the bistability of homogenous states is absent.Comment: 3 pages, 3 figures, accepted to Optics Letters (October 2012

Scalar multi-wormholes

In 1921 Bach and Weyl derived the method of superposition to construct new axially symmetric vacuum solutions of General Relativity. In this paper we extend the Bach-Weyl approach to non-vacuum configurations with massless scalar fields. Considering a phantom scalar field with the negative kinetic energy, we construct a multi-wormhole solution describing an axially symmetric superposition of $N$ wormholes. The solution found is static, everywhere regular and has no event horizons. These features drastically tell the multi-wormhole configuration from other axially symmetric vacuum solutions which inevitably contain gravitationally inert singular structures, such as `struts' and `membranes', that keep the two bodies apart making a stable configuration. However, the multi-wormholes are static without any singular struts. Instead, the stationarity of the multi-wormhole configuration is provided by the phantom scalar field with the negative kinetic energy. Anther unusual property is that the multi-wormhole spacetime has a complicated topological structure. Namely, in the spacetime there exist $2^N$ asymptotically flat regions connected by throats.Comment: 11 pages, 13 figure

Boundary values as Hamiltonian variables. I. New Poisson brackets

The ordinary Poisson brackets in field theory do not fulfil the Jacobi identity if boundary values are not reasonably fixed by special boundary conditions. We show that these brackets can be modified by adding some surface terms to lift this restriction. The new brackets generalize a canonical bracket considered by Lewis, Marsden, Montgomery and Ratiu for the free boundary problem in hydrodynamics. Our definition of Poisson brackets permits to treat boundary values of a field on equal footing with its internal values and directly estimate the brackets between both surface and volume integrals. This construction is applied to any local form of Poisson brackets. A prescription for delta-function on closed domains and a definition of the {\it full} variational derivative are proposed.Comment: 26 pages, LaTex, IHEP 93-4

Compton scattering sequence reconstruction algorithm for the liquid xenon gamma-ray imaging telescope (LXeGRIT)

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is a balloon born experiment sensitive to \g -rays in the energy band of 0.2-20 MeV. The main detector is a time projection chamber filled with high purity liquid xenon (LXeTPC), in which the three-dimensional location and energy deposit of individual \g -ray interactions are accurately measured in one homogeneous volume. To determine the \g -ray initial direction (Compton imaging), as well as to reject background, the correct sequence of interactions has to be determined. Here we report the development and optimization of an algorithm to reconstruct the Compton scattering sequence and show its performance on Monte Carlo events and LXeGRIT data.Comment: To appear in: Hard X-Ray, Gamma-Ray, and Neutron Detector Physics II, 2000; Proc. SPIE, vol. 4141; R.B. James & R.C. Schirato, ed

Signature of Fermi arc surface states in Andreev reflection at the WTe2_2 Weyl semimetal surface

We experimentally investigate charge transport through the interface between a niobium superconductor and a three-dimensional WTe$_2$ Weyl semimetal. In addition to classical Andreev reflection, we observe sharp non-periodic subgap resistance resonances. From an analysis of their positions, magnetic field and temperature dependencies, we can interpret them as an analog of Tomasch oscillations for transport along the topological surface state across the region of proximity-induced superconductivity at the Nb-WTe$_2$ interface. Observation of distinct geometrical resonances implies a specific transmission direction for carriers, which is a hallmark of the Fermi arc surface states.Comment: 5 pages, some misprints has been correcte

Ion Association Effects of Lipophilic Quaternary Ammonium Salts in Ion-Exchange and Potentiometric Selectivity

Strong effects of ion association on the anion-exchange selectivity in systems water -solutions of high quaternary ammonium salts (QAS) in organic solvents as well as on the potentiometric selectivity of plasticized polyvinylchloride (PVC) membranes containing QAS as ion exchangers have been established experimen¬tally and substantiated theoretically. Based on the linear Gibbs energy relations (LGER) and ion association theory by Eigen-Denison-Ramsey-Fuoss, an approach to separate estimation of the parts played by the solvation and ion association fac¬tors in ion-exchange and potentiometric selectivity has been proposed. The results obtained serve as the basis for revision of the formed notions about QAS as of «nonselective» ion exchangers and enable the development of methods for the con¬trol of ion-exchange and potentiometric selectivity using the factor of ion associa¬tion. Specifically, it has been demonstrated that variation of steric accessibility of the QAS exchange center is a powerful means to control the selectivity. It has been found experimentally that due to varying steric accessibility of the QAS exchange center the selectivity values have changed by 3 orders of magnitu¬de in case of single- charged anions exchanged for single-charged ones and by more than 7 orders - in case of double-charged anions exchanged for the sing¬le-charged ones. The above-mentioned effects revealed also in the potentiometric selectivity of QAS-based PVC membranes and to some extent - in the potentiometric selectivity of the membranes based on neutral anion carriers, doped with QAS for provision of anion permselectivity, are of great practical importance for the development of ISE with improved selectivity