Worldsheet Instantons and a Null String Limit of Born-Infeld Theory

For a superstring theory in four spacetime dimensions, we propose a modification of the Born-Infeld action that possesses a well-defined tensionless limit. We interpret this as describing the effective target space dynamics of null strings on a D3-brane. We argue that such a modification can be induced by nonperturbative contributions from instantons in the worldsheet sigma-model describing string propagation on the brane.Comment: 11 pages; Comments and references adde

Spectrum of confining strings in SU(N) gauge theories

We study the spectrum of the confining strings in four-dimensional SU(N) gauge theories. We compute, for the SU(4) and SU(6) gauge theories formulated on a lattice, the string tensions sigma_k related to sources with Z_N charge k, using Monte Carlo simulations. Our results are consistent with the sine formula sigma_k/sigma = sin k pi/N / sin pi/N for the ratio between sigma_k and the standard string tension sigma. For the SU(4) and SU(6) cases the accuracy is approximately 1% and 2%, respectively. The sine formula is known to emerge in various realizations of supersymmetric SU(N) gauge theories. On the other hand, our results show deviations from Casimir scaling. We also discuss an analogous behavior exhibited by two-dimensional SU(N) x SU(N) chiral models.Comment: Latex, 34 pages, 10 figures. Results of new SU(4) simulations added. The new data are included in the analysis, leading to improved final estimates for SU(4). Conclusions unchange

World-sheet instantons in the 2D black hole

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Unstable vortices in 2D WZNW model

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Instanton-induced effective Lagrangian in the Gauge-Higgs theory

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Approximation algorithms for Euclidean group TSP

In the Euclidean group Traveling Salesman Problem (TSP), we are given a set of points P in the plane and a set of m connected regions, each containing at least one point of P. We want to find a tour of minimum length that visits at least one point in each region. This unifies the TSP with Neighborhoods and the Group Steiner Tree problem. We give a (9.1a+1)-approximation algorithm for the case when the regions are disjoint a-fat objects with possibly varying size. This considerably improves the best results known, in this case, for both the group Steiner tree problem and the TSP with Neighborhoods problem. We also give the first O(1)-approximation algorithm for the problem with intersecting regions

Three-dimensional hemisphere-structured LiSn0.0125Mn 1.975O4 thin-film cathodes

Three-dimensional (3D) high surface area LiSn0.0125Mn 1.975O4 thin film cathodes have been fabricated in order to increase a charge capacity in the Li-ion battery. Metal oxide films were deposited by RF magnetron sputtering on three types of hemisphere-structured templates fabricated via spin-coating with polystyrene (PS) beads. Compared to standard planar battery design, the capacities of the close-packed, linked, and isolated 3D films are higher by 1.4, 2.6, and 2.1 times, respectively, which correlates with the corresponding increase of the specific surface area of the templates. The linked hemisphere cathode film shows an improved discharge capacity of 67.6 μA h μm- 1 cm- 2 without significant degradation of the cyclic retention. © 2014 Elsevier B.V.1541sciescopu