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Search Results: 1 - 10 of 488722 matches for " M. V. Ramallo "
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An effective-charge model for the trapping of impurities of fluids in channels with nanostructured walls
M. V. Ramallo
Physics , 2011,
Abstract: We present model equations for the trapping and accumulation of particles in a short cylindrical channel with nanostructured inner walls when a fluid passes through, carrying a moderate load of impurities. The basic ingredient of the model is the introduction of a phenomenological "effective-charge density" of the walls, related to the electrical charges exposed in the nanotexture, and which is gradually reduced as the flow runs through the channel and the trapped impurities cover the internal walls. By solving the proposed equations, three regimes are predicted for the channel: a linear or clean-filter regime, a logarithmic or half-saturation regime, and the saturation limit. It is proposed that experimentally testing these regimes may help to understand the enhanced trapping capability observed in many diverse nanotextured channel structures.
Topological current algebras in two dimensions
J. M. Isidro,A. V. Ramallo
Physics , 1993, DOI: 10.1016/0370-2693(93)91033-J
Abstract: Two-dimensional topological field theories possessing a non-abelian current symmetry are constructed. The topological conformal algebra of these models is analysed. It differs from the one obtained by twisting the $N=2$ superconformal models and contains generators of dimensions $1$, $2$ and $3$ that close a linear algebra. Our construction can be carried out with one and two bosonic currents and the resulting theories can be interpreted as topological sigma models for group manifolds
gl(N,N) Current Algebras and Topological Field Theories
J. M. Isidro,A. V. Ramallo
Physics , 1993, DOI: 10.1016/0550-3213(94)90259-3
Abstract: The conformal field theory for the $gl(N,N)$ affine Lie superalgebra in two space-time dimensions is studied. The energy-momentum tensor of the model, with vanishing Virasoro anomaly, is constructed. This theory has a topological symmetry generated by operators of dimensions 1, 2 and 3, which are represented as normal-ordered products of $gl(N,N)$ currents. The topological algebra they satisfy is linear and differs from the one obtained by twisting the $N=2$ superconformal models. It closes with a set of $gl(N)$ bosonic and fermionic currents. The Wess-Zumino-Witten model for the supergroup $GL(N,N)$ provides an explicit realization of this symmetry and can be used to obtain a free-field representation of the different generators. In this free-field representation, the theory decomposes into two uncoupled components with $sl(N)$ and $U(1)$ symmetries. The non-abelian component is responsible for the extended character of the topological algebra, and it is shown to be equivalent to an $SL(N)/ SL(N)$ coset model. In the light of these results, the $G/ G$ coset models are interpreted as topological sigma models for the group manifold of $G$
Superconformal current algebras and topological field theories
J. M. Isidro,A. V. Ramallo
Physics , 1994, DOI: 10.1016/0370-2693(94)91296-3
Abstract: Topological conformal field theories based on superconformal current algebras are constructed. The models thus obtained are the supersymmetric version of the $G/G$ coset theories. Their topological conformal algebra is generated by operators of dimensions $1$, $2$ and $3$ and can be regarded as an extension of the twisted $N=2$ superconformal algebra. These models possess an extended supersymmetry whose generators are exact in the topological BRST cohomology.
Flux Stabilization of D-branes in a non-threshold bound state background
J. M. Camino,A. V. Ramallo
Physics , 2002, DOI: 10.1016/S0370-2693(02)01652-0
Abstract: We study some configurations of brane probes which are partially wrapped on spheres transverse to a stack of non-threshold bound states. The latter are represented by the corresponding supergravity background. Two cases are studied: D(10-p)-branes in the background of (D(p-2), Dp) bound states and D(8-p)-branes in the (NS5, Dp) geometry. By using suitable flux quantization rules of the worldvolume gauge field, we determine the stable configurations of the probe. The analysis of the energy and supersymmetry of these configurations reveals that they can be interpreted as bound states of lower dimensional objects polarized into a D-brane.
Critical temperatures for superconducting phase-coherence and condensation in La2-xSrxCuO4
N. Coton,M. V. Ramallo,F. Vidal
Physics , 2013,
Abstract: A pivotal ongoing debate about cuprate superconductors (HTS) is the location of the transition temperatures for the superconducting wave function phase coherence and condensation, Tphase and Tcond. This shall elucidate which of two very different interactions dictate the macroscopic superconducting phase diagram of HTS: either those between normal-state carriers or those between preformed vortices and antivortices. Here, we present unambiguous experimental determinations of Tphase and Tcond in the prototypical HTS La2-xSrxCuO4 as a function of the doping level x. Tphase is measured as a sharp change in the exponent alpha of the voltage-current characteristics (V proportional to I^alpha). Tcond is determined from the critical rounding of the ohmic resistivity above Tphase. Our measurements indicate that the transition to macroscopic superconductivity is accompanied by phase coherence due to vortex-antivortex binding and also that, for all x, Tcond lies only a few Kelvin above Tphase, limiting then the shift of the transition due to vortex-antivortex correlations.
Giant Gravitons with NSNS B field
J. M. Camino,A. V. Ramallo
Physics , 2001, DOI: 10.1088/1126-6708/2001/09/012
Abstract: We study the motion of a D(8-p)-brane probe in the background created by a stack of non-threshold (D(p-2), Dp) bound states for $2\le p\le 6$. The brane probe and the branes of the background have two common directions. We show that for a particular value of the worldvolume gauge field there exist configurations of the probe brane which behave as massless particles and can be interpreted as gravitons blown up into a fuzzy sphere and a noncommutative plane. We check this behaviour by studying the motion and energy of the brane and by determining how supersymmetry is broken by the probe as it moves under the action of the background.
M-Theory Giant Gravitons with C field
J. M. Camino,A. V. Ramallo
Physics , 2001, DOI: 10.1016/S0370-2693(01)01431-9
Abstract: We find giant graviton configurations of an M5-brane probe in the D=11 supergravity background generated by a stack of non-threshold (M2,M5) bound states. The M5-brane probe shares three directions with the background and wraps a two-sphere transverse to the bound states. For a particular value of the worldvolume gauge field of the PST formalism, there exist solutions of the equations of motion for which the M5-brane probe behaves as a wave propagating in the (M2,M5) background. We have checked that the probe breaks the supersymmetry of the background exactly as a massless particle moving along the trajectory of its center of mass.
Polynomials for Torus Links from Chern-Simons Gauge Theories
J. M. Isidro,J. M. F. Labastida,A. V. Ramallo
Physics , 1992, DOI: 10.1016/0550-3213(93)90632-Y
Abstract: Invariant polynomials for torus links are obtained in the framework of the Chern-Simons topological gauge theory. The polynomials are computed as vacuum expectation values on the three-sphere of Wilson line operators representing the Verlinde algebra of the corresponding rational conformal field theory. In the case of the $SU(2)$ gauge theory our results provide explicit expressions for the Jones polynomial as well as for the polynomials associated to the $N$-state ($N>2$) vertex models (Akutsu-Wadati polynomials). By means of the Chern-Simons coset construction, the minimal unitary models are analyzed, showing that the corresponding link invariants factorize into two $SU(2)$ polynomials. A method to obtain skein rules from the Chern-Simons knot operators is developed. This procedure yields the eigenvalues of the braiding matrix of the corresponding conformal field theory.
Coset Constructions in Chern-Simons Gauge Theory
J. M. Isidro,J. M. F. Labastida,A. V. Ramallo
Physics , 1992, DOI: 10.1016/0370-2693(92)90480-R
Abstract: Coset constructions in the framework of Chern-Simons topological gauge theories are studied. Two examples are considered: models of the types ${U(1)_p\times U(1)_q\over U(1)_{p+q}}\cong U(1)_{pq(p+q)}$ with $p$ and $q$ coprime integers, and ${SU(2)_m\times SU(2)_1\over SU(2)_{m+1}}$. In the latter case it is shown that the Chern-Simons wave functionals can be identified with t he characters of the minimal unitary models, and an explicit representation of the knot (Verlinde) operators acting on the space of $c<1$ characters is obtained.
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