Quantum skyrmions and the destruction of long-range antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x)

We study the destruction of the antiferromagnetic order in the high-Tc superconductors La(2-x)Sr(x)CuO(4) and YBa(2)Cu(3)O(6+x) in the framework of the CP1-nonlinear sigma model formulation of the 2D quantum Heisenberg antiferromagnet. The dopants are introduced as independent fermions with appropriate dispersion relations determined by the shape of the Fermi surface. The energy of skyrmion topological defects, which are shown to be introduced by doping, is used as an order parameter for antiferromagnetic order. We obtain analytic expressions for this as a function of doping which allow us to plot the curves T_N(x_c)\times x_c and M(x)\times x, for both YBCO and LSCO, in good quantitative agreement with the experimental data.Comment: 4 pages, revtex, 5 embeeded figure

Charge pairing, superconducting transition and supersymmetry in high-temperature cuprate superconductors

We propose a model for high-T$_{c}$ superconductors, valid for $0\leq\delta\leq\delta_{SC}$, that includes both the spin fluctuations of the Cu$^{++}$ magnetic ions and of the O$^{--}$ doped holes. Spin-charge separation is taken into account with the charge of the doped holes being associated to quantum skyrmion excitations (holons) of the Cu$^{++}$ spin background. The holon effective interaction potential is evaluated as a function of doping, indicating that Cooper pair formation is determined by the competition between the spin fluctuations of the Cu$^{++}$ background and of spins of the O$^{--}$ doped holes (spinons). The superconducting transition occurs when the spinon fluctuations dominate, thereby reversing the sign of the interaction. At this point ($\delta = \delta_{SC}$), the theory is supersymmetric at short distances and, as a consequence, the leading order results are not modified by radiative corrections. The critical doping parameter for the onset of superconductivity at T=0 is obtained and found to be a universal constant determined by the shape of the Fermi surface. Our theoretical values for $\delta_{SC}$ are in good agreement with the experiment for both LSCO and YBCO.Comment: RevTex, 4 pages, no figure

Gravitational Waveguides in Cosmology

We discuss the possibility that, besides the usual gravitational lensing, there may exist a sort of gravitational waveguiding in cosmology which could explain some anomalous phenomena which cannot be understood by the current gravitational lensing models as the existence of "brothers" objects having different brilliancy but similar spectra and redshifts posed on the sky with large angular distance. Furthermore, such a phenomena could explain the huge luminosities coming from quasars using the cosmological structures as selfoc-type or planar waveguide. We describe the gravitational waveguide theory and then we discuss possible realizations in cosmology.Comment: 14 pages, latex, submitted to Int. Jou. Mod. Phys.

Search for an Near-IR Counterpart to the Cas A X-ray Point Source

We report deep near-infrared and optical observations of the X-ray point source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have identified a J=21.4 +/- 0.3 mag and Ks=20.5 +/- 0.3 mag source within the 1-sigma error circle, but we believe this source is a foreground Pop II star with Teff=2600-2800 K at a distance of ~2 kpc, which could not be the X-ray point source. We do not detect any sources in this direction at the distance of Cas A, and therefore place 3-sigma limits of R >~ 25 mag, F675W >~ 27.3 mag, J >~ 22.5 mag and Ks >~ 21.2 mag (and roughly H >~ 20 mag) on emission from the X-ray point source, corresponding to M_{R} >~ 8.2 mag, M_{F675W} >~ 10.7 mag, M_{J} >~ 8.5 mag, M_{H} >~ 6.5 mag, and M_{Ks} >~ 8.0 mag, assuming a distance of 3.4 kpc and an extinction A_{V}=5 mag.Comment: 14 pages, 7 figures. Accepted by Ap

Loop-Corrected Compactifications of the Heterotic String with Line Bundles

We consider the E8 x E8 heterotic string theory compactified on Calabi-Yau manifolds with bundles containing abelian factors in their structure group. Generic low energy consequences such as the generalised Green-Schwarz mechanism for the multiple anomalous abelian gauge groups are studied. We also compute the holomorphic gauge couplings and induced Fayet-Iliopoulos terms up to one-loop order, where the latter are interpreted as stringy one-loop corrections to the Donaldson-Uhlenbeck-Yau condition. Such models generically have frozen combinations of Kaehler and dilaton moduli. We study concrete bundles with structure group SU(N) x U(1)^M yielding quasi-realistic gauge groups with chiral matter given by certain bundle cohomology classes. We also provide a number of explicit tadpole free examples of bundles defined by exact sequences of sums of line bundles over complete intersection Calabi-Yau spaces. This includes one example with precisely the Standard Model gauge symmetry.Comment: 47 pages, 6 figures, LaTeX, v2: stability discussion in sect. 2.1 slightly extended, refs. added, v3: normalization of Green-Schwarz term correcte

Statistics of non-linear stochastic dynamical systems under L\'evy noises by a convolution quadrature approach

This paper describes a novel numerical approach to find the statistics of the non-stationary response of scalar non-linear systems excited by L\'evy white noises. The proposed numerical procedure relies on the introduction of an integral transform of Wiener-Hopf type into the equation governing the characteristic function. Once this equation is rewritten as partial integro-differential equation, it is then solved by applying the method of convolution quadrature originally proposed by Lubich, here extended to deal with this particular integral transform. The proposed approach is relevant for two reasons: 1) Statistics of systems with several different drift terms can be handled in an efficient way, independently from the kind of white noise; 2) The particular form of Wiener-Hopf integral transform and its numerical evaluation, both introduced in this study, are generalizations of fractional integro-differential operators of potential type and Gr\"unwald-Letnikov fractional derivatives, respectively.Comment: 20 pages, 5 figure

Witten's Vertex Made Simple

The infinite matrices in Witten's vertex are easy to diagonalize. It just requires some SL(2,R) lore plus a Watson-Sommerfeld transformation. We calculate the eigenvalues of all Neumann matrices for all scale dimensions s, both for matter and ghosts, including fractional s which we use to regulate the difficult s=0 limit. We find that s=1 eigenfunctions just acquire a p term, and x gets replaced by the midpoint position.Comment: 24 pages, 2 figures, RevTeX style, typos correcte

Extracting Spatial Information from Noise Measurements of Multi-Spatial-Mode Quantum States

We show that it is possible to use the spatial quantum correlations present in twin beams to extract information about the shape of a mask in the path of one of the beams. The scheme, based on noise measurements through homodyne detection, is useful in the regime where the number of photons is low enough that direct detection with a photodiode is difficult but high enough that photon counting is not an option. We find that under some conditions the use of quantum states of light leads to an enhancement of the sensitivity in the estimation of the shape of the mask over what can be achieved with a classical state with equivalent properties (mean photon flux and noise properties). In addition, we show that the level of enhancement that is obtained is a result of the quantum correlations and cannot be explained with only classical correlations

The nature of LINER galaxies: Ubiquitous hot old stars and rare accreting black holes

Galaxies, which often contain ionised gas, sometimes also exhibit a so-called low-ionisation nuclear emission line region (LINER). For 30 years this was attributed to a central mass-accreting supermassive black hole (AGN) of low luminosity, making LINER galaxies the largest AGN-sub-population, dominating in numbers over higher luminosity Seyfert galaxies and quasars. This, however, poses a serious problem. While the inferred energy balance is plausible, many LINERs clearly do not contain any other independent signatures of an AGN. Using integral field spectroscopic data from the CALIFA survey, we aim at comparing the observed radial surface brightness profiles with what is expected from illumination by an AGN. Essential for this analysis is a proper extraction of emission-lines, especially weak lines such as the Balmer Hb line which is superposed on an absorption trough. To accomplish this, we use the GANDALF code which simultaneously fits the underlying stellar continuum and emission lines. We show for 48 galaxies with LINER-like emission, that the radial emission-line surface brightness profiles are inconsistent with ionisation by a central point-source and hence cannot be due to an AGN alone. The most probable explanation for the excess LINER-like emission is ionisation by evolved stars during the short but very hot and energetic phase known as post-AGB. This leads us to an entirely new interpretation. Post-AGB stars are ubiquitous and their ionising effect should be potentially observable in every galaxy with gas present and stars older than ~1 Gyr, unless a stronger radiation field from young hot stars or an AGN outshines them. This means that galaxies with LINER-like emission are in fact not a class defined by a property, but rather by the absence of a property. It also explains why LINER emission is observed mostly in massive galaxies with old stars and little star formation.Comment: 8 pages, 7 figure