Skyrmion Superfluidity in Two-Dimensional Interacting Fermionic Systems
In this article we describe a multi-layered honeycomb lattice model of interacting fermions which supports a new kind of parity-preserving skyrmion superfluidity. We derive the low-energy field theory describing a non-BCS fermionic superfluid phase by means of functional fermionization. Such effective theory is a new kind of non-linear sigma model, which we call double skyrmion model. In the bi-layer case, the quasiparticles of the system (skyrmions) have bosonic statistics and replace the Cooper-pairs role. Moreover, we show that the model is also equivalent to a Maxwell-BF theory, which naturally establishes an effective Meissner effect without requiring a breaking of the gauge symmetry. Finally, we map effective superfluidity effects to identities among fermionic observables for the lattice model. This provides a signature of our theoretical skyrmion superfluidy that can be detected in a possible implementation of the lattice model in a real quantum system.

title: A crucial hypothesis for Inflation
abstract: As is well known there are many different inflationary models that can explain the accelerated expansion occurred in the early Universe. It is possible that there exists a fundamental property of this period that could provide the corresponding field theory. Our hypothesis is that the Ricci scalar should assume a positive constant value during Inflation. Considering a single scalar field that drives the inflationary phase, we obtain uniquely the scalar potential and the coupling term between the scalar field and spacetime geometry. For this potential, that is well known in the inflationary scenario, the theoretical prediction of the spectral index ns is in perfect agreement with the experimental data. Interestingly in two dimensions, our model is equivalent to Liouville gravity.

title: Abelian Chern-Simons-Maxwell theory from a tight-binding model of spinless fermions.
abstract: Abelian Chern-Simons-Maxwell theory can emerge from the bosonization of the (2+1)-dimensional Thirring model that describes interacting Dirac fermions. Here we show how the Thirring model manifests itself in the low energy limit of a two-dimensional tight-binding model of spinless fermions. To establish that, we employ a modification of Haldane's model, where the "doubling" of fermions is rectified by adiabatic elimination. Subsequently, fermionic interactions are introduced that lead to the analytically tractable Thirring model. By local density measurements of the lattice fermions we can establish that for specific values of the couplings the model exhibits the confining (2+1)-dimensional QED phase or a topological ordered phase that corresponds to the Chern-Simons theory. The implementation of the model as well as the measurement protocol are accessible with the current technology of cold atoms in optical lattices.

title: Topological effective field theories for Dirac fermions from index theorem
abstract: Dirac fermions have a central role in high energy physics but it is well known that they emerge also as quasiparticles in several condensed matter systems supporting topological order. We present a general method for deriving the topological effective actions of (3+1) massless Dirac fermions living on general backgrounds and coupled with vector and axial-vector gauge fields. The first step of our strategy is standard (in the Hermitian case) and consists in connecting the determinants of Dirac operators with the corresponding analytical indices through the zeta-function regularization. Then, we introduce a suitable splitting of the heat kernel that naturally selects the purely topological part of the determinant (i.e. the topological effective action). This topological effective action is expressed in terms of gauge fields using the Atiyah-Singer index theorem which computes the analytical index in topological terms. The main new result of this paper is to provide a consistent extension of this method to the non Hermitian case where a well-defined determinant does not exist. Quantum systems supporting relativistic fermions can thus be topologically classified on the basis of their response to the presence of (external or emergent) gauge fields through the corresponding topological effective field theories.

title: Non-Abelian Chern-Simons Theory from a Hubbard-like Model
abstract: Here, we provide a simple Hubbard-like model of spin-1/2 fermions that gives rise to the SU(2)-symmetric Thirring model that is equivalent, in the low-energy limit, to the Yang-Mills-Chern-Simons model. First, we identify the regime that simulates the SU(2) Yang-Mills theory. Then, we suitably extend this model so that it gives rise to the SU(2) Chern-Simons theory with level k≥2 that can support non-Abelian anyons. This is achieved by introducing multiple fermionic species and modifying the Thirring interactions, while preserving the SU(2) symmetry. Our proposal provides the means to theoretically and experimentally probe non-Abelian SU(2) level k topological phases.

title: Induced topological phases at the boundary of 3D topological superconductors.
abstract: We present tight-binding models of 3D topological superconductors in class DIII that support a variety of winding numbers. We show that gapless Majorana surface states emerge at their boundary in agreement with the bulk-boundary correspondence. At the presence of a Zeeman field, the surface states become gapped and the boundary behaves as a 2D superconductor in class D. Importantly, the 2D and 3D winding numbers are in agreement, signifying that the topological phase of the boundary is induced by the phase of the 3D bulk. Hence, the boundary of a 3D topological superconductor in class DIII can be used for the robust realization of localized Majorana zero modes.

title: ASCA and ROSAT X-ray Spectra of IC 5063 and NGC 3998
abstract: The results of the 0.1-10 keV spectral analysis of IC 5063 (Narrow Line Radio Galaxy) and NGC 3998 (LINER) are presented. In both cases there is evidence of a FeK$\alpha$ emission line, but its width is poorly constrained. The highly absorbed spectrum of IC 5063 is well described by a double power law model. The X-ray luminosity of a few $\times$ 10$^{43}$ ergs s$^{-1}$ reveals a Sey 1 nucleus seen nearly edge-on, as confirmed by optical and IR studies. NGC 3998 is well fitted by a power law model ($\Gamma$$\sim$1.9) without reflection and weak absorption. The weak X-ray emission detected by ASCA and ROSAT suggests a low-luminosity AGN or an active nucleus in a low-accretion period.

title: ASCA view on high-redshift radio-quiet quasars
abstract: We briefly discuss the latest ASCA results on the X-ray spectral properties of high-redshift radio-quiet quasars.

title: Asca and rosat x-ray spectra of high-redshift radio-loud quasars
abstract: Results are presented on the X-ray properties of 9 high-redshift (1.2 < z < 3.4) radio-loud quasars (RLQs) observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA; 10 observations) and ROSAT (11 observations, for a subset of six quasars). New ASCA observations of S5 0014+81 (z = 3.38) and S5 0836+71 (z = 2.17) and ROSAT observations of PKS 2126-158 for which results were never presented elsewhere are included. A simple model consisting of a power law plus cold, uniform absorption gives acceptable fits to the spectra of all sources. The ASCA spectra of the six brightest objects show evidence for absorption in excess of the Galactic value at a 99% confidence level. Comparison with the ROSAT data suggests that absorption has significantly varied (ΔNH ~ 8 × 1020 cm-2) in the case of S5 0836+71, on a timescale of approximately 0.8 yr in the quasar frame. For the remaining five sources for which ROSAT spectra were available, the two instruments gave consistent results, and the data were combined yielding unprecedented spectral coverage (typically about 0.4-40 keV in the quasar frame) for high-z quasars. This allows us to put severe limits on several different descriptions of the continuum (e.g., broken power law, bremsstrahlung, reflection component). No Fe Kα emission line is detected in any of the ASCA spectra. An absorption edge consistent with Fe Kα at the quasar redshift is marginally detected in S5 0014+81. Possible origins for the observed low energy absorption are discussed. In particular, contributions from the molecular clouds and dust present in our Galaxy (usually disregarded) are carefully considered. In light of the new results for S5 0836+71 and S5 0014+81, absorption intrinsic to the quasars is considered and discussed. The average slope obtained from the eight ASCA spectra in the observed ~0.5-10 keV energy band is Γ0.5-10 keV 1.61 ± 0.04, with a dispersion σ0.5-10 keV 0.10 ± 0.03. The average photon index in the observed 2-10 keV band, where the effect of absorption is negligible, is Γ2-10 keV 1.53 ± 0.05, with a dispersion σ2-10 keV 0.12. Furthermore, the implications of the present results on the calculations of the contribution of quasars to the cosmic X-ray and γ-ray backgrounds are briefly discussed.

title: A deep look at the inner regions of the mini‐BAL QSO PG 1126‐041 with XMM‐Newton
abstract: A long XMM‐Newton observation of the mini‐BAL QSO PG 1126‐041 allowed us to detect a highly ionized phase of X‐ray absorbing gas outflowing at ν∼15000 km s−1. Physical implications are briefly discussed.

title: X-ray source overdensities in Chandra distant cluster fields: a new probe to map the cosmic tapestry?
abstract: The first systematic study of the serendipitous X-ray source density around 10 high z (0.24 2σ per field, and >5σ overall) only on scales 8 � × 8 � and peculiar to cluster fields. The blank field fluctuations, i.e. the most sensitive measurements to date of the X-ray cosmic variance on this scale, indicate 1σ variations within ∼15-25% around the average value. The first marginal evidence that these overdensities increase with cluster redshift is also presented. We speculate that the most likely explanation for what has been observed is that the X-ray sources are AGNs which trace filaments connected to the clusters. Nevertheless other possible explanations such as an "X-ray Butcher-Oemler" effect are not ruled out by the present results. If the association of the overdensities with large-scale structures and their positive relation with cluster redshift is confirmed, these studies could represent a new, direct, way of mapping the cosmic web and its parameters at high z.