96459470
(3+1) -dimensional topological quantum field theory from a tight-binding model of interacting spinless fermions
Currently, there is much interest in discovering analytically tractable (3+1)-dimensional models that describe interacting fermions with emerging topological properties. Towards that end we present a three-dimensional tight-binding model of spinless interacting fermions that reproduces, in the low-energy limit, a (3+1)-dimensional Abelian topological quantum field theory called the BF model. By employing a mechanism equivalent to Haldane's Chern insulator, we can turn the noninteracting model into a three-dimensional chiral topological insulator. We then isolate energetically one of the two Fermi points of the lattice model. In the presence of suitable fermionic interactions, the system, in the continuum limit, is equivalent to a generalized (3+1)-dimensional Thirring model. The low-energy limit of this model is faithfully described by the BF theory. Our approach directly establishes the presence of (2+1)-dimensional BF theory at the boundary of the lattice and it provides a way to detect the topological order of the model through fermionic density measurements.
118598460
Topological effective field theories for Dirac fermions from index theorem
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.
2580056
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.
21372858
Induced topological phases at the boundary of 3D topological superconductors.
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.
117979829
A crucial hypothesis for Inflation
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.
34016084
Abelian Chern-Simons-Maxwell theory from a tight-binding model of spinless fermions.
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.
14932917
ASCA detection of the Fe K edge in the spectrum of the Seyfert 2 galaxy IRAS 04575-7537: a sign of a complex absorber
ASCA X-ray spectral analysis of the Seyfert 2 galaxy IRAS 04575 7537 is presented. The main result is the presence of a significant iron edge at a rest energy of 7.13 +0:21 0:16 keV. The spectrum is flat ( 1.5) with a substantial absorption: NH 10 22 cm 2 and does not require any reflection component. There is also evidence of a narrow Fe K emission line, whose rest energy (E = 6.35 +0:08 0:03 keV) and equivalent width (EW ' 13050 eV) suggest that the line originates from thick and cold matter. The intensity of the line and, in particular, the depth of the iron edge are too strong to be explained by simple trans- mission through the measured absorption column density. This strongly suggests that a model more complex than a single ab- sorbed power law is needed. We propose an absorption model that we call "leaky-total absorber" which can explain the spec- trum flatness, the iron emission line and the edge absorption feature and has also the advantage of having a straightforward physical interpretation in the framework of Unified models. In this model a thick absorber (NH 10 23 cm 2 ), possibly asso- ciated with broad line region (BLR) clouds, partially ( 36%) covers the source continuum with fixed to 1.9 as observed in Sey 1 galaxies. The escaping radiation is then absorbed by a column density NH 10 22 cm 2 , which can be attributed to the torus, seen through its rim.
15356378
Is the X-Ray Spectrum of the Seyfert 2 Galaxy NGC 5252 Intrinsically Flat?
The first X-ray observation of the Seyfert 2 galaxy NGC5252 is reported. ASCA collected $\sim 4000$ photons/detector enough to perform an accurate spectral analysis of this source. The luminosity of NGC5252 is L$_{\rm X}$(0.7-10 keV) $\simeq 2.6 \times 10^{43}$ erg s$^{-1}$, typical of a Seyfert 1 galaxy. A simple description of the spectrum with a single power law is ruled out by the SIS data which shows a strong soft excess at E $\lsimeq$ 1.2 keV. The spectrum is best fitted by models assuming either partial covering of the central source or scattering of the X-ray continuum. The best-fit partial covering model results in a flat ($\Gamma \simeq 1.45 \pm 0.2$) power law continuum emitted by a source almost completely covered (at $\sim$ 94-97\%) by neutral matter ($N_{\rm H} \simeq (4.3 \pm 0.6) \times 10^{22}$ cm$^{-2}$). The detected iron line is remarkably weaker (EW $\sim 90 \pm 60$ eV) than normally found from Seyfert 2 galaxies. In an effort to interpret the observed flat spectrum with an intrinsically steep power law as predicted by unified models, we also included in the models neutral reflection, ionized absorption (warm absorber), and non-uniform cold absorption (dual-absorber). We find that the observed flat continum {\bf and} weakness of the iron line poses a problem for neutral reflection models, whatever the assumed geometry is. The use of an ionized absorber (scattering + warm-absorber model) instead of a neutral absorber seems not justified with the present data. The ``dual-absorber'' model, representing nonuniform (density) absorption along the line of sight, may provide an alternative explanation for the flatness of the spectrum, with no constraints on the Fe K emission line. Another plausible interpretation of the present data is that the X-ray spectrum of NGC5252 is truly intrinsically flat. Its strong
3198083
The low luminosity AGN in the LINER galaxy M 81: BeppoSAX discovery of highly ionized gas
The LINER nucleus of the nearby spiral galaxy M 81 was pointed by BeppoSAX, which caught it at the highest (2- 10) keV flux level observed so far. The LECS, MECS and PDS data, extending over (0.1-100) keV, are used to investi- gate the physical similarities and differences between LINERs and AGNs. The continuum is well fitted by a power law of pho- ton index 1:84, modified by little absorption due to cold material; this extends to (0.1-100) keV the validity of a similar ASCA result. Superimposed on the continuum BeppoSAX de- tects a 6.7 keV emission line (confirming another ASCA result) and an absorption edge at 8:6 keV. Both spectral features are consistent with being produced by iron at the same high ion- ization level, and probably also with the same column density. So, we suggest that they originate from transmission through highly ionized thin material. Concerning the origin of the con- tinuum emission, we do not observe signs of reflection from the optically thick material of an accretion disk, as usually found in Seyfert 1's (a 6.4 keV emission line and a broad bump peaking at 10-20 keV). The low bolometric luminosity of the nucleus of M 81 is consistent with being produced by advection dominated accretion; in this case the X-ray emission should be dominated by Comptonization, rather than by bremsstrahlung, in order to reproduce the steep spectrum observed over the (0.1-100) keV band.
123098211
Measurements of radio pulses from EAS and their dependence on shower parameters
One year's data on radio pulse from EAS at three frequencies (46, 65, 110 MHz) from the Medicina array have been analysed. The dominance of the geomagnetic production mechanism is confirmed. The frequency spectrum between 46 and 65 MHz appears to be flat. The detected electric field strength Ev has been considered as a measure of primary particle energy, and its value is correlated to shower size N and particle density rho at 50 and 100 m from the shower axis in order to study AES longitudinal fluctuations. Errors on each measured quantity are discussed, and it is shown that fluctuations are present.
16664161
Investigating the EGRET-radio galaxies link with INTEGRAL: The case of 3EG J1621+8203 and NGC 6251
The analysis of an INTEGRAL AO2 observation of the error contours of the EGRET source 3EG J1621 + 8203 is presented. The only source found inside the error contours for energies between 20 and 30 keV at 5σ detection significance is the FR I radio galaxy NGC 6251. This supports the identification of NGC 6251 with 3EG J1621 + 8203. The observed flux is higher and softer than observed in the past, but consistent with a variable blazar-like spectral energy distribution.