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.
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.
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.
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.
120919640
Gravitational Behaviour of an Effective Topological Field Theory
Effective topological field theories describe the properties of Dirac fermions in the low-energy regime. In this work, we introduce a new emergent gravity model by considering Dirac fermions invariant under local de Sitter transformations in four-dimensional open manifolds. In the context of Cartan geometry, fermions couple to spacetime through a $Spin(5)$ Cartan connection that can be decomposed in spin connection and tetrads. In presence of a gravitational instanton background, we show that the corresponding effective topological field theory becomes equivalent to a dynamical gravitational theory with positive cosmological constant and Barbero-Immirzi parameter. At classical level and in absence of matter, this theory is compatible with general relativity.
52970930
Variable X-ray absorption in the mini-BAL QSO PG 1126-041
Context. X-ray studies of AGN with powerful nuclear winds are important to constrain the physics of the inner accretion/ejection flow around SMBH, and to understand the impact of such winds on the AGN environment. Aims. Our main scientific goal is to constrain the properties of a va riable outflowing absorber that is thought to be launched nea r the SMBH of the mini-BAL QSO PG 1126-041 using a multi-epoch observational campaign performed with XMM-Newton. Methods. We performed temporally resolved X-ray spectroscopy and simultaneous UV and X-ray photometry on the most complete set of observations and on the deepest X-ray exposure of a mini-BAL QSO to date. Results. We found complex X-ray spectral variability on time scales of both months and hours, best reproduced by means of variable massive ionized absorbers along the line of sight. As a consequence, the observed optical-to-X-ray spectral index is found to be variable with time. In the highest signal-to-noise observa tion we detected highly ionized X-ray absorbing material outflowing much faster (υX∼ 16 500 km s −1 ) than the UV absorbing one (υuv∼ 5 000 km s −1 ). This highly ionized absorber is found to be variable on very short (a few kiloseconds) time scales. Conclusions. Our findings are qualitatively consistent with line driven a ccretion disk winds scenarios. Our observations have opened the time-resolved X-ray spectral analysis field for mini-BA L QSOs; only with future deep studies will we be able to map the dynamics of the inner flow and understand the physics of AGN winds and th eir impact on the environment.
2476266
On the peculiar X-ray properties of the bright nearby radio-quiet quasar PDS 456
BeppoSAX and ASCA observations of the nearby (z = 0.184), high-luminosity, radio-quiet quasar PDS 456 are presented. The X-ray spectrum is characterized by a promi- nent ionized edge at 8-9 keV (originally discovered by RXTE, Reeves et al. 2000) and by a soft excess below 1.5 keV. The lack of any significant iron K emission line suggests for the edge an origin from line-of-sight material rather than from re- flection from a highly ionized accretion disc. The hard X-ray continuum is indeed well modelled by transmission through a highly-ionized medium with a large column density (NHwarm
119193482
A deep look at the inner regions of the mini‐BAL QSO PG 1126‐041 with XMM‐Newton
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.
15297271
BeppoSAX observation of NGC 7674: a new reflection-dominated Seyfert 2 galaxy
The Seyfert 2 galaxy NGC 7674 has been observed within the BeppoSAX Core Programme with the onboard nar- roweldinstrumentsbetween0.1and100keV.Thebroad-band spectrum shows four most relevant spectral components: a) a soft excess below 2 keV; b) a prominent (EW1 keV) Fe line; c) a flat ( obs 1:1) 2{10 keV continuum; d) a 4.5 de- tection above 13 keV. The flat power law spectrum can be very well explained within the current AGN unied models assum- ing a steep ( 2) intrinsic spectrum scattered by warm gas for the low energy band and totally reflected by optically thick cold matter (plausibly a molecular torus) for the high energy band. The case of NGC 7674 adds to the increasing number of so called \Compton-thick" Seyfert 2 galaxies in which the direct emission is totally absorbed and the X-ray luminosity is thus at least one or two orders of magnitude larger than what inferred from the observed flux.
140606740
From (under)ground to space: The birth of Space Science in Italy and the growth of astrophysics in its National Institutions
The first attempt by Italian scientists to carry on Astrophysical observations from Space can be traced back to the early 60ʼs just after the launch of the first satellites. Dominated by Physicists for decades, slowly space astrophysics has become one of the major fields of modern Astronomy. From the first experiments flown on balloons and rockets to the construction of todayʼs sophisticated satellites a historical scenario will be presented in the framework of Italian Science and, more generally, of European and World growth of the field. The first experiments in Universities, the setting of the CNR (National Research Council) labs, later transformed in institutes, to the recent merging of all Astronomical Observatories, will be discussed with mention to historical episodes. Main results and achievements will be outlined. How politics has affected the development of this branch of astronomical research will also be briefly considered within the limits assigned to the present talk. Some of the information contained in this paper comes from original documents, some from interviewing people who played a major role in the events, some from the literature but most of it is from personal reminiscences.