A remark on alpha vacua for quantum field theories on de Sitter space

It is shown that the so-called $\alpha$-vacua which have been proposed as candidates for states of free quantum fields on de Sitter space have infinitely strong fluctuations for typical observables as averaged renormalized energy momentum tensor

Optical and Collective Properties of Excitons in 2D Semiconductors

We study the properties of excitons in 2D semiconductors (2DSC) by numerically solving the Schr\ {o}dinger equation for an interacting electron and hole in the effective mass approximation, then calculating optical properties such as the transition energies, oscillator strengths, and absorption coefficients. Our theoretical approach allows us to consider both direct excitons in monolayer (ML) 2DSC and spatially indirect excitons in heterostructures (HS) consisting of two 2DSC MLs separated by few-layer insulating hexagonal boron nitride (h-BN). In particular, we study indirect excitons in TMDC HS, namely MoS2, MoSe2, WS2, and WSe2; both direct and indirect excitons in the buckled 2D allotropes of silicon, germanium, and tin, known as silicene, germanene, and stanene respectively, or collectively as the Xenes; and both direct and indirect excitons in the anisotropic 2DSC phosphorene, the 2D allotrope of black phosphorus. Our study of indirect excitons in TMDC/h-BN HS was one of the first to study the dependence of the properties of spatially indirect excitons in 2DSC HS with respect to the interlayer separation. When considering excitons in the Xenes, we focused on the dependence of the excitonic properties on the magnitude of an external electric field oriented perpendicular to the Xene monolayer(s), which can be used to tune the band gap of the Xenes in-situ, thereby changing the charge carrier effective mass and thus the properties of the excitons themselves. Interestingly, our results for excitons in the Xenes indicate that freestanding ML Xenes may in fact be excitonic insulators in their ground states, that is, when there is zero external electric field. Furthermore, we predict, based on our results, that the freestanding ML Xenes should undergo a phase transition from the excitonic insulator state to a semiconducting state as the external electric field is increased beyond some critical value which is unique to each material. Lastly, our results show that the anisotropic exciton reduced mass, inherited from the anisotropic effective masses of electrons and holes in phosphorene, causes significant deviations in the eigenstates compared to the isotropic 2D model used for TMDCs and Xenes, and that furthermore, this anisotropy leads to enhanced (suppressed) optical absorption compared to the isotropic exciton, under linearly polarized excitations along the in-plane crystal axes with relatively smaller (larger) charge carrier effective masses. In addition, we were able to extend our theoretical framework to consider both exciton-photon and exciton-exciton interactions in a weakly interacting Bose gas of excitons, thereby allowing for the study of exciton-polaritons in an optical microcavity. Using this extended framework, we calculate the Rabi splitting between upper and lower polaritons in a model microcavity, as well as the critical temperature for the Berezinskii-Kosterlitz-Thouless (BKT) phase transition of a weakly interacting Bose gas of lower polaritons. In particular, we applied these methods to study polaritons in the ML Xenes, once again focusing on the dependence of these quantities on the magnitude of the external electric field. Based on our calculations, we predict that, assuming a particular type of open microcavity which maximizes the exciton-photon interaction strength, both freestanding ML silicene and ML silicene encapsulated by h-BN should support polaritons with relatively large Rabi splittings whose BKT critical temperature is greater than room temperature, such that it should be possible to achieve room-temperature superfluidity of polaritons in these materials for a particular range of values of the external electric field

Deep 1.4 GHZ Follow Up of the Steep Spectrum Radio Halo in Abell 521

In a recent paper we reported on the discovery of a radio halo with very steep spectrum in the merging galaxy cluster Abell 521 through observations with the Giant Metrewave Radio Telescope (GMRT). We showed that the steep spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons and supports a turbulent acceleration scenario. At that time, due to the steep spectrum, the available observations at 1.4 GHz (archival NRAO - Very Large Array - VLA CnB-configuration data) were not adequate to accurately determine the flux density associated with the radio halo. In this paper we report the detection at 1.4 GHz of the radio halo in Abell 521 using deep VLA observations in the D-configuration. We use these new data to confirm the steep-spectrum of the object. We consider Abell 521 the prototype of a population of very-steep spectrum halos. This population is predicted assuming that turbulence plays an important role in the acceleration of relativistic particles in galaxy clusters, and we expect it will be unveiled by future surveys at low frequencies with the LOFAR and LWA radio telescopes.Comment: 11 pages, 3 figures (figure 1 available in gif format only). Requires aastex.cls - Accepted by Ap.

A retrograde approach for liver gene transfer

no abstract availabl

Distribution and abundance of early life stages of squid (Illex argentinus) in the south-west Atlantic

A joint research cruise (Japan-Argentina-Uruguay) was carried out in the South-western Atlantic during August-September 1989 in order to study the winter-spawning and hatchery areas of Illex argentinus, and also the migration pattern of the juveniles towards the continental shelf. A few Rhynchoteuthion larvae were found in subtropical waters of the Brazil Current, next to the Brazil-Malvinas confluence, and in the frontal zone with shelf water, but never at temperatures below 14°C. Large numbers of juveniles found in subantarctic waters (6–10°C) on the shelf were probably migrating southward from their hatchery grounds following the zooplankton concentrations on which they were feeding

The Numerov process over a non-uniform grid

The Numerov process is a solution method applicable to some classes of differential equations, that provides an error term of the fifth order in the grid size with a computational cost comparable to that of the finite-difference scheme. In the original formulation of the method, a uniform grid size is required; the paper shows a procedure for extending its applicability to a non-uniform grid in one dimension. The effectiveness of the procedure is tested on a model problem, and comparisons with other methods are carried out. Finally, it is shown how to extend the applicability of the method to a larger class of equations; among these, the mathematical model of semiconductor devices is important in view of its applications to the integrated-circuit technology

Causal Perturbation Theory and Differential Renormalization

In Causal Perturbation Theory the process of renormalization is precisely equivalent to the extension of time ordered distributions to coincident points. This is achieved by a modified Taylor subtraction on the corresponding test functions. I show that the pullback of this operation to the distributions yields expressions known from Differential Renormalization. The subtraction is equivalent to BPHZ subtraction in momentum space. Some examples from Euclidean scalar field theory in flat and curved spacetime will be presented.Comment: 15 pages, AMS-LaTeX, feynm

The XMM-Newton Detection of Diffuse Inverse Compton X-rays from Lobes of the FR-II Radio Galaxy 3C98

The XMM-Newton observation of the nearby FR-II radio galaxy 3C 98 is reported. In two exposures on the target, faint diffuse X-ray emission associated with the radio lobes was significantly detected, together with a bright X-ray active nucleus, of which the 2 -- 10 keV intrinsic luminosity is (4 -- 8) \times 10^{42} erg s-1. The EPIC spectra of the northern and southern lobes are reproduced by a single power law model modified by the Galactic absorption, with a photon index of 2.2-0.5+0.6 and 1.7-0.6+0.7 respectively. These indices are consistent with that of the radio synchrotron spectrum, 1.73 +- 0.01 The luminosity of the northern and southern lobes are measured to be 8.3-2.6+3.3 \times 10^{40} erg s-1 and 9.2-4.3+5.7 \times 10^{40} erg s-1, respectively, in the 0.7 -- 7 keV range. The diffuse X-ray emission is interpreted as an inverse-Compton emission, produced when the synchrotron-emitting energetic electrons in the lobes scatter off the cosmic microwave background photons. The magnetic field in the lobes is calculated to be about 1.7 \mu G, which is about 2.5 times lower than the value estimated under the minimum energy condition. The energy density of the electrons is inferred to exceed that in the magnetic fields by a factor of 40 -- 50.Comment: 23 pages, 7 figures. Accepted for publication in the Astrophysical Journa