Entangled-photon generation in nano-to-bulk crossover regime

We have theoretically investigated a generation of entangled photons from biexcitons in a semiconductor film with thickness in nano-to-bulk crossover regime. In contrast to the cases of quantum dots and bulk materials, we can highly control the generated state of entangled photons by the design of peculiar energy structure of exciton-photon coupled modes in the thickness range between nanometers and micrometers. Owing to the enhancement of radiative decay rate of excitons at this thickness range, the statistical accuracy of generated photon pairs can be increased beyond the trade-off problem with the signal intensity. Implementing an optical cavity structure, the generation efficiency can be enhanced with keeping the high statistical accuracy.Comment: 9 pages, 3 figure

Helical Magnetic Fields from Inflation

We analyze the generation of seed magnetic fields during de Sitter inflation considering a non-invariant conformal term in the electromagnetic Lagrangian of the form $-\frac14 I(\phi) F_{\mu \nu} \widetilde{F}^{\mu \nu}$, where $I(\phi)$ is a pseudoscalar function of a non-trivial background field $\phi$. In particular, we consider a toy model, that could be realized owing to the coupling between the photon and either a (tachyonic) massive pseudoscalar field and a massless pseudoscalar field non-minimally coupled to gravity, where $I$ follows a simple power-law behavior $I(k,\eta) = g/(-k\eta)^{\beta}$ during inflation, while it is negligibly small subsequently. Here, $g$ is a positive dimensionless constant, $k$ the wavenumber, $\eta$ the conformal time, and $\beta$ a real positive number. We find that only when $\beta = 1$ and $0.1 \lesssim g \lesssim 2$ astrophysically interesting fields can be produced as excitation of the vacuum, and that they are maximally helical.Comment: 17 pages, 1 figure, subsection IIc and references added; accepted for publication in IJMP

A Note on Tsallis Holographic Dark Energy

We explore the effects of considering various infrared (IR) cutoffs, including the particle horizon, Ricci horizon and Granda-Oliveros (GO) cutoffs, on the properties of Tsallis holographic dark energy (THDE) model, proposed inspired by Tsallis generalized entropy formalism \cite{THDE}. Interestingly enough, we find that for the particle horizon as IR cutoff, the obtained THDE model can describe the accelerated universe. This is in contrast to the usual HDE model which cannot lead to an accelerated universe, if one consider the particle horizon as IR cutoff. We also investigate the cosmological consequences of THDE under the assumption of a mutual interaction between the dark sectors of the Universe. It is shown that the evolution history of the Universe can be described by these IR cutoffs and thus the current cosmic acceleration can also been realized. The sound instability of THDE models for each cutoff are also explored, separately.Comment: 12 pages, 31 figure

Generic estimates for magnetic fields generated during inflation including Dirac-Born-Infeld theories

We estimate the strength of large-scale magnetic fields produced during inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis is sufficiently general in the sense that it covers most of conformal symmetry breaking theories in which the electromagnetic field is coupled to a scalar field. In DBI theories there is an additional factor associated with the speed of sound, which allows a possibility to lead to an extra amplification of the magnetic field in a ultra-relativistic region. We clarify the conditions under which seed magnetic fields to feed the galactic dynamo mechanism at a decoupling epoch as well as present magnetic fields on galactic scales are sufficiently generated to satisfy observational bounds.Comment: 7 pages, no figure, accepted in Phys. Rev.

Traversable wormholes with static spherical symmetry and their stability in higher-curvature gravity

The solutions of traversable wormholes and their geometries are investigated in higher-curvature gravity with boundary terms for each case under the presence of anisotropic, isotropic and barotropic fluids in detail. For each case, the effective energy-momentum tensor violates the null energy condition throughout the wormhole throat. The null and weak energy conditions are also analyzed for ordinary matters. The regions that physically viable wormhole solutions can exist are explicitly shown. Furthermore, it is found that the range of the viable regions exhibits an alternating pattern of expansion and contraction. The present analyses can reveal the regions in which traversable wormholes can be constructed for anisotropic, isotropic and barotropic fluids cases with incorporating realistic matter contents, leading to fundamental physics insights into the feasible construction of wormholes in higher-curvature gravity with boundary term.Comment: 11 pages, 8 figure