Cosmological solutions in generalized hybrid metric-Palatini gravity

We construct exact solutions representing a Friedmann-Lema\^itre-Robsertson-Walker (FLRW) universe in a generalized hybrid metric-Palatini theory. By writing the gravitational action in a scalar-tensor representation, the new solutions are obtained by either making an ansatz on the scale factor or on the effective potential. Among other relevant results, we show that it is possible to obtain exponentially expanding solutions for flat universes even when the cosmology is not purely vacuum. We then derive the classes of actions for the original theory which generate these solutions.Comment: 14 pages, 17 figure

Formation and observation of a quasi-two-dimensional dxyd_{xy} electron liquid in epitaxially stabilized Sr2−x_{2-x}Lax_{x}TiO4_{4} thin films

We report the formation and observation of an electron liquid in Sr$_{2-x}$La$_{x}$TiO$_4$, the quasi-two-dimensional counterpart of SrTiO$_3$, through reactive molecular-beam epitaxy and {\it in situ} angle-resolved photoemission spectroscopy. The lowest lying states are found to be comprised of Ti 3$d_{xy}$ orbitals, analogous to the LaAlO$_3$/SrTiO$_3$ interface and exhibit unusually broad features characterized by quantized energy levels and a reduced Luttinger volume. Using model calculations, we explain these characteristics through an interplay of disorder and electron-phonon coupling acting co-operatively at similar energy scales, which provides a possible mechanism for explaining the low free carrier concentrations observed at various oxide heterostructures such as the LaAlO$_3$/SrTiO$_3$ interface

The evolution of tensor perturbations in scalar-tensor theories of gravity

The evolution equations for tensor perturbations in a generic scalar tensor theory of gravity are presented. Exact solution are given for a specific class of theories and Friedmann-Lema\^{i}tre-Robertson-Walker backgrounds. In these cases it is shown that, although the evolution of tensor models depends on the choice of parameters of the theory, no amplification is possible if the gravitational interaction is attractive.Comment: 11 pages, 2 figures, submitted to Physical Review

Pressure-induced Topological Phase Transitions in Rock-salt Chalcogenides

By means of a comprehensive theoretical investigation, we show that external pressure can induce topological phase transitions in IV-VI semiconducting chalcogenides with rock-salt structure. These materials satisfy mirror symmetries that are needed to sustain topologically protected surface states, at variance with time-reversal symmetry responsible for gapless edge states in $\mathcal{Z}_{2}$ topological insulators. The band inversions at high-symmetry points in the Brillouin zone that are related by mirror symmetry, are brought about by an "asymmetric" hybridization between cation and anion $sp$ orbitals. By working out the microscopic conditions to be fulfilled in order to maximize this hybridization, we identify materials in the rock-salt chalcogenide class that are prone to undergo a topological phase transition induced by pressure and/or alloying. Our model analysis is fully comfirmed by complementary advanced \textit{first-principles} calculations and \textit{ab initio}-based tight-binding simulations

A Geometrical Approach to Strong Gravitational Lensing in f(R) Gravity

We present a framework for the study of lensing in spherically symmetric spacetimes within the context of f(R) gravity. Equations for the propagation of null geodesics, together with an expression for the bending angle are derived for any f(R) theory and then applied to an exact spherically symmetric solution of R^n gravity. We find that for this case more bending is expected for R^n gravity theories in comparison to GR and is dependent on the value of n and the value of distance of closest approach of the incident null geodesic.Comment: 9 page

Forward Symplectic Integrators and the Long Time Phase Error in Periodic Motions

We show that when time-reversible symplectic algorithms are used to solve periodic motions, the energy error after one period is generally two orders higher than that of the algorithm. By use of correctable algorithms, we show that the phase error can also be eliminated two orders higher than that of the integrator. The use of fourth order forward time step integrators can result in sixth order accuracy for the phase error and eighth accuracy in the periodic energy. We study the 1-D harmonic oscillator and the 2-D Kepler problem in great details, and compare the effectiveness of some recent fourth order algorithms.Comment: Submitted to Phys. Rev. E, 29 Page

Putting a “C60 Ball” and Chain to Chlorin e6 Improves Its Cellular Uptake and Photodynamic Performances

Chlorin e6 (Ce6) and fullerene (C60) are among the most used photosensitizers (PSs) for photodynamic therapy (PDT). Through the combination of the chemical and photophysical properties of Ce6 and C60, in principle, we can obtain an “ideal” photosensitizer that is able to bypass the limitations of the two molecules alone, i.e., the low cellular uptake of Ce6 and the scarce solubility and absorption in the red region of the C60. Here, we synthesized and characterized a Ce6–C60 dyad. The UV-Vis spectrum of the dyad showed the typical absorption bands of both fullerene and Ce6, while a quenching of Ce6 fluorescence was observed. This behavior is typical in the formation of a fullerene–antenna system and is due to the intramolecular energy, or electron transfer from the antenna (Ce6) to the fullerene. Consequently, the Ce6–C60 dyad showed an enhancement in the generation of reactive oxygen species (ROS). Flow cytometry measurements demonstrated how the uptake of the Ce6 was strongly improved by the conjugation with C60. The Ce6–C60 dyad exhibited in A431 cancer cells low dark toxicity and a higher PDT efficacy than Ce6 alone, due to the enhancement of the uptake and the improvement of ROS generation