Dynamical analysis of the Tsallis holographic dark energy models with event horizon as cut-off and interaction with matter

The model of generalized Tsallis holographic dark energy (which is known to be particular representative of Nojiri-Odintsov HDE) with event horizon as cut-off is investigated using methods of dynamical analysis. We take into consideration possible interaction with dark energy and matter in various forms. Critical points are determined. Cosmological evolution of the Universe depends from interaction parameters. If we use event horizon scale as cutoff quasi-de Sitter expansion is possible only for interaction of type $\sim H(\alpha\rho_{de}+\beta\rho_{m})$ (where $H$ is the Hubble parameter). For interactions $\sim \rho_m \rho_{de} /H$ and $\sim H \rho_{m}^{\alpha}\rho_{de}^{1-\alpha}$ Universe eventually stops ($H\rightarrow 0$) or ends its existence in final singularity ($H\rightarrow\infty$). In first case fraction of dark energy tends to $1$ or constant value lesser than 1 because dynamical equilibrium between matter and dark energy is established on late times.Comment: 23 pp., 9 figs., to appear in Int. J. Mod. Phys.

Unveiling and Manipulating Hidden Symmetries in Graphene Nanoribbons

Armchair graphene nanoribbons are a highly promising class of semiconductors for all-carbon nanocircuitry. Here, we present a new perspective on their electronic structure from simple model Hamiltonians and $\textit{ab initio}$ calculations. We focus on a specific set of nanoribbons of width $n = 3p+2$, where $n$ is the number of carbon atoms across the nanoribbon axis and $p$ is a positive integer. We demonstrate that the energy-gap opening in these nanoribbons originates from the breaking of a previously unidentified hidden symmetry by long-ranged hopping of $\pi$-electrons and structural distortions occurring at the edges. This hidden symmetry can be restored or manipulated through the application of in-plane lattice strain, which enables continuous energy-gap tuning, the emergence of Dirac points at the Fermi level, and topological quantum phase transitions. Our work establishes an original interpretation of the semiconducting character of armchair graphene nanoribbons and offers guidelines for rationally designing their electronic structure

Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/hexagonal boron nitride heterojunctions

Half-metals have been envisioned as active components in spintronic devices by virtue of their completely spin-polarized electrical currents. Actual materials hosting half-metallic phases, however, remain scarce. Here, we predict that recently fabricated heterojunctions of zigzag nanoribbons embedded in two-dimensional hexagonal boron nitride are half-semimetallic, featuring fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity originates from the transfer of charges from hexagonal boron nitride to the embedded graphene nanoribbon. These charges give rise to opposite energy shifts of the states residing at the two edges while preserving their intrinsic antiferromagnetic exchange coupling. Upon doping, an antiferromagnetic-to-ferrimagnetic phase transition occurs in these heterojunctions, with the sign of the excess charge controlling the spatial localization of the net magnetic moments. Our findings demonstrate that such heterojunctions realize tunable one-dimensional conducting channels of spin-polarized Dirac fermions that are seamlessly integrated into a two-dimensional insulator, thus holding promise for the development of carbon-based spintronics

Electrically induced Dirac fermions in graphene nanoribbons

Graphene nanoribbons are widely regarded as promising building blocks for next-generation carbon-based devices. A critical issue to their prospective applications is whether their electronic structure can be externally controlled. Here, we combine simple model Hamiltonians with extensive first-principles calculations to investigate the response of armchair graphene nanoribbons to transverse electric fields. Such fields can be achieved either upon laterally gating the nanoribbon or incorporating ambipolar chemical codopants along the edges. We reveal that the field induces a semiconductor-to-semimetal transition with the semimetallic phase featuring zero-energy Dirac fermions that propagate along the armchair edges. The transition occurs at critical fields that scale inversely with the width of the nanoribbons. These findings are universal to group-IV honeycomb lattices, including silicene and germanene nanoribbons, irrespective of the type of edge termination. Overall, our results create new opportunities to electrically engineer Dirac semimetallic phases in otherwise semiconducting graphene-like nanoribbons

Exciton g-factors in monolayer and bilayer WSe2 from experiment and theory

The optical properties of monolayer and bilayer transition metal dichalcogenide semiconductors are governed by excitons in different spin and valley configurations, providing versatile aspects for van der Waals heterostructures and devices. Here, we present experimental and theoretical studies of exciton energy splittings in external magnetic field in neutral and charged WSe2 monolayer and bilayer crystals embedded in a field effect device for active doping control. We develop theoretical methods to calculate the exciton g-factors from first principles for all possible spin-valley configurations of excitons in monolayer and bilayer WSe2 including valley-indirect excitons. Our theoretical and experimental findings shed light on some of the characteristic photoluminescence peaks observed for monolayer and bilayer WSe2. In more general terms, the theoretical aspects of our work provide additional means for the characterization of single and few-layer transition metal dichalcogenides, as well as their heterostructures, in the presence of external magnetic fields

The Social Scientist Meets the “Believer”: Discussions of God, the Afterlife, and Communism in the Mid-1960s

In this article, I use the transcripts of interviews carried out under the auspices of the Institute of Scientifi c Atheism in the mid-sixties. Informants were asked about diverse aspects of their religious practice and belief, allowing scholars—both then and now—to consider the nature of Soviet “secularization.” Following Charles Taylor, I suggest that this was not simply “a story of loss, of subtraction”; instead, informants' rather heterodox conceptions of the aft erlife indicate moments of individual creativity. In particular, I fi nd that among the poor and marginalized, visions of the aft erlife sometimes articulated a desire for social equality considered missing from Soviet society. I also probe the Soviet state's problematic dependency on atheism. The regime's legitimacy rested on its claim to ensure progress and modernity, and religion— the epitome of backwardness—was a useful antithesis. The interview was a ritual that enacted the superiority of Soviet values (reason, rationality, and enlightenment). And yet the encounter between atheist-interviewer and “believer” could oft en prove unpredictable, suggesting that the religion-atheism binary was in practice rather more brittle than the authorities might have hoped