Quantum phase transition in the one-dimensional extended Peierls-Hubbard model

We consider the one-dimensional extended Hubbard model in the presence of an explicit dimerization $\delta$. For a sufficiently strong nearest neighbour repulsion we establish the existence of a quantum phase transition between a mixed bond-order wave and charge-density wave phase from a pure bond-order wave phase. This phase transition is in the universality class of the two-dimensional Ising model.Comment: 6 pages, 3 figure

Perturbation theory for optical excitations in the one-dimensional extended Peierls--Hubbard model

For the one-dimensional, extended Peierls--Hubbard model we calculate analytically the ground-state energy and the single-particle gap to second order in the Coulomb interaction for a given lattice dimerization. The comparison with numerically exact data from the Density-Matrix Renormalization Group shows that the ground-state energy is quantitatively reliable for Coulomb parameters as large as the band width. The single-particle gap can almost triple from its bare Peierls value before substantial deviations appear. For the calculation of the dominant optical excitations, we follow two approaches. In Wannier theory, we perturb the Wannier exciton states to second order. In two-step perturbation theory, similar in spirit to the GW-BSE approach, we form excitons from dressed electron-hole excitations. We find the Wannier approach to be superior to the two-step perturbation theory. For singlet excitons, Wannier theory is applicable up to Coulomb parameters as large as half band width. For triplet excitons, second-order perturbation theory quickly fails completely.Comment: 32 pages, 12 figures, submtted to JSTA

Conflict between International Treaties: failing to mitigate the effects of introduced marine species.

Humans have changed the face of the earth - we have intentionally altered the locations of species in order to achieve food and economic security (eg, aquaculture of the freshwater fish Tilapia and the marine algae Kappaphycus) while also appealing to our cultural and aesthetic values (eg, the introduction of gorse to New Zealand and Australia). We have accidentally spread pathogens and diseases beyond their natural ranges1 and we have improved our technologies (such as shipping) to such an extent that we can transit our planet in shorter and shorter timeframes. All of these activities have occurred over many hundreds of years and have led in one way or another, to an increasing number of species being introduced beyond their natural ranges. Such introductions are now considered one of the top five threats to native biological diversity. This paper examines how humans have impacted upon the marine environment through the introduction of species beyond their native ranges. Introduced species impact upon native biodiversity, spread diseases and pathogens, and have had economic and social impacts in their ‘new’ ecosystems. Because of the range and extent of introduced species impacts, numerous methods to mitigate the effects of introduced species have been developed and implemented. Within this paper we will examine how two international legal instruments, the Convention on Biological Diversity, 1992 (CBD) and the World Trade Organization’s General Agreement on Tariffs and Trade 1994 (GATT), in particular its associated Agreement on the Application of Sanitary and Phytosanitary Measures (SPS), deal with introduced species. In this context, the paper focuses on the potential for conflict that may arise with the application of these international legal instruments, thus causing a failure to effectively mitigate for the effects of introduced species

Conflict between International Treaties: Failing to mitigate the effects of introduced marine species

Humans have changed the face of the earth - we have intentionally altered the locations of species in order to achieve food and economic security (eg, aquaculture of the freshwater fish Tilapia and the marine algae Kappaphycus) while also appealing to our cultural and aesthetic values (eg, the introduction of gorse to New Zealand and Australia)