Quantum renormalization of high energy excitations in the 2D Heisenberg antiferromagnet

We find using Monte Carlo simulations of the spin-1/2 2D square lattice nearest neighbour quantum Heisenberg antiferromagnet that the high energy peak locations at (pi,0) and (pi/2,pi/2) differ by about 6%, (pi/2,pi/2) being the highest. This is a deviation from linear spin wave theory which predicts equal magnon energies at these points.Comment: Final version, Latex using iopart & epsfi

Correlation and dimensional effects of trions in carbon nanotubes

We study the binding energies of singlet trions, i.e. charged excitons, in carbon nanotubes. The problem is modeled, through the effective-mass model, as a three-particle complex on the surface of a cylinder, which we investigate using both one- and two-dimensional expansions of the wave function. The effects of dimensionality and correlation are studied in detail. We find that the Hartree-Fock approximation significantly underestimates the trion binding energy. Combined with band structures calculated using a non-orthogonal nearest neighbour tight binding model, the results from the cylinder model are used to compute physical binding energies for a wide selection of carbon nanotubes. In addition, the dependence on dielectric screening is examined. Our findings indicate that trions are detectable at room temperature in carbon nanotubes with radius below 8{\AA}

Spin waves in La_2CuO_4: band structure and correlation effects

We calculate the antiferromagnetic spin wave dispersion in the half-filled (electronic density n=1) Hubbard model for a two-dimensional square lattice, using the random phase approximation (RPA) in a broken symmetry (spin density wave) ground state. Our results for the spin wave dispersion, \omega(\vec q), are compared with high-resolution inelastic neutron scattering performed on La_2CuO$_4. The effects of different band structures and different values of the on-site Coulomb interaction on the spin wave spectrum is studied. Particular attention is put on the high energy dispersion values \omega(\pi/2,\pi/2) and \omega(0,\pi).Comment: 4 pages, 2 figure

The city of the future

In the face of the complexity of the interconnected processes involved in the relationships between cities and climate change this report researches the carbon-neutral city through a Bacchi-inspired governmentality lens, where the focus lies on how the physical structure and the technological choices are shaped and made in and through discursive problem representations. The field of carbon-neutral strategies lies in an intersection of physical and technological realities and an administration of social relations. Particular visions of the future are a momentary equilibrium of cultural, historical, and physical relations. Specific socio-technical choices are made as a result of underlying rationales, assumptions and conceptual premises in the strategies shape frame and the understood realm of possibilities for the imagined carbon neutral cities. It is this relation which I seek to scrutinise how the representation of the problem as global, together with premises of ‘future technology’, ‘compensation’ and the ‘limitless growing city’ lays the building block for the visualised futures and thus also for the inherently omitted and silenced elements. I conclude that the choiced trajectories of the Nordic cities towards practical initiatives for a carbon-neutral city are formed within the conjunction of historical, social and geographical situations and the conceptual world constructed by the international and national governed frame