Dynamical properties of a strongly correlated model for quarter-filled layered organic molecular crystals

The dynamical properties of an extended Hubbard model, which is relevant to quarter-filled layered organic molecular crystals, are analyzed. We have computed the dynamical charge correlation function, spectral density, and optical conductivity using Lanczos diagonalization and large-N techniques. As the ratio of the nearest-neighbor Coulomb repulsion, V, to the hopping integral, t, increases there is a transition from a metallic phase to a charge-ordered phase. Dynamical properties close to the ordering transition are found to differ from the ones expected in a conventional metal. Large-N calculations display an enhancement of spectral weight at low frequencies as the system is driven closer to the charge-ordering transition in agreement with Lanczos calculations. As V is increased the charge correlation function displays a collective mode which, for wave vectors close to (pi,pi), increases in amplitude and softens as the charge-ordering transition is approached. We propose that inelastic x-ray scattering be used to detect this mode. Large-N calculations predict superconductivity with d(xy) symmetry close to the ordering transition. We find that this is consistent with Lanczos diagonalization calculations, on lattices of 20 sites, which find that the binding energy of two holes becomes negative close to the charge-ordering transition.

Responsibility to protect or Trojan Horse? The crisis in darfur and humanitarian intervention after Iraq

What does the world's engagement with the unfolding crisis in Darfur tell us about the impact of the Iraq war on the norm of humanitarian intervention? Is a global consensus about a "responsibility to protect" more or less likely? There are at least three potential answers to these questions. Some argue that the merging of strategic interests and humanitarian goods amplified by the intervention in Afghanistan makes it more likely that the world's most powerful states will act to prevent or halt humanitarian crises. Others insist that the widespread perception that the United States and its allies "abused" humanitarian justifications to legitimate its invasion of Iraq has set back efforts to build a global consensus about humanitarian action. A third group argues that the "responsibility to protect" inhibits the potential for abuse and, as a result, consensus is likely to strengthen post-Iraq for precisely this reason. Through a detailed study of the international engagement with Darfur, I suggest that the latter two arguments have merit but need to be adjusted. I argue that the humanitarian intervention norm has changed in two subtle ways. First, while the strength of the norm itself has not changed, the credibility of the United States and U.K. as "norm carriers" has been significantly undermined. Second, while the "responsibility to protect" has been invoked to support international activism, it has also re-legitimated anti-interventionist arguments.

Spin exchange and superconductivity in a t-J(')-V model for two-dimensional quarter-filled systems

The effect of antiferromagnetic spin fluctuations on two-dimensional quarter-filled systems is studied theoretically. An effective t-J(')-V model on a square lattice which accounts for checkerboard charge fluctuations and next-nearest-neighbor antiferromagnetic spin fluctuations is considered. From calculations based on large-N theory on this model it is found that the exchange interaction J(') increases the attraction between electrons in the d(xy) channel only, so that both charge and spin fluctuations work cooperatively to produce d(xy) pairing.