Oxidation of carbon monoxide over Ag (111) by preadsorbed active oxygen studied by XPS and UPS

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Towards a statistical mechanics of nonabelian vortices

A study is presented of classical field configurations describing nonabelian vortices in two spatial dimensions, when a global $SO(3)$ symmetry is spontaneously broken to a discrete group \IK isomorphic to the group of integers mod 4. The vortices in this model are characterized by the nonabelian fundamental group \pi_1 (SO(3)/{\IK}) , which is isomorphic to the group of quaternions. We present an ansatz describing isolated vortices and prove that it is stable to perturbations. Kinematic constraints are derived which imply that at a finite temperature, only two species of vortices are stable to decay, due to `dissociation'. The latter process is the nonabelian analogue of the instability of charge $|q| >1$ abelian vortices to dissociation into those with charge $|q| = 1$. The energy of configurations containing at maximum two vortex-antivortex pairs, is then computed. When the pairs are all of the same type, we find the usual Coulombic interaction energy as in the abelian case. When they are different, one finds novel interactions which are a departure from Coulomb like behavior. Therefore one can compute the grand canonical partition function (GCPF) for thermal pair creation of nonabelian vortices, in the approximation where the fugacities for vortices of each type are small. It is found that the vortex fugacities depend on a real continuous parameter $a$ which characterize the degeneracy of the vacuum. Depending on the relative sizes of these fugacities, the vortex gas will be dominated by one of either of the two types mentioned above. In these regimes, we expect the standard Kosterlitz-Thouless phase transitions to occur, as in systems of abelian vortices in 2-dimensions. Between these two regimes, the gas contains pairs of both types, so nonabelian effects will be important.Comment: 40 pages in a4 LaTeX including 2 tables and 5 uuencoded Postscript figures, QMW-93/15.( The 6th figure, due to its size, is available by directly request from [email protected]. Some typos are corrected and the choice of choosing \r_c has been argued.

Climate Change, Agricultural Production and Civil Conflict: Evidence from the Philippines

Climate change is predicted to affect global rainfall patterns, but there is mixed evidence with regard to the effect of rainfall on civil conflict. Even among researchers who argue that rainfall reduces civil conflict, there is disagreement as to the underlying mechanism. Using data from the Philippines for the period 2001-2009, we exploit seasonal variation in the relationship between rainfall and agricultural production to explore the connection between rainfall and civil conflict. In the Philippines, above-average rainfall during the wet season is harmful to agricultural production, while above-average rainfall during the dry season is beneficial. We show that the relationship between rainfall and civil conflict also exhibits seasonality, but in the opposite direction and with a one-year lag. Consistent with the hypothesis that rebel groups gain strength after a bad harvest, there is evidence that lagged rainfall affects the number of violent incidents initiated by insurgents but not the number of incidents initiated by government forces. Our results suggest that policies aimed at mitigating the effect of climate change on agricultural production could weaken the link between climate change and civil conflict

Election Fraud and Post-Election Conflict: Evidence from the Philippines

Previous studies have documented a positive association between election fraud and the intensity of civil conflict. It is not clear, however, whether this association is causal or due to unobserved institutional or cultural factors. This paper examines the relationship between election fraud and post-election violence in the 2007 Philippine mayoral elections. Using the density test developed by McCrary (2008), we find evidence that incumbents were able to win tightly contested elections through fraud. In addition, we show that narrow incumbent victories were associated with an increase in post-election casualties, which is consistent with the hypothesis that election fraud causes conflict. We conduct several robustness tests and find no evidence that incumbent victories increased violence for reasons unrelated to fraud