N-heterocyclic carbene-palladium and -copper complexes in cross-coupling reactions

Chapter 1 gives the reader a background on cross-coupling reactions, in particular palladium mediated couplings. Furthermore the importance of ligands, including phosphines and N-heterocyclic carbenes (NHC), in such cross-coupling reactions is explored. Chapter 2 provides a background to the reductive lithiation of phosphines, followed by an account of our investigation of BINAP functionalisation by means of reductive elimination.1 The reaction was examined by experimental means and through the use of density functional theory to predict 31P NMR chemical shifts. Chapter 3 provides background on the Heck reaction and selected developments over the years, with particular reference to the use of aryl chlorides in the reaction. A brief discussion of NHC based palladium complex sets the scene for our investigation of a new class of (NHC)-Pd catalysts developed by the Navarro group. Complexes of type (NHC)PdCl2(TEA) (TEA = triethylamine) have been tested for their activity in the Heck reaction, focusing on the scope of the reaction with electron-deficient aryl chlorides and electron-rich aryl bromides.2 Chapter 4 gives an account of the discovery and developments of the Sonogashira reaction. Particular attention is paid to non-classical systems such as palladium-only and copper-only protocols. Herein our investigation into the use of collaborative (NHC)-Pd and (NHC)-Cu in Sonogashira reactions is presented.3 Notable features of this system are the low catalyst loadings and the synthetically convenient conditions in which the reaction can be carried namely non-anhydrous solvents and in air. Publications: (1) Gallop, C. W. D.; Bobin, M.; Hourani, P.; Dwyer, J.; Roe, S. M.; Viseux, E. M. E. J. Org. Chem. 2013, 6522–6528. (2) Gallop, C. W. D.; Zinser, C.; Guest, D.; Navarro, O. Synlett 2014, 2225–2228. (3) Gallop, C. W. D.; Chen, M.-T.; Navarro, O. Org. Lett 2014, 3724–3727

Novel methods of fabrication and metrology of superconducting nanostructures

As metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth $lambda$ is much greater than the film thickness and is comparable with the lateral dimensions of the device

Investigating the intrinsic noise limit of Dayem bridge NanoSQUIDs

NanoSQUIDs made from Nb thin films have been produced with nanometre loop sizes down to 200 nm, using weak-link junctions with dimensions less than 60 nm. These composite (W/Nb) single layer thin film devices, patterned by FIB milling, show extremely good low-noise performance ∼170 nΦ0 at temperatures between 5 and 8.5 K and can operate in rather high magnetic fields (at least up to 1 T). The devices produced so far have a limited operating temperature range, typically only 1–2 K. We have the goal of achieving operation at 4.2 K, to be compatible with the best SQUID series array (SSA) preamplifier available. Using the SSA to readout the nanoSQUIDs provides us with a means of investigating the intrinsic noise of the former. In this paper we report improved white noise levels of these nanoSQUIDs, enabling potential detection of a single electronic spin flip in a 1-Hz bandwidth. At low frequencies the noise performance is already limited by SSA preamplifier noise

Degenerate ground state and anomalous flux hysteresis in an YBa2Cu3O7 grain boundary r.f. SQUID

We report measurements of the flux hysteresis curves and trapped flux distribution in an YBa2Cu3O7 r.f. SQUID containing two closely spaced grain boundary Josephson junctions in parallel. Broadening of the flux distribution from T = 15 K to 30 K is followed by a bifurcation at T = 35 K which corresponds to a degenerate ground state. Above T ~ 40 K the bifurcation disappears, the flux distribution narrows significantly and small secondary loops appear in the hysteresis curves. This behaviour can be modelled qualitatively if we assume a temperature dependent second harmonic term in the current-phase relationship of the junctions.Comment: 10 pages, 7 figure

Antigovernment networks in civil conflicts : how network structures affect conflictual behavior

In this article, we combine a game-theoretic treatment of public goods provision in networks with a statistical network analysis to show that fragmented opposition network structures lead to an increase in conflictual actions. Current literature concentrates on the dyadic relationship between the government and potential challengers. We shift the focus toward exploring how network structures affect the strategic behavior of political actors. We derive and examine testable hypotheses and use latent space analysis to infer actors’ positions vis-à-vis each other in the network. Network structure is examined and used to test our hypotheses with data on conflicts in Thailand from 2001 to 2010. We show the influential role of network structure in generating conflictual behavior

Tunable coaxial cavity resonator for linear and nonlinear microwave characterization of superconducting wires

We discuss experimental results obtained using a tunable cylindrical coaxial cavity constituted by an outer Cu cylinder and an inner Pb-BSCCO wire. We have used this device for investigating the microwave response of the superconducting wire, both in the linear and nonlinear regimes. In particular, by tuning the different modes of the cavity to make them resonant at exactly harmonic frequencies, we have detected the power emitted by the superconducting inner wire at the second- and third-harmonic frequency of the driving field. The results obtained in the nonlinear regime, whether for the microwave surface impedance or the harmonic emission, are qualitatively accounted for considering intergrain fluxon dynamics. The use of this kind of device can be of strong interest to investigate and characterise wires of large dimensions to be used for implementing superconducting-based microwave devices.Comment: 14 pages, 6 embedded figures, accepted for publication in Supercond. Sci. Techno

Learning from the past and stepping into the future : toward a new generation of conflict prediction

Developing political forecasting models not only increases the ability of political scientists to inform public policy decisions, but is also relevant for scientific advancement. This article argues for and demonstrates the utility of creating forecasting models for predicting political conflicts in a diverse range of country settings. Apart from the benefit of making actual predictions, we argue that predictive heuristics are one gold standard of model development in the field of conflict studies. As such, they shed light on an array of important components of the political science literature on conflict dynamics. We develop and present conflict predictions that have been highly accurate for past and subsequent events, exhibiting few false-negative and false-positive categorizations. Our predictions are made at the monthly level for 6-month periods into the future, taking into account the social–spatial context of each individual country. The model has a high degree of accuracy in reproducing historical data measured monthly over the past 10 years and has approximately equal accuracy in making forecasts. Thus, forecasting in political science is increasingly accurate. At the same time, by providing a gold standard that separates model construction from model evaluation, we can defeat observational research designs and use true prediction as a way to evaluate theories. We suggest that progress in the modeling of conflict research depends on the use of prediction as a gold standard of heuristic evaluation