Superconductivity Near a Quantum Critical Point in Ba(Fe,Co)2As2

We will examine the possible link between spin fluctuations and the superconducting mechanism in the iron-based high temperature superconductor Ba(Fe,Co)2As2 based on NMR and high pressure transport measurements.Comment: Invited paper to m2s-IX (2009

Structures for Sophisticated Behaviour: Feudal Hierarchies and World Models

This thesis explores structured, reward-based behaviour in artificial agents and in animals. In Part I we investigate how reinforcement learning agents can learn to cooperate. Drawing inspiration from the hierarchical organisation of human societies, we propose the framework of Feudal Multi-agent Hierarchies (FMH), in which coordination of many agents is facilitated by a manager agent. We outline the structure of FMH and demonstrate its potential for decentralised learning and control. We show that, given an adequate set of subgoals from which to choose, FMH performs, and particularly scales, substantially better than cooperative approaches that use shared rewards. We next investigate training FMH in simulation to solve a complex information gathering task. Our approach introduces a ‘Centralised Policy Actor-Critic’ (CPAC) and an alteration to the conventional multi-agent policy gradient, which allows one multi-agent system to advise the training of another. We further exploit this idea for communicating agents with shared rewards and demonstrate its efficacy. In Part II we examine how animals discover and exploit underlying statistical structure in their environments, even when such structure is difficult to learn and use. By analysing behavioural data from an extended experiment with rats, we show that such hidden structure can indeed be learned, but also that subjects suffer from imperfections in their ability to infer their current state. We account for their behaviour using a Hidden Markov Model, in which recent observations are integrated imperfectly with evidence from the past. We find that over the course of training, subjects learn to track their progress through the task more accurately, a change that our model largely attributes to the more reliable integration of past evidenc

Development of flame retardant synthetic fibres using novel technologies

The dispersion of clays at the nanometer level is known to induce a significant improvement in mechanical properties, flame resistance and barrier properties, compared with pure polymer. Application of ultrasound in polymer processing can be used to improve additive dispersion in polymer melts and solutions but may also initiate chemical reactions and modify the rheological and mechanical properties. This thesis studies the effects of applying ultrasound to the molten fibre-forming polymers, polypropylene (PP) and polyamide 6 (PA6) containing nanoclay and flame retardant additives in order to assess whether improved dispersion generates improved flame retardant properties of derived fabrics. Initially, ultrasound was applied to polypropylene during compounding with up to 5 wt% of ammonium polyphosphate (APP) as flame retardant and 2 wt% of Nanomer 1.3T (Nanocor Inc) as nanoclay. Two ultrasonic probes were used having a power of 50 W at 10, 30, 50 and 100% amplitude and 100W at amplitudes of 20 and 90%. For polyamide 6, the additive selected as flame retardant was aluminium phosphinate (Al-Phos) (Exolit OP 935, Clariant) and the nanoclays, Nanomer 1.3T and Cloisite 25A (Southern Clay Products, USA). Compounded polymer chips of the various formulations were extruded into filaments and tapes by using the Labline extruder and filaments were knitted into fabrics, where possible. Nanoclay dispersion was studied by optical imaging, scanning electron microscopic and electron dispersive scattering or SEM-EDS imaging in terms of silicon or Si-dot mapping for selected tape sample areas using the Datacell software. This calculated the number of Si dots and hence clay particles within a given area and a reduction in Si dot intensity following ultrasound exposure was taken as a measure of increased dispersion in that dispersed particles at the nanolevel were now no longer visible because they are beyond the resolution of the SEM instrument. Results indicated that the 100 W probe with 90% amplitude (ie 90 W power) showed greatest dispersion, so this power was selected for further study. Flammability of the PP tape samples was studied by limiting oxygen index and a modified UL-94 test. Results showed that, in the case of polypropylene, LOI values of samples containing clay and / or flame retardant were not significantly affected by additive or the presence of ultrasound.However, the modified UL-94 test showed that presence of nanoclay and / or flame retardant decreased burning rate which further decreased with ultrasonification. Knitted fabrics were tested for vertical flame spread using the sample ignition test rig described in BS 5438 and results showed rate of burning and burning drips reduced for the ultrasonificated samples. In the case of PA6, the same 100W ultrasonic probe with 90% amplitude was used in the compounding stage. Of particular note was that the LOI of the PA6/25A(2 wt%)/Al-Phos(5 wt%) sample increased more than others and the same sample showed reduced flammability in the vertical flame spread test. However, quality of the filaments was poor due to the extrusion process and some of the samples failed to extrude using the Labline extruder. Subsequent work used the recently acquired Fibre Extrusion Technology (FET) extruder. Based on the above results, PA6 containing 2 wt% of nanoclay and 10 wt% of same flame retardant was used with and without presence of 100 W ultrasonic power at 90% amplitude. Compounded samples were extruded into filaments and tapes and filaments were knitted into fabrics. Properties were characterised as above and the PA6/25A(2%)/AlPhos(10%) sample showed superior performance compared to the others in terms of reduced flammability and this performance was enhanced by ultrasonification during compounding. In order to assess whether chemical degradation had occurred during ultrasonification, the relative viscosity of these samples was measured using a ASTM D 445 Ubbelohde viscometer. The results indicated that chemical degradation is negligible or absent at the probe power used. Differential scanning calorimetry (DSC) was used to determine the degree of crystalinity and values decreased with the addition of clay and flame retardant and further decreased in samples exposed to ultrasound. To extend this PA6 study, ammonium sulphamate (2.5 wt% AS) and dipentaerythritol (1 wt% DP) were selected as flame retardants and various nanoparticles including fumed silica were added. Samples were compounded and extruded into filaments and knitted into fabrics. Of special note is that the PA6/AS(2.5 wt%)/DP(1 wt%)/25A(2 wt%) combination was found to have superior tensile properties as filaments and significantly reduced flame spread in fabric form. In the final part of the thesis, the effect of ultrasound during compounding on core-sheath bicomponent fibres containing combinations of nanoclays and flame retardants was considered. Results showed that the addition of nanoclay or flame retardant to either the core (C) or sheath (S) of the fibres slightly reduced their tensile modulus and elongation-at-break values with respect to a control PA6-C/PA6-S yarn and there was little improvement observed with the ultrasound-exposed samples. The result of vertical flame spread testing showed that the PA6-C / PA6/25A(2 wt%)/Al-Phos (10 wt%)/90W-S sample was superior to all others in terms of their reduced flammability properties defined as minimal burn time, although its values of burn length, flame spread rate and number of drops were not the lowest values

Weighted LDA techniques for I-vector based speaker verification

This paper introduces the Weighted Linear Discriminant Analysis (WLDA) technique, based upon the weighted pairwise Fisher criterion, for the purposes of improving i-vector speaker verification in the presence of high intersession variability. By taking advantage of the speaker discriminative information that is available in the distances between pairs of speakers clustered in the development i-vector space, the WLDA technique is shown to provide an improvement in speaker verification performance over traditional Linear Discriminant Analysis (LDA) approaches. A similar approach is also taken to extend the recently developed Source Normalised LDA (SNLDA) into Weighted SNLDA (WSNLDA) which, similarly, shows an improvement in speaker verification performance in both matched and mismatched enrolment/verification conditions. Based upon the results presented within this paper using the NIST 2008 Speaker Recognition Evaluation dataset, we believe that both WLDA and WSNLDA are viable as replacement techniques to improve the performance of LDA and SNLDA-based i-vector speaker verification

Correcting Experience Replay for Multi-Agent Communication

We consider the problem of learning to communicate using multi-agent reinforcement learning (MARL). A common approach is to learn off-policy, using data sampled from a replay buffer. However, messages received in the past may not accurately reflect the current communication policy of each agent, and this complicates learning. We therefore introduce a 'communication correction' which accounts for the non-stationarity of observed communication induced by multi-agent learning. It works by relabelling the received message to make it likely under the communicator's current policy, and thus be a better reflection of the receiver's current environment. To account for cases in which agents are both senders and receivers, we introduce an ordered relabelling scheme. Our correction is computationally efficient and can be integrated with a range of off-policy algorithms. It substantially improves the ability of communicating MARL systems to learn across a variety of cooperative and competitive tasks

Indexing floodplain effects for flood estimation

Combining flood estimation methodologies with hydraulic models to provide a detailed and spatially coherent representation of flood risk can be problematic. One potential difficulty is that of double-accounting the attenuating effect of floodplain storage. This occurs when effects are represented in both the flood frequency estimation of the flow and also in hydraulic modelling and can be particularly important in the context of the increasing desire to combine hydrological and hydraulic models in a manner that provides a detailed and spatially coherent representation of flood risk. This paper presents an empirically derived index that represents floodplain effects on flood magnitude. A HEC-RAS 1-D hydraulic model was used to generate downstream flow hydrographs in a generalised river reach for defined upstream hydrographs encompassing a range of flows and durations. Geometrical and resistance properties in the reach were systematically varied. Relative attenuations were determined by analysing differences in upstream and simulated downstream hydrographs. The index was derived by relating flood peak attenuations to the channel characteristics in each simulation in a multivariate regression analysis

The Failure of Post-War Reconstruction in Jaffna, Sri Lanka: Indebtedness, Caste Exclusion and the Search for Alternatives

Reconstruction of contemporary war-torn societies can lead to further dispossession and social exclusion, particularly through neoliberal development policies and financialized indebtedness. This dissertation analyses post-war reconstruction of the Jaffna District in the Northern Province of Sri Lanka after the end of a brutal three decade long civil war in May 2009, through a survey and ethnographic study of one village, Pathemany and its oppressed caste quarter, Bharathy Veethy. Drawing on a study of Pathemany village on agrarian change before the war, this study addresses contemporary questions about land, rural incomes, rural debt and caste structure. The study evaluates reconstruction policies through analysis of national and regional data and reports, and scrutinizes indebtedness and new forms of dispossession. It situates the failure of reconstruction in Jaffna within a broader frame of historical changes starting in the late colonial period and the current influences of global economic forces including the impact of the Great Recession of 2008. This work argues that reconstruction policies were an extension of a second wave of neoliberalism in Sri Lanka soon after the war in 2009, which accelerated the liberalization trajectory set by the open economy reforms starting in 1977. An extensive process of financialization with integration of Sri Lanka’s state and private finance with the global capital markets, expansion of credit by national banks and new financial products at the rural level have led to predatory lending and tremendous indebtedness in the countryside. Social institutions and the rural economy in agriculture and fisheries are undermined as development emphasizes infrastructure build-out, credit expansion and self-employment schemes. Reconstruction-led dispossession is leading to outmigration for remittances and contributing to further social exclusion. Caste structure weakened by war-time displacement is strengthening by stealth as public silence on caste relations prevails. The vibrant history of producer co-operatives, including of caste-based occupations in Jaffna, is explored to provide an alternative vision for reconstruction. A critique of the post-conflict discourse of transition reframes reconstruction as a political question, with the war-torn society likely to be shaped by future struggles