Topological quantum phase transitions of attractive spinless fermions in a honeycomb lattice

We investigate a spinless Fermi gas trapped in a honeycomb optical lattice with attractive nearest-neighbor interactions. At zero temperature, mean-field theory predicts three quantum phase transitions, two being topological. At low interactions, the system is semi-metallic. Increasing the interaction further, the semi-metal destabilizes into a fully gapped superfluid. At larger interactions, a topological transition occurs and this superfluid phase becomes gapless, with Dirac-like dispersion relations. Finally, increasing again the interaction, a second topological transition occurs and the gapless superfluid is replaced by a different fully gapped superfluid phase. We analyze these different quantum phases as the temperature and the lattice filling are varied.Comment: 6 pages, 5 figure

Density response and collective modes of semi-holographic non-Fermi liquids

Semi-holographic models of non-Fermi liquids have been shown to have generically stable generalised quasi-particles on the Fermi surface. Although these excitations are broad and exhibit particle-hole asymmetry, they were argued to be stable from interactions at the Fermi surface. In this work, we use this observation to compute the density response and collective behaviour in these systems. Compared to the Fermi liquid case, we find that the boundaries of the particle-hole continuum are blurred by incoherent contributions. However, there is a region inside this continuum, that we call inner core, within which salient features of the Fermi liquid case are preserved. A particularly striking prediction of our work is that these systems support a plasmonic collective excitation which is well-defined at large momenta, has an approximately linear dispersion relation and is located in the low-energy tail of the particle-hole continuum. Furthermore, the dynamic screening potential shows deep attractive regions as a function of the distance at higher frequencies which might lead to long-lived pair formation depending on the behaviour of the pair susceptibility. We also find that Friedel oscillations are present in these systems but are highly suppressed.Comment: 45 pages; 24 figures; published versio

Commercial and Regulatory Aspects of Reverse Hybrid Mail

Driven by market opening and increased competition from electronic communication, postal operators have started extending their scope of business by offering hybrid mail services in addition to physical mail conveyance. This paper discusses commercial and regulatory aspects of reverse hybrid mail, i.e. the electronic delivery and archiving of physical mail messages. It argues that postal operators are well positioned to offer hybrid services due to their established brands and their reputation. The introduction of reverse hybrid mail is able to significantly reduce the cost of postal operations while at the same time fitting customers' needs better than traditional postal services. However, these effects rely on the assumption that a postal operator is actually allowed to introduce an electronic delivery system of letters to entire regions and to thereby partially substitute the physical delivery to the doorstep.Reverse hybrid mail, Regulation, Postal market, Substitution, Intermodal competition

Development and Characterization of a Novel Mouse Model of Single and Repetitive Mild Traumatic Brain Injury

Mild traumatic brain injury (mTBI) or concussion is the most common form of TBI, and although a single concussion rarely results in long-term neurological dysfunction, repeated mild traumatic brain injury (r-mTBI) is a recognized risk factor for later development of neurodegenerative disease. However, the mechanisms contributing to neurodegeneration following TBI remain obscure. Animal models provide a means to examine the factors and mechanisms involved in TBI in experiments that cannot be conducted using human participants. The purpose of this thesis was to develop and fully characterize a reproducible, non-invasive mTBI model that will facilitate the study of repetitive brain injury. In the present study, male wild type mice received a midline concussive blow via an electromagnetic impactor, tuned to produce an injury without fracturing the skull. The injured mice were used to examine the chronic neurobehavioral, neuropathological and biochemical outcomes following single and r-mTBI up to 18 months following injury. Importantly, our findings recapitulate many aspects of human long term TBI sequelae, in particular persistent neuroinflammation, white matter injury, and axonal pathology in the corpus callosum. Our results provide the first evidence that, whilst a single concussion produces transient neurobehavioral changes and pathology which remains static in the period following injury, r-mTBI produces behavioral and pathological changes which continue to evolve many months post injury. There have been a number of clinical studies implicating tau in TBI pathology. As such, we investigated the relationship between tau pathologies after trauma in a transgenic mouse model expressing all 6 isoforms of human tau protein on a null murine background (hTau). Our results revealed that that single and r-mTBI induced a modest cortical increase in the soluble fraction of three different p-tau epitopes at 24 h post last injury. Moreover, this increase was not associated with worse behavioral performance when compared to wild type animals. Therefore, tau hyperphosphorylation appears to have a contributory, but not primary, role in the acute phase post-injury. Additional prospective studies in both humans and animal models are required to characterize the contribution of tau to TBI sequelae. The experimental data presented here suggest that inflammation and axonal injury (as seen in both wild-type and hTau models) appear to play a role in the events following single or repetitive mTBI and strongly correlates with the behavioral changes post-injury. The relationship between a history of mTBI and neuroinflammation are likely to be complex and warrant further work to elucidate their association with neurodegenerative disease. This work represents the development of a novel model, and the demonstration of its relevance to human TBI. This model can now be used for further exploration of TBI-related effects and for evaluation of potential therapeutic and diagnostic approaches, as is discussed throughout the thesis

SYGMA: Stellar Yields for Galactic Modeling Applications

The stellar yields for galactic modeling applications (SYGMA) code is an open-source module that models the chemical ejecta and feedback of simple stellar populations (SSPs). It is intended for use in hydrodynamical simulations and semi-analytic models of galactic chemical evolution. The module includes the enrichment from asymptotic giant branch (AGB) stars, massive stars, SNIa and neutron-star mergers. An extensive and extendable stellar yields library includes the NuGrid yields with all elements and many isotopes up to Bi. Stellar feedback from mechanic and frequency-dependent radiative luminosities are computed based on NuGrid stellar models and their synthetic spectra. The module further allows for customizable initial-mass functions and supernova Ia (SNIa) delay-time distributions to calculate time-dependent ejecta based on stellar yield input. A variety of r-process sites can be included. A comparison of SSP ejecta based on NuGrid yields with those from Portinari et al. (1998) and Marigo (2001) reveals up to a factor of 3.5 and 4.8 less C and N enrichment from AGB stars at low metallicity, a result we attribute to NuGrid's modeling of hot-bottom burning. Different core-collapse supernova explosion and fallback prescriptions may lead to substantial variations for the accumulated ejecta of C, O and Si in the first $10^7\, \mathrm{yr}$ at $Z=0.001$. An online interface of the open-source SYGMA module enables interactive simulations, analysis and data extraction of the evolution of all species formed by the evolution of simple stellar populations.Comment: 18 pages, 10 figures, 3 tables, published in ApJ

The effects of bar-spiral coupling on stellar kinematics in the Galaxy

We investigate models of the Milky Way disc taking into account simultaneously the bar and a two-armed quasi-static spiral pattern. Away from major resonance overlaps, the mean stellar radial motions in the plane are essentially a linear superposition of the isolated effects of the bar and spirals. Thus, provided the bar is strong enough, even in the presence of spiral arms, these mean radial motions are predominantly affected by the Galactic bar for large scale velocity fluctuations. This is evident when comparing the peculiar line-of-sight velocity power spectrum of our coupled models with bar-only models. However, we show how forthcoming spectroscopic surveys could disentangle bar-only non-axisymmetric models of the Galaxy from models in which spiral arms have a significant amplitude. We also point out that overlaps of low-order resonances are sufficient to enhance stellar churning within the disc, even when the spirals amplitude is kept constant. Nevertheless, for churning to be truly non-local, stronger or (more likely) transient amplitudes would be needed: otherwise the disc is actually mostly unaffected by churning in the present models. Finally, regarding vertical breathing modes, the combined effect of the bar and spirals on vertical motions is a clear non-linear superposition of the isolated effects of both components, significantly superseding the linear superposition of modes produced by each perturber separately, thereby providing an additional effect to consider when analysing the observed breathing mode of the Galactic disc in the extended Solar neighbourhood.Comment: 13 pages, 12 figures. MNRAS. Accepted for publication. v2 is the published versio

1/fα1/f^\alpha spectra in elementary cellular automata and fractal signals

We systematically compute the power spectra of the one-dimensional elementary cellular automata introduced by Wolfram. On the one hand our analysis reveals that one automaton displays $1/f$ spectra though considered as trivial, and on the other hand that various automata classified as chaotic/complex display no $1/f$ spectra. We model the results generalizing the recently investigated Sierpinski signal to a class of fractal signals that are tailored to produce $1/f^{\alpha}$ spectra. From the widespread occurrence of (elementary) cellular automata patterns in chemistry, physics and computer sciences, there are various candidates to show spectra similar to our results.Comment: 4 pages (3 figs included

Orocowewin Notcimik Itatcihowin: The Atikamekw Nehirowisiw Code of Practice and the Issues Involved in Its Writing

The Atikamekw Nehirowisiw Nation has for several years been developing a code of practice (orocowewin notcimik itatcihowin) to regulate hunting, fishing and plant harvesting activities in Nitaskinan, its ancestral territory. The Atikamekw Nehirowisiw code of practice is a collective project that sets out to put its territorial regulations in writing. The project's objective is threefold: to ensure the transmission of territorial knowledge and of rules relating to forest activities; to adapt these rules, passed on by ancestors, to the contemporary context; and to have them recognised by non-natives and the governments of other nations, including the governments of Canada and Quebec. This article presents some of the issues related to the process of writing and coding orocowewin notcimik itatcihowin, the Atikamekw Nehirowisiw code of practice; in particular, the importance of the oral tradition as a means of transmitting knowledge is emphasised. In our language, we say "atisokana ki atisokan" – we are infused and transformed by the narratives transmitted orally. This mode of transmission is politically, philosophically and emotionally significant. It is a unique way for us to let the heart speak, through direct contact, without interference. &nbsp