Ferromagnetism of the Hubbard Model at Strong Coupling in the Hartree-Fock Approximation

As a contribution to the study of Hartree-Fock theory we prove rigorously that the Hartree-Fock approximation to the ground state of the d-dimensional Hubbard model leads to saturated ferromagnetism when the particle density (more precisely, the chemical potential mu) is small and the coupling constant U is large, but finite. This ferromagnetism contradicts the known fact that there is no magnetization at low density, for any U, and thus shows that HF theory is wrong in this case. As in the usual Hartree-Fock theory we restrict attention to Slater determinants that are eigenvectors of the z-component of the total spin, {S}_z = sum_x n_{x,\uparrow} - n_{x,\downarrow}, and we find that the choice 2{S}_z = N = particle number gives the lowest energy at fixed 0 < mu < 4d.Comment: v2: Published version. 30 pages latex. Changes in title, abstract, introductio

The Composite Spectrum of Strong Lyman-alpha Forest Absorbers

We present a new method for probing the physical conditions and metal enrichment of the Intergalactic Medium: the composite spectrum of Ly-alpha forest absorbers. We apply this technique to a sample of 9480 Ly-alpha absorbers with redshift 2 < z < 3.5 identified in the spectra of 13,279 high-redshift quasars from the Sloan Digital Sky Survey (SDSS) Fifth Data Release (DR5). Absorbers are selected as local minima in the spectra with 2.4 < tau_Ly-alpha < 4.0; at SDSS resolution (~ 150km/s FWHM), these absorbers are blends of systems that are individually weaker. In the stacked spectra we detect seven Lyman-series lines and metal lines of O VI, N V, C IV, C III, Si IV, C II, Al II, Si II, Fe II, Mg II, and O I. Many of these lines have peak optical depths of < 0.02, but they are nonetheless detected at high statistical significance. Modeling the Lyman-series measurements implies that our selected systems have total H I column densities N_HI ~ 10^15.4cm-2. Assuming typical physical conditions rho / = 10, T = 10^4 - 10^4.5 K, and [Fe/H]= -2 yields reasonable agreement with the line strengths of high-ionization species, but it underpredicts the low-ionization species by two orders of magnitude or more. This discrepancy suggests that the low ionization lines arise in dense, cool, metal-rich clumps, present in some absorption systems.Comment: 7 pages, 4 figures, 1 table, accepted by ApJL, revisions mad

Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations

We have developed an analytical model to describe the evolution of anisotropic galactic outflows. With it, we investigate the impact of varying opening angle on galaxy formation and the evolution of the IGM. We have implemented this model in a Monte Carlo algorithm to simulate galaxy formation and outflows in a cosmological context. Using this algorithm, we have simulated the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM universe. Starting from a Gaussian density field at redshift z=24, we follow the formation of ~20,000 galaxies, and simulate the galactic outflows produced by these galaxies. When these outflows collide with density peaks, ram pressure stripping of the gas inside the peak may result. This occurs in around half the cases and prevents the formation of galaxies. Anisotropic outflows follow the path of least resistance, and thus travel preferentially into low-density regions, away from cosmological structures (filaments and pancakes) where galaxies form. As a result, the number of collisions is reduced, leading to the formation of a larger number of galaxies. Anisotropic outflows can significantly enrich low-density systems with metals. Conversely, the cross-pollution in metals of objects located in a common cosmological structure, like a filament, is significantly reduced. Highly anisotropic outflows can travel across cosmological voids and deposit metals in other, unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap

A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey

We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest absorption. We have developed a novel Locally Calibrated Pixel (LCP) search method that uses adjacent regions of the spectrum to calibrate interlopers and spectral artifacts, which would otherwise limit the measurement of OVI absorption. Despite the challenges presented by searching for weak OVI within the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise, we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2 (the null hypothesis has a chi^2=80 for 9 data points). We interpret our results using synthetic spectra generated from a lognormal density field assuming a mixed quasar-galaxy photoionizing background (Haardt & Madau 2001) and that it dominates the ionization fraction of detected OVI. The LCP search data can be fit by a constant metallicity model with [O/H] = -2.15_(-0.09)^(+0.07), but also by models in which low-density regions are unenriched and higher density regions have a higher metallicity. The density-dependent enrichment model by Aguirre et al. (2008) is also an acceptable fit. All our successful models have similar mass-weighted oxygen abundance, corresponding to [_MW] = -2.45+-0.06. This result can be used to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) = 1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the mass-weighted mean oxygen abundance and the cosmic oxygen density in the Lyman-alpha forest to date and indicates that it contains approximately 16% of metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.Comment: 12 pages, 9 figures. Accepted by ApJ (minor changes

Mini école de médecine : pourquoi vous devriez tenter les séminaires virtuels, animés par des étudiants

Implication Statement Mini Med School (MMS) programs led by medical students provide a unique opportunity for community members to access free and accurate health information while engaging with the education of medical students. Virtually delivered MMS programs reduce barriers to access. 90.3% of participants in a recent MMS enjoyed medical students delivering the seminars. 63.6% of participants preferred virtual MMS seminars, with 31.8% preferring a combination of virtual and in-person delivery. Student-led, virtually delivered MMS programs are an engaging way to support both medical student and community education while strengthening community ties with local medical schools.Énoncé des implications de la recherche Les séminaires de la Mini école de médecine (MÉM) présentés par des étudiants en médecine offrent une occasion unique aux membres de la communauté de profiter d’informations gratuites et précises en matière de santé tout en contribuant à la formation des étudiants. L’offre de séminaires MÉM en mode virtuel rend ces activités plus accessibles. Une grande majorité des participants (90,3 %) à une MÉM tenue récemment ont aimé le fait que des séminaires soient animés par des étudiants, 63,6 % d’entre eux ont préféré les séminaires MÉM virtuels et 31,8 % se sont dits favorables à une combinaison de séminaires virtuels et en personne. Les exposés présentés virtuellement par des étudiants dans le cadre de la MÉM constituent un moyen intéressant de promouvoir aussi bien l’éducation des étudiants que celle de la communauté, tout en renforçant les liens entre cette dernière et les facultés de médecine locales

Momentum distribution of a trapped Fermi gas with large scattering length

Using a scattering length parametrization of the BCS-BEC crossover as well as the local density approximation for the density profile, we calculate the momentum distribution of a harmonically trapped atomic Fermi gas at zero temperature. Various interaction regimes are considered, including the BCS phase, the unitarity limit and the molecular regime. We show that the relevant parameter which characterizes the crossover is given by the dimensionless combination $N^{1/6}a/a_{ho}$, where $N$ is the number of atoms, $a$ is the scattering length and $a_{ho}$ is the oscillator length. The width of the momentum distribution is shown to depend in a crucial way on the value and sign of this parameter. Our predictions can be relevant for experiments on ultracold atomic Fermi gases near a Feshbach resonance.Comment: 6 pages, 2 figures. Submitted to Phys. Rev. A. Added reference

Mind-body and creative arts therapies for people with aphasia: a mixed-method systematic review

Background. In the context of aphasia rehabilitation there is a perceived need for interventions with a reduced linguistic demand targeting well-being. Mind-body and creative arts approaches are holistic and person-centred approaches, primarily relying on means other than verbal exchanges, and promoting self-regulation strategies. Aims. This mixed-method systematic review aimed to evaluate the availability, feasibility, and effectiveness of mind-body and creative arts therapies in promoting well-being for people with aphasia. Eight databases were searched using subject headings and key words. Full-text screening, critical appraisal and data extraction were conducted independently by two reviewers. A segregated synthesis approach was used (i.e. Revised Effect Direction Plot technique; Thematic Synthesis approach). Findings are presented in a narrative and visual form. Main Contribution. Twenty-two studies were included (Mind-body: n=11; Creative-arts: n=11). Heterogeneity of study design and quality, intervention type, procedures and dosage, outcomes, and level of offered communication support were identified. Improvements were noted across a wide range of well-being outcomes with more consistent positive results for anxiety and communication. 128 findings were extracted and synthetised in three broad themes: positive impact on self, empowering multifaceted experience, the relevance of needs-centred adjustments. Conclusion. Provisional findings about the benefits of mind-body and creative arts interventions on aspects of well-being for some individuals with aphasia were identified. However, findings are complex and need to be interpreted cautiously. Facilitators and barriers to these therapies are highlighted with related recommendations for practice. The review poses a demand for further research in the field, implementing rigorous methodology and aphasia-specific support to facilitate inclusion and engagement