Superuniversal transport near a (2+1)(2 + 1)-dimensional quantum critical point

We compute the zero-temperature conductivity in the two-dimensional quantum $\mathrm{O}(N)$ model using a nonperturbative functional renormalization-group approach. At the quantum critical point we find a universal conductivity $\sigma^*/\sigma_Q$ (with $\sigma_Q=q^2/h$ the quantum of conductance and $q$ the charge) in reasonable quantitative agreement with quantum Monte Carlo simulations and conformal bootstrap results. In the ordered phase the conductivity tensor is defined, when $N\geq 3$, by two independent elements, $\sigma_{\mathrm{A}}(\omega)$ and $\sigma_{\mathrm{B}}(\omega)$, respectively associated to $\mathrm{O}(N)$ rotations which do and do not change the direction of the order parameter. Whereas $\sigma_{\mathrm{A}}(\omega\to 0)$ corresponds to the response of a superfluid (or perfect inductance), the numerical solution of the flow equations shows that $\lim_{\omega\to 0}\sigma_{\mathrm{B}}(\omega)/\sigma_Q=\sigma_{\mathrm{B}}^*/\sigma_Q$ is a superuniversal (i.e. $N$-independent) constant. These numerical results, as well as the known exact value $\sigma_{\mathrm{B}}^*/\sigma_Q=\pi/8$ in the large-$N$ limit, allow us to conjecture that $\sigma_{\mathrm{B}}^*/\sigma_Q=\pi/8$ holds for all values of $N$, a result that can be understood as a consequence of gauge invariance and asymptotic freedom of the Goldstone bosons in the low-energy limit.Comment: 6 pages, 4 figures, published versio

Nonperturbative functional renormalization-group approach to transport in the vicinity of a (2+1)(2+1)-dimensional O(NN)-symmetric quantum critical point

Using a nonperturbative functional renormalization-group approach to the two-dimensional quantum O($N$) model, we compute the low-frequency limit $\omega\to 0$ of the zero-temperature conductivity in the vicinity of the quantum critical point. Our results are obtained from a derivative expansion to second order of a scale-dependent effective action in the presence of an external (i.e., non-dynamical) non-Abelian gauge field. While in the disordered phase the conductivity tensor $\sigma(\omega)$ is diagonal, in the ordered phase it is defined, when $N\geq 3$, by two independent elements, $\sigma_{\rm A}(\omega)$ and $\sigma_{\rm B}(\omega)$, respectively associated to SO($N$) rotations which do and do not change the direction of the order parameter. For $N=2$, the conductivity in the ordered phase reduces to a single component $\sigma_{\rm A}(\omega)$. We show that $\lim_{\omega\to 0}\sigma(\omega,\delta)\sigma_{\rm A}(\omega,-\delta)/\sigma_q^2$ is a universal number which we compute as a function of $N$ ($\delta$ measures the distance to the quantum critical point, $q$ is the charge and $\sigma_q=q^2/h$ the quantum of conductance). On the other hand we argue that the ratio $\sigma_{\rm B}(\omega\to 0)/\sigma_q$ is universal in the whole ordered phase, independent of $N$ and, when $N\to\infty$, equal to the universal conductivity $\sigma^*/\sigma_q$ at the quantum critical point.Comment: 25 pages, 4 figure

Ammonia : this is not the end but rather the end of the beginning

Hepatic encephalopathy (HE) represents a wide spectrum of neurological or neuropsychological symptoms caused by liver disease and/or portosystemic shunts. The major role of hyperammonemia in association with systemic inflammation and oxidative stress in the pathogenesis of HE has progressively emerged. However, the cascading downstream effects caused by these pathogenic factors remain unresolved. The underlying abnormalities which are thought to cause HE include modification of glutamatergic and GABAergic neurotransmission, mitochondrial dysfunction, energy impairment, lactate dyshomeostasis, increased blood-brain barrier permeability, brain edema/astrocyte swelling, as well as accumulation of toxic compounds (manganese, bile acids, indols)

Nonperturbative renormalization-group approach preserving the momentum dependence of correlation functions

We present an approximation scheme of the nonperturbative renormalization group that preserves the momentum dependence of correlation functions. This approximation scheme can be seen as a simple improvement of the local potential approximation (LPA) where the derivative terms in the effective action are promoted to arbitrary momentum-dependent functions. As in the LPA the only field dependence comes from the effective potential, which allows us to solve the renormalization-group equations at a relatively modest numerical cost (as compared, e.g., to the Blaizot--Mend\'ez-Galain--Wschebor approximation scheme). As an application we consider the two-dimensional quantum O($N$) model at zero temperature. We discuss not only the two-point correlation function but also higher-order correlation functions such as the scalar susceptibility (which allows for an investigation of the "Higgs" amplitude mode) and the conductivity. In particular we show how, using Pad\'e approximants to perform the analytic continuation $i\omega_n\to\omega+i0^+$ of imaginary frequency correlation functions $\chi(i\omega_n)$ computed numerically from the renormalization-group equations, one can obtain spectral functions in the real-frequency domain.Comment: 19 pages, 10 figures. Final versio

Sympatric Dreissena species in the Meuse River : towards a dominance shift from zebra to quagga mussels

The rapid spread of the quagga mussel, Dreissena rostriformis, in Western Europe is of particular concern since the species is known to have serious ecological and economic impacts, similar to those of the well-established zebra mussel, Dreissena polymorpha. This study aimed (1) to provide an update on the quagga mussel distribution in several Belgian inland waterways, and (2) to check if a shift in dominance between Dreissena species is occurring. Using density measurements and artificial substrate samplers, we compared population dynamics for both species at different time-points based on size-frequency distribution. Our results show that quagga mussels are spreading rapidly throughout Belgium via a number of possible invasion fronts based around large rivers and canals. The quagga mussel became the dominant dreissenid species in both the Meuse River and a number of Belgian canals. In just three years, quagga mussel’s relative abundance increased from 2.9% (±2.9) to 52.6% (±43.1) of the total dreissenid population in the Meuse River. The most rapid increase in abundance has occurred in the Albert Canal, where quagga mussels achieved a mean relative abundance of 80% two years after the first observation. In the Meuse River, the quagga mussel displays a faster growth rate and/or earlier reproduction than the zebra mussel. We discuss different mechanisms that could explain the quagga mussel’s apparent competitive advantage over the zebra mussel

Mild hypothermia in the prevention of brain edema in acute liver failure: mechanisms and clinical prospects

Mild hypothermia (32 degrees C-35 degrees C) reduces intracranial pressure in patients with acute liver failure and may offer an effective adjunct therapy in the management of these patients. Studies in experimental animals suggest that this beneficial effect of hypothermia is the result of a decrease in blood-brain ammonia transfer resulting in improvement in brain energy metabolism and normalization of glutamatergic synaptic regulation. Improvement in brain energy metabolism by hypothermia may result from a reduction in ammonia-induced decrease of brain glucose (pyruvate) oxidation. Restoration of normal glutamatergic synaptic regulation by hypothermia may be the consequence of the removal of ammonia-induced decreases in expression of astrocytic glutamate transporters resulting in normal glutamate neurotransmitter inactivation in brain. Randomized controlled clinical trials of hypothermia are required to further evaluate its clinical impact

From the epidemiology of hepatitis E virus (HEV) within the swine reservoir to public health risk mitigation strategies: a comprehensive review

International audienceAbstractHepatitis E virus (HEV) is the causative agent of hepatitis E in humans, an emerging zoonosis mainly transmitted via food in developed countries and for which domestic pigs are recognised as the main reservoir. It therefore appears important to understand the features and drivers of HEV infection dynamics on pig farms in order to implement HEV surveillance programmes and to assess and manage public health risks. The authors have reviewed the international scientific literature on the epidemiological characteristics of HEV in swine populations. Although prevalence estimates differed greatly from one study to another, all consistently reported high variability between farms, suggesting the existence of multifactorial conditions related to infection and within-farm transmission of the virus. Longitudinal studies and experimental trials have provided estimates of epidemiological parameters governing the transmission process (e.g. age at infection, transmission parameters, shedding period duration or lag time before the onset of an immune response). Farming practices, passive immunity and co-infection with immunosuppressive agents were identified as the main factors influencing HEV infection dynamics, but further investigations are needed to clarify the different HEV infection patterns observed in pig herds as well as HEV transmission between farms. Relevant surveillance programmes and control measures from farm to fork also have to be fostered to reduce the prevalence of contaminated pork products entering the food chain

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La2O3-Ta2O5 system was considered with a focus on the La3TaO7 phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La3−xSrxTaO7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La3TaO7 is a poor oxide ion conductor (σ700 °C = 2 × 10−5S.cm−1), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ700 °C = 2 × 10−4S.cm−1)