Metallic phase in the two-dimensional ionic Hubbard model

We investigate the phases of the ionic Hubbard model in a two-dimensional square lattice using determinant quantum Monte Carlo (DQMC). At half-filling, when the interaction strength or the staggered potential dominate we find Mott and band insulators, respectively. When these two energies are of the same order we find a metallic region. Charge and magnetic structure factors demonstrate the presence of antiferromagnetism only in the Mott region, although the externally imposed density modulation is present everywhere in the phase diagram. Away from half-filling, other insulating phases are found. Kinetic energy correlations do not give clear signals for the existence of a bond-ordered phase

Targeting of cre to the Foxg1 (BF-1) Locus Mediates loxP Recombination in the Telencephalon and Other Developing Head Structures

AbstractThe use of genetics to study the development of the telencephalon and derivatives such as the cerebral cortex has been limited. The telencephalon begins to form midway through gestation, and targeted mutations in genes suspected of playing roles in its development often lead to early phenotypes that preclude analysis of their role at later stages. This problem can be circumvented using a Cre/loxP recombination system. A mouse line was produced in which cre was targeted to the Foxg1 (BF-1) locus, a gene expressed specifically in the telencephalon and discrete head structures. Crosses between Foxg1–Cre mice and three separate loxP reporter mice generated embryos with recombination patterns matching that expected from the normal pattern of Foxg1 expression. Recombination occurs invariably in the telencephalon, anterior optic vesicle, otic vesicle, facial and head ectoderm, olfactory epithelium, mid–hindbrain junction, and pharyngeal pouches. Recombination in some animals also occurs less efficiently in tissues not known to express Foxg1. We show that the genetic background of the parental mice and the loxP target allele can each contribute to differences in the exact pattern of recombination. Collectively, these data show that Foxg1–Cre mice should be useful in the deletion or ectopic expression of any floxed target gene in a Foxg1-like pattern

Monte Carlo Simulations of an Extended Feynman-Kikuchi Model

We present Quantum Monte Carlo simulations of a generalization of the Feynman-Kikuchi model which includes the possibility of vacancies and interactions between the particles undergoing exchange. By measuring the winding number (superfluid density) and density structure factor, we determine the phase diagram, and show that it exhibits regions which possess both superfluid and charge ordering.Comment: 10 pages, 15 figure

Accurately summarizing an outbreak using epidemiological models takes time

Recent outbreaks of monkeypox and Ebola, and worrying waves of COVID-19, influenza and respiratory syncytial virus, have all led to a sharp increase in the use of epidemiological models to estimate key epidemiological parameters. The feasibility of this estimation task is known as the practical identifiability (PI) problem. Here, we investigate the PI of eight commonly reported statistics of the classic Susceptible-Infectious-Recovered model using a new measure that shows how much a researcher can expect to learn in a model-based Bayesian analysis of prevalence data. Our findings show that the basic reproductive number and final outbreak size are often poorly identified, with learning exceeding that of individual model parameters only in the early stages of an outbreak. The peak intensity, peak timing, and initial growth rate are better identified, being in expectation over 20 times more probable having seen the data by the time the underlying outbreak peaks. We then test PI for a variety of true parameter combinations, and find that PI is especially problematic in slow-growing or less-severe outbreaks. These results add to the growing body of literature questioning the reliability of inferences from epidemiological models when limited data are available.Comment: 7 pages, 4 figure

Pair formation and collapse in imbalanced Fermion populations with unequal masses

We present an exact Quantum Monte Carlo study of the effect of unequal masses on pair formation in Fermionic systems with population imbalance loaded into optical lattices. We have considered three forms of the attractive interaction and find in all cases that the system is unstable and collapses as the mass difference increases and that the ground state becomes an inhomogeneous collapsed state. We also address the question of canonical vs grand canonical ensemble and its role, if any, in stabilizing certain phases

Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9

We have performed both in-plane resistivity, Hall effect and specific heat measurements on the thermoelectric cobalt oxide Ca$_{3}$Co$_{4}$O$_{9}$. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to $T_{min}\approx$63 K, a strongly correlated Fermi liquid up to $T^*\approx$140 K, with $\rho=\rho_0+AT^2$ and $A\approx 3.63$ $10^{-2} \mu \Omega cm/K^{2}$, followed by an incoherent metal with $k_Fl\leq 1$ and a high temperature insulator above T$^{**}\approx$510 K . Specific heat Sommerfeld coefficient $\gamma = 93$ mJ/(mol.K$^{2}$) confirms a rather large value of the electronic effective mass and fulfils the Kadowaki-Woods ratio $A/\gamma^2 \approx 0.45$ 10$^{-5}$ $\mu \Omega cm.K^2/(mJ^2mol^{-2})$. Resistivity measurements under pressure reveal a decrease of the Fermi liquid transport coefficient A with an increase of $T^*$ as a function of pressure while the product $A(T^*)^2/a$ remains constant and of order $h/e^2$. Both thermodynamic and transport properties suggest a strong renormalization of the quasiparticles coherence scale of order $T^*$ that seems to govern also thermopower.Comment: 5 pages, 6 figures, accepted for publication in Physical Review

Dopamine D2 Receptor-Mediated Heterologous Sensitization of AC5 Requires Signalosome Assembly

Chronic dopamine receptor activation is implicated in several central nervous system disorders. Although acute activation of Gαi-coupled D2 dopamine receptors inhibits adenylyl cyclase, persistent activation enhances adenylyl cyclase activity, a phenomenon called heterologous sensitization. Previous work revealed a requirement for Gαs in D2-induced heterologous sensitization of AC5. To elucidate the mechanism of Gαs dependency, we expressed Gαs mutants in Gαs-deficient GnasE2−/E2− cells. Neither Gαs-palmitoylation nor Gαs-Gβγ interactions were required for sensitization of AC5. Moreover, we found that coexpressing βARKct-CD8 or Sar1(H79G) blocked heterologous sensitization. These studies are consistent with a role for Gαs-AC5 interactions in sensitization however, Gβγ appears to have an indirect role in heterologous sensitization of AC5, possibly by promoting proper signalosome assembly

Properties of helium bubbles in covalent systems at the nanoscale: A combined numerical and experimental study

International audienceThe properties of nanometric-sized helium bubbles in silicon have been investigated using both spatially resolved electron-energy-loss spectroscopy combined with a recently developed method, and molecular-dynamics simulations. The experiments allowed for an accurate determination of size, aspect ratio, and helium density for a large number of single bubbles, whose diameters ranged from 6 to 20 nm. Very high helium densities, from 60 to 180 He nm −3 , have been measured depending on the conditions, in stark contrast with previous investigations of helium bubbles in metal with similar sizes. To supplement experiments on a smaller scale, and to obtain insights into the silicon matrix state, atomistic calculations have been performed for helium bubbles in the diameter range 1-13 nm. Molecular-dynamics simulations revealed that the maximum attainable helium density is critically related to the strength of the silicon matrix, which tends to yield by amorphization at the highest density levels. Calculations give helium density values for isolated single bubbles that are typically lower than measurements. However, excellent agreement is recovered when the interactions between bubbles and the presence of helium interstitials in the matrix are taken into account. Both experiments and numerical simulations suggest that the Laplace-Young law cannot be used to predict helium density in nanometric-sized bubbles in a covalent material such as silicon

"Pudding mold" band drives large thermopower in Nax_xCoO2_2

In the present study, we pin down the origin of the coexistence of the large thermopower and the large conductivity in Na$_x$CoO$_2$. It is revealed that not just the density of states (DOS), the effective mass, nor the band width, but the peculiar {\it shape} of the $a_{1g}$ band referred to as the "pudding mold" type, which consists of a dispersive portion and a somewhat flat portion, is playing an important role in this phenomenon. The present study provides a new guiding principle for designing good thermoelectric materials.Comment: 5 page

Revealing Superfluid--Mott-Insulator Transition in an Optical Lattice

We study (by an exact numerical scheme) the single-particle density matrix of $\sim 10^3$ ultracold atoms in an optical lattice with a parabolic confining potential. Our simulation is directly relevant to the interpretation and further development of the recent pioneering experiment by Greiner et al. In particular, we show that restructuring of the spatial distribution of the superfluid component when a domain of Mott-insulator phase appears in the system, results in a fine structure of the particle momentum distribution. This feature may be used to locate the point of the superfluid--Mott-insulator transition.Comment: 4 pages (12 figures), Latex. (A Latex macro is corrected