1,120 research outputs found
Non-equilibrium Green's function approach to inhomogeneous quantum many-body systems using the Generalized Kadanoff Baym Ansatz
In non-equilibrium Green's function calculations the use of the Generalized
Kadanoff-Baym Ansatz (GKBA) allows for a simple approximate reconstruction of
the two-time Green's function from its time-diagonal value. With this a drastic
reduction of the computational needs is achieved in time-dependent
calculations, making longer time propagation possible and more complex systems
accessible. This paper gives credit to the GKBA that was introduced 25 years
ago. After a detailed derivation of the GKBA, we recall its application to
homogeneous systems and show how to extend it to strongly correlated,
inhomogeneous systems. As a proof of concept, we present results for a
2-electron quantum well, where the correct treatment of the correlated electron
dynamics is crucial for the correct description of the equilibrium and dynamic
properties
A new approach to hierarchical data analysis: Targeted maximum likelihood estimation for the causal effect of a cluster-level exposure
We often seek to estimate the impact of an exposure naturally occurring or
randomly assigned at the cluster-level. For example, the literature on
neighborhood determinants of health continues to grow. Likewise, community
randomized trials are applied to learn about real-world implementation,
sustainability, and population effects of interventions with proven
individual-level efficacy. In these settings, individual-level outcomes are
correlated due to shared cluster-level factors, including the exposure, as well
as social or biological interactions between individuals. To flexibly and
efficiently estimate the effect of a cluster-level exposure, we present two
targeted maximum likelihood estimators (TMLEs). The first TMLE is developed
under a non-parametric causal model, which allows for arbitrary interactions
between individuals within a cluster. These interactions include direct
transmission of the outcome (i.e. contagion) and influence of one individual's
covariates on another's outcome (i.e. covariate interference). The second TMLE
is developed under a causal sub-model assuming the cluster-level and
individual-specific covariates are sufficient to control for confounding.
Simulations compare the alternative estimators and illustrate the potential
gains from pairing individual-level risk factors and outcomes during
estimation, while avoiding unwarranted assumptions. Our results suggest that
estimation under the sub-model can result in bias and misleading inference in
an observational setting. Incorporating working assumptions during estimation
is more robust than assuming they hold in the underlying causal model. We
illustrate our approach with an application to HIV prevention and treatment
Targeted Estimation and Inference for the Sample Average Treatment Effect
While the population average treatment effect has been the subject of extensive methods and applied research, less consideration has been given to the sample average treatment effect: the mean difference in the counterfactual outcomes for the study units. The sample parameter is easily interpretable and is arguably the most relevant when the study units are not representative of a greater population or when the exposure\u27s impact is heterogeneous. Formally, the sample effect is not identifiable from the observed data distribution. Nonetheless, targeted maximum likelihood estimation (TMLE) can provide an asymptotically unbiased and efficient estimate of both the population and sample parameters. In this paper, we study the asymptotic and finite sample properties of the TMLE for the sample effect and provide a conservative variance estimator. In most settings, the sample parameter can be estimated more efficiently than the population parameter. Finite sample simulations illustrate the potential gains in precision and power from selecting the sample effect as the target of inference. As a motivating example, we discuss the Sustainable East Africa Research in Community Health (SEARCH) study, an ongoing cluster randomized trial for HIV prevention and treatment
Cis-regulatory elements of the mitotic regulator, string/Cdc25
Mitosis in most Drosophila cells is triggered by brief bursts of transcription of string (stg), a Cdc25-type phosphatase that activates the mitotic kinase, Cdk1 (Cdc2). To understand how string transcription is regulated, we analyzed the expression of string-lacZ reporter genes covering approximately 40 kb of the string locus. We also tested protein coding fragments of the string locus of 6 kb to 31.6 kb for their ability to complement loss of string function in embryos and imaginal discs. A plethora of cis-acting elements spread over >30 kb control string transcription in different cells and tissue types. Regulatory elements specific to subsets of epidermal cells, mesoderm, trachea and nurse cells were identified, but the majority of the string locus appears to be devoted to controlling cell proliferation during neurogenesis. Consistent with this, compact promotor-proximal sequences are sufficient for string function during imaginal disc growth, but additional distal elements are required for the development of neural structures in the eye, wing, leg and notum. We suggest that, during evolution, cell-type-specific control elements were acquired by a simple growth-regulated promoter as a means of coordinating cell division with developmental processes, particularly neurogenesis.Dara A. Lehman; Briony Patterson, Laura A. Johnston; Tracy Balzer; Jessica S. Britton; Robert Saint and Bruce A. Edga
Effect of a Patient-Centered Phone Call by a Clinical Officer at Time of HIV Testing on Linkage to Care in Rural Kenya.
In a randomized controlled trial, we tested whether a structured, patient-centered phone call from a clinical officer after HIV testing improved linkage to/re-engagement in HIV care. Among 130 HIV-positive persons, those randomized to the phone call were significantly more likely to link to care by 7 and 30 days (P = .04)
The Influence of the Degree of Heterogeneity on the Elastic Properties of Random Sphere Packings
The macroscopic mechanical properties of colloidal particle gels strongly
depend on the local arrangement of the powder particles. Experiments have shown
that more heterogeneous microstructures exhibit up to one order of magnitude
higher elastic properties than their more homogeneous counterparts at equal
volume fraction. In this paper, packings of spherical particles are used as
model structures to computationally investigate the elastic properties of
coagulated particle gels as a function of their degree of heterogeneity. The
discrete element model comprises a linear elastic contact law, particle bonding
and damping. The simulation parameters were calibrated using a homogeneous and
a heterogeneous microstructure originating from earlier Brownian dynamics
simulations. A systematic study of the elastic properties as a function of the
degree of heterogeneity was performed using two sets of microstructures
obtained from Brownian dynamics simulation and from the void expansion method.
Both sets cover a broad and to a large extent overlapping range of degrees of
heterogeneity. The simulations have shown that the elastic properties as a
function of the degree of heterogeneity are independent of the structure
generation algorithm and that the relation between the shear modulus and the
degree of heterogeneity can be well described by a power law. This suggests the
presence of a critical degree of heterogeneity and, therefore, a phase
transition between a phase with finite and one with zero elastic properties.Comment: 8 pages, 6 figures; Granular Matter (published online: 11. February
2012
First order Mott transition at zero temperature in two dimensions: Variational plaquette study
The nature of the metal-insulator Mott transition at zero temperature has
been discussed for a number of years. Whether it occurs through a quantum
critical point or through a first order transition is expected to profoundly
influence the nature of the finite temperature phase diagram. In this paper, we
study the zero temperature Mott transition in the two-dimensional Hubbard model
on the square lattice with the variational cluster approximation. This takes
into account the influence of antiferromagnetic short-range correlations. By
contrast to single-site dynamical mean-field theory, the transition turns out
to be first order even at zero temperature.Comment: 6 pages, 5 figures, version 2 with additional results for 8 bath
site
Recommended from our members
Costs of streamlined HIV care delivery in rural Ugandan and Kenyan clinics in the SEARCH Studys.
OBJECTIVES/DESIGN:As antiretroviral therapy (ART) rapidly expands in sub-Saharan Africa using new efficient care models, data on costs of these approaches are lacking. We examined costs of a streamlined HIV care delivery model within a large HIV test-and-treat study in Uganda and Kenya. METHODS:We calculated observed per-person-per-year (ppy) costs of streamlined care in 17 health facilities in SEARCH Study intervention communities (NCT: 01864603) via micro-costing techniques, time-and-motion studies, staff interviews, and administrative records. Cost categories included salaries, ART, viral load testing, recurring goods/services, and fixed capital/facility costs. We then modeled costs under three increasingly efficient scale-up scenarios: lowest-cost ART, centralized viral load testing, and governmental healthcare worker salaries. We assessed the relationship between community-specific ART delivery costs, retention in care, and viral suppression. RESULTS:Estimated streamlined HIV care delivery costs were 117/ppy for TDF/3TC/EFV [40%]) and viral load testing (51/ppy), recurring costs (7/ppy). Optimized ART scale-up with lowest-cost ART (24/ppy), and governmental healthcare salaries (163/ppy. We found clinic-to-clinic heterogeneity in retention and viral suppression levels versus streamlined care delivery costs, but no correlation between cost and either retention or viral suppression. CONCLUSIONS:In the SEARCH Study, streamlined HIV care delivery costs were similar to or lower than prior estimates despite including viral load testing; further optimizations could substantially reduce costs further. These data can inform global strategies for financing ART expansion to achieve UNAIDS 90-90-90 targets
- …