104 research outputs found
Adiabatic Approximation in the Density Matrix Approach: Non-Degenerate Systems
We study the adiabatic limit in the density matrix approach for a quantum
system coupled to a weakly dissipative medium. The energy spectrum of the
quantum model is supposed to be non-degenerate. In the absence of dissipation,
the geometric phases for periodic Hamiltonians obtained previously by M.V.
Berry are recovered in the present approach. We determine the necessary
condition satisfied by the coefficients of the linear expansion of the
non-unitary part of the Liouvillian in order to the imaginary phases acquired
by the elements of the density matrix, due to dissipative effects, be
geometric. The results derived are model-independent. We apply them to spin 1/2
model coupled to reservoir at thermodynamic equilibrium.Comment: 24 pages (new version), accepted for publication in Physica
Modeling Host Genetic Regulation of Influenza Pathogenesis in the Collaborative Cross
Genetic variation contributes to host responses and outcomes following infection by influenza A virus or other viral infections. Yet narrow windows of disease symptoms and confounding environmental factors have made it difficult to identify polymorphic genes that contribute to differential disease outcomes in human populations. Therefore, to control for these confounding environmental variables in a system that models the levels of genetic diversity found in outbred populations such as humans, we used incipient lines of the highly genetically diverse Collaborative Cross (CC) recombinant inbred (RI) panel (the pre-CC population) to study how genetic variation impacts influenza associated disease across a genetically diverse population. A wide range of variation in influenza disease related phenotypes including virus replication, virus-induced inflammation, and weight loss was observed. Many of the disease associated phenotypes were correlated, with viral replication and virus-induced inflammation being predictors of virus-induced weight loss. Despite these correlations, pre-CC mice with unique and novel disease phenotype combinations were observed. We also identified sets of transcripts (modules) that were correlated with aspects of disease. In order to identify how host genetic polymorphisms contribute to the observed variation in disease, we conducted quantitative trait loci (QTL) mapping. We identified several QTL contributing to specific aspects of the host response including virus-induced weight loss, titer, pulmonary edema, neutrophil recruitment to the airways, and transcriptional expression. Existing whole-genome sequence data was applied to identify high priority candidate genes within QTL regions. A key host response QTL was located at the site of the known anti-influenza Mx1 gene. We sequenced the coding regions of Mx1 in the eight CC founder strains, and identified a novel Mx1 allele that showed reduced ability to inhibit viral replication, while maintaining protection from weight loss
Meta-analysis of type 2 Diabetes in African Americans Consortium
Type 2 diabetes (T2D) is more prevalent in African Americans than in Europeans. However, little is known about the genetic risk in African Americans despite the recent identification of more than 70 T2D loci primarily by genome-wide association studies (GWAS) in individuals of European ancestry. In order to investigate the genetic architecture of T2D in African Americans, the MEta-analysis of type 2 DIabetes in African Americans (MEDIA) Consortium examined 17 GWAS on T2D comprising 8,284 cases and 15,543 controls in African Americans in stage 1 analysis. Single nucleotide polymorphisms (SNPs) association analysis was conducted in each study under the additive model after adjustment for age, sex, study site, and principal components. Meta-analysis of approximately 2.6 million genotyped and imputed SNPs in all studies was conducted using an inverse variance-weighted fixed effect model. Replications were performed to follow up 21 loci in up to 6,061 cases and 5,483 controls in African Americans, and 8,130 cases and 38,987 controls of European ancestry. We identified three known loci (TCF7L2, HMGA2 and KCNQ1) and two novel loci (HLA-B and INS-IGF2) at genome-wide significance (4.15 Ă 10(-94)<P<5 Ă 10(-8), odds ratio (OR)â = 1.09 to 1.36). Fine-mapping revealed that 88 of 158 previously identified T2D or glucose homeostasis loci demonstrated nominal to highly significant association (2.2 Ă 10(-23) < locus-wide P<0.05). These novel and previously identified loci yielded a sibling relative risk of 1.19, explaining 17.5% of the phenotypic variance of T2D on the liability scale in African Americans. Overall, this study identified two novel susceptibility loci for T2D in African Americans. A substantial number of previously reported loci are transferable to African Americans after accounting for linkage disequilibrium, enabling fine mapping of causal variants in trans-ethnic meta-analysis studies.Peer reviewe
Low-dimensional inverted Si/SiGe modulation-doped electron gases using selective ex-situ ion implantation
A novel fabrication technique for selectively modulation-doping strained-Si quantum wells on relaxed Si1âxGex substrates to produce field effect transistors and low dimensional devices is demonstrated using standard silicon processing techniques. Strainârelaxed Si1âxGex buffers were selectively ion implanted ex-situ through a lithographically patterned resist before being chemically cleaned and replaced in the growth chamber to regrow a quantum well and cap layers. Mobilities of up to 49,900 cm2 Vâ1 sâ1 for a carrier density of 9.72Ă1011 cmâ2 at 1.7 K for selectively doped Hall bars have been demonstrated along with wires using the technique
High-mobility two-dimensional electron gases in Si/SiGe heterostructures on relaxed SiGe layers grown at high temperature
Low-temperature mobilities for two-dimensional electron gases (2DEGs)
formed in tensile-strained Si/SiGe heterostructures are reported, with values up to
2.7 Ă 105 cm2 Vâ1 sâ1 for a density of 4.6 Ă 1011 cmâ2 electrons. The strained
layers were grown at 600 âŚC in a ultra-high-vacuum chemical vapour deposition
system using SiH4 and GeH4 operating at around 20 Pa. The surface morphology
of the layers is also discussed and both the mobility and morphology are linked to
the quality of the virtual substrates. The virtual substrate consists of strain-relaxed
SiGe alloys grown on Si(001) substrates; we show that it is preferable to grow
these substrates at higher temperatures and higher growth rates. For low growth
rates and temperatures the 2DEG mobility as a function of sheet carrier density
was found to be degraded
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