586 research outputs found
Spin gaps and spin-flip energies in density-functional theory
Energy gaps are crucial aspects of the electronic structure of finite and
extended systems. Whereas much is known about how to define and calculate
charge gaps in density-functional theory (DFT), and about the relation between
these gaps and derivative discontinuities of the exchange-correlation
functional, much less is know about spin gaps. In this paper we give
density-functional definitions of spin-conserving gaps, spin-flip gaps and the
spin stiffness in terms of many-body energies and in terms of single-particle
(Kohn-Sham) energies. Our definitions are as analogous as possible to those
commonly made in the charge case, but important differences between spin and
charge gaps emerge already on the single-particle level because unlike the
fundamental charge gap spin gaps involve excited-state energies. Kohn-Sham and
many-body spin gaps are predicted to differ, and the difference is related to
derivative discontinuities that are similar to, but distinct from, those
usually considered in the case of charge gaps. Both ensemble DFT and
time-dependent DFT (TDDFT) can be used to calculate these spin discontinuities
from a suitable functional. We illustrate our findings by evaluating our
definitions for the Lithium atom, for which we calculate spin gaps and spin
discontinuities by making use of near-exact Kohn-Sham eigenvalues and,
independently, from the single-pole approximation to TDDFT. The many-body
corrections to the Kohn-Sham spin gaps are found to be negative, i.e., single
particle calculations tend to overestimate spin gaps while they underestimate
charge gaps.Comment: 11 pages, 1 figure, 3 table
Controlled spontaneous emission
The problem of spontaneous emission is studied by a direct computer
simulation of the dynamics of a combined system: atom + radiation field. The
parameters of the discrete finite model, including up to 20k field oscillators,
have been optimized by a comparison with the exact solution for the case when
the oscillators have equidistant frequencies and equal coupling constants.
Simulation of the effect of multi-pulse sequence of phase kicks and emission by
a pair of atoms shows that both the frequency and the linewidth of the emitted
spectrum could be controlled.Comment: 25 pages including 11 figure
Cross-feeding between intestinal pathobionts promotes their overgrowth during undernutrition
Child undernutrition is a global health issue associated with a high burden of infectious disease. Undernourished children display an overabundance of intestinal pathogens and pathobionts, and these bacteria induce enteric dysfunction in undernourished mice; however, the cause of their overgrowth remains poorly defined. Here, we show that disease-inducing human isolates of Enterobacteriaceae and Bacteroidales spp. are capable of multi-species symbiotic cross-feeding, resulting in synergistic growth of a mixed community in vitro. Growth synergy occurs uniquely under malnourished conditions limited in protein and iron: in this context, Bacteroidales spp. liberate diet- and mucin-derived sugars and Enterobacteriaceae spp. enhance the bioavailability of iron. Analysis of human microbiota datasets reveals that Bacteroidaceae and Enterobacteriaceae are strongly correlated in undernourished children, but not in adequately nourished children, consistent with a diet-dependent growth synergy in the human gut. Together these data suggest that dietary cross-feeding fuels the overgrowth of pathobionts in undernutrition
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