26 research outputs found
Determination of ferroelectric compositional phase transition using novel virtual crystal approach
We employ a new method for studying compositionally disordered ferroelectric
oxides. This method is based on the virtual crystal approximation (VCA), in
which two or more component potentials are averaged into a composite atomic
potential. In our method, we construct a virtual atom with the correctly
averaged atomic size and atomic eigenvalues. We have used our new method to
study the composition dependent phase transition in Pb(Zr_{1-x}Ti_x)O_3 lying
between x=0.5 and x=0.4. We correctly predict the experimentally determined
phase transition from the tetragonal phase to a low-temperature rhombohedral
phase between these two compositions.Comment: 7 pages, 2 figures, Proceedings for Fundamental Physics of
Ferroelectrics, Aspen, CO February 13-20, 200
Accurate construction of transition metal pseudopotentials
We generate a series of pseudopotentials to examine the relationship between
pseudoatomic properties and solid-state results. We find that lattice constants
and bulk moduli are quite sensitive to eigenvalue, total-energy difference and
tail norm errors, and clear correlations emerge. These trends motivate our
identification of two criteria for accurate transition metal pseudopotentials.
We find that both the preservation of all-electron derivative of tail norm with
respect to occupation and the preservation of all-electron derivative of
eigenvalue with respect to occupation {[Phys. Rev. B {\bf 48}, 5031 (1993)]}
are necessary to give accurate bulk metal lattice constants and bulk moduli. We
also show how the fairly wide range of lattice constant and bulk modulus
results found in the literature can be easily explained by pseudopotential
effects.Comment: submitted to Phys. Rev
Virtual-crystal approximation that works: Locating a composition phase boundary in Pb(Zr_{1-x}Ti_3)O_3
We present a new method for modeling disordered solid solutions, based on the
virtual crystal approximation (VCA). The VCA is a tractable way of studying
configurationally disordered systems; traditionally, the potentials which
represent atoms of two or more elements are averaged into a composite atomic
potential. We have overcome significant shortcomings of the standard VCA by
developing a potential which yields averaged atomic properties. We perform the
VCA on a ferroelectric oxide, determining the energy differences between the
high-temperature rhombohedral, low-temperature rhombohedral and tetragonal
phases of Pb(Zr_{1-x}Ti_x)O_3 at x=0.5 and comparing these results to
superlattice calculations and experiment. We then use our new method to
determine the preferred structural phase at x=0.4. We find that the
low-temperature rhombohedral phase becomes the ground state at x=0.4, in
agreement with experimental findings.Comment: 5 pages, no figure
Experimental evaluation of ecological principles to understand and modulate the outcome of bacterial strain competition in gut microbiomes
It is unclear if coexistence theory can be applied to gut microbiomes to understand their characteristics and modulate their composition. Through experiments in gnotobiotic mice with complex microbiomes, we demonstrated that strains of Akkermansia muciniphila and Bacteroides vulgatus could only be established if microbiomes were devoid of these species. Strains of A. muciniphila showed strict competitive exclusion, while B. vulgatus strains coexisted but populations were still influenced by competitive interactions. These differences in competitive behavior were reflective of genomic variation within the two species, indicating considerable niche overlap for A. muciniphila strains and a broader niche space for B. vulgatus strains. Priority effects were detected for both species as strains’ competitive fitness increased when colonizing first, which resulted in stable persistence of the A. muciniphila strain colonizing first and competitive exclusion of the strain arriving second. Based on these observations, we devised a subtractive strategy for A. muciniphila using antibiotics and showed that a strain from an assembled community can be stably replaced by another strain. By demonstrating that competitive outcomes in gut ecosystems depend on niche differences and are historically contingent, our study provides novel information to explain the ecological characteristics of gut microbiomes and a basis for their modulation