10,143 research outputs found
Vibrational modes and lattice distortion of a nitrogen-vacancy center in diamond from first-principles calculations
We investigate vibrational properties and lattice distortion of negatively
charged nitrogen-vacancy (NV) center in diamond. Using the first-principles
electronic structure calculations, we show that the presence of NV center leads
to appearance of a large number of quasilocalized vibrational modes (qLVMs)
with different degree of localization. The vibration patterns and the
symmetries of the qLVMs are presented and analyzed in detail for both ground
and excited orbital states of the NV center. We find that in the high-symmetry
() excited orbital state a pair of degenerate qLVMs becomes unstable,
and the stable excited state has lower () symmetry. This is a direct
indication of the Jahn-Teller effect, and our studies suggest that dynamical
Jahn-Teller effect in the weak coupling regime takes place. We have also
performed a detailed comparison of our results with the available experimental
data on the vibrations involved in optical emission/absorption of the NV
centers. We have directly demonstrated that, among other modes, the qLVMs
crucially impact the optical properties of the NV centers in diamond, and
identified the most important groups of qLVMs. Our results are important for
deeper understanding of the optical properties and the orbital relaxation
associated with lattice vibrations of the NV centers.Comment: 10 RevTeX pages, 10 EPS figure
The Correlation and Effectiveness Between Two Variables that are Considered in Sustainable Investing: ESG Ratings and Credit Ratings
Theory of layered-oxide cathode degradation in Li-ion batteries by oxidation-induced cation disorder
Disorder-driven degradation phenomena, such as structural phase
transformations and surface reconstructions, can significantly reduce the
lifetime of Li-ion batteries, especially those with nickel-rich layered-oxide
cathodes. We develop a general free energy model for layered-oxide
ion-intercalation materials as a function of the degree of disorder, which
represents the density of defects in the host crystal. The model accounts for
defect core energies, long-range dipolar electrostatic forces, and
configurational entropy of the solid solution. In the case of nickel-rich
oxides, we hypothesize that nickel with a high concentration of defects is
driven into the bulk by electrostatic forces as oxidation reactions at the
solid-electrolyte interface reduce nickel and either evolve oxygen gas or
oxidize the organic electrolyte at high potentials (>4.4V vs. Li/Li+). The
model is used in battery cycling simulations to describe the extent of cathode
degradation when using different voltage cutoffs, in agreement with
experimental observations that lower-voltage cycling can substantially reduce
cathode degradation. The theory provides a framework to guide the development
of cathode compositions, coatings and electrolytes to enhance rate capability
and enhance battery lifetime. The general theory of cation-disorder formation
may also find applications in electrochemical water treatment and ion
separations, such as lithium extraction from brines, based on competitive ion
intercalation in battery materials
Liquid Crystal-Solid Interface Structure at the Antiferroelectric-Ferroelectric Phase Transition
Total Internal Reflection (TIR) is used to probe the molecular organization
at the surface of a tilted chiral smectic liquid crystal at temperatures in the
vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are
interpreted using an exact analytical solution of a real model for
ferroelectric order at the surface. In the mixture T3, ferroelectric surface
order is expelled with the bulk ferroelectric-antiferroelectric transition. The
conditions for ferroelectric order at the surface of an antiferroelectric bulk
are presented
A mathematical model for a house integrated with an elevated Chinese Kang heating system
Chinese kang, a potentially energy-efficient domestic
heating system in China, uses high thermal mass to
store surplus heat from the stove during cooking and
releases it later for space heating. In this paper a
preliminary mathematical model is developed for a
House Integrated with an Elevated Kang system
(HIEK). This model considers the transient thermal
behaviors of building envelope, kang system and
indoor air. The macroscopic approach is used to
model the thermal and airflow process for the
elevated kang system. The numerical method for
solving the resultant non-linear equations of HIEK is
proposed and implemented. The HIEK model is
preliminarily evaluated using the measured data from
a field survey, and agreement is reasonably good.
This model can be used to predict the indoor air
temperature for multi-zone HIEK by inputting the
basic parameters like geometry, physical properties
of building and kang. The model can be easily
extended for investigating the thermal performance of
a kang system and its influence on indoor thermal
environment and building energy consumption.
Finally, suggestions for incorporating the kang model
into existing building simulation tools are also
described.published_or_final_versio
A new understanding of the effect of filler minerals on the precipitation of synthetic C–S–H
The filler effect is the most important physical mechanism of mineral admixtures in the early hydration of cement whose chemical properties greatly affect the precipitation of C–S–H. In this study, calcite, strontianite, magnesite, dolomite, quartz, whewellite and whitlockite were selected as the fillers. The morphology and reaction kinetics of synthetic C–S–H precipitated on the surfaces of different fillers were studied via electron microscopy observations and electrical conductivity and ion concentration measurements. The precipitation rate of C–S–H has a positive correlation with the affinity of Ca2+ for adsorption on the fillers, which can be explained by the nucleation barrier of C–S–H. Extremely ordered honeycomb-like morphology of the C–S–H is found on calcite and strontianite surfaces, while less regular leaf-like or honeycomb-like C–S–H is found on whewellite and whitlockite. The ordered C–S–H pattern is related to the lattice cleavage of the ionic compound filler. In the case of quartz, C–S–H prefers growth along the tangential direction, which is quite different from the normal-direction growth on ionic compounds. The in-plane growth of C–S–H on quartz is believed to be induced by a layer of loosely physically adsorbed Ca2+
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