42 research outputs found
Oxygen vacancy induced structural variations of exfoliated monolayer MnO2 sheets
We report findings on the structural stability of exfoliated monolayer MnO2 sheets. Our study reveals that monolayer MnO2 sheets display two specific kinds of structural modification under electron irradiation. An atomic reconstruction (2 x 1) and a phase of MnO, induced by ordered oxygen vacancies, were identified by transmission electron microscopy techniques and further characterized by comparison with density-functional theory calculations. These findings are expected to significantly broaden current knowledge of the structural stability of ultrathin layered sheets
Lattice dielectric response of CdCu{3}Ti{4}O{12} and of CaCu{3}Ti{4}O{12} from first principles
Structural, vibrational, and lattice dielectric properties of
CdCu{3}Ti{4}O{12} are studied using density-functional theory within the local
spin-density approximation, and the results are compared with those computed
previously for CaCu{3}Ti{4}O{12}. Replacing Ca with Cd is found to leave many
calculated quantities largely unaltered, although significant differences do
emerge in zone-center optical phonon frequencies and mode effective charges.
The computed phonon frequencies of CdCu{3}Ti{4}O{12} are found to be in
excellent agreement with experiment, and the computed lattice contribution to
the intrinsic static dielectric constant (~60) also agrees exceptionally well
with a recent optical absorption experiment. These results provide further
support for a picture in which the lattice dielectric response is essentially
conventional, suggesting an extrinsic origin for the anomalous low-frequency
dielectric response recently observed in both materials.Comment: 5 pages; uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/lh_cdct/index.htm
Statistical Mechanics of Vacancy and Interstitial Strings in Hexagonal Columnar Crystals
Columnar crystals contain defects in the form of vacancy/interstitial loops
or strings of vacancies and interstitials bounded by column ``heads'' and
``tails''. These defect strings are oriented by the columnar lattice and can
change size and shape by movement of the ends and forming kinks along the
length. Hence an analysis in terms of directed living polymers is appropriate
to study their size and shape distribution, volume fraction, etc. If the
entropy of transverse fluctuations overcomes the string line tension in the
crystalline phase, a string proliferation transition occurs, leading to a
supersolid phase. We estimate the wandering entropy and examine the behaviour
in the transition regime. We also calculate numerically the line tension of
various species of vacancies and interstitials in a triangular lattice for
power-law potentials as well as for a modified Bessel function interaction
between columns as occurs in the case of flux lines in type-II superconductors
or long polyelectrolytes in an ionic solution. We find that the centered
interstitial is the lowest energy defect for a very wide range of interactions;
the symmetric vacancy is preferred only for extremely short interaction ranges.Comment: 22 pages (revtex), 15 figures (encapsulated postscript
Interstitials, Vacancies, and Supersolid Order in Vortex Crystals
Interstitials and vacancies in the Abrikosov phase of clean Type II
superconductors are line imperfections, which cannot extend across macroscopic
equilibrated samples at low temperatures. We argue that the entropy associated
with line wandering nevertheless can cause these defects to proliferate at a
sharp transition which will exist if this occurs below the temperature at which
the crystal actually melts. Vortices are both entangled and crystalline in the
resulting ``supersolid'' phase, which in a dual ``boson'' analog system is
closely related to a two-dimensional quantum crystal of He with
interstitials or vacancies in its ground state. The supersolid {\it must} occur
for , where is the decoupling field above which
vortices begin to behave two-dimensionally. Numerical calculations show that
interstitials, rather than vacancies, are the preferred defect for , and allow us to estimate whether proliferation also
occurs for B\,\lot\,B_\times.The implications of the supersolid phase for
transport measurements, dislocation configurations and neutron diffraction are
discussed.Comment: 53 pages and 15 figures, available upon request, written in plain TE
Mechanism of dopant distribution: an example of nickel-doped ceria nanoparticles
The microstructure, geometry and accurate distribution of dopants in nickel-doped ceria nanoparticles prepared by spray pyrolysis were characterized. Nickel dopant concentration variation with the distance from the centre of the particle can be approximated as linear with a higher concentration of nickel found closer to the centre. It is suggested that the dopant distribution is mainly controlled by the difference in the diffusion rates of the precursors from the centre to the surface of the particle
Effects if agricultural intensification on breeding success of Corn Buntings (Miliaria calandra)
1. Corn buntings Miliaria calandra have declined steeply in Britain and north-western Europe since the mid-1970s; changes in farming practice are believed to have been partly responsible. 2. We studied nesting corn buntings on the South Downs in west Sussex between 1995 and 1997 to examine the possible effects of agricultural intensification on breeding success. The abundance of invertebrates around individual nests was sampled by sweep-netting in July. 3. Corn buntings provisioning nestlings foraged in grassy margins more than any other habitat relative to their availability within the maximum foraging range. The other habitats used more than expected were spring-sown barley, unintensified grass and set-aside. Those used less than expected included winter-sown wheat and intensively managed grassland. The invertebrates most commonly fed to chicks were more abundant in foraging areas than elsewhere. Their density was negatively correlated with the number of insecticide applications both when cereal fields only were considered and when all foraging habitats were included. 4. The lower the abundance of chick-food invertebrates close to nests, the greater the distance from the nest at which parents foraged, and the longer such trips were in duration. The weights of nestlings, corrected for age using tarsus length, were positively correlated with the abundance of chick-food invertebrates. 5. The probability of nest survival was negatively correlated with the abundance of chick-food invertebrates close to the nest, apparently as a result of an increased risk of predation. 6. Agricultural intensification in Britain, including the increased use of pesticides, has led to a widespread decrease in the availability of chick-food invertebrates on lowland farmland. If our results are typical of corn buntings in an arable environment, this decrease correlates with reduced breeding success. Depending on the mortality rates for fledged chicks and older birds, this reduction may have contributed to the corn buntings' decline and may hamper recovery. 7. Farming practices that increase invertebrate availability ought to benefit breeding corn buntings. Large-scale measures such as set-aside and the spring-sowing of cereals (especially if undersown with grass) depend heavily on overall agricultural policy. Small-scale initiatives might therefore be more feasible; these include the provision of grassy margins or beetle banks and selective spraying of headlands