871 research outputs found
Combining high conductivity with complete optical transparency: A band-structure approach
A comparison of the structural, optical and electronic properties of the
recently discovered transparent conducting oxide (TCO), nanoporous Ca12Al14O33,
with those of the conventional TCO's (such as Sc-doped CdO) indicates that this
material belongs conceptually to a new class of transparent conductors. For
this class of materials, we formulate criteria for the successful combination
of high electrical conductivity with complete transparency in the visible
range. Our analysis suggests that this set of requirements can be met for a
group of novel materials called electrides.Comment: 3 pages, 3 figures, submitted for publicatio
Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation
The electronic properties of single- and multi-cation transparent conducting
oxides (TCOs) are investigated using first-principles density functional
approach. A detailed comparison of the electronic band structure of
stoichiometric and oxygen deficient InO, - and
-GaO, rock salt and wurtzite ZnO, and layered InGaZnO
reveals the role of the following factors which govern the transport and
optical properties of these TCO materials: (i) the crystal symmetry of the
oxides, including both the oxygen coordination and the long-range structural
anisotropy; (ii) the electronic configuration of the cation(s), specifically,
the type of orbital(s) -- , or -- which form the conduction band;
and (iii) the strength of the hybridization between the cation's states and the
p-states of the neighboring oxygen atoms. The results not only explain the
experimentally observed trends in the electrical conductivity in the
single-cation TCO, but also demonstrate that multicomponent oxides may offer a
way to overcome the electron localization bottleneck which limits the charge
transport in wide-bandgap main-group metal oxides. Further, the advantages of
aliovalent substitutional doping -- an alternative route to generate carriers
in a TCO host -- are outlined based on the electronic band structure
calculations of Sn, Ga, Ti and Zr-doped InGaZnO. We show that the
transition metal dopants offer a possibility to improve conductivity without
compromising the optical transmittance
Exciton correlations in coupled quantum wells and their luminescence blue shift
In this paper we present a study of an exciton system where electrons and
holes are confined in double quantum well structures. The dominating
interaction between excitons in such systems is a dipole - dipole repulsion. We
show that the tail of this interaction leads to a strong correlation between
excitons and substantially affects the behavior of the system. Making use of
qualitative arguments and estimates we develop a picture of the exciton -
exciton correlations in the whole region of temperature and concentration where
excitons exist. It appears that at low concentration degeneracy of the excitons
is accompanied with strong multi-particle correlation so that the system cannot
be considered as a gas. At high concentration the repulsion suppresses the
quantum degeneracy down to temperatures that could be much lower than in a Bose
gas with contact interaction. We calculate the blue shift of the exciton
luminescence line which is a sensitive tool to observe the exciton - exciton
correlations.Comment: 27 pages in PDF and DVI format, 8 figure
Quenched Spin Tunneling and Diabolical Points in Magnetic Molecules: II. Asymmetric Configurations
The perfect quenching of spin tunneling first predicted for a model with
biaxial symmetry, and recently observed in the magnetic molecule Fe_8, is
further studied using the discrete phase integral (or
Wentzel-Kramers-Brillouin) method. The analysis of the previous paper is
extended to the case where the magnetic field has both hard and easy
components, so that the Hamiltonian has no obvious symmetry. Herring's formula
is now inapplicable, so the problem is solved by finding the wavefunction and
using connection formulas at every turning point. A general formula for the
energy surface in the vicinity of the diabolo is obtained in this way. This
formula gives the tunneling apmplitude between two wells unrelated by symmetry
in terms of a small number of action integrals, and appears to be generally
valid, even for problems where the recursion contains more than five terms.
Explicit results are obtained for the diabolical points in the model for Fe_8.
These results exactly parallel the experimental observations. It is found that
the leading semiclassical results for the diabolical points appear to be exact,
and the points themselves lie on a perfect centered rectangular lattice in the
magnetic field space. A variety of evidence in favor of this perfect lattice
hypothesis is presented.Comment: Revtex; 4 ps figures; follow up to cond-mat/000311
Angular dependence of novel magnetic quantum oscillations in a quasi-two-dimensional multiband Fermi liquid with impurities
The semiclassical Lifshitz-Kosevich-type description is given for the angular
dependence of quantum oscillations with combination frequencies in a multiband
quasi-two-dimensional Fermi liquid with a constant number of electrons. The
analytical expressions are found for the Dingle, thermal, spin, and amplitude
(Yamaji) reduction factors of the novel combination harmonics, where the latter
two strongly oscillate with the direction of the field. At the "magic" angles
those factors reduce to the purely two-dimensional expressions given earlier.
The combination harmonics are suppressed in the presence of the non-quantized
("background") states, and they decay exponentially faster with temperature
and/or disorder compared to the standard harmonics, providing an additional
tool for electronic structure determination. The theory is applied to
SrRuO.Comment: 5 pages, 2 figures, minor typos correcte
Genetic diversity and stability of the porA allele as a genetic marker in human Campylobacter infection
The major outer-membrane protein (MOMP) of Campylobacter jejuni and Campylobacter coli, encoded by the porA gene, is extremely genetically diverse. Conformational MOMP epitopes are important in host immunity, and variation in surface-exposed regions probably occurs as a result of positive immune selection during infection. porA diversity has been exploited in genotyping studies using highly discriminatory nucleotide sequences to identify potentially epidemiologically linked cases of human campylobacteriosis. To understand the overall nature and extent of porA diversity and stability in C. jejuni and C. coli we investigated sequences in isolates (n=584) obtained from a defined human population (approx. 600 000) over a defined time period (1 year). A total of 196 distinct porA variants were identified. Regions encoding putative extracellular loops were the most variable in both nucleotide sequence and length. Phylogenetic analysis identified three porA allele clusters that originated in (i) predominantly C. jejuni and a few C. coli, (ii) solely C. jejuni or (iii) predominantly C. coli and a few C. jejuni. The stability of porA within an individual human host was investigated using isolates cultured longitudinally from 64 sporadic cases, 27 of which had prolonged infection lasting between 5 and 98 days (the remainder having illness of normal duration, 0–4 days), and 20 cases from family outbreaks. Evidence of mutation was detected in two patients with prolonged illness. Despite demonstrable positive immune selection in these two unusual cases, the persistence of numerous variants within the population indicated that the porA allele is a valuable tool for use in extended typing schemes
Semi-classical spectrum of integrable systems in a magnetic field
The quantum dynamics of an electron in a uniform magnetic field is studied
for geometries corresponding to integrable cases. We obtain the uniform
asymptotic approximation of the WKB energies and wavefunctions for the
semi-infinite plane and the disc. These analytical solutions are shown to be in
excellent agreement with the numerical results obtained from the Schrodinger
equations even for the lowest energy states. The classically exact notions of
bulk and edge states are followed to their semi-classical limit, when the
uniform approximation provides the connection between bulk and edge.Comment: 17 pages, Revtex, 6 figure
Condensation of Ideal Bose Gas Confined in a Box Within a Canonical Ensemble
We set up recursion relations for the partition function and the ground-state
occupancy for a fixed number of non-interacting bosons confined in a square box
potential and determine the temperature dependence of the specific heat and the
particle number in the ground state. A proper semiclassical treatment is set up
which yields the correct small-T-behavior in contrast to an earlier theory in
Feynman's textbook on Statistical Mechanics, in which the special role of the
ground state was ignored. The results are compared with an exact quantum
mechanical treatment. Furthermore, we derive the finite-size effect of the
system.Comment: 18 pages, 8 figure
Classical and Quantum Chaos in a quantum dot in time-periodic magnetic fields
We investigate the classical and quantum dynamics of an electron confined to
a circular quantum dot in the presence of homogeneous magnetic
fields. The classical motion shows a transition to chaotic behavior depending
on the ratio of field magnitudes and the cyclotron
frequency in units of the drive frequency. We determine a
phase boundary between regular and chaotic classical behavior in the
vs plane. In the quantum regime we evaluate the quasi-energy
spectrum of the time-evolution operator. We show that the nearest neighbor
quasi-energy eigenvalues show a transition from level clustering to level
repulsion as one moves from the regular to chaotic regime in the
plane. The statistic confirms this
transition. In the chaotic regime, the eigenfunction statistics coincides with
the Porter-Thomas prediction. Finally, we explicitly establish the phase space
correspondence between the classical and quantum solutions via the Husimi phase
space distributions of the model. Possible experimentally feasible conditions
to see these effects are discussed.Comment: 26 pages and 17 PstScript figures, two large ones can be obtained
from the Author
Prior Mating Experience Modulates the Dispersal of Drosophila in Males More Than in Females
Cues from both an animal’s internal physiological state and its local environment may influence its decision to disperse. However, identifying and quantifying the causative factors underlying the initiation of dispersal is difficult in uncontrolled natural settings. In this study, we automatically monitored the movement of fruit flies and examined the influence of food availability, sex, and reproductive status on their dispersal between laboratory environments. In general, flies with mating experience behave as if they are hungrier than virgin flies, leaving at a greater rate when food is unavailable and staying longer when it is available. Males dispersed at a higher rate and were more active than females when food was unavailable, but tended to stay longer in environments containing food than did females. We found no significant relationship between weight and activity, suggesting the behavioral differences between males and females are caused by an intrinsic factor relating to the sex of a fly and not simply its body size. Finally, we observed a significant difference between the dispersal of the natural isolate used throughout this study and the widely-used laboratory strain, Canton-S, and show that the difference cannot be explained by allelic differences in the foraging gene
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