1,019 research outputs found
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Optimized Effective Potential Model for the Double Perovskites Sr2-xYxVMoO6 and Sr2-xYxVTcO6
In attempt to explore half-metallic properties of the double perovskites
Sr2-xYxVMoO6 and Sr2-xYxVTcO6, we construct an effective low-energy model,
which describes the behavior of the t2g-states of these compounds. All
parameters of such model are derived rigorously on the basis of
first-principles electronic structure calculations. In order to solve this
model we employ the optimized effective potential method and treat the
correlation interactions in the random phase approximation. Although
correlation interactions considerably reduce the intraatomic exchange splitting
in comparison with the Hartree-Fock method, this splitting still substantially
exceeds the typical values obtained in the local-spin-density approximation
(LSDA), which alters many predictions based on the LSDA. Our main results are
summarized as follows: (i) all ferromagnetic states are expected to be
half-metallic. However, their energies are generally higher than those of the
ferrimagnetic ordering between V- and Mo/Tc-sites (except Sr2VMoO6); (ii) all
ferrimagnetic states are metallic (except fully insulating Y2VTcO6) and no
half-metallic antiferromagnetism has been found; (iii) moreover, many of the
ferrimagnetic structures appear to be unstable with respect to the spin-spiral
alignment. Thus, the true magnetic ground state of the most of these systems is
expected to be more complex. In addition, we discuss several methodological
issues related to the nonuniqueness of the effective potential for the magnetic
half-metallic and insulating states.Comment: 15 pages, 9 figure
Local Spectral Weight of a Luttinger Liquid: Effects from Edges and Impurities
We calculate the finite-temperature local spectral weight (LSW) of a
Luttinger liquid with an "open" (hard wall) boundary. Close to the boundary the
LSW exhibits characteristic oscillations indicative of spin-charge separation.
The line shape of the LSW is also found to have a Fano-like asymmetry, a
feature originating from the interplay between electron-electron interaction
and scattering off the boundary. Our results can be used to predict how edges
and impurities influence scanning tunneling microscopy (STM) of one-dimensional
electron systems at low temperatures and voltage bias. Applications to STM on
single-walled carbon nanotubes are discussed.Comment: 15 pages, 10 figues, The latest version in pdf format is available at
http://www.physik.uni-kl.de/eggert/papers/LSW-LL.pd
Etiologic predictive value of a rapid immunoassay for detection of group A streptococcus antigen from throat swabs in patients presenting with a sore throat
Context: A sore throat is a common symptom mainly caused by virus but also by a variety of bacteria such as group A betahaemolytic streptococci (GAS) often resulting in unnecessary antibiotic prescribing. Combinations of symptoms and scores are not specific enough to accurately sort out aetiology. Rapid diagnostic antigen tests (RADT) have demonstrated high sensitivity and specificity in detecting presence of GAS.
Objective: Establish the probability that finding of GAS in a RADT shows a true link between symptoms and GAS while considering carriers of GAS ill from a virus.
Design: Cross-sectional study comparing two groups.
Setting: Emergency department (ED) also managing primary health care cases in a remote rural town with 22,000 residents.
Patients/Participants: 101 consecutive children aged 3-15 years attending for a sore throat as the main complaint and 147 consecutive children of the same age attending the same ED for other reasons than an infection.
Main And Secondary Outcome Measures: Positive and negative Etiologic Predictive Value (EPV).
Results: Positive EPV was 98% (88-100%). Negative EPV was 98% (97-99%). The positive EPV depends on setting and findings in this study and may not be transferable to other settings. It was mathematically shown that negative EPV found in this study is valid in all other reasonable settings and hence can be transferred to any other setting.
Conclusions: The evaluated RADT (Alere Test Pack+Plus With OBC Strep A) is always useful to rule out GAS infection in patients with an uncomplicated sore throat. It is often, depending on setting, useful to rule in a GAS infection in these patients
Etiologic predictive value of a rapid immunoassay for detection of group A streptococcus antigen from throat swabs in patients presenting with a sore throat
Context: A sore throat is a common symptom mainly caused by virus but also by a variety of bacteria such as group A betahaemolytic streptococci (GAS) often resulting in unnecessary antibiotic prescribing. Combinations of symptoms and scores are not specific enough to accurately sort out aetiology. Rapid diagnostic antigen tests (RADT) have demonstrated high sensitivity and specificity in detecting presence of GAS.
Objective: Establish the probability that finding of GAS in a RADT shows a true link between symptoms and GAS while considering carriers of GAS ill from a virus.
Design: Cross-sectional study comparing two groups.
Setting: Emergency department (ED) also managing primary health care cases in a remote rural town with 22,000 residents.
Patients/Participants: 101 consecutive children aged 3-15 years attending for a sore throat as the main complaint and 147 consecutive children of the same age attending the same ED for other reasons than an infection.
Main And Secondary Outcome Measures: Positive and negative Etiologic Predictive Value (EPV).
Results: Positive EPV was 98% (88-100%). Negative EPV was 98% (97-99%). The positive EPV depends on setting and findings in this study and may not be transferable to other settings. It was mathematically shown that negative EPV found in this study is valid in all other reasonable settings and hence can be transferred to any other setting.
Conclusions: The evaluated RADT (Alere Test Pack+Plus With OBC Strep A) is always useful to rule out GAS infection in patients with an uncomplicated sore throat. It is often, depending on setting, useful to rule in a GAS infection in these patients
Arbitrary Choice of Basic Variables in Density Functional Theory. II. Illustrative Applications
Our recent theory (Ref. 1) enables us to choose arbitrary quantities as the
basic variables of the density functional theory. In this paper we apply it to
several cases. In the case where the occupation matrix of localized orbitals is
chosen as a basic variable, we can obtain the single-particle equation which is
equivalent to that of the LDA+U method. The theory also leads to the
Hartree-Fock-Kohn-Sham equation by letting the exchange energy be a basic
variable. Furthermore, if the quantity associated with the density of states
near the Fermi level is chosen as a basic variable, the resulting
single-particle equation includes the additional potential which could mainly
modify the energy-band structures near the Fermi level.Comment: 27 page
Exact zero-point energy shift in the , many modes dynamic Jahn-Teller systems at strong coupling
We find the exact semiclassical (strong coupling) zero-point energy shifts
applicable to the and dynamic Jahn-Teller
problems, for an arbitrary number of discrete vibrational modes
simultaneously coupled to one single electronic level. We also obtain an
analytical formula for the frequency of the resulting normal modes, which has
an attractive and apparently general Slater-Koster form. The limits of validity
of this approach are assessed by comparison with O'Brien's previous
effective-mode approach, and with accurate numerical diagonalizations.
Numerical values obtained for with and coupling
constants appropriate to C are used for this purpose, and are
discussed in the context of fullerene.Comment: 20 pages, 4 ps figure
The influence of the rare earth ions radii on the Low Spin to Intermediate Spin state transition in lanthanide cobaltite perovskites: LaCoO3 vs. HoCoO3
We present first principles LDA+U calculations of electronic structure and
magnetic state for LaCoO3 and HoCoO3. Low Spin to Intermediate Spin state
transition was found in our calculations using experimental crystallographic
data for both materials with a much higher transition temperature for HoCoO3,
which agrees well with the experimental estimations. Low Spin state t6e0
(non-magnetic) to Intermediate Spin state t5e1 (magnetic) transition of Co(3+)
ions happens due to the competition between crystal field t_2g-e_g splitting
and effective exchange interaction between 3 spin-orbitals. We show that the
difference in crystal structure parameters for HoCoO3 and LaCoO3 due to the
smaller ionic radius of Ho ion comparing with La ion results in stronger
crystal field splitting for HoCoO3 (0.09 eV ~ 1000 K larger than for LaCoO3)
and hence tip the balance between the Low Spin and Intermediate Spin states to
the non-magnetic solution in HoCoO3.Comment: 13 pages, 6 figure
Boundary effects on one-particle spectra of Luttinger liquids
We calculate one-particle spectra for a variety of models of Luttinger
liquids with open boundary conditions. For the repulsive Hubbard model the
spectral weight close to the boundary is enhanced in a large energy range
around the chemical potential. A power law suppression, previously predicted by
bosonization, only occurs after a crossover at energies very close to the
chemical potential. Our comparison with exact spectra shows that the effects of
boundaries can partly be understood within the Hartree-Fock approximation.Comment: 4 pages including 4 figures, revised version, to be published in
Phys. Rev. B, January 200
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