8,926 research outputs found
Rich variety of defects in ZnO via an attractive interaction between O-vacancies and Zn-interstitials
As the concentration of intrinsic defects becomes sufficiently high in
O-deficient ZnO, interactions between defects lead to a significant reduction
in their formation energies. We show that the formation of both O-vacancies and
Zn-interstitials becomes significantly enhanced by a strong attractive
interaction between them, making these defects an important source of n-type
conductivity in ZnO.Comment: 12 pages, 4 figure
Photometric defocus observations of transiting extrasolar planets
We have carried out photometric follow-up observations of bright transiting
extrasolar planets using the CbNUOJ 0.6m telescope. We have tested the
possibility of obtaining high photometric precision by applying the telescope
defocus technique allowing the use of several hundred seconds in exposure time
for a single measurement. We demonstrate that this technique is capable of
obtaining a root-mean-square scatter of order sub-millimagnitude over several
hours for a V 10 host star typical for transiting planets detected from
ground-based survey facilities. We compare our results with transit
observations with the telescope operated in in-focus mode. High photometric
precision is obtained due to the collection of a larger amount of photons
resulting in a higher signal compared to other random and systematic noise
sources. Accurate telescope tracking is likely to further contribute to
lowering systematic noise by probing the same pixels on the CCD. Furthermore, a
longer exposure time helps reducing the effect of scintillation noise which
otherwise has a significant effect for small-aperture telescopes operated in
in-focus mode. Finally we present the results of modelling four light-curves
for which a root-mean-square scatter of 0.70 to 2.3 milli-magnitudes have been
achieved.Comment: 12 pages, 11 figures, 5 tables. Submitted to Journal of Astronomy and
Space Sciences (JASS
New attractor mechanism for spherically symmetric extremal black holes
We introduce a new attractor mechanism to find the entropy for spherically
symmetric extremal black holes. The key ingredient is to find a two-dimensional
(2D) dilaton gravity with the dilaton potential . The condition of an
attractor is given by and
and for a constant dilaton ,
these are also used to find the location of the degenerate horizon of
an extremal black hole. As a nontrivial example, we consider an extremal
regular black hole obtained from the coupled system of Einstein gravity and
nonlinear electrodynamics. The desired Bekenstein-Hawking entropy is
successfully recovered from the generalized entropy formula combined with the
2D dilaton gravity, while the entropy function approach does not work for
obtaining this entropy.Comment: 20 pages, 4 figures, Accepted for publication in Physical Review D.
This version includes revisions suggested by the refere
Quantum and Classical Spins on the Spatially Distorted Kagome Lattice: Applications to Volborthite
In Volborthite, spin-1/2 moments form a distorted Kagom\'e lattice, of corner
sharing isosceles triangles with exchange constants on two bonds and
on the third bond. We study the properties of such spin systems, and show that
despite the distortion, the lattice retains a great deal of frustration.
Although sub-extensive, the classical ground state degeneracy remains very
large, growing exponentially with the system perimeter. We consider degeneracy
lifting by thermal and quantum fluctuations. To linear (spin wave) order, the
degeneracy is found to stay intact. Two complementary approaches are therefore
introduced, appropriate to low and high temperatures, which point to the same
ordered pattern. In the low temperature limit, an effective chirality
Hamiltonian is derived from non-linear spin waves which predicts a transition
on increasing , from type order to a new
ferrimagnetic {\em striped chirality} order with a doubled unit cell. This is
confirmed by a large-N approximation on the O() model on this lattice. While
the saddle point solution produces a line degeneracy, corrections
select the non-trivial wavevector of the striped chirality state. The quantum
limit of spin 1/2 on this lattice is studied via exact small system
diagonalization and compare well with experimental results at intermediate
temperatures. We suggest that the very low temperature spin frozen state seen
in NMR experiments may be related to the disconnected nature of classical
ground states on this lattice, which leads to a prediction for NMR line shapes.Comment: revised, section V about exact diagonalization is extensively
rewritten, 17 pages, 11 figures, RevTex 4, accepted by Phys. Rev.
Thermodynamic duality between RN black hole and 2D dilaton gravity
All thermodynamic quantities of the Reissner-Nordstr\"om (RN) black hole can
be obtained from the dilaton and its potential of two dimensional (2D) dilaton
gravity. The dual relations of four thermodynamic laws are also established.
Furthermore, the near-horizon thermodynamics of the extremal RN black hole is
completely described by the Jackiw-Teitelboim theory which is obtained by
perturbing around the AdS-horizon.Comment: 10 pages, 3 figures, version accepted by MPL
- …