175 research outputs found
Random walk on Sierpinski-type multifractals
A method is established which allows the calculation of the walk dimension
for Sierpinski-type multifractals. The multifractal scaling behaviour of the
average time needed to cover a distance in the mentionned multifractals is
shown. For the average-time-multifractal we calculate the Renyi dimensions and
allpy the f(alpha)-formalism.Comment: 9 pages, Postscrip
Pressure-driven magnetic moment collapse in the ground state of MnO
The zero temperature Mott transition region in antiferromagnetic, spin S=5/2
MnO is probed using the correlated band theory LSDA+U method. The first
transition encountered is an insulator-insulator volume collapse within the
rocksalt structure that is characterized by an unexpected Hund's rule violating
`spin-flip' moment collapse. This spin-flip to S=1/2 takes fullest advantage of
the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy
(which increases with decreasing volume) while retaining a sizable amount of
the magnetic exchange energy. While transition pressures vary with the
interaction strength, the spin-flip state is robust over a range of interaction
strengths and for both B1 and B8 structures
On the electronic structure of CaCuO2 and SrCuO2
Recent electronic structure calculations for the prototypical lowdimensional
cuprate compounds CaCuO2 ans SrCuO2 performed by Wu et. al. (J. Phys.: Condens.
Matter v. 11 p.4637 (1999))are critically reconsidered, applying high precision
full-potential bandstructure methods. It is shown that the bandstructure
calculations presented by the authors contain several important
inconsistencies, which make their main conclusions highly questionable.Comment: 4 pages, 3 figures, submitted to J. Phys. Condens. Matte
New insight into the physics of iron pnictides from optical and penetration depth data
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.Comment: 34 pages, 10 figures, submitted to New Journal of Physics
(30.01.2009
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