16 research outputs found
Damping mechanism for the strongly renormalized -axis charge transport in high- cuprate superconductors
We analyze the -axis infrared reflectivity of
LaSrCuO single crystals. The plasma edge near 6 meV,
observed below , is due to Cooper-pair tunneling. This low value of the
plasma edge is consistent with the -axis plasma frequency () obtained
from LDA calculations ( eV) if we take into account that the
single-particle charge transport along the axis is strongly incoherent both
above and below . We find no evidence for a reduction of the -axis
scattering rate () below . Our investigation suggests
, which is exactly opposite to the clean limit.
VSGD.94.6.1Comment: 4 pages, figures on request. Revtex, version 2, Materials Science
Center Internal Report Number VSGD.94.6.
Ab Initio Evidence for the Formation of Impurity d(3z^2-r^2) Holes in Doped La_{2-x}Sr_xCuO_4
Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we
computed the electronic structure of explicitly doped La_{2-x}Sr_xCuO_4 (x =
0.125, 0.25, and 0.5). At each doping level, an impurity hole band is formed
within the undoped insulating gap. This band is well-localized to CuO_6
octahedra adjacent to the Sr impurities. The nature of the impurity hole is
A_{1g} in symmetry, formed primarily from the z^2 orbital on the Cu and p_z
orbitals on the apical O's. There is a strong triplet coupling of this hole
with the intrinsic B_{1g} Cu x^2-y^2/O1 p_{sigma} hole on the same site.
Optimization of the c coordinate of the apical O's in the doped CuO_6
octahedron lead to an asymmetric anti-Jahn-Teller distortion of the O2 atoms
toward the central Cu. In particular, the O2 atom between the Cu and Sr is
displaced 0.26 A while the O2 atom between the Cu and La is displaced 0.10 A.
Contrary to expectations, investigation of a 0.1 A enhanced Jahn-Teller
distortion of this octahedron does not force formation of an x^2-y^2 hole, but
instead leads to migration of the z^2 hole to the four other CuO_6 octahedra
surrounding the Sr impurity. This latter observation offers a simple
explanation for the bifurcation of the Sr-O2 distance revealed in x-ray
absorption fine structure data.Comment: Submitted to Phys. Rev. B. See http://www.firstprinciples.com for
more informatio
Effects of metallic spacer in layered superconducting Sr2(MgTi)O3FeAs
The highly two-dimensional superconducting system
Sr2(MgTi)O3FeAs, recently synthesized in the range of 0.2 < y <
0.5, shows an Mg concentration-dependent . Reducing the Mg concentration
from y=0.5 leads to a sudden increase in , with a maximum ~40 K at
y=0.2. Using first principles calculations, the unsynthesized stoichiometric
y=0 and the substoichiometric y=0.5 compounds have been investigated. For the
50% Mg-doped phase (y=0.5), Sr2(MgTi)O3 layers are completely
insulating spacers between FeAs layers, leading to the fermiology such as that
found for other Fe pnictides. At y=0, representing a phase with metallic
Sr2TiO3 layers, the -centered Fe-derived Fermi surfaces (FSs)
considerably shrink or disappear. Instead, three -centered Ti FSs
appear, and in particular two of them have similar size, like in MgB2.
Interestingly, FSs have very low Fermi velocity in large fractions: the lowest
being 0.6 cm/s. Furthermore, our fixed spin moment calculations
suggest the possibility of magnetic ordering, with magnetic Ti and nearly
nonmagnetic Fe ions. These results indicate a crucial role of
Sr2(MgTi)O3 layers in this superconductivity.Comment: 7 pages; Proceedings of ICSM-201
Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics
To help understand the high activity of silver as an oxidation catalyst,
e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of
methanol to formaldehyde, the interaction and stability of oxygen species at
the Ag(111) surface has been studied for a wide range of coverages. Through
calculation of the free energy, as obtained from density-functional theory and
taking into account the temperature and pressure via the oxygen chemical
potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a
thin surface-oxide structure is most stable for the temperature and pressure
range of ethylene epoxidation and we propose it (and possibly other similar
structures) contains the species actuating the catalysis. For higher
temperatures, low coverages of chemisorbed oxygen are most stable, which could
also play a role in oxidation reactions. For temperatures greater than about
775 K there are no stable oxygen species, except for the possibility of O atoms
adsorbed at under-coordinated surface sites Our calculations rule out thicker
oxide-like structures, as well as bulk dissolved oxygen and molecular
ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Valence fluctuations in CeIn 3 under the effect of pressure
71.20.-b Electron density of states and band structure of crystalline solids, 31.30.Gs Hyperfine interactions and isotope effects, 75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions,