534 research outputs found
Magnetism and superconductivity at LAO/STO-interfaces: the role of Ti 3d interface electrons
Ferromagnetism and superconductivity are in most cases adverse. However,
recent experiments reveal that they coexist at interfaces of LaAlO3 and SrTiO3.
We analyze the magnetic state within density functional theory and provide
evidence that magnetism is not an intrinsic property of the two-dimensional
electron liquid at the interface. We demonstrate that the robust ferromagnetic
state is induced by the oxygen vacancies in SrTiO3- or in the LaAlO3-layer.
This allows for the notion that areas with increased density of oxygen
vacancies produce ferromagnetic puddles and account for the previous
observation of a superparamagnetic behavior in the superconducting state.Comment: 5 pages, 4 figures, to appear in Physical Review B (Rapid
Communications
Non conventional screening of the Coulomb interaction in low dimensional and finite size system
We study the screening of the Coulomb interaction in non polar systems by
polarizable atoms. We show that in low dimensions and small finite size systems
this screening deviates strongly from that conventionally assumed. In fact in
one dimension the short range interaction is strongly screened and the long
range interaction is anti-screened thereby strongly reducing the gradient of
the Coulomb interaction and therefore the correlation effects. We argue that
this effect explains the success of mean field single particle theories for
large molecules.Comment: 4 pages, 5 figure
The parenting task: parent's concerns and where they would seek help
Governments are concerned to promote positive parenting but it is difficult to know how and where to target the necessary support. How should we listen to the concerns expressed by parents themselves? Social work and health care professionals and those involved in developing parenting programmes tend to base their interventions on their experiences with families already in crisis. This paper reports on a survey of the views of two groups of parents: a community sample and a small group of parents involved in a young parent's project. Issues, which concern the parents, are identified as well as consideration of which agencies might be best placed to address these. Parents were most likely to approach their children's school or doctor for information, advice, or support. Parents were found to be reluctant to approach social work agencies
Electronic depth profiles with atomic layer resolution from resonant soft x-ray reflectivity
The analysis of x-ray reflectivity data from artificial heterostructures
usually relies on the homogeneity of optical properties of the constituent
materials. However, when the x-ray energy is tuned to an absorption edge, this
homogeneity no longer exists. Within the same material, spatial regions
containing elements at resonance will have optical properties very different
from regions without resonating sites. In this situation, models assuming
homogeneous optical properties throughout the material can fail to describe the
reflectivity adequately. As we show here, resonant soft x-ray reflectivity is
sensitive to these variations, even though the wavelength is typically large as
compared to the atomic distances over which the optical properties vary. We
have therefore developed a scheme for analyzing resonant soft x-ray
reflectivity data, which takes the atomic structure of a material into account
by "slicing" it into atomic planes with characteristic optical properties.
Using LaSrMnO4 as an example, we discuss both the theoretical and experimental
implications of this approach. Our analysis not only allows to determine
important structural information such as interface terminations and stacking of
atomic layers, but also enables to extract depth-resolved spectroscopic
information with atomic resolution, thus enhancing the capability of the
technique to study emergent phenomena at surfaces and interfaces.Comment: Completely overhauled with respect to the previous version due to
peer revie
Polarization dependence of x-ray absorption spectra in Na_xCoO_2
In order to shed light on the electronic structure of Na_xCoO_2, and
motivated by recent Co L-edge X-ray absorption spectra (XAS) experiments with
polarized light, we calculate the electronic spectrum of a CoO_6 cluster
including all interactions between 3d orbitals. We obtain the ground state for
two electronic occupations in the cluster that correspond nominally to all O in
the O^{-2} oxidation state, and Co^{+3} or Co^{+4}. Then, all excited states
obtained by promotion of a Co 2p electron to a 3d electron, and the
corresponding matrix elements are calculated. A fit of the observed
experimental spectra is good and points out a large Co-O covalency and cubic
crystal field effects, that result in low spin Co 3d configurations. Our
results indicate that the effective hopping between different Co atoms plays a
major role in determining the symmetry of the ground state in the lattice.
Remaining quantitative discrepancies with the XAS experiments are expected to
come from composition effects of itineracy in the ground and excited states.Comment: 10 pages, 4 figure
Linear Response Calculations of Lattice Dynamics in Strongly Correlated Systems
We introduce a new linear response method to study the lattice dynamics of
materials with strong correlations. It is based on a combination of dynamical
mean field theory of strongly correlated electrons and the local density
functional theory of electronic structure of solids. We apply the method to
study the phonon dispersions of a prototype Mott insulator NiO. Our results
show overall much better agreement with experiment than the corresponding local
density predictions.Comment: 4 pages, 2 figure
Spectroscopy of stripe order in La1.8Sr0.2NiO4 using resonant soft x-ray diffraction
Strong resonant enhancements of the charge-order and spin-order
superstructure-diffraction intensities in La1.8Sr0.2NiO4 are observed when
x-ray energies in the vicinity of the Ni L2,3 absorption edges are used. The
pronounced photon-energy and polarization dependences of these diffraction
intensities allow for a critical determination of the local symmetry of the
ordered spin and charge carriers. We found that not only the antiferromagnetic
order but also the charge-order superstructure resides within the NiO2 layers;
the holes are mainly located on in-plane oxygens surrounding a Ni2+ site with
the spins coupled antiparallel in close analogy to Zhang-Rice singlets in the
cuprates.Comment: 4 pages, 3 figure
Flux quantization and superfluid weight in doped antiferromagnets
Doped antiferromagnets, described by a t-t'-J model and a suitable 1/N
expansion, exhibit a metallic phase-modulated antiferromagnetic ground state
close to half-filling. Here we demonstrate that the energy of latter state is
an even periodic function of the external magnetic flux threading the square
lattice in an Aharonov-Bohm geometry. The period is equal to the flux quantum
entering the Peierls phase factor of the hopping
matrix elements. Thus flux quantization and a concomitant finite value of
superfluid weight D_s occur along with metallic antiferromagnetism. We argue
that in the context of the present effective model, whereby carriers are
treated as hard-core bosons, the charge q in the associated flux quantum might
be set equal to 2e. Finally, the superconducting transition temperature T_c is
related to D_s linearly, in accordance to the generic Kosterlitz-Thouless type
of transition in a two-dimensional system, signaling the coherence of the phase
fluctuations of the condensate. The calculated dependence of T_c on hole
concentration is qualitatively similar to that observed in the high-temperature
superconducting cuprates.Comment: 5 pages, 2 figures, to be published in J. Phys. Condens. Matte
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