276 research outputs found
Pressure-induced melting of the orbital polaron lattice in La1-xSrxMnO3
We report on the pressure effects on the orbital polaron lattice in the
lightly doped manganites , with . The
dependence of the orbital polaron lattice on chemical pressure is
studied by substituting Pr for La in
. In addition, we have studied
its hydrostatic pressure dependence in
. Our results strongly
indicate that the hopping significantly contributes to the stabilization of
the orbital polaron lattice and that the orbital polarons are ferromagnetic
objects which get stabilized by local double exchange processes. The analysis
of short range orbital correlations and the verification of the Grueneisen
scaling by hard x-ray, specific heat and thermal expansion data reinforces our
conclusions.Comment: 7 figure
Centers and Cocenters of -Hecke algebras
In this paper, we give explicit descriptions of the centers and cocenters of
-Hecke algebras associated to finite Coxeter groups.Comment: 13 pages, a mistake in 4.2 is correcte
Orbital polaron lattice formation in lightly doped La1-xSrxMnO3
By resonant x-ray scattering at the Mn K-edge on La7/8Sr1/8MnO3, we show that
an orbital polaron lattice (OPL) develops at the metal-insulator transition of
this compound. This orbital reordering explains consistently the unexpected
coexistence of ferromagnetic and insulating properties at low temperatures, the
quadrupling of the lattice structure parallel to the MnO2-planes, and the
observed polarization and azimuthal dependencies. The OPL is a clear
manifestation of strong orbital-hole interactions, which play a crucial role
for the colossal magnetoresistance effect and the doped manganites in general
Cu -edge Resonant Inelastic X-Ray Scattering in Edge-Sharing Cuprates
We present calculations for resonant inelastic x-ray scattering (RIXS) in
edge-shared copper oxide systems, such as CuGeO and LiCuO,
appropriate for hard x-ray scattering where the photoexcited electron lies
above oxygen 2p and copper 3d orbital energies. We perform exact
diagonalizations of the multi-band Hubbard and determine the energies, orbital
character and resonance profiles of excitations which can be probed via RIXS.
We find excellent agreement with recent results on LiCuO and
CuGeO in the 2-7 eV photon energy loss range.Comment: Updated with new data, expanded 9 pages, 9 figure
High-Performance Computer Algebra: A Hecke Algebra Case Study
We describe the first ever parallelisation of an algebraic computation at modern HPC scale. Our case study poses challenges typical of the domain: it is a multi-phase application with dynamic task creation and irregular parallelism over complex control and data structures.
Our starting point is a sequential algorithm for finding invariant bilinear forms in the representation theory of Hecke algebras, implemented in the GAP computational group theory system. After optimising the sequential code we develop a parallel algorithm that exploits the new skeleton-based SGP2 framework to parallelise the three most computationally-intensive phases. To this end we develop a new domain-specific skeleton, parBufferTryReduce. We report good parallel performance both on a commodity cluster and on a national HPC, delivering speedups up to 548 over the optimised sequential implementation on 1024 cores
The Simple Ree groups are determined by the set of their character degrees
Let be a finite group. Let be the set of all complex
irreducible character degrees of In this paper, we will show that if
where is the simple Ree group
then where is an abelian
group. This verifies Huppert's Conjecture for the simple Ree groups
when Comment: 14 pages, to appear in Journal of Algebr
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
A Comparison of Stripe Modulations in LaBaCuO and LaNdSrCuO
We report combined soft and hard x-ray scattering studies of the electronic
and lattice modulations associated with stripe order in
LaBaCuO and LaNdSrCuO. We
find that the amplitude of both the electronic modulation of the hole density
and the strain modulation of the lattice is significantly larger in
LaBaCuO than in LaNdSrCuO
and is also better correlated. The in-plane correlation lengths are isotropic
in each case; for LaBaCuO, \AA\
whereas for LaNdSrCuOF,
\AA. We find that the modulations are temperature independent in
LaBaCuO in the low temperature tetragonal phase. In
contrast, in LaNdSrCuO, the amplitude grows
smoothly from zero, beginning 13 K below the LTT phase transition. We speculate
that the reduced average tilt angle in LaBaCuO results
in reduced charge localization and incoherent pinning, leading to the longer
correlation length and enhanced periodic modulation amplitude.Comment: 6 pages, 4 figure
Stripe order of La1.64Eu0.2Sr0.16CuO4 in magnetic fields studied by resonant soft x ray scattering
We present results on the magnetic field dependence of the stripe order in La1.64Eu0.2Sr0.16CuO4 LESCO . Using resonant soft x ray scattering at the oxygen K edge to probe the 0.259,0,0.648 superlattice reflection, which is commonly associated to charge stripes, we found no pronounced difference in the wave vector, peak widths, and integrated intensity for magnetic fields up to B 6 T. This is in strong contrast to the behavior observed for La1.875Sr0.125CuO4, where a stabilization of the charge modulation in high magnetic fields has been demonstrate
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