120 research outputs found
Resonant elastic soft x-ray scattering in oxygen-ordered YBa_2Cu_3O_{6+delta}
Static charge-density wave (CDW) and spin-density wave (SDW) order has been
convincingly observed in La-based cuprates for some time. However, more
recently it has been suggested by quantum oscillation, transport and
thermodynamic measurements that density wave order is generic to underdoped
cuprates and plays a significant role in YBa_2Cu_3O_{6+delta} (YBCO). We use
resonant soft x-ray scattering at the Cu L and O K edges to search for evidence
of density wave order in Ortho-II and Ortho-VIII oxygen-ordered YBCO. We report
a null result -- no evidence for static CDW order -- in both Ortho-II and
Ortho-VIII ordered YBCO. While this does not rule out static CDW order in the
CuO_2 planes of YBCO, these measurements place limits on the parameter space
(temperature, magnetic field, scattering vector) in which static CDW order may
exist. In addition, we present a detailed analysis of the energy and
polarization dependence of the Ortho-II superstructure Bragg reflection [0.5 0
0] at the Cu L edge. The intensity of this peak, which is due to the valence
modulations of Cu in the chain layer, is compared with calculations using
atomic scattering form factors deduced from x-ray absorption measurements. The
calculated energy and polarization dependence of the scattering intensity is
shown to agree very well with the measurement, validating the approach and
providing a framework for analyzing future resonant soft x-ray scattering
measurements.Comment: 11 pages, 9 figure
Total and Differential Cross Sections for the pp-->pp eta-prime Reaction Near Threshold
The eta-prime meson production in the reaction pp-->pp eta-prime has been
studied at excess energies of Q = 26.5, 32.5 and 46.6 MeV using the internal
beam facility COSY-11 at the cooler synchrotron COSY. The total cross sections
as well as one angular distribution for the highest Q-value are presented. The
excitation function of the near threshold data can be described by a pure
s-wave phase space distribution with the inclusion of the proton-proton final
state interaction and Coulomb effects. The obtained angular distribution of the
eta-prime mesons is also consistent with pure s-wave production.Comment: 6 pages, 5 figures, submitted to Eur. Phys. J.
Polyhedral models for generalized associahedra via Coxeter elements
Motivated by the theory of cluster algebras, F. Chapoton, S. Fomin and A.
Zelevinsky associated to each finite type root system a simple convex polytope
called \emph{generalized associahedron}. They provided an explicit realization
of this polytope associated with a bipartite orientation of the corresponding
Dynkin diagram.
In the first part of this paper, using the parametrization of cluster
variables by their -vectors explicitly computed by S.-W. Yang and A.
Zelevinsky, we generalize the original construction to any orientation. In the
second part we show that our construction agrees with the one given by C.
Hohlweg, C. Lange, and H. Thomas in the setup of Cambrian fans developed by N.
Reading and D. Speyer.Comment: 31 pages, 2 figures. Changelog: 20111106: initial version 20120403:
fixed errors in figures 20120827: revised versio
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
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The structure of blocks with a Klein four defect group
We prove Erdmann’s conjecture [16] stating that every block with a Klein four defect group has a simple module with trivial source, and deduce from this that Puig’s finiteness conjecture holds for source algebras of blocks with a Klein four defect group. The proof uses the classification of finite simple groups
A reevaluation of the coupling to a bosonic mode of the charge carriers in (Bi,Pb)SrCaCuO at the antinodal point
Angle-resolved photoemission spectroscopy (ARPES) is used to study the
spectral function of the optimally doped high-T superconductor
(Bi,Pb)SrCaCuO in the vicinity of the antinodal point
in the superconducting state. Using a parameterized self-energy function, it
was possible to describe both the coherent and the incoherent spectral weight
of the bonding and the antibonding band. The renormalization effects can be
assigned to a very strong coupling to the magnetic resonance mode and at higher
energies to a bandwidth renormalization by a factor of two, probably caused by
a coupling to a continuum. The present reevaluation of the ARPES data allows to
come to a more reliable determination of the value of the coupling strength of
the charge carriers to the mode. The experimental results for the dressing of
the charge carriers are compared to theoretical models.Comment: 12 pages, 7 figure
Electronic phase transitions in PrCaMnO epitaxial thin films revealed by resonant soft x-ray scattering
We report the study of magnetic and orbital order in
PrCaMnO epitaxial thin films grown on
(LaAlO)-(SrAlTaO) (LSAT) (011). In
a new experimental approach, the polarization and energy dependence of resonant
soft x-ray scattering are used to reveal significant modifications of the
magnetic order in the film as compared to the bulk, namely (i) a different
magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an
additional magnetic reordering transition at low temperatures. These
observations indicate a strong impact of the epitaxial strain on the spin
order, which is mediated by the orbital degrees of freedom and which provides a
promising route to tune the magnetic properties of manganite films. Our results
further demonstrate that resonant soft x-ray scattering is a very suitable
technique to study the magnetism in thin films, to which neutron scattering
cannot easily be applied due to the small sample volume.Comment: 5 pages, 3 figure
Optical study of orbital excitations in transition-metal oxides
The orbital excitations of a series of transition-metal compounds are studied
by means of optical spectroscopy. Our aim was to identify signatures of
collective orbital excitations by comparison with experimental and theoretical
results for predominantly local crystal-field excitations. To this end, we have
studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10,
ranging from early to late transition-metal ions, from t_2g to e_g systems, and
including systems in which the exchange coupling is predominantly
three-dimensional, one-dimensional or zero-dimensional. With the exception of
LaMnO3, we find orbital excitations in all compounds. We discuss the
competition between orbital fluctuations (for dominant exchange coupling) and
crystal-field splitting (for dominant coupling to the lattice). Comparison of
our experimental results with configuration-interaction cluster calculations in
general yield good agreement, demonstrating that the coupling to the lattice is
important for a quantitative description of the orbital excitations in these
compounds. However, detailed theoretical predictions for the contribution of
collective orbital modes to the optical conductivity (e.g., the line shape or
the polarization dependence) are required to decide on a possible contribution
of orbital fluctuations at low energies, in particular in case of the orbital
excitations at about 0.25 eV in RTiO3. Further calculations are called for
which take into account the exchange interactions between the orbitals and the
coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved
calculation of orbital excitation energies in TiOCl, figure 16 improved,
references updated, 33 pages, 20 figure
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