1,774 research outputs found
Raman scattering investigation across the magnetic and MI transition in rare earth nickelate RNiO3 (R = Sm, Nd) thin films
We report a temperature-dependent Raman scattering investigation of thin film
rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a
metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic
Neel transition at TN. Our results provide evidence that all investigated
samples present a structural phase transition at TMI but the Raman signature
across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3
and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the
paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is
characterized by a pronounced softening of one particular phonon band around
420 cm-1. This signature is unusual and points to an important and continuous
change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which
stabilizes upon cooling at the magnetic transition. The observed behaviour
might well be a general feature for all rare earth nickelates with TMI =/ TN
and illustrates intriguing coupling mechanism in the TMI > T > TN regime.Comment: Revised & published versio
Comparing hierarchies of total functionals
In this paper we consider two hierarchies of hereditarily total and
continuous functionals over the reals based on one extensional and one
intensional representation of real numbers, and we discuss under which
asumptions these hierarchies coincide. This coincidense problem is equivalent
to a statement about the topology of the Kleene-Kreisel continuous functionals.
As a tool of independent interest, we show that the Kleene-Kreisel functionals
may be embedded into both these hierarchies.Comment: 28 page
Random local strain effects in homovalent-substituted relaxor ferroelectrics: a first-principles study of BaTi0.74Zr0.26O3
We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a
prototype material for relaxors with a homovalent substitution. From a
statistical analysis of relaxed structures, we give evidence for four types of
Ti-atom polar displacements: along the , , or
directions of the cubic unit cell, or almost cancelled. The type of a Ti
displacement is entirely determined by the Ti/Zr distribution in the adjacent
unit cells. The underlying mechanism involves local strain effects that ensue
from the difference in size between the Ti4+ and Zr4+ cations. These results
shed light on the structural mechanisms that lead to disordered Ti
displacements in BaTi(1-x)Zr(x)O3 relaxors, and probably in other BaTiO3-based
relaxors with homovalent substitution.Comment: 5 pages, 4 figure
Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering
We report a Raman scattering investigation of columnar BiFeO3-CoFe2O4
(BFO-CFO) epitaxial thin film nanostructures, where BFO pillars are embedded in
a CFO matrix. The feasibility of a strain analysis is illustrated through an
investigation of two nanostructures with different BFO-CFO ratios. We show that
the CFO matrix presents the same strain state in both nanostructures, while the
strain state of the BFO pillars depends on the BFO/CFO ratio with an increasing
tensile strain along the out-of-plane direction with decreasing BFO content.
Our results demonstrate that Raman scattering allows monitoring strain states
in complex 3D multiferroic pillar/matrix composites.Comment: revised version submitted to Appl. Phys. Let
SmNiO3/NdNiO3 thin film multilayers
Rare earth nickelates RENiO3 which attract interest due to their sharp
metal-insulator phase transition, are instable in bulk form due to the
necessity of an important oxygen pressure to stabilize Ni in its 3+ state of
oxidation. Here, we report the stabilization of rare earth nickelates in
[(SmNiO3)t/(NdNiO3)t]n thin film multilayers, t being the thickness of layers
alternated n times. Both bilayers and multilayers have been deposited by
Metal-Organic Chemical Vapour Deposition. The multilayer structure and the
presence of the metastable phases SmNiO3 and NdNiO3 are evidenced from by X-ray
and Raman scattering. Electric measurements of a bilayer structure further
support the structural quality of the embedded rare earth nickelate layers.Comment: Appl. Phys. Lett. (2011), accepte
An algorithmic approach to the existence of ideal objects in commutative algebra
The existence of ideal objects, such as maximal ideals in nonzero rings,
plays a crucial role in commutative algebra. These are typically justified
using Zorn's lemma, and thus pose a challenge from a computational point of
view. Giving a constructive meaning to ideal objects is a problem which dates
back to Hilbert's program, and today is still a central theme in the area of
dynamical algebra, which focuses on the elimination of ideal objects via
syntactic methods. In this paper, we take an alternative approach based on
Kreisel's no counterexample interpretation and sequential algorithms. We first
give a computational interpretation to an abstract maximality principle in the
countable setting via an intuitive, state based algorithm. We then carry out a
concrete case study, in which we give an algorithmic account of the result that
in any commutative ring, the intersection of all prime ideals is contained in
its nilradical
Imaging Orbital-selective Quasiparticles in the Hund's Metal State of FeSe
Strong electronic correlations, emerging from the parent Mott insulator
phase, are key to copper-based high temperature superconductivity (HTS). By
contrast, the parent phase of iron-based HTS is never a correlated insulator.
But this distinction may be deceptive because Fe has five active d-orbitals
while Cu has only one. In theory, such orbital multiplicity can generate a
Hund's Metal state, in which alignment of the Fe spins suppresses inter-orbital
fluctuations producing orbitally selective strong correlations. The spectral
weights of quasiparticles associated with different Fe orbitals m should
then be radically different. Here we use quasiparticle scattering interference
resolved by orbital content to explore these predictions in FeSe. Signatures of
strong, orbitally selective differences of quasiparticle appear on all
detectable bands over a wide energy range. Further, the quasiparticle
interference amplitudes reveal that , consistent with
earlier orbital-selective Cooper pairing studies. Thus, orbital-selective
strong correlations dominate the parent state of iron-based HTS in FeSe.Comment: for movie M1, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M1.mp4, for movie M2, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M2.mp4, for movie M3, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M3.mp4, for movie M4, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M4.mp4, for movie M5, see
http://www.physik.uni-leipzig.de/~kreisel/osqp/M5.mp
Multi-Atom Quasiparticle Scattering Interference for Superconductor Energy-Gap Symmetry Determination
Complete theoretical understanding of the most complex superconductors
requires a detailed knowledge of the symmetry of the superconducting energy-gap
, for all momenta on the Fermi surface
of every band . While there are a variety of techniques for determining
, no general method existed to measure the signed
values of . Recently, however, a new technique based
on phase-resolved visualization of superconducting quasiparticle interference
(QPI) patterns centered on a single non-magnetic impurity atom, was introduced.
In principle, energy-resolved and phase-resolved Fourier analysis of these
images identifies wavevectors connecting all k-space regions where
has the same or opposite sign. But use of a single
isolated impurity atom, from whose precise location the spatial phase of the
scattering interference pattern must be measured is technically difficult. Here
we introduce a generalization of this approach for use with multiple impurity
atoms, and demonstrate its validity by comparing the
it generates to the determined from single-atom
scattering in FeSe where energy-gap symmetry is established. Finally,
to exemplify utility, we use the multi-atom technique on LiFeAs and find
scattering interference between the hole-like and electron-like pockets as
predicted for of opposite sign
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