619 research outputs found
Recursive self-organizing map as a contractive iterative function system
Recently, there has been a considerable research activity in extending topographic maps of vectorial data to more general data structures, such as sequences or trees. However, the representational capabilities and internal representations of the models are not well understood. We rigorously analyze a generalization of the Self-Organizing Map (SOM) for processing sequential data, Recursive SOM (RecSOM [1]), as a non-autonomous dynamical system consisting off a set of fixed input maps. We show that contractive fixed input maps are likely to produce Markovian organizations of receptive fields o the RecSOM map. We derive bounds on parameter (weighting the importance of importing past information when processing sequences) under which contractiveness of the fixed input maps is guaranteed
Spectromicroscopy of electronic phase separation in KFeSe superconductor
Structural phase separation in AFeSe system has been studied
by different experimental techniques, however, it should be important to know
how the electronic uniformity is influenced, on which length scale the
electronic phases coexist, and what is their spatial distribution. Here, we
have used novel scanning photoelectron microscopy (SPEM) to study the
electronic phase separation in KFeSe, providing a direct
measurement of the topological spatial distribution of the different electronic
phases. The SPEM results reveal a peculiar interconnected conducting
filamentary phase that is embedded in the insulating texture. The filamentary
structure with a particular topological geometry could be important for the
high T superconductivity in the presence of a phase with a large magnetic
moment in AFeSe materials.Comment: 14 pages,3 figure
Three-Dimensional Fermi Surface of Overdoped La-Based Cuprates
We present a soft x-ray angle-resolved photoemission spectroscopy study of
the overdoped high-temperature superconductors LaSrCuO and
LaEuSrCuO. In-plane and out-of-plane components of
the Fermi surface are mapped by varying the photoemission angle and the
incident photon energy. No dispersion is observed along the nodal
direction, whereas a significant antinodal dispersion is identified.
Based on a tight-binding parametrization, we discuss the implications for the
density of states near the van-Hove singularity. Our results suggest that the
large electronic specific heat found in overdoped LaSrCuO can
not be assigned to the van-Hove singularity alone. We therefore propose quantum
criticality induced by a collapsing pseudogap phase as a plausible explanation
for observed enhancement of electronic specific heat
Influence of oxygen-coordination number on the electronic structure of single-layer La-based cuprates
We present an angle-resolved photoemission spectroscopy study of the
single-layer T*-type structured cuprate SmLaSrCuO with unique
five-fold pyramidal oxygen coordination. Upon varying oxygen content,
T*-SmLaSrCuO evolved from a Mott-insulating to a metallic state
where the Luttinger sum rule breaks down under the assumption of a large
hole-like Fermi surface. This is in contrast with the known doping evolution of
the structural isomer LaSrCuO with six-fold octahedral
coordination. In addition, quantitatively characterized Fermi surface suggests
that the empirical rule for octahedral oxygen-coordination
systems does not apply to T*-SmLaSrCuO. The present results
highlight unique properties of the T*-type cuprates possibly rooted in its
oxygen coordination, and necessitate thorough investigation with careful
evaluation of disorder effects.Comment: Accepted for publication in Phys. Rev.
Decoupling of Lattice and Orbital Degrees of Freedom in an Iron-Pnictide Superconductor
The interplay of structural and electronic phases in iron-based
superconductors is a central theme in the search for the superconducting
pairing mechanism. While electronic nematicity, defined as the breaking of
four-fold symmetry triggered by electronic degrees of freedom, is competing
with superconductivity, the effect of purely structural orthorhombic order is
unexplored. Here, using x-ray diffraction (XRD), we reveal a new structural
orthorhombic phase with an exceptionally high onset temperature
( K), which coexists with superconductivity
( K), in an electron-doped iron-pnictide superconductor far
from the underdoped region. Furthermore, our angle-resolved photoemission
spectroscopy (ARPES) measurements demonstrate the absence of electronic nematic
order as the driving mechanism, in contrast to other underdoped iron pnictides
where nematicity is commonly found. Our results establish a new, high
temperature phase in the phase diagram of iron-pnictide superconductors and
impose strong constraints for the modeling of their superconducting pairing
mechanism.Comment: SI available upon reques
Strain-Engineering Mott-Insulating LaCuO
The transition temperature of unconventional superconductivity
is often tunable. For a monolayer of FeSe, for example, the sweet spot is
uniquely bound to titanium-oxide substrates. By contrast for
LaSrCuO thin films, such substrates are
sub-optimal and the highest is instead obtained using
LaSrAlO. An outstanding challenge is thus to understand the optimal
conditions for superconductivity in thin films: which microscopic parameters
drive the change in and how can we tune them? Here we
demonstrate, by a combination of x-ray absorption and resonant inelastic x-ray
scattering spectroscopy, how the Coulomb and magnetic-exchange interaction of
LaCuO thin films can be enhanced by compressive strain. Our experiments
and theoretical calculations establish that the substrate producing the largest
under doping also generates the largest nearest neighbour
hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest
optimising the parent Mott state as a strategy for enhancing the
superconducting transition temperature in cuprates.Comment: 15 pages, 7 figures and 2 tables (including Supplementary
Information
Fate of charge order in overdoped La-based cuprates
In high-temperature cuprate superconductors, stripe order refers broadly to a coupled spin and charge modulation with a commensuration of eight and four lattice units, respectively. How this stripe order evolves across optimal doping remains a controversial question. Here we present a systematic resonant inelastic x-ray scattering study of weak charge correlations in LaSrCuO and LaEuSrCuO. Ultra high energy resolution experiments demonstrate the importance of the separation of inelastic and elastic scattering processes. Long-range temperature-dependent stripe order is only found below optimal doping. At higher doping, short-range temperature-independent correlations are present up to the highest doping measured. This transformation is distinct from and preempts the pseudogap critical doping. We argue that the doping and temperature-independent short-range correlations originate from unresolved electronâphonon coupling that broadly peaks at the stripe ordering vector. In LaSrCuO, long-range static stripe order vanishes around optimal doping and we discuss both quantum critical and crossover scenarios
Quantum Fluctuations in a Weakly Correlated Mott Insulator
Quantum fluctuations in low-dimensional systems and near quantum phase
transitions have significant influences on material properties. Yet, it is
difficult to experimentally gauge the strength and importance of quantum
fluctuations. Here we provide a resonant inelastic x-ray scattering study of
magnon excitations in Mott insulating cuprates. From the thin film of
SrCuO, single- and bi-magnon dispersions are derived. Using an effective
Heisenberg Hamiltonian generated from the Hubbard model, we show that the
single magnon dispersion is only described satisfactorily when including
significant renormalization stemming from quantum fluctuations. Comparative
results on LaCuO indicate that quantum fluctuations are much stronger
in SrCuO suggesting closer proximity to a magnetic quantum critical point.
Monte Carlo calculations suggest an exotic incommensurate magnetic order as the
ground state that competes with the antiferromagnetic N\'eel order. Our results
indicate that SrCuO -- due to strong quantum fluctuations -- is a unique
starting point for the exploration of novel magnetic ground states.Comment: Supplementary Information available upon reques
Uniaxial pressure induced stripe order rotation in La1.88Sr0.12CuO4
Static stripe order is detrimental to superconductivity. Yet, it has been proposed that transverse stripe fluctuations may enhance the inter-stripe Josephson coupling and thus promote superconductivity. Direct experimental studies of stripe dynamics, however, remain difficult. From a strong-coupling perspective, transverse stripe fluctuations are realized in the form of dynamic âkinksââsideways shifting stripe sections. Here, we show how modest uniaxial pressure tuning reorganizes directional kink alignment. Our starting point is La1.88Sr0.12CuO4 where transverse kink ordering results in a rotation of stripe order away from the crystal axis. Application of mild uniaxial pressure changes the ordering pattern and pins the stripe order to the crystal axis. This reordering occurs at a much weaker pressure than that to detwin the stripe domains and suggests a rather weak transverse stripe stiffness. Weak spatial stiffness and transverse quantum fluctuations are likely key prerequisites for stripes to coexist with superconductivity
High-Temperature Charge-Stripe Correlations in LaEuSrCuO
We use resonant inelastic x-ray scattering to investigate charge-stripe
correlations in LaEuSrCuO. By differentiating
elastic from inelastic scattering, it is demonstrated that charge-stripe
correlations precede both the structural low-temperature tetragonal phase and
the transport-defined pseudogap onset. The scattering peak amplitude from
charge stripes decays approximately as towards our detection limit.
The in-plane integrated intensity, however, remains roughly temperature
independent. Therefore, although the incommensurability shows a remarkably
large increase at high temperature, our results are interpreted via a single
scattering constituent. In fact, direct comparison to other stripe-ordered
compounds (LaBaCuO,
LaNdSrCuO and LaSrCuO)
suggests a roughly constant integrated scattering intensity across all these
compounds. Our results therefore provide a unifying picture for the
charge-stripe ordering in La-based cuprates. As charge correlations in
LaEuSrCuO extend beyond the low-temperature
tetragonal and pseudogap phase, their emergence heralds a spontaneous symmetry
breaking in this compound
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