732 research outputs found
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
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
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
Revealing the Orbital Composition of Heavy Fermion Quasiparticles in CeRu2Si2
We present a resonant angle-resolved photoemission spectroscopy (ARPES) study of the electronic band structure and heavy fermion quasiparticles in CeRu2Si2. Using light polarization analysis, considerations of the crystal field environment and hybridization between conduction and f electronic states, we identify the d-electronic orbital character of conduction bands crossing the Fermi level. Resonant ARPES spectra suggest that the localized Ce f states hybridize with eg and t2g states around the zone center. In this fashion, we reveal the orbital structure of the heavy fermion quasiparticles in CeRu2Si2 and discuss its implications for metamagnetism and superconductivity in the related compound CeCu2Si2
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
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 electron–phonon 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
Electronic reconstruction forming a -symmetric Dirac semimetal in CaRuO
Electronic band structures in solids stem from a periodic potential
reflecting the structure of either the crystal lattice or an electronic order.
In the stoichiometric ruthenate CaRuO, numerous Fermi surface
sensitive probes indicate a low-temperature electronic reconstruction. Yet, the
causality and the reconstructed band structure remain unsolved. Here, we show
by angle-resolved photoemission spectroscopy, how in CaRuO a
-symmetric massive Dirac semimetal is realized through a Brillouin-zone
preserving electronic reconstruction. This Dirac semimetal emerges in a
two-stage transition upon cooling. The Dirac point and band velocities are
consistent with constraints set by quantum oscillation, thermodynamic, and
transport experiments, suggesting that the complete Fermi surface is resolved.
The reconstructed structure -- incompatible with
translational-symmetry-breaking density waves -- serves as an important test
for band structure calculations of correlated electron systems
Plasma Kallikrein Mediates Retinal Vascular Dysfunction and Induces Retinal Thickening in Diabetic Rats
Objective: Plasma kallikrein (PK) has been identified in vitreous fluid obtained from individuals with diabetic retinopathy and has been implicated in contributing to retinal vascular dysfunction. In this report, we examined the effects of PK on retinal vascular functions and thickness in diabetic rats. Research Design and Methods: We investigated the effects of a selective PK inhibitor, ASP-440, and C1 inhibitor (C1-INH), the primary physiological inhibitor of PK, on retinal vascular permeability (RVP) and hemodynamics in rats with streptozotocin-induced diabetes. The effect of intravitreal PK injection on retinal thickness was examined by spectral domain optical coherence tomography. Results: Systemic continuous administration of ASP-440 for 4 weeks initiated at the time of diabetes onset inhibited RVP by 42% (P = 0.013) and 83% (P < 0.001) at doses of 0.25 and 0.6 mg/kg per day, respectively. Administration of ASP-440 initiated 2 weeks after the onset of diabetes ameliorated both RVP and retinal blood flow abnormalities in diabetic rats measured at 4 weeks’ diabetes duration. Intravitreal injection of C1-INH similarly decreased impaired RVP in rats with 2 weeks’ diabetes duration. Intravitreal injection of PK increased both acute RVP and sustained focal RVP (24 h postinjection) to a greater extent in diabetic rats compared with nondiabetic control rats. Intravitreal injection of PK increased retinal thickness compared with baseline to a greater extent (P = 0.017) in diabetic rats (from 193 10 m to 223 13 m) compared with nondiabetic rats (from 182 8 m to 193 9 m). Conclusions: These results show that PK contributes to retinal vascular dysfunctions in diabetic rats and that the combination of diabetes and intravitreal injection of PK in rats induces retinal thickening
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