17 research outputs found
Influence of the Fermi Surface Morphology on the Magnetic Field-Driven Vortex Lattice Structure Transitions in YBaCuO0, 0.15
We report small-angle neutron scattering measurements of the vortex lattice
(VL) structure in single crystals of the lightly underdoped cuprate
superconductor YBa2Cu3O6.85. At 2 K, and for fields of up to 16 T applied
parallel to the crystal c-axis, we observe a sequence of field-driven and
first-order transitions between different VL structures. By rotating the field
away from the c-axis, we observe each structure transition to shift to either
higher or lower field dependent on whether the field is rotated towards the
[100] or [010] direction. We use this latter observation to argue that the
Fermi surface morphology must play a key role in the mechanisms that drive the
VL structure transitions. Furthermore, we show this interpretation is
compatible with analogous results obtained previously on lightly overdoped
YBa2Cu3O7. In that material, it has long-been suggested that the high field VL
structure transition is driven by the nodal gap anisotropy. In contrast, the
results and discussion presented here bring into question the role, if any, of
a nodal gap anisotropy on the VL structure transitions in both YBa2Cu3O6.85 and
YBa2Cu3O7
Doping dependence of the chemical potential and surface electronic structure in YBa2Cu3O6+x and La2-xSrxCuO4 using hard x-ray photoemission spectroscopy
The electronic structure of YBa2Cu3O6+x and La2-xSrxCuO4 for various values
of x has been investigated using hard x-ray photoemission spectroscopy. The
experimental results establish that the cleaving of YBa2Cu3O6+x compounds
occurs predominantly in the BaCuO3 complex leading to charged surfaces at
higher x and to uncharged surfaces at lower x values. The bulk component of the
core level spectra exhibits a shift in binding energy as a function of x, from
which a shift of the chemical potential as a function of hole concentration in
the CuO2 layers could be derived. The doping dependence of the chemical
potential across the transition from a Mott-Hubbard insulator to a
Fermi-liquid-like metal is very different in these two series of compounds. In
agreement with previous studies in the literature the chemical potential shift
in La2-xSrxCuO4 is close to zero for small hole concentrations. In YBa2Cu3O6+x,
similar to all other doped cuprates studied so far, a strong shift of the
chemical potential at low hole doping is detected. However, the results for the
inverse charge susceptibility at small x shows a large variation between
different doped cuprates. The results are discussed in view of various
theoretical models. None of these models turns out to be satisfactory.Comment: 18 pages, 15 figure
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Valence-state reflectometry of complex oxide heterointerfaces
Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO3 film on NdGaO3 reveals a pronounced valence-state reconstruction from Co3+ in the bulk to Co2+ at the surface, with an areal density close to 0.5 Co2+ ions per unit cell. An identical film capped with polar (001) LaAlO3 maintains the Co3+ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e− per unit cell to the subsurface CoO2 layer at its LaO-terminated polar surface
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Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6Te10
The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi2Te4 and MnBi4Te7 benchmark the (MnBi2Te4)(Bi2Te3)n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi2Te4 septuple layers (SLs). However, the QAHE realization is complicated in MnBi2Te4 and MnBi4Te7 due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi2Te3 quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi6Te10 (n = 2) with Tc ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi6Te10 system as perspective for the QAHE at elevated temperatures
Author Correction: Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators
A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-
Orbital reflectometry
The occupation of d-orbitals controls the magnitude and anisotropy of the
inter-atomic electron transfer in transition metal oxides and hence exerts a
key influence on their chemical bonding and physical properties. Atomic-scale
modulations of the orbital occupation at surfaces and interfaces are believed
to be responsible for massive variations of the magnetic and transport
properties, but could thus far not be probed in a quantitative manner. Here we
show that it is possible to derive quantitative, spatially resolved orbital
polarization profiles from soft x-ray reflectivity data, without resorting to
model calculations. We demonstrate that the method is sensitive enough to
resolve differences of 3 % in the occupation of Ni e_g orbitals in adjacent
atomic layers of a LaNiO3-LaAlO3 superlattice, in good agreement with ab-initio
electronic-structure calculations. The possibility to quantitatively correlate
theory and experiment on the atomic scale opens up many new perspectives for
orbital physics in d-electron materials
Bringing the margin to the focus: 10 challenges for riparian vegetation science and management
Riparian zones are the paragon of transitional ecosystems, providing critical habitat and ecosystem services that are especially threatened by global change. Following consultation with experts, 10 key challenges were identified to be addressed for riparian vegetation science and management improvement: (1) Create a distinct scientific community by establishing stronger bridges between disciplines; (2) Make riparian vegetation more visible and appreciated in society and policies; (3) Improve knowledge regarding biodiversity—ecosystem functioning links; (4) Manage spatial scale and context-based issues; (5) Improve knowledge on social dimensions of riparian vegetation; (6) Anticipate responses to emergent issues and future trajectories; (7) Enhance tools to quantify and prioritize ecosystem services; (8) Improve numerical modeling and simulation tools; (9) Calibrate methods and increase data availability for better indicators and monitoring practices and transferability; and (10) Undertake scientific validation of best management practices. These challenges are discussed and critiqued here, to guide future research into riparian vegetation
Transitions in mental health care: The European Psychiatric Association contribution to reform in Bulgaria
Background: The Bulgarian Ministry of Health invited the European Psychiatric Association (EPA) to evaluate Bulgarian mental health care service provision in 2018. Bulgarian mental health services face very significant challenges including a legacy of historic underfunding, internal conflicts, poor planning, and the emigration of very high numbers of younger skilled staff that had followed accession to the European Union. There were significant disputes between stakeholders regarding the way forward and had been at least two unsuccessful previous external agency interventions that had attempted to find solutions. /
Method: This EPA position paper describes in detail the EPA mission to Bulgaria including methodology, findings, recommendations, and finally the positive actions and changes that are now underway as a result of the EPA report and intervention aimed at contributing towards improving Bulgarian mental health services. /
Results: After meetings with multiple stakeholders in the Bulgarian mental health system and analysis of data on service delivery, workforce, funding and configuration the EPA Panel agreed a list of twenty recommendations for change. /
Conclusions: The EPA mission, with the collaboration of multiple stakeholders in Bulgaria, was successful in stimulating high level government action to improve mental health services. Despite longstanding differences, it was possible to involve the stakeholders in constructive dialogue. The importance of “speaking with one voice” was a key lesson learned