| 1. |
- Ade, P. A. R., et al.
(författare)
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Planck 2015 results XVII. Constraints on primordial non-Gaussianity
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 594
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Tidskriftsartikel (refereegranskat)abstract
- The Planck full mission cosmic microwave background (CMB) temperature and E-mode polarization maps are analysed to obtain constraints on primordial non-Gaussianity (NG). Using three classes of optimal bispectrum estimators - separable template-fitting (KSW), binned, and modal we obtain consistent values for the primordial local, equilateral, and orthogonal bispectrum amplitudes, quoting as our final result from temperature alone f(NL)(local) = 2.5 +/- 5.7, f(NL)(equil) = 16 +/- 70, and f(NL)(ortho) = 34 +/- 33 (68% CL, statistical). Combining temperature and polarization data we obtain f(NL)(local) = 0.8 +/- 5.0, f(NL)(equil) = 4 +/- 43, and f(NL)(ortho) = 26 +/- 21 (68% CL, statistical). The results are based on comprehensive cross-validation of these estimators on Gaussian and non-Gaussian simulations, are stable across component separation techniques, pass an extensive suite of tests, and are consistent with estimators based on measuring the Minkowski functionals of the CMB. The effect of time-domain de-glitching systematics on the bispectrum is negligible. In spite of these test outcomes we conservatively label the results including polarization data as preliminary, owing to a known mismatch of the noise model in simulations and the data. Beyond estimates of individual shape amplitudes, we present model-independent, three-dimensional reconstructions of the Planck CMB bispectrum and derive constraints on early universe scenarios that generate primordial NG, including general single-field models of inflation, axion inflation, initial state modifications, models producing parity-violating tensor bispectra, and directionally dependent vector models. We present a wide survey of scale-dependent feature and resonance models, accounting for the look elsewhere effect in estimating the statistical significance of features. We also look for isocurvature NG, and find no signal, but we obtain constraints that improve significantly with the inclusion of polarization. The primordial trispectrum amplitude in the local model is constrained to be g(NL)(local) = (9.0 +/- 7.7) x 10(4) (68% CL statistical), and we perform an analysis of trispectrum shapes beyond the local case. The global picture that emerges is one of consistency with the premises of the Lambda CDM cosmology, namely that the structure we observe today was sourced by adiabatic, passive, Gaussian, and primordial seed perturbations.
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| 2. |
- Küspert, Julia, et al.
(författare)
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Pseudogap suppression by competition with superconductivity in La-based cuprates
- 2022
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Ingår i: Physical Review Research. - 2643-1564. ; 4:4
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Tidskriftsartikel (refereegranskat)abstract
- We carried out a comprehensive high-resolution angle-resolved photoemission spectroscopy (ARPES) study of the pseudogap interplay with superconductivity in La-based cuprates. The three systems La2-xSrxCuO4, La1.6-xNd0.4SrxCuO4, and La1.8-xEu0.2SrxCuO4 display slightly different pseudogap critical points in the temperature versus doping phase diagram. We studied the pseudogap evolution into the superconducting state for doping concentrations just below the critical point. In this setting, near optimal doping for superconductivity and in the presence of the weakest possible pseudogap, we uncover how the pseudogap is partially suppressed inside the superconducting state. This conclusion is based on the direct observation of a reduced pseudogap energy scale and re-emergence of spectral weight suppressed by the pseudogap. Altogether these observations suggest that the pseudogap phenomenon in La-based cuprates is in competition with superconductivity for antinodal spectral weight.
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| 3. |
- Sutter, D., et al.
(författare)
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Hallmarks of Hunds coupling in the Mott insulator Ca2RuO4
- 2017
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- A paradigmatic case of multi-band Mott physics including spin-orbit and Hund's coupling is realized in Ca2RuO4. Progress in understanding the nature of this Mott insulating phase has been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide-using angle-resolved photoemission electron spectroscopy-the band structure of the paramagnetic insulating phase of Ca2RuO4 and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund's coupling J = 0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the d(xy) orbital due to c-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund's coupling that together generate the Mott insulating state of Ca2RuO4.
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| 4. |
- Fischer, Hubertus, et al.
(författare)
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Palaeoclimate constraints on the impact of 2 °C anthropogenic warming and beyond
- 2018
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Ingår i: Nature Geoscience. - : Springer Science and Business Media LLC. - 1752-0894 .- 1752-0908. ; 11:7, s. 474-485
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Tidskriftsartikel (refereegranskat)abstract
- Over the past 3.5 million years, there have been several intervals when climate conditions were warmer than during the pre-industrial Holocene. Although past intervals of warming were forced differently than future anthropogenic change, such periods can provide insights into potential future climate impacts and ecosystem feedbacks, especially over centennial-to-millennial timescales that are often not covered by climate model simulations. Our observation-based synthesis of the understanding of past intervals with temperatures within the range of projected future warming suggests that there is a low risk of runaway greenhouse gas feedbacks for global warming of no more than 2 °C. However, substantial regional environmental impacts can occur. A global average warming of 1–2 °C with strong polar amplification has, in the past, been accompanied by significant shifts in climate zones and the spatial distribution of land and ocean ecosystems. Sustained warming at this level has also led to substantial reductions of the Greenland and Antarctic ice sheets, with sea-level increases of at least several metres on millennial timescales. Comparison of palaeo observations with climate model results suggests that, due to the lack of certain feedback processes, model-based climate projections may underestimate long-term warming in response to future radiative forcing by as much as a factor of two, and thus may also underestimate centennial-to-millennial-scale sea-level rise.
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| 5. |
- Horio, M., et al.
(författare)
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Electronic reconstruction forming a C-2-symmetric Dirac semimetal in Ca3Ru2O7
- 2021
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Ingår i: npj Quantum Materials. - : Springer Science and Business Media LLC. - 2397-4648. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Electronic band structures in solids stem from a periodic potential reflecting the structure of either the crystal lattice or electronic order. In the stoichiometric ruthenate Ca3Ru2O7, 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 Ca3Ru2O7 a C-2-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.
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| 6. |
- Horio, M., et al.
(författare)
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Oxide Fermi liquid universality revealed by electron spectroscopy
- 2020
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 102:24
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Tidskriftsartikel (refereegranskat)abstract
- We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr1.3-xLa0.7CexCuO4 (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi liquid cut-off energy scale, we find-in contrast to hole-doped cuprates-a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit-the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.
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| 7. |
- Kramer, K. P., et al.
(författare)
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Band structure of overdoped cuprate superconductors: Density functional theory matching experiments
- 2019
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:22
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive angle-resolved photoemission spectroscopy study of the band structure in singlelayer cuprates is presented with the aim of uncovering universal trends across different materials. Five different hole-and electron-overdoped cuprate superconductors (La1.59Eu0.2Sr0.21CuO4, La1.77Sr0.23CuO4, Bi1.74Pb0.38Sr1.88CuO6+delta, Tl2Ba2CuO6+delta, and Pr1.15La0.7Ce0.15CuO4) have been studied with special focus on the bands with a predominately d-orbital character. Using a light polarization analysis, the e(g) and t(2g) bands are identified across these materials. A clear correlation between the d(3z2-r2) band energy and the apical oxygen distance d(A) is demonstrated. Moreover, the compound dependence of the d(x2-y2) band bottom and the t(2g) band top is revealed. A direct comparison to density functional theory (DFT) calculations employing hybrid exchange-correlation functionals demonstrates excellent agreement. We thus conclude that the DFT methodology can be used to describe the global band structure of overdoped single-layer cuprates on both the hole-and electron-doped side.
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| 8. |
- Horio, M., et al.
(författare)
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Two-dimensional type-II Dirac fermions in layered oxides
- 2018
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Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La1.77Sr0.23CuO4. The Dirac point, constituting the crossing of d(x2-y2) and d(z2) bands, is found approximately one electronvolt below the Fermi level (E-F) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near EF.
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| 9. |
- Giménez-García, Angel, et al.
(författare)
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Pollination supply models from a local to global scale
- 2023
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Ingår i: Web Ecology. - 1399-1183. ; 23:2, s. 99-129
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Tidskriftsartikel (refereegranskat)abstract
- Ecological intensification has been embraced with great interest by the academic sector but is still rarely taken up by farmers because monitoring the state of different ecological functions is not straightforward. Modelling tools can represent a more accessible alternative of measuring ecological functions, which could help promote their use amongst farmers and other decision-makers. In the case of crop pollination, modelling has traditionally followed either a mechanistic or a data-driven approach. Mechanistic models simulate the habitat preferences and foraging behaviour of pollinators, while data-driven models associate georeferenced variables with real observations. Here, we test these two approaches to predict pollination supply and validate these predictions using data from a newly released global dataset on pollinator visitation rates to different crops. We use one of the most extensively used models for the mechanistic approach, while for the data-driven approach, we select from among a comprehensive set of state-of-The-Art machine-learning models. Moreover, we explore a mixed approach, where data-derived inputs, rather than expert assessment, inform the mechanistic model. We find that, at a global scale, machine-learning models work best, offering a rank correlation coefficient between predictions and observations of pollinator visitation rates of 0.56. In turn, the mechanistic model works moderately well at a global scale for wild bees other than bumblebees. Biomes characterized by temperate or Mediterranean forests show a better agreement between mechanistic model predictions and observations, probably due to more comprehensive ecological knowledge and therefore better parameterization of input variables for these biomes. This study highlights the challenges of transferring input variables across multiple biomes, as expected given the different composition of species in different biomes. Our results provide clear guidance on which pollination supply models perform best at different spatial scales-the first step towards bridging the stakeholder-Academia gap in modelling ecosystem service delivery under ecological intensification.
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| 10. |
- Heindl, M. W., et al.
(författare)
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Strong Induced Circular Dichroism in a Hybrid Lead-Halide Semiconductor Using Chiral Amino Acids for Crystallite Surface Functionalization
- 2022
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Ingår i: Advanced Optical Materials. - : Wiley. - 2162-7568 .- 2195-1071. ; 10:14, s. 2200204-
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Tidskriftsartikel (refereegranskat)abstract
- Chirality is a desired property in functional semiconductors for optoelectronic, catalytic, and spintronic applications. Here, introducing enantiomerically-pure 3-aminobutyric acid (3-ABA) into thin films of the 1D semiconductor dimethylammonium lead iodide (DMAPbI3) is found to result in strong circular dichroism (CD) in the optical absorption. X-ray diffraction and grazing incidence small angle X-ray scattering (GISAXS) are applied to gain molecular-scale insights into the chirality transfer mechanism, which is attributed to a chiral surface modification of DMAPbI3 crystallites. This study demonstrates that the CD signal strength can be controlled by the amino-acid content relative to the crystallite surface area. The CD intensity is tuned by the composition of the precursor solution and the spin-coating time, thereby achieving anisotropy factors (gabs) as high as 1.75 × 10–2. Grazing incidence wide angle scattering reveals strong preferential ordering that can be suppressed via tailored synthesis conditions. Different contributions to the chiroptical properties are resolved by a detailed analysis of the CD signal utilizing an approach based on the Mueller matrix model. This report of a novel class of chiral hybrid semiconductors with precise control over their optical activity presents a promising approach for the design of circularly polarized light detectors and emitters.
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