| 1. |
- De Angelis, D., et al.
(författare)
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A sub-100 nm thickness flat jet for extreme ultraviolet to soft X-ray absorption spectroscopy
- 2024
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Ingår i: Journal of Synchrotron Radiation. - : International Union of Crystallography (IUCr). - 0909-0495 .- 1600-5775. ; 31:3 Pt, s. 605-612
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Tidskriftsartikel (refereegranskat)abstract
- Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.
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| 2. |
- Ingle, R. A., et al.
(författare)
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Carbon K-edge x-ray emission spectroscopy of gas phase ethylenic molecules
- 2022
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Ingår i: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 55:4
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Tidskriftsartikel (refereegranskat)abstract
- We report on the C K-edge x-ray absorption spectra and the resonant (RXES) and non-resonant (NXES) x-ray emission spectra of ethylene, allene and butadiene in the gas phase. The RXES and NXES show clear differences for the different molecules. Overall both types of spectra are more structured for ethylene and allene, than for butadiene. Using density functional theory–restricted open shell configuration interaction single calculations, we simulate the spectra with remarkable agreement with the experiment. We identify the spectral features as being due to transitions involving localised 1s orbitals. For allene, there are distinct spectral bands that reflect transitions predominantly from either the central or terminal carbon atoms. These results are discussed in the context of ultrafast x-ray studies aimed at detecting the passage through conical intersections in polyatomic molecules.
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| 3. |
- Chergui, L., et al.
(författare)
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Superfluid-droplet crossover in a binary boson mixture on a ring : Exact diagonalization solutions for few-particle systems in one dimension
- 2023
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Ingår i: Physical Review A. - 2469-9926. ; 108:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the formation of self-bound quantum droplets in a one-dimensional binary mixture of bosonic atoms, applying the method of numerical diagonalization of the full Hamiltonian. The excitation spectra and ground-state pair correlations signal the formation of a few-boson droplet when crossing the region of critical interspecies interactions. The self-binding affects the rotational excitations, displaying a change in the energy dispersion from negative curvature, associated with superfluidity in the many-body limit, to a nearly parabolic curvature indicative of rigid body rotation. We exploit two global symmetries of the system to further analyze the few-body modes in terms of transition matrix elements and breathing mode dynamics. The ground and excited state energies reported here are exact within the respective Hilbert spaces, which have been adapted according to a given set of single-particle basis states and a specific importance threshold of the employed truncation method. We compare these data to the usual ad hoc inclusion of higher-order contributions in the extended Gross-Pitaevskii equation. For the given parameters we find a remarkable agreement between the few-body regime and the thermodynamic limit in one dimension.
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| 4. |
- Aspegren, Markus, et al.
(författare)
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Perfect Zeeman Anisotropy in Rotationally Symmetric Quantum Dots with Strong Spin-Orbit Interaction
- 2024
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Ingår i: Nano Letters. - 1530-6984. ; 24:26, s. 7927-7933
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Tidskriftsartikel (refereegranskat)abstract
- In nanoscale structures with rotational symmetry, such as quantum rings, the orbital motion of electrons combined with a spin-orbit interaction can produce a very strong and anisotropic Zeeman effect. Since symmetry is sensitive to electric fields, ring-like geometries provide an opportunity to manipulate magnetic properties over an exceptionally wide range. In this work, we show that it is possible to form rotationally symmetric confinement potentials inside a semiconductor quantum dot, resulting in electron orbitals with large orbital angular momentum and strong spin−orbit interactions. We find complete suppression of Zeeman spin splitting for magnetic fields applied in the quantum dot plane, similar to the expected behavior of an ideal quantum ring. Spin splitting reappears as orbital interactions are activated with symmetry-breaking electric fields. For two valence electrons, representing a common basis for spin-qubits, we find that modulating the rotational symmetry may offer new prospects for realizing tunable protection and interaction of spin-orbital states.
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| 5. |
- Mewes, L., et al.
(författare)
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Energy relaxation pathways between light-matter states revealed by coherent two-dimensional spectroscopy
- 2020
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Ingår i: Communications Physics. - : Springer Science and Business Media LLC. - 2399-3650. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Coupling matter excitations to electromagnetic modes inside nano-scale optical resonators leads to the formation of hybrid light-matter states, so-called polaritons, allowing the controlled manipulation of material properties. Here, we investigate the photo-induced dynamics of a prototypical strongly-coupled molecular exciton-microcavity system using broadband two-dimensional Fourier transform spectroscopy and unravel the mechanistic details of its ultrafast photo-induced dynamics. We find evidence for a direct energy relaxation pathway from the upper to the lower polariton state that initially bypasses the excitonic manifold of states, which is often assumed to act as an intermediate energy reservoir, under certain experimental conditions. This observation provides new insight into polariton photophysics and could potentially aid the development of applications that rely on controlling the energy relaxation mechanism, such as in solar energy harvesting, manipulating chemical reactivity, the creation of Bose-Einstein condensates and quantum computing. Recent spectroscopic studies have elucidated light-matter interactions in exciton-polaritons at room temperature, yet their precise excited-state dynamics remain unclear. Here, broadband 2D Fourier transform spectroscopy reveals the relaxation between polaritonic states and the role of dark states.
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| 6. |
- Milne, Christopher J., et al.
(författare)
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Disentangling the evolution of electrons and holes in photoexcited ZnO nanoparticles
- 2023
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Ingår i: Structural Dynamics. - : American Institute of Physics (AIP). - 2329-7778. ; 10:6
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy, and ab initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The x-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the x-ray absorption signal is found to rise in similar to 1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of similar to 100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice.
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| 7. |
- Mukherjee, K., et al.
(författare)
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Droplets and supersolids in ultra-cold atomic quantum gases
- 2023
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Ingår i: European Physical Journal: Special Topics. - 1951-6355. ; , s. 3417-3433
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Forskningsöversikt (refereegranskat)abstract
- In this mini-review, we briefly summarize some of the main concepts and ideas behind highly dilute self-bound quantum droplets of both binary and dipolar character. The latter type of systems has more recently led to the experimental discovery of a dipolar supersolid state that allows entirely new insights on this long-sought purely quantum state of matter, with exciting prospects for fundamental research as well as future applied quantum sensing technologies. The first half of the review provides a brief history of droplets and supersolidity in various settings and also discusses the self-binding in binary quantum gases, and the second half of the review summarizes our own recent work in the field, presented at the 2022 FQMT conference in Prague.
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| 15. |
- Viljur, Mari-Liis, et al.
(författare)
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The effect of natural disturbances on forest biodiversity : an ecological synthesis
- 2022
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Ingår i: Biological Reviews. - : Wiley. - 1464-7931 .- 1469-185X. ; 97:5, s. 1930-1947
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Tidskriftsartikel (refereegranskat)abstract
- Disturbances alter biodiversity via their specific characteristics, including severity and extent in the landscape, which act at different temporal and spatial scales. Biodiversity response to disturbance also depends on the community characteristics and habitat requirements of species. Untangling the mechanistic interplay of these factors has guided disturbance ecology for decades, generating mixed scientific evidence of biodiversity responses to disturbance. Understanding the impact of natural disturbances on biodiversity is increasingly important due to human-induced changes in natural disturbance regimes. In many areas, major natural forest disturbances, such as wildfires, windstorms, and insect outbreaks, are becoming more frequent, intense, severe, and widespread due to climate change and land-use change. Conversely, the suppression of natural disturbances threatens disturbance-dependent biota. Using a meta-analytic approach, we analysed a global data set (with most sampling concentrated in temperate and boreal secondary forests) of species assemblages of 26 taxonomic groups, including plants, animals, and fungi collected from forests affected by wildfires, windstorms, and insect outbreaks. The overall effect of natural disturbances on α-diversity did not differ significantly from zero, but some taxonomic groups responded positively to disturbance, while others tended to respond negatively. Disturbance was beneficial for taxonomic groups preferring conditions associated with open canopies (e.g. hymenopterans and hoverflies), whereas ground-dwelling groups and/or groups typically associated with shady conditions (e.g. epigeic lichens and mycorrhizal fungi) were more likely to be negatively impacted by disturbance. Across all taxonomic groups, the highest α-diversity in disturbed forest patches occurred under moderate disturbance severity, i.e. with approximately 55% of trees killed by disturbance. We further extended our meta-analysis by applying a unified diversity concept based on Hill numbers to estimate α-diversity changes in different taxonomic groups across a gradient of disturbance severity measured at the stand scale and incorporating other disturbance features. We found that disturbance severity negatively affected diversity for Hill number q = 0 but not for q = 1 and q = 2, indicating that diversity–disturbance relationships are shaped by species relative abundances. Our synthesis of α-diversity was extended by a synthesis of disturbance-induced change in species assemblages, and revealed that disturbance changes the β-diversity of multiple taxonomic groups, including some groups that were not affected at the α-diversity level (birds and woody plants). Finally, we used mixed rarefaction/extrapolation to estimate biodiversity change as a function of the proportion of forests that were disturbed, i.e. the disturbance extent measured at the landscape scale. The comparison of intact and naturally disturbed forests revealed that both types of forests provide habitat for unique species assemblages, whereas species diversity in the mixture of disturbed and undisturbed forests peaked at intermediate values of disturbance extent in the simulated landscape. Hence, the relationship between α-diversity and disturbance severity in disturbed forest stands was strikingly similar to the relationship between species richness and disturbance extent in a landscape consisting of both disturbed and undisturbed forest habitats. This result suggests that both moderate disturbance severity and moderate disturbance extent support the highest levels of biodiversity in contemporary forest landscapes.
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