| 11. |
- Bielecki, Johan, 1982
(författare)
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Structure and Dynamics in Transition Metal Perovskites — An Optical Spectroscopy Study
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transition metal oxides with the perovskite structure exhibit a vast number of exotic properties due to the large number of competing degrees of freedom at the Fermi surface. Together with couplings between and correlations within these degrees of freedom, this class of materials is of high interest from both a practical and theoretical point of view. In this thesis, the properties of four different perovskite systems and their relation to structure and dynamics is investigated: La1-x CaxMnO3, exhibiting the colossal magnetoresistance effect; LaCoO3, undergoing spin-state transitions influenced by complicated electron-lattice interactions; BiFeO3 that is of the only known room-temperature multiferroic materials; and the proton conductor BaInO3H.Two kinds of optical spectroscopy methods have been used to investigate the structure and dynamics of these transition metal perovskites. Raman spectroscopy measures low-energy excitations and is, in combination with group theoretical selection rules, a sensitive probe of electronic, atomic, and magnetic structure. Ultrafast time-resolved spectroscopy is used to study, in real time, fundamental dynamics and interaction mechanisms on the femtosecond timescale.Utilizing time resolved pump-probe reflectance spectroscopy, thermal and lattice contributions to the spin-state transition are temporarily decouple, and the first explicit observation of the high-spin repulsion in LaCoO3, first conjectured by Goodenough in the 1950's, is presented. This opens up a novel avenue for investigating the spin-state transition in LaCoO3. With the same experimental technique, the electron-lattice, lattice-spin, and electron-spin dynamics are investigated in La1-xCaxMnO3. Incorporating a two-component heat diffusion mechanism it is shown that the rate-equation based model captures all essentials of the >1 ps dynamics.Raman spectroscopy is used to investigate the structural phase-diagram of isovalently substituted BiFeO3. The vibrational frequencies as a function of substitution provides unique input into the controversial phonon assignments of BiFeO3 and the pronounced second-order scattering is explained in terms of LO-modes activated by the Fröhlich interaction. Further, the first phonon assignment is presented and the structural transition during dehydration is investigated for BaInO3H.Complementing the studies on perovskites, the electron-phonon interactions are investigated in a iron-pnictide high temperature superconductor by combining Raman spectroscopy and phonon-phonon calculations. Our results point towards weak electron-phonon couplings and supports the view that a non-phononic glue is responsible for the Cooper pair condensation in iron-pnictides.
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| 12. |
- Bielecki, Johan, 1982, et al.
(författare)
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Two-component heat diffusion observed in LaMnO3 and La0.7Ca0.3MnO3
- 2010
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Ingår i: Physical Review B Condensed Matter. - : Americal Physical Society. - 0163-1829 .- 1095-3795. ; 81:6, s. art. no. 064434-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We investigate the low-temperature electron, lattice, and spin dynamics of LaMnO3 (LMO) and La0.7Ca0.3MnO3 (LCMO) by resonant pump-probe reflectance spectroscopy. Probing the high-spin d-d transition as a function of time delay and probe energy, we compare the responses of the Mott insulator and the double-exchange metal to the photoexcitation. Attempts have previously been made to describe the subpicosecond dynamics of colossal magnetoresistance manganites in terms of a phenomenological three-temperature model describing the energy transfer between the electron, lattice, and spin subsystems followed by a comparatively slow exponential decay back to the ground state. However, conflicting results have been reported. Here we first show clear evidence of an additional component in the long-term relaxation due to film-to-substrate heat diffusion and then develop a modified three-temperature model that gives a consistent account for this feature. We confirm our interpretation by using it to deduce the band gap in LMO. In addition, we also model the nonthermal subpicosecond dynamics, giving a full account of all observed transient features both in the insulating LMO and the metallic LCMO.
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| 13. |
- Gorkhover, Tais, et al.
(författare)
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Femtosecond X-ray Fourier holography imaging of free-flying nanoparticles
- 2018
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Ingår i: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 12:3, s. 150-153
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast X-ray imaging on individual fragile specimens such as aerosols 1 , metastable particles 2 , superfluid quantum systems 3 and live biospecimens 4 provides high-resolution information that is inaccessible with conventional imaging techniques. Coherent X-ray diffractive imaging, however, suffers from intrinsic loss of phase, and therefore structure recovery is often complicated and not always uniquely defined 4,5 . Here, we introduce the method of in-flight holography, where we use nanoclusters as reference X-ray scatterers to encode relative phase information into diffraction patterns of a virus. The resulting hologram contains an unambiguous three-dimensional map of a virus and two nanoclusters with the highest lateral resolution so far achieved via single shot X-ray holography. Our approach unlocks the benefits of holography for ultrafast X-ray imaging of nanoscale, non-periodic systems and paves the way to direct observation of complex electron dynamics down to the attosecond timescale.
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| 14. |
- Granath, Mats, 1972, et al.
(författare)
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Anharmonic softening of Raman active phonons in iron-pnictides: Estimating the Fe isotope effect due to anharmonic expansion
- 2009
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Ingår i: Physical Review B. - 1098-0121. ; 79:23, s. 235103-
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Tidskriftsartikel (refereegranskat)abstract
- We present Raman measurements on the iron-pnictide superconductors CeFeAsO1-xFx and NdFeAsO1-xFx. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interactions, we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B-1g mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum, at least at small wave numbers, is well represented by phonon-phonon interactions without any significant electronic contribution. Even at zero temperature there is a non-negligent effect of interactions on the phonon energy, which for the Fe B-1g mode corresponds to 6 cm(-1) or 3% of the total energy of the mode. We also estimate the anharmonic expansion from Fe (56 -> 54) isotope substitution to Delta a approximate to 5.1 x 10(-4) angstrom and Delta d(Fe-As) approximate to 2.5 x 10(-4) angstrom and the shift of harmonic zero-point fluctuations of bond lengths less than or similar to 3 x 10(-5) angstrom(2), giving a total relative average decrease in electronic hopping integrals of vertical bar delta vertical bar/t less than or similar to 2.0 x 10(-4). For a nonphonon-mediated weak-coupling superconductor this gives an isotope exponent alpha similar to 10(-2). The results pose a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe isotope effect.
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| 15. |
- Hantke, Max Felix, et al.
(författare)
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Rayleigh-scattering microscopy for tracking and sizing nanoparticles in focused aerosol beams
- 2018
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Ingår i: IUCrJ. - 2052-2525. ; 5, s. 673-680
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Tidskriftsartikel (refereegranskat)abstract
- Ultra-bright femtosecond X-ray pulses generated by X-ray free-electron lasers (XFELs) can be used to image high-resolution structures without the need for crystallization. For this approach, aerosol injection has been a successful method to deliver 70-2000 nm particles into the XFEL beam efficiently and at low noise. Improving the technique of aerosol sample delivery and extending it to single proteins necessitates quantitative aerosol diagnostics. Here a lab-based technique is introduced for Rayleigh-scattering microscopy allowing us to track and size aerosolized particles down to 40 nm in diameter as they exit the injector. This technique was used to characterize the 'Uppsala injector', which is a pioneering and frequently used aerosol sample injector for XFEL single-particle imaging. The particle-beam focus, particle velocities, particle density and injection yield were measured at different operating conditions. It is also shown how high particle densities and good injection yields can be reached for large particles (100-500 nm). It is found that with decreasing particle size, particle densities and injection yields deteriorate, indicating the need for different injection strategies to extend XFEL imaging to smaller targets, such as single proteins. This work demonstrates the power of Rayleigh-scattering microscopy for studying focused aerosol beams quantitatively. It lays the foundation for lab-based injector development and online injection diagnostics for XFEL research. In the future, the technique may also find application in other fields that employ focused aerosol beams, such as mass spectrometry, particle deposition, fuel injection and three-dimensional printing techniques.
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| 16. |
- He, Wenxiao, 1985, et al.
(författare)
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Biomimetic nanocrystalline apatite resembling bone
- 2010
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Ingår i: European Cells and Materials: 3rd International NanoBio Conference 2010, ETH Zurich, Switzerland August 24-27 2010. - 1473-2262. ; 20:suppl. 3, s. 106-
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Konferensbidrag (refereegranskat)
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| 17. |
- He, Wenxiao, 1985, et al.
(författare)
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Formation of Bone-like Nanocrystalline Apatite Using Self-Assembled Liquid Crystals
- 2012
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 24:5, s. 892-902
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Tidskriftsartikel (refereegranskat)abstract
- A two-step process using liquid crystalline phases combined with controlled postcrystallization for the preparation of bone-like apatite has been developed. First, amorphous calcium phosphate (ACP) spherules with a diameter of 10.8 +/- 1.4 nm and specific surface area (SSA) in the range of 150-170 m(2)/g were synthesized within a reverse hexagonal liquid crystalline (LC) phase. Second, the ACP spherules were dispersed and aged in Milli-Qwater, where they crystallized into poorly crystalline apatite (PCA). The addition of heparin during aging was explored, which was shown to retard the ACP - PCA conversion. The particle formation within the LC phase was monitored using synchrotron small-angle X-ray scattering, and the formed materials were characterized by X-ray diffraction, conventional and high-resolution transmission electron microscopy, nitrogen adsorption, thermogravimetry with infrared-coupled analysis, and Raman spectroscopy. The PCA formed using the LC aging route presented bone-resembling features, such as,Ca2+ and OH- deficiency, CO32- substitution, poor crystallinity; and ultrahigh SSA of 356 m(2)/g. The resulting particles were compared to hydroxyapatite synthesized via a conventional water-based precipitation method. The LC-aging route exhibited excellent controllability over the CaP crystallization, which enabled facile tailoring of the resulting material properties for different types of application.
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| 18. |
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| 19. |
- Knee, Christopher, 1973, et al.
(författare)
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High Pressure Crystal and Magnetic Phase Transitions in Multiferroic Bi0.9La0.1FeO3
- 2014
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Ingår i: Chemistry of Materials. - : American Chemical Society (ACS). - 1520-5002 .- 0897-4756. ; 26:2, s. 1180-1186
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Tidskriftsartikel (refereegranskat)abstract
- The crystal and magnetic structures of multiferroic Bi0.9La0.1FeO3 have been studied using high resolution neutron powder diffraction in the pressure range 0-8 GPa. Two structural phase transitions are observed. The first, at similar to 1 GPa, transforms the polar R3c structure to an antipolar PbZrO3-like root 2a(p) x 2 root 2a(p) x 2a(p) perovskite superstructure; the second, at similar to 5 GPa, results in a smaller, root 2a(p) x root 2a(p) x 2a(p) unit cell and a structure described with Ibmm (nonstandard setting of Imma) symmetry, in which the a(-)a(-)b(0) octahedral tilt system is retained and the antipolar cation displacements lost. Accompanying the changes in the nuclear structure, the antiferromagnetic spin structure evolves from a cycloid, with a modulation length, lambda approximate to 770 angstrom, to collinear arrangements with the moments aligned along the b-axis (Pbam) and the a-axis (Ibmm) of the orthorhombic unit cells. In comparison with BiFeO3 the transition from a rhombohedral to an orthorhombic structure is suppressed by similar to 3 GPa, reflecting the dilution of the stereochemically active bismuth lone pair by lanthanum. A correlation between the cell contraction of Bi1-xLaxFeO3 (0.0
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| 20. |
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Overview of the JET results
- 2015
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 55:10
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Tidskriftsartikel (refereegranskat)
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