| 1. |
- Andreasson, Jakob, 1975, et al.
(författare)
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Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 78:23, s. 235103-
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Tidskriftsartikel (refereegranskat)abstract
- We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed B-site orthorhombic (space group Pnma) perovskite LaFe0.5Cr0.5O3 but absent in isostructural LaFeO3 and LaCrO3. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the B site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, O-18 substitution is performed on the x=0, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in LaFeO3. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Frohlich interaction. For x=0.02 and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) and La1-ySryFe0.5Cr0.5O3-delta (y=0, 0.16, and 0.5) on the strong electron-phonon coupling present in LaFe0.5Cr0.5O3. The Franck-Condon effect in LaFe0.5Cr0.5O3, is not significantly affected by isovalent A-site substitution, despite the increasing orthorhombic distortion associated with decreasing A-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the A(g) and B-2g phonon modes in AFe(0.5)Cr(0.5)O(3) (A=La, Nd, and Gd) is made based on the present observations and published results for LaCrO3 and AMnO(3). The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.
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| 2. |
- Andreasson, Jakob, 1975
(författare)
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Avoided crossing of rattler modes in thermoelectric materials
- 2008
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Ingår i: Nature materials. ; 7, s. 811-815
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Tidskriftsartikel (refereegranskat)abstract
- Engineering of materials with specific physical properties has recently focused on the effect of nano-sized ‘guest domains’ in a‘host matrix’ that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is aprerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, withoutsimultaneously reducing the charge transport. This is named the ‘phonon glass–electron crystal’ concept and may be realized inhost–guest systems. The guest entities are believed to have independent oscillations, so-called rattlermodes,which scatter the acousticphonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electroncrystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predictedavoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected,and a new explanation for the low L is provided.
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| 3. |
- Andreasson, Jakob, 1975, et al.
(författare)
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Electron-lattice interactions in the perovskite LaFe0.5Cr0.5O3 characterized by optical spectroscopy and LDA plus U calculations
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 80:7, s. 075103-
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Tidskriftsartikel (refereegranskat)abstract
- We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), LDA+U calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite LaFe0.5Cr0.5O3. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first-and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Frohlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish LaFe0.5Cr0.5O3 as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.
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| 4. |
- Andreasson, Jakob, 1975
(författare)
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Electron-Phonon Interactions in Transition Metal Oxides Studied by Resonance Raman Scattering
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, studies of strong electron-phonon interactions in a number of transition metal oxides are presented. Two different electron-phonon interaction mechanisms are identified; Franck-Condon scattering and infrared active longitudinal optical (IR LO) two-phonon activation. The main experimental technique used is temperature dependent resonance Raman scattering and the electron-phonon interactions are studied by the resonant effects they cause on the first and higher order Raman active phonon scattering.The mixed transition metal oxide LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$ with orthorhombic perovskite structure has been studied in detail using resonance Raman scattering with incident photon energies between $\hbar\omega$=1.83 ($\lambda$=676 nm) and $\hbar\omega$=4.13 eV ($\lambda$=300 nm) and variable temperatures. It is established that the characteristic Franck-Condon multi-phonon scattering of a local oxygen breathing mode appears as the photon energy is tuned to the Fe-Cr charge transfer gaps. This interpretation is supported by results obtained from LaFe$_{1-x}$Cr$_{x}$O$_{3}$ (0$\leq$x$\leq$1) which show that the Franck-Condon resonance is critically sensitive to the presence of both Fe and Cr ions in the structure and by Sr-doping in La$_{1-x}$Sr$_{x}$Fe$_{0.5}$Cr$_{0.5}$O$_{3-\delta}$ which shows that the Franck-Condon resonance is sensitive to local effects. In addition to the Franck-Condon effect, Fr\"ohlich interaction induced IR LO two-phonon scattering is observed in the perovskites LaFeO$_{3}$ using $\lambda$=515 nm ($\hbar\omega$=2.41 eV) and LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$ using $\lambda$=334 nm ($\hbar\omega$=3.71 eV). Interestingly, these different resonance effects can be made to mix either by an introduction of a small amount of Cr in LaFeO$_{3}$ or by incident photon energy tuning in LaFe$_{0.5}$Cr$_{0.5}$O$_{3}$The results in the perovskites are complemented by observations in the spin ladder compound Sr$_{14}$Cu$_{24}$O$_{41}$ where an IR LO two-phonon resonance is activated for incident photon energies around the 1.8 eV charge transfer gap. In the spin ladder compound this resonance is linked to the formation of a charge density wave in the copper-oxide ladder layers below T=200 K.In combination, these observations illustrate the complexity of electron-phonon interactions in transition metal oxides and further establish resonance Raman scattering (in particular the higher order phonon scattering) as a sensitive probe of electron-phonon interactions and the relation between local and global effects in correlated material systems.
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| 5. |
- Andreasson, Jakob, 1975, et al.
(författare)
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Franck-Condon higher order lattice excitations in the LaFe(1-x)Cr(x)O3 (x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects
- 2007
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Ingår i: Physical Review B. ; 75, s. 104302-
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Tidskriftsartikel (refereegranskat)abstract
- First and higher order lattice excitiations in the B-site disordered perovskites LaFe(1-x)Cr(x)O3 (x = 0, 0.1, 0.5, 0.9, 1) and La(0.835)Sr(0.165)Fe(0.5)Cr(0.5)O(3-d) are investigated using temperature dependent and polarised inelastic light scattering [lambda = 515 nm (2.41 eV) and 676 nm (1.83 eV)] on oriented crystallites.A peak at approximately 2.4 eV in the imaginary part of the dielectric function of LaFe(0.5)Cr(0.5)O3 is assigned to a charge transfer from Fe 3+ (d5) to Cr 3+ (d3) ions and coupled the appearance of an intense Ag-like mode at approximately 700 cm-1 in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to 7th order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital mediated, electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active perovskite structured manganite LaMnO3. These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the LaFe(1-x)Cr(x)O(3) system as a model system for electron-phonon coupling and higher order Raman scattering in solids.
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| 6. |
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| 7. |
- Andrikopoulos, Prokopis C., et al.
(författare)
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Femtosecond-to-nanosecond dynamics of flavin mononucleotide monitored by stimulated Raman spectroscopy and simulations
- 2020
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 22:12, s. 6538-6552
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Tidskriftsartikel (refereegranskat)abstract
- Flavin mononucleotide (FMN) belongs to the large family of flavins, ubiquitous yellow-coloured biological chromophores that contain an isoalloxazine ring system. As a cofactor in flavoproteins, it is found in various enzymes and photosensory receptors, like those featuring the light-oxygen-voltage (LOV) domain. The photocycle of FMN is triggered by blue light and proceeds via a cascade of intermediate states. In this work, we have studied isolated FMN in an aqueous solution in order to elucidate the intrinsic electronic and vibrational changes of the chromophore upon excitation. The ultrafast transitions of excited FMN were monitored through the joint use of femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy encompassing a time window between 0 ps and 6 ns with 50 fs time resolution. Global analysis of the obtained transient visible absorption and transient Raman spectra in combination with extensive quantum chemistry calculations identified unambiguously the singlet and triplet FMN populations and addressed solvent dynamics effects. The good agreement between the experimental and theoretical spectra facilitated the assignment of electronic transitions and vibrations. Our results represent the first steps towards more complex experiments aimed at tracking structural changes of FMN embedded in light-inducible proteins upon photoexcitation.
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| 8. |
- Bielecki, Johan, 1982, et al.
(författare)
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Electrospray sample injection for single-particle imaging with x-ray lasers
- 2019
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Ingår i: Science advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 5:5
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Tidskriftsartikel (refereegranskat)abstract
- The possibility of imaging single proteins constitutes an exciting challenge for x-ray lasers. Despite encouraging results on large particles, imaging small particles has proven to be difficult for two reasons: not quite high enough pulse intensity from currently available x-ray lasers and, as we demonstrate here, contamination of the aerosolized molecules by nonvolatile contaminants in the solution. The amount of contamination on the sample depends on the initial droplet size during aerosolization. Here, we show that, with our electrospray injector, we can decrease the size of aerosol droplets and demonstrate virtually contaminant-free sample delivery of organelles, small virions, and proteins. The results presented here, together with the increased performance of next-generation x-ray lasers, constitute an important stepping stone toward the ultimate goal of protein structure determination from imaging at room temperature and high temporal resolution.
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| 9. |
- Budelmann, D., et al.
(författare)
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Antiferromagnetic and superconducting proximity effects in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices
- 2003
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 67, s. 140507-
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the interplay between the antiferromagnetic and superconducting order parameters in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices by inelastic light scattering. The ratio of superconducting to antiferromagnetic order is varied through different modulations 4/6, 4/9, and 4/12 of (Y/Pr)Ba2Cu3O7-delta layers. This allows us to identify the proximity effect of the superconducting order parameter into the antiferromagnetic barrier as signified, e.g., by the sharpening of the two-magnon excitation. This proximity effect as well as gap feature and phonon anomalies reveals the delicate interplay between superconducting and antiferromagnetic order parameters.
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| 10. |
- Dicke, B., et al.
(författare)
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Transferring the entatic-state principle to copper photochemistry
- 2018
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Ingår i: Nature Chemistry. - : Springer Science and Business Media LLC. - 1755-4349 .- 1755-4330. ; 10:3, s. 355-362
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Tidskriftsartikel (refereegranskat)abstract
- The entatic state denotes a distorted coordination geometry of a complex from its typical arrangement that generates an improvement to its function. The entatic-state principle has been observed to apply to copper electron-transfer proteins and it results in a lowering of the reorganization energy of the electron-transfer process. It is thus crucial for a multitude of biochemical processes, but its importance to photoactive complexes is unexplored. Here we study a copper complex-with a specifically designed constraining ligand geometry-that exhibits metal-to-ligand charge-transfer state lifetimes that are very short. The guanidine-quinoline ligand used here acts on the bis(chelated) copper(I) centre, allowing only small structural changes after photoexcitation that result in very fast structural dynamics. The data were collected using a multimethod approach that featured time-resolved ultraviolet-visible, infrared and X-ray absorption and optical emission spectroscopy. Through supporting density functional calculations, we deliver a detailed picture of the structural dynamics in the picosecond-to-nanosecond time range.
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