| 1. |
- Sillrén, Per, 1982, et al.
(författare)
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A statistical model of hydrogen bond networks in liquid alcohols
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 136:9, s. 094514-094521
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Tidskriftsartikel (refereegranskat)abstract
- We here present a statistical model of hydrogen bond induced network structures in liquid alcohols. The model generalises the Andersson-Schulz-Flory chain model to allow also for branched structures. Two bonding probabilities are assigned to each hydroxyl group oxygen, where the first is the probability of a lone pair accepting an H-bond and the second is the probability that given this bond also the second lone pair is bonded. The average hydroxyl group cluster size, cluster size distribution, and the number of branches and leaves in the tree-like network clusters are directly determined from these probabilities. The applicability of the model is tested by comparison to cluster size distributions and bonding probabilities obtained from Monte Carlo simulations of the monoalcohols methanol, propanol, butanol, and propylene glycol monomethyl ether, the di-alcohol propylene glycol, and the tri-alcohol glycerol. We find that the tree model can reproduce the cluster size distributions and the bonding probabilities for both mono- and poly-alcohols, showing the branched nature of the OH-clusters in these liquids. Thus, this statistical model is a useful tool to better understand the structure of network forming hydrogen bonded liquids. The model can be applied to experimental data, allowing the topology of the clusters to be determined from such studies.
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| 2. |
- Fu, Yifeng, 1984, et al.
(författare)
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Templated Growth of Covalently Bonded Three-Dimensional Carbon Nanotube Networks Originated from Graphene
- 2012
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Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 24:12, s. 1576-1581
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Tidskriftsartikel (refereegranskat)abstract
- A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.
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| 3. |
- Bielecki, Johan, 1982, et al.
(författare)
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Short-range structure of the brownmillerite-type oxide Ba2In2O5 and its hydrated proton-conducting form BaInO3H
- 2014
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 2:40, s. 16915-16924
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Tidskriftsartikel (refereegranskat)abstract
- The vibrational spectra and short-range structure of the brownmillerite-type oxide Ba2In2O6 and its hydrated form BaInO3H, are investigated by means of Raman, infrared, and inelastic neutron scattering spectroscopies together with density functional theory calculations. For Ba2In2O6, which may be described as an oxygen deficient perovskite structure with alternating layers of InO6 octahedra and InO4 tetrahedra, the results affirm a short-range structure of Icmm symmetry, which is characterized by random orientation of successive layers of InO4 tetrahedra. For the hydrated, proton conducting, form, BaInO3H, the results suggest that the short-range structure is more complicated than the P4/mbm symmetry that has been proposed previously on the basis of neutron diffraction, but rather suggest a proton configuration close to the lowest energy structure predicted by Martinez et al. [J.-R. Martinez, C. E. Moen, S. Stoelen, N. L. Allan, J. Solid State Chem., 180, 3388, (2007)]. An intense Raman active vibration at 150 cm(-1) is identified as a unique fingerprint of this proton configuration.
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| 4. |
- 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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| 5. |
- Bielecki, Johan, 1982, et al.
(författare)
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Structural and magnetic properties of isovalently substituted multiferroic BiFeO3 : Insights from Raman spectroscopy
- 2012
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 86:184422
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Tidskriftsartikel (refereegranskat)abstract
- Raman spectra, supplemented by powder x-ray diffraction and magnetization data of isovalently A- and B-site substituted BiFeO3 in the Bi1−xLaxFeO3 (0≤x≤1), Bi1−xTbxFeO3 (0≤x≤0.2), and Bi0.9Sm0.1Fe1−xMnxO3 (0≤x≤0.3) series, are presented. A good agreement between the structural transitions observed by x-ray diffraction and the vibrational modes observed in the Raman spectra is found over the whole substitutional ranges, and in particular we find spectroscopic signatures of a PbZrO3-type structure for Bi0.8La0.2FeO3. Mode assignments in the substituted materials are made based on Raman spectra of the end-members BiFeO3 and LaFeO3. Moreover, by comparing spectra from all samples with R3c structure, the phonon assignment in BiFeO3 is revisited. A close connection between the degree of octahedral tilt and the Raman shift of the A1 oxygen a−a−a− tilt mode is established. An explanation for the strong second-order scattering observed in Bi1−xLaxFeO3 and Bi1−xTbxFeO3 is suggested, including the assignment of the previously mysterious BiFeO3 mode at 620 cm−1. Finally, the magnetization data indicates a transition from a cycloidal modulated state towards a canted antiferromagnet with increasing A-site substitution, while Bi0.9Sm0.1Fe1−xMnxO3 with x=0 and 0.15 exhibit an anomalous closing of the hysteresis loop at low temperatures. For low A-site substitution levels (x≤0.1) the decreasing Raman intensity of the Fe derived modes correlates with the partial destruction of the spin cycloid as the substitution level increases.
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| 6. |
- 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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| 7. |
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| 8. |
- 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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| 9. |
- Bielecki, Johan, 1982, et al.
(författare)
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Femtosecond optical reflectivity measurements of lattice-mediated spin repulsions in photoexcited LaCoO3 thin films
- 2014
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121 .- 1550-235X. ; 89:3, s. art. no. 035129-
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Tidskriftsartikel (refereegranskat)abstract
- We present results on the temperature dependence of ultrafast electron and lattice dynamics, measured with pump-probe transient reflectivity experiments, of an epitaxially grown LaCoO3 thin film under tensile strain. Probing spin-polarized transitions into the antibonding e(g) band provides a measure of the low-spin fraction, both as a function of temperature and time after photoexcitation. It is observed that femtosecond laser pulses destabilize the constant low-spin fraction (similar to 63%-64%) in equilibrium into a thermally activated state, driven by a subpicosecond change in spin gap Delta. From the time evolution of the low-spin fraction, it is possible to disentangle the thermal and lattice contributions to the spin state. A lattice mediated spin repulsion, identified as the governing factor determining the equilibrium spin state in thin-film LaCoO3, is observed. These results suggests that time-resolved spectroscopy is a sensitive probe of the spin state in LaCoO3 thin films, with the potential to bring forward quantitative insight into the complicated interplay between structure and spin state in LaCoO3.
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| 10. |
- Bielecki, Johan, 1982
(författare)
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Optical Spectroscopy on Correlated Electron Materials
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Strongly correlated materials poses a great challenge to our microscopic understanding of matter, mainly due to the inability of perturbative methods to predict the properties of such materials. This inability stems from strong correlations between electronic, lattice and spin degrees of freedom, leading to a variety of complex and exotic phenomena such as (unconventional) superconductivity, metal-insulator transitions, non-Fermi liquid behavior and colossal magnetoresistance.In this thesis, optical spectroscopy investigations on two kinds of strongly correlated matter is presented: materials exhibiting colossal magnetoresistance (manganites) and superconductivity (iron-pnictides). The research has been done from a fundamental point of view, with the goal to advance our understanding of the fascinating interplay between electrons, phonons and spin fluctuation that is encountered in these materials. The electron-phonon coupling in the newly discovered iron-pnictide class of high temperature superconductors have been studied using Raman spectroscopy assisted by theoretical calculations with the goal to elucidate the role of phonons as the mediating boson between electrons in a Cooper-pair. Our results indicate that the phononic spectra can, to good accuracy, be explained by neglecting completely the electron-phonon coupling. This provides further credentials to the widespread opinion that magnetic fluctuations constitute the attractive force responsible for the destabilization of the Fermi-sea leading to the condensation into superconducting Cooper-pairs. Further analysis shows that, through F doping on the O site, ejecting electrons into the quasi-two dimensional Fe-As plane actually leads to hole-doping at the Fe site, in accord with previous first principle calculations.The fundamental dynamics between the electrons, lattice and spins in two manganite samples have been studied using sub-picosecond time resolved pump-probe reflectance spectroscopy: The parent compound LaMnO3 and the doped La0.7Ca0.3MnO3 exhibiting colossal magnetoresistance. Even though LaMnO3 is a semiconductor and La0.7Ca0.3MnO3 is a double-exchange metal at low temperatures, the observed femtosecond dynamics are very similar. We introduce a phenomenological model that can explain all observed features in both samples using a combination of thermal (>1ps) and non-thermal (
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