| 1981. |
- Westerling, M., et al.
(författare)
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Photoexcitation dynamics in an alternating polyfluorene copolymer
- 2007
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Ingår i: Phys. Rev. B: Condens. Matter Mater. Phys. FIELD Full Journal Title:Physical Review B: Condensed Matter and Materials Physics. ; 75:22
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Tidskriftsartikel (refereegranskat)abstract
- The authors used transient photoinduced absorption on femtosecond to nanosecond time scales as well as delayed fluorescence up to microseconds to study the photogeneration and recombination of charges in thin films of the alternating polyfluorene copolymer poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]. The authors interpret the results using a coupled rate equation model and find that the authors can fit all exptl. results with a single set of parameters. The model includes prompt (
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| 1982. |
- Westerström, Rasmus, et al.
(författare)
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Lack of surface oxide layers and facile bulk oxide formation on Pd(110)
- 2009
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 80:12
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of the Pd(110) surface has been studied from ultrahigh vacuum up to atmospherical pressures by combining scanning tunneling microscopy, low-energy electron diffraction and high-resolution core-level spectroscopy with in situ surface x-ray diffraction, and density-functional theory calculations. Under in situ conditions, we observe a c(2x4) structure which transforms via the formation of antiphase domain boundaries to a "complex" structure with increasing partial oxygen pressure. Contrary to other closed packed and vicinal Pd surfaces investigated so far, no surface oxide is formed, which allows for the formation of the PdO bulk oxide closer to the thermodynamic limit at temperatures relevant for catalysis.
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| 1983. |
- Westerström, Rasmus, et al.
(författare)
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Oxidation and reduction of Pd(100) and aerosol-deposited Pd nanoparticles
- 2011
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969 .- 1098-0121. ; 83:11
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Tidskriftsartikel (refereegranskat)abstract
- Using in situ high-pressure x-ray photoelectron spectroscopy, we have followed the oxidation and the reduction of Pd model catalysts in oxygen and CO pressures in the millibar range. The study includes a Pd(100) single crystal as well as SiOx-supported Pd nanoparticles of 15 or 35 nm diameter, respectively. We demonstrate that nanoparticles also form ultrathin surface oxides prior to the onset of the bulk PdO. The Pd nanoparticles are observed to bulk oxidize at sample temperatures 40 degrees lower than the single-crystal surface. In the Pd 3d(5/2) and the O 1s spectrum, we identify a component corresponding to undercoordinated atoms at the surface of the PdO oxide. The experimentally observed PdO core-level shift is supported by density functional theory calculations. In a CO atmosphere, the Pd 3d(5/2) component corresponding to undercoordinated PdO atoms is shifted by + 0.55 eV with respect to PdO bulk, demonstrating that CO molecules preferably adsorb at these sites. CO coordinated to Pd atoms in the metallic and the oxidized phases can also be distinguished in the C 1s spectrum. The initial reduction by CO is similar for the single-crystal and the nanoparticle samples, but after the complete removal of the oxide we detect a significant deviation between the two systems, namely that the nanoparticles incorporate carbon to form a Pd carbide. Our results indicate that CO can dissociate on the nanoparticle samples, whereas no such behavior is observed for the Pd(100) single crystal. These results demonstrate the similarities, as well as the important differences, between the single crystals used as model systems for catalysis and nm-sized particles on oxide supports.
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| 1984. |
- Westerström, Rasmus, et al.
(författare)
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Tunneling, remanence, and frustration in dysprosium-based endohedral single-molecule magnets
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:6, s. 060406-
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Tidskriftsartikel (refereegranskat)abstract
- Paramagnetic atoms inside nanometer sized fullerenes realize robust, and chemically protected, spin systems. Changing the stoichiometry of the endohedral clusters results in a variety of magnetic ground states, as it is demonstrated for DynSc3-nN@C-80 (n = 1,2,3). All three exhibit distinct hysteresis and qualify as single-molecule magnets. In zero field the magnetization of n = 1 decays via quantum tunneling, while ferromagnetic coupling of the individual dysprosium moments results in remanence for Dy2ScN@C-80 and in a frustrated ground state for n = 3. The latter ground state turns out to be one of the simplest realizations of a frustrated, ferromagnetically coupled, system.
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| 1985. |
- Weston, Daniel, et al.
(författare)
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Classification of ground states and normal modes for phase-frustrated multicomponent superconductors
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:21, s. 214507-
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Tidskriftsartikel (refereegranskat)abstract
- We classify ground states and normal modes for n-component superconductors with frustrated intercomponent Josephson couplings, focusing on n=4. The results should be relevant not only to multiband superconductors, but also to Josephson-coupled multilayers and Josephson-junction arrays. It was recently discussed that three-component superconductors can break time-reversal symmetry as a consequence of phase frustration. We discuss how to classify frustrated superconductors with an arbitrary number of components. Although already for the four-component case there are a large number of different combinations of phase-locking and phase-antilocking Josephson couplings, we establish that there are a much smaller number of equivalence classes where properties of frustrated multicomponent superconductors can be mapped to each other. This classification is related to the graph-theoretical concept of Seidel switching. Numerically, we calculate ground states, normal modes, and characteristic length scales for the four-component case. We report conditions of appearance of new accidental continuous ground-state degeneracies.
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| 1986. |
- Wethekam, S., et al.
(författare)
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Resonant electron capture by C-60 ions at a metal surface with projected band gap
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:12, s. 121416-
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Tidskriftsartikel (refereegranskat)abstract
- C-60(+) and C-60(2+) ions are scattered under grazing incidence from an atomically clean and flat Be(0001) surface at kilo-electron-volt energies. Distances for electron transfer are deduced from shifts of angular distributions for incident C-60(+) and C-60(2+) projectiles, which reflect changes in the interaction potentials at the instants of electron transfers. These distances are consistent with classical over-the-barrier model results indicating that the suppression of charge transfer, observed for atomic projectiles in front of metal surfaces with a projected band gap, is absent for fullerenes.
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| 1987. |
- Wienholdt, S., et al.
(författare)
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Orbital-resolved spin model for thermal magnetization switching in rare-earth-based ferrimagnets
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:2, s. 020406-
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Tidskriftsartikel (refereegranskat)abstract
- The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices.
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| 1988. |
- Wierzbowska, M, et al.
(författare)
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Condensation energy of the homogeneous electron gas from density-functional theory for superconductors
- 2005
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 71:1, s. 1-014509
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Tidskriftsartikel (refereegranskat)abstract
- The condensation energy of homogeneous electron gas is calculated within the density-functional theory for superconductors. Purely electronic considerations include the exchange energy exactly and the correlation energy on a level of the random phase approximation. Singlet superconductivity is assumed, and the Coulomb interaction is studied with a model pairing potential at angular momentum up to l=9 and densities 1less than or equal tor(s)less than or equal to10. The homogeneous gas remains nonsuperconducting up to r(s)similar or equal to9. A very weak negative value of the condensation energy has been found for f waves and higher-l pairing at r(s)=10.
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| 1989. |
- Wierzbowska, M., et al.
(författare)
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Effect of electron correlations in Pd, Ni, and Co monowires
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:3
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the effect of mean-field electron correlations on the band electronic structure of Co, Ni, and Pd ultrathin monatomic nanowires, at the breaking point, by means of density-functional calculations in the self-interaction corrected (SIC) local density approximation (LDA) approach and alternatively by the LDA+U scheme. We find that adding static electron correlations increases the magnetic moment in Pd monowires, but has negligible effect on the magnetic moment in Co and Ni. Furthermore, the number of d-dominated conductance channels decreases somewhat compared to the LDA value, but the number of s-dominated channels is unaffected, and remains equal to one per spin.
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| 1990. |
- Wikberg, J. Magnus, et al.
(författare)
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Dimensionality crossover and frustrated spin dynamics on a triangular lattice
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:22, s. 224411-
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of the magnetic behavior of the layered oxide, LiNi0.65Co0.25Mn0.10O2, through ac and time-dependent susceptibility, dc linear and nonlinear susceptibility as well as neutron-diffraction measurements are presented. A ferrimagneticlike spin ordering appears at 119 K with a spontaneous magnetization coexisting with spin frustration in two dimensions (2D). At lower temperature, a cluster-glass transition is found at 17.4 K indicating a transformation to a completely frustrated state in three dimensions (3D). A dimensionality crossover with temperature, from 2D to 3D, in a magnetically frustrated system has been demonstrated. The observed magnetic behavior is believed to originate from a percolating system of spin clusters defined by disordered and frustrated exchange interactions and the findings conform well with predictions of the percolation cluster model.
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