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| 2. |
- Andreasson, Jakob, 1975, et al.
(author)
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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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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 80:7, s. 075103-
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Journal article (peer-reviewed)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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| 3. |
- Andreasson, Jakob, 1975, et al.
(author)
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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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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 78:23, s. 235103-
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Journal article (peer-reviewed)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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| 4. |
- Bayat, A., et al.
(author)
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Universal single-frequency oscillations in a quantum impurity system after a local quench
- 2015
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In: Physical Review B. - 1098-0121. ; 92:15
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Journal article (peer-reviewed)abstract
- Universal single-frequency oscillations in the local nonequilibrium dynamics of a quantum many-body system are an exceptional phenomenon. In fact, till now, this has never been observed or predicted for the physically relevant case where a system is prepared to be quenched from one quantum phase to another. Here we show how the quench dynamics of the entanglement spectrum may reveal the emergence of such oscillations in a correlated quantum system with Kondo impurities. The oscillations we find are characterized by a single frequency. This frequency is independent of the amount of energy released by the local quench, and scales with the inverse system size. Importantly, the quench-independent frequency manifests itself also in local observables, such as the spin-spin correlation function of the impurities.
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| 5. |
- Bazzanella, Matteo, et al.
(author)
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Ferromagnetism in the one-dimensional Kondo lattice: Mean-field approach via Majorana fermion canonical transformation
- 2014
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 89:3
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Journal article (peer-reviewed)abstract
- Using a canonical transformation, it is possible to faithfully represent the Kondo lattice model in terms of Majorana fermions. Studying this representation, we discovered an exact mapping between the Kondo lattice Hamiltonian and a Hamiltonian describing three spinless fermions interacting on a lattice. This alternative form of the Hamiltonian is suitable for an immediate identification of the competing effects that operate in the Kondo lattice model. In particular, a term describing the double-exchange mechanism appears explicitly in the Hamiltonian. We investigate the effectiveness of this three-fermion representation by performing a zero-temperature mean-field study of the phase diagram at different couplings and fillings for the one-dimensional case, focusing on the appearance of ferromagnetism. The solutions show interesting features that agree in many respects with the known numerical and analytical results. In particular, in the ferromagnetic region connected to the solution at zero electron density, we have a quantitative agreement on the value of the “commensurability parameter” discovered in recent density matrix renormalization group (in one dimension) and dynamical mean-field theory (in infinite dimensions) simulations; furthermore, we provide a theoretical justification for it, identifying a symmetry of the Hamiltonian. This ferromagnetic phase is stabilized by the emergence of a spin-selective Kondo insulator, which is described quite conveniently by the three spinless fermions. We discover, however, that such a phase cannot be the correct description for all the ferromagnetic phases of the one-dimensional Kondo lattice model. We found in fact a different ferromagnetic phase at high filling and low couplings. This phase resembles the Ruderman-Kittel-Kasuya-Yosida (RKKY) ferromagnetic phase existing at vanishing filling, but it incorporates much more of the Kondo effect, making it energetically more favorable than the typical spiral (spin ordered) mean-field ground states. We believe that this second phase represents a prototype for the strange ferromagnetic tongue identified by numerical simulations inside the paramagnetic dome. At the end of the work, we also provide a discussion of possible orders different from the ferromagnetic one. In particular, at half-filling, where we obtain as ground state at high coupling the correct Kondo insulating state.
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| 6. |
- Bhattacharyya, Soumen, et al.
(author)
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Mass-selected photodissociation studies of AlPbn+ clusters (n=7-16): Evidence for the extraordinary stability of AlPb10+ and AlPb12
- 2013
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 87:5
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Journal article (peer-reviewed)abstract
- We report fragmentation pathways and dissociation energies of AlPbn+ (n=7–16) clusters. The clusters are produced with pulsed laser vaporization and studied in a supersonic molecular beam setup. They are mass selected and photodissociated with 532 and 355 nm laser light. Photofragments are thereafter mass separated in a tandem reflectron time-of-flight mass spectrometer. Bare Pbn+ (n = 8–16) clusters preferentially evaporate Pb atoms, with the exception of Pb15+ that fragments by loss of a Pb2 dimer to form the stable Pb13+ cluster. The smallest AlPbn+ (n = 7–11) clusters also show mainly atomic Pb evaporation, whereas the favored fragmentation pathway of the larger clusters (n = 12–16) involves Pb2 and Pb3 fragments. AlPb10+ and AlPb12+ are the most intense fragments of several larger cluster sizes, demonstrating the high stability of these two sizes. Dissociation energies corresponding to the most facile fragmentation channel of AlPbn+ (n = 11–15) are bracketed from the measured laser fluence dependencies of the fragment intensities using constraints imposed by unimolecular reaction rates. ©2013 American Physical Society
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| 7. |
- Bonetti, Stefano, et al.
(author)
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Power and linewidth of propagating and localized modes in nanocontact spin-torque oscillators
- 2012
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:17, s. 174427-
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Journal article (peer-reviewed)abstract
- The integrated power and linewidth of a propagating and a self-localized spin-wave mode excited by spin-polarized current in an obliquely magnetized magnetic nanocontact are studied experimentally as functions of the angle theta(e) between the external bias magnetic field and the nanocontact plane. It is found that the power of the propagating mode increases monotonically with theta(e), while the power of the self-localized mode has a broad maximum near theta(e) = 40 degrees and exponentially vanishes near the critical angle theta(e) = 58 degrees, at which the localized mode disappears. The linewidth of the propagating mode in the interval of angles 58 degrees < theta(e) < 90 degrees, where only this mode is excited, is adequately described by the existing theory, while in the angular interval where both modes can exist the observed linewidth of both modes is substantially broadened due to the telegraph switching between the modes. Numerical simulations and an approximate analytical model give a good semiquantitative description of the observed results.
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| 8. |
- Braunecker, B., et al.
(author)
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Magnetic-field switchable metal-insulator transitions in a quasihelical conductor
- 2013
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In: Physical Review B. - 1098-0121. ; 87:7
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Journal article (peer-reviewed)abstract
- We study Anderson localization in disordered helical conductors that are obtained from one-dimensional conductors with spin-orbit interaction and a magnetic field, or from equivalent systems. We call such conductors "quasihelical" because the spins of the counterpropagating modes are not perfectly antiparallel and have a small spin-wave-function overlap that is tunable by the magnetic field. Due to the overlap, disorder backscattering is possible and allows a localization transition. A conductor can pass through two localization transitions with increasing field, one from the conventionally localized system to the quasihelical conductor (with localization length exceeding the system length), and one at a higher field again to a localized state, due now, however, to backscattering below the magnetic-field induced pseudogap. We investigate these transitions using a unified two-step renormalization group approach. DOI: 10.1103/PhysRevB.87.075151
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| 9. |
- Breitholtz, Marcus, 1974, et al.
(author)
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Overlayer resonance and quantum well state of Cs/Cu(111) studied with angle-resolved photoemission, LEED, and first-principles calculations
- 2007
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In: Physical Review B. - 1098-0121. ; 75:15, s. 9-
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Journal article (peer-reviewed)abstract
- Angle-resolved photoemission spectroscopy and low-energy electron diffraction are used to study submonolayer coverages of Cs on Cu(111) at room temperature (RT) and 170 K. At RT, the Cs saturation coverage is approximately 90% of the full monolayer coverage. The full monolayer is characterized by a quantum well state (QWS) band having an energy of 25 meV below the Fermi level (E-F) in the (Gamma) over bar point and a resonance band extending to energies below the Cu band gap. This is supported by our first-principles calculations. Low-energy electron diffraction shows that the Cs overalyer forms a (2x2) structure over a wide coverage range, in which the QWS has energies from 50 meV above to 25 meV below E-F. The continued energy shift of the QWS after saturation of the diffraction angles is interpreted in terms of vacancies in the overlayer.
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| 10. |
- Bulgakova, Nadya, 1956, et al.
(author)
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Electronic transport and consequences for material removal in ultrafst pulsed laser ablation of materials
- 2004
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In: Physical Review B. - 1098-0121. ; 69
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Journal article (peer-reviewed)abstract
- Fast electronic transport is investigated theoretically based on a drift-diffusion approach for different classes of materials (metals, semiconductors, and dielectrics) under ultrafast, pulsed laser irradiation. The simulations are performed at intensities above the material removal threshold, characteristic for the ablation regime. The laser-induced charging of dielectric surfaces causes a subpicosecond electrostatic rupture of the superficial layers, an effect which, in comparison, is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties.
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