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| 3. |
- Ndiaye, W., et al.
(författare)
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Bulk electronic structure of Mn5Ge3/Ge(111) films by angle-resolved photoemission spectroscopy
- 2013
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 87:16
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Tidskriftsartikel (refereegranskat)abstract
- Mn5Ge3(001) thin films grown on Ge(111)-c(2 x 8) reconstructed surfaces were studied by angle-resolved photoemission using synchrotron radiation in the 14-94 eV photon energy range. The results obtained in the Gamma ALM plane and in the Gamma AHK plane are in agreement with simulations starting with band structure calculations based on the density functional theory. This provides a unique validation of band structure calculations for a proper description of the electronic properties of Mn5Ge3. Only the spectral feature very close to the Fermi level cannot be well explained by the simulation. This departure is discussed in terms of the three-dimensional nature of the sample and of correlation effects.
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| 4. |
- Ndiaye, W., et al.
(författare)
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k dependence of the spin polarization in Mn5Ge3/Ge(111) thin films
- 2015
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 91:12
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Tidskriftsartikel (refereegranskat)abstract
- Mn5Ge3(001) thin films grown on Ge(111) were studied by angle-and spin-resolved photoemission using synchrotron radiation in the 17-40 eV photon energy range. The photoelectron spectra were simulated starting from a first-principles band-structure calculation for the ground state, using the free-electron approximation for the final states, taking into account photohole lifetime effects and k(perpendicular to) broadening plus correlation effects, but ignoring transition matrix elements. The measured spin polarizations for the various k points investigated in the Gamma MLA plane of the Brillouin zone are found to be in fair enough agreement with the simulated ones, providing a strong support to the ground-state band-structure calculations. Possible origins for the departures between either simulations and experiments or previous and present experiments are discussed.
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- Garnier, J., et al.
(författare)
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Toward a wave turbulence formulation of statistical nonlinear optics
- 2012
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Ingår i: Journal of the Optical Society of America B: Optical Physics. - 1520-8540 .- 0740-3224. ; 29:8, s. 2229-2242
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Tidskriftsartikel (refereegranskat)abstract
- During this last decade, several remarkable phenomena inherent to the nonlinear propagation of incoherent optical waves have been reported in the literature. This article is aimed at providing a generalized wave turbulence kinetic formulation of random nonlinear waves governed by the nonlinear Schrodinger equation in the presence of a nonlocal or a noninstantaneous nonlinear response function. Depending on the amount of nonlocal (noninstantaneous) nonlinear interaction and the amount of inhomogeneous (nonstationary) statistics of the incoherent wave, different types of kinetic equations are obtained. In the spatial domain, when the incoherent wave exhibits fluctuations that are statistically homogeneous in space, the relevant kinetic equation is the wave turbulence (Hasselmann) kinetic equation. It describes, in particular, the process of optical wave thermalization to thermodynamic equilibrium, which slows down significantly as the interaction becomes highly nonlocal. When the incoherent wave is characterized by inhomogeneous statistical fluctuations, different forms of the Vlasov equation are derived, which depend on the amount of nonlocality in the system. This Vlasov approach describes, in particular, the processes of incoherent modulational instability and the formation of localized incoherent soliton structures. In the temporal domain, the noninstantaneous nonlinear response function is constrained by the causality condition. It turns out that the relevant kinetic equation has a form analogous to the weak Langmuir turbulence equation, which describes, in particular, the formation of nonlocalized spectral incoherent solitons. In the regime of a highly noninstantaneous nonlinear response and a stationary statistics of the incoherent wave, the weak Langmuir turbulence equation reduces to the Korteweg-de Vries equation. Conversely, in the regime of a highly noninstantaneous response in the presence of a nonstationary statistics, we derive a long-range Vlasov-like kinetic equation in the temporal domain, whose self-consistent potential is constrained by the causality condition. From a broader perspective, this work indicates that the wave turbulence theory may constitute the appropriate theoretical framework to formulate statistical nonlinear optics.
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- Hellsvik, J., et al.
(författare)
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Tuning order-by-disorder multiferroicity in CuO by doping
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:1, s. 014437-
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Tidskriftsartikel (refereegranskat)abstract
- The high Curie temperature multiferroic compound CuO has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so-called "order-by-disorder" mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows us to stabilize the multiferroic phase in nonferroelectric regions of the pristine material phase diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow us to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of multiferroic materials. In addition we present a complete explanation of the CuO phase diagram and a computation on the incommensurability in excellent agreement with experiment without free parameters.
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| 7. |
- Picozzi, S., et al.
(författare)
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Dual nature of improper ferroelectricity in a magnetoelectric multiferroic
- 2007
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 99:22, s. 227201-
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Tidskriftsartikel (refereegranskat)abstract
- Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.
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| 8. |
- Picozzi, Silvia, et al.
(författare)
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Microscopic mechanisms for improper ferroelectricity in multiferroic perovskites : a theoretical review
- 2008
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 20:43, s. 434208-
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Forskningsöversikt (refereegranskat)abstract
- Two microscopic mechanisms helping us to understand the multiferroic behavior of distorted rare-earth manganites are here briefly reviewed. The original work was carried out by means of Hamiltonian modeling and first-principles density functional simulations. Our first topic concerns the link between the Dzyaloshinskii-Moriya interaction and ferroelectricity in incommensurate magnets. We argue that the Dzyaloshinskii-Moriya interaction may play a key role since (i) it induces ferroelectric displacements of oxygen atoms and (ii) it favors the stabilization of a helical magnetic structure at low temperatures. Our second topic concerns the prediction, based on Landau theory, that the symmetry of the zigzag spin chains in the AFM-E (E-type antiferromagnetic) orthorhombic manganites (such as HoMnO3) allows a finite polarization along the c axis. The microscopic mechanism at the basis of ferroelectricity is interpreted through a gain in band energy of the e(g) electrons within the orbitally degenerate double-exchange model. Related Monte Carlo simulations have confirmed that the polarization can be much higher than what is observed in spiral magnetic phases. Density functional calculations performed on orthorhombic HoMnO3 quantitatively confirm a magnetically induced ferroelectric polarization up to similar to 6 mu C cm(-2), the largest reported so far for improper magnetic ferroelectrics. We find in HoMnO3, in addition to the conventional displacement mechanism, a sizable contribution arising from the purely electronic effect of orbital polarization. The relatively large ferroelectric polarization, present even with centrosymmetric atomic positions, is a clear sign of a magnetism-induced electronic mechanism at play, which is also confirmed by the large displacements of the Wannier function centers with respect to the corresponding ions in AFM-E HoMnO3. The final polarization is shown to be the result of competing effects, as shown by the opposite signs of the eg and t(2g) contributions to the ferroelectric polarization.
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