| 1. |
- Cricchio, Francesco, 1981-, et al.
(author)
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Low spin moment due to hidden multipole order from spin-orbital ordering in LaFeAsO
- 2010
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In: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 81:14, s. 140403-
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Journal article (peer-reviewed)abstract
- An antiferromagnetic (AF) low-moment solution, 0.35 μB / Fe, is found in the case of LaOFeAs for an intermediately strong Coulomb interaction U of 2.75 eV. This solution is stabilized over a large moment solution due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. The multipoles are of rank four and can be understood as a type of spin-orbital ordering. Parallels can be drawn to the stabilization of the AF order in, e.g., CaCuO2.
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| 2. |
- Cricchio, Francesco, 1981-, et al.
(author)
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Multipolar and orbital ordering in ferro-pnictides
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Other publication (other academic/artistic)abstract
- The stability of different solutions corresponding to various arrangement of magnetic rank-4 multipoles are evaluated for the ferro-pnictides LaOFeAs, BaFe2As2 and CaFe2As2 compounds. For LaOFeAs, the ground-state solution is identified with a combination of large w4120 and w4120 multipole tensors of the magnetization density which integrates to the correct value of experimental moment. The same procedure is applied to BaFe2As2 and CaFe2 As2 where the lowest energy solution corresponds again to a combination of w4120 and w4120 multipoles, but with a magnetic moment lower than the experimental value. However, for these compounds, our calculated moment constitutes a significant improvement over previous ab-initio calculations that largely overestimate the Fe moment. Moreover for BaFe2As2 and CaFe2As2 , in the total energy curves as function of constrained spin moment, we identify a local minima close to the experimental moment.
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| 3. |
- Cricchio, Francesco, 1981-, et al.
(author)
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Multipolar magnetic ordering in actinide dioxides from first-principles calculations
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Other publication (other academic/artistic)abstract
- The LDA+U method and its multipole decomposition are applied to investigate the ground state of the actinide insulators dioxides UO2 , NpO2 and PuO2 . In case of UO2 , an antifer- romagnetic 3-k type ordering of U moments is identified as one of the lowest energy solutions, in agreement with experiments and recent calculations. For NpO2 , a magnetic multipolar 3-k ordering of triakontadipoles, with a zero net magnetic moment on Np atoms, is found to be one of the most stable solutions. This is in agreement with recent investigations that predict the triakontadipoles to be the hidden order parameter in NpO2 . Also in case of the non-magnetic insulator PuO2 , the ground-state is predicted to be described by a multipolar 1-k ordering of triakontadipoles associated with a magnetization density that integrates to zero on Pu sites.
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| 4. |
- Cricchio, Francesco, 1981-
(author)
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Multipoles in Correlated Electron Materials
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Electronic structure calculations constitute a valuable tool to predict the properties of materials. In this study we propose an efficient scheme to study correlated electron systems with essentially only one free parameter, the screening length of the Coulomb potential. A general reformulation of the exchange energy of the correlated electron shell is combined with this method in order to analyze the calculations. The results are interpreted in terms of different polarization channels, due to different multipoles. The method is applied to various actinide compounds, in order to increase the understanding of the complicate behaviour of 5f electrons in these systems. We studied the non-magnetic phase of δ-Pu, where the spin polarization is taken over by a spin-orbit-like term that does not break the time reversal symmetry. We also find that a non-trivial high multipole of the magnetization density, the triakontadipole, constitutes the ordering parameter in the mysterious hidden order phase of the heavy-fermion superconductor URu2Si2. This type of multipolar ordering is also found to play an essential role in the hexagonal-based superconductors UPd2Al3, UNi2Al3 and UPt3 and in the dioxide insulators UO2, NpO2 and PuO2. The triakontadipole moments are also present in all magnetic actinides we considered, except for Cm. These results led us to formulate a new set of rules for the ground state of a system, that are valid in presence of strong spin-orbit coupling interaction instead of those of Hund; the Katt's rules. Finally, we applied our method to a new class of high-Tc superconductors, the Fe-pnictides, where the Fe 3d electrons are moderately correlated. In these materials we obtain the stabilization of a low spin moment solution, in agreement with experiment, over a large moment solution, due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density.
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| 5. |
- Cricchio, Francesco, 1981-, et al.
(author)
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Polarization of an open shell in the presence of spin-orbit coupling
- 2011
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In: Europhysics letters. - : IOP Publishing. - 0295-5075 .- 1286-4854. ; 94:5, s. 57009-
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Journal article (peer-reviewed)abstract
- There exist many f transition metal-based materials, with large orbital degeneracy and relatively strong spin-orbit coupling, that exhibits some multipolar order. We perform first-principles calculations on these systems within DFT method plus on-site Coulomb correction and we discuss the results in terms of polarization of the density occupation matrix. The fact that higher multipole polarizations play a larger role than the spin polarization indicates a breakdown of Hund's rules and we suggest a new complementary set of rules -Katt's rules, valid in case of large spin-orbit coupling interaction.
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| 6. |
- Cricchio, Francesco, 1981-, et al.
(author)
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The role of triakontadipoles in uranium-based magnetic superconductor materials
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Other publication (other academic/artistic)abstract
- We investigate the ground state of the heavy-fermion actinide compounds UPd2Al3 , UNi2Al3 and UPt3 by means of electronic structure calculations including on-site correlation. The results are analyzed through multipole decomposition. In case of UPd2Al3 and UNi2Al3 we stabilize a solution with an antiferromagnetic arrangement of triakontadipoles with ordering vectors, respectively, q = (0, 0, 1/2 ) and q = ( 1/2 , 0, 1/2 ), whose total magnetic moment agrees with the experimental value. In case of UPt3 we identify a solution with a triakontadipole order whose symmetry is not compatible with the presence of a magnetic moment, in accordance with experiments that detect a very small value of the moment. The Fermi surfaces of the multipole dominated solutions have many common features with those obtained by conventional density functional methods. Finally, for URu2Si2 we are able to stabilize a new solution characterized by a q = (0, 0, 1/2 ) ordering of triakontadipoles and a symmetry that forbids a magnetic moment on uranium site. The Fermi surface of this solution is surprisingly similar to the one of the magnetic AF 1-q case in agreement with recent measurements.
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