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Actinide +IV complexes with six nitrates [AnIV(NO3)6]2− (An = Th, U, Np, and Pu) have been studied by 15N and 17O NMR spectroscopy in solution and first-principles calculations. Magnetic susceptibilities were evaluated experimentally using the Evans method and are in good agreement with the ab initio values. The evolution in the series of the crystal field parameters deduced from ab initio calculations is discussed. The NMR paramagnetic shifts are analyzed based on ab initio calculations. Because the cubic symmetry of the complex quenches the dipolar contribution, they are only of Fermi contact origin. They are evaluated from first-principles based on a complete active space/density functional theory (DFT) strategy, in good accordance with the experimental one. The ligand hyperfine coupling constants are deduced from paramagnetic shifts and calculated using unrestricted DFT. The latter are decomposed in terms of the contribution of molecular orbitals. It highlights two pathways for the delocalization of the spin density from the metallic open-shell 5f orbitals to the NMR active nuclei, either through the valence 5f hybridized with 6d to the valence 2p molecular orbitals of the ligands, or by spin polarization of the metallic 6p orbitals which interact with the 2s-based molecular orbitals of the ligands.

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We study the random transverse field Ising model on a finite Cayley tree. This enables us to probe key questions arising in other important disordered quantum systems, in particular the Anderson transition and the problem of dirty bosons on the Cayley tree, or the emergence of non-ergodic properties in such systems. We numerically investigate this problem building on the cavity mean-field method complemented by state-of-the art finite-size scaling analysis. Our numerics agree very well with analytical results based on an analogy with the traveling wave problem of a branching random walk in the presence of an absorbing wall. Critical properties and finite-size corrections for the zero-temperature paramagnetic-ferromagnetic transition are studied both for constant and algebraically vanishing boundary conditions. In the later case, we reveal a regime which is reminiscent of the non-ergodic delocalized phase observed in other systems, thus shedding some light on critical issues in the context of disordered quantum systems, such as Anderson transitions, the many-body localization or disordered bosons in infinite dimensions.

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This work addresses a class of conjugated hydrocarbons that are expected to be singlet diradicals according to the topological Hückel Hamiltonian while possibly satisfying full on-bond electron pairing. These systems possess two degenerate singly occupied molecular orbitals (SOMOs), but aromaticity brought by properly positioned six-membered rings does prevent Jahn–Teller distortions. Density functional theory (DFT) calculations performed on two emblematic examples confirm the strong bond-length alternation in the closed-shell solutions and the clear spatial symmetry in the open-shell spin-unrestricted determinants, the latter solution always being found to have significantly lower energy. Since the SOMOs are here of different symmetry, the wave function is free from ionic valence-bond component, and spin decontamination of the unrestricted DFT solutions and wave function calculations at the CASSCF-plus-second-order-perturbation level confirm the expected pure diradical character of such molecules. In contrast to disjoint diradicals, the SOMOs of present systems have large amplitudes on neighbor atoms, and we propose to name them entangled pure diradicals, further providing some prescription rules for their design. Additional calculations point out the qualitative contrast between these molecules and the related diradicaloids.

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A complete theoretical analysis using first the simple Hückel model followed by more sophisticated multi-reference calculations on a trinuclear Ni(II) complex (Tp#Ni3HHTP), bearing the non-innocent bridging ligand HHTP3−, is carried out. The three semiquinone moieties of HHTP3− couple antiferromagnetically and lead to a single unpaired electron localized on one of the moieties. The calculated exchange coupling integrals together with the zero-field parameters allow, when varied within a certain range, reproducing the experimental data. These results are generalized for two similar other trinuclear complexes containing Ni(II) and Cu(II). The electronic structure of HHTP3− turns out to be independent of both the chemical nature and the geometry of the metal ions. We also establish a direct correlation between the geometrical and the electronic structures of the non-innocent ligand that is consistent with the results of calculations. It allows experimentalists to get insight into the magnetic behavior of this type of complexes by an analysis of their X-ray structure.

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In the quest of new exotic phases of matter due to the interplay of various interactions, iridates hosting a spin-orbit entangled $j_{\mathrm{eff}}=1/2$ ground state have been in the spotlight in recent years. Also in view of parallels with the low-energy physics of high-temperature superconducting cuprates, the validity of a single- or few-band picture in terms of the $j_{\mathrm{eff}}$ states is key. However, in particular for its structurally simple member Ba$_2$IrO$_4$, such a systematic construction and subsequent analysis of minimal low-energy models are still missing. Here we show by means of a combination of different ab initio techniques with dynamical mean-field theory that a three-band model in terms of Ir-$j_{\mathrm{eff}}$ states fully retains the low-energy physics of the system as compared to a full Ir-$5d$ model. Providing a detailed study of the three-band model in terms of spin-orbit coupling, Hund's coupling and Coulomb interactions, we map out a rich phase diagram and identify a region of effective one-band metal-insulator transition relevant to Ba$_2$IrO$_4$. Compared to available angle-resolved photoemission spectra, we find good agreement of salient aspects of the calculated spectral function and identify features which require the inclusion of non-local fluctuations. In a broader context, we envisage the three- and five-band models developed in this study to be relevant for the study of doped Ba$_2$IrO$_4$ and to clarify further the similarities and differences with cuprates.

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Subjets

Model Hamiltonian derivation First-order spin–orbit coupling Model Hamiltonians Anderson mechanism Magnetism Heavy fermions Cooperative effect Spin-orbit coupling Déplacements chimiques paramagnétiques MOLCAS calculations Binuclear compounds Electron paramagnetic resonance Crystal-field theory and spin Hamiltonians Electronic correlation Bleaney's model MECHANISM Exchange and superexchange interactions Magnetic anisotropy Ionic liquid Magnetic properties Covalency Heptacoordination Manganites Excitation energies Crystal field parameters Calculs ab initio relativistes et corrélés Iridate Density functional theory Lanthanide Coupled cluster calculations Ab initio calculations Relativistic corrections MOLECULAR MAGNETIC-MATERIALS Hyperfine coupling Metal-insulator transition Ground states Effective Hamiltonian theory Electron paramagnetism DOTA ligand Ligand-field theory Hamiltonien modèle Lanthanides Bleaney's theory Spin-orbit interactions Dynamical mean field theory Modeling Basis sets Crystal field theory Dynamical mean-field theory Décontamination de spin Divalent cobalt Finite nucleus effects Model hamiltonian Electron spin Exact diagonalization Free radicals Magnétisme moléculaire Correlated relativistic ab initio calculations Complexes de métaux de transition Anisotropy Dzyaloshinskii-Moriya interaction Calcul ab initio Perturbation theory Anisotropie magnétique Magnetism in organic systems Hyperfine structure MACROCYCLIC POLYARYLMETHYL POLYRADICALS Electronic structure Double exchange model Iodine FOS Physical sciences Actinide NMR Diagonalisations exactes Iridates Magnétisme dans les systèmes organiques Luminescence Magnetic Susceptibility CLUSTERS Actinides Dzyaloshinskii–Moriya interaction Isotropic and anisotropic exchange AB-INITIO Magneto-resistive effects Calculs ab initio Determinants Ab initio calculation Modèle de Bleaney Effets magnéto-résistifs Imidazolium salt Disordered Systems and Neural Networks cond-matdis-nn Electron g-factor Configuration interaction Wave functions Magnetic susceptibility Excited states HIGH-SPIN Configuration interactions High pressure Bleaney

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