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With a lateral bisnaphtho-extended chemical structure, finite 7–13 carbon atom wide armchair graphene nanoribbons (7–13-aGNRs) were on-surface synthesized. For all lengths up to N = 7 monomer units, low-temperature ultrahigh vacuum scanning tunneling spectroscopy and spatial dI/dV maps were recorded at each captured tunneling resonance. The degeneracy of the two central electronic end states (ESs) occurs in a slowly decaying regime with N converging toward zero for N = 6 long 7–13-aGNR (12 bonded anthracenes), while it is N = 2 (4 bonded anthracenes) for seven carbon atoms wide armchair GNRs (7-aGNRs). The two end dI/dV conductance maxima of ESs are also shifted away from strictly two ends of the 7–13-aGNR compared to the 7-aGNR. Using the quantum topology graph filiation between finite length polyacetylene and 7–13-aGNRs wires, we show that this slow decay of 7–13-aGNR ESs is coming from the property of the topological Hückel band matrix that expels the ESs into its eigenvalue spectrum gaps to keep harmony in the core spectrum.

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This article follows earlier studies on the physical origin of magnetic anisotropy and the means of controlling it in polynuclear transition metal complexes. The difficulties encountered when focusing a magnetic field on a molecular object have led to consider the electric field as a more appropriate control tool. It is therefore fundamental to understand what governs the sensitivity of magnetic properties to the application of an electric field. We have already studied the impact of the electric field on the isotropic exchange coupling and on the Dzyaloshinskii–Moriya interaction (DMI). Here, we focus on the symmetric exchange anisotropy tensor. In order to obtain significant values of anisotropic interactions, we have carried out this study on a model complex that exhibits first-order spin–orbit coupling. We will show that (i) large values of the axial parameter of symmetric exchange can be reached when close to the first-order spin–orbit coupling regime, (ii) both correlated energies and wave functions must be used to achieve accurate values of the symmetric tensor components when the DMI is non-zero, and (iii) finally, an interferential effect between the DMI and the axial parameter of symmetric exchange occurs for a certain orientation of the electric field, i.e., the latter decreases in magnitude as the former increases. While DMI is often invoked as being involved in magneto-electric coupling, isotropic exchange and the symmetrical anisotropic tensor also contribute. Finally, we provide a recipe for generating significant anisotropic interactions and a significant change in magnetic properties under an electric field.

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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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Sujets

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

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158