Loading...
Derniers dépôts, tout type de documents
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.
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.
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.
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.
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.
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