- Home

[...]

Sr2IrO4 is characterized by a large spin-orbit coupling, which gives rise to bands with strongly entangled spin and orbital characters, called J1/2 and J3/2. We use light-polarization dependent ARPES to study directly the orbital character of these bands and fully map out their dispersion. We observe bands in very good agreement with our cluster dynamical mean-field theory calculations. We show that the J1/2 band, the closest to the Fermi level EF , is dominated by dxz character along kx and dyz along ky. This is actually in agreement with an isotropic J1/2 character on average, but this large orbital dependence in k-space was mostly overlooked before. It gives rise to strong modulations of the ARPES intensity that we explain and carefully take into account to compare dispersions in equivalent directions of the Brillouin zone. Although the latter dispersions look different at first, suggesting possible symmetry breakings, they are found essentially similar, once corrected for these intensity variations. In articular, the pseudogap-like features close to the X point appearing in the nearly metallic 15% Rh-doped Sr2IrO4 strongly depend on experimental conditions. We reveal that there is nevertheless an energy scale of 30meV below which spectral weight is suppressed, independent of the experimental conditions, which gives a reliable basis to analyze this behavior. We suggest it is caused by disorder.

5d iridium oxides are of huge interest due to the potential for new quantum states driven by strong spin-orbit coupling. The strontium iridate Sr2IrO4 is particularly in the spotlight because of the so-called jeff = 1/2 state consisting of a quantum superposition of the three local t2g orbitals with – in its most simple version – nearly equal population, which stabilizes an unconventional Mott insulating state. Here, we report an anisotropic and aspherical magnetization density distribution measured by polarized neutron diffraction in a magnetic field up to 5 T at 4 K, which strongly deviates from a local jeff = 1/2 picture even when distortion-induced deviations from the equal weights of the orbital populations are taken into account. Once reconstructed by the maximum entropy method and multipole expansion model refinement, the magnetization density shows crossshaped positive four lobes along the crystallographic tetragonal axes with a large spatial extent, showing that the xy orbital contribution is dominant. The analogy to the superconducting copper oxide systems might then be weaker than commonly thought.

The monomer [Ce(COT)] and the dimer [Ce(COT)], with Ce(III) and COT = 1,3,5,7-cyclooctatetraenide, are studied by quantum chemistry calculations. Due to the large spin-orbit coupling, the ground state of the monomer is a strong mixing of σ and π states. The experimental isotropic coupling in the dimer was evaluated by Walter et al. to be J = -7 cm (with a Heisenberg Hamiltonian [Formula: see text]) with a small anisotropic coupling of 0.02 cm. The coupling between the two Ce(III) in the dimer is calculated using CI methods. The low energy part of the spectra are modeled by spin Hamiltonians. All spin Hamiltonians parameters are deduced from ab initio calculations. g factors are calculated for both the pseudodoublet of the monomer and the pseudotriplet of the dimer and their sign have been determined. The magnetic coupling in the dimer is rationalized by a model based on crystal field theory. The kinetic and exchange contributions arising from the different configurations to the isotropic and anisotropic couplings are evaluated. It is shown that the main contribution to isotropic coupling is kinetic and originates from the f-f interaction due to the large transfer integral between those orbitals. However, the f-f interaction plays a non-negligible role. The anisotropic coupling originates from the difference of exchange energy of states arising from the ff configuration and is, in no matter, related to the anisotropy of the local magnetic moments as already pointed by van Vleck for a fictitious s-p system. The analysis of the natural orbitals evidences a superexchange mechanism through a σ orbital of the bridging cycle favored by a local 4f/5d hybridization and that the δ type orbitals, both the HOMOs of the ligands and the virtual f orbitals of the cerium atoms play an important polarization role, and to a less extend the π type orbitals, the HOMOs-1 of the ligands, and the metal f orbitals.

The magnetic anisotropy parameters of a hexacoordinate trigonally elongated Ni(II) complex with symmetry close to D3d are measured using field-dependent magnetization and High-Field and High-Frequency EPR spectroscopy (D = +2.95 cm-1 , |E/D| = 0.08 from EPR). Wavefunction based theoretical calculations reproduce fairly well the EPR experimental data and allows analysing the origin of the magnetic anisotropy of the complex. Calculations on model complexes allows getting insight into the origin of the large increase in the axial magnetic anisotropy (D) when the complex is brought to a prismatic geometry with a symmetry close to D3h. 2

[...]

The density matrix renormalization group in chemistry and molecular physics: Recent developments and new challenges The Journal of Chemical Physics 152, 040903 (2020); https://doi. ABSTRACT Taking as an example the simple CH 3 radical, this work demonstrates the cooperative character of the spin-polarization phenomenon of the closed-shell core in free radicals. Spin polarization of CH σ bonds is not additive here, as spin polarization of one bond enhances that of the next bond. This cooperativity is demonstrated by a series of configuration interaction calculations converging to the full valence limit and is rationalized by analytic developments. The same phenomenon is shown to take place in those diradicals where spin polarization plays a major role, as illustrated in square planar carbo-cyclobutadiene C 12 H 4. The treatment of cooperativity represents a challenge for usual post-Hatree-Fock methods. Published under license by AIP Publishing. https://doi.

[...]

[...]

[...]

Spin-polarization effects may play an important role in free radicals and in the magnetic coupling between radical centers. Starting from restricted open-shell calculations, i.e. a closed-shell description of the non-magnetic core electrons, a low-order perturbation expansion identifies the spin polarization contribution to the energy of mono-radicals and to singlet-triplet energy differences in diradicals. Broken-symmetry (BS) single-determinant calculations introduce only a fraction of spin polarization effects, and in a biased manner, since BS determinants are not spin eigenfunctions. We propose a simple technique to correctly evaluate spin-polarization energies by taking into account the effect of spin-flip components of one-hole one-particle excited configurations. Spin-decontamination corrections is shown to play a non-negligible role in BS evaluation of bond energies. The importance of spin-decontamination is illustrated on cases for which spin polarization is the leading contribution to the singlet-triplet gap, what characterizes twisted conjugated double-bonds and disjoint diradicals.

[...]

[...]

A theoretical framework for the generation of natural orbitals, natural spin orbitals, as well as orbital- and spin-magnetizations from multi-configurational ab-initio wavefunction calculations including spin-orbit coupling is presented. It is shown how these computational orbital and magnetization tools can be used to inter- pret and rationalize the magnetic properties of selected complexes containing tran- sition metals, lanthanides, and actinides.

Basic concepts for calculating electronic paramagnetic resonance are discussed, with a focus on methods that are suitable for molecules containing heavy ele- ments. Inclusion of relativistic effects is essential in such calculations. Selected examples are presented to illustrate practical applications of these theoretical methods.

Nuclear spin Cooperative effect Determinants Méthodes basées sur la fonction d’onde Complexes de métaux de transition Ab initio calculation CLUSTERS Diagonalisations exactes Molécules aimants Free radicals Exchange and superexchange interactions Transition state Molecule-photon collisions Exact diagonalization Calculs ab initio relativistes et corrélés Perturbation theory Anderson mechanism Molecular magnet SYSTEMS AB-INITIO Transition-metal complexes Electron paramagnetic resonance Transition metal complexes Calculs ab initio Configuration interactions Electronic correlation MOLECULAR MAGNETIC-MATERIALS Ab initio calculations Magnétisme dans les systèmes organiques Photodissociation Modèle de double échange Wavefunction based methods Model hamiltonian Crystal-field theory and spin Hamiltonians MECHANISM Spin-orbit interactions Configuration interaction Finite nucleus effects Negative ions Double exchange model Spin–orbit coupling Spectroscopy Zeeman interaction Relativistic and correlated ab initio calculations Relativistic corrections Iodine Spin Hamiltonian Magnetic properties MACROCYCLIC POLYARYLMETHYL POLYRADICALS Crystal field theory Effets magnéto-résistifs HIGH-SPIN Single molecule magnets Magnétisme moléculaire SPINTRONICS Density functional theory Potential energy surfaces Electron paramagnetism Lanthanides Hyperfine structure Théorie du champ cristallin Polarization Magneto-resistive effects Heavy fermions Théorie de la fonctionnelle de la densité UDFT Molecular electronic states Molecular magnetism Hamiltonien modèle Excited states Excitation energies POLYMER Ligand-field theory Wave functions Magnetic anisotropy Triplet state Strongly correlated electron systems Manganites Magnetism in organic systems Basis sets Calcul ab initio Electron spin Spectroscopie Ground states Hyperfine coupling Décontamination de spin Electron g-factor Spin-polarization SUPERCONDUCTIVITY Anisotropie magnétique Spin decontamination Coupled cluster calculations