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Molecules with inversion of the singlet and triplet excited‐state energies are highly promising for the development of organic light‐emitting diodes (OLEDs). To date, azaphenalenes are the only class of molecules where these inversions have been identified. Here, we screen a curated database of organic crystal structures to identify existing compounds for violations of Hund's rule in the lowest excited states. We identify two further classes with this behavior. The first, a class of zwitterions, has limited relevance to molecular emitters as the singlet‐triplet inversions occur in the third excited singlet state. The second class consists of two D2h‐symmetry non‐alternant hydrocarbons, a fused azulene dimer and a bicalicene, whose lowest excited singlet states violate Hund's rule. Due to the connectivity of the polycyclic structure, they achieve this symmetry through aromatic stabilization. These hydrocarbons show promise as the next generation of building blocks for OLED emitters. [ABSTRACT FROM AUTHOR]

Magneto-optics properties of (Co41Fe39B20)x(SiO2)100−x alloys were studied applying both experimental spectral ellipsometry and quantum mechanical theory (incl. molecular calculus) approaches. Magneto-optics (MO) properties of the alloys were derived from the measured parameters of reflected elliptically polarized light. The experimental data were explained applying selection rules for the magnetic quantum number. Theoretical approach based on electron gas in a metal with angular momentum coupled to magnetic field demonstrated applicability of the magnetic quantum number and Hund’s rule for ascertaining the MO alloys properties. In this modeling, the magneto-optics properties of ferromagnetic alloys can be explained being based on derivations for the magnetic quantum number selection rules. Hund’s rule directly influences the dielectric tensor off-diagonal elements signs. The nonrelativistic Schrödinger equation for the Co2Fe2B alloy was numerically solved taking into account spins of Co and Fe atoms as well as orbital moments of wave functions in order to check the theoretical approach. As a result, the MO dispersion curves can be theoretically evaluated using the modified Spicer ratio that includes density of states functions with specific azimuthal and spin quantum numbers.

In a semi-review paper, we show that the hidden Jahn-Teller effect (JTE) and pseudo-JTE (PJTE) in molecular systems and solids, under certain conditions lead to the formation of two coexisting stable space configurations with different magnetic and dielectric properties, switchable by external perturbations. One of the stable configurations has a high space symmetry and a non-zero or higher spin (HS) (non-zero magnetic moment), the other being distorted, but with zero or lower spin (LS). The number of systems with hidden JTE or PJTE is innumerable; we demonstrate this on the (no exhaustible, too) group of systems with half-filed closed-shell degenerate electronic (orbital or band) configurations e² and t³. The spin-crossover-change from the high symmetry HS arrangement to the low-symmetry LS geometry is accompanied by (driven by the PJTE) orbital disproportionation, in which the system prefers spins-paired states with two electrons on the same orbital (and lower symmetry charge distribution) over the Hund's spin-parallel arrangement involving several orbitals. Ab initio calculations previously carried out on a series of molecular systems and clusters in crystals, including CuF3, Si3, Si4, Ge4, C4H4, Na4-, C603-, CuO6 (in two crystal environments, LiCuO2 and NaCuO2), etc., confirmed the general theory and allowed for estimates of the parameter values including relaxation times. The hidden JTE and PJTE are thus general tools for search and studies of polyatomic systems with bistabilities. [ABSTRACT FROM AUTHOR]

Magneto-optics properties of (Co41Fe39B20)x(SiO2)100−x alloys were studied applying both experimental spectral ellipsometry and quantum mechanical theory (incl. molecular calculus) approaches. Magneto-optics (MO) properties of the alloys were derived from the measured parameters of reflected elliptically polarized light. The experimental data were explained applying selection rules for the magnetic quantum number. Theoretical approach based on electron gas in a metal with angular momentum coupled to magnetic field demonstrated applicability of the magnetic quantum number and Hund’s rule for ascertaining the MO alloys properties. In this modeling, the magneto-optics properties of ferromagnetic alloys can be explained being based on derivations for the magnetic quantum number selection rules. Hund’s rule directly influences the dielectric tensor off-diagonal elements signs. The nonrelativistic Schrödinger equation for the Co2Fe2B alloy was numerically solved taking into account spins of Co and Fe atoms as well as orbital moments of wave functions in order to check the theoretical approach. As a result, the MO dispersion curves can be theoretically evaluated using the modified Spicer ratio that includes density of states functions with specific azimuthal and spin quantum numbers. [ABSTRACT FROM AUTHOR]

Abstract A Quasi Quantum Treatment (QQT) of the rotational inelastic NO(X)-He collisions in the pure Hund's case (a) spin orbit state conserving transitions is extended to the mixed Hund's case (a) spin orbit state conserving and changing transitions. To enable this extension a polar and azimuthal angle dependent intermolecular hard-shell PES, V S (R S , γ R , χ R) = E S , has been developed. The DCSs and ICSs calculated by QQT are compared with those obtained from QM exact calculations onto a full R-range ab initio PES. The rotationally inelastic scattering of NO(X) from He presents a paradigm for the rotationally inelastic scattering at a thermal collision energy of molecules residing in a Π electronic rotational eigenstate. [ABSTRACT FROM AUTHOR]

Hund’s multiplicity rule invariably holds for the ground configurations of few-electron atoms as well as those of multi-electron quantum dots. However, the ordering of the corresponding interelectronic repulsions exhibits a reversal in the former but not in the latter system, upon varying the system parameters. Here, we investigate the transition between these two types of behaviour by studying few-electron atoms confined in spherical cavities. “Counter-intuitive” ordering of the interelectronic repulsions is confirmed when the nuclear charge is low enough and the cavity radius is large enough. [ABSTRACT FROM AUTHOR]

We present the modulation of magnetic order on the orbital selective Mott phases (OSMP) and the metal-insulator transitions (MIT) of multi-orbital Hubbard models by employing the rotationally invariant slave-boson methods. We show that at half filling, the well-known paramagnetic (PM) OSMP is completely covered by an antiferromagnetic (AFM) Slater insulator, and the PM Mott phase by an AFM Mott insulator when electron correlation strength varies from intermediate to strong both in two- and three-orbitals Hubbard systems. Away from half-filling, we find that a partial-polarized AFM orbital-selective Slater phase appears in the intermediate correlation regime, and an almost full-polarized AFM OSMP fully covers the paramagnetic OSMP. In addition, the ferromagnetic phase in the three-orbital case is more robust than that in the two-orbital case. These results demonstrate that the modulation of magnetic correlation to the quasiparticle spectra leads to much rich and more interesting MIT scenario in multiorbital correlated systems. [ABSTRACT FROM AUTHOR]

Perovskite cobaltates are widely recognized as potential candidates for their applications in magnetic and magnetoresistive applications. In this report, pseudo-magnetic CaFe0.5Co0.5O3−δ was synthesized at ambient pressure. Various calcinations, heating times, and sintering temperatures were applied in order to achieve the optimized conditions. The optimal sintering temperature with excellent phase and microstructure was found at ∼1050 °C using CaCO3 as precursor. Rietveld analysis was carried out to solve and refine the XRD pattern of the final specimens. The sample with excellent refined values was solved in orthorhombic phase and pbcm space group. From the SEM and EDS analysis, the excellent grain growth with high densification and ABO3 structure was confirmed respectively. [ABSTRACT FROM AUTHOR]

Students collect magnetic susceptibility data to verify that Hund's rule correctly predicts electronic configurations. Systems examined include three commercially available lanthanide(III)-containing complexes of the form M(acac)3(H2O)2 (where M = La(III), Nd(III), and Gd(III), and acac denotes the [CH3C(O)CHC(O)CH3]- anion, i.e., deprotonated acetylacetone) as well as student-synthesized Co(III)- and Fe(III)-containing M(acac)3 analogues. The spin-only formula for the paramagnetic susceptibility cleanly reveals the number of unpaired electrons present in all metal ions investigated herein, except Nd(III), because it also has an orbital magnetic moment. In addition to illustrating the key role that the Pauli principle has in shaping electronic structure, the exercise contrasts the behavior of high- vs low-spin complexes and 3d versus 4f orbitals. Finally, students find that oxidation-reduction and acid-base reactions can be useful in chemical synthesis. [ABSTRACT FROM AUTHOR]

Single crystals of L -alanine cadmium chloride (LACC) and cobalt chloride (Co 2+ ) doped LACC have been grown by the slow evaporation solution growth technique. The grown crystals were subjected to various characterizations such as powder XRD, SXRD, FTIR, UV–vis, EDAX, TG/DTA, VSM, Dielectric and Second Harmonic Generation (SHG) measurements. The lattice parameters of the grown crystals were determined by single crystal X-ray analysis. EDAX analysis confirms the presence of Co 2+ ion in the host material. The functional group and optical behavior of the crystals were identified from FTIR and UV-vis spectrum analysis. Electrical parameters such as dielectric constant, dielectric loss have been studied. The thermal stability of the compound was found out using TGA/DTA analysis. Second Harmonic Generation of the samples was confirmed by Kurtz-Perry powder technique. Magnetic properties of the crystals studied by VSM were also reported. The encouraging results show that the cobalt chloride doped LACC crystals have greater potential applications in optical devices. [ABSTRACT FROM AUTHOR]

The interface magnetization of n-type BaTiO3/La0.7Sr0.3MnO3 heterojunction is selectively probed by magnetic second-harmonic generation at 80 K. The injection of minority spins at the interface causes a sudden, reversible transition of the spin alignment of interfacial Mn ions from ferromagnetic to antiferromagnetic exchange coupled, while the bulk magnetization remains unchanged. We attribute the emergent interfacial antiferromagnetic interactions to weakening of the doubleexchange mechanism caused by the strong Hund's rule coupling between injected minority spins and local magnetic moments. The effect is robust and may serve as a viable route for electronic and spintronic applications. [ABSTRACT FROM AUTHOR]

The non-monotonic behavior of the electron repulsion energy and the inter-electronic distance, as a function of the internuclear separation, in the 3Π u excited state of the hydrogen molecule has been assessed by explicit calculation and analysis of the electron-pair density distribution functions from high level ab initio full configuration interaction wave functions, for both the 3Π u and the 1Π u states. Additionally, Hund's rule as applied to these two states has been accounted for in terms of simple electronic shielding effects induced by wave function antisymmetrization. [ABSTRACT FROM AUTHOR]

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In the present paper we address the problem of optical isomerism embodied in the socalled 'Hund's paradox', which points to the difficulty to account for chirality by means of quantum mechanics. In particular, we explain the answer to the problem proposed by the theory of decoherence. The purpose of this article is to challenge this answer on the basis of a conceptual analysis of the phenomenon of decoherence, that reveals the limitations of the theory of decoherence to solve the difficulties posed by optical isomerism and, in general, by quantum measurement. [ABSTRACT FROM AUTHOR]

In this review article I discuss two aspects of magnetism in small metal clusters. The first question discussed is whether simple metal clusters, that obey electronic shell models and mimic properties of elemental atoms, also obey Hund's rule of maximum spin multiplicity. The second question is whether small clusters of 4 d transition metal atoms, that are non-magnetic in the bulk, have magnetic ground states. The question arises because calculations showed that small V clusters are magnetic although the bulk metal is not. We discuss known results on Rh clusters in detail to show that small clusters are generally magnetic, but it is difficult to unequivocally identify the ground state due to the presence of many isomers and spin states that are very close in energy. [ABSTRACT FROM AUTHOR]

We report the derivation of line-strength formulae for fine- and hyperfine-resolved electric-quadrupole rotation–vibration transitions in Hund's case b molecules using the formalism of spherical tensor algebra. These expressions may serve in the analysis of spectra of linear molecules, in particular diatomic molecules such as H2, N2and their ions, which are of current interest in precision spectroscopy. [ABSTRACT FROM AUTHOR]

The present paper deals with the theoretical analysis of superconducting transition temperature ( T) within two-orbital-per-site model Hamiltonian in iron-based superconductors. The Green's function approach within the Bardeen-Cooper-Schrieffer (BCS) mean-field approximation has been applied. The expression of superconducting transition temperature has been obtained as a function of hopping parameters, onsite Coulomb interactions, and Hund's coupling term. It is pointed out that the transition temperature decreases with increasing nearest neighbor hopping ( t), next nearest neighbor hopping ( t), and also with onsite Coulomb interactions. While on increasing nearest neighbor hopping between π-orbitals ( t), the transition temperature initially increases linearly and acquire a maximum value (at t = 0.065 eV), thereafter T shows suppression. The present transition temperature analysis can be connected with the pressure-dependent (nearest and next nearest neighbor coupling) parameters (in the band structure) in iron-based systems. Further, the transition temperature increases with increasing the hybridization energy, Hund's coupling term, and carrier density (< n>) per site. Finally, the theoretically obtained results on T have been viewed in terms of experimental variation of T under pressure and existing theoretical predictions in iron-based superconductors. [ABSTRACT FROM AUTHOR]

The analysis of the relation between structure and properties of the substances is very important in engineering. This paper objective is to provide, from the educational point of view, the location of the elements in the Periodic Table, from its electronic distribution. Some theoretical methods were applied during the investigation, such as the analysis and criticism of the sources, the systemic method, as well as other empiric ones, such as the observation and the opinion of the experts. All these methods made possible the achievement of an innovative general procedure that has the advantage of unifying those already existing and incorporating the study of the elements named "rare soils" that have many applications in last generation technologies. With the implementation of this new procedure have achieved good academic results, in the learning of the relationship between structure and properties of substances, these results constitute a modest contribution to the teaching of chemistry. [ABSTRACT FROM AUTHOR]

In most magnetically-ordered iron pnictides, the magnetic moments lie in the FeAs planes, parallel to the modulation direction of the spin stripes. However, recent experiments in hole-doped iron pnictides have observed a reorientation of the magnetic moments from in-plane to out-of-plane. Interestingly, this reorientation is accompanied by a change in the magnetic ground state from a stripe antiferromagnet to a tetragonal nonuniform magnetic configuration. Motivated by these recent observations, here we investigate the origin of the spin anisotropy in iron pnictides using an itinerant microscopic electronic model that respects all the symmetry properties of a single FeAs plane. We find that the interplay between the spin-orbit coupling and the Hund's rule coupling can account for the observed spin anisotropies, including the spin reorientation in hole-doped pnictides, without the need to invoke orbital or nematic order. Our calculations also reveal an asymmetry between the magnetic ground states of electron- and hole-doped compounds, with only the latter displaying tetragonal magnetic states. [ABSTRACT FROM AUTHOR]

The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M--M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund's rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χMT ≈ 53 cm3 K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field 57Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm-1 and a magnetic blocking temperature of 2.9 K (0.059 K/min). [ABSTRACT FROM AUTHOR]

The recently proposed concept of a Hund's metal-a metal in which electron correlations are driven by Hund's rule coupling-can be used to explain the exotic magnetic and electronic behaviour of strongly correlated electron systems of multi-orbital metallic materials. Tuning the abundance of parameters that determine these materials is, however, experimentally challenging. Here, we show that the basic constituent of a Hund's metal-a Hund's impurity-can be realized using a single iron atom adsorbed on a platinum surface, a system that comprises a magnetic moment in the presence of strong charge fluctuations. The magnetic properties can be controlled by using the tip of a scanning tunnelling microscope to change the binding site and degree of hydrogenation of the 3d transition-metal atom. We are able to experimentally explore a regime of four almost degenerate energy scales (Zeeman energy, temperature, Kondo temperature and magnetic anisotropy) and probe the magnetic excitations with the microscope tip. The regime of our Hund's impurity can be tuned from an emergent magnetic moment to a multi-orbital Kondo state, and the system could be used to test predictions of advanced many-body theories for non-Fermi liquids in quantum magnets or unconventional superconductors. [ABSTRACT FROM AUTHOR]

We present optical transmission measurements that reveal a charge gap of 0.86 eV in the local-moment antiferromagnetic insulator BaMn2As2, an order of magnitude larger than previously reported. Density functional theory plus dynamical mean-field theory (DFT + DMFT) calculations correctly reproduce this charge gap only when a strong Hund's coupling is considered. Thus, BaMn2As2 is a member of a wider class of Mn pnictide compounds that are Mott-Hund insulators. We also present optical reflectance for metallic 2%-K-doped BaMn2 AS2 that we use to extract the optical conductivity at different temperatures. The optical conductivity σ1(ω) exhibits a metallic response that is well described by a simple Drude term. Both σ(ω → 0,T) and ρ(T) exhibit Fermi-liquid temperature dependencies. From these measurements, we argue that a more strongly correlated Hund-metal version of the parent compounds of the iron pnictide superconductors has not yet been realized by doping this class of Hund insulators. [ABSTRACT FROM AUTHOR]

We present a continuous-time Monte Carlo impurity solver for multiorbital impurity models which combines a strong-coupling hybridization expansion and a weak-coupling expansion in the Hund's coupling parameter J. This double-expansion approach allows to treat the dominant density-density interactions U within the efficient segment representation. We test the approach for a two-orbital model with static interactions, and then explain how the double expansion allows to simulate models with frequency dependent U(ω) and J(ω). The method is used to investigate spin-state transitions in a toy model for fullerides, with repulsive bare J but attractive screened J. [ABSTRACT FROM AUTHOR]

We analyze the stability of excitonic ground states in the two-band Hubbard model with additional electron-phonon and Hund's rule couplings using a combination of mean-field and variational cluster approaches. We show that both the interband Coulomb interaction and the electron-phonon interaction will cooperatively stabilize a charge density wave (CDW) state which typifies an "excitonic" CDW if predominantly triggered by the effective interorbital electron-hole attraction or a "phononic" CDW if mostly caused by the coupling to the lattice degrees of freedom. By contrast, the Hund's rule coupling promotes an excitonic spin density wave. We determine the transition between excitonic charge and spin density waves and comment on a fixation of the phase of the excitonic order parameter that would prevent the formation of a superfluid condensate of excitons. The implications for exciton condensation in several material classes with strongly correlated electrons are discussed. [ABSTRACT FROM AUTHOR]

We study spatial correlation effects in multiorbital systems, especially in a paramagnetic metallic state subject to Hund's coupling. We apply a cluster extension of the dynamical mean-field theory (DMFT) to the three-orbital Hubbard model away from half filling, where previous single-site DMFT studies revealed that local correlation effects caused by Hund's coupling bring about unusual strongly correlated metallic behaviors. We find that Hund's coupling significantly affects the nonlocal correlations too; it strongly modulates the electron distribution in the momentum space so as to make a momentum region almost half filled and hence strongly correlated. It leads to an anomalous electronic state distinct both from the Fermi liquid and the Mott insulator. We identify the mechanism of the anomalous state with the intersite ferromagnetic correlations induced by Hund's coupling. [ABSTRACT FROM AUTHOR]

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials CaRuO3 and SrRuO3 is presented. Phase diagrams are determined for both materials as a function of Hubbard repulsion U and Hund's rule coupling J . Metallic and insulating phases are found, as are ferromagnetic and paramagnetic states. The locations of the relevant phase boundaries are determined. Based on the computed phase diagrams, Mott dominated and Hund's dominated regimes of strong con-elation are distinguished. Comparison of calculated properties to experiments indicates that the actual materials are in the Hund's coupling dominated region of the phase diagram so can be characterized as Hund's metals, in common with other members of the ruthenate family. Comparison of the phase diagrams for the two materials reveals the role played by rotational and tilt (GdFeO3-type) distortions of the ideal perovskite structure. The presence of magnetism in SrRuO3 and its absence in CaRuO3 despite the larger mass and larger tilt/rotational distortion amplitude of CaRuO3 can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-7" properties of CaRuO3 to those of SrRuO3 provides insight into the effects of magnetic order on the properties of a Hund's metal. The study provides a simultaneous description of magnetism and correlations and explicates the roles played by band theory and Hubbard and Hund's interactions. [ABSTRACT FROM AUTHOR]

The interplay of nonlocal Coulomb repulsion and Hund's coupling in the d-orbital manifold in frustrated triangular lattices is analyzed by a multiband extended Hubbard model. We find a rich phase diagram with several competing phases, including a robust pinball liquid phase, which is an unconventional metal characterized by threefold charge order, bad metallic behavior, and the emergence of high-spin local moments. Our results naturally explain the anomalous charge-ordered metallic state observed in the triangular layered compound AgNiO2. The potential relevance to other triangular transition-metal oxides is discussed. [ABSTRACT FROM AUTHOR]

We study itinerant ferromagnetism in a t2g multiorbital Hubbard system in the cubic lattice, which consists of three planar oriented orbital bands of dxy, dyz, and dzx. Electrons in each orbital band can only move within a two-dimensional plane in the three-dimensional lattice parallel to the corresponding orbital orientation. Electrons of different orbitais interact through the on-site multiorbital interactions including Hund's coupling. The strong-coupling limit is considered in which there are no doubly occupied orbitais but multiple on-site occupations are allowed. We show that in the case in which there is one and only one hole for each orbital band in each layer parallel to the orbital orientation, the ground state is a fully spin-polarized itinerant ferromagnetic state, which is unique apart from the trivial spin degeneracy. When the lattice is reduced into a single two-dimensional layer, the dzx and dyz bands become quasi-one-dimensional while the dxy band remains two-dimensional. The ground-state ferromagnetism also appears in the strong-coupling limit as a generalization of the double-exchange mechanism. Possible applications to the systems of SrRuO3 and LaA1O3/SrTiO3 interface are discussed. [ABSTRACT FROM AUTHOR]

The electronic structure of Li2RuO3 was investigated using x-ray emission and absorption spectroscopy and by theoretical calculations employing two approaches: the local density approximation (LDA) and a combination of LDA with the cluster extension of dynamical mean-field theory (LDA+DMFT). The evolution of the spectral properties with the strength of electronic correlations is analyzed. We show that for moderate values of on-site Coulomb repulsion U and intra-atomic Hund's rule exchange JH, Li2RuO3 is in an orbital-selective strongly correlated state in the sense that a part of the t2g manifold (i.e., xz/yz) behaves as local atomic orbitais susceptible to Hubbard correlations, while the remaining (xy) orbitais must be described as bond-centered molecular orbitais. Both theoretical approaches succeed in explaining the x-ray data, and a comparison of the theoretical and experimental spectra provides a reasonable estimate of the possible correlation strength (U) and Hund's coupling (JH) in Li2RuO3. [ABSTRACT FROM AUTHOR]

The possibility of a spontaneous spin-triplet paired phase of the Fulde-Ferrell-Larkin-Ovchinnikov type is studied. As it is shown in a system with the dominant interband pairing and two distinct Fermi surface sheets, the Fermi wave-vector mismatch can be compensated by a nonzero center-of-mass momentum of the Cooper pairs. This idea is examined with the use of a model which describes the two hole-like bands in the iron-based superconductor. It is shown that for the proper range of model parameters, the minima of the free energy appear which correspond to a nonzero Cooper pair momentum. Different superconducting gap symmetries are analyzed, and the corresponding phase diagrams are shown. [ABSTRACT FROM AUTHOR]

Universal aspects of the Hund’s rule induced spin-triplet pairing are analysed within the two-band Hubbard model on a square lattice. According to our calculations, this pairing mechanism in conjunction with the correlation effect can result in stability of the paired phase in the so-called purely repulsive interactions regime, in which there is no effectively attractive interaction. Furthermore, even though all of the interaction terms are of intrasite character, the pairing contains both intra- and inter-site components. In effect, the gap parameter has a mixture ofs-waveandextended s-wavesymmetries. The calculations have been carried out with the use of the Statistically Consistent Gutzwiller Approximation developed by us in recent years. [ABSTRACT FROM PUBLISHER]

We study the emergence of quasiparticles in Hund's metals with an SU(M) × SU(AO-symmetric Kondo impurity model carrying both spin and orbital degrees of freedom. We show that the coupling of the impurity spin to the conduction electrons can beferromagnetic, notably for hole-doped iron pnictides. We derive the weak-coupling renormalization group (RG) equations for arbitrary representations of SU(M) × SU(A). A ferromagnetic spin coupling results in a protracted RG flow, accounting for the surprising particle-hole asymmetry that is observed in the iron-pnictide systems. We establish the low coherence scale 7k, which depends on the filling through the impurity representation. We also discuss the temperature dependence of the spin and orbital susceptibilities. Finally, we argue that this mechanism explains the strong valence dependence of the coherence scale observed in dilute transition-metal magnetic alloys. [ABSTRACT FROM AUTHOR]

We employ a combination of density functional theory and dynamical mean-field theory to investigate the electronic structure of the recently synthesized insulator BaCrO3. Our calculations show that Hund's coupling is responsible for strong correlation effects, which are however not sufficient to turn the system insulating without breaking any symmetry. The Hund's correlated metal is however unstable with respect to orbital ordering, which indeed makes the system insulating. The orbitally ordered insulator favors Jahn-Teller distortions and a secondary magnetic ordering. [ABSTRACT FROM AUTHOR]

We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard U, at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase. [ABSTRACT FROM AUTHOR]

We investigate the electronic structure of a perovskite-type Pauli paramagnet SrMoO3 (t2g2) thin film using hard x-ray photoemission spectroscopy and compare the results to realistic calculations that combine density functional theory within the local-density approximation (LDA) with dynamical mean-field theory (DMFT). Despite the clear signature of electron correlations in the electronic specific heat, the narrowing of the quasiparticle bands is not observed in the photoemission spectrum. This is explained in terms of the characteristic effect of Hund's rule coupling for partially filled t2g bands, which induces strong quasiparticle renormalization already for values of Hubbard interaction which are smaller than the bandwidth. This interpretation is supported by DMFT model calculations including Hund's rule coupling, which show a renormalization of low-energy quasiparticles without affecting the overall bandwidth. The photoemission spectra show additional spectral weight around -2.5 eV that is not present in the LDA+DMFT results, pointing to a source of correlations that is not present in our calculations that include only on-site interactions. We interpret this weight as a plasmon satellite, which is supported by the measured core-level spectra that all show satellites at this energy. [ABSTRACT FROM AUTHOR]

A recently discovered magnetic semiconductor Ba1-xKx(Zn1-yMny)2As2, with its decoupled spin and charge doping, provides a unique opportunity to elucidate the microscopic origin of the magnetic interaction and ordering in dilute magnetic semiconductors (DMSs). We show that (i) the conventional density functional theory accurately describes this material, and (ii) the magnetic interaction emerges from the competition of the short-range superexchange and a longer-range interaction mediated by the itinerant As holes, coupled to Mn via the Schrieffer-Wolff p-d interaction representing an effective Hund's rule coupling JHeff. The key difference between the classical double exchange and the actual interaction in DMSs is that an effective JHeff, as opposed to the standard Hund's coupling JH, depends on the Mn d-band position with respect to the Fermi level, and thus allows tuning of the magnetic interactions. The physically transparent description of this material may also be applicable in more complicated DMS systems. [ABSTRACT FROM AUTHOR]

We investigate the phase diagrams of the spin-orbital d9 Kugel-Khomskii model for increasing system dimensionality: from the square lattice monolayer, via the bilayer to the cubic lattice. In each case we find strong competition between different types of spin and orbital order, with entangled spin-orbital phases at the crossover from antiferromagnetic to ferromagnetic correlations in the intermediate regime of Hund's exchange. These phases have various types of exotic spin order and are stabilized by effective interactions of longer range which follow from enhanced spin-orbital fluctuations. We find that orbital order is in general more robust while spin order melts first under increasing temperature, as observed in several experiments for spin-orbital systems. [ABSTRACT FROM AUTHOR]

For some years, Slater's theory of many-electron atoms has been strongly criticized, particularly in connection with the explanation of Hund's rules. This criticism is discussed. An analysis of the energies of spectroscopic transitions suggests that, in those areas of application where it has proved most useful, the theory can justifiably be retained. [ABSTRACT FROM AUTHOR]

Hund's rule is one of the fundamentals of the correlation physics at the atomic level, determining the ground state multiplet of the electrons. It consists of three laws: (i) maximum S (total spin), (ii) maximum L (total orbital angular momentum) under the constraint of (i), and (iii) the total angular momentum J is ∣ L - S∣ for electron number less than half, while J = L + S for more than half due to the relativistic spin-orbit interaction (SOI). In real systems, the electrons hop between the atoms and gain the itinerancy, which is usually described by the band theory. The whole content of theories on correlation is to provide a reliable way to describe the intermediate situation between the two limits. Here we propose an approach toward this goal, i.e., we study the two-atom systems of three t2g orbitals and see how the Hund's rule is modified by the transfer integral t between them. It is found that the competition between t and the Hund's coupling J at each atom determines the crossover from the molecular orbital limit to the strong correlation limit. Especially, the focus is on the generalization of the third rule, i.e., the inter- and intra-atomic SOI's in the presence of the correlation. We have found that there are cases where the effective SOI's are appreciably enhanced by the Hund's coupling. The conditions for the enhancement are the intermediate Hund's coupling and the filling of four or five electrons. [ABSTRACT FROM AUTHOR]

The general chemistry curriculum includes a prelude that consumes nearly all of the first semester and occupies the first third of the typical textbook. This necessary prelude to the main event is comparable in scope to precalculus though not broken out as a formal 'prechemistry' course. Atomic orbitals account for much of this prelude-to-chemistry. By tradition, orbital theory is conveyed to the student in three disjunct pieces, presented in the following illogical order: the Pauli principle, the Aufbau principle, and Hund's rule. (Often the n + l rule is tossed into the mix as well, though with no fixed place in the scheme). In the early twentieth century, as various researchers announced new insights into the atom at unpredictable intervals, no one could have been faulted for teaching orbitals in such a manner, catch-as-catch-can. A hundred years on, the vestiges of that (presumed) practice look wrong, and are indefensible. In the approach advocated here, orbitals would be taught as a single hierarchical rule-set, with the parts coherently sequenced as Aufbau-Hund-Pauli (and with Madelung's n + l rule rehabilitated as part of Aufbau, no longer a free-floating mnemonic aid only). Logic aside, pragmatism offers its own argument for adopting this scheme: A tighter approach to Aufbau can lighten the 'prechemistry' burden significantly and bring the student that much sooner to chemistry itself. [ABSTRACT FROM AUTHOR]

We show that in transition-metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give, e.g., long-range magnetic order or stay paramagnetic. A similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called the orbital-selective Peierls transition. This can occur in the case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupling JH. Yet another consequence of this picture is that for an odd number of electrons per dimer there exists competition between the double-exchange mechanism of ferromagnetism and the formation of a singlet dimer by the electron on one orbital, with the remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule couphng, typical for 3d compounds, whereas the second is more plausible for 4d-5d compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which the orbital-selective phase seems to be realized. [ABSTRACT FROM AUTHOR]

Abstract: Ferrite powder samples of Cr x Fe3−x O4 (0.0≤x≤1.0) were prepared by chemical co-precipitation, and calcined in a tube furnace with argon-flow at 1723K for 2h. X-ray diffraction patterns indicated that all the samples had an (A)[B]2O4 single phase cubic spinel structure with a space group. Magnetic measurements indicated that the magnetization of the samples decreased with the Cr doping level. A new model for the magnetic ordering in these samples was employed to explain the dependence of the magnetization and cation distribution on the Cr doping level; namely, taking into consideration constraints arising from Hund's rules and from the spin direction of the itinerant 3d electrons, the directions of the Cr2+ and Cr3+ cation magnetic moments were taken to lie antiparallel to the moments of the Fe cations within the same sub-lattice (A or B sub-lattice). [Copyright &y& Elsevier]

We have investigated the pressure-induced spin-state transition in Co2+ systems in terms of a competition between Hund's exchange energy (J) and crystal-field splitting (ΔCF). First, we show the universal metastability of the low-spin state in octahedrally coordinated Co2+ systems. Then we present the strategy to search for a Co2+ system, for which the mechanism of spin-state and metal-insulator transitions is governed not by Mott physics but by J versus ΔCF physics. Using CoCl2 as a prototypical Co2+ system, we have demonstrated the pressure-induced spin-state transition from high-spin to low-spin, which is accompanied with insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic transitions. Combined with the metastable character of Co2+ and the high compressibility nature of CoCl2, a transition pressure as low as 27 GPa can be identified on the basis of J versus ΔCF physics. [ABSTRACT FROM AUTHOR]

We demonstrate the stability of the spin-triplet paired s-wave (with an admixture of extended s-wave) state for the limit of purely repulsive interactions in a degenerate two-band Hubbard model of correlated fermions. The repulsive interactions limit represents an essential extension of our previous analysis (2013 New J. Phys.15 073050), regarded here as I. We also show that near the half-filling the considered type of superconductivity can coexist with antiferromagnetism. The calculations have been carried out with the use of the so-called statistically consistent Gutzwiller approximation (SGA) for the case of a square lattice. We suggest that the electron correlations in conjunction with the Hund's rule exchange play the crucial role in stabilizing the real-space spin-triplet superconducting state. A sizable hybridization of the bands suppresses the homogeneous paired state. [ABSTRACT FROM AUTHOR]

In a system where selective Mott localization is realized, some electrons show a gap to charge excitations while others do not. A hybridization between these two kind of electrons will lead to a smoothening of this sharp difference and can even bring the system back to a complete delocalization. We show here that there is a large region of parameters at finite hybridization where the selective localization persists and the system shows a partial filling of the selective gap with incoherent states, giving rise to a pseudogap. This result is illustrated here in a two-orbital Hubbard model with Hund's coupling, but is based on quite general assumptions and should hold for a larger class of systems, and possibly be a paradigm for the pseudogap mechanism in cuprates. [ABSTRACT FROM AUTHOR]