Hund's Interaction Research Articles

Analytical slave-spin mean-field approach to orbital selective Mott insulators

We use the slave-spin mean-field approach to study particle-hole symmetric one- and two-band Hubbard models in presence of Hund's coupling interaction. By analytical analysis of Hamiltonian, we show that the locking of the two orbitals vs.\,orbital-selective Mott transition can be formulated within a Landau-Ginzburg framework. By applying the slave-spin mean-field to impurity problem, we are able to make a correspondence between impurity and lattice. We also consider the stability of the orbital selective Mott phase to the hybridization between the orbitals and study the limitations of the slave-spin method for treating inter-orbital tunnellings in the case of multi-orbital Bethe lattices with particle-hole symmetry.

Infrared spectroscopic study ofCaFe0.7Co0.3O3

Temperature-dependent infrared spectroscopy has been investigated for ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ which undergoes a ferromagnetic transition at ${T}_{C}\ensuremath{\approx}177\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. It is observed that the spectral weight is transferred from $\ensuremath{\sim}4800--14 000{\mathrm{cm}}^{\ensuremath{-}1}$ to $\ensuremath{\sim}0$--$4800{\mathrm{cm}}^{\ensuremath{-}1}$ as the temperature is lowered around ${T}_{C}$. Such a large-range spectral weight transfer is attributed to the Hund's interaction. The phonons in ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ show minor asymmetric line shapes, implying relatively weak electron-phonon coupling compared with the parent compound ${\mathrm{CaFeO}}_{3}$. The optical conductivity also reveals a broad peak structure in the range of $\ensuremath{\sim}700--1500{\mathrm{cm}}^{\ensuremath{-}1}$. Fit by the model of single-polaron absorption, the broad peak is interpreted by the excitation of polarons. From the fitting parameters of the polaron peak, we estimate the electron-phonon coupling constant $\ensuremath{\alpha}\ensuremath{\sim}0.4--0.5$, implying that ${\mathrm{CaFe}}_{0.7}{\mathrm{Co}}_{0.3}{\mathrm{O}}_{3}$ falls into the weak-coupling regime.

Momentum-dependent susceptibilities and magnetic exchange in bcc iron from supercell dynamical mean-field theory calculations

We analyze the momentum- and temperature dependences of the magnetic susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by a combination of density functional theory and dynamical mean-field theory (DFT+DMFT). By considering a general derivation of the orbital-resolved effective model for spin degrees of freedom for Hund's metals, we relate momentum-dependent susceptibilities in the paramagnetic phase to the magnetic exchange. We then calculate non-uniform orbital-resolved susceptibilities at high-symmetry wave vectors by constructing appropriate supercells in the DMFT approach. Extracting the irreducible parts of susceptibilities with respect to Hund's exchange interaction, we determine the corresponding orbital-resolved exchange interactions, which are then interpolated to the whole Brillouin zone. Using the spherical model we estimate the temperature dependence of the resulting exchange between local moments.

Experimental evidence for importance of Hund's exchange interaction for incoherence of charge carriers in iron-based superconductors

Angle-resolved photoemission spectroscopy (ARPES) is used to study the scattering rates of charge carriers from the hole pockets near Gamma in the iron-based high-Tc hole doped superconductors KxBa1-xFe2As2 x=0.4 and KxEu1-xFe2As2 x=0.55$ and the electron doped compound Ba(Fe1-xCox)2As2 x=0.075. The scattering rate for any given band is found to depend linearly on energy, indicating a non-Fermi liquid regime. The scattering rates in the hole-doped compound are considerably larger than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times bigger than the binding energy indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron and hole doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-Tc superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.

Hund Interaction, Spin-Orbit Coupling, and the Mechanism of Superconductivity in Strongly Hole-Doped Iron Pnictides.

We present a novel mechanism of s-wave pairing in Fe-based superconductors. The mechanism involves holes near d_{xz}/d_{yz} pockets only and is applicable primarily to strongly hole doped materials. We argue that as long as the renormalized Hund's coupling J exceeds the renormalized interorbital Hubbard repulsion U^{'}, any finite spin-orbit coupling gives rise to s-wave superconductivity. This holds even at weak coupling and regardless of the strength of the intraorbital Hubbard repulsion U. The transition temperature grows as the hole density decreases. The pairing gaps are fourfold symmetric, but anisotropic, with the possibility of eight accidental nodes along the larger pocket. The resulting state is consistent with the experiments on KFe_{2}As_{2}.

Nematicity and Magnetism in FeSe and Other Families of Fe-Based Superconductors

Nematicity and magnetism are two key features in Fe-based superconductors, and their interplay is one of the most important unsolved problems. In FeSe, the magnetic order is absent below the structural transition temperature $T_{str}=90$K, in stark contrast that the magnetism emerges slightly below $T_{str}$ in other families. To understand such amazing material dependence, we investigate the spin-fluctuation-mediated orbital order ($n_{xz}\neq n_{yz}$) by focusing on the orbital-spin interplay driven by the strong-coupling effect, called the vertex correction. This orbital-spin interplay is very strong in FeSe because of the small ratio between the Hund's and Coulomb interactions ($\bar{J}/\bar{U}$) and large $d_{xz},d_{yz}$-orbitals weight at the Fermi level. For this reason, in the FeSe model, the orbital order is established irrespective that the spin fluctuations are very weak, so the magnetism is absent below $T_{str}$. In contrast, in the LaFeAsO model, the magnetic order appears just below $T_{str}$ both experimentally and theoretically. Thus, the orbital-spin interplay due to the vertex correction is the key ingredient in understanding the rich phase diagram with nematicity and magnetism in Fe-based superconductors in a unified way.

Spin-density functional theories and their+Uand+Jextensions: A comparative study of transition metals and transition metal oxides

Previous work on the physical content of exchange correlation functionals that depend on both charge and spin densities is extended to elemental transition metals and a wider range of perovskite transition metal oxides. A comparison of spectra and magnetic moments calculated using exchange correlation functionals depending on charge density only or on both charge and spin densities, as well as the $+U$ and $+J$ extensions of these methods confirms previous conclusions that the spin-dependent part of the exchange correlation functional provides an effective Hund's interaction acting on the transition metal $d$ orbitals. For the local spin density approximation and spin-dependent Perdew-Burke-Ernzerhof generalized gradient approximation, the effective Hund's exchange is found to be larger than 1 eV. The results indicate that at least as far as applications to transition metals and their oxides are concerned, $+U$, $+J$ and +dynamical mean field theory extensions of density functional theory should be based on exchange-correlation functionals of charge density only.

Influence of the electronic structure on the transport properties of some iron pnictides

Abstract An important feature of the iron-based pnictides is their multi-band electronic structure with both electron and hole bands at the Fermi level. The size of these pockets can be changed by different types of substitution, resulting in a variety of original magnetic and electronic properties. The contributions of both types of carriers will thus have important consequences on the evolution of the transport properties versus temperature and doping. It has been pointed out that Hund's rule interaction plays a prominent role in the physics of these compounds by allowing a strong orbital differentiation between the 3d Fe orbitals. As a result, a description in terms of more or less correlated electrons was proposed and may have important consequences on the scattering lifetimes of the different carriers. Finally, the presence of very flat bands at the Fermi level may induce a semiconductor-like behavior, with a change in carrier concentration with temperature. In this paper, we will review the evolution of transport properties with chemical doping/substitution in iron pnictides. We will more particularly focus on the 122 family (Ba(Sr,Ca)Fe2As2) and the 111 LiFeAs compound for which sizeable single crystals required for transport measurements are available. The combined resistivity, Hall effect and magnetoresistance data will be analyzed in association with electronic structure calculations, angle-resolved photoemission measurements and quantum oscillations. In spite of the strong interplay between antiferromagnetism and superconductivity in most part of their phase diagram, direct signatures of spin fluctuations are difficult to identify in the transport properties of iron pnictides. We will show that measurements of the longitudinal magnetoresistance provide a powerful tool for studying the coupling between the charge carriers and the spin degrees of freedom.

Kondo physics in a Ni impurity embedded in O-doped Au chains

By means of ab initio calculations we study the effect of O-doping of Au chains containing a nanocontact represented by a Ni atom as a magnetic impurity. In contrast to pure Au chains, we find that with a minimun O-doping the $5d_{xz,yz}$ states of Au are pushed up, crossing the Fermi level. We also find that for certain O configurations, the Ni atom has two holes in the degenerate $3d_{xz,yz}$ orbitals, forming a spin $S=1$ due to a large Hund interaction. The coupling between the $5d_{xz,yz}$ Au bands and the $3d_{xz,yz}$ of Ni states leads to a possible realization of a two-channel $S=1$ Kondo effect. While this kind of Kondo effect is commonly found in bulk systems, it is rarely observed in low dimensions. The estimated Kondo scale of the system lies within the present achievable experimental resolution in transport measurements. Another possible scenario for certain atomic configurations is that one of the holes resides in a $3d_{z^2}$ orbital, leading to a two-stage Kondo effect, the second one with SU(4) symmetry.

Electronic correlations, magnetism, and Hund's rule coupling in the ruthenium perovskitesSrRuO3andCaRuO3

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials CaRuO$_3$ and SrRuO$_3$ 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 correlation 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 (GdFeO$_3$-type) distortions of the ideal perovskite structure. The presence of magnetism in SrRuO$_3$ and its absence in CaRuO$_3$ despite the larger mass and larger tilt/rotational distortion amplitude of CaRuO$_3$ can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-$T$ properties of CaRuO$_3$ to those of SrRuO$_3$ 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.

Fluctuation-driven magnetic hard-axis ordering in metallic ferromagnets.

We demonstrate that the interplay between soft electronic particle-hole fluctuations and magnetic anisotropies can drive ferromagnetic moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic two-band model with local Coulomb and Hund's interactions and a spin-orbit-induced easy plane anisotropy. The phase diagram is calculated within the fermionic quantum order-by-disorder approach, which is based on a self-consistent free-energy expansion around a magnetically ordered state with unspecified orientation. Quantum fluctuations render the transition of the easy-plane ferromagnet first order below a tricritical point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy, and the moments flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering along the magnetic hard direction.

Two-fluid description for iron-based superconductors

We present a two-fluid description for iron-based superconductors, which contains an itinerant electron Fermi-liquid and a local moment spin-liquid, coupled together via an effective Hund's rule interaction. We examine the low-energy collective behavior of such a system. We find that an electron–spinon composite mode emerges in the intermediate coupling regime, which may account for the hump-dip behavior observed in the recent scanning tunneling spectroscopy experiments. The superconductivity and spin-density-wave phases are consistently described within the same framework. Possible experimental tests are also proposed.

Interplay of many-body and single-particle interactions in iridates and rhodates

Motivated by recent experiments exploring the spin-orbit-coupled magnetism in $4d$- and $5d$-band transition metal oxides, we study magnetic interactions in Ir- and Rh-based compounds. In these systems, the comparable strength of spin-orbit coupling (SOC), crystal field splitting (CF) and Coulomb and Hund's coupling leads to a rich variety of magnetic exchange interactions, leading to new types of ground states. Using a strong coupling approach, we derive effective low-energy super-exchange Hamiltonians from the multi-orbital Hubbard model by taking full account of the Coulomb and Hund's interactions in the intermediate states. We find that in the presence of strong SOC and lattice distortions the super-exchange Hamiltonian contains various kinds of magnetic anisotropies. Here we are primarily interested in the magnetic properties of Sr$_2$IrO$_4$ and Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ compounds. We perform a systematic study of how magnetic interactions in these systems depend on the microscopic parameters and provide a thorough analysis of the resulting magnetic phase diagram. Comparison of our results with experimental data shows good agreement. Finally, we discuss the parameter space in which the spin-flop transition in Sr$_2$IrO$_4$, experimentally observed under pressure, can be realized.

Helical magnetic structures in cubic perovskites

One-dimensional helical structures with a finite Hund interaction have been considered in detail. The analytical expressions for the electron spectrum of charge carriers in simplest one-dimensional and three-dimensional helical structures in a simple cubic lattice, which simulates the manganese sublattice in the crystal structure of perovskite, have been derived in the tight-binding approximation. It has been shown that this approach can explain qualitatively the formation of the antiferromagnetic A structure in LaMnO3.

Highly sensitive spin-crossover transition in a metal-organic molecular crystal

Metal-organic molecular systems are fascinating alternatives to conventional spintronic materials such as transition-metal oxides and diluted magnetic semiconductors, having the advantage of their structural diversity and chemical sensitivity. Here we demonstrate, based on advanced density functional theory calculations, that a cobalt-organic-molecular crystal exhibits particular spin-crossover phenomenon, which is extremely sensitive to external pressure as well as the surrounding water ligands' coordination. We found keen competition between crystal-field splitting and on-site Hund's exchange interaction, resulting in the highly-sensitive spin-crossover transition. Our finding paves the way to soft spintronics, in which molecular spin representing a bit can be easily flipped by small changes in external pressure and temperature at low energy cost.

Magnetic Couplings, Optical Spectra, and Spin-Orbit Exciton in5dElectron Mott InsulatorSr2IrO4

Based on the microscopic model including spin-orbit coupling, on-site Coulomb and Hund's interactions, as well as crystal field effects, we investigate the magnetic and optical properties of Sr(2)IrO(4). Taking into account all intermediate state multiplets generated by virtual hoppings of electrons, we calculate the isotropic, pseudodipolar, and Dzyaloshinsky-Moriya coupling constants, which describe the experiment quite well. The optical conductivity σ(ω) evaluated by the exact diagonalization method shows two peaks at ~0.5 and ~1.0 eV in agreement with experiment. The two-peak structure of σ(ω) arises from the unusual Fano-type overlap between the electron-hole continuum of the J(eff)=1/2 band and the intrasite spin-orbit exciton observed recently in Sr(2)IrO(4).

Role of rotational symmetry in the magnetism of a multiorbital model

Effect of rotationally-invariant Hund's rule coupling on a magnetism of multiorbital Hubbard models is studied within a dynamical mean field theory framework. Comparison of static magnetic susceptibilities and local densities of states of two- and three-orbital models of a complete rotationally invariant Coulomb interaction and a "density-density" Hartree type interaction shows the different role of spin-flip interactions for different band fillings. In the particle-hole symmetric case the Mott-Hubbard physics dominates due to the strong effective Coulomb interaction, while for the multiple electronic configurations away from half-filling (two electrons in the three band model) the formation of local magnetic moments due to Hund's exchange interaction becomes the most significant effect for itinerant magnetic systems. A shift of the temperature of magnetic ordering due to the rotationally-invariant Hund's rule coupling is found to be the largest in a three-orbital model with a two-electron occupancy where the single particle spectrum is metallic and is not sensitive to different forms of the Coulomb vertex. A larger enhancement of the effective mass in a model with a rotationally-invariant interaction is discussed. In the half-filled case we find a drastic change in the density of states close to the Mott transition which is related to the spin-flip Kondo fluctuations in a degenerate orbital case, while the corresponding shift of the magnetic transition temperature is relatively small. It is shown that a change in the ground state degeneracy due to a different symmetry of the Coulomb interaction in the density-density model leads to a breakdown of the quasiparticle peak at the Fermi level in the proximity of a Mott transition on the metallic side. We discuss the relevance of rotationally-invariant Hund's interaction in the transition metal magnetism.

Kondo Resonance Narrowing ind- andf-Electron Systems

We develop a simple scaling theory for the effect of Hund's interactions on the Kondo effect, showing how an exponential narrowing of the Kondo resonance develops in magnetic ions with large Hund's interaction. Our theory accounts for the exponential reduction of the Kondo temperature with spin S of the Hund's coupled moment, first observed in d-electron alloys in the 1960s, and more recently encountered in numerical calculations on multiband Hubbard models. We discuss the consequences of Kondo resonance narrowing for the Mott transition in d-band materials, particularly iron pnictides, and the narrow ESR linewidth recently observed in ferromagnetically correlated f-electron materials.

Effect of Jahn-Teller coupling on Curie temperature in the double-exchange model

We consider the two-band double exchange model for manganites with Jahn-Teller (JT) coupling and explore the suppression of the ferromagnetism because of the JT distortion. The localized spins of the $\emph{t}_{2g}$ electrons are represented in terms of the Schwinger bosons, and two spin-singlet Fermion operators are introduced instead of the $e_{g}$ electrons' operators. In terms of the new Fermi fields the on-site Hund's interaction is in a diagonal form and one accounts for it exactly. Integrating out the spin-singlet fermions, we derive an effective Heisenberg model for a vector which describes the local orientations of the total magnetization. The exchange constants are different for different space directions and depend on the density $n$ of $\emph{e}_{g}$ electrons and JT energy. At zero temperature, with increasing the density of the $\emph{e}_{g}$ electrons the system undergoes phase transition from ferromagnetic phase $(0<n<n_c)$ to A-type antiferromagnetic phase $(n_c<n)$. The critical value $n_c$ decreases as JT energy is increased. At finite temperature we calculate the Curie temperature as a function of electron density for different JT energy. The results show that JT coupling strongly suppresses the spin fluctuations and decreases the Curie temperature.

Spin, charge, orbital and lattice degrees of freedom in quasi-cubic manganites: A brief review

The rich phase diagram of La1-xCaxMnO3 is the consequence of the interplay of spin, charge, orbital and lattice degrees of freedom, giving rise to several magnetic phases, Jahn–Teller distortions, spin-canting and phase separation, and orbital and charge order. Each Mn ion is in a mixed valent state of two magnetic configurations, Mn4+ and Mn3+. We study the phase diagram using mean-field slave bosons for the itinerant eg-electrons in two orbitals with excluded multiple occupancy of each site, Hund's rule interaction between the eg and t2g states and Jahn–Teller coupling of the eg orbitals to the lattice.