Strong Hund's Coupling Research Articles

The superexchange interaction influence on the magnetic ordering in manganites

Manganese oxides such as La 1 −x Sr x MnO 3 and La 1− x Ca x MnO 3 have been attracting considerable attention since the discovery of colossal magnetoresistance (CMR). These materials crystallize in the perovskite-type lattice structure where the crystal field breaks the symmetry of the atomic wave function of the manganese d electrons. The energetically lower t 2g levels are occupied by three localized electrons. Due to a strong Hund coupling their spins are aligned, forming a localized corespin with S=3 /2. The electron configuration of the Mn 3 + ions is t 2 g 3 e g 1 , whereas for Mn 4 + ions the e g electron is missing. Due to a hybridization of the e g wave function with the oxygen 2p orbitals, the e g electrons are itinerant and can move from a Mn 3 + ion to a neighboring Mn 4 + via a bridging O 2 − . In this paper, we present the ferromagnetic (FM) properties of the manganites obtained with the Monte Carlo working on a 2D system, using the ferromagnetic Kondo lattice model (FKLM), including the antiferromagnetic (AF) superexchange (SE) interaction. We investigate the spin–spin correlation function dependence with the electron density, the temperature dependence of structure factor, without the SE interactions and finally, the SE interaction influence of the FM ordering.

Bond electronic polarization induced by spin

We study theoretically the electric polarization induced by noncollinear spin configurations in the limit of strong Hund coupling. We employ a model of two magnetic ions sandwitching an oxygen ion. It is shown that there appears a longitudinal polarization P_x along the bond, which is roughly proportional to (m_r^x)^2-(m_l^x)^2 where m_{r(l)}^x is the x-component of the spin orientation vector at right (left) magnetic ion. A numerical study of the model Hamiltonian yields both longitudinal and transverse electric dipole moments. The transverse polarization is shown to have a non-uniform as well as a uniform component, with the latter being consistent with the previous theory. The longitudinal polarization is non-uniform and oscillating with the period half that of the spin order, but the local magnitude is typically much larger than the uniform transverse polarization and may be detected by X-ray/neutron scattering experiments.

A new theory of doped manganites exhibiting colossal magnetoresistance

Rare earth manganites doped with alkaline earths, namely $Re_1_-_xAxMnO_3$, exhibit colossal magnetoresistance, metal insulator transitions, competing magnetic, orbital and charge ordering, and many other interesting but poorly understood phenomena. In this article I outline our recent theory based on the idea that in the presence of strong Jahn-Teller, Coulomb and Hund's couplings present in these materials, the low-energy electronic states dynamically reorganize themselves into two sets: one set (l) which are polaronic, i.e., localized and accompanied by large local lattice distortion, and another (b) which are non-polaronic and band-like. The coexistence of the radically different l and b states, and the sensitive dependence of their relative energies and occupation upon doping x, temperature T, magnetic field H, etc., underlies the unique effects seen in manganites. I present results from strong correlation calculations using dynamical mean-field theory and simulations on a new 2-fluid model which accord with a variety of observations.

Linear Dichroism in X-Ray Absorption and Orbital Orderings

Linear Dichroism in X-Ray Absorption and Orbital Orderings

Effective spinless fermions in the strong-coupling Kondo model

Starting from the two-orbital Kondo-lattice model with classical ${t}_{2g}$ spins, an effective spinless fermion model is derived for strong Hund coupling ${J}_{H}$ with a projection technique. The model is studied by Monte Carlo simulations and analytically using a uniform hopping approximation. The results for the spinless fermion model are in remarkable agreement with those of the original Kondo-lattice model, independent of the carrier concentration, and even for moderate Hund coupling ${J}_{H}.$ Phase separation, the phase diagram in uniform hopping approximation, as well as spectral properties including the formation of a pseudogap are discussed for both the Kondo-lattice and the effective spinless fermion model in one and three dimensions.

Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density

We studied ferromagnetism in the one-dimensional Hubbard model with doubly degenerate atomic orbitals by means of the density-matrix renormalization-group method and obtained the ground-state phase diagrams. It was found that ferromagnetism is stable from low to high (0< n < 1.75) electron density when the interactions are sufficiently strong. Quasi-long-range order of triplet superconductivity coexists with the ferromagnetic order for a strong Hund coupling region, where the inter-orbital interaction U'-J is attractive. At quarter-filling (n=1), the insulating ferromagnetic state appears accompanying orbital quasi-long-range order. For low densities (n<1), ferromagnetism occurs owing to the ferromagnetic exchange interaction caused by virtual hoppings of electrons, the same as in the quarter-filled system. This comes from separation of the charge and spin-orbital degrees of freedom in the strong coupling limit. This ferromagnetism is fragile against variation of band structure. For high densities (n>1), the phase diagram of the ferromagnetic phase is similar to that obtained in infinite dimensions. In this case, the double exchange mechanism is operative to stabilize the ferromagnetic order and this long-range order is robust against variation of the band-dispersion. A partially polarized state appears in the density region 1.68<n<1.75 and phase separation occurs for n just below the half-filling (n=2).

LiV2O4: evidence for two-stage screening

LiV2O4, a frustrated mixed valent metal (d1↔d2), is argued to undergo two spin-screening processes. The first quenches the effective spin to produce the spin 12 behavior seen below room temperature [1], while the second produces the heavy Fermi liquid character seen at low temperatures [2]. We present a t–J model with strong Hund's coupling of the strongly correlated d-electrons. Valence fluctuations of the Hubbard operators (S=12↔1) combined with the frustration of the underlying corner-shared tetrahedral vanadium lattice are the essential components of our model.

Incommensurate Structures in the Double-Exchange System

We study a relation between the gauge invariance and incommensurate structures of the double-exchange model at zero temperature. The phenomena which are related to them are discussed. are localized (t2g) spins which are treated as classical vectors directed along (θi ,φ i) in the spherical coordinates. The i or j is the index of the site on an arbitrary lattice under consideration. The second term is the Hund's rule coupling between the eg electron and the localized t2g spin. Moreover, J(≥0) is the direct exchange coupling strength between t2g spins. We denote the spin configuration by a set {θi ,φ i} (i =1 , 2, ··· ,N ,where N is the total number of the sites). In the real materials,the eg electron is almost polarized because of the strong Hund coupling, JH/t,and the limit JH/t →∞ is widely used. In the limit,the eg electron spin perfectly aligns along the t2g spin direction and the electron has effectively only single component,i.e.,spinless fermion. One of the standard methods to treat the

Quantum spin assisted tunneling in half-metallic manganite tunnel junctions

The electron tunneling in half metallic manganite tunnel junctions is studied by using a quantum mechanically treated double exchange model. We show that the stimulation of spin excitations, caused by the strong Hund's coupling between the conduction $e_g$ electrons and the localized quantum spins of the manganite ions, would assist electrons to tunnel through the junction even with the antiparallel aligned magnetizations of the electrodes. This mechanism gives rise to an extra tunnel conductance, in addition to that predicted by Julliere's model. Our theory is in good agreement with the voltage dependence of the tunnel conductance in manganite tunnel junctions observed by experiments.

Ferromagnetism of Sulfur Deficient BaVS3-δ

Ferromagnetic properties of sulfur deficient BaVS 3-δ with δ= 0.05, 0.10, 0.15 and 0.20 were measured to estimate the Curie temperature and the saturation magnetization. The averaged saturation magnetization is less than 0.4 µ B / V, which is far smaller than 1 µ B expected for S = 1/2. 51 V NMR experiments revealed the coexistence of ferromagnetic and nonmagnetic (or spin-singlet) phases. A strong Hund coupling is suggested as a possible origin of the ferromagnetic interaction in the sulfur deficient system, referring to the temperature dependence of the resistivity.

Temperature-induced MIT in doubly degenerate Hubbard model

In this paper the doubly degenerate model of narrow band material is studied. By means of generalized Hartree–Fock approximation in the Green function technique the cases of strong and weak Hund's coupling are considered. The metal–insulator transitions in the model at different integer values of electron concentrations are studied.

CPA susceptibility of the double exchange model

We use a many-body coherent potential approximation (CPA) to study the paramagnetic state of the double exchange model with finite local spins S. Our CPA treats dynamical effects due to the quantum nature of the local spins in a local approximation, becomes exact in the atomic limit, and reduces to Kubo's one-electron CPA in the empty band limit and to dynamical mean field theory (DMFT) in the classical S→∞ limit. We solve the CPA equations of motion for all S in the strong Hund coupling limit, determining all quantities self-consistently. We show that the CPA does not give a ferromagnetic transition for any S; this is due to the local spin probability distribution function P( S z ) being inaccurately obtained. We bypass this difficulty by extrapolating P( S z ) from the exact (within DMFT) S=∞ result, and obtain a magnetic transition for all S with the expected Curie laws at band filling n=0, 1 .

Metal-insulator transition in a doubly orbitally degenerate model with correlated hopping

In the present paper, we propose a doubly orbitally degenerate narrow-band model with correlated hopping. The peculiarity of the model is taking into account the matrix element of electron-electron interaction, which describes intersite hoppings of electrons. In particular, this leads to the concentration dependence of the effective hopping integral. The cases of the strong and weak Hund's coupling are considered. By means of a generalized mean-field approximation the single-particle Green function and quasiparticle energy spectrum are calculated. Metal-insulator transition is studied in the model at different integer values of the electron concentration. With the help of the obtained energy spectrum, we find energy gap width and criteria of metal-insulator transition.

Effect of the Orbital Level Difference in Doped Spin-1 Chains

Doping of a two-orbital chain with mobile S=1/2 Fermions and strong Hund's rule couplings stabilizing the S=1 spins strongly depends on the presence of a level difference among these orbitals. By DMRG methods we find a finite spin gap upon doping and dominant pairing correlations without level-difference, whereas the presence of a level difference leads to dominant charge density wave (CDW) correlations with gapless spin-excitations. The string correlation function also shows qualitative differences between the two models.

Phase diagram of an extended Kondo lattice model for manganites: The Schwinger-boson mean-field approach

We investigate the phase diagram of an extended Kondo lattice model for doped manganese oxides in the presence of strong but finite Hund's coupling and on-site Coulomb interaction. By means of the Schwinger-boson mean-field approach, it is found that, besides magnetic ordering, there will be nonuniform charge distributions, such as charge ordering and phase separation, if the interaction between electrons prevails over the hybridization. Which of the charge ordering and phase separation appears is determined by a competition between effective repulsive and attractive interactions due to virtual processes of electron hopping. Calculated results show that strong electron correlations caused by the on-site Coulomb interaction as well as the finite Hund's coupling play an important role in the magnetic ordering and charge distribution at low temperatures.

Monte Carlo study of the magnetic phase diagram of a double-exchange model with the background superexchange interaction

The magnetic phase diagrams of the double-exchange model with an antiferromagnetic superexchange interaction between localized spins are investigated on a three-dimensional 3D lattice by using a Monte Carlo technique. The transition temperature ${T}_{c}$ is obtained from the calculated susceptibility and is used to determine the phase boundaries. As a result, the antiferromagnetic phase is realized, and the dependence of the ferromagnetic ${T}_{c}$ on the doping concentration exhibits a maximum near a doping of $x\ensuremath{\sim}1/3.$ In contrast to the mean-field predictions, if the magnetic field is included, the ferromagnetic ordering is strongly enhanced, and the antiferromagnetic ordering is not replaced by a canted phase but is suppressed at low doping levels. Moreover, the relevance of this complicated spin ordering to the phase separation observed in the ferromagnetic Kondo lattice model with strong Hund coupling is studied further.

Effective Hamiltonian for an extended Kondo-lattice model and a possible origin of charge ordering in half-doped manganites

An effective Hamiltonian is derived in the case of the strong Hund coupling and on-site Coulomb interaction by means of a projective perturbation approach. A physical mechanism for charge ordering in half-doped manganites ${(R}_{0.5}{X}_{0.5}{\mathrm{MnO}}_{3})$ is proposed. The virtual process of electron hopping results in antiferromagnetic superexchange and a repulsive interaction, which may drive electrons to form a Wigner lattice. The phase diagram of the ground state of the model is presented at half doping. In the case of formation of a Wigner lattice, we prove that spins of electrons are aligned ferromagnetically as well as that the localized spin background is antiferromagnetic. The influence of the on-site Coulomb interaction is also discussed.

Coexistence of antiferromagnetic and ferromagnetic phase for ferromagnetic Kondo lattice model

To study the proposed phase separations in doped manganites, we performed Monte-Carlo calculations for the ferromagnetic Kondo lattice model with strong Hund's coupling between conduction electrons and localized spins. For the practical calculations, we adopted a one dimensional lattice and treated the spins of the localized t2g electrons semi-classically. A direct evidence of the phase separation is observed from a snapshot of the spatial dependence of localized spins. No indication of the canted or spiral phases is found in the results of simulations. Further, the calculated results of the spin structure factor in the phase separation region are well compared with recent experiments.

Crystal structure and magnetoresistance of Na-doped LaMnO3

The crystal structure and magnetoresistance of are investigated. La1-xNaxMnO3 crystallizes in a rhombohedrally distorted perovskite structure and exhibits a sharp ferromagnetic transition as well as a negative magnetoresistance at around room temperature. On the basis of alternating-current susceptibility and resistivity measurements as well as a comparison with La1-xNaxMnO3 compounds, it is proposed that Na doping tends to drive the system from a regime characterized by strong Hund coupling and strong electron-phonon coupling to one characterized by weak Hund coupling and weak electron-phonon coupling.

Properties of Manganites: Band and Percolation Approaches

Properties of the parent compound, LaMnO3 are greatly affected by strong Hund's coupling. The result of this coupling is antiferromagnetic ordering (with a low Neel temperature) along one of the cubic axes and, as a consequence, a picture of disconnected ferromagnetic layers. Furthermore, the cooperative Jahn–Teller effect turns the compound into a band insulator. At small doping (La1−x Ca x MnO3, x≪1) the band insulator state coexists with the presence of localized holes. An insulator-ferromagnetic metal transition occurs at the percolation point x c=0.16.