Superexchange interaction in rare earth dielectrics

The second quantization method is applied to the analysis of superexchange interaction in rare earth dielectrics. Introduction of double tensor operators associated with the second quantization operators makes it possible to write the superexchange interaction Hamiltonian in the effective form of the total magnetic moment interaction of ions . As an example, the obtained Hamiltonian is used to explain the magnetic properties of rare earth pnicudes of the yttrium group.Translated from Izvestiya VUZ. Fizika, No. 12, pp. 78–82, December, 1973.The authors thank B. V. Karpenko and V. Ya. Mitrofanov for useful discussions.     

顺磁性物质CeF3中磁化率和费尔德常数的定量计算

本文首先计及晶场对顺磁性物质CeF_3中Ce~(3+)离子4f基态和5d激发态能级的影响,进而考虑了间接交换作用有效场II_v和外磁场H_e共同作用下导致的晶场基态能级的进一步分裂,然后应用基态双能级跃迁模型定量计算了CeF_3中的比法拉第旋转θ_F,费尔德常数F和磁化率X以及它们与温度的依赖关系.计算表明,CeF_3中的磁光效应及其温度特性与Ce~(3+)离子之间的间接交换作用密切相关,在60相似文献   

Suppression of antiferromagnetic interactions through Cu vacancies in Mn-substituted CuInSe2 chalcopyrites

Stoichiometric and Cu-poor Cu(0.95-x)Mn(0.05)InSe(2) (x = 0-0.20) compounds were synthesized by high-temperature, solid-state reactions. The presence of copper vacancies is revealed by Rietveld refinements of combined neutron and x-ray powder diffraction data. The antiferromagnetic interaction is depressed by the copper deficiency, which may be explained as the competition between the antiferromagnetic Mn-Se-Mn superexchange interaction and the hole-mediated ferromagnetic exchange induced by the copper vacancy. The introduction of copper vacancies is proposed to be a viable route to impart carrier-mediated ferromagnetic exchange in the chalcopyrite-based dilute magnetic semiconductors.     

Superexchange mechanism of energy transfer between neighboring lanthanide ions in dielectric crystals

It is shown that unusually rapid energy transfer between neighboring lanthanide ions in dielectric crystals is caused by the efficient short-range interaction, which is similar to the magnetic superexchange interaction. Both these interactions are related to virtual transition of electrons between lanthanide ions via common bridge ligands. A general method is developed for calculating matrix elements of the effective superexchange Hamiltonian, and the rates of energy transfer in exchange-coupled pairs of lanthanide ions are estimated. The possibility of using the superexchange model of energy transfer for analysis of up-conversion in lanthanide crystals is discussed.     

Charge Ordering Due to Antiferromagnetic Correlation in Quarter-Filled Manganites

In the present paper, we study the zero-temperature phase diagram of the doped perovskite manganites at filling $x=0.5$ by the real-space Hartree-Fock approximation method. Our purpose is to resolve a controversial issue arising recently on the origin of the charge ordered phases in these systems. We find that the antiferromagnetic superexchange interaction between the localized spins plays the central role in producing the concerned phases. Our results confirm some speculations on this issue.     

Exact and numerical results for a dimerized coupled spin- 1/2 chain

We establish exact results for coupled spin-1/2 chains for special values of the four-spin interaction V and dimerization parameter delta. The first exact result is at delta = 1/2 and V = -2. Because we find a very small but finite gap in this dimerized chain, this can serve as a very strong test case for numerical and approximate analytical techniques. The second result is for the homogeneous chain with V = -4 and gives evidence that the system has a spontaneously dimerized ground state. Numerical diagonalization and bosonization techniques indicate that the interplay between dimerization and interaction could result in gapless phases in the regime 0相似文献   

Inhomogeneity-induced enhancement of the pairing interaction in cuprate superconductors

Scanning tunneling spectroscopy has recently discovered a positive correlation between the magnitude of the superconducting gap and positions of dopant oxygen atoms in Bi-based cuprates. We propose a microscopic mechanism that could be responsible for this effect. In particular, we demonstrate that the dopant-induced spatial variation of the atomic levels always enhances the superexchange interaction.     

Influence of Dzyaloshinskii-Moriya and Kaplan-Shekhtman-Entinwohlman-Aharony superexchange interactions on ground state properties of the one-dimensional spin-Peilers model in open chain

The effects of the Dzyaloshinskii-Moriya (DM) and the Kaplan-Shekhtman-Entinwohlman-Aharony (KSEA) superexchange interactions on the ground state properties of the one-dimensional spin-Peilers system in open chain are studied by using the Lanczos numerical method.The study concentrates mainly on the influence of systemic dimerisation in open chain.The results show that systemic ground state energy density varies with dimerisation parameter δ in different DM interactions,and there exists a special point δ c where the DM interaction has no influence on the systemic dimerisation,no matter whether the DM interaction is relative or irrelative to systemic dimerisation (η=1 or η=0).The KSEA interaction has no fixed special point,but the points of intersection are dense relatively in a certain numberical value range,and sparse in other numberical value ranges.So we can conclude that the antisymmetric anisotropy DM interaction differs from the symmetric anisotropy KSEA interaction,but they are analogous in the sense of the influence of systemic dimerisation in open chain.     

Ising model for the alloys according to the effective field theory

FeAl alloys in their disordered structural phase have been investigated through an Ising model where besides exchange interactions between nearest-neighbors Fe atoms, a superexchange interaction mediated by Al atoms is also taken into account. The model has been approximately treated according to the effective field theory. Although the phase diagram, as a function of Al concentration, is similar to the one previously obtained from Bogoliubov variational approach for the free energy, a different behavior for the superexchange interaction is achieved, which can also be physically accepted for this system.     

Spin-vibronic superexchange in Mott-Hubbard fullerides

In the Mott-Hubbard cubic fulleride Li3(NH3)6C60 the superexchange energy is found to be much smaller than the rotational quantum for Jahn-Teller deformations at fullerene sites. This gives rise to a new type of superexchange interaction involving threefold degenerate vibronic ground states of C3-60 ions. In contrast with spin-orbital models, the spin-vibronic superexchange can be only antiferromagnetic and shows a significant vibronic reduction of the superexchange amplitude, in agreement with magnetic susceptibility data. As a function of the transfer parameters, two quadrupolar fully dynamical vibronic orders with quenched vibronic moments on sites develop in the ground state.     

Coexistence of antiferromagnetic and ferromagnetic orders at remanent state in epitaxial multiferroic Bi2FeCrO6 nanostructures

We report the local electronic and magnetic properties of Bi(2)FeCrO(6) nanostructures by element-specific polarized x-ray techniques. Sizable magnetic ordering in the remanent state is observed at room temperature for both Fe and Cr ions. The Bi(2)FeCrO(6) system offers an example of d(5)-d(3) magnetic superexchange interaction with a magnetic order for both Fe and Cr, which are both formally in the +?3 valence state. The results suggest a coexistence of antiferromagnetic and ferromagnetic superexchange interaction between Fe and Cr spins in the nanostructures at the remanent state and at room temperature.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Spin order due to orbital fluctuations: cubic vanadates

We investigate the highly frustrated spin and orbital superexchange interactions in cubic vanadates. The fluctuations of t(2g) orbitals trigger a novel mechanism of ferromagnetic interactions between spins S = 1 of V3+ ions along one of the cubic directions which operates already in the absence of Hund's rule exchange J(H), and leads to the C-type antiferromagnetic phase in LaVO3. The Jahn-Teller effect can stabilize the orbital ordering and the G-type antiferromagnetic phase at low temperatures, but large entropy due to orbital fluctuations favors again the C phase at higher temperatures, as observed in YVO (3).     

Dynamics of solitary wave scattering in the Fermi-Pasta-Ulam model

In this Letter, we study the scattering dynamics of a pair of solitary waves in the Fermi-Pasta-Ulam model with interaction potential V(x)=alphax(2)/2+x(4)/4 and establish a quantitative connection between the scattering property and the energy transport behavior. The energy and momentum conservation laws are obtained and the scattering rates of solitary waves are calculated. Our studies suggest that the anharmonic limit model with alpha=0 can be taken as a paradigm model for studying lattice solitary waves.     

Magnetic Properties and Spin Crossover in Transition Metal Oxides with d 5 Ions at High Pressures

We analyze the influence of cooperative effects on the magnetic properties and spin crossover between the high-spin (HS) term S = 5/2 and low-spin (LS) term S = 1/2 in Mott–Hubbard dielectrics with 3d5 ions under high pressures. Two cooperation mechanisms (superexchange interaction and effective interaction via the elastic system) are considered. The sign of the exchange interaction changes because of the crossover from the antiferromagnetic in the HS state to the ferromagnetic in the LS state. In view of the large difference between the ionic radii of the HS and LS states, the systems with spin crossover acquire an additional strong coupling via the elastic system. Using the Hubbard operator representation and considering the electronic states of the two terms simultaneously, we obtain the effective Hamiltonian with allowance for the cooperative effects. The magnetic phase diagram and the spin crossover are investigated in the mean field approximation. It is shown that the inclusion of cooperative effects at low temperatures leads to a first-order phase transition between the antiferromagnetic HS state and the ferromagnetic LS state. At higher temperatures, more complicated sequences of phase transitions are possible upon an increase in pressure, including the HS paramagnet–HS antiferromagnet–LS paramagnet and HS antiferromagnet–LS paramagnet–LS ferromagnet transitions.     

Pressure-induced transition from localized electron toward band antiferromagnetism in LaMnO(3)

The temperature dependence of the ac susceptibility under pressure has been used to track the Néel temperature T(N) of the Mott insulators LaMnO3, CaMnO3, and YCrO3. Bloch's rule relating T(N) to volume V, viz., alpha=dlog(T(N)/dlog(V=-3.3, is obeyed in YCrO3 and CaMnO3; it fails in LaMnO3. This breakdown is interpreted to be due to a sharp increase in the factor [U(-1)+(2Delta)(-1)] entering the superexchange perturbation formula. A first-order change at 7 kbar indicates that the transition from localized-electron to band magnetism is not smooth.     

Phase diagram of the disordered RKKY model in dilute magnetic semiconductors

We consider ferromagnetism in spatially randomly located magnetic moments, as in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect exchange RKKY interaction. We obtain, via Monte Carlo calculations, the magnetic phase diagram as a function of the impurity moment density n(i) and the relative carrier concentration n(c)/n(i). As evidenced by the diverging correlation length and magnetic susceptibility, the boundary between ferromagnetic and nonferromagnetic phases constitutes a line of zero temperature critical points which can be viewed as a magnetic percolation transition. In the dilute limit, we find that bulk ferromagnetism vanishes for n(c)/n(i) >0.1. We also incorporate the local antiferromagnetic direct superexchange interaction between nearest neighbor impurities and examine the impact of a damping factor in the RKKY range function.     

Electron pairing by cooperation of superexchange and polaron effects

This paper proposes a pairing mechanism relevant to high T_c superconductors with strong electron correlations. The Hubbard model is extended to account for a large electron-phonon interaction. It is shown that the band width is much reduced by the polaron effect, but that the superexchange interaction is not reduced. The cooperation of superexchange and polaron effects makes an anisotropic singlet pair a stable bound state, with the largest binding energy in the one-dimensional system.     

Macroscopic degeneracy and emergent frustration in a honeycomb lattice magnet

Using a hybrid method based on fermionic diagonalization and classical Monte Carlo techniques, we investigate the interplay between itinerant and localized spins, with competing double- and superexchange interactions, on a honeycomb lattice. For moderate superexchange, a geometrically frustrated triangular lattice of hexagons forms spontaneously. For slightly larger superexchange a dimerized ground state is stable that has macroscopic degeneracy. The presence of these states on a nonfrustrated honeycomb lattice highlights novel phenomena in this itinerant electron system: emergent geometrical frustration and degeneracy related to a symmetry intermediate between local and global.     

Spin dynamics and orbital state in LaTiO3

A neutron scattering study of the Mott-Hubbard insulator LaTiO3 ( T(N) = 132 K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant J = 15.5 meV and a small Dzyaloshinskii-Moriya interaction ( D = 1.1 meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO3.