Showing papers on "Exchange interaction published in 1989"

Motion of a single hole in a quantum antiferromagnet

Abstract: We formulate a quasiparticle theory for a single hole in a quantum antiferromagnet in the limit that the Heisenberg exchange energy is much less than the hopping matrix element, J\ensuremath{\ll}t. We consider the ground state of the spins to be either a quantum N\'eel state or a d-wave resonating-valence-bond (RVB) state. We show in a self-consistent perturbation theory that the hole spectrum is strongly renormalized by the interactions with spin excitations. The hole can be described by a narrow quasiparticle band located at an energy of order -t with a quasiparticle residue of order J/t and a bandwidth of order J. Above the quasiparticle band is an incoherent band of width of order t. Our results indicate that the energy scale for any coherent phenomenon involving the holes is \ensuremath{\delta}J, where \ensuremath{\delta} is the doping concentration. In the N\'eel state we perform a spin-wave expansion on an anisotropic Heisenberg model. In the Ising limit we reproduce previously known results and then expand perturbatively about that limit. In this expansion we find that the holes have a quasiparticle residue of ${J}_{z}$/t and a bandwidth of ${J}_{\ensuremath{\perp}}$. In the Heisenberg limit we employ a ``dominant pole'' approximation in which we ignore contributions to the self-energy from the incoherent part of the hole spectrum. A similar technique is used to study the d-wave RVB state. The relevance of our results to recent optical experiments is discussed.

Phase diagram of the frustrated spin-1/2 Heisenberg antiferromagnet in 2 dimensions.

Abstract: A Lanczos technique is used to study the frustrated spin-1/2 Heisenberg model on square lattices of 16 and 20 sites. Frustration is introduced by an interaction along the diagonals of the plaquettes with coupling J2 greater than or equal to 0. For large J2, the ground state breaks (spontaneously) the lattice rotational symmetry. For intermediate values of J2, the squares of order parameters associated with spin-Peierls and 'twisted'states have a peak, while a similar quantity for a chiral state shows no interesting structure.

Large exchange interactions in the electron gas of GaAs quantum wells.

Abstract: Inelastic-light-scattering measurements show that exchange Coulomb interactions in the two-dimensional electron gas of GaAs microstructures are more important than previously anticipated. Small-wave-vector spectra from modulation-doped quantum wells exhibit unexpected single-particle intersubband transitions in addition to collective spin-density and charge-density modes. From the measured spectral energies the direct and exchange intersubband Coulomb interactions are determined and found to be of comparable strengths.

Zero-temperature ordering in two-dimensional frustrated quantum Heisenberg antiferromagnets

Abstract: La phase magnetiquement desordonnee du modele d'Heisenberg pour un reseau carre, accessible aux developpements des dimers, se revele spontanement dimerisee dans la configuration «en colonne» predite par Read et Sachdev. Les estimations du parametre d'ordre de dimerisation sont une fraction substantielle de celui de l'etat entierement dimerise

Valence-electron excitations in the alkali metals

Abstract: To study the dynamical response of the valence-electron gas in nearly-free-electron metals, we carried out an electron-energy-loss investigation on Na, K, Rb, and Cs. On polycrystalline samples we measured the volume plasmon dispersion, which showed strong deviations not only from the random-phase approximation, but deviations as well from the predictions of the current theories for Fermi liquids, including short-range exchange and correlation. In addition, anomalous dispersion of the plasmon half-width was found. Both results may be traced back to exchange and correlation effects not properly treated by the current Fermi-liquid theories. Furthermore, we observed the excitation of intraband transitions in Na and Rb, the dispersion of which yields the effective band mass for unoccupied states. The results for Na are at variance with recently published enhancement of the band mass for the occupied states. To elucidate the influence of band structure, measurements were also performed on a single-crystalline Na film and we observed the so-called zone-boundary collective state in Na.

3d-5d band magnetism in rare earth transition metal intermetallics: LuFe2

Abstract: Self-consistent energy band calculations for LuFe2 are reported. LuFe2 is found to order magnetically according to the Stoner criterion and the calculated moment is 2.85 mu B/formula unit in agreement with experiment. Spin-orbit coupling was included in the calculation and the spin and induced orbital contributions to the moment at the Fe site also found to be in agreement with experiment. The total moment is calculated to include a contribution of -0.41 mu B at the Lu site, which as yet has not been observed. The mechanism responsible for the ferrimagnetic alignment of the 3d and 5d spin densities is investigated in detail, and the Lu-5d moment related to 3d-5d hybridisation.

Nonadditive effects in HF and HCl trimers

Abstract: Nonadditive effects are calculated for (HF)3 and (HCl)3 complexes and analyzed via the combination of perturbation theory of intermolecular forces with Mo/ller–Plesset perturbation theory (MPPT). In both systems the nonadditivity is dominated by the self‐consistent field (SCF) deformation effect, i.e., mutual polarization of the monomer wavefunctions. Heitler–London exchange and correlation effects are of secondary importance. Three‐body terms exhibit much lesser basis set dependence than the two‐body effects and even quite moderate basis sets which are not accurate enough for treatment of two‐body forces can yield three‐body effects of quantitative quality. This is due in large measure to the additivity of strongly basis set dependent components such as uncorrelated and correlated electrostatics and dispersion. Various approximate models for the three‐body potentials and total interaction in the (HF)3 cluster are analyzed from the point of view of their ability to predict the orientation dependence of in...

Magnetization reversal in CoCr perpendicular thin films

Abstract: Computer simulation has been utilized to study magnetization reversal processes in CoCr perpendicular films. The model is based on the columnar structure of the film. Each column is considered to be a single crystal with perpendicular uniaxial crystalline anisotropy. The model assumes that each column is always uniformly magnetized even during its magnetization reversal. The gyromagnetic equation of motion with phenomenological Landau–Lifshitz damping is utilized to describe the magnetization rotation of this coupled system. The study focuses on the collective magnetization reversal modes of the particles due to magnetostatic interactions and intergranular exchange coupling. Low nucleation fields occur which are characterized by a planar chain nucleation mode. This yields coercivities equal to or less than the crystalline anisotropy field 2K/M. It is argued that, due to this collective process, the uniform rotation reversal mechanism for the individual particles in the film is energetically more favorable than nonuniform reversal, such as curling. Intercolumn exchange coupling significantly reduces the coercivity and an increase of the exchange coupling strength changes the magnetization behavior from ‘‘particulate’’ to ‘‘continuous.’’

Finite-Temperature Ferromagnetism of Nickel

Abstract: By use of a generalized Hubbard model, we investigate the influence of electron correlations on the temperature dependence of the quasiparticle properties of ferromagnetic Ni. The one-particle energies of the model Hamiltonian are taken from a realistic band-structure calculation. The model contains only two parameters, Hubbard U and the interband exchange J. It is approximately solved by use of a self-consistent moment method. We find a ferromagnetic ground state, mainly caused by the uppermost d subband, a magnetic moment at T=0 of 0.56${\ensuremath{\mu}}_{B}$, a Curie temperature of ${T}_{C}$=635 K, a Brillouin-type magnetization curve, a strict Curie-Weiss behavior of the paramagnetic susceptibility, a satellite peak some 6 eV below the chemical potential \ensuremath{\mu} as a consequence of strong electron correlations in the uppermost d subband, a temperature-dependent spin polarization of the satellite (\ensuremath{\simeq}75% at T=0), temperature-dependent exchange splittings at the top of the d band (0.23--0.36 eV at T=0), and an enhancement factor of the electronic specific heat q(T=0) \ensuremath{\simeq}-0.56. All these results are in excellent agreement with the experiment. For the first time the full temperature dependence of the quasiparticle band structure and the quasiparticle density of states of ferromagnetic Ni are presented.

Analysis of the potential energy surface of Ar–NH3

Abstract: The combination of supermolecular Mo/ller–Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potential energy surface of Ar–NH3. Anisotropy of the self‐consistent field (SCF) potential is determined by the first‐order exchange repulsion. Second‐order dispersion energy, the dominating attractive contribution, is anisotropic in the reciprocal sense to the first‐order exchange, i.e., minima in one nearly coincide with maxima in the other. The estimated second‐order correlation correction to the exchange effect is nearly as large as a half ΔESCF in the minimum and has a ‘‘smoothing’’ effect on the anisotropy of e(20)disp. The model which combines ΔESCF with dispersion energy (SCF+D) is not accurate enough to quantitatively describe both radial and angular dependence of interaction energy. Comparison is also made between Ar–NH3 and Ar–PH3, as well as with the Ar dimer.

Energy partitioning of the self‐consistent field interaction energy of ScCO

Abstract: The SCF interaction energy for ScCO was partitioned into the electrostatic, polarization, exchange, and charge‐transfer terms via the Morokuma analysis This definition of polarization does not satisfy the Pauli exclusion principle and therefore, there is nothing to prevent the valence electrons of one fragment from collapsing into the core orbitals of the other fragment To correct this problem, the polarization and charge‐transfer terms were calculated in the presence of exchange The dominant interactions were exchange repulsion, electrostatic attraction, and distortion of the wave function by polarization and charge transfer Pi backbonding was the dominant distortion for the 4 Σ− 4s1 3dπ2 state, while sigma distortions dominated the 4s2 3d 1 states

Solvent influence on the magnetic field effect of polymethylene-linked photogenerated radical ion pairs

Abstract: The solvent viscosity and polarity dependence of the magnetic field effect in polymethylene‐linked radical ion pairs, which were generated by photoinduced intramolecular electron transfer in compounds of the type pyrene–(CH2)n–N,N‐dimethylaniline, has been studied. A stochastic Liouville equation is used, in which the dynamics of the polymethylene chain, the spin Hamiltonian as a function of the varying radical distance (exchange interaction), and a distance‐dependent back electron transfer rate are incorporated. The results are compared with predictions made on the basis of the (static) subensemble approximation.

Statistics of holons in the quantum hard-core dimer gas

Abstract: The nature of holons, and in particular, their statistics, are studied in the context of a two-dimensional quantum hard-core dimer gas. This model has been shown to embody the low-energy physics of the short-ranged resonating-valence-bond state. We find that, depending on detailed energetic considerations, the holons either bind a half flux quantum of "statistical flux," in which case they are fermions, or they are free, in which case they are bosons. The exchange energy is shown to favor a fermionic hole while the hole kinetic energy (which generally exceeds the exchange energy) favors bosonic holes. Finally, it is shown that even in the bosonic case, flux quantization is in units of $\frac{\mathrm{hc}}{2e}$.

One-Dimensional Isotropic Spin-1/2 Heisenberg Magnet with Ferromagnetic Nearest-Neighbor and Antiferromagnetic Next-Nearest-Neighbor Interactions

Abstract: We study the ground-state and finite-temperature properties of the one-dimensional isotropic spin-1/2 Heisenberg magnet with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions which compete with each other. Extrapolating the exact results for finite-size systems of up to 20 spins, we estimate the ground-state energy and the ground-state two-spin correlation function in the infinite-size limit. Using the cluster transfer matrix method, we also calculate the temperature dependence of the internal energy and the specific heat and that of the inverse correlation length and the wave number, by both of which the asymptotic behavior of the two-spin correlation function in the long-distance limit is characterized. We compare the results of these calculations with those obtained previously for the case where the nearest-neighbor interaction is antiferromagnetic.

Experimental evidence of the Dzyaloshinsky-Moriya antisymmetric exchange interaction in the one-dimensional Heisenberg antiferromagnet KCuF3: EPR measurements

Abstract: Based on the authors' results of EPR experiments, they point out that KCuF3, a one-dimensional Heisenberg antiferromagnet in spite of its pseudo-cubic crystal structure, has a Dzyaloshinsky-Moriya (DM) antisymmetric exchange interaction Sigma dijSi*Sj between spins on the c-axis; the direction of dij is perpendicular to the c-axis. The observed EPR linewidth shows (2+sin2 theta )-like angular behaviour ( theta is the angle between the c-axis and the external field), which coincides well with the theory of line broadening due to the DM interaction with dij perpendicular to c-axis. The linewidth shows a rapid decrease with lowering temperature and its value extrapolated to T=0 K from the paramagnetic region is zero. As the theory by Soos et al. (1977) shows, this temperature dependence also indicates that the EPR line is governed by the DM interaction; i.e. it arises from the four-spin correlation functions ((Sialpha Sjbeta -Sibeta Sjalpha )2) (where alpha and beta =x, y and z) in the second moment due to the DM interaction which is different from ((Sialpha Sjbeta +Sibeta Sjalpha )2) of the perturbation terms of both the dipole-dipole and the anisotropic exchange interactions.

Spin glasses and nonergodicity

Abstract: This review is devoted to spin glasses, i.e., disordered magnets with randomly distributed ferro- and antiferromagnetic exchange interactions. Extensive experimental data and the theory developed for a simple model with an infinite exchange interaction range show that, as the temperature is reduced, the competition between different types of disorder leads to a nonergodic (or quasi-nonergodic) state in spin glasses. The fundamentals of the statistical mechanics of nonergodic systems are established. The review also examines the connection between the problem of spin glasses and those of combinatorial optimization, as well as the use of spin-glass models in biology. Attempts to construct a theory of real spin glasses with a distance-dependent exchange interactions are discussed.

Spin-polarized electron-energy-loss spectroscopy on Ni.

Abstract: High-resolution spin-polarized electron-energy-loss data on Ni(110) are presented. Exchange scattering is found to be the dominant loss mechanism. The Stoner excitation spectrum shows a sharp threshold around 65-meV energy loss (Stoner gap). The energy-loss probability for spin-down electrons is up to 3 times larger than for spin-up electrons. An interpretation in terms of the Ni 3d band structure is given.

4f Electron effects in high Tc RBa2Cu3O7 − δ (R = rare earth) superconductors

Abstract: Experiments on high TcRBa2Cu3O7 − δ (R = rare earth; δ ≈ 0.1) compounds have revealed a variety of interesting effects that involve the 4f electrons of the R ions. Schottky anomalies due to the partial lifting of the degeneracy of the Hund's rules ground state multiplets of the R ions by the crystalline electric field (CEF) are found in the low temperature specific heat. With some exceptions, the anisotropy of the room temperature paramagnetic susceptibility correlates with the sign of the second order Stevens factor of the CEF hamiltonian. Specific heat anomalies due to the antiferromagnetic ordering of the R3+ ions in RBa2Cu3O7 − σ compounds with R  Nd, Sm, Dy and Er can be described well by the anisotropic twodimensional Ising model with an exchange interaction parameter ratio that ranges from approximately 50 for neodymium to approximately 4 for dysprosium. The magnetic ordering temperatures and shapes of the magnetic specific heat anomalies change markedly when the superconductivity of the compounds is quenched by increasing the oxygen vacancy concentration to δ ≳ 0.5, indicating that RKKY and/or superexchange interactions are involved in the magnetic ordering of the R3+ ions in these materials. The anomalous pressure dependence of Tc and the normal state electrical resistivity in the (Y1 − xPrxx)Ba2Cu3O7 − δ system suggest that the praseodymium 4f states are hybridized with valence band states, which may be responsible for the lack of metallic behavior and superconductivity in PrBa2Cu3O7 − δ. The compound PrBa2Cu3O7 − δ appears to exhibit some form of complex antiferromagnetic order at approximately 16 K and has a γT contribution to the specific heat at low temperatures with a large γ value of approximately 169 mJ (mol praseodymium)−1 K−2, reminiscent of heavy fermion behavior.

Exchange-induced spin polarization of conduction electrons in paramagnetic metals

Abstract: The conduction electrons in Au are shown to be spin polarized at distances up to 10 nm from an interface with Fe by use of an electron tunneling technique. A simple model is proposed to explain this effect.

Effective Hamiltonian for CuO2 layer

Abstract: The effective Hamiltonian for CuO 2 layer is derived based on Cu: d x 2 - y 2 and O: p σ orbitals including the Coulomb interactions between Cu and O sites ( V ) as well as the on-site Coulomb repulsion at Cu ( U d ) and O ( U p ) sites. It is assumed that Cu ++ is stable under doping. We found a region where parameters permit an attractive interaction between two holes located in the neighboring sites of the same Cu atom resulting in the simultaneous enhancement of the antiferromagnetic exchange interaction between a Cu spin and a hole spin. The special case with vanishing V and U p , which is of interest in the context of RVB, is discussed without recourse to the classification of symmetric and nonbonding orbitals.

Long-range interactions in the compressible Heisenberg chain.

Abstract: We investigate the effects of long-range Kac-Baker-type interactions between the ions on the nonlinear coupled magnetic and elastic excitations of the compressible Heisenberg chain. We found analytical expressions for the dynamics of the low-temperature\char21{}long-wavelength nonlinear excitations of the elastic and magnetic modes. The dependence on the long-range interaction parameter r shows the existence of low-energy excitations near an upper limit r=${r}_{c}$. Finally, we performed a particle analogy by associating a property of mass to the elastic pulse: the existence of coupled solitons is found to be related to the finiteness of an effective mass.

Mean-field theory for vacancies in a quantum antiferromagnet.

Abstract: The vacancies in a quantum antiferromagnet have dipolar interactions mediated by antiferromagnetic (AF) spin waves. An effective Hamiltonian incorporating this effect is studied within a mean-field approximation. The ground state is found to be an {ital s}- or {ital d}-wave superconductor. In the strong-coupling regime the superconductivity coexists with the incommensurate spiral AF order. The analysis is limited to the regime where the AF correlation length is larger than the distance between vacancies.

EPR study of polycrystalline superconductors with YBa2Cu3O7 structure

Abstract: Electron paramagnetic resonance (EPR) of Gd3+, Eu2+, and copper ions has been investigated in the high-Tc superconductor with YBa2Cu3O7−α structure. It has been established that the system is heterogeneous at 0.15≤δ≤0.5 and consists of metallic and dielectric regions. The former arises due to oxygen enrichment while the later due to oxygen deficiency. The integral of exchange interaction between Gd3+ localized moments and conduction electrons Jsf=0.016 eV has been determined from the normal state temperature dependence of Gd3+ EPR linewidth for metallic regions. Tc depression by gadolinium-localized moments for GdBa2Cu3O7−α was estimated to be ΔTc⋍−2K. Anomalies in linewidth temperature dependence upon transition from the normal to the superconducting state have given information about the value and temperature behavior of the superconductor's energy gap. The model, which gives the opportunity to understand some peculiarities of the EPR signal for YBa2Cu3O7−α samples, is proposed in terms of several bottlenecked spinsubsystems: spin-liquid in CuO planes and Cu2+-O− and Cu2+-O2− fragments in CuO chains.

Magnetisation measurements of the synthetic olivine single crystals A2SiO4 with A=Mn, Fe or Co

Abstract: Magnetisation measurements of three synthetic single-crystals A2SiO4 of the olivine family are presented (A=Mn, Fe and Co). All three compounds order antiferromagnetically in the range 45-65 K. For A=Mn, weak ferromagnetism is present. For A=Fe and Co, the magnetisation is strongly anisotropic. A model is proposed, which extends existing published explanations of the magnetic structures. The magnetic cations are represented by spin Hamiltonians, and their interactions by mean fields. Analytical relationships are derived between Curie temperatures, Curie constants and Neel temperatures on the one hand, and spin Hamiltonian parameters on the other. From the latter parameters the maximum possible information on orbital levels is derived. This is enough to calculate magnetic entropies in good agreement with published data, together with specific heat measurements, and to point out the limits of validity of the linearisation processes that are necessary to obtain usable analytical relations between experimental and atomic parameters. Below TN, the susceptibilities are analysed using the conclusions of previous magnetic structure studies, especially those concerning the role of competing exchanges, single-ion anisotropies, and symmetry requirements.