Showing papers on "Exchange interaction published in 1995"
TL;DR: In this article, it was shown that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator.
Abstract: Evidence is presented that within the density-functional theory orbital polarization has to be treated on an equal footing with spin polarization and charge density for strongly interacting electron systems. Using a basis-set independent generalization of the LDA+U functional, we show that electronic orbital ordering is a necessary condition to obtain the correct crystal structure and parameters of the exchange interaction for the Mott-Hubbard insulator ${\mathrm{KCuF}}_{3}$.
3,523 citations
TL;DR: The band edge exciton structure is calculated, including the effects of the electron-hole exchange interaction and a nonspherical shape, in CdSe quantum dots to show the importance of exciton spin dynamics in the recombination mechanism.
Abstract: We use external magnetic fields to identify the band edge emitting state in CdSe quantum dots. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk, is strongly enhanced by quantum confinement and allows the observation of an optically passive "dark" excitonic state.
700 citations
TL;DR: The evolution of the spectral weight of the two-dimensional Hubbard model is studied from insulating to metallic behavior, traced back to one and the same many-body origin: the doping-dependent antiferromagnetic spin-spin correlation.
Abstract: On the basis of quantum Monte Carlo results the evolution of the spectral weight $A(\mathbf{k},\ensuremath{\omega})$ of the two-dimensional Hubbard model is studied from insulating to metallic behavior. As observed in recent photoemission experiments for cuprates, the electronic excitations display essentially doping-independent features; a quasiparticlelike dispersive narrow band of width of the order of the exchange interaction $J$ and a broad valence- and conduction-band background. The continuous evolution is traced back to one and the same many-body origin: the doping-dependent antiferromagnetic spin-spin correlation.
141 citations
TL;DR: In this paper, the possibility of exchange interaction between ferromagnetic films across nonmagnetic metallic interlayers has been considered for a long time and was clearly identified and characterized in Fe/Cr structures and rare earth yttrium multilayers.
122 citations
TL;DR: The influence of a coupling parameter turning on the electron-electron interaction on the difference between Kohn-Sham and Hartree-Fock kinetic and exchange energies is investigated and the formulation and validity of the Hohenberg-Kohn theorem in basis set representations are discussed.
Abstract: Kohn-Sham wave functions yielding the Hartree-Fock ground-state densities of alkaline-earth and noble-gas atoms are calculated. From the Kohn-Sham wave functions the corresponding noninteracting kinetic energies and the exchange energies are calculated according to the density-functional definition. The difference between the density-functional and the Hartree-Fock exchange energies for a given electron density is found to be surprisingly small. This justifies, at least for the systems studied here, the common practice of using Hartree-Fock exchange energies as reference values to assess the quality of today's approximate density functionals for the exchange energy. The calculated Kohn-Sham wave functions give energies with the atomic Hamiltonian operators which are de facto identical to the corresponding values obtained from the optimized potential method. The influence of a coupling parameter turning on the electron-electron interaction on the difference between Kohn-Sham and Hartree-Fock kinetic and exchange energies is investigated. Various hybrid schemes which combine the Kohn-Sham and the Hartee-Fock methods are compared. For the atomic systems investigated here, the different exchange and correlation energies occurring in these schemes are found in most cases to deviate very little from the standard Kohn-Sham exchange and correlation energies. Finally, the formulation and validity of the Hohenberg-Kohn theorem in basis set representations are discussed.
110 citations
TL;DR: In this article, a uniaxial spin system on the square lattice, where the spins are oriented perpendicular to the lattice and are coupled by both a dipole-dipole interaction and an exchange interaction, is studied.
Abstract: A uniaxial spin system on the square lattice, where the spins are oriented perpendicular to the lattice and are coupled by both a dipole-dipole interaction and an exchange interaction, is studied. The subtle interplay of the exchange and dipolar interaction in two dimensions destabilizes the ferromagnetic ground state of the nearest-neighbor Ising model and gives rise to a sequence of striped phases. An analytic expression for the leading terms in an asymptotic expansion of the ground-state energy for the striped phase is derived for the discrete lattice. Comparison with the corresponding results for a previously proposed checkerboard state show that the striped phase is the ground state. The results are shown to be in excellent agreement with earlier numerical results. The finite-temperature phase diagram is obtained for a finite lattice using Monte Carlo simulation techniques and the corresponding structure-factor patterns discussed.
91 citations
TL;DR: In this paper, the temperature dependence of the magnetic susceptibility χ and the 59 Co Knight shift 59 K in a single ion model with the low-spin 1 A 1 ground state above which the high-spin 5 T 2 state is located was analyzed.
Abstract: 59 Co and 139 La NMR measurements have been performed to study the microscopic magnetic properties closely related with the spin state of a trivalent cobalt ion in LaCoO 3 . The temperature-independent 59 Co and 139 La Knight shifts below ∼ 30 K clearly revealed the presence of a nonmagnetic ground state at low temperatures. We calculated the temperature dependence of the magnetic susceptibility χ and the 59 Co Knight shift 59 K in a single ion model with the low-spin 1 A 1 ground state above which the high-spin 5 T 2 state is located. Then, both the trigonal crystal-field and spin-orbit interactions are taken into account. From this analysis, we infer the presence of an antiferromagnetic exchange interaction between the high-spin states. The characteristic temperature dependence of the 59 Co hyperfine coupling constant is also discussed.
74 citations
TL;DR: The study of exciton spin relaxation in GaAs quantum wells by picosecond time-resolved photoluminescence spectroscopy has found that spin relaxation is strongly affected by excitonic localization, and the excitation-energy dependence of the polarization decay time indicates that exchange interaction is the leading spin-relaxation mechanism at low temperature.
Abstract: We have investigated exciton spin relaxation in GaAs quantum wells by picosecond time-resolved photoluminescence spectroscopy, in order to determine the origin of spin-relaxation processes and their dependence on excitonic localization. The studies have been done as a function of excitation and detection energy, lattice temperature, and carrier density. The excitation-energy dependence of the polarization decay time indicates that exchange interaction is the leading spin-relaxation mechanism at low temperature. However, we have not been able to explain the high-temperature (T\ensuremath{\ge}50 K) dependence of the polarization decay time with either the D'yakonov-Perel' mechanism or with the exchange interaction. We have also found that spin relaxation is strongly affected by excitonic localization.
52 citations
TL;DR: In this paper, the degree of linear polarization and time behaviour of photoluminescence of very small size CdSe nanocrystals under size-selective excitation were investigated.
Abstract: We have investigated the degree of linear polarization and time behaviour of photoluminescence of very small size CdSe nanocrystals under size-selective excitation. At low temperature we observe a luminescence line Stokes-shifted by a few meV and its phonon replica. The degree of linear polarization is positive at short times then becomes negative and remains constant and equal to -15%. The following model is proposed. The exciton state is split by electron-hole exchange interaction into two states with the optically forbidden state at lower energy. Laser light creates excitons in the upper energy state. The luminescence line arises from the lower energy state where excitons have relaxed after a few tens of ps. Recombination is made possible through a phonon assisted virtual transition to the confined B-exciton state. As a result the polarization degree is negative at long times. The value of the exchange energy obtained from the luminescence Stokes shift (8 meV in 40A-diameter nanocrystals) is much larger than the bulk value (0.1 meV). The enhancement of exchange interaction by quantum confinement and the value of the degree of polarization are in good agreement with theoretical estimates.
50 citations
TL;DR: In this paper, the effect of surface dilution on the critical behaviour of a ferromagnetic Ising film in transverse surface and bulk fields is studied within the framework of an effective field theory based on a single-site cluster method.
46 citations
TL;DR: The oscillatory behavior of the interlayer exchange interaction is observed in Co/Au/Co sandwiches; the measured values of the pseudoperiod and of the attenuation length agree well with recent determination by magneto-optical and magnetoresistive measurements and with theoretical predictions.
Abstract: Magnetic anisotropies and interlayer exchange interaction are derived from Brillouin-scattering spectra of various thin films with one or two Co magnetic layers, evaporated in ultrahigh vacuum. The oscillatory behavior of the interlayer exchange interaction is observed in Co/Au/Co sandwiches; the measured values of the pseudoperiod (9.6 \AA{}) and of the attenuation length (12 \AA{}) agree well with recent determination by magneto-optical and magnetoresistive measurements and with theoretical predictions.
TL;DR: In this article, the authors discuss the high magnetic field magnetization processes in ferromagnetic compounds of the f-d type, that is, in compounds consisting of rare-earth and transition metal elements of the Fe group.
Abstract: Publisher Summary This chapter discusses the high magnetic field magnetization processes in ferromagnetic compounds of the f–d type—that is, in compounds consisting of rare-earth and transition metal elements of the Fe group. The processes that are connected with the variation of mutual orientation of their magnetic sublattices and that reveal themselves through characteristic kinks and jumps in magnetization curves are focused in the chapter. Magnetic symmetry of a crystal changes when the orientation of magnetic sublattices spontaneously varies. A phase transition accompanied by a change of the magnetic symmetry takes place. The phase transitions are referred to as “field induced phase transitions” (FIPT) that may be of 1st or 2nd order. FITP connected with breaking of a collinear alignment of sublattices in ferrimagnetics are also considered in the chapter. It is assumed that the magnetic anisotropy of these materials is small in the sense that the energy of a magnetic anisotropy is smaller in comparison with the energy of the exchange interaction of sublattices. The magnetic anisotropy in such materials is best manifested near a compensation point, where critical fields of transitions to a canted phase tend to zero. In this connection, H–T phase diagrams are very complicate near Tc and composed of curves of the 1st and 2nd order phase transitions and of critical, tricritical points, and others.
TL;DR: In this article, the authors studied the one-dimensional quantum statistics at the limit of isotropic exchange interaction in the unpaired electron system of a ferromagnetic organic radical crystal, 3-(4-chlorophenyl)-1,5-diphenyl-6-oxoverdazyl.
Abstract: One-dimensional quantum statistics has been studied at the limit of isotropic exchange interaction in the unpaired electron system of a ferromagnetic organic radical crystal, 3-(4-chlorophenyl)-1,5-diphenyl-6-oxoverdazyl. The characteristic field dependence of the magnetic heat capacity of this crystal is quantitatively explained by the theoretical values derived from the Bethe-ansatz technique applied to the quantum transfer matrix, with a unique intrachain exchange constant J / k B =5.5 K. By the simultaneous measurement of magnetic susceptibility and heat capacity, the three-dimensional ordering at T c =0.21 K, which is triggered by a weak interchain coupling ∣ z J '∣/ k B =3×10 -2 K, is confirmed.
TL;DR: In this paper, the effects of anisotropies and exchange interactions on compensation temperature in a ferrimagnetic bilayer system with disordered interfaces consisting of spin-12 and spin-1 atoms are investigated by the use of the standard mean-field theory.
TL;DR: A generalized Bloch-Maxwell formulation for laser-field-coupled semiconductors is derived from a two-band model which includes direct Coulomb interactions, and intrinsic adiabatic inversion, adiABatic following, anomalous Rabi cycling, and unique, as well as fast, optical switching, all of which depend upon the coherent Coulomb exchange interaction.
Abstract: A generalized Bloch-Maxwell formulation for laser-field-coupled semiconductors is derived from a two-band model which includes direct Coulomb interactions. The momentum-dependent, microscopic, electron-hole equations of motion in the time-dependent Hartree-Fock approximation and neglecting interband exchange interactions form the starting point for the formulation. A self-consistent set of coupled equations in four dynamical variables for the medium, together with the electric-field amplitude coupled through the Maxwell wave equation in a semiclassical approximation, are obtained to lowest order in the coherent Coulomb exchange interaction and the density-of-states distribution. Intrinsic optical bistability is predicted in a steady state due to a carrier density-dependent redshift of the band edge due explicitly to coherent Coulomb exchange. Integration of the dynamical equations for conditions which correspond to the ultrafast time regime exhibit intrinsic adiabatic inversion, adiabatic following, anomalous Rabi cycling, and unique, as well as fast, optical switching, all of which depend upon the coherent Coulomb exchange interaction.
TL;DR: In this paper, the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation was investigated and it was shown that the luminescence line closest to the laser arises from the recombination of the optically forbidden A-exciton.
Abstract: We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser arises from the recombination of the optically forbidden A-exciton. Radiative recombination occurs via a phonon-assisted virtual transition to theB-exciton state. The electron-hole exchange energy obtained from the experimental results strongly increases with decreasing nanocrystal size as expected from the increasing overlap of the electron and hole wave functions. The size dependence is in good agreement with the theoretical predictions.
TL;DR: In this paper, the Curie temperature of dysprosium films epitaxially grown on yttrium and on erbium is shown to increase continuously with the epitaxial strain induced along the c-axis.
Abstract: The Curie temperature of dysprosium films epitaxially grown on yttrium and on erbium is shown to increase continuously with the epitaxial strain induced along the c-axis. This variation is correlated to modifications of the α strains, of the γ distortion and of the exchange energy barrier. The evolution of these different terms is discussed from the experimental thermal variations of the lattice parameters and turn angles determined by neutron diffraction.
TL;DR: In this paper, the spin-catalysis is introduced in order to describe and classify a wide range of phenomena in which chemical reactions are promoted by substances assisting in inducing spin changes and overcoming spin-prohibition.
Abstract: The crucial role of electron spin in the control of the reaction channels in the region of activated complexes can easily be inferred from the general principles of chemical bonding. Magnetic perturbations could change spin at the intermediate stages of a reaction or in the region of activation barriers and could hence influence the reaction rate through spin switching of the reaction paths. Spin-orbit coupling is one of the most important intrinsic magnetic perturbations in molecules; its role in chemical reactivity is here shown by a few typical examples. Spin-orbit coupling induced spin flip could also be important in catalysis by transition metals. General qualitative arguments predict great enhancements of the spin-orbit coupling in catalytic complexes with transition metal compounds. The concept of spin-catalysis is introduced in order to describe and classify a wide range of phenomena in which chemical reactions are promoted by substances assisting in inducing spin changes and overcoming spin-prohibition. This concept is based on results of quantum chemical calculations with account of spin-orbit coupling and configuration interaction in the intermediate complexes. Besides spin-orbit coupling, the role of intermolecular exchange interaction with open shell catalysts is stressed. The catalytic action would definitely depend on the efficiency of spin uncoupling inside the reacting substrate molecule and this could be induced by magnetic and exchange perturbations.
TL;DR: In this article, a two-step model was proposed to describe the dichroic absorption effects in terms of an additive contribution to the backscattering amplitude caused by the exchange interaction, which was directly correlated to the spin-moment of the neighboring atom.
TL;DR: In this paper, it was shown that a kinetic energy functional can be used as an exchange energy functional and, conversely, exchange used as a K energy functional, with a proper transformation of energy as a functional of the density.
TL;DR: In this paper, the magnetic properties of 4-methacryloylamino-2,2,6, 6,6-tetramethylpiperidine-1-oxyl (MATMP) have been investigated by means of adiabatic heat capacity calorimetry in the 008-24 K range.
TL;DR: Electronic band-structure calculations of simple metals (Li, Na, K, Rb, and Ca), where the exact Kohn-Sham density-functional exchange (EXX) potential combined with the LDA (local-density approximation) correlation potential is presented.
Abstract: We present electronic band-structure calculations of simple metals (Li, Na, K, Rb, and Ca), where we use the exact Kohn-Sham density-functional exchange (EXX) potential combined with the LDA (local-density approximation) correlation potential. The method is the same as one exploited in the previous papers [Phys. Rev. B 50, 14 816 (1994) ; Phys. Rev. Lett. 74, 2989 (1995)]. Surprisingly, in contrast to the case of semiconductors and insulators, the resulting eigenvalues of valence bands are mostly in agreement with the corresponding LDA eigenvalues. In the case of Ca, where the empty d band exists above the Fermi energy, however, the EXX calculation modifies the position of the d bands relative to the s bands considerably, implying the importance of treating the correlation energy on the same footing as the exchange energy.
TL;DR: In this article, the EPR spectra of paramagnetic single crystals having two or more anisotropic ions per unit cell with effective spin S = 1 2, located at symmetry related but magnetically nonequivalent positions, and coupled by exchange interactions are analyzed.
TL;DR: In this article, specific heat measurements in the temperature range 0.05 K to 0.95 K for the copper-amino acid complex Cu(L-alanine)2 were reported.
Abstract: We report specific heat measurements in the temperature range 0.05 K
TL;DR: In this article, a double exchange and isotropic exchange model was developed to account for the magnetic properties of the hexanuclear octahedral clusters [Fe6(μ3-X8)(PEt3)6]+ (X = S, Se).
Abstract: A model that considers the double exchange and isotropic exchange is developed to account for the magnetic properties of the hexanuclear octahedral clusters [Fe6(μ3-X8)(PEt3)6]+ (X = S, Se), in which one extra electron added to the [Fe6(μ3-X8)(PEt3)6]2+ is delocalized over six structurally equivalent iron centres. The ground state of the complexes is S = 7/2 and, in our model, it can arise from either a ferromagnetic or an antiferromagnetic exchange interaction between the unpaired spins on Fe, provided that the sign of the transfer integral p is negative.
TL;DR: In this article, the magnetic properties of hexagonal RGa 2 compounds are summarized, considering recent experimental results obtained on single crystals, and an analysis of exchange interactions throughout the series as well as a qualitative description of the Fourier transform of the exchange interaction, J ( q ), are made.
TL;DR: In this paper, the heat capacity of the intermetallic compounds PrNi 2, GdNi 2 and ErNi 2 has been calculated with a model that takes into account the energy splitting of the ground multiplet due to the crystal electric field (CEF) and the exchange interaction in a molecular field approximation.