Showing papers on "Exchange interaction published in 1993"
TL;DR: Simulation of Enzyme Reactions Using Valence Bond Force Fields and Other Hybrid Quantum/Classical Approaches as mentioned in this paper is an example of a hybrid approach that combines classical and quantum approaches.
Abstract: Simulation of Enzyme Reactions Using Valence Bond Force Fields and Other Hybrid Quantum/Classical Approaches
765 citations
TL;DR: Analysis of the time dependence of optical intensities of both circular polarizations, including competing relaxation mechanics from exciton exchange and from single-particle spin flip into optically inactive states, leads to characteristic shapes reflecting their relative importance.
Abstract: A theory of exciton spin dynamics is given in terms of the exchange spin-flip mechanism taking full account of the confinement of the quantum well. Exciton spin relaxation belongs to the motional narrowing class, with a characteristic inverse proportionality to the momentum scattering time of the exciton center of mass. Analysis of the time dependence of optical intensities of both circular polarizations, including competing relaxation mechanics from exciton exchange and from single-particle spin flip into optically inactive states, leads to characteristic shapes reflecting their relative importance. The calculated well-width dependence from our theory of the exciton spin relaxation leads to polarization intensities in good agreement with measurement. Theoretical electric and magnetic field dependences have yet to be tested against experiment.
476 citations
TL;DR: In this paper, a general scheme was established within the effective mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect.
Abstract: A general scheme is established within the effective-mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect. This effect is found to appear most pronounced in the quantum-dot structure in comparison with the quantum-well and quantum-wire structures. A formula of the lowest exciton energy in the strong confinement regime is derived and the significance of the dielectric confinement effect is clarified. We investigate the dependence of the binding energy and the oscillator strength of the lowest-energy excitonic state on the quantum-dot radius, the electron-to-hole mass ratio, and the dielectric-constant ratio between the quantum dot and the surrounding medium. The subband mixing effect due to the electron-hole Coulomb interaction gives a finite oscillator strength to excitonic transitions which are forbidden in the absence of the Coulomb interaction. This effect is shown unambiguously in the calculated excitonic energy spectra. Furthermore, the electron-hole exchange interaction in a quantum dot is discussed. The short-range part of the exchange energy is shown to increase in proportion to the inverse of the volume of the quantum dot as the quantum-dot size is reduced. On the other hand, the long-range part of the exchange energy is found to be sensitively dependent on the shape of the quantum dot. In particular, it vanishes for the optically allowed excitonic states in a spherical quantum dot.
326 citations
TL;DR: In this paper, a review of magnon properties of yttrium-iron garnet (YIG), a classical object for experimental studies in magnetism, is presented, and a new method of approximate calculation of the magnon spectra in magnets with large unit cell and to obtain by means of this method some basic properties of YIG.
306 citations
TL;DR: The exchange energy for pairs of helium and neon atoms was calculated with recently proposed exchange-energy functionals for Hartree-Fock electron densities, and compared with the exact exchange energy.
Abstract: The exchange energy for pairs of helium and neon atoms was calculated with recently proposed exchange-energy functionals for Hartree-Fock electron densities, and compared with the exact (Hartree-Fock) exchange energy. While all of the functionals calculate the total exchange energies to within 1%, most of the functionals give very poor results for ΔE exc , the exchange-energy contribution to the interaction energy. At the positions of the energy minima for the atom pairs, most functionals give ΔE exc in error by over 100%. The reason for the difference in accuracy between the total exchange energy and ΔE exc is that the total exchange energy is dominated by the high density and small gradient regions near the nuclei, while ΔE exc is dominated by the low density and large gradient regions between the atoms
177 citations
TL;DR: In this article, localized electron states with unpaired spin located within or at the interfaces of an otherwise nonmagnetic metallic spacer layer are assumed to be intrinsic to the ideal multilayer structure rather than due to impurities or structural defects.
Abstract: We postulate localized‐electron states with unpaired spin located within or at the interfaces of an otherwise nonmagnetic metallic spacer layer. Loose exchange coupling of these spins to two ferromagnets mediates a non‐Heisenberg exchange coupling between them which includes a biquadratic term. A particular version of the model assumes that each interfacial layer of magnetic atoms is weakly exchange coupled to the remainder of the ferromagnets. This model permits interpretation of the biquadratic‐coupling data of Gutierrez et al. for Fe/Al/Fe trilayers and of Fuss et al. for Fe/Au/Fe. According to this interpretation, the observed biquadratic coupling is intrinsic to the ideal multilayer structure rather than due to impurities or structural defects.
162 citations
TL;DR: In this article, the exchange anisotropy of oxide passivated Co particles was studied using shifted hysteresis loops after the sample was field cooled (FC), and the shift in the FC loop exhibited a maximum at a particle size of 80 A (shift=10.7 kOe) and is related to the amount of surface oxidation.
Abstract: Oxide passivated Co particles have been prepared by vapor deposition in the size range of 50–350 A. A strong exchange anisotropy was observed due to the core‐shell structure of the Co particles, where the core consists of metallic Co and the shell of Co oxides. The exchange anisotropy of the particles was studied using shifted hysteresis loops after the sample was field cooled (FC). The shift in the FC loop exhibited a maximum at a particle size of 80 A (shift=10.7 kOe) and is related to the amount of surface oxidation. The shift in the loops disappeared around 150 K in all the samples irrespective of their particle size, and this is attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell.
130 citations
TL;DR: A thin film of EuSe was used as a funnel barrier between the normal metal Ag and superconducting Al to show tunneling characteristics and enhanced Zeeman spliffing of the Al quasiparficle states because of the exchange interaction at the Al-EuSe.
Abstract: A thin film of EuSe was used as a funnel barrier between the normal metal Ag and superconducting Al. Tunneling characteristics at 0.45 K showed the following: (1) the absence of exchange splitting of the EuSe conduction band in zero magnetic field, where EuSe is antiferromagnetic; (2) field-dependent spin polarization of the tunneling electrons as high as 97% at H≥1.2 T; (3) enhanced Zeeman spliffing of the Al quasiparficle states because of the exchange interaction at the Al-EuSe; (4) a decrease in tunnel resistance by as much as 75% in magnetic field ≥1.7 T
127 citations
TL;DR: In this paper, a direct comparison of the splitting of free exciton states with the magnetic moment of the Mn2+ ion system was made, showing excellent agreement with simple exchange interaction model throughout the composition range studied.
121 citations
TL;DR: It is shown how the model, applicable to a trilayer film such as Fe/Cr/Fe or Co/Ru/Co, generates an infinite sum of terms with coefficients A 12 ,B 12 ,... in the coupling energy, extending beyond the Heisenberg-like A 12 term of bilinear coupling between the moments of the ferromagnets.
Abstract: We consider a free-electron model of exchange coupling between transition-metal ferromagnetic films separated by a paramagnetic metal spacer. The minority-spin energy bands in the ferromagnets are matched to those of the paramagnetic spacer. The majority-spin electrons experience a repulsive potential arising from the lack of corresponding states in the spacer. The height of the potential barrier is equal to the exchange-energy gap in the ferromagnets. We show how the model, applicable to a trilayer film such as Fe/Cr/Fe or Co/Ru/Co, generates an infinite sum of terms with coefficients A 12 ,B 12 ,... in the coupling energy, extending beyond the Heisenberg-like A 12 term of bilinear coupling between the moments of the ferromagnets
89 citations
TL;DR: The spectrum of a one-dimensional chain of SU(n) spins positioned at the static equilibrium positions of the particles in the corresponding classical Calogero system with an exchange interaction inversely proportional to the square of their distance is studied in this article.
Abstract: The spectrum of a one-dimensional chain of SU(n) spins positioned at the static equilibrium positions of the particles in the corresponding classical Calogero system with an exchange interaction inversely proportional to the square of their distance is studied. As in the translationally invariant Haldane-Shastry model the spectrum is found to exhibit a very simple structure containing highly degenerate 'super-multiplets'. The algebra underlying this structure is identified and several sets of raising and lowering operators are given explicitly. On the basis of this algebra and numerical studies the authors give the complete spectrum and thermodynamics of the SU(2) system.
TL;DR: This model of M-component quantum rotors coupled by Gaussian-distributed random, infinite-range exchange interactions suggests that the critical properties of the transverse-field Ising model (believed to be identical to the M→1 limit) are the same as those of the M=∞ quantum rotor.
Abstract: We examine a model of M-component quantum rotors coupled by Gaussian-distributed random, infinite-range exchange interactions. A complete solution is obtained at M=\ensuremath{\infty} in the spin-glass and quantum-disordered phases. The quantum phase transition separating them is found to possess logarithmic violations of scaling, with no further modifications to the leading critical behavior at any order in 1/M; this suggests that the critical properties of the transverse-field Ising model (believed to be identical to the M\ensuremath{\rightarrow}1 limit) are the same as those of the M=\ensuremath{\infty} quantum rotors.
TL;DR: In this article, a strong interlayer exchange interaction is observed in polycrystallineantiferromagnetic CoO•NiO thin-film superlattices with short repeat distances.
Abstract: A strong interlayer exchange interaction is observed in polycrystallineantiferromagnetic CoO‐NiO thin‐film superlattices. This was determined by measuring the exchange field H e resulting from coupling these superlattices with Ni81Fe19. The temperature above which H e is zero (the blocking temperature) is taken as a measure of the ordering temperature of the superlattice. In CoO‐NiO superlattices with short repeat distances the CoO and NiO layers order at a single temperature that is between the ordering temperatures of bulk CoO and NiO. These ordering temperatures are the same as for Co x Ni1−x O alloy films with the same overall composition. The temperature dependence of H e in some of the superlattice exchange couples deviates from the linear behavior expected for cubic antiferromagnets. In addition, the exchange anisotropy induced by some CoO‐NiO superlattices is greater than that induced by Co x Ni1−x O. The higher H e and nonlinear temperature dependence suggest that the interlayer coupling has a strong effect on the magnetocrystalline anisotropy of the layers within the superlattice. Thick‐NiO/thin‐CoO/Ni81Fe19trilayers were produced to investigate the thickness dependence of the oxide‐oxide interaction. When the CoO layers are sufficiently thin (≤20 A), the CoO ordering temperature approximates the NiO value, indicating quite strong coupling throughout the CoO layer. The effect of the coupling is much weaker for thicker CoO layers (≥30 A).
TL;DR: The electronic properties of substitutional 3d transition-metal impurities in II-VI semiconductors have been studied using the cluster and Anderson impurity models with configuration interaction and it is shown that the photoemission and inverse-photoemission spectra, exchange interaction between the 3d magnetic moment and the host band states, and donor and acceptor ionization energies can be reproduced with the same set of parameters.
Abstract: The electronic properties of substitutional 3d transition-metal impurities in II-VI semiconductors have been studied using the cluster and Anderson impurity models with configuration interaction. It is shown that the photoemission and inverse-photoemission spectra, d-d optical-absorption spectra, exchange interaction between the 3d magnetic moment and the host band states, and donor and acceptor ionization energies can be reproduced with the same set of parameters, which show systematic variation with expected chemical trends. The importance of multiplet effects in the formation of donor and acceptor levels within the band gap is demonstrated.
TL;DR: Low-energy electron-diffraction data are presented, which show that the bcc Cu(001) overlayer follows a nearly perfect bcc lattice.
Abstract: Ferromagnetic-resonance (FMR) and surface magneto-optical Kerr-effect (SMOKE) studies of the exchange coupling in bcc Fe/Cu/Fe(001) structures are presented. It is shown that the interfaces in bcc Fe/Cu/Fe(001) trilayers grown on a Ag(001) single-crystal substrate can be significantly improved by choosing an appropriate growth procedure. Low-energy electron-diffraction data are presented, which show that the bcc Cu(001) overlayer follows a nearly perfect bcc lattice. The exchange coupling in bcc Fe/Cu/Fe trilayers was studied as a function of interlayer thickness ranging from 7 to 14 monolayers. Quantitative data from FMR and SMOKE measurements are compared. The interpretation of magnetization loops for Fe/Cu/Fe trilayers requires the simultaneous presence of bilinear and biquadratic exchange coupling between the magnetic layers. Computer calculations were used to determine the strength of the bilinear and biquadratic exchange couplings. It is shown that the strength of the biquadratic exchange coupling increases with increasing terrace width. The measured values of the bilinear and biquadratic exchange coupling were compared with a model recently proposed by Slonczewski, which treats the exchange coupling in trilayers with imperfect interfaces. Slonczewski's model was used to deconvolute the data to obtain the intrinsic behavior of the bilinear exchange coupling in bcc Fe/Cu/Fe(001) trilayers. It is shown that the exchange coupling unobscured by interface roughness exhibits a strong short-wavelength oscillatory behavior that is in agreement with recent first-principles band calculations. Composite Ag-Cu structures were also studied. The presence of a few atomic layers of Ag(001) added to the Cu layer rapidly decreases the exchange coupling.
TL;DR: In this article, the phase transitions of finite and infinite ferroelectric systems using the Ising model in a transverse field were studied using the mean-field layer approximation and the results for the Curie temperature were expressed in terms of general determinants.
Abstract: The author studies the phase transitions of finite and infinite ferroelectric systems using the Ising model in a transverse field. The mean-field layer approximation is used and the results for the Curie temperature are expressed in terms of general determinants. The effects of surface modification are introduced through a surface exchange constant J0 and a surface transverse field Omega 0. It is shown that above a critical curve in J0- Omega 0 space, surface polarization is possible, even when there is no phase transition in the bulk. The dependence of the Curie temperature on the film thickness is obtained for two types of surface modification.
TL;DR: Spin-restricted and unrestricted post-Hartree-Fock calculations were carried out for clusters of triplet methylene and nitroxide radicals in this paper, where the UHF-based methods such as UMP and QCISD followed by approximate spin projection provided reasonable energy differences between the high and low-spin states of these species.
TL;DR: The spectrum of a one-dimensional chain of $SU(n)$ spins positioned at the static equilibrium positions of the particles in a corresponding classical Calogero system with an exchange interaction inversely proportional to the square of their distance is studied in this paper.
Abstract: The spectrum of a one-dimensional chain of $SU(n)$ spins positioned at the static equilibrium positions of the particles in a corresponding classical Calogero system with an exchange interaction inversely proportional to the square of their distance is studied. As in the translationally invariant Haldane--Shastry model the spectrum is found to exhibit a very simple structure containing highly degenerate ``super-multiplets''. The algebra underlying this structure is identified and several sets of raising and lowering operators are given explicitely. On the basis of this algebra and numerical studies we give the complete spectrum and thermodynamics of the $SU(2)$ system.
TL;DR: The existence of an oscillatory exchange coupling in this controversial system is confirmed and magneto-optical Kerr-effect data from which the oscillation period of the exchange coupling can be determined to be about 9 A are presented, consistent with theory and the SPNR data.
Abstract: By probing the magnetisation profile of high-quality molecular-beam-epitaxy-grown Co/Cu(111) superlattices with spin-polarised neutron reflectivity (SPNR), we have observed a coherent antiferromagnetic spin structure and confirm that this coupling behavior sensitively depends on the Cu sparer thicknesses. We present magneto-optical Kerr-effect data from which the oscillation period of the exchange coupling can be determined to be about 9 A, consistent with theory and the SPNR data. With these results we confirm the existence of an oscillatory exchange coupling in this controversial system
TL;DR: In this paper, an analysis of the exchange energy of a homonuclear molecular ion was carried out in a model problem consisting of the one-dimensional limit of H2+ and it was demonstrated that the use of the infinite polarization expansion for the localized wave function in the Holstein-Herring formula yields an approximate exchange energy which at large internuclear distances R has the correct leading behavior to O(e−R).
Abstract: In order to clarify questions about the calculation of the exchange energy of a homonuclear molecular ion, an analysis is carried out of a model problem consisting of the one‐dimensional limit of H2+. It is demonstrated that the use of the infinite polarization expansion for the localized wave function in the Holstein–Herring formula yields an approximate exchange energy which at large internuclear distances R has the correct leading behavior to O(e−R) and is close to but not equal to the exact exchange energy. The extension to the n‐dimensional double‐well problem is presented.
IBM1
TL;DR: Parkin et al. as discussed by the authors used the spin-current method to calculate the oscillatory exchange energy that couples two semi-infinite ferromagnets with exchange-split parabolic bands which are joined by a nonmagnetic metallic spacer.
TL;DR: In this paper, the Coulombic and exchange contributions to the intermediate and long-range interaction potentials for two lowest states (X 1 1 Σ g + and a 3 Σ u + ) of Li 2 were investigated.
Abstract: This article critically surveys the best available data for the two lowest states (X 1 1 Σ g + and a 3 Σ u + ) of Li 2 . These two states both dissociate to two ground-state Li atoms. Since both states are known precisely up to near dissociation (a rare situation), they can be used to determine the separate Coulombic and exchange contributions to the intermediate and long-range interaction potentials. This article represents the first such experimental determination for any atomic interaction. This work clearly shows that exchange is exponential over 2 orders of magnitude in energy and that accurate theoretical calculations of Coulombic and exchange contributions agree well with experiment for this benchmark system
TL;DR: In this paper, the fine structure of excitons is modified in quantum wells due to the reduced symmetry and increased Coulomb interaction, and the dispersion relations for localized and resonant polaritons as well as the radiative lifetime for the resonant modes are derived.
Abstract: The fine structure of excitons, caused by the electron-hole exchange interaction, is modified in quantum wells due to the reduced symmetry and increased Coulomb interaction. We present a systematic formulation of the exchange interaction for quantum-well excitons and perform numerical calculations of the fine-structure splitting and dispersion for Aln 0.3 Ga 0.7 As/GaAs quantum wells. The coupling of quantum-well excitons with the electromagnetic field is formulated in second quantization. By solving the coupled equations of motion for excitons and photons we calculate the dispersion relations for localized and resonant polaritons as well as the radiative lifetime for the resonant modes
TL;DR: The present results allow us to confirm the validity of spin-wave theory up to temperatures of 0.5 T c and to predict the phase-transition temperature according to an Ising-like model with a renormalized exchange constant.
Abstract: Using a classical Monte Carlo simulation on the two-dimensional Heisenberg model with exchange anisotropy, we have obtained reliable results for several properties: spontaneous magnetization, total energy, self-correlation, susceptibility, and correlation functions. An improved algorithm has been used in order to eliminate artifacts in such quantities below the critical temperature. The present results allow us to confirm the validity of spin-wave theory up to temperatures of 0.5 T c and, simultaneously, to predict the phase-transition temperature according to an Ising-like model with a renormalized exchange constant
TL;DR: A survey of the current theoretical understanding of the spin relaxation processes of electrons and holes in confined systems of quasi-two dimensions is given in this article, where the theoretical framework is used to form a bridge between the electronic properties and the recent time-resolved polarization measurements.
Abstract: A survey of the current theoretical understanding of the spin relaxation processes of electrons and holes in confined systems of quasi-two dimensions is given. The theoretical framework is used to form a bridge between the electronic properties and the recent time-resolved polarization measurements. Spin relaxation mechanisms surveyed include single-particle spin flips via spin-orbit interaction and electron-hole-pair spin flips via exchange interaction.
TL;DR: Magnetic susceptibilities of 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) radicals have been measured in the temperature range between 4.0 and 300 K.
TL;DR: Inelastic light scattering spectra of the one-dimensional electron gas in GaAs quantum wires embedded in a strong perpendicular magnetic field show long-wavelength collective excitations and display multiple structures that indicate the magnetoroton density of states.
Abstract: Inelastic light scattering spectra of the one-dimensional electron gas in GaAs quantum wires embedded in a strong perpendicular magnetic field show long-wavelength collective excitations and display multiple structures that indicate the magnetoroton density of states. The observed shift of the q∼0 intersubband magnetoplasmons from the cyclotron frequency is the signature of 1D behavior. At low temperatures spin polarization of the 1D system is revealed by the exchange enhancement of spin-flip excitations
TL;DR: The results support that the recently observed reentrant behavior to an insulating phase around [nu]=1/3 in [ital p]-doped GaAs/AlGaAs is a consequence of an increased stability of the Wigner crystal due to the effects of Landau-level mixing.
Abstract: Using a variational quantum Monte Carlo method, we study the two-dimensional Wigner crystal induced by a strong magnetic field in the fractional quantum Hall effect regime. Effects of exchange, intra-Landau-level correlation, and inter-Landau-level mixing on the total energy and their dependence on the carrier mass and magnetic field strength are calculated. Our results support that the recently observed reentrant behavior to an insulating phase around \ensuremath{
u}=1/3 in p-doped GaAs/AlGaAs is a consequence of an increased stability of the Wigner crystal due to the effects of Landau-level mixing.
TL;DR: In this article, the missing of nearest-neighbour manganese spins adjacent to the transition from magnetic to nonmagnetic materials results in a reduced probability of finding antiferromagnetically coupled Mn-Mn clusters.
Abstract: We have performed magneto-optical studies on a series of thin CdTe/(CdMn)Te single quantum wells at low temperatures and in external magnetic fields up to 7.5 T. As a consequence of the high manganese content of the barrier layers ( x 20%) the antiferromagnetic exchange interaction between the manganese spins exhibit spin-glass behaviour. By comparison of the Zeeman splitting of barrier and quantum well excitons, we found a reduced antiferromagnetic coupling of Mn-spins near the heterointerface. We assume that the missing of nearest-neighbour manganese spins adjacent to the transition from magnetic to nonmagnetic materials results in a reduced probability of finding antiferromagnetically coupled Mn-Mn clusters. The mechanism of interface-related frustrated antiferromagnetism leads to the astonishing result of comparable or even larger Zeeman splittings of quantum well excitons compared with those observed in barrier layers. If we introduce a second interface for the magnetic layer as can be realized with ultrathin superlattices, missing nearest neighbours produce a more pronounced frustration of antiferromagnetic coupling due to the quasi two-dimensional character of the magnetic layer, which is correlated with a higher percolation threshold.
TL;DR: In this paper, the dependence of spin relaxation in quantum-well structures on an external electric field along the growth direction and on the well thickness was investigated, and it was shown that exciton spin relaxation is dominated by electronhole exchange interaction.
Abstract: We report measurements of the dependence of exciton spin relaxation in quantum‐well structures on an external electric field along the growth direction and on the well thickness. The results show that exciton spin relaxation is dominated by electron‐hole exchange interaction, and provide a quantitative understanding of various spin‐relaxation rates and their dependence on electric field and well thickness.