Showing papers on "Exchange interaction published in 1996"

Enhancement of electron-hole exchange interaction in CdSe nanocrystals: A quantum confinement effect

Abstract: Photoluminescence of CdSe nanocrystals in a glass matrix was investigated at low temperature with size-selective excitation. The emission spectrum consists of a line a few meV below the excitation laser energy (denoted the F line) and a two-phonon replica superimposed on a broadband. The energy difference between the excitation energy and the F-line position increases with decreasing nanocrystal size. From the analysis of the time behavior of the luminescence and the degree of linear polarization, we attribute the F line to the recombination of the optically forbidden A exciton. Radiation recombination is made possible through a phonon-assisted virtual transition to the confined B-exciton state. The experimental degree of linear polarization is in good agreement with the theoretical calculations. The value of the electron-hole exchange energy obtained from the energy separation between the excitation energy and the F line is much larger than the bulk value and reaches 24 meV in 30-\AA{}-diam. nanocrystals. The size dependence of the exchange energy is in good agreement with the theoretical prediction in the limit of small nanocrystals. \textcopyright{} 1996 The American Physical Society.

Gradient expansion of the exchange energy from second-order density response theory.

Abstract: The basic idea behind the present work is that an atom is not a linear perturbation of the electron gas. We have thus analyzed the exchange energy of the inhomogeneous electron gas to third order in the deviation from a constant density. We give the symmetry properties obeyed by the corresponding second-order response function Lx, and demonstrate how Lx gives rise to gradient corrections to the exchange energy. The expansion, which is taken up to sixth order in the density gradient, also includes the Laplacian of the density. In the case of a statically screened Coulomb interaction, we have calculated the coefficients of second- and fourth-order gradient terms both analytically and numerically. In analogy with the corresponding results from linear-response theory, the fourth-order coefficient is shown to diverge as the screening is made to vanish. For the bare Coulomb interaction we have not succeeded in obtaining analytical results, and, due to numerical problems at small-q vectors, our numerically obtained coefficients have an estimated uncertainty of 20%. (Less)

Hybrid classical quantum force field for modeling very large molecules

Abstract: A coherent computational scheme on a very large molecule in which the subsystem that undergoes the most important electronic changes is treated by a semiempirical quantum chemical method, though the rest of the molecule is described by a classical force field, has been proposed recently The continuity between the two subsystems is obtained by a strictly localized bond orbital, which is assumed to have transferable properties determined on model molecules The computation of the forces acting on the atoms is now operating, giving rise to a hybrid classical quantum force field (CQFF) which allows full energy minimization and modeling chemical changes in large biomolecules As an illustrative example, we study the short hydrogen bonds and the proton-exchange process in the histidine-aspartic acid system of the catalytic triad of human neutrophil elastase The CQFF approach reproduces the crystallographic data quite well, in opposition to a classical force field The method also offers the possibility of switching off the electrostatic interaction between the quantum and the classical subsystems, allowing us to analyze the various components of the perturbation exerted by the macromolecule in the reactive part Molecular dynamics confirm a fast proton exchange between the three possible energy wells The method appears to be quite powerful and applicable to other cases of chemical interest such as surface reactivity of nonmetallic solids © 1996 John Wiley & Sons, Inc

Polaron‐polaron interactions in diluted magnetic semiconductors

Abstract: The bound magnetic polaron (BMP) is the characteristic collective state of diluted magnetic semiconductors. Isolated BMP are well understood, but their interactions are only beginning to be explored. Recent polaron magnetization experiments on p‐ZnMnTe suggest a ferromagnetic polaron‐polaron interaction, in contrast to the invariably antiferromagnetic impurity exchange interaction in conventional semiconductors. To investigate this question theoretically, we have developed a simplified model of polaron pairs whose central feature is competition between the usual, antiferromagnetic, virtual‐hopping interaction, and the loss of carrier‐magnetic ion exchange energy, by intermediate ions, when the polaron moments are antiferromagnetically aligned. The model is sufficiently simple that its partition function can be calculated in detail. With reasonable parameters, it predicts a ferromagnetic polaron‐polaron interaction at low temperatures.

Ground states with cluster structures in a frustrated Heisenberg chain

Abstract: We examine the ground state of a Heisenberg model with arbitrary spin S on a one-dimensional lattice composed of diamond-shaped units. A unit includes two types of antiferromagnetic exchange interaction which frustrate each other. The system undergoes phase changes when the ratio between the exchange parameters varies. In some phases, strong frustration leads to larger local structures or clusters of spins than a dimer. We prove for arbitrary S that there exists a phase with four-spin cluster states, which was previously found numerically for a special value of in the S = 1/2 case. For S = 1/2 we show that there are three ground-state phases, and determine their boundaries.

Molecule-based magnetic materials : theory, techniques, and applications

Abstract: Molecule-Based Magnets: An Introduction Exchange Interaction of Organic Spin Systems Theoretical Approaches to Molecular Magnetism Magnetic-Susceptibility Measurement Techniques Superconducting Quantum Interference Device Studies in Molecule-Based Magnetism Continuous Wave and Fourier Transform Pulsed Electron Magnetic Resonance Spectroscopy in Organic-Molecular Magnetism: Theory and Applications Heat-Capacity Calorimetry of Molecule-Based Magnetic Materials Use of Mssbauer Effect Spectroscopy in the Study of the Low-Dimensionality Magnetism and Long-Range Order of Magnetic Systems of Contemporary Interest Design, Synthesis, and Characterization of pi-Cross- Conjugated Polycarbenes with High-Spin Ground States Origin of Superparamagnetic-Like Behavior in Large Molecular Clusters Magnetostructural Correlations in Dinuclear Cu(II) and Ni(II) Complexes Bridged by pi[2-1,1-Azide and pi[2-Phenoxide The Azido Ligand: A Useful Bridge for Designing High-Dimensional Magnetic Systems Molecular Magnetic Materials and Small Clusters Containing N-Donor Chelated Metal Species Combined with Hexacyanometallate, Tris-oxalatometallate, and Related Bridging Groups Design of Organic-Based Materials with Controlled Magnetic Properties Progress Toward "Conditional" Magnetic Materials: Magnetic Properties of (m-Pyridyl Nitronyl Nitroxide)[2HBR and Cu[2(OH)[3, (alkylcarboxylate) Intrachain Ferromagnetic Spin Alignment in pi-Conjugated Polyradicals with a Poly(phenylenevinylene) Chain Very High Spin Polyradicals Assembly of Imino Nitroxides with Cu(I) Halides Weak-Ferromagnetism and Ferromagnetism in Tetrafluorotetracyanoquinodimethanide Salts Spin Transition Molecular Materials for Display and Data Processing Improved Synthesis of the V(tetracyanoethylene)[x*y(solvent) Room-Temperature Magnet: Doubling of the Magnetization at Room Temperature Design and Magnetism of New Bimetallic Assemblies with Fe(III)-CN-Ni(II) Linkages

Generalized gradient approximation for the relativistic exchange-only energy functional

Abstract: A generalized gradient approximation (GGA) for the exchange energy functional of relativistic many-electron systems, including both a longitudinal and a transverse contribution, is presented. The resulting longitudinal exchange energies for atoms reproduce the corresponding exact values, obtained via the relativistic optimized-potential method, with the same accuracy that has been found for GGAs in the nonrelativistic context. In addition, it is shown that transverse contributions to the self-consistent exchange potential should not be neglected in the case of gradient-corrected functionals. \textcopyright{} 1996 The American Physical Society.

Exchange coupling between ferromagnetic fcc Ni81Fe19 and antiferromagnetic bcc CrMnPt films

Abstract: We studied an antiferromagnetic CrMnPtx [(Cr:Mn≂1:1) in atomic percent] film for an exchange‐biased layer, focusing especially on the relationships between the exchange coupling properties of the CrMnPtx (top)/Ni81Fe19(bottom) films and the character of its CrMnPtx film. The best Pt content to obtain a large exchange coupling of the CrMnPtx film was 5.0–8.0 at. %. Typically, the exchange coupled 50 nm CrMnPt5−8/40 nm Ni81Fe19 films exhibited a relatively large exchange coupling field of ∼22 Oe and a high blocking temperature of ∼380 °C. Besides, the CrMnPt5−8 film deposited on the Ni81Fe19 film had a considerably high resistivity of ∼300–350 μΩ cm. These large exchange coupling and high resistivity values were obtained only when the α‐phase with a disordered bcc structure was stabilized in the CrMnPtx film by the underlying fcc Ni81Fe19 film. The Pt within the CrMnPtx film might localize the Mn magnetic moment. As to why the CrMnPtx film having the Pt content of 5.0–8.0 at. % could give the Ni81Fe19 film a large exchange coupling, this was attributed to the nearest neighbor Mn–Mn atomic distance within the CrMnPt5−8 film being the same as the distance at which the Mn–Mn exchange interaction showed the maximum negative value. Furthermore, the decrease in size of the exchange coupling field and lowered blocking temperature for tCrMnPt<30 nm (tCrMnPt: CrMnPtx film thickness) were thought to originate from a decrease of antiferromagnetic CrMnPtx anisotropy with decreasing tCrMnPt.

Effect of Coulomb Interaction on the X-Ray Magnetic Circular Dichroism Spin Sum Rule in Rare Earths

Abstract: A deviation from the spin sum rule, which relates the integrated intensity of the X-ray magnetic circular dichroism (MCD) signal to the expectation value of the spin operator S-z ((S-z)), is numerically calculated in the case of the 3d --> 4f absorption for rare earths from the trivalent Ce to Tm. The calculation on the basis of an atomic model including the full multiplet shows that the deviation increases from similar to 60. To for Ce with increasing 4f electron number, reaches at similar to 230% for Sm and drops to less than similar to 10% for Gd and heavier rue earths An interplay of the 3d-4f exchange interaction. the 3d core spin-orbit interaction and the 4f electron configuration in the initial ground state Is shown to cause the chemical trend of the deviation from the sum rule.

Metallic ferromagnetism in a single-band model: Effect of band filling and Coulomb interactions.

Abstract: A single-band tight-binding model with on-site repulsion and nearest-neighbor exchange interaction has been proposed as a simple model to describe metallic ferromagnetism. Here we extend previously obtained exact-diagonalization studies for a one-dimensional \textonehalf{}-filled band system to other band fillings, and consider the effect of including various other Coulomb matrix elements in the Hamiltonian that are expected to be of appreciable magnitude in real materials. Results of exact diagonalization and mean-field theory for the one-dimensional case are compared. As the band filling decreases from \textonehalf{}, the tendency to ferromagnetism is found to decrease in exact diagonalization, while mean-field theory predicts the opposite behavior. A nearest-neighbor Coulomb repulsion term is found to suppress the tendency to ferromagnetism; however, the effect becomes small for large on-site repulsion. A pair hopping interaction enhances the tendency to ferromagnetism. A nearest-neighbor hybrid Coulomb matrix element breaks electron-hole symmetry and causes metallic ferromagnetism to occur preferentially for more than half-filled rather than less-than-half-filled bands in this model. Mean-field theory is found to yield qualitatively incorrect results for the effect of these interactions on the tendency to ferromagnetism. The implications of these results for the understanding of ferromagnetism in real materials is discussed.

Neutron scattering study of the magnetic structure of Cs2CuCl4

Abstract: The magnetic structure in the ordered phase of the nearly one-dimensional Heisenberg antiferromagnet has been measured using elastic neutron scattering. crystallizes in the orthorhombic Pnma space group with spin chains running along the crystallographic b-direction. Below the ordering temperature the magnetic structure is incommensurate along the chain direction with a temperature-independent ordering wavevector q = (0, 0.472, 0) (rlu}). The occurrence of an incommensurate structure is shown to be the consequence of frustration on the spins induced by the exchange interaction between chains. Group theory is used to determine the possible magnetic structures compatible with the symmetry of the crystal. The results show that at T = 0.3 K the spin ordering is cycloidal with spins rotating in a plane that contains the propagation direction b. A mean-field calculation of the magnetic ground-state energy including exchange anisotropy effects is used to study the stability of the observed structure. Values for the interchain exchange constants that are consistent with the features of the magnetic structure are proposed.

Electron paramagnetic resonance governed by the Dzyaloshinsky - Moriya antisymmetric exchange interaction in

Abstract: Several reports of electron paramagnetic resonance experiments performed on the spin - Peierls compound have appeared so far, but none of them have precisely explained the origin of the line broadening and the temperature dependence of both the linewidth and the resonance field, all of which differ from those of conventional one-dimensional Heisenberg antiferromagnets in which the dipolar or anisotropic exchange interaction brings about line broadening. In the present report, it is clarified that the antisymmetric exchange interaction, with -axis (magnetic chain), between nearest-neighbour Cu spins on the c-axis governs all factors which characterize the EPR line of , i.e., the value and the angular dependence of the linewidth at high temperatures where the short-range order is completely absent, the temperature dependence of the linewidth, and the resonance field, as well as the high-temperature lineshape. The present conclusion indicates that the crystal symmetry of this compound is lower than that given by the space group Pbmm which was reported by Vollenkle et al in 1967, because the symmetry Pbmm does not allow the antisymmetric exchange interaction mentioned above.

Neutron Scattering Study of Magnetic Excitations in the Spin S = 1 One-Dimensional Heisenberg Antiferromagnet Y2BaNiO5

Abstract: Magnetic excitations of the ideal one-dimensional, spin S =1, antiferromagnet Y 2 BaNiO 5 are studied by means of inelastic neutron scattering on single crystal sample. Our experimental results at low temperature indicate that Y 2 BaNiO 5 can be regarded as the best isotropic Haldane-system sofar studied with the intrachain exchange energy J / k B =-24.1 meV and the averaged gap Δ( T =7 K)=8.5 meV. Y 2 BaNiO 5 exhibits a good magnetic one-dimensionality with the inter-/intrachain exchange ratio of | J ′ / J |≤10 -4 and has only very small easy-axis and in-plane single ion anisotropy of | D / J |=0.03 and | E / J |∼0.01. In addition the temperature dependence of the Haldane gap Δ( T ) and its damping rate was determined.

High-Temperature Study of Octahedral Cation Exchange in Olivine by Neutron Powder Diffraction

Abstract: Time-of-flight, neutron powder diffraction to 1000°C provides precise octahedral site occupancies and intersite distribution coefficients for MnMgSiO 4 and MnFeSiO 4 olivines. Intersite exchange occurs in minutes down to 500°C. Equilibrium distribution coefficients show that manganese ordering into the larger octahedral site decreases with increasing temperature. Exchange energies are 15.7 and 10.1 kilojoules per mole for magnesium-manganese and iron-manganese, respectively. Distribution coefficients deduced for FeMgSiO 4 olivine suggest an exchange energy of 4.8 kilojoules per mole. Intersite exchange energies are consistent with diffusion coefficients in the order iron > magnesium > manganese. Geothermometry based on magnesium-manganese and iron-manganese exchange may be possible only for samples equilibrated below 500°C.

Electron Paramagnetic Resonance Knight Shift in Semimagnetic (Diluted Magnetic) Semiconductors.

Abstract: The investigations of the physical properties of magnetic ions diluted in metallic matrices with their local magnetic moments (if formed) coupled to the conducting carriers by s-d exchange interaction have led to several outstanding experimental and theoretical discoveries. These include, in particular, the Kondo effect, the Anderson virtual bound state (VBS) model, the Ruderman-KittelKasuya-Yosida (RKKY) indirect exchange interaction via free carriers, and the formation of metallic spin glasses. Since the very beginning, electron paramagnetic resonance (EPR) was used as an experimental tool to study these systems. In the pioneering works of Owen, Dyson, and Yosida [1‐ 3] the basic ideas of EPR on local moments in a conducting matrix were developed and quantified in the phenomenological Bloch-Hasegawa equations as reviewed, e.g., by Barnes [4]. One of the standard effects anticipated in these works is the carrier concentration induced shift of the position of the EPR resonance field of the local magnetic moment —the analog of the well known Knight shift in nuclear magnetic resonance. This effect can be expressed as a change of the local moment’s g factor: Dg › JsdrsEFdge, where Jsd is the local moment ‐ conducting carrier exchange integral, rsEFd is the density of states at the Fermi level EF , and ge is the g factor of conducting carriers. The change of the g factor is accompanied by the Korringa contribution to the width of the resonance line: DH › spy ¯ h df JsdrsEF dg 2 kBT with a characteristic linear temperature dependence. The Korringa contribution to the EPR linewidth was identified in a large number of diluted magnetic metallic systems [4] including semimagnetic semiconductors [5 ‐ 9] with both Mn 21 as well as Gd 31 , Eu 21 , and Fe 31 ions. The observation of the carrier concentration induced shift of the resonance field is quite rare and, in fact, is well documented only in strongly paramagnetic metals like Pd:Gd [4]. The experimental observation usually made is to find the resonance at a position different than expected for a given magnetic ion in other (nonconducting) matrices and assign the difference to the effect of the Knight shift. Also, in metals, a direct experimental proof by the observation of the carrier concentration induced shift scaling with the density of states at the Fermi level is not possible because of the inherent lack of control of the concentration of carriers. In this Letter we will show for the first time that a straightforward proof for the EPR Knight shift can be established for diluted magnetic systems with well controlled semimetallic electron properties. By changing the concentration of carriers in PbTe:Mn 21 , a strongly degenerated IV-VI semiconductor, we observe the shift of the resonance field being of different sign for holes and for electrons and scaling as a density of states at the Fermi level. This first observation of the EPR Knight shift in semimagnetic semiconductors gives new possibilities to determine experimentally both sign and magnitude of the Jsd exchange integrals. The method is unique in its ability to provide the information in the limit of strong dilution. It also has no limitations brought about by the high mobility required in the (usually applied) magneto-optical methods, so it is, in particular, suitable for disordered systems.

Magnetic Properties of 1,5-Dimethylverdazyl Radical Crystals. Ferromagnetism in 3-(4-Chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl Radical Crystal

Abstract: Magnetic properties of organic radical crystals, 3-(4-chlorophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-CDTV) and 3-(4-bromophenyl)-1,5-dimethyl-6-thioxoverdazyl (p-BDTV), have been studied by measuring the heat capacity and ac susceptibility above 0.13 K in external magnetic fields of 0−30 kOe. p-CDTV has been found to be a new ferromagnet with a Curie temperature of Tc = 0.68 ± 0.02 K. p-CDTV behaves as a quasi-one-dimensional Heisenberg ferromagnet with the intrachain exchange interaction of 2J/kB = +12.0 ± 0.6 K above the transition temperature Tc. The interchain exchange interaction zJ‘/kB was estimated to be +0.21 ± 0.02 K, where z is the number of interchain bonds per spin. p-BDTV, on the other hand, behaves as a one-dimensional Heisenberg antiferromagnet with negative exchange interaction of 2J/kB = −41.2 K.

Non-perturbative approaches to magnetism in strongly correlated electron systems

Abstract: The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate lattice structure and/or kinetic energy of the electrons. We furnish evidence of the stability of itinerant ferromagnetism in the pure Hubbard model for appropriate lattices at electronic densities not too close to half-filling and large enough U. Already a weak direct exchange interaction, as well as band degeneracy, is found to reduce the critical value of U above which ferromagnetism becomes stable considerably. Using similar numerical techniques the Hubbard model with an easy axis is studied to explain metamagnetism in strongly anisotropic antiferromagnets from a unifying microscopic point of view.

High‐nuclearity mixed‐valence magnetic clusters: A general solution of the double exchange problem

Abstract: We report here a general solution of the double‐exchange problem in the high‐nuclearity mixed valence systems containing arbitrary number P of the electrons delocalized over the network of N (P

Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO3

Abstract: The microscopic description of the spin-Peierls transition in pure and doped CuGeO3 is developed taking into account realistic details of crystal structure. It it shown that the presence of side-groups (here Ge) strongly influences superexchange along Cu−O−Cu path, making it antiferromagnetic. Nearest-neighbour and next-nearest neighbour exchange constantsJ nn andJ nnn are calculated. Si doping effectively segments the CuO2-chains leading toJ nn (Si)≃0 or even slightly ferromagnetic. Strong sensitivity of the exchange constants to Cu−O−Cu and (Cu−O−Cu)−Ge angles may be responsible for the spin-Peierls transition itself (“bond-bending mechanism” of the transition). The nature of excitations in the isolated and coupled spin-Peierls chains is studied and it is shown that topological excitations (solitons) play crucial role. Such soltons appear in particular in doped systems (Cu1−x Zn x GeO3, CuGe1−x Si x O3) which can explain theT SP (x) phase diagram.

Biquadratic exchange interactions in the Europium monochalcogenides

Abstract: We have examined the biquadratic exchange interaction strengths in the Europium monochalcogenides EuO, EuS, EuSe and EuTe using magnetization data of the paramagnetic phase and elaborate the consequences this additional interaction mechanism has on the magnetic phase diagrams of EuSe and EuTe. It is shown that the cubic susceptibility χ3 obeys a Curie-Weiss law at suffciently high temperatures and that the associated Curie-Weiss temperature θ3 is a measure for the biquadratic interaction strength. For all these materials the biquadratic interaction is ferromagnetic (θ3 > 0). This leads to a conflicting situation in the case of EuTe for which θ1 < 0. We attribute the peculiar observation, that the MnO superstructure reflection intensities as observed with neutron scattering correspond only to 0.6 of that moment expected for perfect magnetic order, to the presence of biquadratic interactions. The critical field Bc follows a T2 law in the spin-wave regime (T < 0.8 K) for EuTe and EuSe but for these two materials with an antiferromagnetic ground state the cubic susceptibility χ3 diverges at a temperature T* which is 2.5 K and 1.2 K above the ordering temperature, respectively. In the temperature range Tc < T < T* the magnetization curves exhibit some weak but definite anomaly which might be interpreted as a field-induced transition into the ferromagnetic state. A new multicritical point has been identified along the critical field curve Bc of EuSe.

Magnetic polaron in ferro- and antiferromagnetic semiconductors

Abstract: The temperature-dependent quasiparticle spectrum of a single conduction electron exchange coupled to a ferro- or antiferromagnetically ordered localized-spin system (e.g., EuO, EuTe) is calculated by a moment-conserving Green function technique. In the weak coupling regime the exchange interaction leads to an almost rigid shift of the Bloch dispersion. The induced spin splitting of the conduction band states is proportional to the magnetization 〈${\mathit{S}}^{\mathit{z}}$〉 of the localized-spin system. As soon as the coupling constant exceeds a critical value an additional splitting of the quasiparticle dispersion for each spin projection sets in due to different elementary excitations. One is based on a repeated emission and reabsorption of a magnon by the conduction electron resulting in an effective attraction between magnon and electron. This gives rise to a polaronlike quasiparticle (``magnetic polaron''). Another excitation is due to a direct magnon emission or absorption by the electron thereby flipping its own spin (``scattering states''). For the exactly calculable special case of a ferromagnetically saturated spin system (T=0 K), the magnetic polaron appears only in the \ensuremath{\downarrow} spectrum and turns out to be a stable quasiparticle. For finite temperatures it gets a finite lifetime. In antiferromagnetic systems each quasiparticle band exhibits an additional ``Slater splitting'' due to the reduced magnetic Brillouin zone. The predicted strong correlation effects in the excitation spectrum require unconventional interpretations of respective inverse photoemission experiments. \textcopyright{} 1996 The American Physical Society.

Spin relaxation of electrons in p -doped quantum wells via the electron-hole exchange interaction

Abstract: We have performed a calculation of the spin-relaxation time of photoexcited electrons in p-doped quantum wells with the spin flip due to the electron-hole exchange interaction. A comparision with the same process taking place in bulk semiconductors is presented, as well as a comparision with other spin-flip channels that compete with the exchange channel. Photoexcitation of electrons in p-doped samples populates electronic states with momentum larger than the hole Fermi momentum. For these electrons, the calculated spin-relaxation times are of the order of hundreds of picoseconds, as observed experimentally. However, we have found a strong increase in these times with decreasing electronic kinetic energy, which makes this spin-flip process ineffective for electrons approaching the conduction-band edge. The reason is that the exclusion principle reduces the phase space available for the degenerate holes to be scattered by slow electrons. In quantum wells, the lifting of the heavy-hole--light-hole degeneracy at the top of the valence band provides an additional reduction in hole scattering, lengthening the electron spin-relaxation times. \textcopyright{} 1996 The American Physical Society.

Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO$_3$

Abstract: The microscopic description of the spin-Peierls transition in pure and doped CuGeO_3 is developed taking into account realistic details of crystal structure. It it shown that the presence of side-groups (here Ge) strongly influences superexchange along Cu-O-Cu path, making it antiferromagnetic. Nearest-neighbour and next-nearest neighbour exchange constants $J_{nn}$ and $J_{nnn}$ are calculated. Si doping effectively segments the CuO_2-chains leading to $J_{nn}(Si)\simeq0$ or even slightly ferromagnetic. Strong sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be responsible for the spin-Peierls transition itself (``bond-bending mechanism'' of the transition). The nature of excitations in the isolated and coupled spin-Peierls chains is studied and it is shown that topological excitations (solitons) play crucial role. Such solitons appear in particular in doped systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the $T_{SP}(x)$ phase diagram.

Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain

Abstract: In this second of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain we present exact results for strongly correlated tight-binding electrons. In the limit of a strong on-site interaction $U$ we map the Hubbard model onto the Harris-Lange model which can be solved exactly in one dimension in terms of spinless fermions for the charge excitations. The exact solution allows for an interpretation of the charge dynamics in terms of parallel Hubbard bands with a free-electron dispersion of band-width $W$, separated by the Hubbard interaction $U$. The spin degrees of freedom enter the expressions for the optical absorption only via a momentum dependent but static ground state expectation value. The remaining spin problem can be traced out exactly since the eigenstates of the Harris-Lange model are spin-degenerate. This corresponds to the Hubbard model at temperatures large compared to the spin exchange energy. Explicit results are given for the optical absorption in the presence of a lattice distortion $\delta$ and a nearest-neighbor interaction $V$. We find that the optical absorption for $V=0$ is dominated by a peak at $\omega=U$ and broad but weak absorption bands for $| \omega -U | \leq W$. For an appreciable nearest-neighbor interaction, $V>W/2$, almost all spectral weight is transferred to Simpson's exciton band which is eventually Peierls-split.

Theory of non‐Heisenberg exchange: Results for localized and itinerant magnets

Abstract: A general formulation of intersite (layer) exchange coupling in bulk (multilayer) materials is proposed based on the treatment of the spin (layer) rotation as a perturbation in terms of the force theorem and multiple scattering theory. The expansion of the intersite (layer) exchange interaction energy gives expressions for the bilinear and biquadratic exchange. For metals, the approach is illustrated by linear muffin‐tin orbital calculations of exchange coupling constants in the ferromagnetic 3D‐metals and the fcc phase of bulk Fe. Long range oscillations of strongly volume dependent exchange coupling in fcc Fe appears to be the origin of the spin‐density‐wave instability in this metastable phase. The correctness of expressions for the limit of localized magnets is demonstrated by calculations for the antiferromagnetic insulator NiO. In contrast with other theories, this method can be used for both nonmagnetic and magnetic spacers in metallic multilayers, as is illustrated by calculations of the interfaci...

Positronium - hydrogen atom scattering using the static exchange model

Abstract: Scattering of positronium atoms by hydrogen atoms has been investigated using the static exchange model and the first Born approximation (FBA) using only the exchange interaction. The FBA elastic total cross section at very low energy is very close to the estimated value of Massey and Mohr in the zero energy limit. We report total elastic, differential and quenching cross sections using the static exchange model for the energy range 0.068 - 100 eV. The present static exchange results are found to differ from the corresponding results of Fraser. The ratio of the quenching to total elastic cross section approaches the value 0.25 with increasing energy as predicted by Massey and Mohr.

Lifting of the ground-state degeneracy by crystal-field interactions in the pyrochlore

Abstract: The results of susceptibility and neutron inelastic scattering measurements on the pyrochlore compound are presented. The crystal-field ground state of the ion has zero spontaneous magnetization, and a virtually dispersionless crystal-field excitation is observed. It is concluded that the crystal-field interaction dominates the exchange interaction in , lifting the expected degeneracy of the frustrated ground state.

Change of Magnetism in the Two-Dimensional Heisenberg Ferromagnet K2CuF4 Observed at High Pressures

Abstract: The mechanism responsible for determining the exchange interaction to be ferromagnetic or antiferromagnetic in Cu 2+ compounds is closely correlated with the antiferrodistortive (AFD) or ferrodistortive (FD) order of Cu 2+ hole orbitals. One of the representative two-dimensional Heisenberg ferromagnets, K 2 CuF 4 , has an ideal AFD orbital ordering. To investigate the possibility of the magnetic phase transition due to a change in the orbital ordering from AFD to FD at high pressures, susceptibility measurements of this compound have been made under various pressures up to 13 GPa over the temperature range from 1.5 to 18 K. A sudden decrease in the Curie temperature starting from 7 GPa was observed. Furthermore, a large decrease in the susceptibility was observed at pressures above 10 GPa. These results indicate a pressure induced ferromagnetic to antiferromagnetic phase transition occurring at about 8–9 GPa as a result of a change of orbital state from AFD to FD order.