Short-range Exchange Interactions Research Articles

High resolution spectroscopy of yellow 1S excitons in Cu 2O

The 1S ortho- and para-exciton of the yellow series in Cu 2O are studied by high resolution spectroscopy. We observe that the 3-fold ortho-exciton ( Γ 5 +-symmetry) is not degenerate. For a general k-direction there are three resonances with line width down to 1 μeV. The splitting is quantitatively explained by taking into account the long range and short range exchange interaction. The group velocity of the quadrupole polariton is directly determined with use of 20 ns pulses with still high spectral resolution (Δ E≈0.1 μeV). Group velocities down to 4×10 4 m/s are quantitatively described by a one-oscillator quadrupole polariton model. The para-exciton ( Γ 2 +-symmetry) is seen in absorption in magnetic fields down to 1 T. In selected crystals it shows a line width of 0.2 μeV.

Exchange interactions in R2M17 (M=Fe, Co, Ni) heavy rare-earth compounds

Band structure calculations were performed on R2M17 compounds where R is a heavy rare earth or Y and M=Fe, Co or Ni, by using TB-LMTO-ASA method. The 4f–5d–3d exchange interactions were analysed. The magnetic moments of transition metals, at various sites, were determined and analysed in correlation with their local environments. The R5d band polarizations are linearly dependent on De Gennes factor and are due both to local 4f–5d and to 5d–3d short-range exchange interactions.

Rare-Earth Based Materials: Electronic Structures and Magnetic Properties

The electronic structures and magnetic properties of some rare-earth-transition metal compounds having technical interest is analyzed. The exchange interactions of 4f-5d-3d type is discussed in correlation with crystal structure and transition metal partner. The contributions to 5d band polarization due to local 4f-5d and short-range 5d-3d exchange interactions are estimated. Finally we analyze the magnetic behaviour of transition metals in rare-earth compounds.

Isoelectronic traps in heavily doped GaAs:(In,N)

GaAs samples doped with low indium and nitrogen contents were investigated by continuous-wave (CWPL) and time-resolved photoluminescence (TRPL) at low temperature (10 K). The simultaneous incorporation of doping amounts of both indium and nitrogen elements in GaAs creates new isoelectronic traps that are not present in indium-free GaAs:N. These traps are built from nitrogen-related defects of GaAs:N now perturbated by one additional indium atom. They are believed to involve a preferential orientation of the In atom and of one N atom in near-neighbor positions. The experiments are consistent with the assumption that the clusters are perturbed by a single In atom but that a definitive assignment cannot be given without addition theoretical modeling. Optical feature characteristics of the exciton bound to these kinds of isoelectronic traps are reported: (i) the exciton fine structure with dipole-forbidden and dipole-allowed states and (ii) the train of phonon replicas that is typical of isoelectronic traps. The observation of the tine structure of the trapped excitons is made possible because of the thermodynamic equilibrium between the two populations of excitons (dipole-allowed and dipole-forbidden states) in the conditions of our measurement. It gives us a direct measurement of the short-range exchange interaction (∼0.7 meV). The TRPL experiments allow us to characterize the exciton recombination dynamics for the different isoelectronic traps and notably the transfer mechanisms of excitons among these various traps.

Coherent spin dynamics of polaritons in semiconductor microcavities

The influence of the strong light-matter coupling in semiconductor III–V microcavities on polariton spin dynamics is studied in the spontaneous emission regime. Under resonant pulsed excitation, we observe a quenching of spin and alignment relaxation when the polariton is photon-like. These behaviours are attributed to the very small value of the long-range electron–hole exchange term of Coulomb interaction within the excitonic component of the quasi-particle and to the weakness of polariton–polariton Coulomb scattering via the inter-exciton short-range exchange interaction. We investigate then the polariton coherent spin dynamics. Coherence decay occurs in two steps: at short time delay, the polariton keeps the phase memory of the excitation pulse. We show that it is thus possible to manipulate the polariton spin and alignment within the optical dephasing time T2 using the temporal coherent control technique. In a second step (t > T2), optical coherence vanishes, but spin coherences survive and spin quantum beats experiments can be performed within the spin relaxation time under transverse magnetic field. We observe an electron–hole spin correlation within the excitonic component of the quasi-particle under resonant excitation; an increase of the absolute value of the electron effective transverse Landé g-factor with the excitonic character of polaritons is evidenced, which is the consequence of the strong coupling regime. The main experimental features are well accounted by a model taking into account only two classes of excitations in the lower polariton branch.

Mechanisms of exciton spin relaxation in quantum dots

We study the phonon-assisted relaxation processes (longitudinal relaxation time T1) within the radiative doublet of the heavy-hole-exciton in asymmetrical quantum dots. Two different relaxation mechanisms are considered: the exciton spin–acoustic phonon coupling via the strain-dependent short-range exchange interaction and the second-order quasielastic interaction between charge carriers and LO phonons. For zero magnetic fields and low temperatures, the calculated relaxation times for typical QDs are very long compared to the exciton lifetime yet they are strongly reduced in high magnetic fields (of the order of a few Tesla) and high temperatures T ≥ 100 K.

Magnetic phase transition and size effect in thin magnetic film

L× L× LN multilayered film structure with long-range dipole–dipole interactions, short-range exchange interactions and magnetic surface anisotropy is considered. Based on Monte Carlo simulation and the local energy minimum methods, spin configuration, magnetization, specific heat of the systems are calculated. The phase diagram from perpendicular to in-plane magnetization with varying temperature is presented. The ferromagnetic, striped, antiferromagnetic and disordered phases exist in the system. The strong size effect of phase transition is observed. It is found that the results from local energy minimum method agree well with ones from Monte Carlo simulation.

Short-range exchange and long-range dipole interactions in a triangular planar model

The planar triangular antiferromagnet with nearest neighbor (NN) exchange interaction $Jl0$ exhibits a ``chiral'' phase transition in addition to the Berezinskii-Kosterlitz-Thouless phase transition. We show that the dipole-dipole interaction which is always present in actual compounds modifies considerably the behavior of the model. Indeed, an analytic low temperature expansion and a Monte Carlo simulation show that a succession of ferromagnetic (F), striped (AF1, AF2), and $120\ifmmode^\circ\else\textdegree\fi{}$ three-sublattice (T) configurations occur at increasing $|J|/\ensuremath{\omega}$ where $\ensuremath{\omega}$ is the dipole interaction strength. Standard long-range order is present in F, AF1, and AF2 configurations, whereas a chiral order is present in the T configuration. The transition to the paramagnetic (P) phase is continuous for the F phase, and first order for the AF1 and T phases. The T-P chiral phase transition seems to be continuous only for $\ensuremath{\omega}=0.$

Luminescence spectroscopy of theGd-rich Ce3-, Tb3- and Mn2-doped phosphate glasses

Absorption, emission and excitation spectra and luminescence decay kinetics of Na(K)GdCe, NaGdTb and NaGdMn phosphate glasses and their dependence on the temperature (in the 1.7-300 K range) and on the glass composition have been studied. The processes of energy migration through the Gd 3+ ions and following energy transfer in Gd 3+ -impurity pairs have been compared for the three types of the glasses studied. It has been suggested that the Gd 3+ → Tb 3+ energy transfer occurs only in the closest pairs through a very short-range exchange interaction, while the Gd 3+ → Ce 3+ and Gd 3+ → Mn 2+ energy transfer is possible also in more separated pairs mainly due to the longer-range multipolar Gd 3+ -impurity interaction.

On the magnetic behaviour of GdCo2xCux compounds

The GdCo2–xCux compounds crystallize in a cubic C15 type structure for x ≤ 0.20. The compounds are ferrimagnetically ordered. The mean cobalt magnetic moments, determined at 4.2 K, are little dependent on composition. The saturation moments per formula unit, at 4.2 K, are in rather good agreement with those obtained from band structure calculations. The Gd 5d band polarizations were analyzed. The contributions from 4f–5d, local exchange M5d(Gd), and 5d–3d short range exchange interactions, M5d(0), were determined. The M5d(0) values are proportional to cobalt magnetizations.

Coherent spin states and spin quantum beats in semiconductor microcavities

The present study evidences the influence of the strong light-matter coupling in semiconductor III–V microcavities on polariton spin dynamics at low excitation. Under resonant excitation, we observe a quenching of spin and alignment relaxation when the polariton is photon like. These behaviours are attributed, respectively, to the very small value of the long-range electron–hole exchange term of Coulomb interaction within the excitonic component of the quasi-particle and to the weakness of polariton–polariton Coulomb scattering via the inter-exciton short-range exchange interaction. Spin dynamics are also investigated under transverse magnetic field. We observe an electron–hole spin correlation within the excitonic component of the quasi-particle under resonant excitation, and an increase of the absolute value of the electron effective transverse Landé g-factor with the excitonic character of polaritons, which can be derived from a model taking into account only two classes of excitations in the lower polariton branch.

Magnetism and x-rays: a vision of the future (abstract only)

In this talk I will give my personal view of important areas in magnetism research where x-rays have the ability to make key contributions. The selected areas deal with the manipulation of magnetization on the nanoscale with unprecedented speeds, i.e. on the picosecond and even femtosecond timescale. This development is driven by the need of information technology to store information more efficiently and process it faster. Implementation of this goal requires concepts that deviate from the classical concepts of switching the magnetization with Oersted fields created by current flow through wires or coils. The new concepts are based on the generation of spin-polarized electrons and spin states with a finite lifetime. For example, the generation of entangled spin states holds promise for futuristic concepts such as quantum computing, while spin-polarized currents can be used to switch the magnetization in nanostructures through the strong short-range quantum mechanical exchange interaction. I will demonstrate how such phenomena can be studied by means of advanced x-ray techniques that provide high spatial resolution, through real and reciprocal space imaging, in conjunction with ultra-short pulses. The envisioned x-ray studies utilize existing synchrotron radiation sources and, in the future, soft x-ray free-electron lasers.

Dynamics of electronic energy transfer from the S2 state of azulene to the S2 state of zinc porphyrin

Electronic energy transfer between the S2 state of azulene as donor and the S2 state of zinc porphyrin as acceptor in dichloromethane and CTAB micelles has been investigated. In dichloromethane high S2–S2 energy transfer efficiency, which cannot be explained using the Forster theory, is observed. An inhomogeneous distribution of acceptors surrounding the donor, leading to short-range exchange interaction and higher multipole interaction is proposed. In CTAB micelles, Forster's mechanism is found to agree well with the observed energy transfer efficiency when a surface-uniform distance distribution between donor and acceptor is assumed. The implications of S2–S2 energy transfer in our system for designing efficient molecular devices is discussed.

Magnetization orientation dependence of the quasiparticle spectrum and hysteresis in ferromagnetic metal nanoparticles

We use a microscopic Slater-Koster tight-binding model with short-range exchange and atomic spin-orbit interactions that realistically captures generic features of ferromagnetic metal nanoparticles to address the mesoscopic physics of magnetocrystalline anisotropy and hysteresis in nanoparticle quasiparticle excitation spectra. Our analysis is based on qualitative arguments supported by self-consistent Hartree-Fock calculations for nanoparticles containing up to 260 atoms. Calculations of the total energy as a function of magnetization direction demonstrate that the magnetic anisotropy per atom fluctuates by several percents when the number of electrons in the particle changes by one, even for the largest particles we consider. Contributions of individual orbitals to the magnetic anisotropy are characterized by a broad distribution with a mean more than two orders of magnitude smaller than its variance and with no detectable correlations between anisotropy contribution and quasiparticle energy. We find that the discrete quasiparticle excitation spectrum of a nanoparticle displays a complex non-monotonic dependence on an external magnetic field, with abrupt jumps when the magnetization direction is reversed by the field, explaining recent spectroscopic studies of magnetic nanoparticles. Our results suggests the existence of a broad cross-over from a weak spin-orbit coupling to a strong spin-orbit coupling regime, occurring over the range from approximately 200- to 1000-atom nanoparticles.

Reaction of Metastable Ar*(3P2) and Kr*(3P2) Atoms with Water Vapor: Excitation Functions for Electronic Quenching Collisions

In this paper, we examine the functional nature of the state-specific cross section for electronic quenching σQ(E) in collisions of metastable noble gas atoms [Ar*(3P2) and Kr*(3P2)] with ground-state water molecules confined to a scattering-gas cell. The relative kinetic energy range studied is E = 0.0463−0.772 eV for Ar*(3P2) and 0.0432−0.692 eV for Kr*(3P2). These beam-gas-luminescence experiments incorporate a novel state-specific monitor system employing Xe gas. Photon emission from a state-specific Xe state produced in quenching collisions with the respective metastable noble gas (Ng*) atoms is a direct measure of the total disappearance of the metastable state under scrutiny in the reaction with water vapor. We use the classical orbiting and absorbing sphere models with empirically derived parameters [London dispersion Cdisp, induction Cind (spherically averaged or aligned dipole), and absorbing sphere radius ras] to predict energy-dependent cross sections for the total quenching reactions. The underlying premise of the models is that although the quenching process itself is envisioned to be a short-range two-electron exchange interaction, it is the long-range attractive forces between the respective collision partners that are critical in determining whether such short-range processes ultimately occur. Values of σQ determined from the models are in very good agreement with our experiment. This work supersedes our previous study for Ar*(3P2,0) [Novicki, S.; Krenos, J. J. Chem. Phys. 1988, 89, 7031].

Modern quantum theory and computer simulation in nanotechnologies: quantum topology approaches to kinematic and dynamic structures of self-assembling processes

Some exact mathematical statements and decision of the problem of self-assemble effects in nanosystems are given within the framework of the quantum topological approach. Obtained results are as follows. An elementary self-assembling process on microscopic level of material is a confined compact chemical particle evolution that has quantum mechanical kinematic and dynamic structures. The process has a domain in the event space allocated given duration and some spatial–temporary organization, which result from the fundamental indeterminacy principle of quantum theory and finiteness of electromagnetic signal velocity. On mesoscopic level, there are topological intersections between compact chemical particles, which form topology of quantum statistical processes of self-assembling within extended exchanging cluster nanosystem. Basic reasons of self-organizing are short-range exchange interactions between chemical particles and stochastic influences of quantum statistical phase fluctuations of exchange statistical operators of their intersections. The only formation exchange clusters of chemical particles cause occurrence of a level, which evolves under the laws of quantum statistics. Exchange pasting of compact chemical particles within cluster sets information resources of nanosystem. On macroscopic level of self-assembling in materials, there are only classical fundamental forces between separate compact nanosystems, which form thermo-statistical system of self-assemblage units. So there is hierarchy of self-organizing processes in the form of a triad feedback levels: quantum mechanical level inside chemical particles, quantum statistical level inside exchange clusters of nanosystems, and thermo-statistical level of self-assembly system imbedded in thermostat. Computer simulations of self-assembling confinement of H atom inside nanosystem illustrate theory.

Effect of magnetic interactions on the magnetic properties of ball-milled SmCo 5+NiO powders

An enhancement of the coercivity, H C, has been observed in SmCo 5 when ball-milled with antiferromagnetic NiO powders. Moreoever, after short-term milling, these composites exhibit very high values of the squareness without need for particle alignment. Classical Δ M plots show that short-range exchange interactions are predominant for fields H< H C, while long-range magnetostatic interactions prevail for H> H C.

Fine Structure of Exciton Levels in Strained Quantum Dots

The excitonic ground state in single quantum dots with structural anisotropy is studied in dependence on the magnitude of the electron–hole short-range exchange interaction. Structural anisotropy is described phenomenologically by lattice deformation. The fine structure of the exciton levels is due to an interplay of the deformations and exchange interaction and is the more pronounced the smaller the quantum dots are. The lattice deformations are the reason for optical anisotropy in strained quantum dots. The exchange interaction, however, suppresses the polarization effects which are much more significant in large quantum dots. An additional decrease of the optical anisotropy arises from the coexistence of structural and epitaxial deformations.

Critical Phenomena of Ferromagnetic Transition in Double-Exchange Systems

Critical phenomena of ferromagnetic transition at finite temperatures are studied in double-exchange systems. In order to investigate strong interplay between charge and spin degrees of freedom, Monte Carlo technique is applied to include fluctuations in a controlled and unbiased manner. By using finite-size scaling analysis, critical exponents and transition temperature are estimated for a model with Ising spin symmetry in two dimensions. The obtained exponents are far distinct from the mean-field values, but consistent with those of spin models with short-range exchange interactions. The universality class of this transition belongs to that of short-range interaction with the same spin symmetry. We also discuss the case for three dimensions. The results are compared with experimental results in perovskite manganites which show colossal magnetoresistance.

ChemInform Abstract: Monte Carlo Models of the Magnetization Reversal in Thin Films with Strong Perpendicular Anisotropy

A variety of different models and Monte Carlo algorithms are used to study the magnetization reversal in thin films with perpendicular magnetization. The thin film is represented by a two-dimensional micromagnetic grid or array of classical Ising spins, with long-range dipolar and a short-range effective exchange interaction arising from the contribution of the domain-wall energy. The selection of an appropriate model requires some knowledge of the defects that may be present in the film and their mechanism of interaction with the domain boundary. Magnetic viscosity and equilibrium domain structures are simulated using the Metropolis algorithm with or without an additional energy barrier. Other algorithms are useful when the process of nucleation and growth of magnetic domains involves a wide distribution of relaxation times. We study the relationship between the methods and discuss their relative merits and range of application.