Showing papers on "Exchange interaction published in 2005"

Effects of dielectric confinement and electron-hole exchange interaction on excitonic states in semiconductor quantum dots

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.

Vanishing Magnetic Interaction in Ferromagnetic Thin Films

Abstract: In this thesis the magnetic and structural properties of ultra-thin 3d transition metals films have been investigated, in particular Fe, Ni and Co films. X-ray Magnetic Circular Dichroism (XMCD) has provided element specific spin (ms ) and orbital (ml ) moments per atom by utilizing the magneto optic sum-rules. Element specific hysteresis curves have been measured by means of X-ray Resonant Magnetic Scattering (XRMS), and the local crystallographic structure has been investigated using Extended X-ray Absorption Fine Structure (EXAFS).By performing XMCD on Fe/Ag(100) we observe a spin reorientation from in-plane to out-of-plane as the Fe thickness is lowered. At temperatures below 300K it occurs around 5-7 mono layers (ML) of Fe. While reorienting the magnetization out-of-plane the orbital moment increases with 125% but only a minor increase (5%) of the spin moment is observed. Extended X-ray Absorption Fine Structure (EXAFS) measurements indicate that films 6 ML and thicker have a bulk-like bcc structure. For the thin out-of-plane films, the local crystallographic structure is more complicated.The spin reorientation of the Au/Co/Au tri-layer system has been studied as a function of temperature, Co layer and Au cap thickness. An unexpected behavior of the orbital moment upon spin reorientation is found in these systems. An ex-situ prepared sample shows a smooth spin reorientation from an in-plane to an out-of-plane easy magnetization direction as the temperature is lowered from 300K to 200K. In-situ prepared samples have also been investigated and a novel phase diagram has been identified. The Au/Co interface has been explored during the Au capping by means of photoemission measurements.In the bi- and tri-layer system of Fe and Ni we have been able to manipulate the spin reorientation by varying the Fe and Ni thickness. A novel non-collinear interlayer exchange interaction for 3d ferro magnets in direct contact has been discovered for a set of samples. This exchange interaction is found to be strongly dependant on the preparation conditions.

Singlet-triplet decoherence due to nuclear spins in a double quantum dot

Abstract: We have evaluated hyperfine-induced electron spin dynamics for two electrons confined to a double quantum dot. Our quantum solution accounts for decay of a singlet-triplet correlator even in the presence of a fully static nuclear spin system, with no ensemble averaging over initial conditions. In contrast to an earlier semiclassical calculation, which neglects the exchange interaction, we find that the singlet-triplet correlator shows a long-time saturation value that differs from 1/2, even in the presence of a strong magnetic field. Furthermore, we find that the form of the long-time decay undergoes a transition from a rapid Gaussian to a slow power law (similar to 1/t(3/2)) when the exchange interaction becomes nonzero and the singlet-triplet correlator acquires a phase shift given by a universal (parameter independent) value of 3 pi/4 at long times. The oscillation frequency and time-dependent phase shift of the singlet-triplet correlator can be used to perform a precision measurement of the exchange interaction and Overhauser field fluctuations in an experimentally accessible system. We also address the effect of orbital dephasing on singlet-triplet decoherence and find that there is an optimal operating point where orbital dephasing becomes negligible.

The performance of hybrid density functionals in solid state chemistry: the case of BaTiO3

Abstract: The performance of hybrid exchange functionals is investigated within the framework of Density Functional Theory (DFT), in describing the properties of cubic and tetragonal ferroelectric phases of BaTiO3. A variable fraction ξ of non-local HF exchange is mixed with Becke's GGA formulation, and coupled with the LYP correlation functional. Standard DFT functionals (LDA, PW, B3LYP) and uncorrelated HF are also added for comparison. Inclusion of the non-local HF exchange has a noticeable effect on the calculated electronic density, and several observables show clear trends as a function of the mixing parameter ξ; these include not only the band gap, but also lattice parameter and bulk modulus, structural distortions, relative ionic sizes, electronic polarizability and polarization of the tetragonal phase. In this context, it is shown that hybrid functionals yield results that are generally in better agreement with experiment than local (LDA and GGA) DFT formulations. A higher fraction of HF exchange than the ...

Magnetic state and electronic structure of the δ and α phases of metallic Pu and its compounds

Abstract: By the local density approximation with on-site Coulomb repulsion U LDA+U method with spin-orbit coupling LDA+U+SO the magnetic state and electronic structure have been investigated for plutonium in and phases and for the Pu compounds PuN, PuCoGa5, PuRh2, PuSi2, PuTe, and PuSb. In agreement with experiment we found for metallic plutonium in both phases a nonmagnetic ground state with Pu ions in f 6 configuration with zero values of spin, orbital, and total moments. This result is determined by a strong spin-orbit coupling in the 5f shell. It leads to the clear splitting of 5f states into f 5/2 and f 7/2 subbands even in the LDA calculation. The Fermi level is in a pseudogap between them, so that the f 5/2 subshell is already almost completely filled with six electrons before Coulomb correlation effects are taken into account. The competition between spin-orbit coupling and the exchange Hund interaction favoring a magnetic ground state in the 5f shell is so delicately balanced that a small increase less than 15% of the exchange interaction parameter value from JH=0.48 eV obtained in the constrained LDA calculation would result in a magnetic ground state with nonzero spin and orbital moment values. For the Pu compounds investigated in the present work, a predominantly f 6 configuration with nonzero magnetic moments was found in PuCoGa5, PuSi2, and PuTe, while PuN, PuRh2, and PuSb have the f 5 configuration with sizable magnetic moment values. Whereas the pure jjcoupling scheme was found to be valid for metallic plutonium, an intermediate coupling scheme is needed to describe the 5f shell in Pu compounds. The results of our calculations show that the exchange interaction term in the Hamiltonian should be treated in a general matrix form for Pu and its compounds.

Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

Abstract: We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot.

Abstract: We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots. The spectrum exhibits a $p$-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric $e\mathrm{\text{\ensuremath{-}}}e$ and $e\mathrm{\text{\ensuremath{-}}}h$ exchange interactions.

First-principles study of exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z = Al, Ge)

Abstract: We report the parameter-free, density functional theory calculations of interatomic exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z = Al, Ge). To calculate the interatomic exchange interactions we employ the frozen-magnon approach. The Curie temperatures are calculated within the mean-field approximation to the classical Heisenberg Hamiltonian by solving a matrix equation for a multi-sublattice system. Our calculations show that, although a large magnetic moment is carried by Mn atoms, competing ferromagnetic (inter-sublattice) and antiferromagnetic (intra-sublattice) Mn–Mn interactions in Mn2VAl almost cancel each other in the mean-field experienced by the Mn atoms. In Mn2VGe the leading Mn–Mn exchange interaction is antiferromagnetic. In both compounds the ferromagnetism of the Mn subsystem is favoured by strong antiferromagnetic Mn–V interactions. The obtained value of the Curie temperature of Mn2VA li si ngood agreement with experiment. For Mn2VGe there is no experimental information available and our calculation is ap rediction.

First-principles study of exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge)

Abstract: We report the parameter-free, density functional theory calculations of interatomic exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge). To calculate the interatomic exchange interactions we employ the frozen-magnon approach. The Curie temperatures are calculated within the mean-field approximation to the classical Heisenberg Hamiltonian by solving a matrix equation for a multi-sublattice system. Our calculations show that, although a large magnetic moment is carried by Mn atoms, competing ferromagnetic (inter sublattice) and antiferromagnetic (intra sublattice) Mn-Mn interactions in Mn2VAl almost cancel each other in the mean-field experienced by the Mn atoms. In Mn2VGe the leading Mn-Mn exchange interaction is antiferromagnetic. In both compounds the ferromagnetism of the Mn subsystem is favored by strong antiferromagnetic Mn-V interactions. The obtained value of the Curie temperature of Mn2VAl is in good agrement with experiment. For Mn2VGe there is no experimental information available and our calculation is a prediction.

Optical properties of a semiconductor quantum dot with a single magnetic impurity: photoinduced spin orientation

Abstract: We describe the optical resonant manipulation of a single magnetic impurity in a self-assembled quantum dot. We show that using the resonant pumping one can address and manipulate selectively individual spin states of a magnetic impurity. The mechanisms of resonant optical polarization of a single impurity in a quantum dot involve anisotropic exchange interactions and are different than those in diluted semiconductors. A Mn impurity can be prepared in a given spin state and can act as qubit. The limiting factors for the spin manipulation are the electron-hole exchange interaction and finite temperature.

Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom.

Abstract: The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).

Spin rotation in alpha-Fe2O3 nanoparticles by interparticle interactions.

Abstract: Nanoparticles of alpha-Fe2O3 (hematite) typically have the sublattice magnetization directions in the hexagonal (001) plane below the Ne el temperature. By use of Mo ssbauer spectroscopy we have found that for agglomerated particles the sublattice magnetization may be rotated of the order of 15 degrees out of plane, depending on the particle size. The spin rotation can be explained by exchange interaction between neighboring particles with nonparallel (001) planes. The results imply that interparticle interactions can lead to spin directions deviating from the easy axis defined by the magnetic anisotropy.

Magnetic exchange interactions in quantum dots containing electrons and magnetic ions.

Abstract: We present a theory of magnetic exchange interactions in quantum dots containing electrons and magnetic ions. We find the interaction between the electron and Mn ion to depend strongly on the number of electrons. It can be switched off for closed shell configurations and maximized for partially filled shells. However, unlike the total electron spin S which is maximized for half-filled shells, we predict the exchange interaction to be independent of the filling of the electronic shell. We show how this unusual effect manifests itself in quantum dot addition and excitation spectrum.

Magnetic and structural properties of Co nanocluster thin films

Abstract: In this work we report on the magnetic characterization of thin films composed of gas-phase cobalt nanoclusters deposited on surfaces. Measurements of magnetization curves at ambient temperature indicate a strong exchange interaction between the clusters, while at cryogenic temperatures an exchange bias field appears. The latter confirms the existence of a ferromagnetic/antiferromagnetic core-shell system. Temperature-dependent magnetization measurements under zero-field-cooled conditions showed a rather broad maximum situated around 200 K. Magnetic force microscopy indicates the formation of a correlated super-spin-glass sCSSGd resulting from the frustration between the interparticle exchange interaction and the randomly oriented intraparticle anisotropy. The approach to saturation of the magnetization curves at 295 K is consistent with a CSSG.

Polarization of valence band holes in the (Ga,Mn)as diluted magnetic semiconductor.

Abstract: We report on direct measurements of the impurity band hole polarization in the diluted magnetic semiconductor (Ga,Mn)As. The polarization of impurity band holes in a magnetic field is strongly enhanced by antiferromagnetic exchange interaction with Mn ions. The temperature dependence of the hole polarization shows a strong increase of this polarization below the Curie temperature. We show that the ground state of the impurity band is formed by uniaxial stress split F=+/-1 states of antiferromagnetically coupled Mn ions (S=5/2) and valence band holes (J=3/2). The gap between the Mn acceptor related impurity band and the valence band is directly measured in a wide range of Mn content.

Dilute magnetic semiconductors

Abstract: We describe exchange interactions in dilute magnetic semiconductors (DMS) based on ab initio calculations. The electronic structure of DMS is calculated on the basis of the density functional theory using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). We show that there are two classes of DMS with very different properties. In systems with localized majority d-states deep in the valence band, the ferromagnetism is induced by Zener's p–d exchange interaction. This interaction is weak, but long ranged. For systems with impurity bands in the gap, the ferromagnetism is driven by Zener's double exchange mechanism. This interaction is very strong, but short ranged. Sophisticated Monte Carlo methods show that, for small concentrations, the percolation effect should be included to estimate the Curie temperatures of DMS. In particular, the ferromagnetism is strongly suppressed in double exchange systems due to the absence of percolation for low concentrations.

Localized electronic excitations in NiO studied with resonant inelastic X-Ray scattering at the Ni M threshold: evidence of spin flip.

Abstract: We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M-2,M-3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to dd excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125 +/- 15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.

The Casimir force at high temperature

Abstract: The standard expression of the high-temperature Casimir force between perfect conductors is obtained by imposing macroscopic boundary conditions on the electromagnetic field at metallic interfaces. This force is twice larger than that computed in microscopic classical models allowing for charge fluctuations inside the conductors. We present a direct computation of the force between two quantum plasma slabs in the framework of non-relativistic quantum electrodynamics including quantum and thermal fluctuations of both matter and field. In the semi-classical regime, the asymptotic force at large slab separation is identical to that found in the above purely classical models, which is therefore the right result. We conclude that when calculating the Casimir force at non-zero temperature, fluctuations inside the conductors cannot be ignored. Aspects of this subject are treated in a companion letter by Jancovici B. and Samaj L., Casimir force between two ideal-conductor walls revisited (Europhys. Lett., 72 (2005) 35).

Dynamics within the exciton fine structure of colloidal CdSe quantum dots.

Abstract: Evidence for an interaction between the quantum dot exciton fine structure states F = ±1 is obtained by measuring the dynamics of transitions among those states, exciton spin relaxation or flipping An ultrafast transient grating experiment based on a crossed-linear polarization grating is reported By using the quantum dot selection rules for absorption of circularly polarized light, it is demonstrated that it is possible to detect transitions between nominally degenerate fine structure states, even in a rotationally isotropic system The results for colloidal CdSe quantum dots reveal a strong size dependence for the exciton spin relaxation rate from one bright exciton state (F = ±1) to the other in CdSe colloidal quantum dots at 293 K, on a time scale ranging from femtoseconds to picoseconds, depending on the quantum dot size The results are consistent with an interaction between those states attributed to a long-range contribution to the electron−hole exchange interaction

Direct equivalence between quantum phase transition phenomena in radiation-matter and magnetic systems: Scaling of entanglement

Abstract: We show that the quantum phase transition arising in a standard radiation-matter model (Dicke model) belongs to the same universality class as the infinitely coordinated, transverse-field XY model The effective qubit-qubit exchange interaction is shown to be proportional to the square of the qubit-radiation coupling A universal finite-size scaling is derived for the corresponding two-qubit entanglement (concurrence) and a size-consistent effective Hamiltonian is proposed for the qubit subsystem

Exchange spring structures and coercivity reduction in FePt∕FeRh bilayers: A comparison of multiscale and micromagnetic calculations

Abstract: Calculations of magnetization reversal mechanism and coercivity reduction in exchange coupled FePt∕FeRh bilayers are presented It is shown by comparison with atomistic model calculations that the use of a standard micromagnetic model leads to an underestimation of the exchange energy at the interface, leading to a reduced coercivity decrease for small interfacial exchange energy constant This is due to the failure of the domain wall (DW) to penetrate the hard FePt phase in the micromagnetic calculations A multiscale model is proposed based an atomic level simulation in the interface region coupled with a micromagnetic approach elsewhere This leads to improved calculations of DW structures at the interface, allowing a detailed study of the magnetization reversal mechanism The new approach predicts a saturation in the coercivity reduction as a function of interface exchange energy at 4% of the bulk value, which is associated with complete continuity of the DW across the interface

Ab initio study of mirages and magnetic interactions in quantum corrals

Abstract: The state of the art ab initio calculations of quantum mirages, the spin polarization of surface-state electrons, and the exchange interaction between magnetic adatoms in Cu and Co corrals on Cu(111) are presented We find that the spin polarization of the surface-state electrons caused by magnetic adatoms can be projected to a remote location and can be strongly enhanced in corrals, compared to an open surface Our studies give clear evidence that quantum corrals could permit one to tailor the exchange interaction between magnetic adatoms at large separations and magnetic states of corrals, and could produce a mirage at a remote location We show that the spin polarization of surface-state electrons caused by magnetic adatoms placed in the corral focus is projected to an empty focus Our study presents clear evidence that the long-range exchange in- teraction between magnetic adatoms is strongly affected by confined electronic states of corrals The possibility of tailoring the exchange interaction by modifying the corral geometry is demonstrated The spin polarization of the electron gas in the empty focus of the Co corral used in the experimental setup of Manoharan et al (3) is revealed Adatoms and corrals destroy the two-dimensional (2D) periodicity of the ideal surface Heller et al (11) have shown in their studies of ''quantum stadium'' that the multiple-scattering approach is physically motivated to treat the electronic states of an arbitrary corral geometry and arbitrary placed adatoms in 2D systems Therefore, we believe that an ab initio method based on the multiple- scattering theory is well suited for calculations of magnetic adatoms in quantum corrals Our approach is based on the density functional theory (DFT) in the local spin density approximation and multiple-scattering approach using the Korringa-Kohn-Rostoker Green's function method for ada- toms and clusters on surfaces (12) Although the DFT does not account for properties of dynamical origin like the Kondo effect, it is an accurate method to determine static quantities (13) Therefore, our calculations are related to electronic and magnetic properties of quantum corrals above the Kondo temperature We treat an ideal surface as an infinite 2D perturbation of bulk Taking into account the 2D periodicity of the ideal surface, we calculate the structural Green's function by solving a Dyson equation self-consistently (12) This func- tion is then used as the reference Green's function in the Dyson equation for the self-consistent calculations of the Green's function of the corral (with or without adatoms) in a real space representation:

Spin-Polarized Current Induced Torque in Magnetic Tunnel Junctions

Abstract: We present tight-binding calculations of the spin torque in non-collinear magnetic tunnel junctions based on the non-equilibrium Green functions approach. We have calculated the spin torque via the effective local magnetic moment approach and the divergence of the spin current. We show that both methods are equivalent, i.e. the absorption of the spin current at the interface is equivalent to the exchange interaction between the electron spins and the local magnetization. The transverse components of the spin torque parallel and perpendicular to the interface oscillate with different phase and decay in the ferromagnetic layer (FM) as a function of the distance from the interface. The period of oscillations is inversely proportional to the difference between the Fermi-momentum of the majority and minority electrons. The phase difference between the two transverse components of the spin torque is due to the precession of the electron spins around the exchange field in the FM layer. In absence of applied bias and for a relatively thin barrier the perpendicular component of the spin torque to the interface is non-zero due to the exchange coupling between the FM layers across the barrier.

Observation of the dark exciton in CdSe quantum dots

Abstract: We use external magnetic fields to identify the band edge emitting statc in CdSe quantum dms. 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.

On modulational instability of nonlinear waves in 1D ferromagnetic spin chains

Abstract: We report a theoretical study of modulational instability of extended nonlinear spin waves in a one-dimensional ferromagnetic chain. The investigation is made both analytically within the framework of the linear stability analysis and also numerically by means of molecular dynamics simulations. Using a Holstein?Primakoff transformation for the spin operators, the Hamiltonian, which is constituted by a Heisenberg exchange term, a biquadratic exchange energy, an anisotropic energy and a Zeeman term, is bosonized. Then we derive a discrete nonlinear Schr?dinger-like equation for the spin-wave motion. Using a linear stability analysis, we establish the stability criteria of the spin waves in such a ferromagnetic chain. From our numerical simulations of the discrete spin chain for the onset of instability, it emerges that the analytical predictions are correctly verified. For a long timescale, depending on the strength of the biquadratic exchange interaction relative to the exchange energy and the anisotropy energy, on the one hand an intrinsic localized wave train can be created displaying properties of the breather motion. On the other hand, due to the increasing size of the instability domain, with increase of the biquadratic parameter, the instability can fully develop and the linear stability fails; consequently, the time evolution of the modulated spin waves can show both regular and chaotic behaviour.

Linear polarisation inversion: A signature of Coulomb scattering of cavity polaritons with opposite spins

Abstract: The polarisation of exciton-polariton emission is used to determine parameters of the interaction Hamiltonian of cavity polaritons. Linear polarisation of the polariton emission under linearly polarised excitation is theoretically analysed and shown to be determined by the ratio of the interaction strengths of polaritons with opposite and parallel spins. The experimentally observed inversion of the linear polarisation of the emission with respect to that of the exciting light indicates the dominant role of the exchange interaction in the polariton-polariton scattering. A comparison with the theory yields the sign of the term in the Hamiltonian describing the interaction of two polaritons with opposite spins, as well as its relative value with respect to the main term responsible for the scattering of polaritons with parallel spins. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton-spin relaxation in quantum dots due to spin-orbit interaction

Abstract: We present theoretical results for the spin relaxation of exciton-bound electrons and holes in weakly confining quantum dots. The relaxation is driven by the spin-orbit interaction in the conduction band and the linear in the momentum term in the valence band, respectively. The relaxation occurs between the optically active (bright) and inactive (dark) exciton states due to acoustic-phonon-assisted spin flips. The exchange splitting between the bright and dark states acts as a constant external magnetic field. A sequential flip of the (exciton-bound) electron and hole spins results in the spin-flip transition between the bright exciton states (i.e., an exciton-spin relaxation). We find that the spin relaxation time for an exciton-bound electron is several orders of magnitude faster than for a single electron. The resulting exciton spin-relaxation time is also several orders of magnitude faster than the one in small dots which is driven by the electron hole exchange interaction. We obtain the dependence of the exciton-spin relaxation time on dot size and temperature.

Exciton spin manipulation in inAs/GaAs quantum dots : Exchange interaction and magnetic field effects

Abstract: We report quantum beat phenomena between different polarization states in InAs quantum dots, observed in time-resolved photoluminescence and transient dichroism measurements. The polarization of the emitted light can be changed from linear to circular by applying a small magnetic field. Following quasiresonant excitation the electron and hole spin states remain stable during the exciton lifetime, independent of the applied magnetic field. We show that the exciton spin coherence is partially preserved in the energy relaxation process. These experiments enable us to measure the anisotropic exchange energy delta(Exy) and the exciton g factor. We find delta(Exy)30 µeV and g2.5 for the quantum dot excitons.

Exchange interaction in quantum rings and wires in the Wigner-crystal limit

Abstract: We present a controlled method for computing the exchange coupling in correlated one-dimensional electron systems based on the relation between the exchange constant and the pair-correlation function of spinless electrons. This relation is valid in several independent asymptotic regimes, including the low-electron-density case, under the general condition of a strong spin-charge separation. Explicit formulas for the exchange constant are obtained for thin quantum rings and wires with realistic Coulomb interactions by calculating the pair-correlation function via a many-body instanton approach. A remarkably smooth interpolation between high- and low-electron-density results is shown to be possible. These results are applicable to the case of one-dimensional wires of intermediate width as well. Our method can be easily generalized to other interaction laws, such as the inverse distance squared one of the Calogero-Sutherland-Moser model. We demonstrate excellent agreement with the known exact results for the latter model and show that they are relevant for a realistic experimental setup in which the bare Coulomb interaction is screened by an edge of a two-dimensional electron gas.