Showing papers on "Exchange interaction published in 2002"

Fine structure of neutral and charged excitons in self-assembled In(Ga)As/(Al)GaAs quantum dots

Abstract: The fine structure of excitons is studied by magnetophotoluminescence spectroscopy of single self-assembled In(Ga)As/(Al)GaAs quantum dots. Both strength and orientation of the magnetic field are varied. In a combination with a detailed theoretical analysis, these studies allow us to develop a comprehensive picture of the exciton fine structure. Symmetry of the dot structures as well as its breaking cause characteristic features in the optical spectra, which are determined by the electron-hole exchange and the Zeeman interaction of the carriers. The symmetry breaking is either inherent to the dot due to geometry asymmetries, or it can be obtained by applying a magnetic field with an orientation different from the dot symmetry axis. From data on spin splitting and on polarization of the emission we can identify neutral as well as charged exciton complexes. For dots with weakly broken symmetry, the angular momentum of the neutral exciton is no longer a good quantum number and the exchange interaction lifts degeneracies within the fine-structure manifold. The symmetry can be restored by a magnetic field due to the comparatively strong Zeeman interactions of electron and hole. For dots with a strongly broken symmetry, bright and dark excitons undergo a strong hybridization, as evidenced by pronounced anticrossings when states within the manifold are brought into resonance. The fine structure can no longer be described within the frame developed for structures of higher dimensionality. In particular, the hybridization cannot be broken magnetically. For charged excitons, the exchange interaction vanishes, demonstrating that the exchange splitting of a neutral exciton can be switched off by injecting an additional carrier.

Mechanisms of Spin-Polarized Current-Driven Magnetization Switching

Abstract: The mechanisms of the magnetization switching of magnetic multilayers driven by a current are studied by including exchange interaction between local moments and spin accumulation of conduction electrons. It is found that this exchange interaction leads to two additional terms in the Landau-Lifshitz-Gilbert equation: an effective field and a spin torque. Both terms are proportional to the transverse spin accumulation and have comparable magnitudes.

Bound magnetic polaron interactions in insulating doped diluted magnetic semiconductors

Abstract: The magnetic behavior of insulating doped diluted magnetic semiconductors (DMS's) is characterized by the interaction of large collective spins known as bound magnetic polarons. Experimental measurements of the susceptibility of these materials have suggested that the polaron-polaron interaction is ferromagnetic, in contrast to the antiferromagnetic carrier-carrier interactions that are characteristic of nonmagnetic semiconductors. To explain this behavior, a model has been developed in which polarons interact via both the standard direct carrier-carrier exchange interaction (due to virtual carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due to the interactions of polarons with magnetic ions in an interstitial region). Using a variational procedure, the optimal values of the model parameters were determined as a function of temperature. At temperatures of interest, the parameters describing polaron-polaron interactions were found to be nearly temperature-independent. For reasonable values of these constant parameters, we find that indirect ferromagnetic interactions can dominate the direct antiferromagnetic interactions and cause the polarons to align. This result supports the experimental evidence for ferromagnetism in insulating doped DMS's.

Analysis of the magnetic coupling in binuclear complexes. II. Derivation of valence effective Hamiltonians from ab initio CI and DFT calculations

Abstract: Most interpretations of the magnetic coupling J between two unpaired electrons rest upon simple valence models that involve essentially the ferromagnetic direct exchange contribution, Kab, and the antiferromagnetic effect of the delocalization resulting from the interaction between neutral and ionic determinants, tab, whose energy difference is U. Ab initio valence-only calculations give very poor estimates of J, whatever the definition of the magnetic orbitals, and large CI expansions are required to evaluate it properly. It is, however, possible to define valence effective Hamiltonians from the knowledge of the eigenenergies and the eigenvectors of these accurate CI calculations. When applied to four different complexes, this strategy shows that spin polarization may change the sign of the direct exchange interaction, Kab, and that dynamical correlation results in a dramatic reduction of the effective repulsion U. The present article also shows how Kab, tab, and U effective parameters can be extracted from density functional theory (DFT) calculations and that the typical overestimation of J in DFT can be attributed to an excessive lowering of the effective on-site repulsion.

Analytical determination of the [Ln-aminoxyl radical] exchange interaction taking into account both the ligand-field effect and the spin-orbit coupling of the lanthanide ion (Ln = DyIII and HoIII).

Abstract: Numerous compounds in which a paramagnetic LnIII ion is in an exchange interaction with a second spin carrier, such as a transition metal ion or an organic radical, have been described. However, except for GdIII, very little has been reported about the magnitude of the interactions. Indeed, for these ions both the ligand-field effects and the exchange interactions between the magnetic centers become relevant in the same temperature range; this makes the analysis of the magnetic behavior of such compounds more difficult. In this study, quantitative analyses of the thermal variations of the static isothermal initial magnetic susceptibility measured on powdered samples of the {Ln(NO3)3[organic radical]2} (Ln=DyIII and HoIII) compounds were performed. The ligand-field effects on the Ln ions were taken into account, and the exchange interactions within a molecule were treated exactly within an appropriate Racah formalism. Values of the intramolecular {Ln–aminoxyl radical} exchange parameter have thus been rigorously deduced for both the Dy Kramers and Ho non-Kramers ion-based compounds. Ferromagnetic {Ln–radical} interactions are found for both the Dy and Ho derivatives with J=8 cm−1 and J=4.5 cm−1, respectively.

Effect of optical spin injection on ferromagnetically coupled Mn spins in the III-V magnetic alloy semiconductor (Ga,Mn)As.

Abstract: We report on the new type of photoinduced magnetization in ferromagnetic (Ga, Mn)As thin films. Optically generated spin-polarized holes change the orientation of ferromagnetically coupled Mn spins and cause a large change in magnetization, being 15% of the saturation magnetization, without the application of a magnetic field. The memorization effect has also been found as a trace after the photoinduced magnetization. The observed results suggest that a small amount of nonequilibrium carrier spins can cause collective rotation of Mn spins presumably through the p-d exchange interaction.

Fine structure of charged and neutral excitons in InAs- Al 0.6 Ga 0.4 As quantum dots

Abstract: Photoluminescence spectra from single self-assembled InAs quantum dots embedded in an ${\mathrm{Al}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ matrix are reported. The spectra consist of a higher-energy linearly polarized doublet, with large splitting of the order of 1 meV, and a lower-energy unpolarized line. The polarized lines are explained in terms of exciton recombination at asymmetric dots, with the splitting due to the anisotropic exchange interaction. The lower-energy unsplit, unpolarized line is ascribed to recombination at the same dots but in the presence of an excess charge, which results in zero net electron spin and hence quenching of the exchange interaction. The conclusions are fully supported by magneto-optical investigations.

Magnetization-step studies of antiferromagnetic clusters and single ions: Exchange, anisotropy, and statistics

Abstract: A magnetic cluster is a group of magnetic ions (“spins”) that interact with each other but which, to a good approximation, do not interact with other magnetic ions. Such clusters are responsible for many of the interesting and useful properties of a large number of molecular crystals, and of dilute magnetic materials below the percolation concentration. In a molecular crystal the magnetic clusters are usually all of one type. In a dilute magnetic material, on the other hand, many cluster types are present. The magnetization-step (MST) method is a relatively new form of spectroscopy for measuring intracluster magnetic interactions, mainly exchange constants and anisotropy parameters. In dilute magnetic materials this method also yields the relative populations of different cluster types. This review focuses on the principles and applications of the MST method to relatively small clusters, no more than a dozen spins or so. It covers only MSTs from spin clusters in which the dominant exchange interaction is antiferromagnetic (AF), and MSTs from isolated magnetic ions. Such MSTs are the result of changes of the magnetic ground state, caused by energy-level crossings in a magnetic field H. At a sufficiently low temperature, each change of the ground state leads to a MST. Magnetic clusters may be classified by size. The smallest is a “single,” consisting of one isolated magnetic ion. Next are “pairs” (dimers), followed by “triplets” (trimers), “quartets” (tetramers), etc. Although the classification by size is useful, clusters of the same size may have different intracluster interactions, and also different geometrical shapes. More detailed classifications of magnetic clusters are therefore also needed. A cluster “type” specifies both the size of the cluster and the set of all intracluster magnetic interactions which are nonzero. Different geometries of clusters of the same type correspond to different “configurations.” MSTs from isolated spins (singles) are discussed first. When subjected to certain types of single-ion anisotropy, e.g., uniaxial hard-axis anisotropy, singles give rise to MSTs. Examples of anisotropy parameters which were determined from such MSTs are presented. An interesting application of MSTs from singles is the determination of the populations of Jahn–Teller distortions which are energetically equivalent at H=0 but are inequivalent at finite H. For clusters larger than singles, the strongest intracluster interaction is usually the isotropic exchange. Using a model with one isotropic exchange constant J, predictions for MSTs from pairs, open and closed triplets, and the six possible types of quartets, are presented. Observations of some of these MSTs, and the exchange constants derived from them, are discussed. Recent studies of MSTs from AF rings in molecular crystals are summarized. The remainder of the review is devoted to a detailed discussion of MSTs in dilute magnetic materials, exemplified by the dilute magnetic semiconductors (DMSs). The theory for MSTs in these materials is based on various cluster models (each specifying the exchange constants that are included), and on the assumption of a random distribution of the magnetic ions. The latter assumption is needed for calculations of the populations of various cluster types. The simplest cluster model includes only the largest isotropic exchange constant, usually J1 between nearest neighbors (NNs). This J1 model accounted for much of the early MST data in Mn-based II–VI DMSs. These early data yielded values of J1, showed that the distribution of the Mn ions was random, and explained the difference between the apparent and true saturation values of the magnetization. Following these early successes the “pure” J1 model was improved in several ways: (1) Some effects of the weaker exchange interactions with distant neighbors (DNs) were treated approximately. (2) Weak anisotropies, and the Dzyaloshinski–Moriya interaction, were added to the model. (3) A spread in the values of J1, due to alloy disorder and/or a lower crystal symmetry, was included. (4) The possibility a nonrandom magnetic-ion distribution was considered, and methods of observing nonrandomness experimentally, and quantifying the degree of nonrandomness, were devised. (5) Cluster probabilities in molecular beam epitaxy (MBE)- grown quantum structures, particularly near interfaces, were considered. Experimental data relating to each of these improvements of the J1 model are presented. Very recent works focused on a direct determination, using MSTs, of the relatively small DN exchange constants. Most of these experiments on DNs required a magnetometer operating in a dilution refrigerator, near 20 mK. The data interpretations were based on cluster models with up to five exchange constants. These models involve hundreds of cluster types, even when clusters with more than four spins are excluded. Clusters with more than four spins were treated approximately. Elaborate computer programs for computing all cluster probabilities and energy levels were required. The results for the DN exchange constants Ji in Mn-based II–VI DMSs disagree with all previous theoretical predictions. Specifically, the next-nearest-neighbor exchange constant J2 is not the second-largest exchange constant. The distance dependence of the Ji is material dependent, unlike the universal behavior predicted by all theories which considered this issue. The experimental results are partially explained by the Yu–Lee and Wei–Zunger theories, which include the directional dependence of the exchange interaction in addition to the distance dependence. The directional dependence leads to a reduction of J2. Electronically accessible tables for cluster types and their probabilities are included as EPAPS. These tables are for all clusters with up to four spins, in both the fcc cation lattice and in the (ideal) hcp cation structure. For fcc the tables include 16 different cluster models with exchange interactions up to the fifth neighbor. For hcp, 64 cluster models with up to eight exchange constants (corresponding to interactions up to the fourth neighbor in fcc) are included. Tables for quintets in the special case of the NN model in fcc and hcp are also included.

Anisotropic exchange in LiCuVO 4 probed by ESR

Abstract: We investigated the paramagnetic resonance in single crystals of ${\mathrm{LiCuVO}}_{4}$ with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing ${\mathrm{CuO}}_{6}$ octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, agree nicely with the parameters obtained from the angular dependence of the linewidth. The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish ${\mathrm{LiCuVO}}_{4}$ as one-dimensional compound at high temperatures.

Quantum Dots with Rashba Spin-Orbit Coupling

Abstract: We present results on the effects of spin-orbit coupling on the electronic structure of few-electron interacting quantum dots. The ground-state properties as a function of the number of electrons in the dot N are calculated by means of spin-density functional theory. We find a suppression of Hund's rule due to the competition of the Rashba effect and exchange interaction. Introducing an in-plane Zeeman field leads to a paramagnetic behavior of the dot in a closed-shell configuration and to spin texture in space.

Exchange interactions and Curie temperature in (Ga,Mn)As

Abstract: We use supercell and frozen-magnon approaches to study the dependence of the magnetic interactions in (Ga,Mn)As on the Mn concentration. We report the parameters of the exchange interaction between Mn spins and the estimates of the Curie temperature within the mean-field and random-phase approximations. In agreement with experiment we obtain a nonmonotonous dependence of the Curie temperature on the Mn concentration. We estimate the dependence of the Curie temperature on the concentration of the carries in the system and show that the decrease of the number of holes in the valence band leads to a fast decrease of the Curie temperature. We show that the hole states of the valence band are more efficient in mediating the exchange interaction between Mn spins than the electron states of the conduction band.

Magnetochemical origin for Invar anomalies in iron-nickel alloys

Abstract: Zero- and finite-temperature (T) first-principles calculations versus composition (c) show that magnetochemical effects lead to Invar anomalies in Fe-(Ni, Co, Pt) alloys. Chemical short- or long-range order and negative interatomic exchange interaction of electrons in antibonding majority-spin states force the face-centered-cubic lattice to compete simultaneously for a smaller volume (from antiferromagnetic tendencies) and a larger volume (from Stoner ferromagnetic tendencies). The resulting additional negative lattice anharmonicity is very large for Fe-(Ni, Co) while absent for Fe-Pt. Our results explain the T- and c-dependent behavior of Invar properties, including the lattice softening and thermal expansion of Fe-Ni. In addition, the occurrence of a noncollinear spin structure at $T=0 \mathrm{K}$ near Invar can be understood on the basis of our results.

T-matrix analysis of biexcitonic correlations in the nonlinear optical response of semiconductor quantum wells

Abstract: A detailed numerical analysis of exciton-exciton interactions in semiconductor quantum wells is presented. The theory is based on the dynamics-controlled truncation formalism and evaluated for the case of resonant excitation of 1s-heavy-hole excitons. It is formulated in terms of standard concepts of scattering theory, such as the forward-scattering amplitude (or T-matrix). The numerical diagonalization of the exciton-exciton interaction matrix in the 1s-approximation yields the excitonic T-matrix. We discuss the role of the direct and exchange interaction in the effective two-exciton Hamiltonian, which determines the T-matrix, evaluated within the 1s-subspace, and also analyze the effects of the excitonic wave function overlap matrix. Inclusion of the latter is shown to effectively prevent the 1s-approximation from making the Hamiltonian non-hermitian, but a critical discussion shows that other artefacts may be avoided by not including the overlap matrix. We also present a detailed analysis of the correspondence between the excitonic T-matrix in the 1s-approximation and the well-known T-matrix governing two-particle interactions in two dimensional systems via short-range potentials.

Fine structure of the trion triplet state in a single self-assembled semiconductor quantum dot

Abstract: The emission from the charged biexciton is used to monitor the energy structure of the trion triplet state in a negatively charged CdSe/ZnSe quantum dot. The isotropic part of the electron–hole exchange interaction regroups the otherwise sixfold degenerated state in three Kramers doublets. The energy separation between the radiative pairs with total spin projection Fz=±3/2 and ±1/2 is 1.6 meV. The anisotropic part mixes states with ΔFz=±2 resulting in a partly linearly polarized transition dipole. Application of magnetic field lifts the degeneracy of the Kramers doublets. The g factors also manifest the anisotropic state mixing.

Spin dynamics of a two-dimensional metal in a superconducting state: Application to the high- T c cuprates

Abstract: We show that a two-dimensional (2D) metal in a superconducting (SC) state with wave-vector-dependent gap behaves as a quantum-spin liquid characterized by a collective mode of resonance type. Formally the mode exists whatever is a form of Fermi surface (FS). However, when the latter is spherical the effect is rather formal: any factor leading to an even weak electron scattering, like finite T, impurity, etc., easily kills the mode. On the contrary, for the type of FS observed in the high-${T}_{c}$ cuprates (characterized by a proximity to the saddle-point wave vectors) the resonance mode exists in a true sense being well separated from the electron-hole continuum and well defined. The effect is due to the effective 1D dimension in a proximity of electronic topological transition quantum critical point. The physical nature of the collective mode varies progressively between two possibilities. In the case of a weak interaction, the resonance mode is a typical effect of a two-particle bound state that appears slightly below the bottom of the two-particle continuum. The effect exists even in the case of infinitesimally weak interaction (of relevant sign). In the case of a strong or intermediate interaction, it is a critical mode existing as a precursor of spin-density wave (SDW) instability. The gap of the collective mode is not related directly to the value of the SC gap (being, however, limited by the latter from above) but rather to a proximity to SDW instability. We predict also a second collective mode lying inside the two-particle continuum. The existence of these two modes and the particular form of their dispersion in the case corresponding to the underdoped cuprates ${(d}_{{x}^{2}\ensuremath{\sim}{y}^{2}}$ symmetry of SC gap, exchange interaction of antiferromagnetic sign, etc.) allows one to understand the main features observed experimentally in the underdoped cuprates, namely the so-called ``41 meV resonance peak'' and ``24 meV incommensurability'' and their two-dimensional wave vector dependences, the doping evolution of the spin dynamics and many other features. The theory also predicts new features to be tested experimentally.

Spin excitations in La 2 CuO 4 : Consistent description by inclusion of ring exchange

Abstract: We consider the square lattice Heisenberg antiferromagnet with plaquette ring exchange and a finite interlayer coupling leading to a consistent description of the spin-wave excitation spectrum in La 2 CuO 4 . The values of the in-plane exchange parameters, including ring exchange J@, are obtained consistently by an accurate fit to the experimentally observed in-plane spin-wave dispersion, while the out-of-plane exchange interaction is found from the temperature dependence of the sublattice magnetization at low temperatures. The fitted exchange interactions J= 151.9 meV and J@=0.24J give values for the spin stiffness and the Neel temperature in excellent agreement with the experimental data.

Magnetic properties of a homologous series of vanadium jarosite compounds.

Abstract: Redox-based, hydrothermal synthetic methodologies have enabled the preparation of a new series of stoichiometrically pure jarosites of the formula, AV3(OH)6(SO4)2 with A = Na+, K+, Rb+, Tl+, and NH4+. These jarosites represent the first instance of strong ferromagnetism within a Kagome layered framework. The exchange interaction, which is invariant to the nature of the A+ ion (ϑCW ≈ +53(1) K), propagates along the d2 magnetic sites of the triangular Kagome lattice through bridging hydroxyl groups. An analysis of the frontier orbitals suggests this superexchange pathway to possess significant π-orbital character. Measurements on a diamagnetic host jarosite doped with magnetically dilute spin carriers, KGa2.96V0.04(OH)6(SO4)2, reveal significant single-ion anisotropy for V3+ ion residing in the tetragonal crystal field. This anisotropy confines the exchange-coupled moments to lie within the Kagome layer. Coupling strengths are sufficiently strong to prevent saturation of the magnetization when an external f...

Peierls-like transition induced by frustration in a two-dimensional antiferromagnet.

Abstract: We show that the introduction of frustration into the spin- $1/2$ two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via a next-nearest-neighbor exchange interaction can lead to a Peierls-like transition, from a tetragonal to an orthorhombic phase, when the spins are coupled to adiabatic phonons. The two different orthorhombic ground states define an Ising order parameter, which is expected to lead to a finite temperature transition. Implications for ${\mathrm{Li}}_{2}{\mathrm{VOSiO}}_{4}$, the first realization of that model, will be discussed.

A Hamiltonian Model for Linear Friction in a Homogeneous Medium

Abstract: We introduce and study rigorously a Hamiltonian model of a classical particle moving through a homogeneous dissipative medium at zero temperature in such a way that it experiences an effective linear friction force proportional to its velocity (at small speeds). The medium consists at each point in a space of a vibration field modelling an obstacle with which the particle exchanges energy and momentum in such a way that total energy and momentum are conserved. We show that in the presence of a constant (not too large) external force, the particle reaches an asymptotic velocity proportional to this force. In a potential well, on the other hand, the particle comes exponentially fast to rest in the bottom of the well. The exponential rate is in both cases an explicit function of the model parameters and independent of the potential.

Impurity-semiconductor band hybridization effects on the critical temperature of diluted magnetic semiconductors

Abstract: We have studied the critical temperature of diluted magnetic semiconductors by means of Monte Carlo simulations and coherent-potential-approximation (CPA) calculations. In our model for this system, the magnetic ions couple with the carriers through an antiferromagnetic exchange interaction J and an electrostatic interaction W. The effective impurity potential J-W controls the hybridization between the magnetic impurities and the hole charge on the dopants. We find that the critical temperature depends substantially on the hole charge on the magnetic impurities. The CPA critical temperature is always lower than that obtained in the Monte Carlo simulations, although all trends in the simulation results are reproduced in the CPA calculations. Finally we predict the existence of pockets of phase segregation instability close to the carrier's band edges.

Exchange bias with perpendicular anisotropy in (Pt/Co)/sub n//FeMn multilayers

Abstract: Sputtered (Pt-Co)/sub n/-FeMn multilayers exhibit both perpendicular magnetic anisotropy and exchange bias at room temperature. Their magnetic properties after in-plane and perpendicular-to-plane magnetic annealing have been studied by means of extraordinary Hall effect and magnetic force microscopy or polar Kerr microscopy. A crossover from in-plane to perpendicular anisotropy was observed as a function of number of repeats n. The dynamics of magnetization switching has been investigated on both sides of the hysteresis loops. A clear asymmetry exists in the dynamics depending whether the field is parallel or antiparallel to the exchange bias field. Similarly, strong differences in the domain patterns at the coercive field were observed. Exchange-biased (Pt-Co)-Pt-(Pt-Co)-FeMn spin-valves were also investigated. Well-separated hysteresis loops were associated with the "soft" and pinned components of the stack. These structures were used to study the interlayer coupling through the Pt spacer.

From the square lattice to the checkerboard lattice: Spin-wave and large-n limit analysis

Abstract: Within a spin wave analysis and a fermionic large-n limit, it is shown that the antiferromagnetic Heisenberg model on the checkerboard lattice may have different ground states, depending on the spin size S. Through an additional exchange interaction that corresponds to an intertetrahedra coupling, the stability of the Neel state has been explored for all cases from the square lattice to the isotropic checkerboard lattice. Away from the isotropic limit and within the linear spin-wave approximation, it is shown that there exists a critical coupling for which the local magnetization of the Neel state vanishes for any value of the spin S. One the other hand, using the Dyson-Maleev approximation, this result is valid only in the case S=½ and the limit between a stable and an unstable Neel state is at S=1. For S=½, the fermionic large-n limit suggests that the ground state is a valence-bond solid build with disconnected four-spin singlets. This analysis indicates that for low spin and in the isotropic limit, the checkerboard antiferromagnet may be close to an instability between an ordered S=0 ground state and a magnetized ground state.

Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density

Abstract: We studied ferromagnetism in the one-dimensional Hubbard model with doubly degenerate atomic orbitals by means of the density-matrix renormalization-group method and obtained the ground-state phase diagrams. It was found that ferromagnetism is stable from low to high $(0lnl1.75)$ electron density when the interactions are sufficiently strong. The quasi-long-range order of triplet superconductivity coexists with the ferromagnetic order for a strong Hund coupling region, where the interorbital interaction ${U}^{\ensuremath{'}}\ensuremath{-}J$ is attractive. At quarter-filling $(n=1),$ the insulating ferromagnetic state appears, accompanying orbital quasi-long-range order. For low densities $(nl1),$ ferromagnetism occurs owing to the ferromagnetic exchange interaction caused by virtual hoppings of electrons, the same as in the quarter-filled system. This comes from separation of the charge and spin-orbital degrees of freedom in the strong-coupling limit. This ferromagnetism is fragile against variation of band structure. For high densities $(ng1),$ the phase diagram of the ferromagnetic phase is similar to that obtained in infinite dimensions. In this case, the double exchange mechanism is operative for stabilizing the ferromagnetic order and this long-range order is robust against variation of the band dispersion. A partially polarized state appears in the density region $1.68\ensuremath{\lesssim}n\ensuremath{\lesssim}1.75$ and phase separation occurs for n just below the half-filling $(n=2).$

Interlayer exchange interactions, SU(4) soft waves, and skyrmions in bilayer quantum Hall ferromagnets

Abstract: The exchange Coulomb interaction is the driving force for quantum coherence in quantum Hall systems. We construct a microscopic Landau-site Hamiltonian for the exchange interaction in bilayer quantum Hall ferromagnets, which is characterized by the SU(4) isospin structure. By taking a continuous limit, the Hamiltonian gives rise to the SU(4) nonlinear sigma model in the von Neumann-lattice formulation. The ground-state energy is evaluated at filling factors v= 1,2,3,4, It is shown at v= 1 that there are three independent soft waves, where only one soft wave is responsible for the coherent tunneling of electrons between the two layers. It is also shown at v= I that there are three independent skyrmion states apart from the translational degree of freedom. They are CP 3 skyrmions enjoying the spin-charge entanglement confined within the lowest Landau level.

All Optical Implementation of Multi-Spin Entanglement in a Semiconductor Quantum Well

Abstract: We use ultrafast optical pulses and coherent techniques to create spin entangled states of non-interacting electrons bound to donors (at least three) and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the exchange interaction between localized excitons and paramagnetic impurities, can in principle be applied to entangle a large number of spins.

Critical properties of thin quantum and classical Heisenberg films

Abstract: The dependence of the critical temperature T c (l,Δ) on film thickness I and ratio of surface (J,) to bulk (J b ) exchange interaction strengths Δ≡J s /J b -I in quantum and classical Heisenberg models are studied by using the effective-field theory within the framework of the differential operator technique. It is found that for A Δ c , T c is larger than both the bulk T b c and the surface T s c critical temperatures of the corresponding semi-infinite systems and, as the film thickness I further increases, T c decreases and approaches, for large value of l, the surface magnetic transition T s c .

Interaction of electrons with spin waves in the bulk and in multilayers (invited)

Abstract: The exchange interaction between electrons and magnetic spins is considerably enhanced near interfaces, in magnetic multilayers. As a result, a dc current can be used to generate spin oscillations. We review theory and experimental evidence. The s–d exchange interaction causes a rapid precession of itinerant conduction-electron spins s around the localized spins S of magnetic electrons. This s precession has been observed directly [Weber et al., Science 291, 1015 (2001)] with electron beams through Fe, Co, and Ni films. Because of it, the time-averaged interaction torque between s and S vanishes. Thus, electrons do not interact at all with long-wavelength spin waves, in the bulk. An interface between a magnetic layer and a spacer causes a local coherence between the precession phases of different electrons, in a region within 10 nm from the interface [J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996)]; [L. Berger, Phys. Rev. B 54, 9353 (1996)]. Also, a second magnetic layer with pinned S is used to ...

Interaction Of Electrons With Spin Waves In The Bulk And In Multilayers

Abstract: The exchange interaction between electrons and magnetic spins is considerably enhanced near interfaces, in magnetic multilayers. As a result, a dc current can be used to generate spin oscillations. We review theory and experimental evidence. The s-d exchange interaction causes a rapid precession of itinerant conduction-electron spins s around the localized spins S of magnetic electrons. Because of the precession, the time-averaged interaction torque between s and S vanishes. An interface between a magnetic layer and a spacer causes a local coherence between the precession phases of differnt electrons, within 10 nm from the interface, and restores the torque. Also, a second magnetic layer with pinned S is used to prepare s in a specific direction. the current-induced drive torque of s on S in the active layer may be calculated from the spin current (Slonczewski) or from the spin imbalance Delta-mu (Berger). Spin current and Delta-mu are proportional to each other, and can arise from Fermi-surface translation, as well as from expansion/contraction.