Showing papers on "Spin states published in 1999"

Electrical spin injection in a ferromagnetic semiconductor heterostructure

Abstract: Conventional electronics is based on the manipulation of electronic charge. An intriguing alternative is the field of ‘spintronics’, wherein the classical manipulation of electronic spin in semiconductor devices gives rise to the possibility of reading and writing non-volatile information through magnetism1,2. Moreover, the ability to preserve coherent spin states in conventional semiconductors3 and quantum dots4 may eventually enable quantum computing in the solid state5,6. Recent studies have shown that optically excited electron spins can retain their coherence over distances exceeding 100 micrometres (ref. 7). But to inject spin-polarized carriers electrically remains a formidable challenge8,9. Here we report the fabrication of all-semiconductor, light-emitting spintronic devices using III–V heterostructures based on gallium arsenide. Electrical spin injection into a non-magnetic semiconductor is achieved (in zero magnetic field) using a p-type ferromagnetic semiconductor10 as the spin polarizer. Spin polarization of the injected holes is determined directly from the polarization of the emitted electroluminescence following the recombination of the holes with the injected (unpolarized) electrons.

Atomic-scale magnetic modeling of oxide nanoparticles

Abstract: We present a method for atomic-scale modeling of the magnetic behavior of ionic magnetic solids. Spin distributions and net magnetic moments are calculated for nanoparticles of ferrimagnetic ${\mathrm{NiFe}}_{2}{\mathrm{O}}_{4}$ and $\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3},$ and antiferromagnetic NiO as a function of applied field. Calculations incorporate crystal structures and exchange parameters determined from bulk data, bulk anisotropy for core spins, reasonable estimates for the anisotropy of surface spins, and finite temperatures simulated by random perturbations of spins. Surface spin disorder was found in the case of ferrimagnetic spinel nanoparticles, due to broken exchange bonds at the surface. The calculations also demonstrate that surface anisotropy enhances the coercivity of such particles only when surface spin disorder is present. Simulated thermal perturbations were used to characterize the distribution of energy barriers between surface spin states of such particles. The distribution of barriers can explain the macroscopic quantum tunneling like magnetic relaxation at low temperatures found experimentally. Calculations on NiO nanoparticles predict eight, six, or four-sublattice spin configurations in contrast to the two-sublattice configuration accepted for bulk NiO. Relatively weak coupling between the multiple sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts, in qualitative agreement with experiment.

High-Nuclearity Magnetic Clusters: Generalized Spin Hamiltonian and Its Use for the Calculation of the Energy Levels, Bulk Magnetic Properties, and Inelastic Neutron Scattering Spectra.

Abstract: A general solution of the exchange problem in the high-nuclearity spin clusters (HNSC) containing arbitrary number of exchange-coupled centers and topology is developed. All constituent magnetic centers are supposed to possess well-isolated orbitally non-degenerate ground states so that the isotropic Heisenberg-Dirac-Van Vleck (HDVV) term is the leading part of the exchange spin Hamiltonian. Along with the HDVV term, we consider higher-order isotropic exchange terms (biquadratic exchange), as well as the anisotropic terms (anisotropic and antisymmetric exchange interactions and local single-ion anisotropies). All these terms are expressed as irreducible tensor operators (ITO). This allows us to take full advantage of the spin symmetry of the system. At the same time, we have also benefitted by taking into account the point group symmetry of the cluster, which allows us to work with symmetrized spin functions. This results in an additional reduction of the matrices to diagonalize. The approach developed here is accompanied by an efficient computational procedure that allows us to calculate the bulk magnetic properties (magnetic susceptibility, magnetization, and magnetic specific heat) as well as the spectroscopic properties of HNSC. Special attention is paid to calculate the magnetic excitations observed by inelastic neutron scattering (INS), their intensities, and their Q and temperature dependencies. This spectroscopic technique provides direct access to the energies and wave functions of the different spin states of the cluster; thus, it can be applied to spin clusters in order to obtain deep and detailed information on the nature of the magnetic exchange phenomenon. The general expression for the INS cross-section of spin clusters interacting by all kinds of exchange interactions, including also the single-ion zero-field splitting term, is derived for the first time. A closed-form expression is also derived for the particular case in which only the isotropic exchange interactions are involved. Finally this approach has been used to model the magnetic properties as well as the INS spectra of the polyoxometalate anion [Ni(9)(OH)(3)(H(2)O)(6)(HPO(4))(2)(PW(9)O(34))(3)](16)(-), which contains a central magnetic cluster formed by nine exchange-coupled Ni(II) ions surrounded by diamagnetic phosphotungstate ligands (PW(9)O(34))(9)(-).

Single-Molecule Magnet Behavior of a Tetranuclear Iron(III) Complex. The Origin of Slow Magnetic Relaxation in Iron(III) Clusters

Abstract: The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = −0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To investigate the origin of the magnetic anisotropy we have evaluated the projection of the single-ion and dipolar contributions to the zfs of the ground state. The zfs tensors of the three structurally independent iron(III) centers have been calculated from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded...

Introduction to unconventional superconductivity

Abstract: Part I: Cooper Pairing 1. Spin States of Pairs 2. Superfluid Helium-3 Phases 3. Superconducting States in Crystals 4. Energy Gap and Critical Temperature 5. Low Temperature Behaviour of Thermodynamical Values 6. Manifestations of Parity of ElectronsNumber 7. Spin Susceptibility and Knight Shift 8. Landau Expansion of Free Energy 9. Multicompoent Superconducting States 9. Ginzburg-Landau Equations and the Problem of Upper Critical Field 10. Boundary Conditions and Surface Superconductivity 11.Meissner and Mixed States in Nonconventional Superconductors 12. Magnetic Superconductors 13. Josephson EffectPart II: Gor'kov Equations 14. Ginzburg-Landau Functional 15. Upper Critical Field in p-wave Superconductors 16. Boundary Condtions for the Order Parameter 17. Influence of Impurities 18. Thermal Conductivity of Nonconventional Superconductors

Iron Chemistry of a Pentadentate Ligand That Generates a Metastable Fe(III)-OOH Intermediate.

Abstract: In an effort to gain more insight into the factors controlling the formation of low-spin non-heme FeIII−peroxo intermediates in oxidation catalysis, such as activated bleomycin, we have synthesized a series of iron complexes based on the pentadentate ligand N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). The following complexes have been prepared: [(N4Py)FeII(CH3CN)](ClO4)2 (1), [(N4Py)FeIICl](ClO4) (2), [(N4Py)FeIIIOMe](ClO4)2 (3), and [(N4Py)2Fe2O](ClO4)4 (4). Complexes 1 and 2 have low- and high-spin FeII centers, respectively, whereas 3 is an FeIII complex that undergoes a temperature-dependent spin transition. The iron centers in the oxo-bridged dimer 4 are antiferromagnetically coupled (J = −104 cm-1). Comparison of the crystal structures of 1, 3, and 4 shows that the ligand is well suited to accommodate both FeII and FeIII in either spin state. For the high-spin FeIII complexes 3 and 4 the iron atoms are positioned somewhat outside of the cavity formed by the ligand, while in ...

Observation of the Distribution of Molecular Spin States by Resonant Quantum Tunneling of the Magnetization

Abstract: Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(\xi_H) of magnetic energy bias \xi_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(\xi_H) (around the resonant condition \xi_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.

Quantum nondemolition measurement of spin via the paramagnetic Faraday rotation

Abstract: We propose a method for quantum nondemolition (QND) measurement of one component of spin by the measurement of polarization rotation of the off-resonant light induced by a paramagnetic Faraday rotation effect We show that nonclassical states of spins such as the squeezed spin state, the Dicke state, and macroscopic quantum superposed state of spin can be realized corresponding to particular results of QND measurements Furthermore, it is shown that a nonclassical state of light such as the twin photon state can also be realized with this method

Structural approach of the features of the spin crossover transition in iron (II) compounds

Abstract: We have determined the crystal structures, both in high and low spin state, of four Fe(PM-L) 2 (NCS) 2 complexes, where PM is N-2′-pyridylmethylene and the aromatic subunit L is 4-aminoterphenyl (TeA), 4-(phenylazo)aniline (AzA), 4-aminobiphenyl (BiA) or 4-(phenylethynyl)aniline (PEA). As previously reported, these compounds undergo a spin crossover at low temperature with different features of transition: very smooth and incomplete for Fe(PM-TeA) 2 (NCS) 2 , smooth with almost no hysteresis for Fe(PM-AzA) 2 (NCS) 2 , unusually abrupt for Fe(PM-BiA) 2 (NCS) 2 and abrupt with a very large hysteresis (37 K) for Fe(PM-PEA) 2 (NCS) 2 . In Fe(PM-BiA) 2 (NCS) 2 , Fe(PM-TeA) 2 (NCS) 2 and Fe(PM-AzA) 2 (NCS) 2 the spin conversion is not associated with a large structural phase transition and the space group is the same above and below the temperature of transition: orthorhombic Pccn for the two first and monoclinic P2 1 /c for the third. On the other hand, Fe(PM-PEA) 2 (NCS) 2 undergoes a change in the crystal symmetry from P2 1 /c (high spin) to Pccn (low spin) which corresponds to a strong re-organisation of the iron atom network. The evolution as a function of temperature of the FeN 6 core as well as of the intramolecular characteristics are almost identical in all four compounds. To a first approximation, the crystal packing is similar in all of the structures except that the P2 1 /c structures develop an asymmetrical molecular environment. Nevertheless, a close examination of the intermolecular interactions, classified as intra- and inter-sheet, show some differences. The intrasheet and the intersheet interactions are stronger in Fe(PM-BiA) 2 (NCS) 2 and Fe(PM-PEA) 2 (NCS) 2 than either in Fe(PM-TeA) 2 (NCS) 2 where no ‘second’ neighbour intrasheet contacts are created, or in Fe(PM-AzA) 2 (NCS) 2 where the intersheet interactions are weak. Thus, the abruptness of the transition is attributed to the combination of close intrasheet and intersheet contacts. The hysteresis effect in Fe(PM-PEA) 2 (NCS) 2 is connected to the phase transition which could occur due to an irregular iron atom network associated with very short carbon-carbon intermolecular contacts at high temperature, not found in Fe(PM-AzA) 2 (NCS) 2 which shows the same irregular iron atom network.

Core-Level Satellites and Outer Core-Level Multiplet Splitting in Mn Model Compounds

Abstract: We report a systematic study of the Mn 2p, 3s and 3p core-level photoemission and satellite structures for Mn model compounds. Charge-transfer from the ligand state to the 3d metal state is observed and is distinguished by prominent shake-up satellites. We also observe that the Mn 3s multiplet splitting becomes smaller as the Mn oxidation state increases, and that 3s-3d electron correlation reduces the branching ratio of the 7S:5S states in the Mn 3s spectra. In addition, as the ligand electronegativity decreases, the spin state purity is lost in the 3s spectra as evidenced by peak broadening. Our results are best understood in terms of the configuration-interaction (CI) model including intrashell electron correlation, charge-transfer and final-state screening.

Spin gap and magnetic coherence in a clean high-temperature superconductor

Abstract: A notable aspect of high-temperature superconductivity in the copper oxides is the unconventional nature of the underlying paired-electron state. A direct manifestation of the unconventional state is a pairing energy—that is, the energy required to remove one electron from the superconductor—that varies (between zero and a maximum value) as a function of momentum, or wavevector1,2: the pairing energy for conventional superconductors is wavevector-independent3,4. The wavefunction describing the superconducting state will include the pairing not only of charges, but also of the spins of the paired charges. Each pair is usually in the form of a spin singlet5, so there will also be a pairing energy associated with transforming the spin singlet into the higher-energy spin triplet form without necessarily unbinding the charges. Here we use inelastic neutron scattering to determine thewavevector-dependence of spin pairing in La2−xSrxCuO4, the simplest high-temperature superconductor. We find that the spin pairing energy (or ‘spin gap’) is wavevector independent, even though superconductivity significantly alters the wavevector dependence of the spin fluctuations at higher energies.

Collective oscillations of an interacting trapped Fermi gas

Abstract: We calculate the effects of two-body interactions on the low-frequency oscillations of a normal Fermi gas confined in a harmonic trap. The mean-field contribution to the collective frequencies is evaluated in the collisionless regime using a sum-rule approach. We also discuss the transition between the collisionless and hydrodynamic regimes with special emphasis on the spin dipole mode in which two atomic clouds occupying different spin states oscillate in the opposite phase. The spin dipole mode is predicted to be overdamped in the hydrodynamic regime. The relaxation time is calculated as a function of temperature and the effects of Fermi statistics are explicitly pointed out.

Silicon vacancy in SiC: A high-spin state defect

Abstract: We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet 3T1. For the singly negative VSi, we find a high-spin ground-state 4A2 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.

Magnetic and Transport Properties of LaCo1-xNixO3-Comparison with La1-xSrxCoO3

Abstract: We have investigated magnetization, neutron scattering, resistivity, thermoelectric power and Hall effect in LaCo 1- x Ni x O 3 ( x =0.05 and 0.1) single crystals. At low temperatures, they show spin-glass behavior with a strong ferromagnetic correlation. Around 500 K, spin state transition from the intermediate spin state ( S =1) to the high spin state ( S =2) accompanied by an insulator-to-metal transition is observed as in undoped LaCoO 3 . These magnetic properties of LaCo 1- x Ni x O 3 are very similar to those previously reported for La 1- x Sr x CoO 3 . The low-temperature transport properties of both LaCo 1- x Ni x O 3 and La 1- x Sr x CoO 3 exhibit the characteristics of an insulator near the insulator-to-metal transition. The sign of the carrier is verified to be positive in both systems from the thermoelectric power measurement. However, the sign of the Hall coefficient of LaCo 1- x Ni x O 3 is negative. These transport properties are discussed in terms of small polaron hopping mechanism.

Interspin Crossing and Reactivity

Abstract: The change in spin states makes the difference! A change in the spin state sometimes accounts for the lowest energy pathways in reactions where a crossing of ground- and excited-state surfaces occurs along the reaction coordinate (see schematic representation). This reaction paradigm is illustrated with a number of reactions from the areas of organic, inorganic, and organometallic chemistry.

Spin isomerisation of para-substituted phenyl cations

Abstract: The singlet and triplet potential energy surfaces of a series of p-X-substituted aryl cations (X = H, CN, CH3, F, OH, NH2) are investigated computationally at the B3LYP/6-31G(d) level of theory. The first four species are found to be ground state singlets, the last has a triplet ground state, and the spin states of the OH derivative are almost isoenergetic. The minimum energy crossing points (MECPs) between the two surfaces are found to lie very little above the higher of the two minima in all cases, and the spin-orbit coupling is significant at those points. Therefore, it is expected that aryl cations will rapidly convert to their most stable spin state, and that in cases of near degeneracy such as for p-HO-C6H4+, the states may interconvert rapidly enough to both be accessible in thermal reactions.

Role of Long-Range Interaction in Photoinduced Spin-State Transitions in Spin-Crossover Complexes

Abstract: We explain theoretically the peculiarities of photoinduced low-spin (LS)→high-spin (HS) transitions in spin-crossover complexes, such as the threshold behavior in the excitation light intensity for phase conversion and the existence of the incubation period. Our investigation is based on a model where a single complex is described by the two electronic states and the breathing oscillation mode with the long-range interaction among the complexes. As the spin transition processes, we take into account the LS→HS photoexcitation process and the HS→LS nonradiative decay process. The rate for the latter process is sensitive to the LS fraction in the crystal because the potential barrier for the HS→LS decay is dependent on the LS fraction through the long-range interaction among the complexes; the lifetime of the metastable HS state becomes longer as the LS fraction decreases. Such sensitivity leads to the nonlinear temporal evolution for the LS fraction, which coincides with the experimental results.

The spin state of the neutral silicon vacancy in 3C–SiC

Abstract: Recent theoretical studies show that the neutral silicon vacancy (VSi) in cubic silicon carbide (3C–SiC) exhibits negligible Jahn–Teller distortion. This provides an opportunity to study the energy sequence of different multiplets in a vacancy with genuine Td symmetry. Calculations using the local spin density approximation give a spin triplet as ground state. The determination of the true ground state requires, however, the incorporation of configuration interactions. Using multiconfigurational self-consistent field calculations we show that the ground state of the neutral VSi0 in 3C–SiC is a spin singlet. The calculated energy difference, ∼0.1 eV, in favor of the singlet spin state would still allow the experimental observation of the triplet state at high temperature.

Photodetachment studies of extended excited states in i-xen clusters (n=1-54)

Abstract: We present a comprehensive experimental study of bound excited states in I−Xen clusters (n=1–54), using photoelectron spectroscopy and energy-dependent action spectroscopy. Starting at n=4, the electron detachment action spectra developed peaks lying in energy lower than the vertical binding energy. This behavior has been shown for both final spin states of the neutral iodine. It indicates the existence of bound electronic states extended over the xenon cluster. The peaks lying in energy below the J=1/2 detachment continuum of the iodine (n>4) were detected over the whole cluster size range of I−Xen using electron detachment action spectroscopy. For the bound states relating to the J=3/2 continuum, in the size range n=4–12, thermionic emission has marked the existence of bound states. For n>12, these states were stabilized and detected via two-photon excitations. The picture, unveiled from the above photoelectron and action spectra, is the gradual evolution of bound excited states on the xenon solvent clu...

Broken cylindrical symmetry in binary mixtures of Bose-Einstein condensates

Abstract: We provide Monte Carlo simulation and analytic interface arguments for the absence of cylindrical symmetry in the density difference of a binary mixture of Bose-Einstein condensates in traps in the phase segregated regime. For different spin states of ${}^{87}\mathrm{Rb},$ the total density remains very symmetric however. There is a metastable symmetric phase in a substantial part of this phase segregated regime. The boundary between the symmetric and the nonsymmetric phase is a function of the relative concentration. The effect of different confining potentials is investigated.

Band termination in the N = Z Odd-Odd Nucleus 46 V

Abstract: High spin states in the odd-odd $N=Z$ nucleus ${}^{46}\mathrm{V}$ have been identified. At low spin, the $T=1$ isobaric analog states of ${}^{46}\mathrm{Ti}$ are established up to ${I}^{\ensuremath{\pi}}{=6}^{+}.$ Other high spin states, including the band terminating state, are tentatively assigned to the same $T=1$ band. The $T=0$ band built on the low-lying ${3}^{+}$ isomer is observed up to the ${1f}_{7/2}$-shell termination at ${I}^{\ensuremath{\pi}}{=15}^{+}.$ Both signatures of a negative parity $T=0$ band are observed up to the terminating states at ${I}^{\ensuremath{\pi}}{=16}^{\ensuremath{-}}$ and ${I}^{\ensuremath{\pi}}{=17}^{\ensuremath{-}},$ respectively. The structure of this band is interpreted as a particle-hole excitation from the ${1d}_{3/2}$ shell. Spherical shell model calculations are found to be in excellent agreement with the experimental results.

Elastic anomalies with the spin-state transitions in LaCoO3

Abstract: Ultrasonic measurements of LaCoO 3 were performed from 4.2 to 600 K. We observed two lattice softenings related to the spin-state transitions of LaCoO 3 around 100 and 500 K. The temperature dependence of the elastic modulus is well explained by a model involving three spin states coupled with the lattice. The parameters obtained from the present ultrasonic measurement are in good agreement with those previously obtained from lattice expansion.

Cooperativity in a spin transition ferrous polymer: Interacting domain model, thermodynamic, optical and EPR study

Abstract: A new thermodynamic model is proposed in order to account for the high spin \(\) low spin conversion in metal-organic polymers. The model, based on the idea that the spin conversion occurs in interacting domains of like-spin metal ions, allows to explain most of the important features of various types of spin conversion. The sine qua non condition of the existence of spin transitions with hysteresis is obtained. In the case of very large cooperativity, the model predicts unusual behaviour of the spin conversion system due to a low-temperature metastable high spin state. Existence of such a state is interesting in the context of the light induced excited spin state trapping recently observed in some ferrous compounds. The model is applied to interpret the spin transition in polycrystalline ferrous polymer [Fe1-yCuy (Htrz)2trz] (BF4) with y = 0.00, 0.01 and 0.10, detected by differential scanning calorimetry, optical reflectivity and electron paramagnetic resonance. The domain size and the interaction energy between the domains are estimated as, respectively, n = 11 and \(\) for the y = 0 compound. As the copper content is growing, n and \(\) tend to decrease, resulting in transformations of the shape of hysteresis loop which becomes less steep, narrows and shifts to lower temperatures. The electron paramagnetic resonance gives further evidence of the presence of like-spin domains.