Localized Spin States Research Articles

Manipulation of exciton spin states in a dilute magnetic semiconductor by nanostructured ferromagnets

AbstractA hybrid of nanostructured Fe/Tb multilayer ferromagnets and ZnCdMnSe dilute magnetic semiconductor (DMS) quantum well was studied by magneto‐micro‐photoluminescence (µPL) spectroscopy. When the sample is magnetized in an external magnetic field an additional fringe field from the nanomagnets effectively modulates the local exciton spin states in the quantum well (QW) due to the giant Zeeman effect. Observation of 1.5% of remanent PL polarization degree from the QW gives a direct proof of remanent exciton spin polarization in the DMS. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two paramagnetic iron states at the Verwey phase transition in magnetite

At the Verwey phase transition (VPT) region a wide line of the ferromagnetic resonance (FMR) and a narrow EPR spectrum were observed. The EPR line at 125 K occurred and then disappeared below 85 K. This unexpected phenomenon is observed only if iron charge state transformation is present. Simulation of both FMR as well as EPR spectra has indicated the presence of two different iron clusters in the Fe 3O 4 structure. Best fit of both spectra yielded g eff = 2.4 and 4.1 for FMR, and g EPR = 2.8 and 3.4 for EPR. The presence of two-component (EPR, FMR) spectra at about T V is due to a frustration of charge distribution in the B-sublattice as a result of the orthorhombic Pmca pseudosymmetry constraints on the atomic positions in monoclinic symmetry cell P2/c. The paramagnetic (EPR) center observed is an admixtured Fe 2+ state in antiferromagnetically ordered A-sublattice ferromagnetically coupled with the B-lattice. Two paramagnetic defects observed (EPR) of localized spin states are evoked by two valency states of Fe 2.4 and Fe 2.6 below T v . They can originate from two types of Fe 4O 4 cubes: “electron-rich” and” electron-poor” at a ratio 3:1 [J.P. Wright, J.P. Attfield, P.G. Radaelli, Phys. Rev. Lett. 87 (2001) 266401; Phys. Rev. B 66 (2002) 214422] or from two types of bond dimerization (H. Seo, M. Ogata, H. Fukuyama, Phys. Rev. B65 (2002) 85107).

Synthesis and characterization of cationic iron complexes supported by the neutral ligands NPi-Pr3, NArPi-Pr3, and NSt-Bu3

This paper compares the local geometries, spin states, and redox properties of a series of iron complexes supported by neutral, tetradentate NP3(tris(phosphine)amine) and NS3(tris(thioether)amine) ligands. Our consideration of an Fe-mediated N2fixation scheme similar to that proposed by Chatt for molybdenum motivates our interest in systems of these types. This report specifically describes the synthesis and characterization of cationic Fe(II) chloride complexes supported by the neutral ligands NPi-Pr3(NPi-Pr3 = [N(CH2CH2P-i-Pr2)3]), NArPi-Pr3(NArPi-Pr3 = [N(2-diisopropylphosphine-4-methylphenyl)3]), and NSt-Bu3(NSt-Bu3 = [N(CH2CH2S-t-Bu)3]). The solid-state structures, electrochemistry, and magnetic properties of these complexes are reported. Whereas the NPi-Pr3and NArPi-Pr3ligands provide pseudotetrahedral S = 2 ferrous cations [Fe(NPi-Pr3)Cl]PF6(1[PF6]) and [Fe(NArPi-Pr3)Cl]BPh4(2[BPh4]) featuring a long Fe—N bond distance, the NSt-Bu3ligand gives rise to a trigonal bipyramidal structure with a S = 1 ground state and a much shorter Fe–N interaction. The complexes 1[BPh4] and 2[BPh4] can be reduced under CO to give rise to the five-coordinate Fe(I) monocarbonyls [Fe(NPi-Pr3)CO]BPh4(4[BPh4]) and [Fe(NArPi-Pr3)CO]BPh4(5[BPh4]). The solid-state structures and electrochemistry of 4[BPh4] and 5[BPh4] are described, as is the EPR spectrum of 4[BPh4]. The synthesis and characterization of the hydride–dinitrogen complex [Fe(NPi-Pr3)(N2)(H)]PF6(6[PF6]) has also been accomplished and its properties are also reported.Key words: nitrogenase, iron, polydentate phosphines, thioether ligands, N2chemistry, nitrogen, Fe(I).

Valence and magnetic transitions inYbInCu4 probedusing 63Cu nuclear quadrupole resonance under high pressure

Applying pressure to the valence fluctuating compoundYbInCu4 lowers the valence transition temperature and stabilizes the high temperature (HT)localized spin state. For pressure above 2.4 GPa, the valence transition iscompletely suppressed and the magnetically ordered state arises as a newground state below 2.4 K. In order to elucidate microscopically the relationbetween the valence and magnetic transitions in f electron system, we performed63Cu NQRmeasurements of YbInCu4 under pressure up to 2.5 GPa. The experimental results provide clearevidence for the onset of the magnetic ordering in the pressure-stabilizedHT state above 2.3 GPa. The nuclear spin–lattice relaxation rate1/T1 in the HTstate shows 1/T1 = constant behaviour down to 3.0 K and a divergence increase just below 2.4 K.We concluded that the pressure-stabilized localized electron state ofYbInCu4 transforms directly into a magnetically ordered state, without displaying any intermediateheavy Fermi liquid behaviour.

Expansion Around the Mean Field in Quantum Magnetic Systems

We introduce a new definition of ordered phase in a magnetic system based on properties of the local spin state probability. A statistical functional associated to this quantity depends both on amplitude and direction of the local magnetization. We show that it is possible to introduce an expansion at fixed magnetization amplitude in the inverse of lattice coordination number if the direction is selected by an extremum condition. In the limit of infinite coordination number we recover the mean field results. First order corrections are derived for the Ising model in the presence of a transverse field and for the XY model. Our results concerning critical temperature and order parameter compare favorably with other approaches.

Small-tunneling-amplitude boson-Hubbard dimer. II. Dynamics

We present analytical relations for the quantum evolution of the number difference of bosons between the two sites of a double-well potential, by using perturbative results for small tunneling amplitudes in the two-mode approximation. Results are obtained for two different initial conditions: completely localized states and coherent spin states. In the former case both the short and the long time behavior is investigated and the characteristic Bohr frequencies of the energy spectrum are determined. In the latter case we calculate the short time-scale evolution of the number difference. The analytical expressions compare favorably with direct numerical solutions of the quantum problem. Finally, we discuss the corresponding Gross-Pitaevskii (i.e., mean-field) dynamics and we point out the differences between the full quantum evolution and the mean-field evolution.

Local magnetic properties and spin state of [formula omitted] NMR study

59 Co NMR studies have been made to clarify the spin state and local magnetic properties of YBaCo 2O 5.5, a new cobalt oxide with an oxygen-deficient and A-site ordered perovskite structure. In the antiferromagnetic state we observed four Co NMR spectra with internal fields of 216, 24, 19, and 0 kOe. This means that the crystal structure at low temperatures does not have the 2×1×1 superstructure observed at room temperature.

Structural Order Parameter in the Pyrochlore Superconductor Cd2Re2O7

It is shown that both structural phase transitions in Cd2Re2O7, which occur at T_{s1}=200 K and T_{s2}=120 K, are due to an instability of the Re tetrahedral network with respect to the same doubly degenerate long-wavelength phonon mode. The primary structural order parameter transforms according to the irreducible representation E_u of the point group O_h. We argue that the transition at T_{s1} may be of second order, in accordance with experimental data. We obtain the phase diagram in the space of phenomenological parameters and propose a thermodynamic path that Cd2Re2O7 follows upon cooling. Couplings of the itinerant electronic system and localized spin states in pyrochlores and spinels to atomic displacements are discussed.

Fast Kawasaki spin exchange limit of spin-facilitated kinetic Ising models.

We study a Fock-space operator technique for describing the stochastic kinetics of a spin-facilitated kinetic Ising model. We focus in particular on the diffusion (fast Kawasaki exchange) limit in which the kinetics can be described by a single mean field evolution equation. We derive some general criteria for the approximative validity of mean field theory for the case of a nondiverging diffusion coefficient of the local spin states.

Time scales of phonon-induced decoherence of semiconductor spin qubits

Decoherence of a shallow donor electron spin in silicon caused by electron-lattice interaction is studied. We find that there are two time scales associated with the evolution of the electron spin density matrix: the fast but incomplete decay due to the interaction with non-resonant phonons followed by slow relaxation resulting from spin flips accompanied by resonant phonon emission. We estimate both time scales as well as the magnitude of the initial drop of coherence for P donor in Si and argue that the approach used is suitable for evaluation of phonon induced decoherence for a general class of localized spin states in semiconductors.

Theory of scanning tunneling microscopy measurement of single spin decoherence in a superconductor.

Localized spin states in conventional superconductors at low temperatures are expected to have a long decoherence time due to the strong suppression of spin relaxation channels. We propose a scanning tunneling microscopy (STM) experiment allowing the direct measurement of the decoherence time of a single spin in a conventional superconductor. The experimental setup can be readily applied to general-purpose spin-polarized STM and to local spin relaxation spectroscopy. A possible extension of the setup to a quantum information processing scheme is discussed.

Local magnetic states in La 1− xSr xMnO 3 and Nd 1− xSr xMnO 3

Local magnetic state of Mn atoms in ferromagnetic perovskite-type manganites is investigated with magnetic circular dichroism (MCD) of Mn 2p core-level absorption (XAS). Small but clear differences in XAS and MCD spectra are found between La 0.84Sr 0.16MnO 3 and Nd 0.53Sr 0.47MnO 3. The electronic states of Mn atoms, especially the local spin and orbital states are discussed.

Mott transition with antiferromagnetic ordering in ammoniated alkali C 60 superconductors: NMR studies

13 C -NMR measurements were performed for ammoniated A 3 C 60 ( A= alkali metals ) superconductors, (NH 3) x A 3C 60. For (NH 3)K 3C 60, the Curie–Weiss-type T dependence of susceptibility and T-independent nuclear relaxation rate, 1/ T 1=const., were observed at high Ts above 150 K, indicating the localized spin state of C 60 molecular spins. The extremely broadened spectra for 13 C -NMR was observed below T N ∼45 K , providing unambiguous evidence for an antiferromagnetic order with a moment of μ 0∼1 μ B / C 60 . The present NMR measurements clearly showed that the intercalation of NH 3 makes (NH 3)K 3C 60 a Mott–Hubbard insulator with AF order below T N ∼45 K . We also discuss the partially ammoniated A 3C 60.

Temperature dependence of the resistivity in the double-exchange model

The resistivity around the ferromagnetic transition temperature in the double-exchange model is studied by the Schwinger-boson approach. The spatial spin correlation responsible for scattering of conduction electrons are taken into account by adopting the memory function formalism. Although the correlation shows a peak lower than the transition temperature, the resistivity in the ferromagnetic state monotonically increases with increasing temperature due to a variation of the electronic state of the conduction electron. In the paramagnetic state, the resistivity is dominated by the short-range correlation of scattering and is almost independent of the temperature. It is attributed to a cancellation between the nearest-neighbor spin correlation, the fermion bandwidth, and the fermion kinetic energy. This result implies the importance of the temperature dependence of the electronic states of the conduction electron as well as the localized spin states in both ferromagnetic and paramagnetic phases.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO 8:LiClO 4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle–carbon particle and carbon particle–polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory.

Interplay between Hund coupling and Hubbard interaction inSr2RuO4

We formulate a microscopic model describing the combined effect of the Hund coupling and the local Coulomb repulsion on the ground-state properties of the layered perovskite compound ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. Considering that the properties of this compound are mainly determined by the electron dynamics in the ruthenium-oxygen planes, we have performed an exact diagonalization study on small planar Ru-O clusters, based on the use of the Lanczos algorithm. We show that the minimum value of the Hund coupling $J$ needed to have on each ruthenium atom a local triplet spin state, is a rapidly decreasing function of the intra-orbital $d$-electron Coulomb repulsion $U$. In addition, from the analysis of the off-site spin-spin correlation function, ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is found to behave as a paramagnetic metal, in agreement with the experiments.

Characterization of polyparaphenylene (PPP)-based carbons

Polyparaphenylene (PPP)-based carbons heat-treated at temperatures (THT) from 600 °C up to 3000 °C have been characterized both structurally and in terms of their physical properties. Special attention is given to PPP heat-treated at 700 °C (denoted by PPP-700), since samples heat-treated to this temperature were observed to have exceptionally high lithium affinities when electrochemically doped with Li. At low THT below 700 °C, it is found that the local structure of PPP-based samples can be characterized mostly in terms of a disordered polymer. As a result of heat treatment to high temperature, PPP-based carbon shows graphitization behavior with regard to x-ray diffraction d002 (graphite c-axis d-spacing) development and to the increase of the Raman IG/ID intensity ratio (where IG and ID are the integrated intensities of the 1580 cm−1 and 1360 cm−1 Raman modes, respectively), as is found in so-called graphitizing carbons. However, development of the c-axis crystallite size (Lc) is restricted to very small values, in the range of so-called nongraphitizing carbons, while the a-axis crystallite size (La) attains values up to roughly 120 Å for heat treatments near 3000 °C. These structural properties of PPP-based carbons are consistent with the observed electrical characteristics and their dependence on THT. Low temperature magnetic susceptibility measurements were analyzed, along with Raman spectra, allowing for the characterization of disorder in terms of localized spin states for several heat-treated PPP samples. By interpreting the results of these various characterization techniques, we are able to present an insightful perspective on the nature of PPP-based carbons and the role of PPP-700 as an effective lithium host material for secondary battery applications.

Magnetic Phase Diagram of a Generalized Kondo Lattice Model with Classical Localized Spins

We study the magnetic ground state of a generalized Kondo lattice model with classical localized spins, which includes antiferromagnetic exchange interaction J between localized spins. We determine the magnetic phase diagram on the plane of J and fermion concentration n , examining all possibilities of planar order of the localized spins in two dimensions. We find canted ferromagnetic phases, diagonal spiral phases, stripe spiral phases as well as the Neel ordered phase and the ordinary ferromagnetic phase. The densities of states of the fermions for these localized spin state are calculated. For small n , we analytically examined the stability of the Neel state and the ferromagnetic state in detail in any dimensions. We also discuss the t - J limit, J K / t →∞, for small n .

Alternate appearance of localized spin and itinerant electron magnetic states in γMn-Ni alloys as observed from the temperature dependence of the electrical resistivity

An alternate appearance of localized spin magnetic states and itinerant electron magnetic states was observed in electrical resistivities of antiferromagnetic γMn-24.9, -23.0 and -21.0 at% Ni alloys at temperatures from 300 K to the Néel points. The lattice vibration with unpaired 3d-electrons and the spin density fluctuation of itinerant 3d-electrons change complementarily with magnetic fields from 0 to 0.23 Wb/m 2 in the localized magnetic state. The complementary change is larger in the localized magnetic state of higher temperatures and of lower 3d-electron density. The cause of the complicated crystallographic structures and magnetic state of the alloys is discussed from the viewpoint of the complementary change and its dependence on temperature and density of 3d-electrons.

Electron states in the two-band Hubbard model: Paramagnetic phase

The electron energy spectrum of the copper-based oxides is calculated in the two-band Hubbard model. The conditions for the onset of a localized spin state are studied. The effect of doping is also studied.