Ground-state Orientation Research Articles

The role of spin–orbit coupling in optically induced ultrafast magneticdynamics11This paper is dedicated to Professor K Baberschke on the occasion of his65th birthday.

Spin–orbit coupling (SOC) plays a major role in solid-state magnetism. Its effectsrange from completely defining the splitting of f states in rare earths tothe correct prediction of the ground-state orientation of the magneticmoment in transition metals. The breaking of symmetries that it produces isthe main tool for addressing magneto-optical effects. In this paper wetheoretically study the possibility of all-optical spin switching. By simulating theeffect of an ultrashort laser pulse on an electronic system, we show thecrucial role of SOC in the spin dynamics occurring in this regime. The roleof other parameters is also analysed. Finally, we show the possibility ofobtaining an all-optical spin switching in the subpicosecond time regime.

A DFT study of Zr-S rotational barriers of (η5-C5H5)2Zr(Cl)(SR); the origin of an inverse steric effect

We have previously reported the experimental rotational barriers about the M–S bonds of 16e– bent metallocene monothiolates (η5-C5H5)2Zr(Cl)(SR) (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3] (1a–d): 32, 33, 35, and 26 kJ mol–1, respectively. The ground-state orientation about the Zr–S bonds of 1 that maximizes S(pπ) [Formula: see text] M(dπ) bonding (Cl-Zr-S-R [Formula: see text] 90°) also maximizes Cp « R (Cp = η5-C5H5) steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R [Formula: see text] 0°) is one that minimizes S(pπ) [Formula: see text] M(dπ) bonding and maximizes Cl « R steric interaction. Deviation from a ground-state orientation that is ideal for S(pπ ) [Formula: see text] M(dπ ) bonding might be expected as the size of the R-group and Cp « R steric interaction increases. The present study employs hybrid density function computational methods to reproduce the experimental rotational barriers. The computational rotational barriers decrease in the order 1a (R = CH3) > 1b (R = CH2CH3) > 1c (R = CH(CH3)2) at low temperature. However, opposing enthalpic factors reverse this trend at higher temperatures. The aberrant trend (unexpectedly low rotational barrier) that is observed for the 1d (R = C(CH3)3) derivative is due to dominant ground-state steric (enthalpic) effects. We conclude the thiolate ligand of 1d is misdirected in the ground-state with respect to the other thiolate derivatives.Key words: misdirected ligands, metallocene, thiolate, structure, bonding, dynamics.

Quantum Hall ferromagnets: Induced topological term and electromagnetic interactions

The $\ensuremath{\nu}=1$ quantum Hall ground state in materials such as GaAs is well known to be ferromagnetic in nature. The exchange part of the Coulomb interaction provides the necessary attractive force to align the electronic spins spontaneously. The gapless Goldstone modes are the angular deviations of the magnetization vector from its fixed ground-state orientation. Furthermore, the system is known to support electrically charged spin skyrmion configurations. It has been claimed in the literature that these skyrmions are fermionic owing to an induced topological Hopf term in the effective action governing the Goldstone modes. However, objections have been raised against the method by which this term has been obtained from the microscopics of the system. In this paper, we use the technique of the derivative expansion to derive, in an unambiguous manner, the effective action of the angular degrees of freedom, including the Hopf term. Furthermore, we have coupled perturbative electromagnetic fields to the microscopic fermionic system in order to study their effect on the spin excitations. We have obtained an elegant expression for the electromagnetic coupling of the angular variables describing these spin excitations.

Misdirected π-donor ligands: (η 5-C 5H 5) 2Zr(Cl) (SR) with sterically more demanding R groups have lower rotational barriers

Rotational barriers about the M-S bonds of 16-electron bent metallocene monothiolates (η 5-C 5H 5) 2Zr(Cl) (SR) (R = −CH 3, −CH 2CH 3, −CH(CH 3) 2, −C(CH 3) 3) ( 1a–d) have been measured by dynamic 1H NMR methods: 32, 33, 35 and 26 kJ mol −1, respectively. The ground-state orientation about the Zr-S bonds of 1 that maximizes Spπ → Mdπ bonding (Cl-Zr-S-R ≈ 90°) also maximizes CpR steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R ≈ 0°) is one that minimizes Spπ → Mdπ bonding and maximizes ClR steric interaction. Deviation from a ground-state orientation that is ideal for Spπ → Mdπ bonding might be expected as the size of the R group and CpR steric interaction increases. Thus, the aberrant trend for the R = −C(CH 3) 3 derivative could be attributed to a ground-state steric effect where the sterically demanding −C(CH 3) 3 group forces an unfavorable (misdirected) orientation for Mdπ-Spπ bonding, but a favorable orientation with respect to CpR and ClR steric interactions. However, the solid-state structures of (η 5-C 5H 5) 2Zr(SR) 2 (R = −CH 3, −CH 2CH 3, −CH(CH 3) 2, −C(CH 3) 3) ( 2a–d) exhibit regular variation of their metric parameters as evidenced by their Zr-S-C bond angles of 108, 109, 113, and 124° and S-Zr-S′ bond angles of 97, 99, 100 and 106°, respectively. Neither the S′-Zr-S-R torsion angles nor the dihedral angles that describe the relationship between the S/Zr/S′ and Cp(centroid)/Zr/Cp′ (centroid) planes (both indicators of the relative orientation of the Zr dπ acceptor orbital and the thiolate S pπ donor orbital) reflect the steric demand of the R group. Thus, the size of the R group imposes a measured effect on the geometry of 2 and the tert-butyl group is not extraordinary. Although the enthalpic and entropic effects could not be deconvoluted for rotation about the Zr-S bond of 1 in the present study, literature precedents suggest that both enthalpic and entropic effects may play a role in determining the irregular trend that is observed.

Structure, dynamics, and phase transitions in the fullerene derivatives C60O and C61H2.

The effect of perturbing the icosohedral symmetry of ${\mathrm{C}}_{60}$ by the addition of the side groups -O and -${\mathrm{CH}}_{2}$ upon orientational order-disorder and glass transitions in solid ${\mathrm{C}}_{60}$ has been studied by a combination of high-resolution capacitance dilatometry and single-crystal x-ray and powder inelastic neutron scattering. Both fullerene derivatives ${\mathrm{C}}_{60}$O (epoxide) and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ (6,5-annulene) are shown to undergo a sequence of transitions similar to that found in pure ${\mathrm{C}}_{60}$, i.e., a first-order orientational ordering transition just below room temperature followed by an orientational glass transition at lower temperatures. Although the exact origin of the glass transition in ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ is unclear, the glass transition in ${\mathrm{C}}_{60}$O has the same origin as that in ${\mathrm{C}}_{60}$, with a significantly higher degree of order due to a larger energy difference between pentagon and hexagon orientations. The dilatometric data at the glass transition indicate that, in contrast to ${\mathrm{C}}_{60}$, the ground-state orientation of both ${\mathrm{C}}_{60}$O and ${\mathrm{C}}_{61}$${\mathrm{H}}_{2}$ molecules is that with the smallest volume, also demonstrating a significant influence of the side groups upon the details of the structure. A possible explanation of these differences in terms of steric effects is proposed. \textcopyright{} 1996 The American Physical Society.

Reflection spectroscopy of spin-polarized atoms near a dielectric surface

We compare the interactions of spin-polarized Na with glass surfaces that are either bare or silicone coated. For this purpose, we reflect a laser beam that is resonant with the Na atoms from the interface, using an angle of incidence close to the critical angle of total internal reflection. Polarization-selective detection of the reflected light provides information about the ground-state orientation of the atoms, which is created by optical pumping with a second laser beam. Our theoretical description treats the atomic vapor as a homogeneous anisotropic medium characterized by a macroscopic magnetization. Comparative experimental studies performed with silicone-coated and bare glass surfaces provide evidence that wall relaxation can be observed by reflection spectroscopy.

Self-induced planar and cylindrical splitting of a laser beam in sodium vapor.

An almost linearly polarized laser beam focused into a sodium vapor cell may split into different patterns ranging from two spots to a spot and a ring of opposite circular polarization. The patterns are controlled by static magnetic fields of the order of the earth's field. The behavior is explained, allowing for a longitudinal component of the dielectric polarization in the paraxial wave equation. Numerical results reveal the delicate balance between light shift, influence of the magnetic field, and the creation of ground-state orientation by the competing polarization components.

Radiation trapping: An alternative mechanism for chaos in a nonlinear optical resonator.

The dynamical behavior of a sodium-filled Fabry-P\'erot resonator in a static magnetic field is studied systematically over a braod parameter range. The experiments reveal that all apparent types of nonlinear oscillations form a distinct scenario, including a parameter range of generalized bistability. The theoretical descripton includes the influence of radiation trapping on the dynamics of the ground-state orientation. Numerical simulations yield an almost quantitative reproduction of the experimentally observed scenario, with the inclusion of chaotic oscillations, and show the dominant role of radiation trapping as a nonlinear mechanism leading to chaotic behavior.

Role of radiation trapping in degenerate four-wave-mixing experiments.

The process of degenerate four-wave mixing is studied experimentally in dense sodium vapor in a rare-gas atmosphere. The influence of trapped fluorescence light on the ground-state orientation is shown to be responsible for the observed strong reduction of saturation phenomena with increased sodium density. The interpretation is based on a simple model and is supported by results obtained by suppressing the fluorescence.

Nonlinear effects of radiation trapping in ground-state oriented sodium vapor.

Effects of radiation transport in the interaction of a laser beam with sodium vapor in an argon atmosphere are reported. Investigations of the temporal and spatial evolution of ground-state orientation, which is produced by optical pumping with a circularly polarized near-resonant laser beam, reveal a strong deviation from the expected pure diffusive behavior. Transverse profiles are narrowed considerably with increasing sodium density as well as with increasing pump intensity. The temporal decay of orientation is also drastically altered. A simplified nonlinear and nonlocal theoretical model of radiation trapping gives excellent qualitative agreement with the observations and allows an intuitive interpretation.

Propagation of light in a [formula omitted] resonant medium

The propagation of light in a resonant medium with multiple sublevels depends on the populations of and coherences between theses sublevels. We analyze the case of a J= 1 2 ↔J′= 1 2 transition with arbitrary ground-state polarization in the limit of a weak probe beam. It is found that transverse components of the ground-state orientation lead to a longitudinal component in the eigenpolarizations; a laser beam propagating through an optically pumped medium should therefore be displaced laterally.

N-Pentenyl glycosides as mediators in the asymmetric synthesis of monosubstituted chiral nonracemic tetrahydrofurans and .gamma.-lactones

n-Pentenyl glycosides can be oxidatively hydrolyzed by treatment with N-bromosuccinimide, previous work having been focused on the usefulness of the resulting glycosyl moiety. In this paper, attention is focused on asymmetric induction in the 2-(bromomethyl) furan that is liberated. The enantiomeric excess depends strongly on the orientations at the anomeric centers and at C2, as well as on the protecting group on the C2 oxygen. α-Anomers display higher asymmetric induction, and rationalization of this observation is based on the assumption that the molecule reacts from the favored ground-state orientation, wherein the exo anomeric effect is displayed