Numerical simulation study on spin resonant depolarization due to spin-orbit coupling

The spin polarization phenomenon in lepton circular accelerators had been known for many years.It provides a new approach for physicists to study the spin feature of fundamental particles and the dynamics of spin-orbit coupling,such as spin resonances.We use numerical simulation to study the features of spin under the modulation of orbital motion in an electron storage ring.The various cases of depolarization due to spin-orbit coupling through an emitting photon and misalignment of magnets in the ring are discussed.     

Numerical simulation study about spin resonant depolarization due to spin—orbit coupling

Spin polarization phenomenon in lepton circular accelerators had been known for many years. It gives new approach for physicists to study about spin feature of fundamental particles and dynamics of spin-orbit coupling, such as spin resonances. We use numerical simulation to study the feature of spin under the modulation of orbital motion in electron storage ring. The various cases of depolarization due to spin-orbit coupling through emitting photon and misalignment of magnets in the ring are discussed.     

Spin-echo measurements of the conduction electrons self-diffusion coefficient in a metal

The principle possibility of spin echo measurements of conduction electron orbital motion parameters is experimentally demonstrated, using metallic lithium as an example.     

Quantum dissipative Rashba spin ratchets

We predict the possibility to generate a finite stationary spin current by applying an unbiased ac driving to a quasi-one-dimensional asymmetric periodic structure with Rashba spin-orbit interaction and strong dissipation. We show that under a finite coupling strength between the orbital degrees of freedom the electron dynamics at low temperatures exhibits a pure spin ratchet behavior, i.e., a finite spin current and the absence of charge transport in spatially asymmetric structures. It is also found that the equilibrium spin currents are not destroyed by the presence of strong dissipation.     

Spin-to-orbital angular momentum conversion and spin-polarization filtering in electron beams

We propose the design of a space-variant Wien filter for electron beams that induces a spin half-turn and converts the corresponding spin angular momentum variation into orbital angular momentum of the beam itself by exploiting a geometrical phase arising in the spin manipulation. When applied to a spatially coherent input spin-polarized electron beam, such a device can generate an electron vortex beam, carrying orbital angular momentum. When applied to an unpolarized input beam, the proposed device, in combination with a suitable diffraction element, can act as a very effective spin-polarization filter. The same approach can also be applied to neutron or atom beams.     

Atomistic spin dynamic method with both damping and moment of inertia effects included from first principles

We consider spin dynamics for implementation in an atomistic framework and we address the feasibility of capturing processes in the femtosecond regime by inclusion of moment of inertia. In the spirit of an s-d-like interaction between the magnetization and electron spin, we derive a generalized equation of motion for the magnetization dynamics in the semiclassical limit, which is nonlocal in both space and time. Using this result we retain a generalized Landau-Lifshitz-Gilbert equation, also including the moment of inertia, and demonstrate how the exchange interaction, damping, and moment of inertia, all can be calculated from first principles.     

Dynamics of topological solitons in two-dimensional ferromagnets

Dynamical topological solitons are studied in classical two-dimensional Heisenberg easy-axis ferromagnets. The properties of such solitons are treated both analytically in the continuum limit and numerically by spin dynamics simulations of the discrete system. Excitation of internal mode causes orbital motion. This is confirmed by simulations.     

Electron spin precession in semiconductor quantum wires with Rashba spin-orbit coupling

The influence of the Rashba spin-orbit coupling on the electron spin dynamics is investigated for a ballistic semiconductor quantum wire with a finite width. We monitor the spin evolution using the time-dependent Schrödinger equation. The pure spin precession characteristic of the 1D limit is lost in a 2D wire with a finite lateral width. In general, the time evolution in the latter case is characterized by several frequencies and a nonrigid spin motion.Received: 16 April 2003, Published online: 11 August 2003PACS: 73.21.Hb Quantum wires - 73.22.Dj Single particle states     

Laser-induced orbital and spin excitations in ferromagnets: insights from a two-level system

A recent time-resolved measurement showed that laser-induced orbital and spin excitations proceed in unison and the spin-orbit ratio is held constant during demagnetization. Here a two-level model shows that these orbital and spin excitations originate from state population and state interference effects. For an addressed state, spin and orbital dynamics are solely from the state interference, where the spin and orbital momenta oscillate with the laser frequency and match the dipole moment exactly, an unambiguous test case for the time-resolved magneto-optical Kerr effect. For an undressed state, the interference effect introduces a rapid beating in orbital momentum, which is observed in the first-principles calculation in fcc Ni. The state population change leads to a constant spin-orbit ratio, which explains the linear dependence between spin and orbital momentum changes within 2 ps upon the arrival of a pump pulse in Fe.     

A theoretical study on effects of electron spin relaxation in pulsed nutation experiments for a quartet state in liquid solution

An effect of spin relaxation on electron spin nutation was analyzed theoretically for a quartet state in liquid solution. Modulation of zero-field interactions by random rotational motion is considered as a source of electron spin relaxation under an assumption of a short correlation time. We solved the quantum mechanical equation of motion under a nutation pulse and calculated a nutation spectrum. The results showed that the nutation frequency was strongly affected by spin relaxation. We discuss dependencies of several parameters, such as a rotational correlation time, a microwave field intensity and frequency, and a zero-field interaction on nutation frequency.     

Electric dipole spin resonance for heavy holes in quantum dots

We propose and analyze a new method for manipulation of a heavy-hole spin in a quantum dot. Because of spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy holes such as the effective g factor, mass, spin-orbit coupling constants, spin relaxation, and decoherence times.     

Non-Markovian Dynamics of a Localized Electron Spin Due to the Hyperfine Interaction

We review our theoretical work on the dynamics of a localized electron spin interacting with an environment of nuclear spins. Our perturbative calculation is valid for arbitrary polarization p of the nuclear spin system and arbitrary nuclear spin I in a sufficiently large magnetic field. In general, the electron spin shows rich dynamics, described by a sum of contributions with exponential decay, nonexponential decay, and undamped oscillations. We have found an abrupt crossover in the long-time spin dynamics at a critical shape and dimensionality of the electron envelope wave function. We conclude with a discussion of our proposed scheme to measure the relevant dynamics using a standard spin–echo technique.     

Relativistic electron vortex beams: angular momentum and spin-orbit interaction

Motivated by the recent discovery of electron vortex beams carrying orbital angular momentum (AM), we construct exact Bessel-beam solutions of the Dirac equation. They describe relativistic and nonparaxial corrections to the scalar electron beams. We describe the spin and orbital AM of the electron with Berry-phase corrections and predict the intrinsic spin-orbit coupling in free space. This can be observed as a spin-dependent probability distribution of the focused electron vortex beams. Moreover, the magnetic moment is calculated, which shows different g factors for spin and orbital AM and also contains the Berry-phase correction.     

Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites

We derive a realistic microscopic model for doped colossal magnetoresistance manganites, which includes the dynamics of charge, spin, orbital and lattice degrees of freedom on a quantum mechanical level. The model respects the SU(2) spin symmetry and the full multiplet structure of the manganese ions within the cubic lattice. Concentrating on the hole doped domain ( 0≤x≤0.5) we study the influence of the electron-lattice interaction on spin and orbital correlations by means of exact diagonalisation techniques. We find that the lattice can cause a considerable suppression of the coupling between spin and orbital degrees of freedom and show how changes in the magnetic correlations are reflected in dynamic phonon correlations. In addition, our calculation gives detailed insights into orbital correlations and demonstrates the possibility of complex orbital states. Received 4 September 2002 / Received in final form 8 November 2002 Published online 31 December 2002     

Electron and nuclear spin dynamics in antiferromagnetic molecular rings

We study theoretically the spin dynamics of antiferromagnetic molecular rings, such as the ferric wheel Fe10. For a single nuclear or impurity spin coupled to one of the electron spins of the ring, we calculate nuclear and electronic spin correlation functions and show that nuclear magnetic resonance (NMR) and electron spin resonance (ESR) techniques can be used to detect coherent tunneling of the Néel vector in these rings. The location of the NMR/ESR resonances gives the tunnel splitting and its linewidth an upper bound on the decoherence rate of the electron spin dynamics. We illustrate the experimental feasibility of our proposal with estimates for Fe10 molecules.     

Polarization fields and phase space densities in storage rings: Stroboscopic averaging and the ergodic theorem

A class of orbital motions with volume preserving flows and with vector fields periodic in the “time” parameter θ is defined. Spin motion coupled to the orbital dynamics is then defined, resulting in a class of spin-orbit motions which are important for storage rings. Phase space densities and polarization fields are introduced. It is important, in the context of storage rings, to understand the behavior of periodic polarization fields and phase space densities. Due to the 2π time periodicity of the spin-orbit equations of motion the polarization field, taken at a sequence of increasing time values θ,θ+2π,θ+4π,…, gives a sequence of polarization fields, called the stroboscopic sequence. We show, by using the Birkhoff ergodic theorem, that under very general conditions the Cesàro averages of that sequence converge almost everywhere on phase space to a polarization field which is 2π-periodic in time. This fulfills the main aim of this paper in that it demonstrates that the tracking algorithm for stroboscopic averaging, encoded in the program SPRINT and used in the study of spin motion in storage rings, is mathematically well-founded. The machinery developed is also shown to work for the stroboscopic average of phase space densities associated with the orbital dynamics. This yields a large family of periodic phase space densities and, as an example, a quite detailed analysis of the so-called betatron motion in a storage ring is presented.     

Coherent single electron spin control in a slanting Zeeman field

We consider a single electron in a 1D quantum dot with a static slanting Zeeman field. By combining the spin and orbital degrees of freedom of the electron, an effective quantum two-level (qubit) system is defined. This pseudospin can be coherently manipulated by the voltage applied to the gate electrodes, without the need for an external time-dependent magnetic field or spin-orbit coupling. Single-qubit rotations and the controlled-NOT operation can be realized. We estimated the relaxation (T1) and coherence (T2) times and the (tunable) quality factor. This scheme implies important experimental advantages for single electron spin control.     

Electron-spin and electron-orbital dependence of the tunnel coupling in laterally coupled double vertical dots

We employ a new laterally coupled, vertical double dot with a tunable tunnel-coupling gate in a parallel configuration to study the electron spin and orbital dependence of quantum mechanical tunnel coupling on the size of the honeycomb vertices in the small electron numbers regime. We find a transition from the weak coupling regime, where fluctuations in tunnel coupling due to varying electron configuration dominate the anticrossings, to a regime where the two dots coalesce. We apply a magnetic field to ascertain the orbital angular momenta of the Fermi surface eigenstates, which correlate with anticrossing size, and we identify spin pairs with congruent behavior.