Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.     

Spin Dynamics in Disordered Solids

Modern state of studies in spin dynamics of statically disordered media is presented. Next four fundamental problems are attended mainly: 1) delocalization of nuclear polarization in subsystem of impurity nuclei (it is exemplified in model nuclear spin system ⁸Li-⁶ Li in the LiF single crystal) 2) nuclear relaxation via paramagnetic impurities in crystals of arbitrary space dimension d 3) free induction decay and EPR line form function at d ≤ 3; and 4) form function of the hole, burned on the wing of the dipolar EPR line.     

Two-Hole Dynamics in Spin Ladders

We present an analytic theory for the energy spectrum of a two-leg spin ladder doped with two holes. Starting from a pseudo-fermion-bond-boson representation of the corresponding t _1,2 –J _1,2 Hamiltonian we apply a diagrammtic approach adapted to the limit of strong rung coupling, which includes both, the coupling of holes to the spin background as well as the two-hole interactions. The two-hole spectrum is calculated and the formation of bound states is discussed. Additionally the evolution of the spin gap of the ladder upon doping is analyzed. A comparison with existing exact diagonalization data is presented and good agreement is found.     

Spin Dynamics in ZnO-Based Materials

In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.     

Possible Spin Pumping Effects on Spin Torque Induced Magnetization Switching in Magnetic Tunneling Junctions

Dependence of spin torque induced magnetization switching upon interfacial insulating layers properties of magnetic tunneling junctions (MTJ) are studied. For the same magnetic properties and patterning geometric dimensions, changes in MTJ interfacial insulating layers properties reveal interesting magnetization switching behaviors. These behaviors cannot be explained by conventional Landau-Lifshitz-Gilbert equation with a spin torque term and an intrinsic ferromagnetic relaxation damping. However the magnetization switching dynamics can be understood through assumption of spin pumping effects in magnetic tunneling junctions. This is not only important for fundamental understanding of spin and electronic transport in MTJ but also important for practical trade-offs between critical switching current and MTJ resistance for spin torque random access memory.     

Coherent Spin Dynamics and Spin Polarized Transport in Doped Semiconductors

We provide an overview of measurements that elucidate the effects of interactions, quantum confinement, reduced dimensionality, and interfacial geometries on coherent electronic spin dynamics and spin transport in doped semiconductors. The experiments focus on a variety of doped semiconductor systems, ranging from bulk n-GaAs crystals to modulation doped II-VI magnetic semiconductor quantum wells. In particular, the latter provide model systems in which electron gases are strongly exchange-coupled to an engineered distribution of magnetic moments, hence allowing one to systematically tailor spin interactions between confined electronic states, magnetic ions, and nuclei. Two complementary techniques including state-of-the-art spin dynamical probes having high temporal (~100 fs) and spatial (~100 nm) resolution, and low-temperature magneto-transport, are used to survey a variety of physical phenomena in these systems.     

Spin Pumping Effect in Superfluid 3He A1

A fountain effect is a common phenomenon in both ³He and ⁴He superfluids. Unique to superfluid ³He is the magnetic fountain effect, which has been used to determine the spin direction of the condensate in ³He A₁ phase. Here we present a pressure driven fountain effect in A₁ phase. The experimental cell is composed of a large reservoir connected to a small detector chamber through superleak channels of width of 20 μm. One wall of the detector chamber houses a movable circular 6 μm thick membrane which serves as a sensitive capacitive pressure sensor and also acts as a spin pump. In A₁ phase, a DC voltage applied on the capacitor induces a simultaneous mass and spin superfluid current into the small chamber. After equilibration, removal of the DC voltage causes a sudden pressure drop followed by a slow relaxation. The sudden drop is a consequence of reversed superfluid flow through the superleak. The observed decay times during the slow relaxation agree with those obtained in magnetically induced spin flow experiment. These observations show that the slow relaxation stems from spin relaxation in the absence of applied field gradient.     

Spin Dynamics of 3He in Aerogel

We report continuous-wave NMR measurements on superfluid ³ He contained inside high porosity aerogel at 3.24 MPa, in a 28.4 mT magnetic field, and down to 0.9 mK. Three different purities of ³ He were used: pure ³ He, ³ He with enough ⁴ He to replace the first localized layer, and ³ He with enough ⁴ He to replace both localized layers. Below 2.26mK, the NMR spectrum does not consist of a single Lorentzian, but a distribution of NMR resonant frequencies both above and below the Larmor frequency, _L . Upon cooling, the component of the spectrum at frequencies below the Larmor frequency moves to higher frequencies over a narrow temperature range and does not return to < _L upon warming. The higher frequency component of the spectrum displays two resolvable peaks which become a complex structure upon removal of the localized ³ He spins. The spectrum changes as the magnetic field is rotated about the axis of the aerogel sample, indicating that the ³ He is able to detect a preferred direction in the aerogel. The shape of the spectrum shows no rotation or magnetic field hysteresis. We find that the average frequency shifts of our NMR spectra do not depend on the presence of the localized ³ He spins. These frequency shifts and the presence of a temperature-independent magnetization for the liquid ³ He suggest that the aerogel stabilizes a single equal-spin-pairing state in our experiments.     

Spin Dynamics in Narrow-Gap Semiconductor Epitaxial Layers

We report on investigation of the spin dynamics in InAs and InSb films grown on GaAs at a temperature range from 77 K to 290 K. For both materials, the large lattice mismatch with the GaAs substrate results in the formation of an interface accumulation layer with a large defect concentration, which strongly affects the spin relaxation in these areas. Moreover, the native surface defect in the InAs films resulted in an additional charge accumulation layer with high conductivity, but very short spin lifetime. In contrast, in InSb layers, the surface states introduce a depletion region. We have correlated the spin relaxation with a multi-layer analysis of the transport properties, and find that in a 1 μm thick InAs film, approximately 70% of the total current flows through the interface and surface accumulation layers, which have sub-picosecond lifetimes, whereas in InSb films of the same thickness, the semiconducting layer carries more than 90% of the total current, and the spin lifetime in the accumulation layer is only slightly less than that of the central semiconducting layer. We suggest that InSb could be a more attractive candidate for spintronic applications than InAs.     

Tunable Spin Current Through a Vibrating Molecular Quantum Dot with Spin Bias and Spin Flip Scattering

By applying the Lang-Firsov canonical transformation and the Keldysh nonequilibrium Green’s function approach, the effect of the spin-flip scattering on the spin current through a vibrating molecular quantum dot with spin bias is theoretically investigated. We can obtain the spin current from the output terminal, and find that the sign of the spin current can be changed by adjusting the spin-flip strength, and a pure spin current can be generated via the charge bias and the spin bias. In the presence of the electron-phonon interaction, the positions of the current peaks are shifted and the spin current is remarkably suppressed, which leads to the Franck-Condon blockade. Furthermore, it is found that the competition between the EPI and the spin-flip scattering jointly determines the character of the spin current. These results offer us a way to manipulate the spin current in the spin current setup. The proposed device should be realizable with use of the present technology at low temperature.     

Novel Aspects of Spin-Polarized Transport and Spin Dynamics

There is a renewed interest to study spin-polarized transport and spin dynamics in various electronic materials. Motivation to examine the spin degrees of freedom (mostly in electrons, but also in holes and nuclei) comes from various sources: ranging from novel applications which are either not feasible or ineffective with conventional electronics, to using the spin-dependent phenomena for exploring fundamental properties of solid-state systems. Taken in a broader context, term spintronics is addressing various aspects of these efforts and stimulating new interactions between different subfields of condensed matter physics. Recent advances in material fabrication made it possible to introduce the nonequilibrium spin in novel class of systems, including ferromagnetic semiconductors, high temperature superconductors, and carbon nanotubes—which leads to a question of how could such a spin be utilized. For this purpose it is important to extend the understanding of spin-polarized transport and spin dynamics to consider inhomogeneous systems, various heterostructures, and the role of interfaces. This article presents some views on novel aspects of spin-polarized transport and spin dynamics (referring also to the topics which were addressed at the conference Spintronics 2001) and suggests possible future research directions.     

Josephson Tunneling Current in Spin Polaron Theory

Using an effective Hamiltonian derived from spin polaron theory, the Josephson tunneling current between two high temperature superconductors is calculated. The method makes use of the Matsubara Green??s function and a canonical transformation analogous to the Schrieffer?CWolff transformation. The Josephson tunneling current is then obtained in terms of the energy gap function and the tunneling matrix, and it was found to have some similarities with the BCS result.     

Simulations of Non-linear Spin Dynamics in Spin Polarized Dilute 3He–4He Mixtures

Studies of spin dynamics of highly polarized dilute mixtures of ³He in superfluid ⁴He have been performed by various researchers over the past three decades. One series of experiments performed at Cornell University in the early 1990’s revealed a novel long timescale excitation. We present the numerical solution of the non-linear Leggett spin dynamics equation in one spatial dimension subject to boundary conditions consistent with the Cornell experiments. Experimentally observed phenomena are composed of trains of bursts in the transverse magnetization lasting several seconds. The simulations capture the time evolution of the individual bursts localized in time. Preliminary results of two dimensional simulations are also presented.     

A Critical Examination of the Spin Dynamics in High-Tc Cuprates

A critical examination of the spin dynamics in high-T _c cuprates is made in the light of recent inelastic neutron scattering results obtained by different groups. The neutron data show that incommensurate magnetic peaks in YBCO belong to the same excitation as the resonance peak observed at (/a, /a). Being observed only in the superconducting state, the incommensurability is rather difficult to reconcile with a stripe picture. We also discuss the link between the resonance peak spectral weight and the superconducting condensation energy.     

Spin Dynamics in the Ferromagnetic Phase of Colossal Magnetoresistive Oxides

A review is given that focuses on the spin dynamics in the ferromagnetic regime of the magnetoresistive oxides. At small wave vectors the quadratic dispersion relation is remarkably isotropic throughout the composition range, with a gap that is too small to measure with conventional neutron scattering techniques. At larger wave vectors the spin waves are strongly damped in the ground state, in contrast to expectations based on a simple half-metallic band structure, while the temperature dependence of the damping and renormalization of the spin waves is anomalous. An unusual spin-diffusion component to the fluctuation spectrum develops as the Curie temperature is approached, which appears to be related to the formation of polarons in the system and the consequent charge localization. Near and above T_C the diffuse scattering from polarons is directly observed, as well as the correlations between the polarons.     

Spin Dynamics of a Fermi Liquid in an Electric Field

The influence of an external electric field on the spin dynamics of an electrically neutral Fermi liquid is considered, the mechanism of such an influence being the relativistic spin-orbital interaction. As a result, Leggett's equations for the spin dynamics of weakly polarized Fermi liquids are generalized to the case of non-zero external electric field. In addition, we obtained the transverse spin dynamics equation for strongly spin-polarized liquids in an electric field at zero temperature. In both situations covariant derivatives depending on the electric field are shown to be substituted for spatial gradients in line with the SU(2) gauge invariance of the microscopic Hamiltonian. The new equations are applied to the study of spin flow along a channel, where an electric field is found to bring about an additional phase shift of the order of magnitude of the phase shift in superfluid ³He-B but growing with time.     

Pumping Current Noise in Ballistic Nanostructures: Acoustoelectric Current in a Point Contact

We consider current noise in a non-biased ballistic nanostructure irradiated by an AC field, which is assumed to be a stationary random process to imitate time averaging in the experiment. The AC field creates a current, which contains DC and AC components, the latter vanishing after time averaging. Correspondingly, the correlator describing current noise has two contributions. The first contribution to it is responsible for the fluctuations of the DC current. These fluctuations happen because of electron inelastic transitions between scattering states. Transitions agitated by the thermal bath lead to a thermal noise spectrum, which disappears at zero temperature; its magnitude is renormalized compared to equilibrium. Transitions initiated by the AC field lead to a noise spectrum, which is a convolution of the thermal one and that of the AC field. The magnitude of this noise is proportional to the power of the AC field and is finite at zero temperature. The second contribution to the noise correlator is due to fluctuations of the AC current. This noise does not depend on temperature, and its spectrum reproduces that of the AC field.     

First-Principle Calculation of Spin Current in Arsenic Nitride Nanoribbons

Journal of Superconductivity and Novel Magnetism - In this paper, first-principle calculation methods based on density functional theory are used to study the electronic structure of arsenic...     

Excitation of Nonlinear Spin Oscillations in an Antiferromagnet under the Action of Pulsed Terahertz Pumping

Technical Physics Letters - We present a model of excitation of nonlinear spin oscillations in an antiferromagnet under the action of terahertz pulses of an electromagnetic field. It is shown that,...