Electron Spin Precession Research Articles

Direct detection of optically-induced microwave spin precession in Fe(III) halogenates

Picosecond optical excitation of electron spin precession has been observed at room temperature by detecting an induced EMF for acetone solutions of tetraethylammonium iron(III) tetrachloride ((C 2H 5) 4NFeCl 4), tetraethylammonium iron(III) tetrabromide ((C 2H 5) 4NFeBr 4), and iron(III) trichloride (FeCl 3). A detection bandwidth of 7 GHz allowed the free induction decay (FID) signal with a sub-nanosecond decay time. Magnetic field dependence of the FID frequency for (C 2H 5) 4NFeCl 4 is well fitted with the Zeeman energy splitting given by the S= 5 2 spin Hamiltonian with a g-value of 2.015±0.007 and a zero fine structure constant. A Fourier transform of the FID waveform shows a Lorentzian shape. The dephasing time T 2 coincides with the longitudinal decay time T 1 and is independent of the magnetic field, indicating that the spin system we studied is under the motional narrowing regime. The optical technique allows Fourier transformed (FT) ESR without microwave excitation at an arbitrary magnetic field.

Realization of a Room-Temperature Spin Dynamo: The Spin Rectification Effect

We demonstrate a room-temperature spin dynamo where the precession of electron spins in ferromagnets converts energy from microwaves to a bipolar current of electricity. The current/power ratio is at least 3 orders of magnitude larger than that found previously for spin-driven currents in semiconductors. The observed bipolar nature and intriguing symmetry are fully explained by the spin rectification effect via which the nonlinear combination of spin and charge dynamics creates dc currents.

Gate tunability of stray-field-induced electron spin precession in aGaAs∕InxGa1−xAsquantum well below an interdigitated magnetized Fe grating

Time-resolved Faraday rotation is used to measure the coherent electron spin precession in a $\mathrm{Ga}\mathrm{As}∕{\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum well below an interdigitated magnetized Fe grating. We show that the electron spin precession frequency can be modified by applying a gate voltage of opposite polarity to neighboring bars. A tunability of the precession frequency of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}∕\mathrm{V}$ has been observed. Modulating the gate potential with a gigahertz frequency allows the electron spin precession to be controlled on a nanosecond timescale.

Electronic Spin Precession and Interferometry from Spin-Orbital Entanglement in a Double Quantum Dot

A double quantum dot inserted in parallel between two metallic leads can entangle the electron spin with the orbital (dot index) degree of freedom. An Aharonov-Bohm orbital phase can be transferred to the spinor wave function, providing a geometrical control of the spin precession around a fixed magnetic field. A fully coherent behavior occurs in a mixed orbital-spin Kondo regime. Evidence for the spin precession can be obtained, either using spin-polarized metallic leads or by placing the double dot in one branch of a metallic loop.

Strain effects during coherent spin transport via moving quantum dots

AbstractThe geometry of the sample is shown to dictate the importance of strain on the spin‐orbit splitting of the conduction band for electrons transported via dynamic quantum dots (DQDs) generated by surface acoustic waves (SAWs). Photoluminescence measurements monitoring the Larmor precession of electron spins in the absence of an applied magnetic field are used to characterize the spin‐orbit splitting of the conduction band. We show that, in thick quantum wells (QWs), the Dresselhaus spin‐orbit contribution to the precession frequency reduces and may become comparable to the one associated with the strain field induced by the SAW. We attribute deviations in the measured precession frequency to strain effects, which become more pronounced for QWs located closer to the surface because of the larger SAW strain field. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonuniform Rashba-Dresselhaus spin precession along arbitrary paths

Electron spin precession in nonuniform Rashba-Dresselhaus two-dimensional electron systems along arbitrary continuous paths is investigated. We derive an analytical formula to describe the spin vectors (expectation values of the injected spin) in such conditions using a contour-integral method. The obtained formalism is capable of dealing with the nonuniformity of the Rashba spin-orbit field due to the inherent random distribution of the ionized dopants, and can be applied to curved one-dimensional quantum wires. Interesting examples are given, and the modification to the spin precession pattern in a Rashba-Dresselhaus channel when taking the random Rashba field into account is shown.

Experimental realization of a ballistic spin interferometer based on the Rashba effect using a nanolithographically defined square loop array

The gate-controlled electron spin interference was observed in nanolithographically defined square loop (SL) arrays fabricated using In$_{0.52}$Al$_{0.48}$As/In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As quantum wells. In this experiment, we demonstrate electron spin precession in quasi-one-dimensional channels that is caused by the Rashba effect. It turned out that the spin precession angle $\theta$ was gate-controllable by more than 0.75$\pi$ for a sample with $L=1.5\mu$m, where $L$ is the side length of the SL. Large controllability of $\theta$ by the applied gate voltage as such is a necessary requirement for the realization of the spin FET device proposed by Datta and Das [Datta {\it et. al.}, Appl. Phys. Lett. {\bf 56}, 665 (1990)] as well as for the manipulation of spin qubits using the Rashba effect.

Cavity enhanced Faraday rotation of semiconductor quantum dots

Dielectric vertical cavities are used to study the spin dynamics of molecularly self-assembled colloidal CdSe quantum dots (QDs). A quality factor dependent enhancement of Faraday rotation (∼25×) is observed and attributed to optically excited spins interacting with multiple passes of the cavity photons. This enables dynamical measurements at extremely low powers on relatively small numbers of quantum confined spins. In CdSe QDs, measurements reveal that spectroscopic contributions from exciton and electron spin precession depend on the power of excitation. We demonstrate that this scheme is amenable to chemically synthesized systems as a means to increase detection sensitivity.

COMBINED EFFECTS OF RASHBA SPIN-ORBIT INTERACTION STRENGTH AND ITS EXTENDED LENGTH IN QUANTUM WIRES

The combined effects of Rashba spin-orbit interaction (SOI) strength and the length of SOI excited region, which can be controlled by the external gate voltage deposited on the heterostructure, and on the electron spin precession in quasi-one-dimensional quantum wires are investigated by evaluating the relative conductance change, i.e. the ratio of difference of the conductances of the spin-up and spin-down polarized electrons to the total conductance. It is found that for proper wire width, electron spin precession can be smoothly achieved by co-adjusting the SOI strength and the length of SOI excited region. However, for wide quantum wire and strong SOI, the electron spin precession is significantly reduced due to the appearance of inter-subband mixing.

Spin relaxation mechanism of strain-induced GaAs quantum dots studied by time-resolved Kerr rotation

We observed electron spin precession under magnetic field in single-layer quantum dots (QDs) by highly sensitive time-resolved Kerr rotation measurement. The spin lifetime is longer than that for the quantum well (QW). This is a result of the additional spatial confinement of electrons in QDs. Below $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the spin lifetime is almost constant, and is 7 times shorter than the carrier lifetime. This suggests that the strong electron-hole exchange interaction dominates over the electron spin lifetime in QDs at low temperature.

Effects of strain, electric, and magnetic fields on lateral electron-spin transport in semiconductor epilayers

We construct a spin-drift-diffusion model to describe spin-polarized electron transport in zinc-blende semiconductors in the presence of magnetic fields, electric fields, and off-diagonal strain. We present predictions of the model for geometries that correspond to optical spin injection from the absorption of circularly polarized light, and for geometries that correspond to electrical spin injection from ferromagnetic contacts. Starting with the Keldysh Green's function description for a system driven out of equilibrium, we construct a semiclassical kinetic theory of electron-spin transport in strained semiconductors in the presence of electric and magnetic fields. From this kinetic theory we derive spin-drift-diffusion equations for the components of the spin-density matrix for the specific case of spatially uniform fields and uniform electron density. We solve the spin-drift-diffusion equations numerically and compare the resulting images with scanning Kerr microscopy data of spin-polarized conduction electrons flowing laterally in bulk epilayers of $n$-type GaAs. The spin-drift-diffusion model accurately describes the experimental observations. We contrast the properties of electron-spin precession resulting from magnetic and strain fields. Spin-strain coupling depends linearly on electron wave vector, and spin-magnetic field coupling is independent of electron wave vector. As a result, spatial coherence of precessing spin flows is better maintained with strain than with magnetic fields, and the spatial period of spin precession is independent of the applied electrical bias in strained structures whereas it is strongly bias-dependent for the case of applied magnetic fields.

Highly sensitive time-resolved Kerr rotation measurements on single-layer quantum dots

We construct the highly sensitive Kerr rotation system to observe electron spin precession in single-layer quantum dots. The system consists of a photoelastic modulator, a balanced detector and tandem double lock-in amplifiers. The angle resolution achieves 5×10 −6 degree, and is highest among the reported so far. To evaluate the possibility of measuring spin relaxation in single-layer quantum dots, we studied the strain-induced GaAs quantum dots. We observe successfully the electron spin precession in the single-layer quantum dots under resonant excitation.

Studies on the temperature and energy dependence of g factor in GaAs by femtosecond laser absorption quantum beats

Time-resolved elliptically polarized pump-probe spectroscopy developed by us is used to study spin relaxation dynamics of electrons in an intrinsic GaAs under an external magnetic field. Absorption quantum beats are observed and attributed to Larmor precession of electron spin around the magnetic field. The frequency of this quantum beat is used as a new method to measure electronic g factor with high accuracy. In this paper, this new method is utilized to investigate the temperature and energy dependence of electronic g factor in the intrinsic GaAs. It is found that electronic g factor increases with the temperature and energy of electrons, which is guitc different from the dependence predicted by k·p theory. An empirical formula is given to describe the temperature and energy dependence of g factor by fitting experimental data.

Nearly degenerate time-resolved Faraday rotation in an interacting exciton system

We report experimental studies of nearly degenerate time-resolved Faraday rotation (TRFR) of electron spin precession in an interacting exciton system. We show that many-body interactions between excitons strongly modify the TRFR response through the coupling of the spin coherence to two-exciton states. The striking difference between TRFR and transient differential absorption (DA) further reveals that exciton-exciton interactions play crucial but qualitatively different roles in TRFR and DA.

Resonant Spin Splitting in III-V Semiconductors

We present flrst-principles studies of the zero fleld spin splitting of energy bands in typical III{V semiconductors. Our calculations reveal that the strain induces linear-k spin splitting of the conduction band in the i point, which is linear in strain, and determine the magnitude of the so-called acoustic phonon constant that characterizes the magnitude of the spin splitting. In addition, we show that optical phonons lead to spin-∞ip processes and we present quantitative results for the spin-phonon deformation potentials in GaAs. Most importantly, the calculations show that the linear-k spin splitting can be resonantly enhanced when bands cross in a particular point of the Brillouin zone. This resonant enhancement of the bulk inversion asymmetry coupling constant by more than one order of magnitude was observed in both valence and conduction bands and can be steered by the application of the external stress. This allows tailoring of the spin relaxation and spin precession of conduction electrons in nanostructures to a much larger extent than was hitherto assumed.

Spin-polarized electron transport through nanometer-scale Al grains

We investigate spin-polarized electron tunneling through ensembles of nanometer-scale Al grains embedded between two Co reservoirs at 4.2 K, and observe tunneling-magnetoresistance (TMR) and effects from spin precession in the perpendicular applied magnetic field (the Hanle effect). The spin-coherence time $({T}_{2}^{\ensuremath{\star}})$ measured using the Hanle effect is of order ns. The dephasing is attributed to electron spin precession in local magnetic fields. Dephasing process does not destroy TMR, which is strongly asymmetric with bias voltage. The asymmetric TMR is explained by spin relaxation in the Al grains and asymmetric electron dwell times.

Spin precession of electrons reflected from a ferromagnetic surface

The reflection of spin-polarized electrons from polycrystalline ferromagnetic films is investigated in a “complete” scattering experiment, where a spin-polarized electron source is combined with a spin polarization detector. The spin polarization vector of the electrons is shown to precess during reflection about the magnetization direction of the ferromagnetic film. Consequently, a torque is generated on the magnetization by the reflected electrons.

Measuring Electronic Spin-Precession in Magnetic Thin Films by Spin-Polarized Low-Energy Electron Microscopy

Extract HTML view is not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button. Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Optoelectronic control of spin dynamics at near-terahertz frequencies in magnetically doped quantum wells

We use time-resolved Kerr rotation to demonstrate the optical and electronic tuning of both the electronic and local moment (Mn) spin dynamics in electrically gated parabolic quantum wells derived from II-VI diluted magnetic semiconductors. By changing either the electrical bias or the laser energy, the electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic field of 3 T and at a temperature of 5 K. The corresponding range of the electrically-tuned effective electron g-factor is an order of magnitude larger compared with similar nonmagnetic III-V parabolic quantum wells. Additionally, we demonstrate that such structures allow electrical modulation of local moment dynamics in the solid state, which is manifested as changes in the amplitude and lifetime of the Mn spin precession signal under electrical bias. The large variation of electron and Mn-ion spin dynamics is explained by changes in magnitude of the sp−d exchange overlap.

Long-lived spin coherence states in semiconductor heterostructures

We study evolution of electron spin coherence having nonhomogeneous direction of spin polarization vector in semiconductor heterostructures. It is found that the electron spin relaxation time due to the D'yakonov-Perel' relaxation mechanism essentially depends on the initial spin polarization distribution. This effect has its origin in the coherent spin precession of electrons diffusing in the same direction. We predict a long spin relaxation time of a novel structure: a spin coherence standing wave and discuss its experimental realization.