GaAs Spin Research Articles

Room temperature spin relaxation length in spin light-emitting diodes

We investigate the spin relaxation length in GaAs spin light-emitting diode devices under drift transport at room temperature. The spin-polarised electrons are injected through a MgO tunnel barrier from a Fe/Tb multilayer in magnetic remanence. The decrease in circular polarization with increasing injection path length is investigated and found to be exponential, supporting drift-based transport. The spin relaxation length in our samples is 26 nm, and a lower bound for the spin injection efficiency at the spin injector/GaAs interface is estimated to be 25 ± 2%.

Preparation characterization of MnSb–GaAs spin LED

Spin light-emitting-diodes (spin-LEDs) composed of an epitaxial MnSb layer having in-plane magnetization and GaAs–AlGaAs double hetero-structures were prepared using a two-chamber MBE system. Electroluminescence extracted through the cleaved side walls showed elliptically polarized light emission with circular polarization of 2.1% at 30K without an auxiliary external magnetic field.

GaAs spin injector microcantilever probe assembly via a releasable “epitaxial patch technology”

We demonstrate that GaAs spin injector microcantilever probes can be produced using a novel releasable “epitaxial patch technology” assembly approach as apposed to a monolithic fabrication route i.e. surface/bulk micromachining. The GaAs microcantilevers are attached to fused silica support wafers; thus taking advantage of the optical and spin properties of GaAs and the mechanical properties of the silica. The microcantilever probes have been characterized for their spin injection properties and also used as atomic force microscopy (AFM) probes to evaluate their imagining capabilities. The assembly involved the production of released epitaxial semiconductor “patches” having a micrometer thickness. These patches are then assembled onto a pre-fabricated fused silica support wafer. AFM characterization revealed a resonant frequency and quality factor equal to commercial silicon-based AFM probes together with high quality images; the spin injection characterization demonstrated injected photocurrents of tens of nA and a spin polarization of ∼15%.

Oscillatory spin-polarized tunnelling from silicon quantum wells controlled by electric field

Spin-dependent electronic transport is widely used to probe and manipulate magnetic materials and develop spin-based devices. Spin-polarized tunnelling, successful in ferromagnetic metal junctions, was recently used to inject and detect electron spins in organics and bulk GaAs or Si. Electric field control of spin precession was studied in III-V semiconductors relying on spin-orbit interaction, which makes this approach inefficient for Si, the mainstream semiconductor. Methods to control spin other than through precession are thus desired. Here we demonstrate electrostatic modification of the magnitude of spin polarization in a silicon quantum well, and detection thereof by means of tunnelling to a ferromagnet, producing prominent oscillations of tunnel magnetoresistance of up to 8%. The electric modification of the spin polarization relies on discrete states in the Si with a Zeeman spin splitting, an approach that is also applicable to organic, carbon-based and other materials with weak spin-orbit interaction.

Coherent population trapping of an electron spin in a single negatively charged quantum dot

Coherent population trapping (CPT) refers to the steady-state trapping of population in a coherent superposition of two ground states that are coupled by coherent optical fields to an intermediate state in a three-level atomic system1. Recently, CPT has been observed in an ensemble of donor-bound spins in GaAs (ref. 2) and in single nitrogen-vacancy centres in diamond3 by using a fluorescence technique. Here, we report the demonstration of CPT of an electron spin in a single quantum dot. The observation demonstrates both the CPT of an electron spin and the successful generation of Raman coherence between the two spin ground states of the electron4,5,6. This technique can be used to initialize, at about a gigahertz rate, an electron spin state in an arbitrary superposition by varying the ratio of the Rabi frequencies between the driving and probe fields. The results show the potential importance of charged quantum dots for a solid-state approach to the implementation of electromagnetically induced transparency7,8, slow light9, quantum information storage10 and quantum repeaters11,12.

Penetration depth model for optical alignment of nuclear spins in GaAs

A framework for predicting bulk NMR quantities in semi-insulating GaAs under optical alignment conditions is developed by combining literature penetration depth data with simple kinetic equations. The model accounts for the major photoconductivity and NMR intensity variations with photon energy, including the peak near 1.5 eV. With the fitting parameters fixed, the photon-flux dependence of the NMR intensity is predicted quantitatively up to an energy shift. At the highest fluxes, we see the NMR intensity level off, suggesting that it is possible to completely fill the relevant electronic reservoir. The model also quantitatively fits the time dependence of the light-induced hyperfine shift. The experimental and modeling results have implications toward understanding the microscopic mechanism, and are consistent with nuclear polarization via bound electrons. Finally, the model makes experimentally testable predictions for the photon energy and flux dependences of the spin temperature and hyperfine shift of the NMR line.

Anisotropic Spin Transport in (110) GaAs Quantum Wells

Mobile piezoelectric potentials are used to coherently transport electron spins in GaAs (110) quantum wells (QW) over distances exceeding 60 microm. We demonstrate that the dynamics of mobile spins under external magnetic fields depends on the direction of motion in the QW plane. This transport anisotropy is an intrinsic property of moving spins associated with the bulk inversion asymmetry of the underlying GaAs lattice.

Magnetoresistance of fully epitaxial MnAs∕GaAs lateral spin valves

The authors report the growth, fabrication, and characterization of lateral MnAs∕GaAs spin valves where Schottky tunnel barriers enable all-electrical spin injection and detection. Through a difference in geometric aspect ratio for the MnAs contacts, parallel and antiparallel alignment between the contact magnetization is obtained by varying the external magnetic field. Temperature-dependent conductivity measurements indicate that tunneling is the dominant transport mechanism for the MnAs∕GaAs Schottky diode polarizer and analyzer contacts. Peak magnetoresistances of 3.6% at 10K and 1.1% at 125K are observed for a 0.5μm channel length spin valve.

Passive magnetic shielded spin polarized electron source with optical electron polarimeter

A new GaAs(100) spin polarized electron source with an optical polarimeter, which is employed in the field of polarized electron and gas atom collision, is presented in detail. The apparatus is passive-magnetic-shielded by a box and a cylinder made of nickel–iron–molybdenum soft magnetic alloy without Helmholtz coil arrangement. And a uniformly distributed residual magnetic field of less than 5×10−7 T is obtained near the collision area. The spin polarized electron beam is transmitted and focused onto collision point from photocathode by a set of electron optics with more than 25% transmission 95 cm distance through an 1 mm diameter aperture. Construction and operation of the apparatus, such as vacuum and magnetic shielding system, photocathode, laser optics, electron optics and polarimeter are discussed. The polarization of the spin polarized electron beam is determined to be 30.8±3.5% measured with a He optical polarimeter.

Spin Noise Spectroscopy in GaAs

We observe the noise spectrum of electron spins in bulk GaAs by Faraday-rotation noise spectroscopy. The experimental technique enables the undisturbed measurement of the electron-spin dynamics in semiconductors. We measure exemplarily the electron-spin relaxation time and the electron Landé g factor in -doped GaAs at low temperatures and find good agreement of the measured noise spectrum with a theory based on Poisson distribution probability.

Optical polarization of nuclear spins in GaAs

The photon energy, irradiation time, and sample temperature dependences of laser-enhanced NMR spectra from semi-insulating GaAs at 9.4 T were measured. These data were used to test the existing model for the mechanism of optical nuclear polarization---namely, optical excitation of shallow donor states and subsequent spin diffusion of polarization throughout the bulk. Features of the present results, such as their uniqueness to semi-insulating GaAs and pumping data obtained via laser irradiation above the band gap, are inconsistent with this model. An additional model is proposed that includes localized paramagnetic centers as storage sites for electron spin polarization, but relies upon delocalized electron states to accomplish the bulk nuclear spin polarizations. The delocalized states could be free excitons.

Spatial imaging of magnetically patterned nuclear spins in GaAs

We exploit ferromagnetic imprinting to create complex laterally defined regions of nuclear spin polarization in lithographically patterned MnAs/GaAs epilayers grown by molecular beam epitaxy (MBE). A time-resolved Kerr rotation microscope with approximately 1 micron spatial resolution uses electron spin precession to directly image the GaAs nuclear polarization. These measurements indicate that the polarization varies from a maximum under magnetic mesas to zero several microns from the mesa perimeter, resulting in large (10**4 T/m) effective field gradients. The results reveal a flexible scheme for lateral engineering of spin-dependent energy landscapes in the solid state.

Relaxation of photoinjected spins during drift transport in GaAs

We studied the transport of photoinjected spins in GaAs by time-resolved photoluminescence measurements. At low temperatures, the spin polarization after drift transport of 4 μm is found to decrease as the applied electric field E increases to a few kV/cm, and it disappears when E exceeds 3 kV/cm. The origin of the field-dependent spin relaxation is discussed.

Transport and lifetime enhancement of photoexcited spins in GaAs by surface acoustic waves.

We demonstrate spin transport and spin lifetime enhancement in GaAs quantum wells induced by the traveling piezoelectric field of a surface acoustic wave (SAW). Spin transport lengths of about 3 microm corresponding to spin relaxation times during transport over 1 ns are observed, which are considerably longer than the exciton spin diffusion lengths in the absence of a SAW. The slow spin relaxation is attributed to a reduced electron-hole exchange interaction, when the carriers are spatially separated by the lateral potential modulation induced by the SAW.

Room-temperature ultrafast carrier and spin dynamics in GaAs probed by the photoinduced magneto-optical Kerr effect

The picosecond and subpicosecond dynamics of spins in intrinsic and n-doped GaAs is investigated at room temperature using the time-resolved, pump and probe, photoinduced, near-resonant magneto-optical Kerr effect between 1.44 and 1.63 eV. Three components with different temporal and spectral behavior are distinguished in both Kerr rotation and ellipticity, and their origin is discussed in relation with theoretical predictions and simulations. Two contributions are attributed to the splitting of the spin sublevels. The first one, present only as long as pump and probe pulses coincide in time, is a coherent response accounted for in terms of the optical Stark effect, whereas the longer decay of the second one, up to 35 ps, is attributed to electron spin relaxation. The third component is assumed to arise from the difference in population of the photoexcited states and decays within a time smaller than the pulse duration, which is attributed to the energy relaxation of electrons towards the bottom of the conduction band through carrier-carrier and carrier-phonon scattering.

Coupled quantum dots as quantum gates

We consider a quantum-gate mechanism based on electron spins in coupled semiconductor quantum dots. Such gates provide a general source of spin entanglement and can be used for quantum computers. We determine the exchange coupling $J$ in the effective Heisenberg model as a function of magnetic $(B)$ and electric fields, and of the interdot distance $a$ within the Heitler-London approximation of molecular physics. This result is refined by using $\mathrm{sp}$ hybridization, and by the Hund-Mulliken molecular-orbit approach, which leads to an extended Hubbard description for the two-dot system that shows a remarkable dependence on $B$ and $a$ due to the long-range Coulomb interaction. We find that the exchange $J$ changes sign at a finite field (leading to a pronounced jump in the magnetization) and then decays exponentially. The magnetization and the spin susceptibilities of the coupled dots are calculated. We show that the dephasing due to nuclear spins in GaAs can be strongly suppressed by dynamical nuclear-spin polarization and/or by magnetic fields.

Effect of spin split off level on hole subband states of GaAs/AlGaAs quantum wells

To investigate the bulk state mixing due to the presence of the quantum well interfaces, we employ an exact solution for the hole subband dispersion of a [001] symmetric GaAs/AlGaAs quantum well, within the 6×6 Luttinger model, which extends previous work by Andreani, Pasquarello and Bassani for the 4×4 case. Numerical results are presented for the hole subband states and in-plane dispersion curves of a 66 Å AlAs-GaAs-AlAs quantum well, which show anticrossing behaviour. The nature of anticrossings between the 1 st/2 nd and the 8 th/9 thsubbands is investigated by studying the bulk state decomposition of the envelope functions. In the vicinity of the 1 st/2 nd subband anticrossing, there is a negligible bulk GaAs spin split off band content. In contrast, in the region of the anticrossing for the higher 8 th and 9 th subband states, there is considerable GaAs spin split off band amplitude, as the GaAs spin split off band is nearby in energy.

The GaAs spin polarized electron source

The design, construction, operation, and performance of a spin polarized electron source utilizing photoemission from negative electron affinity (NEA) GaAs are presented in detail. A polarization of 43±2% is produced using NEA GaAs (100). The polarization can be easily modulated without affecting other characteristics of the electron beam. The electron beam intensity depends on the intensity of the exciting radiation at 1.6 eV; beam currents of 20 μA/mW are obtained. The source is electron optically bright; the emittance phase space (energy-area-solid angle product) is 0.043 eV mm2 sr. The light optics, electron optics, and cathode preparation including the GaAs cleaning and activation to NEA are discussed in depth. The origin of the spin polarization in the photoexcitation process is reviewed and new equations describing the depolarization of photoelectrons in the emission process are derived. Quantum yield and polarization measurements for both NEA and positive electron affinity surfaces are reported. The important considerations for interfacing he polarized electron source to an experiment are illustrated by its application to polarized low energy electron diffraction (PLEED). The advantages of this spin polarization modulated electron gun for PLEED are clearly demonstrated by sample PLEED results for W(100) and ferromagnetic Ni(110). A comparison with other polarized electron sources shows that the GaAs spin polarized electron source offers many advantages for a wide range of applications.