Time- and spatially-resolved Faraday rotation spectroscopy is used to measure the magnitude and direction of the momentum-dependent spin splitting in strained InGaAs epilayers. The epilayers are lattice-matched to the GaAs substrate and designed to reduce inhomogeneous effects related to strain relaxation. Measurements of momentum-dependent spin splitting as a function of electron spin drift velocity along [100], [010], [110] and [1$\overline{1}$0] directions enable separation of isotropic and anisotropic effective magnetic fields that arise from uniaxial and biaxial strain along $\langle$110$\rangle$. We relate our findings to previous measurements and theoretical predictions of spin splitting for inversion symmetry breaking in bulk strained semiconductors.

The potential for spin-based information processing in spintronic devices has spurred significant research on spin coherence in semiconductors. We review some recent advances in the physics of semiconductor spin coherence, focusing on phenomena in both bulk and heterostructure systems. Coherent spin ensembles can be generated and manipulated optical, electrical, and magnetic interactions. Optical measurement techniques enable both temporal and spatial resolution of spin dynamics, allowing observation of electrical control and generation of spin polarization, including the recent discovery of the spin Hall effect. Spin dynamics are also studied for spins confined in quantum dots.

The effect of uniaxial tensile strain on spin coherence in n-type GaAs epilayers is probed using time-resolved Kerr rotation, photoluminescence, and optically detected nuclear magnetic resonance spectroscopies. The band gap, electron spin lifetime, electron g factor, and nuclear quadrupole splitting are simultaneously imaged over millimeter scale areas of the epilayers for continuously varying values of strain. All-optical nuclear magnetic resonance techniques allow access to the strain-induced nuclear quadrupolar resonance splitting in field regimes not easily addressable using conventional optically detected nuclear magnetic resonance.

Time-resolved Kerr rotation spectroscopy as a function of pump-probe distance, voltage and magnetic field is used to measure the momentum-dependent spin splitting energies in GaAs and InGaAs epilayers. The strain of the samples can be reproducibly controlled in the cryostat using three- and four-point bending applied with a mechanical vise. We find that the magnitude of the spin splitting increases linearly with applied tension and voltage. A strain-drift diffusion model is used to relate the magnitude of the measured spin-orbit splitting to the amount of strain in the sample.

The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

An electron propagating through a solid carries spin angular momentum in addition to its mass and charge. Of late there has been considerable interest in developing electronic devices based on the transport of spin that offer potential advantages in dissipation, size and speed over charge-based devices1. However, these advantages bring with them additional complexity. Because each electron carries a single, fixed value (- e) of charge, the electrical current carried by a gas of electrons is simply proportional to its total momentum. A fundamental consequence is that the charge current is not affected by interactions that conserve total momentum, notably collisions among the electrons themselves2. In contrast, the electron's spin along a given spatial direction can take on two values, ± ℏ/2 (conventionally ↑,↓), so that the spin current and momentum need not be proportional. Although the transport of spin polarization is not protected by momentum conservation, it has been widely assumed that, like the charge current, spin current is unaffected by electron–electron (e–e) interactions. Here we demonstrate experimentally not only that this assumption is invalid, but also that over a broad range of temperature and electron density, the flow of spin polarization in a two-dimensional gas of electrons is controlled by the rate of e–e collisions.

We report on the electromagnetic response of digital ferromagnetic heterostructures sDFHd: systems with d-doped MnAs layers separated by GaAs spacers of variable thickness syd. The gross features of the infrared conductivity of DFH samples are consistent with the notion that these digital structures are GaAs/ Ga 1˛xMnxAs superlattices. This conclusion is supported by a combination of spectral weight analysis and effective medium theory. The optical properties of DFH also provide insights into the evolution of their critical temperature with GaAs spacing. In DFH a low-lying gap materializes in the energy dependent conductivity, which is interpreted as a mobility gap resulting from Anderson localization.

We describe a new means for all-electrical generation of spin polarization in semiconductors. In contrast with spin injection of electrons by tunneling through a reverse-biased Schottky barrier, we observe accumulation at the metal-semiconductor interface of forward-biased ferromagnetic Schottky diodes, which is consistent with a theory of spin-dependent reflection off the interface. Spatiotemporal Kerr microscopy is used to image the electron spin and the resulting dynamic nuclear polarization that arises from the nonequilibrium carrier polarization.

We demonstrate a scheme for optically patterning nuclear-spin polarization in semiconductor∕ferromagnet heterostructures. A scanning time-resolved Kerr rotation microscope is used to image the nuclear-spin polarization that results when GaAs∕MnAs epilayers are illuminated with a focused laser having a Gaussian profile. Rather than tracking the intensity profile of the laser spot, these images reveal that the nuclear polarization forms an annular lateral structure having circular symmetry with a dip rather than a peak at its center.

We have measured the optical constants of Ga1-xMnxAs from 0.62 to 6 eV, using spectroscopic ellipsometry. The second derivatives of the dielectric function are examined through a critical point analysis, allowing us to inspect interband transitions from different points in k space. The evolution of the band structure over a broad doping range is determined. Specifically, the E-1 critical point shifts to higher energies with increased doping of Mn, while all other critical points appear unaffected. The evolution of the critical points results from the interplay between band-gap renormalization due to ionized impurities and sp-d hybridization of the Mn induced impurity band with GaAs valence and conductions bands.

We employ optical pump-probe spectroscopy to investigate the voltage dependence of spontaneous electron and nuclear spin polarizations in hybrid MnAs/n-GaAs and Fe/n-GaAs Schottky diodes. Through the hyperfine interaction, nuclear spin polarization that is imprinted by the ferromagnet acts on conduction electron spins as an effective magnetic field. We demonstrate tuning of this nuclear field from <0.05 to 2.4 kG by varying a small bias voltage across the MnAs device. In addition, a connection is observed between the diode turn on and the onset of imprinted nuclear polarization, while traditional dynamic nuclear polarization exhibits relatively little voltage dependence.

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. A time-resolved Kerr rotation microscope with \ensuremath{\sim}1 \ensuremath{\mu}m 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 $(\ensuremath{\sim}{10}^{4}\mathrm{T}/\mathrm{m})$ effective field gradients. The results reveal a flexible scheme for lateral engineering of spin-dependent energy landscapes in the solid state.

Epitaxial (001) EuO thin films have been grown on (001) Si utilizing an intermediate, epitaxial SrO buffer layer by molecular-beam epitaxy. Four-circle x-ray diffraction reveals nearly phase-pure samples. Magnetic measurements indicate that the EuO layer is ferromagnetic, with a transition temperature (68 K) close to the bulk value and a saturation magnetic moment of 4.7 Bohr magnetons per Eu atom. The magneto-optic Kerr effect observed is also comparable to bulk EuO. Such heterostructures have potential as a means to inject spin-polarized electrons into silicon for use in spintronics applications.

Infrared spectroscopy is used to study the doping and temperature dependence of the intragap absorption in the ferromagnetic semiconductor Ga1-xMnxAs, from a paramagnetic, x=0.017 sample to a heavily doped, x=0.079 sample. Transmission and reflectance measurements coupled with a Kramers-Kronig analysis allow us to determine the optical constants of the thin films. All ferromagnetic samples show a broad absorption resonance near 200 meV, within the GaAs band gap. We present a critical analysis of possible origins of this feature, including a Mn-induced impurity band and intervalence band transitions. The overall magnitude of the real part of the frequency dependent conductivity grows with increasing Mn doping, and reaches a maximum in the x=0.052 sample where T-C saturates at the highest value (similar to70 K) for the series. We observe spectroscopic signatures of compensation and track its impact on the electronic and magnetic state across the Mn phase diagram. The temperature dependence of the far infrared spectrum reveals a significant decrease in the effective mass of itinerant carriers in the ferromagnetic state. A simple scaling relation between changes in the mass and the sample magnetization suggest that the itinerant carriers play a key role in producing the ferromagnetism in this system.