David D. Awschalom

Local manipulation of nuclear spin in a semiconductor quantum well

The shaping of nuclear spin polarization profiles and the induction of nuclear resonances are demonstrated within a parabolic quantum well using an externally applied gate voltage. Voltage control of the electron and hole wave functions results in nanometer-scale sheets of polarized nuclei positioned along the growth direction of the well. Applying rf voltages across the gates induces resonant spin transitions of selected isotopes. This depolarizing effect depends strongly on the separation of electrons and holes, suggesting that a highly localized mechanism accounts for the observed behavior.

Infrared probe of the insulator-to-metal transition in Ga1−xMnxAs and Ga1−xBexAs

We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or nonmagnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga1−xMnxAs, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be-doped samples, however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band.

Optical Manipulation, Transport and Storage of Spin Coherence in Semiconductors

The drive to build a framework for coherent semiconductor spintronic devices provides a strong motivation for understanding the coherent evolution of spin states in semiconductors [1,2]. The fundamental aim in this context is to discover regimes in which carefully prepared quantum states based upon spin can evolve coherently long enough to allow the storage, manipulation and transport of quantum information in devices. Such devices might exploit, for instance, the interference between two coherently-occupied spin states whose time variation occurs at a frequency ΔE/h, where ΔE is their energy separation. Since typical spin splittings in semiconductors are in the range of meV, the rapidly varying oscillations of a classical observable such as the spin orientation (magnetization) can occur at GHz-THz frequencies, providing the basis for ultrafast devices. Another possibility is that this quantum interference may actually be used as part of a calculation within the context of quantum computing algorithms [3]. It is hence crucial to develop experimental tools that probe spin coherence in semiconductors and that allow one to map out schemes for its manipulation, storage and transport. The previous chapter formulated the theoretical foundations underlying coherent spin dynamical phenomena in semiconductors and introduced specific mechanisms that may be responsible for spin relaxation and spin decoherence, pointing out the important physical distinctions between longitudinal and transverse spin relaxation times (T 1 and T 2, respectively) [4]. We note that it is the latter timescale that is of direct relevance to coherent spin devices and hence we focus on experimental techniques that probe the transverse spin relaxation time in semiconductors.

Magnetic anisotropy in arrays of nanometer-scale iron particles

Arrays of ferromagnetic iron particles have been fabricated by a combination of chemical vapor deposition and scanning tunneling microscopy. Hall magnetometry and MFM measurements have been used to determine their magnetic properties. The measured switching fields indicate magnetization reversal by curling from which an exchange length, /spl lambda//sub ex/=3.1 nm, may be estimated. The cylindrically shaped particles have an easy magnetization direction (EMD) along their long axis due to shape anisotropy. The magnetic anisotropy is evaluated by fitting calculated magnetization curves to those measured with applied field perpendicular to the particles EMD.

Fabrication and Characterization of Modulation-Doped ZnSe/(Zn,Cd)Se (110) Quantum Wells: A New System for Spin Coherence Studies

We describe the growth of modulation-doped ZnSe/(Zn,Cd)Se quantum wells on (110) GaAs substrates. Unlike the well-known protocol for the epitaxy of ZnSe-based quantum structures on (001) GaAs, we find that the fabrication of quantum well structures on (110) GaAs requires significantly different growth conditions and sample architecture. We use magnetotransport measurements to confirm the formation of a two-dimensional electron gas in these samples, and then measure transverse electron spin relaxation times using time-resolved Faraday rotation. In contrast to expectations based upon known spin relaxation mechanisms, we find surprisingly little difference between the spin lifetimes in these (110)-oriented samples in comparison with (100)-oriented control samples.

Development of Spintronic Bandgap Materials

The development of Ge/Si quantum dots with high spatial precision has been pursued, with the goal of developing a platform for “spintronics bandgap materials”. Quantum dots assemblies were grown by molecular beam epitaxy on carbon-templated silicon substrates. These structures were characterized by atomic force microscopy. Vertically gated structures were created on systems with up to six well-defined quantum dots with a controlled geometric arrangement, and low-temperature (mK) transport experiments were performed. These experiments showed evidence for a crossover from diamagnetic to Zeeman energy shifts in resonant tunneling of electrons through electronic states in the quantum dots.

Spin control in semiconductors: Variations on a theme

The manipulation of electron spins in semiconductors is a core concept in semiconductor spintronics. We review recent experiments that show how optical, electrical and exchange fields allow the control of spin-dependent phenomena in semiconductor devices. The first example addresses the all-electrical generation of electron spin polarization in conventional semiconductors via the spin-orbit interaction. Our experiments show that current-induced spin polarization and the spin Hall effect can be observed even in wide band gap semiconductors such as ZnSe, despite a relatively weak spin-orbit coupling parameter. The next example shows how circularly polarized photons allow us to both pump and probe spin polarized states in semiconductor microcavity lasers, resulting in the surprising finding that the spin dephasing time is correlated with the Q-factor of the cavity and the onset of stimulated emission. Finally, we discuss how we exploit the exchange interaction between local moments and band electrons to manipulate electronic and local moment spin dynamics in magnetic semiconductor quantum structures.

Confinement engineering ofs−dexchange interactions inGa1−xMnxAs∕AlyGa1−yAsquantum wells

Recent measurements of coherent electron spin dynamics reveal an antiferromagnetic s-d exchange coupling between conduction band electrons and electrons localized on Mn{2+} impurities in GaMnAs quantum wells. Here we discuss systematic measurements of the s-d exchange interaction in GaMnAs/AlGaAs quantum wells with different confinement potentials using time-resolved Kerr rotation. Extending previous investigations of the dependence of the s-d exchange, N{0}alpha, on well width, we find that its magnitude also depends on well depth. Both phenomena reduce to a general dependence on confinement energy, described by a band-mixing model of confinement-induced kinetic exchange in the conduction band.