S. Mack

Visualizing Critical Correlations Near the Metal-Insulator Transition in Ga1-xMnxAs

Metal-Insulator Transition At near-zero temperatures, some materials undergo a metal-insulator transition and their electronic properties change from conducting to insulating. In the dilute magnetic semiconductor Ga1−xMnxAs, a promising spintronics material, the metal-insulator transition is driven by the substitution of Ga atoms with Mn. While disorder clearly plays a key role in this transition, the influence of electron-electron correlations has been far from clear. Richardella et al. (p. 665; see the Perspective by Fiete and de Lozanne) used scanning tunneling microscopy to study the electronic states of this system. The autocorrelation function of the local density of states exhibited a power law (rather than an exponential) decay at Fermi energy. Thus, electron-electron interactions are indeed crucial for understanding dilute magnetic semiconductors. Scanning tunneling microscopy reveals the import role of electron-electron interactions in a dilute magnetic semiconductor. Electronic states in disordered conductors on the verge of localization are predicted to exhibit critical spatial characteristics indicative of the proximity to a metal-insulator phase transition. We used scanning tunneling microscopy to visualize electronic states in Ga1-xMnxAs samples close to this transition. Our measurements show that doping-induced disorder produces strong spatial variations in the local tunneling conductance across a wide range of energies. Near the Fermi energy, where spectroscopic signatures of electron-electron interaction are the most prominent, the electronic states exhibit a diverging spatial correlation length. Power-law decay of the spatial correlations is accompanied by log-normal distributions of the local density of states and multifractal spatial characteristics.

Spin-seebeck effect: a phonon driven spin distribution.

Here we report on measurements of the spin-Seebeck effect of GaMnAs over an extended temperature range alongside the thermal conductivity, specific heat, magnetization, and thermoelectric power. The amplitude of the spin-Seebeck effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and the phonon-drag contribution to the thermoelectric power of the GaMnAs, demonstrating that phonons drive the spin redistribution. A phenomenological model involving phonon-magnon drag explains the spatial and temperature dependence of the measured spin distribution.

Stoichiometric growth of high Curie temperature heavily alloyed GaMnAs

Heavily alloyed, 100nm Ga1−xMnxAs (x>0.1) films are obtained via low-temperature molecular beam epitaxy by utilizing a combinatorial technique which allows systematic control of excess arsenic during growth. Reproducible electronic, magnetic, and structural properties are optimized in a narrow range of stoichiometric growth conditions. In contrast to a prediction of the Zener model of hole-mediated ferromagnetism, the Curie temperature of the stoichiometric material is independent of x (for x>0.1), while substitutional Mn content is proportional to x within a large window of growth conditions.

Epitaxial EuO thin films on GaAs

We demonstrate the epitaxial growth of EuO on GaAs by reactive molecular beam epitaxy. Thin films are grown in an adsorption-controlled regime with the aid of an MgO diffusion barrier. Despite the large lattice mismatch, it is shown that EuO grows well on MgO(001) with excellent magnetic properties. Epitaxy on GaAs is cube-on-cube and longitudinal magneto-optic Kerr effect measurements demonstrate a large Kerr rotation of 0.57°, a significant remanent magnetization, and a Curie temperature of 69 K.

Drift and diffusion of spins generated by the spin Hall effect

Electrically generated spin accumulation due to the spin Hall effect is imaged in n-GaAs channels using Kerr rotation microscopy, focusing on its spatial distribution and time-averaged behavior in a magnetic field. Spatially resolved imaging reveals that spin accumulation observed in transverse arms develops due to the longitudinal drift of spin polarization produced at the sample boundaries. One- and two-dimensional drift-diffusion modeling is used to explain these features, providing a more complete understanding of observations of spin accumulation and the spin Hall effect.

Ferromagnetism and infrared electrodynamics of Ga1-xMnxAs

We report on the magnetic and the electronic properties of the prototype dilute magnetic semiconductor Ga

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

_{1-x}

Oscillatory spin polarization and magneto-optical Kerr effect in Fe₃O₄ thin films on GaAs(001).

Mn

Spin control of drifting electrons using local nuclear polarization in ferromagnet-semiconductor heterostructures.

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Infrared conductivity of hole accumulation and depletion layers in (Ga,Mn)As- and (Ga,Be)As-based electric field-effect devices

As using infrared (IR) spectroscopy. Trends in the ferromagnetic transition temperature