ABSTRACT
We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.
ABSTRACT
Variations in the width of a quantum well (QW) are known to be a source of broadening of the exciton line. Using low temperature near-field optical microscopy, we have exploited the dependence of exciton energy on well width to show that in GaAs QWs, these seemingly random well-width fluctuations actually exhibit well-defined order-strong long-range correlations appearing laterally, in the plane of the QW, as well as vertically, between QWs grown one on top of the other. We show that these fluctuations are correlated with the commonly found mound structure on the surface. This is an intrinsic property of molecular beam epitaxial growth.
ABSTRACT
A near-field scanning optical microscope for operation within a storage Dewar is described. It was designed for studies of opaque samples and operates in the collection mode. Illumination can be either through the tip or from the side via a separate fiber. Scans can be begun within 2 h after start of cooldown. Its rigid design allows high resolution and long scans with no additional vibration isolation. To illustrate its performance, measurements of photoluminescence in GaAs/AlGaAs heterostructures are presented. The signal and noise levels for the two illumination modes are examined.
