The spin polarization of electrons photoemitted from (110) GaAs by irradiating with circularly polarized light of energy 1.5 & heu & 3.6 eU was measured by Mott scattering. The GaAs surface was treated with cesium and oxygen to obtain a negative electron af6nity (NEA), The spectrum of spin polarization P(hem) exhibits a peak (P = 40%) at threshold arising from transitions at I, and positive (P = 8%) and negative (P = -8%) peaks at 3.0 and 3.2 eV, respectively, arising from transitions at L (A). Anomalous behavior, consisting of a depolarization at threshold and an increase and shift in the peak polarization to 54% at 1.7 eV, is attributed to a small positive electron a%nity (PEA) characteristic of some samples. Restriction of the photoelectron emission angle by the PEA leads directly to the anomalously high P. Results of calculations show that P cannot be increased above 50% for emission arising from transitions at I in NEA GaAs. Our detailed interpretation of the spectra indicates how spin-polarized photoemission can be used to study the spindependent aspects of electronic structure. The outstanding qualities of NEA GaAs as a source of spinpolarized electrons are discussed and compared with other sources.

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The evolution of the surface potential during homoepitaxial growth of Fe(001) is studied by scanning tunneling microscopy and reflection high energy electron diffraction. The observed morphology exhibits a non-self-affine collection of moundlike features that maintain their shape but coarsen as growth proceeds. The characteristic feature separation I. is set in the submonolayer regime and increases with thickness t as L(t) -to'"-o" . During the coarsening phase, the mounds are characterized by a magic slope and a lack of reflection symmetry. These observations are shown to be described by a continuum growth equation without capillarity. PACS numbers: 68.55.Jk, 05.70.Ln, 61.16.ChThe basic mechanisms of epitaxial growth onto Hat single crystal surfaces have been known for over fifty years [1]. In the absence of surface defects, growth occurs by the nucleation, growth, and coalescence of two-dimensional (single atom height) islands. Atoms from an external beam arrive at the surface, transfer the heat of condensation to the substrate, and begin activated surface diffusion. Pairs of migrating atoms collide randomly over the surface and may bind to form dimers. The dimers may or may not dissociate thermally before other atoms diffuse to join them. Eventually, stable nuclei form that grow in size as other atoms arrive and attach. Individual island growth continues until nearby islands begin to impinge upon one another and coalesce. Layer completion occurs as freshly deposited particles fill in the gaps between coalesced islands. In principle, this scenario repeats for each layer so that the surface roughness varies periodically in time. But in fact, shot noise in the deposition Aux randomly induces nucleation of new stable nuclei on top of existing islands before layer completion occurs. The progressive roughening of the growth front implied by this picture has been the subject of numerous experimental and theoretical studies in recent years [2,3]. Part of the impetus for many of these studies has been the theoretical expectation [4] that noise-induced roughening during crystal growth might lead to self-affine surfaces [5]. Very recently, however, experimental data obtained from GaAs(001) [6],Cu(001) [7],Ge(001) [8], and Pt (111) [9] have revealed that a completely different morphological scenario can occur for homoepitaxy onto high quality single crystal surfaces. The surface morphology here takes the form of a collection of fairly regular three-dimensional mounds characterized by a well-defined separation distance L(t). The mere existence of the single scale length L(r) implies that these surfaces are not self-affine. According to Villain [10], the origin of this behavior can be traced to an intrinsic instability of the Oat surface that occurs in the island nucleation and coalescence regime whenever energy barriers to the transport of diffusing atoms downward over step edges exceed the usual surface diffusion barrier on a Oat terrace. Since atoms on an incomplete layer that recoil from these barriers are more likely t...

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End states--the zero-dimensional analogs of the two-dimensional states that occur at a crystal surface--were observed at the ends of one-dimensional atom chains that were self-assembled by depositing gold on the vicinal Si(553) surface. Scanning tunneling spectroscopy measurements of the differential conductance along the chains revealed quantized states in isolated segments with differentiated states forming over end atoms. A comparison to a tight-binding model demonstrated how the formation of electronic end states transforms the density of states and the energy levels within the chains.

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