Island embryo regression driven by a beam of self-ions in the linear regime.

The kinetics of island growth and regression are discussed under the approximation of linear response, including the Gibbs-Thompson potential, for a reacting assembly of adatoms and advacancies (thermal defects) on a surface irradiated with a beam of self-ions. First the quasistatic growth or shrinkage rate, for islands of size n less than the critical size n, is calculated for the driven system, exactly, for linear response. This result is employed to determine successively: (i) the regression rate of driven embryo islands with n < n; and (ii) the structure of the steady state decay chain established when embryos of a particular size n0 < n are created by ion beam impacts. The changed embryo distribution caused by irradiation differs markedly from the populations of the embryos at equilibrium.

Real-time imaging of surface evolution driven by variable-energy ion irradiation.

We describe the design of a tandem instrument combining a low-energy electron microscope (LEEM) and a negative ion accelerator. This instrument provides video rate imaging of surface microtopography and the dynamics of its evolution during irradiation by energetic ions, at temperatures up to 1700 K. The negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range 0-5 keV, and with current densities up to 30 muA/cm2 ( approximately 2 x 10(14)ions/cm2 s or approximately 0.2 ML/s). The LEEM operates at a base pressure in the 10(-9)Pa range. We describe the design and operating principles of the instrument and present examples of Pt(111) and Si(001) self-ion irradiation experiments.

Sublimation of atomic layers from a chromium surface.

We employ low-energy electron microscopy to study the kinetics of thermal etching, or sublimation, of Cr(001) at approximately 1100 K. Atomic layers are removed from the surface by spontaneous nucleation and growth of two-dimensional vacancy islands, by rotation of spiral steps, and by island decay. The growth rates of vacancy islands and the rotation frequencies of double spirals are measured as a function of temperature, and the results are correlated with activation barriers of surface processes. Mass transport between the surface and bulk is shown to be unimportant.

Why is diffusion in metals and on metal surfaces universal?

Recent experiments that determine mass diffusion D(s) on the close packed surfaces of vacuum compatible metals are reviewed. The results turn out to be approximately universal when scaled to homologous temperatures T/T(m), with T(m) the melting temperature. Similar behaviour for vacancy-driven diffusion in bulk metals has been recognized for decades. Remarkably, the uncertainty with which this scaling occurs is only ∼10%. Possible origins of the universality are discussed.

Thermally activated stripe reconstruction induced by O on Nb (011).

We report scanning tunneling microscopy and low energy electron microscopy (LEEM) observations for thin films of Nb (011) of stripe-phase behavior by two variants of an O-induced reconstruction. Stripes occur for thin films but not bulk crystals. At low temperatures the less-favored variant is thermally activated as single stripes on surface heterogeneities. Near T0 = 1505 K, where the reconstruction is lifted, the stripes crowd to form a periodic array with a temperature dependent spacing. LEEM permits quantitative insight into stripe behavior and reveals novel details of stripes interacting with topographic features such as steps, facets, and dislocations.

Magnetostatic effects in the nucleation of rare earth ferromagnetic phases.

It has been reported that superheating, supercooling, and explosive kinetics coupled to other degrees of freedom occur at the ferromagnetic transitions of Er and Dy, and that metastable phases occur during the transition kinetics of Er. We explain these observations in terms of magnetostatic energy, which requires highly eccentric nuclei in the homogeneous nucleation of magnetic transitions in heavy rare earths. The magnetostatics favor transitions through ferrimagnetic intermediaries. The unusual kinetics derive from effective spin lattice relaxation.

Antistructure and point defect response in the recovery of ion-irradiated Cu3Au.

The response of the point defect and antistructure systems to ion beam irradiation is investigated using methods of linear response on thin single crystals of ordered Cu3Au grown by molecular beam epitaxy. We demonstrate that antisite evolution, as measured by electrical resistance, quantitatively determines both the defect populations and diffusion in the irradiation field, and we explore new linear and nonlinear response processes as the antistructure system is driven from equilibrium.

Protein conformation from electron spin relaxation data.

Electron spin relaxation data from five ferric proteins are analyzed in terms of the fractal model of protein structures. Details of this model are presented. The results lead to a characterization of protein structures by a single parameter, the fractal dimension, d. This structural parameter is shown to determine the temperature dependence of the Raman electron spin relaxation rate, which varies as T3 + 2d. Computations of d are made using x-ray data for 17 proteins. The results range from d = 1.76 for lysozyme to d = 1.34 for ferredoxin. These values are compared with values of d obtained from the present electron spin relaxation data on five ferric proteins. Typical results are d = 1.34 +/- 0.06 from relaxation data and 1.34 +/- 0.05 from x-ray data for ferredoxin; d = 1.67 +/- 0.03 from relaxation data and 1.66 +/- 0.05 from x-ray data for ferricytochrome c. The data thus support the theoretical model. Applications of this spin resonance technique to the study of changes in protein conformation are discussed.