Chris Pearson

Hot scanning tunneling microscope study of B type step edges and small silicon islands on Si(001)

We have used a scanning tunneling microscope to image step edges as well as small Si islands on the Si(001)‐2×1 reconstructed surface at temperatures up to 700 K. We count the changes in the step edge configuration and extract activation energies for the dominant processes. We also observe fluctuations in Si island size at rates that are higher than those we observe on step edges. To aid visualization of the dynamics, we display consecutive images of fluctuating islands and step edges as movies.

A compact micropositioner for use in ultrahigh vacuum

We have designed and built a low profile compact micropositioner for use in air or ultrahigh vacuum. The dimensions are 3.1×1.9×0.75 cm3 with a 1.4 cm range of motion. The device is actuated by two piezoelectric tubes, and utilizes both stick slip and sequential clamping to attain linear motion in one dimension. For motion in two dimensions the flat profile allows units to be stacked. The micropositioner is capable of climbing an incline of 30° while carrying a 17 g load. It is driven forward by staggered 0–250 V signals with a fast rise time and slow decay time. We obtained step sizes from 30 to 400 nm with standard deviation less than 15 nm, and have demonstrated a maximum average velocity of 0.14 mm/s.

Strain mediated reconstructions and indium segregation on InGaAs∕GaAs(001) alloy surfaces at intermediate lattice mismatch

In vacuo scanning tunneling microscopy is used to investigate the surface reconstructions of pseudomorphic InGaAs alloys at intermediate values of compressive strain. The coverage of different reconstructions varies with film thickness, concomitant with changes in composition and strain at the surface arising from In segregation and changes in surface morphology. Thin samples exhibit mainly disordered (1×3) reconstructions along with small regions of incommensurate (1×2). With increasing thickness, the (1×3) transforms into more regular (4×3) or c(4×6), whose coverage mirrors the increase and saturation of In surface composition. Regions of α2(2×4) reconstructions are also present, and their coverage initially increases with In surface composition, but later decreases upon saturation of In at the surface. This decrease is concurrent with the onset of surface roughening, suggesting that the α2(2×4) reconstruction is strain stabilized.

Quantized lattice dynamic effects on the spin-Peierls transition

The density matrix renormalization group method is used to investigate the spin-Peierls transition for Heisenberg spins coupled to quantized phonons. We use a phonon spectrum that interpolates between a gapped, dispersionless (Einstein) limit to a gapless, dispersive (Debye) limit. A variety of theoretical probes are used to determine the quantum phase transition, including energy gap crossing, a finite size scaling analysis, bond order auto-correlation functions, and bipartite quantum entanglement. All these probes indicate that in the antiadiabatic phonon limit a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type is observed at a non-zero spin-phonon coupling,

The coexistence of surface reconstruction domains on strained heteroepitaxial films

g_{\text c}

Direct tests of microscopic growth models using hot scanning tunneling microscopy movies.

. An extrapolation from the Einstein limit to the Debye limit is accompanied by an increase in

Si(001) Step Dynamics.

g_{\text c}

Imaging the evolution of lateral composition modulation in strained alloy superlattices

for a fixed optical (

Elastically Induced Coexistence of Surface Reconstructions

q=\pi

\textbf{Surface phase stability and surfactant behavior of InAsSb alloy surfaces.}

) phonon gap. We therefore conclude that the dimerized ground state is more unstable with respect to Debye phonons, with the introduction of phonon dispersion renormalizing the effective spin-lattice coupling for the Peierls-active mode. We also show that the staggered spin-spin and phonon displacement order parameters are unreliable means of determining the transition.