Single Molecule Transistors
H. Park

Single-molecule transistors incorporating individual divanadium molecules. Background: optical micrograph of an array of single-molecule transistors. Upper right inset: a cartoon of a divanadium molecule bridging two gold electrodes. Lower left inset: a schematic diagram of a single-molecule transistor.

Manipulation of Conductivity in DNA
M. Tinkham and H. Park

a) Topographic image of a DNA strand; b) Phase shift image indicating that the conductivity has increased by several orders of magnitude compared to pure l-DNA. In previous work using a novel contactless scanning conductance method, we imaged the conductivity of l-DNA strands and found that they are insulating, apparently ruling out their use in “wiring” nano-circuits. Recently, we have discovered that by substituting Zn2+ ions into the base pairs of l-DNA the conductivity of the DNA strand increased sufficiently to appear on our scanning conductance probe. Further work will be needed to explore possible applications of such DNA with enhanced conductivity.

Electronic States in Stretched DNA Macromolecules
E. Kaxiras

The amplitude of the electronic wavefunction of stretched poly-CG DNA macromolecule is shown on a cylindrical surface anchored at one of the guanine carbon atoms. The contours show the electron concentration. Stretching of the molecule along its axis leads to significant changes in the overlap between electrons on adjacent bases, which has a dramatic effect on the conductivity of the structure, reducing it by as much as 6 orders of magnitude for an elongation of 90% relative to its natural length.

Simulations of Coherent Electron Flow
E. Heller and R.M. Westervelt

Flow of electrons into a two-dimensional electron gas–each electron follows its own trajectory, and the color indicates its semiclassical phase. Toward the lower right, farthest from the source, the phases no longer align due to thermal broadening in their velocity distribution.

Metallic ErSb Nanoparticles on a GaSb Surface
A.C. Gossard

Semimetallic ErSb nanoparticles were grown epitaxially on a GaSb surface by molecular beam epitaxy. The size and density of the metal nanoparticles can be controlled by the growth technique. The particles can also be overgrown and embedded in a semiconductor film. Narayanamurti will use BEEM to characterize the films and Kaxiras will do calculations to understand their structural and electronic properties.

Controlling Spin Flow through a Single Electron Transistor with the Kondo Effect
M. Kastner

The figures show the differential conductance resulting from the Kondo effect in a single electron transistor (SET). An unpaired electron in the quantum dot inside the SET forms a weak chemical bond with electrons in the leads. A magnetic field breaks this bond between 0 and 4T, splitting the Kondo peak. At a higher field 14T one has a sharp threshold—for electrons with low energy near Vds= 0 only one spin direction can pass through the SET.
At higher energies, electrons with opposite spin can pass too. This effect may be useful in controlling electron spins for quantum computing.

Now you See it, Now you Don’t
C.M. Marcus and B.I. Halperin

The conductance of GaAs/AlGaAs quantum dots vs. magnetic field reveals an important difference between large and small dots: the conductance peak at B = 0 produced by strong spin-orbit coupling for large dots becomes a minimum for small dots. These data demonstrate that spin-orbit effects are suppressed in small dots.

Live Presentations at the Current Science & Technology Center Museum of Science, Boston
C. Alpert and J. Rosenberg

Joel also presents “True Science Stories” about current controversies in the news:

1. The Hendrick Schön investigation
2. The Bubble Fusion Controversy
3. The Synthesis of Element 118

Other science news topics presented recently include:

• Creation of antihydrogen at CERN this fall
• 2002 Nobel Prizes in Chemistry and Physics
• Science Journals, Copyright, and the Internet

A video of Joel’s presentation, The Bubble Fusion Controversy, may be viewed on the CS&T website: http://www.mos.org/cst/article/3550/