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Number 565, November 14, 2001 by Phil Schewe, James Riordon, and Ben Stein

Quantum Acoustics

The flatland world of electrons residing at the two-dimensional interface between semiconductor planes is an integral part of quantum-well lasers found in many popular electro-optical devices such as grocery scanners and CD players. But the physics at work in these two dimensional electron gases (2DEG) is far from exhausted.

Two years ago a team of physicists used subtle sound waves (surface acoustic waves, or SAW) rippling through one of the semiconductor planes used to confine the electrons to form up the electrons into orderly lines (in effect "quantum wires") and also to transport controllably these formations (see Physical Review Focus).

Now the team, consisting of physicists at the Institute of Semiconductor Physics in Novosibirsk, Russia (contact Sasha Govorov, temporarily at Ohio University, govorov@helios.ohiou.edu) and the Ludwig Maximilans University in Munich (Achim Wixforth, achim.wixforth@physik.uni-muenchen.de ), propose to use two such surface acoustic waves, oriented at right angles, to confine the electrons to essentially zero-dimensional pockets which can be maneuvered around. Thus initially free electrons are organized into quantum wires and dots by intense sound waves.

Furthermore, the train of wires or dots might be able to move through the "quantum film" (the planar region between the semiconductor layers) without resistance; alternatively it can be said that the sound waves move without dissipation, thus constituting an example of self-induced acoustic transparency.

The researchers, who are presently testing their scheme, also hope to combine this ability to position electrons or deliver them selectively to quantum dots with other processes, such as the conversion of light waves into electron-hole (exciton) objects useful for processing optically-encoded information (see Update 321 and accompanying animation; and Ludwig Maximilans website). (Govorov et al., Physical Review Letters, 26 November 2001.)

Singing Like a Canary

Singing like a canary requires little thought, but simple actions, to yield complex vocal physics, researchers have found, yielding potential insights into how humans generate speech sounds.

Human speech and the songs of many bird species share a central similarity: the skills are not present at birth, but are only learned through early-life experiences.

To determine how brain activity leads to the production of sound, scientists strive to understand how much of the sound comes from complicated instructions from the brain and how much comes from complex physics of vocal organs.

Now, a US-Argentina research collaboration (Gabriel Mindlin, University of Buenos Aires, Gabriel@birkhoff.df.uba.ar) has designed a simple physical model that accurately reproduces notes of a canary song.

The researchers modeled the canary's vocal organ, called the syrinx. According to previous experimental evidence, the syrinx generates sound through vibrations of its labial "folds"---flaps of tissue which open and close the air passage between the throat and the lungs.

In their model, the researchers make the key assumption that these labial folds behave like a simple spring, moving back and forth to change the size of the air passage. They further assume that a canary controls its vocalizations through two actions: changing the pressure of the air from the lungs and using muscles to modify the stiffness of the folds.

By varying these two parameters, the researchers found that the spring-like labia could produce faithful recreations of three canary notes. Therefore, simple changes to a basic system, rather than sophisticated instructions from the brain, can reproduce the rich, complex vocal physics which give rise to complicated sounds. (Gardner et al., Physical Review Letters, 12 November 2001.)

The Femtosecond Delay in Electron Collective Motion

The femtosecond delay in the advent of collective motion among electrons in a semiconductor has been observed, for the first time, by an experiment at the Technical University of Munich.

In general, multi-body interactions, whether at the electron level or planetary level, cause a change in properties different from that observed when only two bodies are present.

For example, electrons moving through a semiconductor crystal have very different solo and collective motions; each particle's effective mass and charge become modified by the changes it induces in the surrounding lattice, such as by causing subtle vibrations (phonons) to draw near. The electron is no longer just its former self but has become a corporate entity (particle plus collective motions), or quasiparticle. This alteration, sometimes referred to by the name of screening or dressing, does not happen instantly.

The Munich group, using ultrafast laser pulses, first excited a plasma of electrons and holes and then, with a secondary probe pulse bouncing off the collective motions of the quasiparticles, monitored the growth of the screening process on a femtosecond basis. They observed that it takes tens of femtoseconds for the screening to be complete. (Huber et al., Nature, 15 November 2001.)