An Anomalous Acoustoelectric Effect

An anomalous acoustoelectric effect has been discovered by a Russia-Poland-Ukraine collaboration (A.V. Goltsev, Ioffe Physical Technical Institute, St. Petersburg, goltsev@gav.ioffe.rssi.ru).

When an acoustic wave propagates through an electrically conducting surface, it can drag electric charge along with it, just as wind drags autumn leaves along a street. This "acoustic wind" is known more formally as the acoustoelectric (AE) effect.

Studying the electric current produced by the AE effect can provide important information on how electrically charged particles interact with the crystal lattice of a conducting material. Such materials include "manganites," manganese-based compounds that can exhibit "colossal magnetoresistance," in which electrical conductivity becomes tremendously sensitive to external pressure and applied magnetic fields.

Towards these ends, the researchers investigated the AE effect in a manganite thin film atop a lithium-niobium-oxygen (LNO) substrate. They observed an unusual effect: sending an acoustic wave in a certain direction produced a much weaker electric current than expected in that direction.

The researchers discovered why: in addition to the ordinary acoustic wind, a countervailing wind was flowing in a direction opposite to the acoustic wave. The countervailing wind arose from the fact that the substrate was "piezodielectric," in which electric fields were generated in response to pressure. When the acoustic wave created an alternating pattern of compression and expansion in the substrate, the compressed regions produced electric fields pointing in the direction of the countervailing wind. These fields interacted with the electrons on the thin film. Since the manganites increase their conductivity dramatically when compressed, this encouraged a flow of electrons in the countervailing direction.

While this anomalous AE effect is probably too weak for technological applications, measuring it could provide a new method for studying the effects of applied pressure on a conducting material. This could be useful in those cases when employing conventional methods for those measurements is difficult, as is the case for thin films or quantum wells, wires, or dots. (Ilisavskii et al., Physical Review Letters, 1 October 2001.)