An Anomalous Acoustoelectric Effect
Sam Wormley
The American Institute of Physics Bulletin of Physics News
Number 557 September 20, 2001 by Phillip F. Schewe, Ben Stein,
and James Riordon
AN ANOMALOUS ACOUSTOELECTRIC EFFECT has been
discovered by a Russia-Poland-Ukraine collaboration (A.V.
Goltsev, Ioffe Physical Technical Institute, St. Petersburg,
gol...@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)