Acoustoelectric currents in manganite thin films

Abstract

Recently an anomalous acoustoelectrical (AE) effect produced by surface acoustic waves (SAW) in thin La 0.67Ca 0.33MnO 3 films deposited on a piezoelectric LiNbO 3 substrate was discovered [Phys. Rev. Lett. 87 (2001) 146602]. The SAW, launched along the distinct crystallographic axis (+ z) of the structure, induces a dc current I AE in the film, which in the ordinary AE effect would be odd with respect to the SAW wave vector q . However, in the investigated case I AE contains, along with the ordinary odd component, an anomalous one, which is even in q . The anomalous effect dominates near the metal–insulator transition, while the ordinary effect prevails at high and low temperatures. The anomalous effect appears to depend on strong modulation of the film conductivity resulting from the elastic deformations carried by the SAW. We have investigated the influence of an in-plane magnetic field H on the magnitude of I AE, and found an increase of about 50% when H was parallel to q . The increase was limited to the anomalous component of I AE. We speculate that this may be caused by magnetic field induced changes in the conductivity dependence on pressure. The ordinary AE current was left essentially unchanged. Apparently the boost given to this component by the increase of conductivity in a magnetic field is compensated by the corresponding SAW attenuation decrease. We have also conducted I AE measurements with the SAW split into two components of equal amplitude, propagating in the film in opposite directions. As a result, we have obtained cancellation of the ordinary I AE component and augmentation of the anomalous one, i.e. we have confirmed our understanding of the AE effects.