Influence of the second subband on thesurface acoustic wave transmission in the quantum Hall regime

Abstract

The attenuation of surface acoustic waves (SAWs) propagatingthrough an asymmetric two-layer system is investigated in thequantum Hall regime. The position of the Fermi level is variedin the vicinity of the population threshold of the secondsubband by changing the electron density and by injecting a dccurrent. For large currents, the quantum Hall effect (QHE)breaks down. The SAW attenuation in the breakdown regime revealsthat the vanishing conductivity of the QHE is maintained in theinterior of the sample despite its absence from the resistance.The inhomogeneous breakdown of the QHE becomes more apparentwhen the Fermi level approaches the second subbandlevel. When the electron population in the second subband issignificantly large, the SAW transmission develops giganticstructures at the quantum Hall states. A drastic variation ofthe screening properties associated with the relocation ofelectrons between the two layers is responsible for thebehaviour. The observation of a hysteretic behaviour evidencesthat the SAW-generated electric fields are strong enough toinfluence the interlayer charge transfer.