High-frequency hopping conductivity in the quantum Hall effect regime: Acoustical studies

The high-frequency conductivity of Si delta-doped GaAs/AlGaAs heterostructu res is studied in the integer quantum Hall effect (QHE) regime, using acous tic methods. Both the real and the imaginary parts of the complex conductiv ity are determined from the experimentally observed magnetic held and tempe rature dependencies of the velocity and the attenuation of a surface acoust ic wave. It is demonstrated that in structures with carrier density (1.3- 2 .8) X 10(11) cm(-2) and mobility (1 - 2) X 10(5) cm(2)/V s the mechanism of low-temperature conductance near the QHE plateau centers is hopping. It is also shown that at magnetic fields corresponding to tilling factors 2 and 4, the doped Si delta layer efficiently shunts the conductance in the two-d imensional electron gas (2DEG) channel. A method to separate the two contri butions to the real part of the conductivity is developed, and the localiza tion length in the 2DEG channel is estimated within the context of a neares t-neighbour hopping model.