Talk given by

Dr. Pavel Alekseev 
Ioffe Institute 
St. Petersburg, Russia

Abstract:

At low temperatures, in very clean two-dimensional samples, the electron mean free path for collisions with static defects and phonons becomes greater than the sample width. Under this condition, the electron transport can occur by formation of a viscous flow of an electron fluid. We study the viscous flow of 2D electrons in a magnetic field perpendicular to the 2D layer. We calculate the viscosity coefficients as the functions of magnetic field and temperature. The decrease of the diagonal viscosity in magnetic field leads to a very strong negative magnetoresistance which is temperature and size dependent.

Our analysis demonstrates that this viscous mechanism is responsible for the giant negative magnetoresistance recently observed in the ultrahigh-mobility GaAs quantum wells. We also calculate the time dispersion of the viscosity coefficients in a magnetic field and the corresponding ac response of a viscous electron fluid. For a sufficiently pure sample, the ac response exhibits an anti-resonance at the frequency equal to twice the cyclotron frequency, which can be related to the observed resonances in photovoltage and photoresistance of the ultrahigh-mobility GaAs quantum wells.