Negative magnetoresistance in viscous flow of two-dimensional electrons

Aalto Quantum Physics Seminar (Nanotalo). Speaker: Dr. Pavel Alekseev (Ioffe Institute, Russia).

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 classical magnetic fields perpendicular to the 2D layer. We calculate the viscosity coefficients as the functions of magnetic field and temperature. The decrease of the diagonal viscosity coefficient in magnetic field leads to a strong negative magnetoresistance, which is temperature- and size-dependent. Our analysis demonstrates that such viscous mechanism is responsible for the giant negative magnetoresistance recently observed in the ultra-high mobility GaAs quantum wells. This indicates that 2D electrons in that structures in moderate magnetic fields should be treated as a viscous fluid. We also calculate the time dispersion of the viscosity coefficients in magnetic field and the corresponding ac response of a 2D viscous electron fluid. For a sufficiently pure sample, the ac response exhibits an anti-resonance at the frequency equal to twice the cyclotron frequency. Possibly, this anti-resonance is responsible for the resonance and the feature recently observed in photovoltage and photoresistance of the ultrahigh-mobility GaAs quantum wells. This can be the second evidence that electrons in such structures in moderate magnetic fields form a viscous liquid.