Asymmetric nonlinear conductance of quantum dots with broken inversion symmetry

Coherent electron transport in open, asymmetric (triangular) quantum dots is studied experimentally and theoretically in the nonlinear response regime. The nonlinear dot conductance is found to be asymmetric with respect to zero bias voltage. This conductance asymmetry is related to the nonsymmetric effect of an applied electric field on the quantum electron states inside the dot and on their coupling to the states in the electron reservoirs. The direction of the asymmetry depends sensitively on the amplitude of an applied ac voltage, on the Fermi energy and on the magnetic field, and is suppressed at temperatures above a few Kelvin. Quantum dots can therefore be viewed as ratchets, that is, devices in which directed particle flow is induced by nonequilibrium fluctuations, in the absence of (time-averaged) external net forces and gradients. A quantum mechanical model calculation reproduces the key experimental observations. The magnitude of the conductance asymmetry is found to depend strongly on the electric field distribution inside the dot. In addition to exact calculations, an approximation is presented which makes it possible to qualitatively predict the nonlinear behavior from the energy dependence of the conductance in the linear response regime. We also discuss a semiclassical explanation for our observations and comment on limits of quantum-interference induced rectification.