Nonlinear conductance at small driving voltages in quantum point contacts

Conductance asymmetry of quantum point contacts at near-zero applied biases is reported. This non-linearity in the current-voltage characteristics depends in sign and magnitude on the quantization condition of the point contact (i.e., the gate voltage). The variation in the conductance is of the order of tens of ohms against a background of several kOMEGA, and occurs in a very small bias region, Absolute value of V much less than (E(n)-E(n-1))/e around zero bias, where E(n) is the nth one-dimensional subband energy in the quantum point contacts. The non-Ohmic and asymmetric behavior causes a rectified dc signal as the response to an applied ac current. Second-harmonic-generation measurements also confirm the observation. The phenomenon is discussed in light of recent mechanisms for nonlinearities in point contacts due to the influence of an electric field on a two-level scatterer or on an intermode interference between mode-mixing points.