Vector-asymmetry of the quantum point contact conductance

We have investigated a non-linearity of quantum point contacts (QPCs) by measuring their differential resistance, dV dI, and the rectification, i.e. a dc voltage response Vrect to an applied ac curent. The differential resistance has an almost step-like behavior, i.e. the resistance is constant in a wide range of small currents but depends on current direction. The value of the step is of the order of a few tens of Ohms against a background of several kΩ and the step occurs over a very small bias less than kT e. This current asymmetry causes an almost linear dependence of the rectified voltage on applied ac current amplitude. The step height varies strongly with gate-voltage and is to some extent correlated with the quantized conductance plateaus. © 1992.