Symmetry of two-terminal nonlinear electric conduction.

The well-established symmetry relations for linear transport phenomena cannot, in general, be applied in the nonlinear regime. Here we propose a set of symmetry relations with respect to bias voltage and magnetic field for the nonlinear conductance of two-terminal electric conductors. We experimentally confirm these relations using phase-coherent, semiconductor quantum dots.

Strong, ultranarrow peaks of longitudinal and hall resistances in the regime of breakdown of the quantum hall effect

With unusually slow and high-resolution sweeps of magnetic field, strong ultranarrow (width down to 100 &mgr;T) resistance peaks are observed when high currents are applied through quantum Hall samples. The peaks are dependent on the directions and even the history of magnetic field sweeps, indicating the involvement of a very slow physical process. Such a process and the sharp peaks are, however, not predicted by existing theories. We also find that the sharp resistance peaks are influenced by the nuclear spin flips.

Experimental tunneling ratchets

Adiabatically rocked electron ratchets, defined by quantum confinement in semiconductor heterostructures, were experimentally studied in a regime where tunneling contributed to the particle flow. The rocking-induced electron flow reverses direction as a function of temperature. This result confirms a recent prediction of fundamentally different behavior of classical versus quantum ratchets. A wave-mechanical model reproduced the temperature-induced current reversal and provides an intuitive explanation.